There were some windows (W2 CVI parts) laying around the ATF. I measured the zero degree transmission for 1064 and 532 by sticking them in the output path of the Mach Zehnder (between the Mach Zehnder box and the PD Readout box).

I left the cover on the PD Readout box, so am not positive that I did not misalign the beams onto the PDs, though I was using at roughly normal incidence (less than 10 degrees), so I doubt it.

Results are for normal incidence, which is not what the coatings were designed for in every case. Beam polarization here should be P.

I played around with the second one a bit to see if I could get the IR to go through. It did not seem to work. Maybe a user error due to lateness? Maybe a previous user error in labeling? Maybe CVIs coatings just happened to do that! Looks like I won't use a 45P window at 0 deg!

I appraised the lab's lens collection, and here is what I found in mounted lenses. There are more in the lens kit, but I didn't look at those as they were beyond my laziness horizon.

Hmm.. I probably should have done that in some sort of sensible order...

The power transmission of the optical window for the IRLab cryostat was measured to be 0.966+/-0.002 at 2004nm. (Attachment 1)

A chopper powermeter was set to the QE measurement setup (Attachment 2). The window was held with a mount as shown in Attachmnent 3. The laser source was excited with the pumping current of 101.04mA. The output power was  monitored with a Thorlabs DET10D (PD#2 with Amp#2) attached at the 10% side of the 90:10 beamsplitter. The detected photocurrent after subtracting the dark current of 15.7uA was 152uA. The power meter detected the power around 0.95mW, while the power with the window inserted was around 0.91~0.92.

PD1       Window   No Window
[V]       [mW]     [mW]
-0.855    0.913    0.944
-0.855    0.906    0.951
-0.855    0.914    0.947
-0.855    0.922    0.950
-0.855    0.913    0.949
-0.855    0.912    0.948
-0.855    0.920    0.946
-0.855    0.915    0.946
-0.855    0.916    0.951
-0.855    0.915    0.952
-0.855    0.919    0.947
-0.855    0.921    0.944
-0.855    0.916    0.948
Note: PD1 had the dark output of -0.0809V.
Note2: The power meter readings had the fluctuation of +/-0.005 mW

I prepared some basic optics for a PD QE enhancement experiment.

Specifically, two half wave plates, a PBS, a BS, a PD mount, and a stage for the PD mount are prepared.

The PD mount has a glued connector for PDs for replacing them easily.

The sage for the PD mount has three micrometers for moving PDs accurately to three axes.

A male pin assignment for a DC power supply of a circuit for the PD is confirmed.

As shown in the following image, #1 pin, #2 pin, and #5 or #9 pin should be connected to +15 V, -15 V, and GND, respectively.

In addition, for aligning the optics, a CCD camera and a lens for the camera are also prepared.

All things are placed on an optical bench without being aligned.

I will align the optics and test the PD circuit and the camera with laser light.

To pick off adequate laser power for our PD QE enhancement experiment, the first HWP in front of the laser was rotated.

The inititial angle of the first HWP was 48 degree.

For measuring the laser power, some optics were aligned as shown in the following figure using a CCD camera.

In this figure, the beam dump #1 is placed for dummping the laser for the other expetiment that is made on the same optical bench as the PD QE experiment, and the Irises are placed for dumping unnecessary light.

I plan to place HWPs and PBSs additionally and to measure the polarization and the laser power at the beginning of next week.

For measuring  a current beam profile, the setup as shown in Fig. 1 was prepared.

At first, laser powers at Point 1 and 2 were measured and the results were

* at Point 1: 17.71 mW

* at Point 2: 16.15 mW

The difference may be come from the loss of HWP #2 and the beam damped by Iris.

(Laser power just in front of the Iris cannot be measured because of the space constraint.)

Then the beam profile was measured as shown in Fig. 2.

The measurement was perfomed from 0 cm to 25.4 cm (11 points) and in the measurement the angles of HWP #1 and #2 were 48.1 degree and 22.4 degree, repsectively.

(The zero point is set after the PBS as shown in Fig. 1.)

As shown in Fig. 2, the beam profile, in especially y-direction, seems to be a bit strange.

This is condiered to be an effect of a kind of saturation, but this is a just guess.

The measured data and the fitted curves are shown in Fig. 3. 

The beam measued yesterday was very elliptic.

Thus, for obtaining a round beam, we decided to use a single mode fiber.

The single mode fiber was aligned with KojiA's instruction.

(There is the specific method in the last of this log)

As a result, the obtained mode matching ratio was 64.5%. (Input power for the fiber was 18.7 mW, and output power of the fiber was 12.07 mW.)

Then, the setup as shown in Fig. 1 was prepared.

In this setup, the beam profile after the single mode fiber was measured as shown in Fig. 2.

When Fig. 2 is observed, there is still a kind of lines in the y-direction and we consider it comes from the problem of camera.

The measured data and fitted curves are shown in Fig. 3.(Zero point of the distance was set at the position of the collimator #2)

Specific method to align the single mode fiber

1. place the collimator #1 at the focal point of the laser

2. align the laser light (IR light) to the collimator #1 roughly with steering mirror

3. unset the collimator #2 and set a fiber illuminator instead of the collimator #2

4. at just after the Iris #2, align the laser from the fiber illuminator (Red light) to the IR light with knobs of the collimator #1 holder.

5. at just before the collimator #1, align the IR light to the Red light with the steering mirror #1

6. repeat step 4. and 5. a few times

7. unset the fiber illuminator and confirm the IR laser is ouput from the collimator #2. (If the IR light cannot be observed, go back to step 3.)

8. set a power meter after the collimator #2

9. maximize the IR light power using yaw knobs of the collimator #1 holder and the steering mirror #1

10. maximize the IR light power using pitch knobs of the collimator #1 holder and the steering mirror #1

11. repeat step 9. and 10.

12. If a good mode matching ratio can be obtained, this fiber alignment is finished. If the good mode matching ratio cannot be obtained, change the Lens #1 and re-start from step 1.

For adjusting the beam, the setup was prepared as shown in Fig. 1.

Our taget is 80 um beam waist in radius as shown in Fig. 2.

(Please note that the axes of Fig. 1 and Fig. 2 do not have the same zero point.)

The waist position is not much took care to if the position can be accessed easily.

In our simulation using the beam measured yesterday, when the lens #2 (f=-200 mm) and the lens #3 (f=150 mm) are placed at z = 0.260 m and z = 0.474 m, respectively, the target radius could be obtained at z = 0.74 m.

However, in this setup we cannot obtain the target beam and the waist size seemed to be larger and the waist position seemed to be farther.

Thus the lenses were moved iteratively.

Finally, the lens #2 and #3 are placed at z = 0.114 m and z = 0.483 m, respectively, and the measured beam is as shown in Fig. 3.

(The beam profiler seemed to be saturated. Thus the additional ND filter (O.D.=4.0) is put and the saturation seemed to be extinguished as shown in Fig. 4.)

As a result, the beam waist radius and the beam waist position are 86 um and z = 0.750 m (for the x-direction) and 89 um and z = 0.750 m (for the y-direction), respectively.

These parameters can still be improved by refining the positions of the lens #2 and #3.

For measuring the polarization, the setup as shown in Fig. 1 was prepared.

The angle of the HWP #2 was 30 degree.

Rotating the angle of the HWP #3, I measured the laser power with a power meter and a PD.

And I fitted the measured data to the function, .

Here \theta is the angle of the HWP #3.

The result was shown in Fig. 2 and the paremeters were determined as

(with the power meter) a = 7.965 +/- 0.0005 mW, b = -0.002 +/- 0.003 mW, phi = -40.65 +/- 0.01,

(with the PD) a = 1373 +/- 3, b = -1 +/ 2, phi = 40.52 +/- 0.03. 

Accoding to this result, the S-pol. and the P-pol are obtained at 40.6 degree and 85.6 degree of the angle of the HWP #2, respectively.

And the calbration constant of the PD from voltage to power is determined roughly as 5.8*10^(-3) W/V. (Systematic errors have not yet been concerned.)

According to the polariztion measurement, we found that the laser is not linear polarization.

Thus the QWP #2 was placed after collimator #2 as shown in Fig. 1 and the linear polarization laser was obtained.

Then the PD, C30665GH, with the glass window was set on the PD mount and placed at the center of the rotational stage as shown in Fig. 1.

The PD position is 5 cm after the beam waist and, at this position, the beam sizes in x-direction and y-direction are 215 um and 210 um, respectively.

In this setup, there are two reflected lights which may be come from the glass window and the PD.

Changing the laser incident angle to the PD, I measured the PD reflectivities and the PD efficiencies for P-pol. and S-pol.

The results are shown in Fig. 2 and Fig. 3.

The reason why the measurement end before the tasty part like ~40deg is that the beam is clipped.

Possibly, the light may already be clipped at ~25deg.

The alignment, i.e. the centering of the light to the PD, was optimized before the measurement as possible as we could.

The measurement was done only by rotating the rotational stage, i.e. I didn't use the steering mirror.

(Perhaps, I should use the steering mirror for optimizing the alignment at every angle.)

The incidnet power was 11.4 +/- 0.1 mW.

I meausred the powers of the one light, the other light (with the Iris #3), and the two lights (without the Iris #3) at all incident angles.

The errors in Fig. 2 is determined by the error of the measured incident and reflected light power. The position dependency on the power meter is not took into consideration.

When the PD responsivity for 1064 nm and the PD calibration constant (W/V) are 0.78 A/W and 5.8*10^(-3) W/V (see elog:2017), respectively, is assumed, the PD EQE is about 0.9 between -30 degree and 40 degree in the incident angle, according to

 ,

where I is the photocurrent, h is Planck's constant, c is the speed of light, phi is the incident power, n is the index of refraction of air, e is the elementary electronic charge, and lambda is the wavelength of the laser.

For calibrating the output voltage to EQE, the electric current must be known but, so far, I don't know the resistance in the photo detector circuit because the circuit is not made by myself. Thus I must investigate the circuit.

This is why I used "0.78 A/W" and "5.8*10^(-3) W/V" to estimate the current for a moment. The calibration constant "5.8*10^(-3) W/V" is determined without taking the errors, such as the position dependency on the power meter, the fluctuation of the laser light and so on, into consideration. (Of course, this is a problem.)

The errors in Fig. 3 is determined by the error of the reading error of the oscilloscope and the incident power. The position dependency on the power meter is not took into consideration.

We aligned the single mode fiber again and We obtained the linear polarization without QWP as shown in Fig. 1.

P-pol. and S-pol. are able to be obtined at 34.8 deg and 81.0 deg in angle of HWP #3, respectively.

Then, We checked the PD read-out circuit and the circuit diagram is shown in Fig. 2.

Important resistanve values are as follows:

R4 = 20.2 Ohm, R5 = open, R11 = 1.000 kOhm, R16 = 9.82 kOhm, R7 = 50.2 Ohm.

In this circuit, the current generated by PD (I_PD) can be calibrated from the DC output voltage (V_out) using the following equation:

(In other words, the trans-impedance of the readout circuit is 219 Ohm.)

After that, we measured the DC output voltage at every 10 deg in the incident angle with the window glass as shown in Fig. 3.

The PD position is 5 cm after the beam waist and, at this position, the beam sizes in x-direction and y-direction are 215 um and 210 um, respectively.

The incident angle is determined with +/- 0.5 deg error by following way:

step1 (in this step, PD is alomost perpendicular to the beam) seeing reflected light from the PD, we centerd the beam to the PD using the steering mirror. 

step2 We miscenterd the beam to the border of the PD, and checked the incident angle seeing the reflected light. The incident angle was 0.5 deg (-0.5 deg in the opposite side.) Then the beam was re-centerd to the PD by the steering mirror.

step3 The PD is tilted by the rotational stage (the angle of the stage is called "alpha"), and, if the beam is miscenterd, we centerd the beam to the PD using steering mirror (the angle caused by the steering mirror is called "beta"). Thus the incident angle is "alpha + beta"

step4 Without touching the steering mirror, the PD is rotated to be perpendicular to the beam (i.e. "alpha" became 0 deg).

step5 We checked whether the beam is still on the PD or not (in today's measurement, the beam is on the PD in this step). If the beam is on the PD, the incident angle at step3 is estimated as follows,

alpha - 0.5 < alpha + beta (incident angle) < alpha + 0.5 (degree)

Therefore, the incident angle is determined with +/- 0.5 deg error.

We are going to measure again the reflectivity tomorrow with this method.

D980454-00(QE).pdf

The reflecrivities and the EQEs of the PD (C30665GH, with the glass window) for S-pol and P-pol were measured at every 10 deg in incident angle as shown in Fig. 1 and Fig. 2.

In this measurement, for alignmnet the steering mirror was used and it was confirmed that the beam was not clipped.

The EQE is obtained from the DC output voltage which was read from the oscilloscope using the following equation (see also elog2019):

,

where I_PD is the photocurrent, V_out is the DC output voltage, h is Planck's constant, c is the speed of light, phi is the incident power, n is the index of refraction of air, e is the elementary electronic charge, and lambda is the wavelength of the laser.

Can you calculate the IQE from EQE and reflectivity (by ignoring scatter loss)?

Can you measure the EQE/IQE/reflectivity at the angle where the EQE goes maximum for the P pol?

Can you use larger fonts in the plots?

I calculated IQEs using the measured EQEs and reflectivities and the following formula:

,

where R is the reflectivity of the PD, and T is the transmittance of the PD.

Here T is assumed to be zero and the scattering loss is ignored as you said.

The obtained IQEs are shown in Fig. 1.

Also the EQE, IQE, and reflectivity at a few angles near the angle where the EQE goes maximum for the P pol were measured as shown in Fig. 2 and Fig. 3.

The angle where the EQE goes maximum for the P pol is -51 deg.

So far, in these plots, the systematic errors because of the power meter are not considered.

And I replaced the figures with the figures having larger fonts.

QE_Ref_P.txt QE_Ref_S.txt

AroundPeak.txt

QEs (with the glass window) with the reflector were measured as shown in Fig. 1.

The radius of curvature of the reflector is 25 cm and the reflector was placed at a distance of 5 cm from the PD.

At the reflector position, the beam size is 403 um in x-direction and 391 um in y-direction.

And, at the PD position, the beam size of the light reflected by the reflector is 435 um in x-direction and 422 um in y-direction.

When Fig. 1 is observed, the QEs in S-pol are less enhanced than that in P-pol.

This is because there are a few lights which are reflected by the PD and have almost the same power, in other words in this measurement only one or two beams are re-input to the PD by reflector.

In fact, we can see the two peak output voltages of the PD when the reflector are rotated in the yaw direction.

QE_enh.txt

The glass window of the PD was removed and is placed in our shelf.

The reflectivity, EQE, and IQE of the PD were measured without the glass window as shown in following figures in the same setup as the measurements of the PD with glass window (see also elog2019, elog2020, and elog2022).

The laser power incident to the PD was 11.4 +/- 0.1 mW.

Ref_QE_wow.txt QE_cal3_NK.m

The enhanced QE of the PD without the glass window was measured in the same setup as elog2023.

QE_enh.txt QE_enh_ref.txt QE_enh_cal_NK.m

My interpretation of this is that we get ~2% increase in the peak EQE for p-pol by doing the extra bounce. Is that correct? If so, its a pretty good result.

I wonder if you can do some more angles to see if there are any features in the angular dependence.

It would also be interesting to modify the beam size to see if there is any change in the EQE. What is the estimated beam size now in the x & y directions? Does the reflected beam overlap the first beam? It would be good if the 2nd reflection from the PD can be steered to not go back into the fiber.

For the circuit, the R=20 Ohms makes it nicely so that the PD bias voltage doesn't change that much, which is good.

Is there a diagram with the reflector in place?

Is it possible to use the camera to take an image of the reflection from the PD? I wonder if its Gaussian or messy.

What about repeating this experiment with the 1.5 micron laser now? Or maybe a HeNe where the IQE is lower?

> My interpretation of this is that we get ~2% increase in the peak EQE for p-pol by doing the extra bounce. Is that correct? If so, its a pretty good result.

Your interpretation for the current result is collect. However, there are still several errors which aren't took into consideration, such as the error of the resistances and so on. Thus, for saying that the EQE is increased by about 2%, we must obtain more accurate data or analyse the results more preciselly.

> I wonder if you can do some more angles to see if there are any features in the angular dependence.

We plan to do a finer angular scan with the more accurate setup.

> It would also be interesting to modify the beam size to see if there is any change in the EQE. What is the estimated beam size now in the x & y directions? Does the reflected beam overlap the first beam? It would be good if the 2nd reflection from the PD can be steered to not go back into the fiber.

Actually, it is not easy to change the beam size immediatelly because the lenses and the PD are fixed. However, we plan to change the beam size for observing the BRDF. Thus in that time we will measure the EQE changing the beam size.

The estimated beam size of the first incident light on the PD is 215 um in the x direction and 210 um in the y direction, and the size of the second reflected light on the PD is 436 um in the x direction and 422 um in the y direction.

The second reflected light is aligned not to overlap the first incident light. However, so far, the distance of the two beams cannot not be measured because the second light is so weak that we cannot see.

> Is there a diagram with the reflector in place?

This is the diagram with the reflector.

> Is it possible to use the camera to take an image of the reflection from the PD? I wonder if its Gaussian or messy.

It's possible. We will measure the beam profile of the reflection light from the PD for confirming if the PD can be regarded to be well-polished and flat.

> What about repeating this experiment with the 1.5 micron laser now? Or maybe a HeNe where the IQE is lower?

For now, we don't have the plan to change the wavelength. Before the wavelength is changed, we would like to do the whole measurement includidng the noise estimation such as the back scattering noise. Once we establish the whole experimental method, we will change the wavelength and the PD.

If we want to check soon the EQE with a different beam size, we may want just to add (for a kind of quick check) an additional lens in the path without changing what we already have.

Rich removed the window from PD on Friday. The basics steps for the removal are the following:

0. PD in a socket, it helps;

1. In the beginning you open the screws on the cutter up so they just hold the photodiode;

2. Make a light little line first at desired height and make sure that it is a circle and not a helix;

3. You do not want to go start right away full force on it, you want to make tiny little incremental cuts. Eventually it just falls apart;

The beam shape reflected by the PD was measured.

The measurement was done 5 cm and 15 cm far from the PD at 15 deg, 45 deg, and 60 deg in incident angle.

(Please note that only at 15 deg, the measurement was done at 5 cm because of the space constraint.)

The results are shown in Figs. 1 and 2 and Tabs. 1 and 2.

When Figs. 1 and 2 are observed, the beam shape reflected by the PD looks still gaussian.

In the simulation, the beam size is estimated as, at 5 cm, 403 um in x-direction and 391 um in y-direction and, at 15 cm, 792 um in x-direction and 766 um in y-direction, respectively.

When Tabs. 1 and 2 are observed, the measured beam size looks not so different from the simulated one.

Tab. 1 Beam size in x-direction

	15 deg	45 deg	60 deg
5 cm (6 cm)	470 um	406 um	411 um
15 cm	736 um	625 um	608 um

The QEs were measured again at every 10 deg incident angle.

The power meter to measure the incident angle was S401C (Thorlabs).

Output voltage was read by the degital multimeter 77 IV (Fluke).

The results are shown in Figs. 1--3.

(About the reflectivities, the data measured in elog2024 are used for now. Those data are obtained with S130C (Thorlabs. error is +/- 7%.))

In Fig. 3, the ratio of the enhanced QE and the not-enhanced QE is shown.

In these figures, the error of the S401C (3%), 77 IV (about 0.4% (depends on the measure value)), and resistances of the readout circuit (assumed to be 1%) are taken into consideration and they are assumed to be independent.

These results are consistent to the data we have measured by last week in the error.

Current errors of the EQEs in Figs. 1 and 2 about +/- 4% and they are mainly determined by the systematic error of the power meter.

Figure 3 claims that the QEs are improved with +/- 0.6% error using our new method.

In the spec sheet, the systematic error of RM9 (power meter, Ophir) is 5%.

This is not very good.

However, the power meter was calibrated in the company at 808 nm wavelength and the deviation was 0.6%

This is very good.

If the 0.6% error could be applied to our power measurement, the systematic error would be improved.

Thus we inquired the error of RM9 to Ophir.

The answer was that the error is not 0.6% but is 5% at 1064 nm.

For confirming if the BRDF of the PD (C30665GH) can be measured, the setup as shown in Fig. 1 is established.

For now, incident angle to the PD is 0 deg for simplicity.

The beam size on the PD (C30665GH) is estimated as 215 um in the x direction and 210 um in the y direction.

If the reflected light is incident to the lens, the light is focued at the PD (PDA100A).

For determining theta_s and aligning the lens, metal tube, and PD (PDA100A), the incident angle is set to theta_s/2 temporally and seeing the reflected light they are placed without PD.

Specifically, the reflected light is cented to the lens and the output of the tube.

After that, the PD (PDA100A) is placed and the incident angle is set to 0 deg.

The distance between lens and PD is 31 cm for theta_s>0, and 23 cm for theta_s<0.

These distances are determined by space constraint.

And also for space constraint, the BRDF cannot be measured at the theta_s beteween 40 and 80 deg.

The incident power is also measured by the PD (PDA100A).

The gain of the PD (PDA100A) was 0 dB at the incident power measurement and 70 dB at the scattered light power measurement.

The BRDF is obtained with folloing equation,

,

where P_S is the power of the scatterd light, P_i is the power of the incident light, theta_i is the incident angle, dOmega is the solid angle of the detector, r_l is the radius of the lens, and d is the distance between the PD (C30665GH) and the lens.

With the setup and the method, the BRDF is measured for P-pol and S-pol as shown in Figs. 2 and 3.

The error of the Figs. 2 and 3 are determined by the systematic error of the PD and the distance error.

When Figs. 2 and 3 are observed, the BRDF looks to be able to be measured.

However the data quality can be improved.

Thus we are going to scan finer angles and change the incident angle from tomorrow.

The BRDFs at 0 deg and 15 deg in incident angle were measured for the wide angle with the same way as elog2034 and Fig. 1 but finer scaning.

The results for P-pol and S-pol are shown in FIgs. 2 and 3.

It looks that there is no difference for polarization.

The tendency that the BRDF increases from -75 deg is consistent to the previous result of the BRDF measurement of the viewport used in LIGO.

Between -40 deg and -70 deg, we face the sensitivity limit of the PD (PDA100A). (In this range, the estimated power is 1 nW and the power range of the PDA100A is 500 pW in the specsheet.)

For comparing the two BRDF, the BRDF plot in which the BRDF at 15 deg incident angle is shifted by 30 deg is shown as Fig. 4.

In this plot, the BRDF for P-pol is picked. 

It looks that there is no difference between 0 deg and 15 deg in incident angle from -40 deg to 40 deg.

The BRDF of the PD (C30665GH) for 15 deg incident angle was measured arond the peak with the same was as elog2035 and elog2034 but with the additaional Iris as shown in Fig. 1.

For calculating the BRDF. the fomular in elog2034 is used. 

(Please note that r_l becoms the radius of the Iris in front of the PDA100A and d becomes the distance between PD (C30665GH) and the Iris)

The Iris diameter is 3.5 mm.

There is no difference for polarization as the wider scan.

The result for p-pol is shown in Fig. 2 with the wider scan (the relsult of the elog 2034). This result was wrong.

There is a asymmetry around the -30 deg and the data around -35 deg are not consistent to the previous data.

We think these are come from the slight tilt of the Iris.

We will confirn that after tomorrow.

The result shown in the elog2036 looks not to consistent to the previous result.

Thus we checked the alignment of the Iris.

First, the effect of the tilt of the Iris was checked and it was found that the tilt effect is very small.

Second, we found that the center of the Iris was off from the center of the lens.

This off effect is dominant in the BRDF measurement.

The off was about 1 mm. However, the diameter of the Iris was 3.5 mm and 1 mm is about 30% and large to 3.5 mm.

Thus we re-align the Iris and measure the BRDF of the PD (C30665GH) at the 15 deg incident angle again as shown in Fig. 1.

Figure 1 shows that the new result is consistent to the previous wide range measurement in the error.

The reason why the error at -29 deg and -31 deg is very large is the effect of the main reflected beam.

The result that the new data arond peak and the previous wide range data are combined for p-pol and s-pol is also attached.

Note that the incident beam is at 0 deg and the angle is that we must take care in terms of the back scattering.

The reflectivity of the PD (C30665GH) at 15 deg incident angle was measured precisely with PDA100A as shown in Fig. 1.

Ref_wow_again.txt

The BRDF at 15 deg incident angle with the Iris in the incident path was measured for p-pol with the same way as elog2034.

(Please note that, in this measurement, d is the distance between the PD (C30665GH) and the Iris in fron of the PDA100A.)

The setup is shown in Fig. 1.

The result is shown in Fig. 2.

Figure 2 indicates that the Iris reduced the BRDF by about 10 times.

In terms of the BRDF, the mount of the back scattering of our technique can be estimated using the result of elog2038 and elog2039.

In our new technique, there are two effect of the back scattering:

1. the first incident light effect,

2. the second reflection light effect.

If the reflector is placed on the 5 cm far from the PD, the two effect is estimated as follows.

The BRDF of the first effect is roughly estimated as 5*10^(-5) [1/st] form the result around 0deg of elog2039.

The BRDF of the second effect is roughly estimated as 1.5*10^(-4) (= 3*10^(-3) * 0.05) [1/st] from the result at 31 deg of elog2039 (2*10^(-3) [1/st]) and the reflectivity at 15 deg of elog2038 (0.05).

The Iris 1 diameter was 1.4 mm.

For confirming this estimation is reasonable, the back scattering effect was measured with the setup as shown in Fig. 1.

In this setup, the PBS and the QWP are used as a isolator.

The Iris 1 is place on the beam waist.

We measured with/without Iris 1 and with/without the reflector, i.e. in four pattern.

For calculating the BRDF, the formula in elog2034 is used.

For r_l, the Iris radius in front of the PDA100A (5.83 mm) is used and, for d, the distance between the PD (C30665GH) and the Iris in front of the PDA100A (35.56 cm) is used.

The result is shown in Fig. 2.

Taking the difference of the result between w/ Iris, w/o reflector and w/ Iris, w/ reflector, we can obtain the effect of the reflector, i.e. the second effect.

The difference is (1.0 +/- 0.8)*10^(-4) [1/st].

This is comparable of the upper estimation. 

From these estimations, the second effect is larger by a factor of two or three.

However, considering the BRDF result arond the peak of elog 2039, we can reduce the second effect easily and dramatically by the reflector placed closer to the PD (e.g. 2.5 cm).

We will check this with more precise measurement.

With the chopper, the BRDF at 15 deg incident angle with the Iris on the laser path was measured again.

This is because the BRDF with the Iris on the laser path was limited by the sensitivity even at the scattering angle very close to the peak (e.g. 35 deg).

The setup up is shown in Fig. 1.

The diameter of the Iris2 was 7 mm at -25, -27, and -33 deg, and was 3.5 mm at -29 and -31 deg.

In angles wider than -35 deg, the Irs 2 wad removed for obtaining larger light and the lens diameter is 1.905 cm.

We determined the choppwer frequency as 253 Hz, considering the PD dark noise. 

The PD dark noise at 253 Hz is about 100 uV as shown in Fig. 2.

The peak value at 253 Hz is read and the value is calbrated to the voltage of the PD.

The calibration constant is determined cahnging the laser power and measureing the DC value of the PD (without chopper) and the peak value of the FFT analyzer and the constant is determined as 0.80 +/- 0.01. (from FFT to PD)

The result of the BRDF is shown in Fig. 3.

Using the setup for the measurement of the back scattering (in elog2040), the relative distance between the 1st incident beam and the 2nd reflected beam was measured.

First, the 1st incident beam position was detemined by aligning the reflector and overlapping the 2nd reflected beam on the 1st incidnet beam with the monitor of the value of the multimeter ("worst angle").

After that, the reflector was aligned onto the point that the back scattering is smallest and the relative angle of the knob of the reflector mount was noted ("best angle").

Using the value of the relative angle and some geometrical features of the mount, the relative reflector angle between "worst angle" and "best angle" was determined 0.4 degree, i.e. the incident angle to the reflector was 0.4 deg.

From this value, relative distance between the 1st incident beam and the 2nd reflected beam on the PD was determined 0.8 mm as shown in Fig. 1.

And the 1st incident beam and the 2nd reflected beam on the Iris was determined as 1.6 mm.

This means the main reflected beam was dumped by the Iris. (The Iris apature radius is 0.65 mm.)

The distance on the PD is designed as 1 mm.

The design value and the measured value is consistent.

For obtainning the design value, the 1st incident beam should be miscentered a bit more.

Using the improved reflectivity measurement, the IQE is calculated again (see also elog2031).

The QEs are calcurated with the same way as elog2020 and elog2022 and the results are shown in Figs. 1 and 2.

Figure 1 shows that, for p-pol, the not-enhanced EQE and the IQE are 0.90 +/- 0.03 and 0.94 +/- 0.04 at 15 deg, respectively.

Figure 2 shows that, for p-pol, the EQE is enhanced 4.0 +/- 0.4% and, between 30 deg and 50 deg, the EQE is enhanced almost up to the IQE.

The BRDF with Iris was measured with the chopper and FFT analyzer again because in the previous measuremt it is suspected that there may be a beam clip.

This time, the BRDF was measured after is is confirmed that the are no clipping using a IR view.

The result is shown in Fig. 1.

The error estimation has not yet be done.

The two data shown in Fig. 1 are consistent in the skirt ares by are not consistent arond the peak.

Thus, we checked the clipping effect by making a clip intentionally and measuring the BRDF at several point.

When there is a clip, the previous BRDF is reproduced.

There, we conclude that there was a clip in the previous measurement.

(This is the work on Thursday.)

Background:

We observed large discrepancy of the back reflection between the values measured with a BS and estimated from the BRDF. The back-scattering from the PD (C30665GH) without the reflector measured in elog2040 (2*10^(-3) [1/sr]) was about 40 times larger than the value expected from the BRDF measurement (elog2045, 5*10^(-5) [1/sr]). We thought this came from the ampbient light, scattering from other optics (e.g. PBS), and scattering from the chopper. We tried to measure the back reflection again with a refined setup.

What we did:

As the first attempt, for dumping the scattering light from the chopper, a "wall" to dump the light with the aluminum foil was mede and was placed between the lens and PBS as shown in Fig. 1. Before the wall was placed, the back reflection was about 162.6 mVrms (PDA100A gain 70dB). After the wall was placed, the back reflection was improved to about 82.5 mVrms (PDA100A gain 70dB).

Secondly, the chopper was tilted to reduce the direct reflection. Then the back reflection was reduced to about 3.48 mVrms (PDA100A gain 70dB).

Thirdly, we found that the incident beam was clipped at the incident Iris and the level was 0.4% in terms of the transmitted light power. (The Iris had been opened 1.2 mm in diameter.) The clip was small but could not be negligible. Thus we opened the Iris to 1.8 mm in diameter and the clip level was less than 0.1%, which is the detection limit of the degital multimater used to measure the output signal of the PD (C30665GH).

In order to obtain a reference calibration, we placed an HR mirror right before the target PD and measured the reflected power on the measurement PD (PDA100A). 

All the numbers were measured with an FFT analyzer to read the peak value at the chopping frequency.

After that, we measured the back reflection with the same way as elog2040.

Measurement:

We measured the back scattering in following conditions:

1. d_ref = 5 cm, theta_ref = 0.8 deg, without the reflector,

2. d_ref = 5 cm, theta_ref = 0.8 deg, with the reflector,

3. d_ref = 5 cm, theta_ref = 1.7 deg, without the reflector,

4. d_ref = 5 cm, theta_ref = 1.7 deg, with the reflector,

5. d_ref = 2 cm, theta_ref = 4.3 deg, without the reflector,

6. d_ref = 2 cm, theta_ref = 4.3 deg, with the reflector,

where the paremeters are explanes in Fig. 2. In conditions 3--6, using the CCD camera, the primary incident beam on the PD aligned was  on 1 mm far from the boundary of the PD, i.e. the beam was aligned at 0.5 mm off from the center of the PD as shown in Fig. 2. (I'm sorry but in the condition 1 and 2 the beam position was unknown.) The theta_ref is determined as widely as we can keeping the output signal of the PD (C30665GH) maximum.

For calculating the BRDF. the fomular in elog2034 is used.

(Please note that r_l becoms the radius of the Iris in front of the PDA100A and d becomes the distance between PD (C30665GH) and the Iris)

The Iris diameter was 1.143 cm, the distance between PD (C30665GH) and the Iris, d, was 43.1 cm.

Result:

The measured value with HR mirror in front of the PD (C30665GH) was 2.400 Vrms (PDA100A gain was 0 dB.)

The voltages measured in the conditions were as follows (PDA100A gain was 70 dB).

1: 3.192 +/- 0.007mVrms

2: 27.3 +/- 0.01 mVrms

3: 3.347 +/- 0.004 mVrms

4: 3.786 +/- 0.008 mVrms

5: 3.881 +/- 0.009 mVrms

6: 3.846 +/- 0.007 mVrms. 

The BRDF results are shown in Figs. 3 and 4.

The error estimation was not yet be done.

Discussion:

* Primary incident beam effect

When we see the values measured in the condtions 1, 3, and 5, the primary incident beam effect is about 8*10^(-4) [1/sr] which is still larger about 10 times larger than the expected value from the  BRDF measurement, 5*10^(-5) [1/sr].

This means that, for observing the back reflection of the primary incident beam, we must reduce the noise, i.e. the scattering which does not come from the PD.

* Secondary reflected beam effect

When we see the the gaps of the value between condition 1 and 2, between 3 and 4, and between 5 and 6, when theta_ref is made larger, the secondary reflection beam effect was reduced.

The difference between condition 3 and 4 is 1.1*10^(-4) [1/sr] and the expected secondary reflection light effect from the BRDF measurement was 0.8*10^(-4)  [1/sr].

They look consistent.

The difference between condition 5 and 6 can not be seen in current sensitivity and the expected secondary reflection light effect from the BRDF measurement was 1*10^(-5) [1/sr]

Thus it is reasonable that there is no difference between 5 and 6. 

The difference between 3 and 5 is considered to come from the laser power drift.

Therefore, the secondary reflected beam effecte can be explained with the BRDF measurement.

Background:

We are still suffering from excess amount of back reflection compared with the predicted number from the BRDF measurement. Assuming this light is not coming from the target PD, we want to reduce the scattering from other optics. As an attempt, we decided to use a BS instead of the combination of a cube PBS and a QWP. This way we can reduce the number of the optics involved, particularly the PBS which may cause more scattering than others.

What we did:

The PBS and the QWP were removed, and then a 50:50 BS was inserted as shown in Fig. 1.

The distance between the target PD and Iris2 was measured to be 38.1 cm. The diameter of Iris2 was set to be 7 mm.

As a part of the calibration, the incident power on the target PD was measured using PDA100A (Gain 0dB). The measured value was 3.973 V with no chopper blocking the beam.

An HR mirror was then placed before the target PD to measure the reflected light power with the chopper still halted and the iris 1 is oped using PDA100A (Gain 0dB) which is placed at the position as shown in Fig. 1. It was measured to be 1.253 V. This means that the reflectivity of the BS is about 0.315. This reduction is considered to be the clipping at Iris1 as the returning beam has the bigger beam size (i.e. the iris worked as a sort of mode cleaner).

(==> There might be a misunderstanding because of my not clear explanation. So, I edited this paragraph again. KN)

Then, the chopper was started at 253 Hz. The reflected light from the HR mirror was measured using a FFT analyzer to be 642 mVrms (PDA100A gain 0 dB).
(==> mVrms? KA ==> mVrms. I fixed. KN)

Throughout the measurement, the opening of Iris1 was adjusted such that the clip level was less than 0.1% of the incident power (see also elog2047).This yielded the Iris1 diameter of 1.8 mm.

Finally, the back reflection was measured in the same way as elog 2040 , and also with Iris1 opend as much as possible.

Result:

The measured voltage with Iris1 with the diameter of 1.8 mm: 720 uVrms (PDA100A gain 70 dB)
==> This corresponds to the BRDF of 1.4 x 10^-3 [1/sr].

The measured voltage with Iris1 completely open: 640 uVrms (PDA100A gain 70 dB)
==> This corresponds to the BRDF of 1.2 x 10^-3 [1/sr].

These values are much (x40) larger than the BRDF value expected from the BRDF measurement (5x10^-5 [1/sr], see also elog2045).
In fact, these are much (x2) larger than the ones with the PBS & the QWP (8x10^-4 [1/sr], see also elog 2047).

(It is not so clear how these numbers were calculated. How these calibrations were used? How did you incorporate the BS transmissivity and reflectivity??? KA)

==> For calculating the BRDF, the fomular as shown elog2034 is used. Seeing the fomular, we need the ratio of the incident light power and the scattered (reflected, in this measurement) light power, the incident angle, and the solid angle. The ratio of the incident light power and the scattered light power can be obtained by taking the ratio of the measured voltage with the chopper as a part of the calibration, Vin (642 mVrms), and the measured voltage with Iris1 with the diameter of 1.8 mm (720 uVrms) or with Iris1 completely open (640 uVrms), Vsca, considering the difference of the PDA100A gain. In this calculation the BS reflectivity and transmissivity do not appear, see also Fig. 2. In addition, using the incident angle, 15 deg, and the solid angle from the PD(C30665GH) to the Iris2, 2.6x10^-4 sr, the BRDF can be calculated.

And when the Iris was closed, the value was larger. This means that the Iris may make a scattering.
(==> This didn't make sense compared with the numbers above. KA ==> The numbers were written conversely. I fixed. KN)

[Ed.: KA]

To reduce the scattering, we did some try and error.

The setup was same as the elog2048.

First, to reduce the scattering from the Iris 1, in front of the Iris 1 the aluminum foil which has the hole for the laser was placed.

We made several foils, different hole sizes and different shapes.

We cannot found the reasonable tendency, but with the best foil the measured scatterd value became 350 uVrms (BRDF = 7.1 * 10^(-4) [1/sr]).

This is the comparable value to the BRDF measured with PBS and QWP (elog2047).

After that, we changed the steering mirror #2 from Newfocus 5104 to Newport 10Q20HE.1.

Then, without aluminum foil, i.e. in completely same setup as elog2048, the measured scatterd value became 11 mVrms.

And we tried to place the best aluminum foil and the measured valued as reduced to 820 uVrms.

I set up a measurement of the DET10D QE at 2004nm. I supplied the Eblana 2004nm fiber-coupled laser diode with 45mA of current. I first measured the output with the Thorlabs power meter and then measured output with the DET10D photodiode. Both systems are fiber-coupled (although both fiber couplers are screwed on rather than glued on).

The power was 0.34mW = 3.425E15 photons per second

The photodetector output was run through a 50 Ohm resistor which was then run through a SR560 with 100x gain at DC. The measured voltage from the SR560 was 2.183V. Hence the photocurrent was 4.36E-4A  = 2.7E15 electrons per second

Therefore, the QE was 0.79 at 2004nm. This agrees with the manufacturers curve. However, I notice that there:

• is a window over the photodiode (so we might lose a few percent there). It's not clear from the manual whether this is AR coated or not.
• the voltage from the SR560 fluctuates a bit when I rotate the fiber coupler that is attached to the front (I measured the QE when this was maximized). This tells me that we’re not centered on the photodiode and a fraction of the fiber output isn’t getting onto the active area of the PD.  

Last night, Andrew and I measured the QE of two regular InGaAs photodiodes at 2004nm using the fiber-coupled Eblana laser diode as a source.

The two diodes in question were a Thorlabs PDA10CS (adjustable gain set to +60dB) and a fiber-coupled NewFocus 1811. 

Incident power:

We set the current to 55mA which provided 495 micro Watts of power at 2004nm. Each photon has an energy of 9.9E-20J. This corresponds to 5.0E15 photons per second.

Photodiode response:

Each diode had internal transimpedance gain (R_T) and the output was connected to an SR560. The voltage difference between the diode being OFF and ON was measured. 

Electrons per Amp = 6.25E18

Quantum Efficiency:

• 1811: 2.7E10 / 5E15 = 5.4E-6 electrons per photon
• PDA10CS: 8E10 / 5E15 = 16E-6 electrons per photon

The silver lining here is that the 'extended' InGaAs only has a 3x higher QE than your standard InGaAs.

Are you sure you got all the light on the 1811? Don't you have a larger aperture diode to use than that little thing?

Also, I think the new focus 1811 DC transimpedance is 1 k, not 10 k.

**[Internal Quantum Efficiency added]

[Koji, Shalika]

Further measurements were done after elog:2419 for Quantum Efficiency of Extended InGaAs Photodiodes(X8906). A Laser of wavelength 2um was used with an incident power of 0.80+0.02mW.  The Ophir RM9 power meter was used to check the incident power and also measure the reflectivity.

Attachment 1: The Setup. A Fibre launcher was used to project the laser along with a converging lens of the focal length of 40.0 mm which was further arranged with a subsequent converging lens of  150mm focal length. A mirror was used to reflect the laser light on the photodiode at an angle of 45^o. The bias voltage was provided to pin 4 of photodiode using a Sallen Key low pass filter and the output at pin 3 of the photodiode was fed to a transimpedance amplifier (with a gain of 5.1k) which converted the photocurrent to voltage.

Attachment 2: The Quantum Efficiency is plotted with respect to different bias voltages, It was observed that the quantum efficiency increases with an increase in bias voltage. An External Quantum Efficiency of 77.4% was observed at 1V(maximum bias voltage for the photodiode). The Internal Q.E was observed to be 83.8% taking into account Reflectivity of (60.0+1) uW at an angle of 17deg. 

Attachment 3: To recreate all data

The QE and dark current of all the InAsSb sensors were measured. All the measurements were done in room temperature.
- The incident beam power of the 2004nm beam was 0.95mW.
- The beam was focused down to 50um gaussian radius, which was confirmed by DataRay BeamR.
- The angle of incidence was ~0deg.

- The element side (nominally Pin 2, 3, or 6) were connected to the vias boltage (negative) and the common ground was connected to the transimpedance amplifier (Shalika OP140 R=5100Ohm)
- The dark current was highly dependent on the reverse bias voltage. The QE was also bias dependent.
- Sb3512 A2 have different behavior compared to others. Alex mentioned that Sb3512 is the test batch. We can exclude this sensor from the test.
- The best QE was ~0.7 for Sb3513 A3 P2 (Pink) and Sb3513 A2 P6 (Purple). Both have the area of 500um^2. These two particular elements have low dark current of <1mA. The dark noise of this specific sensor should be measured.

Some issues of the measurements

- The transimpedance amp (TIA) has suspicious behavior. The saturation voltage was ~17V rather than <-15V. This indicates that the voltage regulators possibly have leakage of the input voltage (+/-18V) to the output line. This needs to be checked, particularly before the dark noise test.

- TIA saturation: The bias voltages could not be raised to ~1V for some PDs because of the dark noise and the saturation of the TIA. The transimpedance should be lowered by a factor of ~5.
- Because of the low bias voltages of these saturated cases, the max QEs were not reached. This also prevented from checking if there was any clipping loss. This should be checked again with the lower transimpedance.

- TBD: The angular dependence and the reflectivity of the sensor should be checked. It is difficult to carry out these tests without a sensor card.

The lenses were arranged so that the spot on the PD can become smaller. A quick measurement on a (500um)^2 element showed the QE of ~80%

With the strong focusing lens of f=40mm, the beam was once expanded to a few mm. Then f=75mm lens focuses the beam to ~30um (radius). (See Attachments 1&2)

With this new beam, the QE was quickly checked. The new measurement is indicated as "Sb3513 A2P6new" in the plot. It showed the QE of ~80%.
The AOI was scanned to find any maximum, but the AOI of 0deg was the best at least with the given beam. I'm not sure yet why 500umx500um requires such small beam radius like 30um. Awesome

InAsSb PD QE Test

The relationship between the spot radius and the apparent QE (EQE) was measured.

1) The spot size was checked with DataRay Beam'R2. The beam scanner was mounted on the post with a micrometer stage in the longitudinal direction. (Attachment1 upper plot)
It was confirmed that the beam is focused down to ~22um. The incident power was about 0.9mW.

2) The InAsSb detector (Sb3513A2) was mounted on the PD holder and then mounted on the stage+post. The photocurrent was amplified by a FEMTO's transimpedance amp (V/A=1e3Ohm). The dark current and the total photocurrent were measured at each measurement point with the beam aligned to the PD every time. The estimated EQEs were plotted in the lower plot of the attachment.

Note that P2, P3, and P6 elements have the size of (500um)^2, (750um)^2, and (1000um)^2, respectively.

The absolute longitudinal position of the sensor was of course slightly different from the position of the beam scanner. So the horizontal axis of the plots was arbitrary adjuted based on the symmetry.

The remarkable feature is that the QE goes down with small spot size. This is suggesting a nonlinear loss mechanism such as recombination loss when the carrier density is high.

With the present incident power, the beam size of 100um is optimal for all the element sizes. For the larger elements, a bigger beam size seems still fine.

The next step is to estimate the clipping loss and the saturation threshold with the Gaussian beam model.

Clipping and saturation were investigated by the semi-analytical model. In the analysis, the waist radius of 20um at the micrometer position of 8mm is used.

1) Clipping

Firstly, the clipping loss was just geometrically calculated. Here the saturation issue was completely ignored.　The elements P6, P3, and P2 have the sizes of (500um)^2, (750um)^2m, and (1000um)^2, respectively. However, these numbers could not explain the clipping loss observed at the large spot sizes. Instead, empirically the effective sizes of (350um)^2, (610um)^2, and (860um)^2 were given to match the measurement and the calculation. This is equivalent to have 70um of an insensitive band at each edge of an element (Attachment 1). These effective element sizes are used for the calculation throughout this elog entry.

2) Saturation modeling

To incorporate the saturation effect, set a threshold power density. i.e. When the power density exceeds the threshold, the power density is truncated to this threshold. (Hard saturation)

Resulting loss was estimated using numerical integration using Mathematica. When the threshold power density was set to be 0.85W/mm^2, the drop of QE was approximately matched at the waist (Attachment 2). However, this did not explain the observed much-earlier saturation at the lower density. This suggests that the saturation is not such hard.

In order to estimate the threshold power density, look at the beam size where the first saturation starts. The earlier sagging of the QE was represented by the threshold density of 0.1W/mm^2. (Attachment 3)

The QE of the (500um)^2 element has been tested with a half-power (0.51mW) instead of 0.92mW.
It is clear that the central dip depth is reduced by the lower power density.

The PD mounts were delivered from ProtoLabs. The order was sent on Tue last week and it's here on Monday. Excellent!
And the quality looks pretty good.

The surfaces are sandblasted. Do we want to do any process on the bottom surface to reduce the thermal resistance?

An indium solder string also came in.

System diagram of the PD QE test with the IRLabs cryostat.

PT-SE (MS/PT-SE) connector data sheets

Connector/receptacles/tools https://www.peigenesis.com/images/content/pei_tabs/amphenol/pt-ptse-series/new-thumbs/123-146_pt_series.pdf
Amphenol catalog http://www.amphenol-industrial.com/images/catalogs/PT.pdf

Detoronics Hermeic Sealed Connectors (DT02H-18-*PN) http://www.hselectronics.com/pdf/Detoronics-Hermetic-Connectors.pdf

AF8 crimping tool (expensive!) https://www.mouser.com/ProductDetail/DMC-Tools/AF8?qs=gvhpkjpQEVSjrLbsepewjg%3D%3D
AF8 alternative https://www.jrdtools.com/?gclid=Cj0KCQiA2vjuBRCqARIsAJL5a-IQ9ztCEYKdo645v_RhUBJS3eMIars1LubjlKZoorS-lnx6ClDDiMUaAlZiEALw_wcB

Thermistor link: https://www.tec-microsystems.com//Download/Docs/Thermistors/TB04-222%205%25%20Thermistor_Specification_upd2018.pdf

TEC spec: Mounted TEC type: 2MD04-022-08/1 https://www.tec-microsystems.com/products/thermoelectric-coolers/2md04-series-thermoelectric-coolers.html
2MD04-022-08/1 dTmax = 96, Qmax = 0.4W, Imax = 0.7A,  Umax = 2.0, ACR = 2.29 Ohm