Quadrant Photodiode (QPD)

Reference: PDQ80S1 Quadrant Detector System Operating Manual

www.thorlabs.com

QPD is the device to track laser beam movement for precise dis-placement measurement.

Introduction

• Also known as quadrant and bi-cell detectors, these devices have two or four distinct photosensitive elements separated by a minuscule gap.

• A light spot illuminating just one element only produces photocurrent in that element. When the spot is translated across the surface of the detector, the energy becomes dis-tributed between adjacent elements.

• The ratio between the photocurrent outputs from these el-ements determines the relative position of the spot on the surface.

• It's important to note that the detector only provides posi-tion information over a linear distance of the spot diameter. Elsewhere, it is known to be in a specific segment, but not exactly where. Because of this, when working with lasers, defocusing may be required in order to obtain maximum range.

[3]

Specifications

Operating principle

When the beam is centered on the detector, x, y difference signals come to zero

Beam incident angle should be normal

Trapped Bead Movement Measurement

• Measuring lateral displacement with QPD– Irrespective of the particle size we can assume the lateral shift

of the particle moves the peak intensity to the cell located in the shift direction.

QPD

Lens

Bead

Brownian motion of bead

Con’t

– We need to properly set the axial location of the QPD such that the QPD signal is nulled (all quadrants of equal value) irrespec-tive of the location of the spherical particle that coincides with the beam waist center.

– Axial displacement can be measured by the change in the power caused by defocusing at the QPD.

Q1

Q4

Q2

Q3

Measurement plane of QPD

x

y

Considerations during QPD calibration

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1. Position the beam to the center of quadrants where x and y difference signals are closely

zero. 2. From the center, move the beam to the left until the x difference signal no longer in-

creases and note the value that is negative x limit.3. Repeat the above step moving the beam to the right of center to determine positive x

limit.4. Repeat step 2 and 3 to determine the positive

and negative y limits. 5. Maximum measurement area should not be inclined. If inclined, check the beam is incident with normal to the quadrants.6. Calculate calibration factor (S/nm) by moving

the beam with known distance by using manual

stage.

Maximum measurement area

QPD calibration procedure (manual stage, laser source)

• Beam shape: QPD is optimized for stable circular beam• Beam size: 1mm ~ 3.9 mm. It depends on sensor area of the device. Minimum diameter

should be much larger than the band gap of quadrants. Maximum diameter should be smaller than the half of sensor area. Use lens to control the beam size

Beam selection

QPD APIPC

APIUSB communication PDQ.hUSB.husb.lib

Function List and Flow chart

• USBinitPDQ80S1();• PDQSendScanInterval();• PDQWriteHAlignmentWindow();• PDQWriteVAlignmentWindow();• PDQSendScanInterval();• PDQStartScan();• PDQReadScan();• PDQReadScan();• USBUninit();

USBinitPDQ80S1()

PDQSendScanInterval()

PDQStartScan()

PDQReadScan()

PDQStopScan()

AcquisitionComplete?

Y

N

Example code

QPD (Hamamatsu)

QPD is the device to track laser beam movement for precise dis-placement measurement.

Specifications (QPD sensor)

Specifications (NI-DAQ)

Number of Channels 16 SE/8 DI Sample Rate 1.25 MS/s Resolution 16 bits Simultaneous Sampling No Maximum Voltage Range -10..10 V       Range Accuracy 1920 µV       Range Sensitivity 112 µV Minimum Voltage Range -100..100 mV       Range Accuracy 52 µV       Range Sensitivity 6 µV Number of Ranges 7 On-Board Memory 4095 samples

Operating principle

When the beam is centered on the detector, x, y difference signals come to zero

Beam incident angle should be nor-mal

IR laserQPD1

QPD2P-polarized beamS-polarized beam

Dual QPD system (QPD1, QPD2)

Circuit of the QPD module

• Circuit of the QPD mod-ule– D1: QPD sensor– J1: Voltage input of

QPD(power supply)

• Op-amp (U1~U7)– voltage amplifier with

differential inputs – reduce the noise signal

• Each signals are calcu-lated in the circuit (X, Y, Sum)– 3 output voltage (J2,

J3, J4)

QPD control with NI DAQ

SHC68-68-EPM CableSCB-68 Connector Block PCI-6250 card

Programming using DAQmx API

QPD sensor and electrical circuit module

+

X - SignalY - Signal Sum - Signal

Power SupplyInput

Voltage

NI DAQ (PCI-6250)

• PCI-6250 is a high-speed multifunction M Series data acqui-sition (DAQ) board optimized for superior accuracy at fast sampling rates– 16 analog inputs, 1 MS/s (Multichannel)– improved measurement accuracy, resolution,

and sensitivity by choosing high-accuracy M Series.

• PC-BASED DATA ACQUISITION

Libraries for NI DAQ - DAQmx driver software interactive data-logging soft-

ware

NI DAQ (SCB-68)

• The SCB-68 is a shielded I/O connector block for interfacing I/O signals to plug-in DAQ devices with 68-pin connectors. Combined with the shielded cables, the SCB-68 provides rugged, very low-noise signal termination

NI DAQ (SCB-68)

• Connecting the SCB-68 with QPD

– AI 0~AI 2 (QPD 1)• AI 0: x signal• AI 1: y signal• AI 2: sum signal

– AI 0~AI 5 (QPD 2)• AI 3: x signal• AI 4: y signal• AI 5: sum signal

– AI 8~AI 13 • GND (0 volt)

QPD1

QPD2

Trapped Bead Movement Measurement

• Freq(Hz) : Setting the sampling rate

• Duration: Measuring time for QPD

• Persistence: Tracking the signal

• Start scan: Starting the QPD scan

• Start save: Generating text file– Max save count: 6000000

QPD2(S-polarized beam)

QPD1(P-polarized beam)

NIDAQmx Functions For QPD Data Aquistion

• Task Configuration/Control: DAQmxCreateTask (), DAQmxS-

tartTask(), DAQmxStopTask(), DAQmxClearTask()

• Channel Creation: DAQmxCreateAIVoltageChan()

• Timing: DAQmxCfgSampClkTiming()

• Read: DAQmxReadAnalogF64()

Sample programIn project settings, link, “NIDAQmx.lib”