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PHYSICS-400 SPECIAL PROBLEMS IN

PHYSICS

FIBER LOOP CAVITY RING DOWN

SPECTROSCOPY

Fırat İdikut

1575356

SUPERVISOR: Assoc. Prof. Dr. HAKAN ALTAN

JAN 24,2012

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Contents FIBER LOOP CAVITY RING DOWN SPECTROSCOPY .................................................................................. 3

a. Free-Space Systems ......................................................................................................................... 3

b. Fiber Based Systems ……………………………………………………………………………………………………………………..4

EQUIPMENT LIST ..................................................................................................................................... 6

EXPERIMENTAL WORK............................................................................................................................. 7

DISCUSSION AND CONCLUSION ............................................................................................................ 19

REFERENCES .......................................................................................................................................... 20

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FIBER LOOP CAVITY RING DOWN SPECTROSCOPY

Cavity ring down spectroscopy(CRDS) is a technique which is based on the light

absorption of the species in the gas liquid and solid phases where the light goes back and

forth in the cavity. FLCRDS technique differs from the other spectral analysis techniques such

as fluorescent, absorption and emission spectroscopy of the material. CRDS is typically multi-

pass through the sample, and single color. All materials have their own characteristics at

different wavelengths. One of them is the absorption of the incoming light. These

spectroscopy systems can be classified into two.

a. Free-Space Systems

In this type of CRDS, two highly reflected dielectric mirrors are used for focusing

the single mode light pulse into a cavity. The reason for using the single mode light pulse is

to make our system optically stable. Otherwise, it is not possible to focus a multimode pulse

into the cavity. After a few round trips, the pulse will escape from the system, so this system

is unstable.

Figure 1: Free-space CRDS[6]

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In figure 1, the working principle of the system is shown. The cavity is empty.

Because of the small absorption of the air and the reflective loss of the dielectric mirrors,

intensity decaying is observed in every round trip of the trapped pulse in the cavity. This

technique is suitable for measuring the low absorption typically in trace gases. There are

different techniques and new ways in CRDS, all of them depend on measuring the ring down

time. The main aspect of all these techniques uses the intensity decaying and ring down time

in order to determine the type of the material.

b. Fiber-Based Systems

In these types of cavities, a single laser pulse is coupled into the fiber. Instead of

making the round trips of the pulse between two mirrors, the pulse travels into the fiber

cavity. The fiber-based cavity can be classified into two. The first one is the linear fiber cavity

and the second is fiber loop cavities.

Figure 2: Fiber-based CRDS[2]

In figure 2.a, the cavity is formed by using two Bragg grating mirrors and the

laser coupled into the cavity. In figure 2.b, instead of directly coupling of the pulse, the pulse

is injected into the system by using the coupler. The Coupler has two outputs which divide

the intensity of the light at the rate of 99:1 theoretically. 99% of the light is travelling into

the loop and the measurement is made from the second output (1%). The main

disadvantage of the CDRS is the trade-off the high efficiency with high loss. The

measurement of the ring down time can be made be done with great precision if the

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insertion, absorption, bending and splicing losses can be reduced. This is done by using the

formula below, the measurement of the ring down time (τ)[4,5];

eqn1

In this equation, C0 is the speed of the light in the fiber core, L is the length of the fiber,

A=α*L is the absorption of the fiber core, B=β*d is the absorption of the sample. L and d are

the length of the fiber and the thickness of the cavity, and α, β are the absorption

coefficients of fiber and sample. The third term in the bracket is the loss coefficient of the

fiber coil with radius R, here η is the amplitude bending loss. lnTs is the splicing loss.

Figure 3:Ring down pulse of the 58 m fiber loop with 1550 nm DFB laser and 200nm pulse width. [3]

In this project, we used a nanosecond pulsed laser and form a fiber loop cavity to measure

ring down time. In our FLCRDS, the nanosecond laser light at 1535nm is coupled into the

loop and the relative exponential intensity decaying is measured by InGaAs PIN detector.

The tau where the

intensity is I0/e

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Figure 4: the Cavity ring down system

.

EQUIPMENT LIST

1. Cobalt Tango 1535 nm DPSSL laser. (3.3nm pulse width, 29mW putput power,

3.33kHz repetition rate)

2. 50 and 20m fiber spool.

3. Fiber splicer: Fujikura 60S

4. Photodiode: ET3010 InGaAs PIN detector. (Electro-Optics Technology)

5. X, Y, Z stage (Meller-Griot)

6. 2x2 coupler [99:1] (lightel Technology)

1

4

6

12

8

7

5

7

7. Collimator (lens coupled fiber collimator)

8. Digital Oscilloscope

9. Power meter (Thorlabs PM100D)

10. IR-Spectrum Analyzer (Steller Net InGaAs 1024-channel Spectrum Analyzer)

11. GRIN lenses

EXPERIMENTAL WORK

Before we starte our experiment, we characterized the laser in order to compare the

manufacture specifications with measurements. According to our tests, the laser has the

output as same as manufacture specification, 20mW. The spectrum of the laser is

1533nm measured and the pulse shape is q-switched shape. The pulse length is about

6.4ns. In the manual, the length is stated as 3.33ns pulse. The time between the pulses is

300micro second. After the laser was characterized, we set our experimental setup.

Figure 5: Design of Our Initial System

InGaAS PIN

diode

LASER (Cobalt

TANGO)

Oscilloscope

coupler

Fiber cable

disk with

R=7.25cm

and L=50m

99% 1%

collimator

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While coupling the laser into the system, we achieved to couple about 75% of the power.

We lost about 25% at the beginning. After that we spliced black-colored fiber with coupler

and we measure the output power of the blue fiber.

Figure 6: 2x2 coupler. The laser is input from black the power split is shown.

Afterwards we spliced the 50m fiber with the 0.00dB loss; we measured the output of 50m

fiber to be 10-10.5 mW. The possible reasons of the power loss are bending loss, the

absorption loss and wrong wrapping. The second splicing is between the red head and

coupler. This splicing is also successfully done with 0.00dB. After finishing the splicing we set

our oscilloscope and started the measurement.

Figure 7: First measurement with the initial system shown.

1%

99%

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In the figure above, we can see four peaks. The distance between the pulses is 250ns. The

fiber cable has the refractive index n, 1.5. By using these data, the length of the fiber in this

setup is calculated.

V and C are the speed of the light into the medium and into the free space. L is the Length of

the fiber. By using these equations:

V=3*108(m/s)/1.5

V=2*108 (m/s)

L=2*108(m/s)*25 (ns)

L=50m

It is verified that the length of the fiber is in agreement with the experimentally

measured value. The loss of the system is greater than 9dB which means that in every turns

the system lost is more than half of its power.

According to the equation below, the loss of each trips can be determined

In this equation, P0 is the initial power with respect to the next one. It is not clearly seen the

peak values. Because of that, for this calculation, the amplitude ratio of the peaks is

estimated. By looking at the grids in the figure, the third pulse is about 15 times greater than

the forth. The ratio of the powers of the pulses is unitless, so the amplitude ratio of the

peaks is used for determined the loss in dB.

10log(1/15)= -11.7 [dB]

In the figure 4, the second peak is much greater than the first peak. The reason why our

initial system failed is that we only used one 2x2 coupler in this system. This coupler has the

property when the input legs change, the output power division of the legs also switches. At

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the end, after the three round trips system, no power remained in the loop. The figure

below clarifies this property.

Figure 8: The laser pulse first coupled into the system from the black input leg. The measurement is done by the red output and the pulse makes round trips from blue to clear input direction.

Figure 9: It indicates the separation ratio of the intensity in each round trip. The above chart explains our data very well in Figure 7.

%1

%99 %1

%99

INPUT

Next round trips

INPUT

1st

(i=0)pulse

100%

2nd

(İ=1),3rd

(i=2),4th

(i=3) pulses

İ=1, İ=2, i=3

İ=0 (1%)

İ=1(98%)

İ=2(0.01%)

İ=3(0.0001%)

İ=0 (99%)

İ=1 (0.01%)

İ=2(0.0001%)

İ=3(0.000001)

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Our next step in this project was to redesign the system so that we can observe a nice

decay. We wrapped 20m fiber spool and used two 2x2 coupler. The new system is shown

the figure below. The photo of the system is shown in Figure 4.

Figure 10: The new design of the system

Figure 11: The new design of the system

In this design, we used two 2x2 couplers. We coupled red output of the first

coupler to the black input of the second coupler. By doing so, in every round trip, the light

coupled to the second coupler into the black input leg so that output power separation rate

of the red and the blue ends are not switched.

LASER( Cobalt

Tango with

1535nm) Oscillosope

Diameter=13.5cm

L=20m fiber spool

InGaAS PIN

photodiode

FC coupled

collimator

couplers

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Figure 12: The tau is measured as where the intensity becomes I/e of the initial intensity I.

Figure 13: Ring down time observed for a 20m fiber spool with radius 13,5cm.

In figure 13, we can calculate the total length of the fiber. The index of refraction of the fiber

core is 1.5 and the time separation between two pulses is 121ns.

121ns

D=135mm

The tau(τ)

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V=3*108(m/s)/1.5

V=2*108 (m/s)

L=2*108(m/s)*121 (ns)

L=24.2m

It is obviously seen that we have some extra fiber length. This extra path comes from the

couplers. Each couplers has two meters extra fiber.

To calculate the ring down time, we found the peaks of the figure 11 and put the peaks into

scatter graph. After that, we fit exponential function on the graph. The time decay constant

tau is calculated as 2,071µs.

Figure 14: the peaks were plotted into the scattered graph and eqn5 was fitted on the graph.

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In our system, there are some factors causing the experimental loss. These are the

splicing, fiber core absorption, and the bending loss. However, the most dominant one

among them is the bending loss. Therefore, we wanted to examine the bending loss factor.

To do so, we wrapped three more spools having approximately the same length, 20m.

The wrapping a spool with fiber cable is very hard, because the fiber is very weak.

Under some amount of the tension, it can be easily broken. One of the important points for

wrapping the spool, fiber has to been wrapped without overlap. The overlapping leads to

extra loss. To prevent this issue, we designed a mechanical system on the optical table. It

was very efficient and fast method for wrapping spool with fiber. The system is illustrated in

the figure below

Figure 15: the screws make the tension stable. Using tweezers provides us sensitive control during wrapping and prevents overlap.

Major Spool

The spool

screws

tweezer

1 meter

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By using this method, we prepared the spools. After that, we observed the ring downs for

different spools. The measurements are shown below.

Figure 16: The Ring Down for 80mm spool

Figure 17: Calculation of τ for spool having 80mm diameter

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Figure 18: The Ring Down for 63,5mm spool

Figure 19: Calculation of τ for spool having 63.5mm diameter

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Figure 20: The Ring Down for 48mm spool

Figure 21: Calculation of τ for spool having 63.5mm diameter

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TABLE 1

D=48mm D=63.5mm D=80mm D=135mm

Y0 0 0 0 0

A 1,21554 1,24186 1,24707 0,27407

τ 1,96379E-6 1,98433E-6 1,99697E-6 2,07154E-6

Error of τ 3,24659E-8 3,6865E-8 3,66141E-8 8,42505E-8

In the table 1, we summarized the τ values of each spools. It is observed when the

diameter of the spool increases, the bending loss decreases. These measurements clearly

show that the bending loss has great effect on the system.

In this project, our final step was to generate a cavity and determine the absorption

coefficient of the some samples. To do so, we could first determine the cavity thickness by

the eqn1. In this equation, if we let the d alone, we can calculate the thickness of the cavity.

The new form of the equation is the following form:

The only unknown parameter is d in eqn2. We neglected the connector loss term

(lnTs) because it is negligible. However, we haven’t done this step in this project. In the

conclusion part, the reasons will be explained.

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DISCUSSION AND CONCLUSION

In this project, the fiber loop cavity ring down spectroscopy has been studied to

determine the absorption loss of the liquid and gasses. 1535nm cobalt tango pulse laser was

used as a power source and the measurement was done by In GaAS PIN photodiode. Initially,

equipment was tested before designing the cavity. Later, our initial design was set. In this

design, it was observed that the system did not work properly. Four pulses were observed

and amplitude difference between the pulses was extremely high. On the other hand, the

pulses were equally distributed into the figure. By analyzing this data, the fiber length, used

in this design, was calculated. After the data analysis was done, the system was redesigned.

This time, two 2x2 couplers and 20m fiber cable were used in figure 11. The outcome of this

design was very reasonable. The ring down was successfully achieved. In figure 12, the CD

case having 13.5cm diameter was used. The peak distances and ring down time were

calculated. According to the results of the calculation, the peak separation is about 121ns,

which is corresponds to 24.2 fiber length with the speed of light into the fiber 2*108(m/s).

There are 4.2 m extra fibers although 20m fiber spool is spliced because the couplers add the

extra paths into the system. By selecting the peaks and plot these peaks into scattered line,

it is achieved to fit exponential function on the graph and to find the time constant τ. The

time is approximately 2.071µs in CD case. After the ring down was observed, the next step

was to determine the effect of the possible loss on the system. The connector and fiber

cable absorption loss are negligible. The only big loss comes from the bending loss. When

the light travels into the fiber, bending of the fiber causes the light escaping from the fiber

core. To observe how much it affects our system, the new spools are wrapped having the

diameters 48.5, 63.5,80 mm. To prepare these spools, a mechanical system was designed

which is illustrated in figure 15. After the spools were done, three different ring downs were

observed. In table 1, the ring down times is illustrated. It is clearly observed when the

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diameter of the spool increases, the ring down time also increases. This means that the

bending loss is inversely proportional to the diameter of the spool. However, in figures 16,18

and 20, the thee or five peaks do not decay exponentially because the detector is saturated.

In figure 20, this outcome can be observed. Before the decaying start, some peaks were

observed. In spite of this error, the bending effect still proven. The final step of this project

was to generate a cavity. It could have been done in this term but it could not. The GRIN

lenses were used for generating the cavity. Nevertheless, coupling the cavity with GRIN lens

was not accomplished. The output power of the green lens diverges and coupling this

diverging beam into the input green lens was not easy. The next term, we believe that we

will generate the cavity by using the same method or trying new techniques and then we will

determine the absorption coefficient of the some liquids and gases.

REFERENCES

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7595-7621

[2] H. Waechter, J. Litman , A. Cheung, J.A. Barnes, H. Loock, “Chemical Sensing Using Fiber

Cavity Ring-Down Spectroscopy, Sensors” 2010, 10,pg 1716-1742

[3] G. steward, K. Atherton, H. Yu and B. Brian,” An investigation of an optical fiber amplifier

loop for intra-cavity and ring-down cavity loss measurements”,Meas. Sci. Technol

2001,12,pg 845

[4] R. Stephan Brown,I. Kozin, Z. Tong, R.D. Oleschuk and H. Peter Loock “Fiber loop ring

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spectroscopy:A sensitive absorption technique for small liquid samples”,Rev. Sci. Instrum.

2003,74, pg 4821-4825

[6] G. Berden, R. Engeln, Cavity Ring-Down Spectroscopy Techniques and Applications, Wiley,

1st edition,2009, pg Xii preface

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[7] R.Li,H-P Loock and R.D Oleschuk “Capillary Elecrophoresis Absorption Detection using

Fiber Loop Ring Down Spectroscapy” Anal. Chem. 2006,78,pg 5685-5692

[8] P.B. Tarsa,P. Rabinowitz and K.K Lehmann “Evanecent field absorption in a passive optical

fiber resonator using continuous-wave cavity ring-down spectroscopy” 2003, pg 1-7

[9] W.R Simpson “Continuous wave cavity ring-down spectroscopy applied to in situ

detection of dinitrogen pentoxide (N2O5)” Rev. Sci . Instrum,2003,74, pg 1-12

[10] J.J. Scheer, J. B. Paul, A. O’Keefe, and R.J. Saykally “ Cavity Ringdown Laser Absorption

Spectroscopy: History, Development, and Application to Pulsed Molecular Beam” Chem Rev

1996,97, pg 25-51