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Chapter 1: An Overview of TFPDAS4

TFPDAS4 is designed to be the easy-to-use and powerful application center to use in combination with a Tandem-

Fabry-Pérot interferometer that is developed and sold by JRS Scientific Instruments.

This chapter gives an overview on how TFPDAS4 works and what you can do with it. If you are new to TFPDAS4, this

chapter is for you. If you are familiar with the TFPDAS4 features and interface, feel free to skip this chapter. This chap-

ter does not provide all detailed information and instructions needed to work with TFPDAS4. Once you are familiar

with the interface of TFPDAS4, you will find in depth explanations and instructions in later chapters.

This chapter covers:

• A first look at TFPDAS4

• TFPDAS4 main window in more details

• Structure of the menu bar

• First time using TFPDAS4

• What you need to do if none of the parameters were configured yet

Chapter 1: An Overview of TFPDAS4_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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9

A First Look at TFPDAS4

Spectrum window: shows the signal detected

by the photo detector

Cursor Coordinates: displays the frequency and

number of counts recorded at the location of the red

cursor in the spectrum window

Active Stabilization: list of the

main parameters that ensure a

stable operation of the

interferometer: the Dither actual,

the Finesse Optimizer Signal, and

the Drift Stabilizer Signal

Piezo Status: shows current

positions of all of the piezo-electric

actuators that control the mirror tilt

and spacing in their maximum

travel range of ±5V

Number of Scans: displays a running

total of the number of scans

Active Stabilization Status: use three buttons to change

status of active stabilization routine: green LED - stabilization

running; red LED - stabilization turned off

Chapter 1: An Overview of TFPDAS4 - A First Look at TFPDAS4_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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10

TFPDAS4 Main Window in More Detail

Chapter 1: An Overview of TFPDAS4 - TFPDAS4 Main Window in More Detail_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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11

Spectrum window

GHz or FSR

lin

ea

r o

r lo

ga

rith

mic

sca

le

The most important section of the main window, of course, is the graph that shows the signal detected by the photo detec-tor.

In a running measurement, the detected signal can either be accumulated as the number of scans increases or the signal de-tected during one single scan of the interferometer stage can be displayed. You can toggle between these two options with the shortcut Control + W or using the menu SPECTRUM > show

accumulated scans.

When accumulating a spectrum, the x-axis can be displayed ei-ther in units of the free spectral range (FSR) or in GHz. Switching between the units can be done in the menu SPECTRUM > X

Axis. The y-axis displays the detected photon counts. You can change the scale of the y-axis between linear and logarithmic in the menu SPECTRUM > Y Axis or with the shortcut Control + Y.

Cursor coordinates

The red cursor in the graph can be dragged to any desired posi-tion with the mouse. The Cursor Coordinates section in the upper left corner of the TFPDAS4 main window displays the fre-quency and number of counts recorded at the cursor location. This allows you to accurately determine the frequency of a measured signal.

The frequency is given in GHz or FSR depending on the units chosen for the x-axis.

DRAG

CHECK frequency

of signal

Number of scans

The Number of Scans box displays a running total of the num-ber of scans that have been counted in the current accumula-tion, e.g., the interferometer performed a total of 2446 scans.

If you are in the reflection or transmission mode, the number of scans from the last measured spectrum is displayed.

teste

Chapter 1: An Overview of TFPDAS4 - TFPDAS4 Main Window in More Detail_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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12

Active stabilization

To ensure high performance of the Tandem-Fabry-Pérot inter-ferometer over a long period of time, TFPDAS4 is equipped with an Active Stabilization routine. This routine is divided into three separate parts. The main parameters required to evaluate the condition and the status of the stabilization are displayed in the main window of TFPDAS4.

• The Finesse Optimizer makes small adjustments to the piezo voltages to achieve and maintain optimum align-ment of the etalon mirrors. The FO Signal is the area un-der the reference peak, which provides a measure for the mirror parallelism and the synchronization of both etalons.

• The Drift Stabilizer ensures that the main reference peak remains at the 0-GHz position, which is achieved by keep-ing the DS Signal as close to zero as possible.

• To allow the system react to disturbances of different magnitude, the voltage step size used for the stabilization can be adjusted dynamically using the Dynamic Dither. During stabilization, the piezo voltages controlling the mirror tilt and spacing are varied by the amount of the Dither actual in the positive and in the negative direc-tion. For both directions, the FO signal is evaluated and then the piezo voltage is corrected in the direction that improves the FO signal. In order to compensate for major disturbances as fast as possible, the dither actual is changed dynamically depending on the current value of the FO signal. If the FO signal drops below a user-specified threshold, the dither actual will be increased. Once the FO signal improves, the dither actual value will be decreased.

All three parts of the Active Stabilization can be turned on and off by pressing the circular LED buttons in the TFPDAS4 main window.

Active Stabilization

OFF

Active Stabilization

ON

For detailed information concerning the stabilization routines and the related parameters, we refer to the corresponding chap-ter: Stabilization.

Piezo status

On the lower left section of the main window, the current posi-tions of all of the piezo-electric actuators that control the mirror tilt and spacing are shown.

If one of the piezo voltages reaches the border of the possible scan range between -5 V and 5 V, i.e., the edge of the yellow bar, the indicator above will turn red and the message Piezos out of range will be displayed.

If this happens, it will be necessary to open the interferometer and align the etalon mirrors using the motor compensation as described on page 28 of the TFP-1 Operator Manual by JRS Scientific Instruments.

http://www.jrs-si.ch/products.php


Structure of Menu Barmenu pathmenu path shortcut brief descriptionsave spectrum

measurement data file Control + Ssave actual data with all parameters in .mdf format that was specifically designed

for TFPDAS4

spreadsheet file Control + Shift + S save data in measurement graph in .txt format

MAIN AutosaveMAIN

Load spectrum load single measurement done previously

Data inspector

EXIT Control + Q exit TFPDAS4

Reflection Mode Control + R switch to reflection mode

Transmission

ModeControl + T switch to transmission mode

TFP Auto Alignment run auto alignment

scan width small (large) scan width Control + Z change scan width in transmission or reflection mode

Open Box Mode switch to open box mode for securely opening the interferometer

ADVANCED open input shutter Control + Shift + K

start new spec-

trumControl + M start new spectrum

continue spec-

trumControl + Shift + M continue paused accumulation

clear spectrum Control + C clear accumulated scans and start new accumulation

ROI show ROIs Control + B show regions of interest

SPEC-

show accumu-

lated scansControl + W switch between showing accumulated counts or the counts of one single scan

TRUM pause accumula-

tion of scanspause accumulation but still measure new spectra

Get Number of

TAG BITS

X Axis...Frequency (GHz) display x-axis in units of gigahertz

X Axis...FSR display x-axis in units of free spectral range

Y Axis...autoscale

Control + Yautoscale y-axis to highest intensity measured

Y Axis...logarithmic

Control + Ydisplay y-axis in a logarithmic scale

Chapter 1: An Overview of TFPDAS4 - Structure of Menu Bar_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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13

menu pathmenu path shortcut brief descriptionPA-

Alignment Control + F5open window containing all of the parameters related to the auto alignment and

the alignment modePA-

RAME- Sample Control + F6 open window in which you can type in information about the sample

TER Scan Control + F7 open window containing all of the parameters related to acquisition mode

Stabilize Control + F8 open window containing all of the parameters related to the stabilization

COM-

PLEX

define and start

scanopen window to define new complex scan

SCAN analyze scan open window to analyze data from a previous measurement

Dark count rate measure dark count rate

DIA-

GNOS-

Piezo Calibration

(Z)calibrate voltage applied to the scanning Z piezo

TICS Piezo Calibration

(dZ)calibrate voltage applied to the dZ piezo

Chapter 1: An Overview of TFPDAS4 - Structure of Menu Bar_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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14

First Time Using TFPDAS4

Before you work with TFPDAS4 you must have a working knowledge of how to handle and align a Tandem-Fabry-Pérot interferometer developed by JRS Scientific Instruments. If you have any questions concerning the alignment of the interferometer, we recommend reading the operator manual provided on the homepage by JRS Scientific Instruments: TFP-1 Operator Manual.

This section tells you what you need to do when you use the TFPDAS4 software the very first time.

Step 1: Start TFPDAS4

To start TFPDAS4 click on the TFPDAS4.exe icon under C:\Programs\TFPDAS4-Project.

The main window of TFPDAS4 together with the TFPDAS4 Welcome screen will pop up. To close the TFPDAS4 Welcome screen click with the mouse or press any key of the key-board.

In this section, we assume that all of the parameters that need to be set in order to al-low for a reliable performance of TFPDAS4 were set during the installation. If this was not the case, please continue with the next section: What you need to do if none of the parameters were configured yet.

Step 2: Align etalon mirrors

Before you can start a measurement, the interferometer mirrors need to be aligned. You can either do this manually using the remote control or run the auto alignment routine.

If you decide to do the alignment manually, choose TFP > Reflection Mode to bring the interferometer in the reflection mode: This option will only work if the stage, that changes the optical path inside the interferometer, is automized. If this is not the case, you will need to change the optics manually:

Switch to Reflection Mode

Pause scanning of interferometer with

ESC button on keyboard.

Bring switch on outside of the interfer-

ometer housing to ALIGN position.

Go to TFP > Reflection Mode.

Chapter 1: An Overview of TFPDAS4 - First Time Using TFPDAS4_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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15

TFPDAS4 main window



The graph in the main window of TFPDAS4 will probably show a noisy but continuous signal. Only in case of aligned or partially aligned etalon mirrors small dips may be visible in the reflection signal.

In order to align the etalon mirrors, use the game controller or the hand box provided by JRS Scientific Instruments to change the voltages applied to the X and Y piezos of etalon 1 and 2. The voltage applied to the dZ piezo will only change the spacing of etalon 2. A variation in the dZ voltage will allow to synchronize the transmission of eta-lon 2 to the transmission of etalon 1, i.e., to move the dip associated with etalon 2 to the same position as the dip associated with etalon 1. Changing the voltage applied to the Z piezo will simultaneously change the mirror spacing of both etalons. That is why the dips associated with etalon 1 and 2 will both shift together if you vary the Z piezo voltage. Adjust the voltage applied to the Z piezo to move the overlapping dips of etalons 1 and 2 to the center of the scan range.

For further details on how to align the etalon mirrors manually, we refer to the chapter designated to this topic: Mirror Alignment Using the USB Game Controller

Important: The USB Game Controller needs to be connected to the personal computer when it is still turned off.

Once the etalon mirrors are aligned and synchronized, you will see a single dip in the reflection signal.

Switch to TFP > Transmission Mode in the main window of TFPDAS4.

If the optical stage inside your interferometer is not automized, you will need to change the optics manually:

Switch to Transmission Mode




ometer housing to TANDEM position.

Go to TFP > Transmission Mode.


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16

Reflection Mode

etalon mirrors not aligned

Reflection Mode

etalon mirrors aligned and

transmission synchronized

Transmission Mode

right after alignment of the

etalon mirrors

no dip visible two dips overlapping one transmission peak

visible

Instead of aligning the etalon mirrors manually, you can also choose to do the align-ment of the mirrors all automatically: Go to TFP > Auto Alignment.

If the movement of the optical stage inside the interferometer is automized, the Auto Alignment routine will bring the interferometer in a condition for starting a meas-urement within a few minutes. You can find the details on how the auto alignment routine operates in the corresponding section of this manual: Auto Alignment

When the etalon mirrors are aligned - either manually or using the auto alignment routine - and the interferometer is scanning in transmission mode, a single peak will be visible in the graph of the TFPDAS4 main window.

Step 3: Activate stabilization

This transmission peak may be rather small after you switched to the transmission mode. This indicates that the (3+3) beam path inside the interferometer is not aligned to its optimum. Therefore, TFPDAS4 is equipped with an Active Stabilization routine which will automatically optimize and stabilize the overall alignment and performance of the interferometer.

To activate the stabilization routines click on each of the three circular LED buttons under Active Stabilization in the main window so that they shine green.


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17

ready to start a

measurement

Stabilization OFF Stabilization ON

OPTIMIZATION

peak height

remains constant

Stabilization ON

Step 4: Start measurement

When a nice peak is visible in the transmission signal that is also centered around the 0-GHz position, the interferometer is in a condition to start a measurement. Go to SPECTRUM > Start new spectrum and a new measurement will start.

For the Scan Range, i.e., the covered frequency range during the measurement, as well as for the Regions of Interest (ROIs), default values are set. You can use the shortcut Control + B to display the regions of interest on the graph in the TFPDAS4 main window. In order to change the scan range or the regions of interest we refer to the corresponding section of this manual: Regions of Interest


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18

What you need to do if none of the parameters were configured yet

Step 1: Calibration of the Z piezo voltage

The Z piezo voltage simultaneously determines the mirror spacing of both etalons. The scanning of the stage is realized by applying a saw-tooth voltage to the Z piezo. The offset of this saw tooth voltage determines the position of the elastically scattered light which we refer to as the reference peak.

In order to allow for a proper scanning of the Z piezo, the applied voltage range needs to be translated into the correct frequency range. This is done with the parameter Scan Piezo (Z) Calibration in the PARAMETER > Scan Parameter menu. When start-ing TFPDAS4 the very first time, the value of this parameter will most probably not be correct.

You can check this when the interferometer is in reflection mode and a dip associated with etalon 1 is visible: Move the offset of the scanning Z piezo so that two transmis-sion orders, i.e., two dips associated with etalon 1, are visible. Check the distance be-tween the two transmission orders. If this distance does not equal one free spectral range (FSR), the calibration of the scanning Z piezo is not correct.

To determine the correct value for the parameter Scan Piezo (Z) Calibration, chose the menu DIAGNOSTICS > Piezo Calibration (Z). This routine will run automatically and will set the calibration to its proper value. You can find a detailed description of this calibration procedure in the corresponding chapter of this manual: Piezo Calibra-tion (Z)

When the calibration is finished and you go back to the reflection mode, the distance between two transmission orders of etalon 1, i.e., between two dips associated with etalon 1, must equal one free spectrum range. If this is still not the case, run the DIAG-

NOSTICS > Piezo Calibration (Z) again.

Important: If for some reason the old value of the Scan Piezo (Z) Calibration parameter equals zero, the calibration routine will not be able to determine the cor-rect value. Just type in any value for the Scan Piezo (Z) Calibration parameter in the PARAMETER > Scan menu and then run the calibration routine.

Chapter 1: An Overview of TFPDAS4 - What you need to do if none of the parameters were configured yet_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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19

offset scanamplitudeZ piezo voltage timeNo dip attributed to etalon 2!

must equal 1 FSR!

etalon 1order n

etalon 1order n+1

Step 2: Calibration of the dZ piezo voltage

The dZ piezo only changes the mirror spacing of etalon 2 and is used to synchronize the transmission functions of both etalons. Align etalon 1 and 2 to observe a dip for each etalon in the reflection signal. If you now change the voltage applied to the dZ piezo, you will observe that only the dip associated to etalon 2 will move; the dip asso-ciated to etalon 1 will remain unchanged.

In order to translate a certain voltage range into a shift in free spectral range, a calibra-tion of the dZ piezo voltage is necessary. This calibration value is determined by the parameter Piezo (dZ) Calibration in the menu PARAMETER > Parameter Scan.

Chapter 1: An Overview of TFPDAS4 - What you need to do if none of the parameters were configured yet_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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20

etalon 1 etalon 2

change dZ only one dip will move

Chapter 2: Modes of Operation

JRS Scientific Instruments distinguishes between two different optical systems inside the interferometer housing: In

the tandem mode, the light has to pass the (3+3) beam path and is detected at the photodetector as one single

peak only in case of good mirror alignment. If the optics are switched to JRS Scientific Instruments' align mode, the

beam path inside the interferometer is changed in such a way that the light cannot pass the mask that is mounted

right behind etalon 1. Therefore, the light is only detected at the photo detector if it is reflected off of both etalons.

In this configuration, a noisy and continuous signal will be seen at the photomultiplier showing none, one, or pos-

sibly two dips depending on the alignment of the Tandem-Fabry-Pérot interferometer.

TFPDAS4 is equipped with different modes of operation that correspond to these two optical systems and set the

current operating status of the interferometer. But TFPDAS4 is not limited to just two modes of operations but distin-

guishes between a total of five modes of operations:

• Reflection Mode: corresponds to align mode

• Transmission Mode: corresponds to tandem mode

• Auto Alignment: automized routine that will reliably bring your Tandem-Fabry-Pérot interferometer into a condition of suffi-

cient alignment to start a measurement within only a few minutes

• Acquisition Mode: actual measurement mode

• Open Box Mode: mode for maintenance of the interferometer

These modes of operations determine the operating status of the interferometer, in particular, the voltage range applied to the scan-ning Z piezo and the performance of the double shutter at the entrance pinhole of the interferometer housing.

Important: If your interferometer is not equipped with an automized stage with which the optics can be moved remotely, the commands for changing the optics will need to be deactivated in the software. If this is not done, TFPDAS4 will try to communi-cate with the optical stage and will wait indefinitely for feedback that will not come. This will lead to a crash of TFPDAS4. A button is included that will deactivate all communication to the optical stage. You find this in the first tab of the Alignment Pa-

rameters window which can be called in the menu bar under PARAMETER > Alignment or with the shortcut Control + F5.

Chapter 2: Modes of Operation - Reflection Mode_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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21

This chapter covers:

• Reflection Mode

• How to set up a USB game controller

• Mirror alignment using the hand box or USB game controller

• Which dip belongs to which etalon?

• What if I observe two dips that are not one FSR apart yet both move when I change the Z piezo voltage?

• Parameters related to the Reflection Mode

• Transmission Mode

• Auto Alignment

• Acquisition Mode

• Open Box Mode


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22

Reflection Mode


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23

In reflection mode, the optical beam inside the inter-

ferometer is switched to a configuration in which the

light of the reference beam is reflected from the first

etalon, then guided via a beamsplitter to the second

etalon where it is again reflected. Finally, the light is

sent to the photo detector where it is detected as a

noisy but continuous signal. When the etalon mirrors

are aligned or partially aligned, one or two dips in the

detected signal are visible.

In the reflection mode configuration, the photo detec-

tor essentially detects the product of the reflection

functions of the two etalons:

(1 − AiryE1) · (1 − AiryE2),

where AiryE1 and AiryE2 represent the Airy transmission

functions of etalons 1 and 2, respectively, which de-

pend on the etalon spacing L. The functions

1 − AiryE1,E2

are equal to one, except in those situations when the

etalon spacing is an integer multiple n of half the laser

wavelength λ: L = n · λ/2. For these etalon spacings, the

resonance of each etalon can be seen as a respective

dip in the detected signal.

Since the depth of the dips is a measure of the mirror

parallelism of each etalon, the reflection mode is

used to optimize the alignment and synchroniza-

tion of both etalons. Improving the depth of one dip

leads to an enhancement of the mirror parallelism of

the corresponding etalon. The two etalons are in series

so each etalon causes its own dip in the reflection sig-

nal, which can be optimized separately by varying the

piezo voltages X1-Y1 and X2-Y2 for etalons 1 and 2, re-

spectively.

The dip of the first etalon can be centered at the mid-

dle of the scan range, which corresponds to the posi-

tion of zero frequency shift, by adjusting the Z-offset of

the scan piezo. Modification of the dZ piezo voltage

changes the mirror spacing of etalon 2 only and, thus,

is used to synchronize the transmission function of the

second etalon to that of the first etalon.

TFPDAS4 main window when the interferometer is scanning in

Reflection Mode

etalon 1 etalon 2

Over the entire scan range a signal is detected at the photo detector. The two dips

that are visible in the otherwise continuos reflection signal are attributed to the good

alignment of the etalon mirrors.

scan range


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24

How to Set Up the USB Game Controller

The TFPDAS4 software allows for the use of a USB game controller to move the piezos and, thus, align the etalon mirrors. The first time you use the game controller you will have to configure the buttons. The steps to do this are as follows:

Step 1: Connect game controller to computer

Check to make sure that the game controller is connected to one of the USB ports of the TFPDAS4 computer.

Step 2: Call Configuration window

Go to PARAMETER > Alignment.

This will pop up a the Alignment Parameter window. Select the third tab on the top en-titled Hand Box.

Within this pane, select the check box beside Use USB Controller and then click on the Configure button.

This will pop up the Configuration window.

Important:You will need to restart your computer after connecting the game controller. If you forget to do this, the game controller will not be recognized as new hardware and cannot be used.

Chapter 2: Modes of Operation - How to Set Up the USB Game Controller_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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25

Alignm

ent

Parameter

window

Step 3: Configure game controller

Now you are ready to configure the controller. Follow the instructions on the top of the Configuration window from left to right.

Initially all of the gray boxes should read not allocated.

Even if the joysticks of the game controller are not moved, they will never remain at the absolute zero position but will always send a small voltage signal. That is why it is necessary to define a combinational use of one joystick and one button to change any of the piezo voltages.

To configure the joystick and button on the controller that will change the X1 piezo voltage, first check the box for X1 on the configuration window. Next move the left joystick on the controller up and down. The left gray box on the screen should now read Y axis. Then press button 5 and the right gray box should read Button 5:Y axisnot allocated Button 5not allocatedProceed to the next piezos and configure each using the same proce-dure. When the controller is fully configured, the software window should look like this: Y axisZ axis rotation Button 5Button 5Button 6Button 6Button 8Button 7Z axis rotationY axisZ axis rotationY axisNote that Z axis rotation corresponds to up-down motion of the right joystick.


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26

Configuration

window

button 5

button 6

button 8

button 7

Y axis

Step 4: Confirm changes

Click on the OK button on the Configuration window and then on the Alignment Pa-

rameter window to accept the changes and return to the main window of TFDPAS4.

The USB game controller is now ready to use...

Important: If the status of the gray boxes do not change when you press a but-ton on the game controller, it is most probably related to a wrong device index. TFPDAS4 needs to know at which USB port it should listen for commands coming

from the game controller. Use the Cancel button to go back to the Alignment Parameter window and change the USB device index. Usually, it should be a number between 0 and 10.

Now try to do the configuration again.

Important: If you ever need to buy a new game controller, pay attention that buttons 5, 6, 7, and 8 are just digital buttons and NO analogue buttons!


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27

Mirror Alignment Using the USB Game Controller

The mirror tilt as well as the mirror spacing can be controlled remotely via changing the piezo voltages X1, Y1, X2, Y2, dZ, and Z using a hand box provided by JRS Scientific Instruments or a commercially available USB game controller.

The following section will give a step-by-step description on how to align the etalon mirrors using the game controller:

Step 1: Switch to reflection mode

Switch to TFP > Reflection Mode.

Pause scanning of

interferometer with ESC

button on keyboard.

Bring switch on outside of the

interferometer housing to

ALIGN position.


Pause scanning of interferometer

with ESC button on keyboard.

Bring switch on outside of the

interferometer housing to

TANDEM position.


Switch to Reflection Mode Switch to Transmission Mode

Step 2: Mirror alignment - etalon 1

Modifying the X1 and Y1 piezo voltages adjusts the mirror tilt of the first etalon in the x- and y-directions and will cause either an enhancement or a degeneration of the first etalon's dip. Adjust the piezo voltages X1 and Y1 step by step and one after the other and observe how the dip in the reflection signal changes.

Change the X1 and Y1 piezo voltages using the game controller: X1: move left joystick while pressing button 5

Y1: move right joystick while pressing button 5

Important: If your interferometer is not equipped with an automated stage for remotely changing the optical path, you will always need to change the op-tics manually using the switch on the side of the interferometer housing:Chapter 2: Modes of Operation - Mirror Alignment Using the USB Game Controller_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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28

When the movement of the optical stage

is finished, the reflection signal shown in

the main window will look like this.

Step 3: Mirror alignment - etalon 2

Modifying the X2 and Y2 piezo voltages adjusts the mirror tilt of the second etalon in the x- and y-directions and will cause either an enhancement or a degeneration of the second etalon's dip. Adjust the piezo voltages X2 and Y2 step by step and one after the other and observe how the dip in the reflection signal changes.

Change the X2 and Y2 piezo voltages using the game controller: X2: move left joystick while pressing button 6

Y2: move right joystick while pressing button 6

Step 4: Synchronization - etalon 2

The transmission of etalon 2 needs to be synchronized with the transmission of etalon 1. This can be achieved by adjusting the dZ piezo voltage: This will only move the dip associated with etalon 2 towards the position of the dip attributed to etalon 1. Excel-lent synchronization, i.e., a good overlap of both etalons is crucial for high perform-ance of the Tandem-Fabry-Pérot interferometer. Use TFP > scan width or the short cut Control + Z to decrease the scan region to the small scan width to obtain a higher resolution for overlapping the dips.

Change the dZ piezo voltage using the game controller: dZ: move right joystick while pressing button 8

Important: If the entire graph is all white after you switched to the reflec-tion mode, you will need to change the scale of the y-axis: Use the and arrows on the key board or type in a new value for the maximum of the y-axis to define a larger intensity range.Important: If no dips appear when you change the X and Y piezo voltages, it is probably necessary to open the interferometer and align the etalon mir-rors using the motor compensation as described on page 28 of the TFP-1

Operator Manual by JRS Scientific Instruments.Chapter 2: Modes of Operation - Mirror Alignment Using the USB Game Controller

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29

etalon 1

Etalon 1 is well aligned

when a deep and narrow

dip can be seen.

etalon 1 etalon 2

Both etalons are aligned

when two narrow dips are visible.

SHIFT

dZ PIEZO





Step 5: Set 0 GHz Line - etalon 1 & 2

The Z piezo moves the entire scan stage, on which the second mirror of both etalons are mounted, and thus controls the mirror spacing of both Fabry-Pérot interferometers at the same time. To set the zero frequency shift to the correct position, vary the Z piezo voltage to shift the position of the first etalon's dip to the middle to the scan range.

Change the Z piezo voltage using the game controller: Z: move left joystick while pressing button 7

Interferometer aligned

The etalon mirrors are now aligned and the transmission functions of both etalon are synchronized to transmit light simultaneously.

Step 6: Set 0 GHz Line - etalon 1 & 2

Switch to TFP > Transmission Mode.

If your interferometer is not equipped with an automated stage for remotely changing the optical path, you will always need to change the optics manually using the switch on the side of the interferometer housing. Please refer to the beginning of this section.

Tip: Step 5 of the manual alignment routine is not absolutely necessary!You can directly switch to TFP > Transmission when both etalons are synchro-nized, i.e., after step 4. If the Drift Stabilizer is activated, the transmission peak will automatically be moved to the center of the scan range.

Chapter 2: Modes of Operation - Mirror Alignment Using the USB Game Controller_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

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30

CENTER

etalon 1 & 2

SHIFT

Z PIEZO

etalon 1 & 2

Finesse optimization

Since in transmission mode, the light of the reference beam does not pass each etalon just once but instead passes the entire (3+3) beam path, an even higher accuracy of the mirror alignment and synchronization is required than in reflection mode.

If all the stabilization routines are turned on, i.e., all the circular buttons under Active Stabilization shine green, the Finesse Optimizer will increase the area underneath the transmission peak and, thus, optimize the overall alignment and stability of the interferometer.

Ready to start a measurement...

Summary: Overview on piezo voltages

X1: controls mirror tilt for etalon 1 in the horizontal direction

effect on interferometer effect on alignment

changes depth of dip(s) associated with etalon 1

combination game controller

press button 5 and move left joystick

Y1: controls mirror tilt for etalon 1 in the vertical direction


press button 5 and move right joystick

X2: controls mirror tilt for etalon 2 in the horizontal direction



Y2: controls mirror tilt for etalon 2 in the vertical direction



dZ: controls mirror spacing of etalon 2

changes position of dip(s) associated with etalon 2


Z: controls mirror spacing of etalon 1 and 2 simultaneously

changes position of all dips


Chapter 2: Modes of Operation - Mirror Alignment Using the USB Game Controller_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

31

FINESSE

OPTIMIZING

Transmission Mode

Which dip belongs to which etalon?

If the Tandem-Fabry-Pérot interferometer is in reflection mode and you observe two distinct dips that are not one free spectral range apart from each other, here is an easy way how to check which dip belongs to which etalon:

Change the dZ piezo voltage!

A variation in the dZ piezo voltage will only change the mirror spacing of etalon 2. Therefore, only the dip associated with eta-lon 2 will move if you modify the dZ piezo voltage:

etalon 2

Chapter 2: Modes of Operation - Which dip belongs to which etalon?_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

32

What if I observe two dips that are not one FSR apart yet both move when I change the Z piezo

voltage?

In rare cases it may happen that you observe two dips that are not one FSR apart from each other, yet they still both move when you change the dZ piezo voltage. If this happens, there are several things that you should check:

Reason 1: You see two transmission orders of etalon 2

If you see two dips and both move when you change the dZ voltage, check the dis-tance between the dips! Is it roughly 1.1 FSR, you probably see two transmission or-ders of etalon 2.

Etalon 2 is mounted under an angle of 6°. A change ∆d of the mirror spacing of etalon 1 results in a smaller change of the mirror spacing of etalon 2 by ∆d cos(6°). That is why two transmission orders of etalon 2 are more than one free spectral range apart.

Reason 2: The mirror spacing in TFPDAS4 does not match the physical

spacing of the mirrors

If you see two dips that both move when you change the dZ voltage and are not 1.1 FSR apart, check the mirror spacing!

In order to correctly translate the scan voltages applied to the Z piezo into frequencies, TFPDAS4 needs to know the coarse spacing of the mirrors. The coarse mirror spacing is changed with the gauge on the side of the inter-ferometer housing.

This spacing parameter must be correctly defined in the software:

You can either do this in the menu PARAMETER > Scan or in the menu PARAMETER > Sample. The value must be given in units of millimeter.

If you change the spacing in one of the two windows it will automatically be updated in the other window.

Chapter 2: Modes of Operation - What if I observe two dips that are not one FSR apart yet both move when I change the Z piezo voltage?_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

33

etalon 2 -

transmission

order n

etalon 2 -

transmission

order n+1

Reason 3: The calibration of the Z piezo is not correct

You checked the spacing parameter and still you see two dips that both move when you change the dZ voltage and are not 1.1 FSR apart. Check the calibration of the scanning Z piezo!

The calibration for the Z piezo voltage may not be correct. If that is the case, the volt-age applied to the scan stage does not correspond to the appropriate range in FSR. For further information on how to calibrate the scanning Z piezo, refer to the section Piezo Calibration (Z).

Chapter 2: Modes of Operation - What if I observe two dips that are not one FSR apart yet both move when I change the Z piezo voltage?_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

34

Parameter Related to the Reflection Mode

Chapter 2: Modes of Operation - Transmission Mode_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

35

All of the reflection/transmission mode parameters, that are de-scribed in more detail below, are set under the first tab of the Alignment Parameter window. This can be called in the menu bar under PARAMETER > Alignment or with the shortcut Control + F5.

As mentioned previously, the reflection mode is for alignment purposes only. The performance of the Tandem-Fabry-Pérot in-terferometer desired for alignment purposes, especially the scanning speed, is often much different than the settings used in the acquisition mode in which the actual spectrum is meas-ured. Consequently, the parameters determining the perform-ance of the interferometer can be defined separately for the reflection/transmission modes and for the acquisition mode.

In the reflection and transmission modes, the interferometer is scanned with a frequency that can be chosen independently from the scanning speed used in the acquisition mode. The overall scan frequency is determined by two parameters: the Acquisition time per channel as well as the Align Channels

for each of the reflection and transmission modes. Generally, it is recommended to choose a fast scanning frequency for the alignment of the Tandem-Fabry-Pérot interferometer. This will allow for a much faster response when changing the voltages applied to the etalon piezos with the remote control, i.e., either the hand box or the game controller.

A crucial part for the mirror alignment is the synchronization of the transmission of both etalons. To aid in this critical step of the alignment, the scan width in the reflection and transmission mode can be changed from the large to the small scan width using either the menu item TFP > scan width or the shortcut Control + Z. The number of channels that the scan range is di-vided into in the reflection and transmission mode is given by the Align Channels parameter and remains fixed. Therefore, reducing the scan range leads to a higher resolution and switch-ing to a smaller scan width allows for a higher precision when overlapping both dips.

If your interferometer is not equipped with an automized stage with which the optics can be moved remotely, the commands for changing the optics will need to be deactivated in the software. If this is not done, TFPDAS4 will try to communicate with the optical stage and will wait indefinitely for feedback that will not come. This will lead to a crash of TFPDAS4. In the Parame-

ter Alignment window a button is included that will deactivate all communication to the optical stage.

If your interferometer is equipped with an automized stage for changing the optical path, it may happen that the signal report-ing the current status of the stage is mixed up. Is this the case, TPFDAS4 will think that the interferometer is in reflection mode when it actually is in transmission mode and vice versa. There-fore, a button is implemented in the software that allows you to invert the Check if align signal generated by the interfer-ometer.

Transmission Mode


_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

36

In the transmission configuration, only the reference beam is sent into the Tandem-Fabry-Pérot interferometer and the optical stage inside is in the position to allow sixfold passage of the light through both etalons:

blocked

If the mirrors are well aligned, the transmission peak of the ref-erence beam should be visible in the center of the main window of TFPDAS4 defining the zero gigahertz line.

Since the area under the peak is a measure for the mirror paral-lelism and synchronization of both etalons, the transmission mode is used to keep the interferometer aligned when no measurements are running.

The transmission mode is set up with the same number of channels as the reflection mode and with the same scanning ranges in order to cycle quickly through the stabilization algo-rithm and to converge rapidly into an optimally aligned state. You can check easily to see whether the stabilization routines are activated. In the main window of TFPDAS4, if all three circular LEDs under Active Stabilization shine green then all stabiliza-tion routines are running. If any of the LEDs glow red, the corre-

sponding stabilization routine can be activated by pressing on the LED.

In the transmission mode, the Z piezo scans across the 0 GHz line. If any elastically scattered light coming from the sample enters the interferometer, its high intensity will destroy the photo detector. The system is designed so that only the much weaker light of the reference beam will pass through the double shutter at the entrance of the interferometer while the Z piezo is scanning in transmission mode.

Nevertheless, it might be desirable to allow the sample beam to enter the interferometer while still in transmission mode. If you want to manually align the etalon mirror parallelism, for exam-ple, you may want to see the elastically scattered light inside the interferometer while the Z piezo is scanning. In the menu TFP >

ADVANCED it is possible to open the input shutter and let the sample beam enter the interferometer at any time.

Important: If you do decide to use this feature, you

need to be especially sure that you know what you are do-ing! Before opening the input shutter, you must to take pre-cautions to protect the photo detector with some kind of shutter or even disconnect its power.

Because opening the input shutter and letting the sample beam enter the interferometer can cause a lot of harm to your photo detector, this feature is password protected. Please contact us if you are interested in using this Advanced feature of TFPDAS4.

TFPDAS4 main window when the interferometer is scanning in

Transmission Mode

Only if the mirrors of both etalons are well

aligned and synchronized, a signal will be

detected at the photo detector.

Reference Peakelastically scattered light that

defines zero frequency shift

scan range


_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

37

Parameter Related to the Transmission Mode

Chapter 2: Modes of Operation - Parameter Related to the Transmission Mode_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

38

All of the reflection/transmission mode parameters, that are de-scribed in more detail below, are set under the first tab of the Alignment Parameter window. This can be called in the menu bar under PARAMETER > Alignment or with the shortcut Control + F5.

As mentioned previously, the reflection mode is for alignment purposes only. The performance of the Tandem-Fabry-Pérot in-terferometer desired for alignment purposes, especially the scanning speed, is often much different than the settings used in the acquisition mode in which the actual spectrum is meas-ured. Consequently, the parameters determining the perform-ance of the interferometer can be defined separately for the reflection/transmission modes and for the acquisition mode.

In the reflection and transmission modes, the interferometer is scanned with a frequency that can be chosen independently from the scanning speed used in the acquisition mode. The overall scan frequency is determined by two parameters: the Acquisition time per channel as well as the Align Channels

for each of the reflection and transmission modes. Generally, it is recommended to choose a fast scanning frequency for the alignment of the Tandem-Fabry-Pérot interferometer. This will allow for a much faster response when changing the voltages applied to the etalon piezos with the remote control, i.e., either the hand box or the game controller.

A crucial part for the mirror alignment is the synchronization of the transmission of both etalons. To aid in this critical step of the alignment, the scan width in the reflection and transmission mode can be changed from the large to the small scan width using either the menu item TFP > scan width or the shortcut Control + Z. The number of channels that the scan range is di-vided into in the reflection and transmission mode is given by the Align Channels parameter and remains fixed. Therefore, reducing the scan range leads to a higher resolution and switch-ing to a smaller scan width allows for a higher precision when overlapping both dips.

If your interferometer is not equipped with an automized stage with which the optics can be moved remotely, the commands for changing the optics will need to be deactivated in the software. If this is not done, TFPDAS4 will try to communicate with the optical stage and will wait indefinitely for feedback that will not come. This will lead to a crash of TFPDAS4. In the Parame-

ter Alignment window a button is included that will deactivate all communication to the optical stage.

If your interferometer is equipped with an automized stage for changing the optical path, it may happen that the signal report-ing the current status of the stage is mixed up. Is this the case, TPFDAS4 will think that the interferometer is in reflection mode when it actually is in transmission mode and vice versa. There-fore, a button is implemented in the software that allows you to invert the Check if align signal generated by the interfer-ometer.

Auto Alignment

Chapter 2: Modes of Operation - Auto Alignment_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

39

TFPDAS4 is equipped with an Auto Alignment routine

that brings your Tandem-Fabry-Pérot interferometer

into conditions for starting a measurement within only

a few minutes, simply by pressing one button. For both

etalons the optimum mirror tilt is detected in the re-

flection mode and the transmission functions are syn-

chronized. The routine automatically switches to the

transmission mode and then optimizes the overall

transmission of the interferometer.

The Auto Alignment consists of three separate steps:

• First, a coarse search for the transmission dips is

performed, i.e., the mirror tilts are varied to find

transmission dips for etalon 1 and 2.

• Once these dips are found, their depths and

therefore the mirror parallelism is optimized in a

fine alignment cycle.

• In the last step, the transmission of both etalons is

synchronized to allow efficient sixfold passage of

the light.

On the next pages, you can find a step by step descrip-

tion of the auto alignment routine. Since this a fully

automated routine, you do not have to do any of the

steps yourself. Instead you can sit back and watch the

software do the work for you.

All of the parameters related to the auto alignment

routine can be changed in the second tab of the PA-

RAMETER > Alignment window, which you can under

the corresponding menu item or with the shortcut

Control + F5.

Step 1: Switch to Reflection Mode

The auto alignment routine will automatically bring your interferometer into the re-flection mode.

Switch to Reflection Mode




ometer housing to ALIGN position.


Step 2: Coarse mirror alignment - etalon 1

The piezo voltages X2 and Y2, which regulate the mirror tilt of etalon 2, are set to the maximum value of 5V. This way, no dip caused by a transmission of etalon 2 will be de-tected during the alignment of etalon 1 and, once a dip is found, it can be unambigu-ously attributed to etalon 1.

To obtain the proper mirror tilt for etalon 1, the X1 and Y1 piezo voltages are varied gradually following a circular alignment scheme in the X1-Y1-coordinate plane. The degree by which the voltages are changed depends on the parameter Auto coarse step [V]. The bigger the value of this parameter, the farther apart two scan points are from each other.

The Auto coarse step [V] must be chosen carefully: If it is too big, the step size for varying the piezo voltages might be too large so that the optimal mirror tilt might not be found. If the step size is too small, the search for the appropriate piezo volt-ages will take a very long time, which is also not desirable.

Important: If your interferometer is not equipped with an automated stage for remotely changing the optical path, you will always need to change the op-tics manually using the switch on the side of the interferometer housing:Chapter 2: Modes of Operation - Auto Alignment_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

40

etalon 1

threshold for dip detection

The threshold for the dip detection for etalon 1 is

defined by the parameter Threshold factor coarse E1. It

is given as a percent of the average reflection signal.

Once etalon 1 transmits enough light, i.e., when the minimum in the reflection signal reaches below the predefined Threshold factor coarse E1, good values for the X1 and Y1 piezo voltages are found.

The voltage offset for the scanning Z piezo is changed to shift the transmission posi-tion of etalon 1 to -1/4 FSR with respect to the center of the scan area.

The final values for the X1, Y1, and Z piezo voltage are stored.

Step 3: Coarse mirror alignment - etalon 2

The piezo voltages X1 and Y1 are set to the maximum value of 5V. This way, no dip caused by a transmission of etalon 1 will be de-tected during the alignment of etalon 2.

To obtain the proper mirror tilt for etalon 2, the same procedure is used as for etalon 1 except that now the X2 and Y2 piezo volt-ages are varied gradually following a circular alignment scheme in the X2-Y2-coordinate plane.

Important: This change in the transmission position of etalon 1 is only cor-rect if the value of the parameter Scan Piezo (Z) Calibration is well cali-brated. Refer to the section Piezo Calibration (Z).Chapter 2: Modes of Operation - Auto Alignment

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

41

etalon 1

The indicator for the threshold will shortly

blink green. In addition, a vertical cursor will

appear indicating the position of the found

minimum.

etalon 1

-1/4 FSR CENTER

SHIFT

Z PIEZO

etalon 2

threshold for dip detection

The threshold for the dip detection for etalon 2 is defined

by the parameter Threshold factor coarse E2. It is given

as a percent of the average reflection signal and can be

chosen independently from the threshold for etalon 1.

Once etalon 2 transmits enough light, i.e., when the minimum in the reflection signal reaches below the predefined Threshold factor coarse E2, good values for the X2 and Y2 piezo voltages are found.

The transmission position of etalon 2 is shifted via changing the dZ piezo voltage to +1/4 FSR with respect to the center of the scan area.

Step 4: Fine mirror alignment - etalon 1& 2

Now, the values for the X1, Y1, and Z piezo voltages, that were found after step 2, are loaded. The dips of both etalons are now as far apart as possible, i.e., +1/2 FSR.

At this stage of the auto alignment routine, both etalons transmit light such that the dips reach below the threshold factor coarse E1 and E2, respectively.

The next step is to optimize the mirror parallelism. This is done by simultaneously vary-ing the X and Y tilt of both etalons in much smaller steps. In one optimization cycle, the piezo voltages X1 (Y1) and X2 (Y2) are changed first in the positive and then the negative direction with a step size given by the parameter Auto fine step [V]. For the test steps in the ± directions, the depth of the dips for each etalon are compared sepa-rately and the voltages are adjusted by a factor of Auto fine step [V]/Auto fine slope in the direction that improves the alignment.

Important: This change in the transmission position of etalon 1 is only cor-rect if the value of the parameter Piezo (dZ) Calibration is well calibrated. Refer to the section Piezo Calibration (dZ).Chapter 2: Modes of Operation - Auto Alignment

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

42

etalon 2

The indicator for the threshold will shortly blink

green. In addition, a vertical cursor will appear

indicating the position of the found minimum.

etalon 2

+1/4 FSRCENTER

SHIFT

dZ PIEZO

etalon 1 etalon 2

To not lose the found dips during the optimization

procedure, the routine stops as soon as one of the dips

reaches above the Threshold factor fine.

OPTIMIZING

Step 5: Synchronization

After a certain amount of optimization cycles given by the parameter Auto fine cycles, the transmission functions of both etalons are synchronized. This is done via changing the dZ piezo voltage by the amount that corresponds to -1/2 FSR.

The synchronized transmission of both etalons is now shifted to the middle of the scan range by changing the offset applied to the scanning Z piezo. This offset determines the 0-GHz position in future measurements.

Step 6: Switch to transmission mode

After the synchronization of the transmission functions of etalon 1 and 2 is complete, the optics inside the interferometer are automatically switched to the transmission mode.

Since in transmission mode, the light of the reference beam does not pass each etalon just once but instead passes the entire (3+3) beam path, an even higher accuracy of the mirror alignment and synchronization is required than in the reflection mode. That is why the Finesse Optimizer routine is now taking care for further optimizing the sta-bility of the etalon mirrors.

Switch to Transmission Mode




ometer housing to TANDEM position.


Ready to start a measurement...

Important: If your interferometer is not equipped with an automated stage for remotely changing the optical path, you will always need to change the op-tics manually using the switch on the side of the interferometer housing:Chapter 2: Modes of Operation - Auto Alignment

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

43

etalon 1 etalon 2

CENTER

b: SHIFT

Z PIEZO

SHIFT

dZ PIEZO

FINESSE

OPTIMIZING

Acquisition Mode

Chapter 2: Modes of Operation - Acquisition Mode_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

44

The actual measurement is performed in the Acquisi-

tion Mode. In the acquisition mode, the optical stage

inside the interferometer is such that the light is

transmitted through both etalons, the same as for the

transmission mode.

It is very important to know that in the acquisition

mode the input shutter of the interferometer is

opened at certain positions of the scanning process

so that the light coming from the sample can enter the

interferometer and closed for other positions. To en-

sure that the photo detector is protected, the input

shutter must be closed at scan positions where the

elastically scattered light can pass the etalons of the

interferometer. Therefore, the mirror distance corre-

sponding to the 0-GHz line needs to be known pre-

double

shutter

cisely. For this reason, an acquisition can only be

started after a reference peak centered at the 0-GHz

line is observed in the transmission mode.

If one tries to acquire a spectrum when the interfer-

ometer is not in the transmission mode, a dialog box

will pop up and inform the operator that they must

switch to the transmission mode or run the auto

alignment routine. If you are not familiar with these

modes, please read the corresponding sections in this

manual before attempting to acquire a spectrum.

In this section, we assume that the Tandem-Fabry-

Pérot interferometer is aligned and that the central

peak of the reference beam is visible in the transmis-

sion mode.

Regions of Interest

While accumulating a spectrum, the light coming from the sample will enter the interferometer only inside the Re-

gions of interest (ROI). Outside of those regions, the input shutter of the interferometer is closed and only the refer-

ence beam enters the Tandem-Fabry-Pérot interferometer. Therefore, at least one region of interest has to be de-

fined in order to measure any signal from the sample. With TFPDAS4, the user is not limited to just one region of in-

terest; the user may define as many ROI's as they like. The position, width and scanning speed within each ROI can

be defined in different ways.

If you only would like to display the regions of interest on the spectrum you can either go to the menu SPECTRUM > ROI (Regions of Interest) > show ROIs or use the short-cut Control + B. All current regions of interest will be shown in the spectrum by cur-sors that define their borders. In the picture below, four regions of interest are defined. The scanning speed of the each ROI can be checked in the PARAMETER > Scan menu. The borders of an existing region of interest can be modified - even during a running measurement - by just dragging the cursor with the mouse to a new position. To make it easy to distinguish between different regions of interest, the cursors for each ROI can have different colors.

More advanced manipulations of the regions of interest, like adding, deleting or changing the scanning speed, can be done in the menu PARAMETER > Scan. The re-gions of interest that are currently defined are listed in the table Regions of Interest (ROI) on the right side of the Scan Parameters window. The Scan Range graph shows the position of the regions of interest within the scan range, similar to what you see when the regions of interest are displayed during a running measurement in the TFPDAS4 main window.

Chapter 2: Modes of Operation - Regions of Interest_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

45

RO

I 1

RO

I 2

RO

I 3

RO

I 4

refe

ren

ce p

ea

ke

last

ica

lly

sca

tte

red

lig

ht

Scan

Parameters

window

How to Define a New Region of Interest

Step 1: Open scan parameter window

Go to the PARAMETER > Scan menu.

This will pop up a window listing all the parameters related to the acquisition mode in which an actual measurement is accumulated.

Step 2: Define right border

Type the right border of the region of interest that you want to define in the Regions

of interest table.

If you fill in the value in FSR, the program will automatically update the corresponding value in GHz and vice versa.

Step 3: Define left border

Type the left border of the region of interest that you want to define in the Regions of

interest table.

If you fill in the value in GHz, the program will automatically update the corresponding value in FSR and vice versa.

Step 4: Set T Factor

In order to complete the definition of a new region of interest, you also need to choose its T Factor, which determines the scanning time per channel within the associated frequency range:

In the acquisition mode, each channel is scanned for a certain period of time which is given by the parameter Time per Channel. Inside a region of interest, this scan time for each channel is multiplied with the T Factor, resulting in a slower scanning of the stage. With a Time per Channel of 0.5 ms and a T Factor of 5, for example, the meas-uring time within the region of interest will be 5 times longer, i.e., 2.5 ms per channel.

Tip: A new region of interest can only be created as long as it does not intersect with any existing region of interest and does not reach beyond the reference peak or the scan range.


_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

46

5 times slower

scanning

inside ROI

At this point, the newly defined region of interest will be shown by two cursors in the graph on the left of the Scan Parameters window.

Step 5: Change color (optional)

The color of the cursors can be changed by simply clicking on the corresponding Color field in the table. A color box will appear and you can click on your desired color.

Step 6: Return to TFPDAS4 main window

Once you have created all desired regions of interest, press the OK but-ton to close the Scan Parameters window and return to the main win-dow of TFPDAS4.

All of the regions of interest can be displayed in the spectrum window by pressing the shortcut Control + B or using the menu SPECTRUM > ROI (Regions of Interest) >

show ROIs.

Tip: If either the left or the right border or the T Factor is not set, the col-umn ACTION of the table will say delete, which indicates that the region of interest is not fully defined yet. The region of interest is only valid if all three values are defined!Tip: If you change any region of interest in the PARAMETER > Scan menu while a measurement is running, the measurement will be restarted and the accumulated data will be lost! The program will ask if you are sure about the changes before they are actually applied.


_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

47

newly defined

ROI

How to Modify an Existing Region of Interest

Before changing any region of interest, make sure that the scan range is chosen ap-propriately!

The left limit and right limit of the scan range can be changed below the graph on the left side of the PARAMETER > Scan menu pane. The scan range may be over any range that you wish. The only two limitations are that the reference peak, i.e., the zero gigahertz position, must always lie within the scan range and the scanning Z piezo must not exceed its travel range of ±5 V.

If you fill in the values in GHz, the program will automatically update the correspond-ing values in FSR and vice versa.

The red button below the graph allows you to change the x-axis of the graph from FSR to GHz.

Option 1: Change values in ROI table

You can change the size of the region of interest by typing different values for the bor-ders into the Regions of Interest table on the right side of the PARAMETER > Scan menu pane.

If you fill in the value in GHz, the program will automatically update the corresponding value in FSR and vice versa.

Important: You may not move any border into another region of interest! If you try, the program will tell you which regions of interest intersect and will set the border to its former value. The same will happen if you try to extend a region of interest over the reference beam or beyond the scan region.Tip: The scanning time inside a region of interest can only be changed by typing a different T Factor into the ROI table!

Chapter 2: Modes of Operation - How to Modify an Existing Region of Interest_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

48

Option 2: Drag cursors in Scan Range graph of the Scan Parameters

window

You can change the size of the region of interest by dragging the cursors that indicate the ROI's borders in the graph of the Scan Parameters window with the mouse.

If you move the cursors, the values in the table will be updated automatically.

Option 3: Use the ACTION column in ROI table

If you press on click in the Action column of the Region of Interest table, a small window will appear that provides more advanced op-tions for modifying an existing region of interest:

Delete ROI simply deletes the selected ROI. It is not, however, possi-ble to delete the last existing region of interest because at least one region of interest needs to be defined in order to measure a spec-trum.

Split in 2 ROIs divides the existing ROI into three equal parts. It creates two new ROIs, one in the first third, the other in the last third of the pre-vious ROI. Using this feature is often quicker than typing in the values for a new region of interest.

original ROI split ROIs


_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

49

drag with

mouse

Add (Anti)Stokes mirrors the se-lected region of interest with re-spect to the 0-GHz line. This option works provided that the new region of interest lies within the scan re-gion and does not intersect with any existing region of interest.

original

ROI

added

Anti Stokes

Option 4: Drag cursors in the Spectrum graph of the TFPDAS4 main

window

You can change the borders of existing regions of interest by dragging the cursors with the mouse in the spectrum graph of the TPFDAS4 main window.

It is possible to change the regions of interest by dragging the cursors even while a measurement is running and a spectrum is accumulated.


_______________________________________________________________________________________________________________________________________________________________________________________________________________________________

50

drag with

mouse

Open Box Mode

The open box mode is for maintenance purposes only. When you call this mode, the shutter in front of the photo

detector will close by default.

Careful: Since shutters are not installed in all interferometers, you still have to be careful and take precautions before you

open the interferometer box to ensure that your photo detector is not harmed.

In the open box configuration, the scanning stage is idle, i.e., the scan amplitude is zero and the voltage applied to

the Z piezo is static. The voltages of all piezos can be centered at 0 V with one button. This is quite useful if you need

to align the etalon mirrors using the motor compensation as described on page 28 of the TFP-1 operator manual

by JRS Scientific Instruments.

The optics inside the interferometer can be

changed from Reflection to Transmission

Mode remotely. Again, this feature will only

work if the stage that changes the optical path

inside your interferometer is automized.

In addition, it is possible to switch the input

shutter of the interferometer between sample

beam and reference beam, which will let only

the light from the respective light path enter

the interferometer.

To remotely control the piezo voltages press

the Activate hand box button. This will also

be necessary if you work with a game control-

ler instead of the hand box.

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http://www.jrs-si.ch/downloads/TFP-1%20Manual.pdf

http://www.jrs-si.ch/downloads/TFP-1%20Manual.pdf

Sample Parameters

The PARAMETER > Sample menu can be used for the description of the sample. Nearly all of the parameters in this

section serve for information purposes only and do not influence the performance of TFPDAS4.

The only important parameter in the Sample

Parameters window is the Spacing.

The coarse spacing of the etalon mirrors can

be varied manually with the switches on the

outside of the interferometer housing and is

indicated at the dial gauge on the side of the

interferometer housing. More information on

how to change the coarse mirror spacing can

be found in the next section.

Every time you change the coarse spacing

of the etalon mirrors, you need to make

sure you do not forget to update the Spac-

ing parameter in the software.

This Spacing parameter is critical for the cal-

culation of the correct frequencies. When you

observe that the frequencies that you measure do not match the frequencies that you expect, you should check to

see if the Spacing is correct.

Tip: The Spacing (mm) parameter can be changed in the PARAMETER > Scan window as well. If you change the spacing in

one of the two windows it will automatically be updated in the other window.

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52

How to change to coarse spacing of the etalon mirrors

Step 1: Physically change coarse mirror spacing

You can change the coarse mirror spacing with the switches on the side of the inter-ferometer housing:

Turn the knob to the Z position, then use the ADJUST switch to in- or decrease the mirror spacing.

Step 2: Check coarse mirror spacing

The gauge on the side of the interferometer housing states the coarse mirror in units of millimeter, in the picture, for example, 7.5 mm.

Step 3: Adjust Spacing parameter in software

Define the value of the coarse mirror spacing for the usage in the software:

You can either do this in the menu PARAMETER > Scan or in the menu PARAMETER >

Sample. The value must be given in units of millimeter.

If you change the spacing in one of the two windows it will automatically be updated in the other window.

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53

increasedecrease

coarse mirror

spacing

Chapter 3: Measurement Tools

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55

All the features relevant to the start of a simple meas-

urement can be found in the menu SPECTRUM.

A new spectrum, i.e., a new accumulation, can be

started using the shortcut Control + B or the menu

item SPECTRUM > start new spectrum.

Important: Starting a new measurement is only

possible if the interferometer is scanning in trans-

mission mode and the peak in the center of the scan

range is visible and stabilized.

The stage will start scanning the frequency range that

is defined by the Scan range parameter in the section

PARAMETER > Scan where also the Regions of Inter-

est are defined.

The regions of interest specify the frequency range in

which the light coming from sample can enter the in-

terferometer. At all other times, the double shutter

blocks the light coming from the sample and only the

light from the much weaker reference beam enters

the interferometer. You can display the currently de-

fined regions of interest with the menu item ROI (Re-

gions of Interest) > show ROIs or with the shortcut

Control + B. Colored cursors will appear in the meas-

urement graph indicating the borders of each region

of interest. During a running accumulation, the bor-

ders of these regions of interest can be changed by

dragging the corresponding cursor to a desired fre-

quency position. But careful: Two regions of interest

may not intersect and a region of interest must never

extend across the reference peak, i.e., the elastically

scattered light defining the position of the zero giga-

hertz frequency shift. For more advanced manipula-

tion of the Regions of Interest refer to the corre-

sponding section of this manual.

If you did not define a certain Number of scans in the

menu PARAMETER > Scan, the accumulation will run

infinitely.

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56

You can clear a running accumulation under the menu

SPECTRUM > clear spectrum. A new accumulation of

the detected signal will start automatically.

There are two ways the measured data can be dis-

played in the measurement graph in the main window

of TFPDAS4: The measured data can either be accumu-

lated scan after scan or the counts from one single

scanning process of the stage can the displayed. You

can toggle between these two data representations

using the SPECTRUM > show accumulated Scans

menu item or the shortcut Control + W. With SPEC-

TRUM > pause accumulation of scans the scanning of

the stage continues but the measured data is not

added to the currently displayed spectrum.

The axes of the measurement graph in the main win-

dow of TFPDAS4 can be changed between Frequency

(GHz) and FSR for the x-axis and between a linear and

logarithmic scale for the y-axis.

The SPECTRUM > Get Number of TAG BITS menu is

only needed if your setup is equipped for time re-

solved Brillouin light scattering spectroscopy meas-

urements.

Chapter 4: Stabilization

In order to ensure the stability of your Tandem-Fabry-Pérot Interferometer over long periods of time, it is necessary

to maintain parallel alignment of the mirrors as well as the correct spacing of the etalons. TFPDAS4 is equipped with

an Active Stabilization routine that is used to maintain stability. The routine is divided into three separate parts:

• The Finesse Optimizer maintains optimum alignment of the etalon mirrors. The FO Signal is equal to the area

under the reference peak and serves as a measure of the mirror parallelism and the synchronization of both

etalons.

• The Drift Stabilizer ensures that the reference peak remains centered at the 0-GHZ position, which is regu-

lated by adjusting the offset voltage applied to the Z piezo to the keep the DS Signal close to zero.

• The Dynamic Dither feature allows the stabilization routines to dynamically adjust so that they are able to re-

act to disturbances of different strengths. During stabilization, the piezo voltages that control the mirror tilts,

i.e., X1, Y1, X2, and Y2, and the spacing of etalon 2, i.e., dZ, are varied by the Dither actual in the positive as

well as in the negative direction. The FO signal is evaluated after each adjustment and then a correction signal

is set to change the piezo voltage in the direction that improves the FO signal.

In order to compensate for major disturbances as fast as possible, the dither actual value is adjusted dynami-

cally depending on the value of the FO signal: When the interferometer is

reasonably well aligned, small values of the dither actual are chosen that are

sufficient to maintain stability, whereas, if the FO signal drops dramatically

the dither actual value will increase immediately so that the interferometer

will come back to a state of reasonable stability more quickly.

The indicators showing the current status of the stabilization routines are dis-

played in the main window of TFPDAS4 in the section Active Stabilization.

The LED button next to the Dither actual refers to the Dynamic Dither, the one

next to the FO Signal displays the status of the Finesse Optimizer, and the last LED

button is associated with the Drift Stabilizer.

Active Stabilization status in

TFPDAS main window

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57

All three parts of the active stabilization routine can be switched on and off by pressing the corresponding LED

in the TFPDAS4 main window.

In this chapter, the three stabilization algorithms are described in more detail in the order that they are listed in the

Active Stabilization window. The stabilization window can be called in the menu of TFPDAS4 under PARAMETER >

Stabilize.

Active Stabilization window

This window contains all of the parameters related to the stabilization routines. Once the stabilization parameters

have been set up for your specific interferometer, most will not usually need to be changed with the exception of

the FO Dither threshold. This parameter has to be adjusted every time you change the intensity of the reference

beam.

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58

Finesse Optimizer (FO)

The Finesse Optimizer maintains the optimum alignment of the etalon mirrors. The algorithm changes the voltages applied to the X, Y, and dZ piezos of the etalons in a systematic way and compares the area underneath the reference peak, which is an ex-cellent measure for the mirror parallelism, after each step. One stabilization cycle for the finesse optimizer operates as follows:

After each spectrum is scanned, the area underneath the reference peak is summed up over a region of FO Width, centered on the 0-GHz position. The FO width is specified in units of the free spectral range. This summed signal is the FO Signal, which is always displayed in the main window of TFPDASS4. The value of the FO signal reflects the alignment of the interfer-ometer: the higher the value, the better the transmission after the sixfold passage. It is important to remember that any change in the intensity of the reference beam will automatically result in a change of the FO signal. Consequently, the FO signal only provides a useful measure of the interfer-ometer alignment as long as the intensity of the reference beam remains constant.

During one stabilization cycle, the voltages that control the mirror parallelism and their relative spacing are optimized using five degrees of freedom: X1, Y1, X2, Y2 and dZ. The routine changes the voltages of each of these piezos in turn, first by plus Dither actual, second by minus Dither actual. The values of the FO signal for variation of a given piezo voltage in the plus and minus direction are compared and then the voltage of the corresponding piezo is corrected to optimize the signal before proceed-ing to the next piezo. In this way, each of the degrees of freedom is optimized in turn. Since an accurate synchronization of both etalons is crucial for a high transmission, the dZ voltage is adjusted twice during one cycle to ensure that the transmission peaks of both etalons coincide. Overall, a full stabilization cycle consists of 12 separate scans, as shown in the table on the left.

After each plus/minus pair of scans, the associated FO signals are compared and a cor-rection signal is applied to the corresponding piezo in the direction that improves the signal. To avoid potential instabilities, the change in the piezo voltage should not be too large. The magnitude of the correction signal is chosen in two different ways, de-pending on the values of various parameters and the magnitude of the FO signal:

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scan number piezo voltage changed amount of change1 X1 - dither actual2 X1 + dither actual3 Y1 - dither actual4 Y1 + dither actual5 dZ - dither actual6 dZ + dither actual7 X2 - dither actual8 X2 + dither actual9 Y2 - dither actual10 Y2 + dither actual11 dZ - dither actual12 dZ + dither actual

If Regula Falsi (RF) is chosen, the magnitude of the piezo voltage change is equal to the dither voltage multiplied by the FO Tiltslope. Since the correction step must not be too big, the value for the FO tiltslope should be chosen to be between 1 and 5. If it has a larger value, the system may become unstable and even loose transmission en-tirely.

The Newton interpolation (NI) option uses linear interpolation and, thus, takes the intensity difference of the two FO signals into account. After the test steps in the nega-tive and the positive directions, the difference between the measured FO signals is calculated and normalized by their sum. This ratio is multiplied by the FO Tiltslope (NI) as well as the actual dither to obtain the correction value for the corresponding piezo voltage. This method yields correction values that are very small in case the FO signals after both test steps are almost the same and much larger values when they are not.Since the relative change in the FO signal is taken into account when using Newton Interpolation, the values for the FO tiltslope (NI) can be larger than in case of the Reg-ula Falsi method. Still, they should not be too large or the system may become unsta-ble.

Which algorithm is employed depends on the value of the FO signal. The Newton in-terpolation is more precise but it is also more prone to error if the signal-to-noise ratio is poor. Thus it is only used when the FO signal reaches or exceeds the threshold specified by the Minimum FO Signal for NI.

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Drift Stabilizer (DS)

The 0-GHz position is maintained via applying small voltage offsets to the scanning Z piezo. Since this piezo is susceptible to small temperature changes, shifts of the 0-GHz position need to be compensated for. To account for this, TFPDAS4 is equipped with the Drift Stabilizer algorithm, which maintains the 0-GHz position of the spectrum at the preset channel.

To make sure that the drift stabilizer is enabled, check if the circular LED button next to the DS Signal indicator in the main window of TFPDAS4 shines green. If it is not green, press the LED button to activate the drift stabilizer.

The DS Signal is a measure of how well the reference peak is positioned at the chan-nel corresponding to the zero frequency shift and is displayed in the main window of TFPDAS4. To determine this value, the region given by the DS Width centered around the 0-GHz line is divided into two equal parts. The reference peak counts are summed up separately on each side of the 0-GHz line. The difference of these two values is normalized by their sum to obtain the DS signal, which ranges between -1 and 1. If the DS signal is zero, it means, that the number of counts in both halves of the DS width is exactly the same and, thus, that the reference peak is perfectly centered at the 0-GHz position. If the reference peak is completely shifted to the left (right), the DS signal will be close to -1 (1), respectively.

If there a shift in the 0-GHz position is observed by the stabilization routine, this will be compensated for by applying a voltage offset applied to the Z piezo. The DS signal is multiplied by the DS Slope and then subtracted from the current Z piezo offset. That is why the DS slope should not be too large. The applied voltage must be within the working range of the Z piezo, from -5 V to 5 V, or this will lead to substantial instabili-ties of the system. Therefore, the DS slope should not be set a value larger than 30.

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61

Dynamic Dither (DD)

Occasionally a particularly strong disturbance might lead to a rapid misalignment of the etalon mirrors and, thus, to a strong decrease of the interferometer performance. In order to react to short term perturbations like this, the Dither, i.e., the value of the mirror tilt during the Finesse Optimization can be adjusted dynamically.

A large dither guarantees high stability but the overall transmission of your Tandem-Fabry-Pérot Interferometer will be poor. A low dither maximizes the transmission at the expense of low stability. Therefore, a criteria needs to be established to define when to increase or decrease the Dither actual value.

If the FO signal is small, the transmission through the sixfold pass will be low, indicat-ing that the mirrors are poorly aligned. The parameter FO Dither Threshold deter-mines whether the FO signal is sufficiently high enough or not: If the FO signal drops below this threshold due to some disturbance, the dither voltage will be multiplied with the FO Dither factor inc in order to obtain the optimum mirror alignment more quickly.

The value of the Dither actual parameter is always displayed in the main window of TFPDAS4.

As soon as it reaches the value of the FO Dither max, its value will not increase further. The mirror alignment should improve and the FO signal should increase. As soon as the FO signal ex-ceeds the FO dither threshold, the actual dither will again be multiplied by the FO Dither factor dec until it reaches the FO Dither min value.

Since both the increase and decrease of the actual dither in-volve multiplication with the parameters FO Dither factor inc and FO Dither factor dec, these values need to be larger and smaller than 1, respectively.

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