https://goo.gl/photos/FaqFC7tuxz76m2236

OMC_Lab/252

I beieve that they are Exelitas C30642.
FYI: There also is one on the former-gyro optical table. This one doesn't have the cap.

Measure the profile of the back propagation beam by injecting a beam from the other side.
This gives you how the input mode should be.

Jenne's laser at the 40m PD testing table is a fiber coupled 1064nm DL.

But you just can couple 5~10% of the beam from the other side of the fiber to know the mode at the input side.
It does not require too much effort if you have the fiber testing illuminator to align the beam.

Correction: If the diode is 3mm x 3mm, it is Excelitas C30665.

I thought the ants we usually have around us are Argentine Ants. I agree that they are not primarily attracted to sugar but like protains(raw meet etc).

Steve found "Terro" and it worked pretty efficiently at the 40m. I bought the same at home and it worked brilliant. This attracts ants and prevent them to spread randomly. So it should be placed at their entrance points. Otherwise, you just increase the number of ants in your lab. But in a few days, a colony will become completely silent.

Someone (Aidan?) bought Terro for the subbasement labs a while ago, if I'm correct.

This means that you want to make the SHG crystal longer. Is that true? If so, can you change the temperature for the optimal phase matching by tuning the 1064 crystal temperrature? I suspect you need to cool the YAG crystal, but I am not sure what is the thero-optic constant of the SHG crystal, and how much you can gain from this.

Great recovery job!

On the Friday cleaning, we vacated the east optical table. The Si scatterometer was disassembled and the Si block was moved and stored to the cryo lab.

[Aidan, Chris, Koji]

We went down to the lab to check the situation of the setups for 2um laser measurement and stabilization and the new cryostat.

[2um laser frequency noise measurement]

• Looked at the add-on transimpedance amps: Something was wrong with them. The power bypass caps are attached to the "hot" supply lines in parallel (both sides of the caps are soldered to a same line). And some power supply lines have no voltage. This circuit is not necessary to be bipolar. To be fixed (KA)
• We temporarily connected one of the thorlabs 2um InGaAs biased detectors to an SR560. It showed reasonable output: DC/AC response OK & no nonsense.
• The AOM was bypassed and the homodyne fringe was checked.
  The fringe visibility was low (~10%) and was dependent on the stress applied to the delayline fiber.
  Suspected polarization rotation somewhere -> ToDo: Check the polarization states of the output beams.
• ToDo: Check should be done with each component. how much are the output power, output polarization, dependence/fluctuation of the polarization, etc. 
  We might be able to use the 2um Faraday Isolators (as PBSs) for the measuement.
• Checked the fringing of the fiber delayline Mach Zehnder. We observed one fringe per sec level fluctuation.
• Laser current actuation was checked and it turned out that it is so strong and sufficient to lock the delayline fringe.

[2um AOM]

• The fiber coupled AOM gave us a reasonable amount of DC/AC actuation of the laser intensity.
• The power of the 1st order output has the dependence on the "freq input" of the driver. This is probably because of the matching between the fiber coupling and the deflection angle, which is freq dependent.
• When the freq input is 8.8V_DC, the 1st order output has the maximum efficiency. The efficiency was 96%@990mV_DC input to the modulation in.
• The AOM actuation bandwidth was tested to be ~MHz, at least.
• We are not supposed to give more than 1V to the modulation in while we want to apply 8.8V to the freq input. Incorrect plugging may cause the damage of the modulation input port. The setup needs to be improved with a protection circuits / AOM driver circuit.
• Our understanding is that the modulation input has a 50Ohm input impedance while the freq input has high-Z
• The next step towards the intensity stabilization is low noise photodetector circuits and proper interface to the AOM driver.
• Also we want to set up TECs and other circuits for the LaserComponents PDs.

[Cryostat]

• Cleaning: there are many components are scattered on the table.
• Plan:
  • Move the Zack rack to the next of the optical table (or somewhere)
  • Move the yellow chemical cabinet to the place where the rack was. We can pile up some plastic boxes on it.
  • Remove the delicate optics from the steel table.
  • Place heavy cryostat components on the steel table.
  • Connect the cryo cooler to the cryostat. How do we do that? Fisrt rigidly attach for testing and then move to soft attaching?
  • Replace the optical windows to the 2um ones (2"). The current ones are for 1.5um.
  • We need a 2" 50/50 BS at 2um. Lenses and steering mirrors are in hand.

The transimpedance amps for the 2um (unamped) InGaAs detectors were made and evaluated.

Attachment 1: The circuit diagram

The usual transimpedance configuration. The detector (Thorlabs DET-10D) is an extended InGaAs which is sensitive up to 2.2um. I believe the detector is biased to 1.8V although it is not obvious and the 12V battery is used. The feedback resistor was chosen to be 5kOhm so that the circuit can handle up to ~2mA (~1.7mW). The feedback capacitance pf 100pF for compensation was chosen kind of arbitrary to keep the circuit stable and also the RC cut off to be more than 100kHz. The output resistance is 100Ohm. The selection of the opamp is described below.

Attachment 2: The amplifier noise Part I

The amplifier noise (the first unit called Amp #2)  was evaluated with the opamp swapped with OP27 (BJT), LT1128 (BJT), OPA604 (FET), and LT1792 (FET), chosen from the 40m stock. For the given environment, the FET amps exhibited better performance while the BJT amps suffered from more line noise coupling and the larger 1/f noise. Particularly, LT1792 reached at the level of ~2pA/rtHz, with lower line noises. This looks the best among them. Note that the 5kOhm feedback resistor gives 1.8pA/rtHz current noise. 

Attachment 3: The amplifier noise Part II

Then the second unit (called Amp #1) was made. This unit has more high-frequency noise. It turned out that the noise was coming from the power supply which was the +/-12V from the rear panel of an SR560 which was connected to the AC power. The noise dramatically went away with the battery mode operation of SR560 (by disconnecting the AC power). The floor level was 2.2pA/rtHz and it was slightly higher than the quadratic sum of Johnson noise of 5kOhm and the voltage noise of the amp (4nV/rtHz). This noise level was just sufficient for the purpose of the 2um detector.

Attachment 4: The detector noise levels

Now the detector #1 and #2 were paired with the amp #1 and #2, respectively. In fact the detector 1/f noise was way too large compared to the amplifier noise. There is no hope to detect shot noise level of the mA photocurrent. 

Attachment 5: The detector response

The detector response of each PD+AMP pair was measured using Jenne's laser and Thorlabs PD10A (~150MHz). There was some systematic error of the absolute level calibration, therefore the transfer functions were adjusted so that they have 5kOhm transimpedance at ~1kHz. The phase delay is ~30deg at 100kHz. This partially comes from the combination of 100pF//5kOhm and the ~4MHz bandwidth gain of the opamp. If we want faster response we need to modify these.

The amplifier sets for the thorlabs 2um PDs were delivered to the lab.

- PD1 and Amp1, PD2 and Amp2 are the proper combination. If a high quality power supply is used, it is not an issue.
- The cables for the external bench supply or the 9V batteries have been made.

We received the TO-66 sockets for LaserComponents PDs (Andon Electronics F425-1009-01-295V-R27-L14 Qty.10). It is made of FRP. It is very nicely made.

Great! Can you convert this into the laser frequency noise Hz/rtHz? I believe this [rad/rtHz] was still the measured phase noise and was neither the laser phase noise nor frequency noise yet.

Questions:

1) Has the DC output voltages of the regulators checked?

2) What's the target voltages of the regulator circuits? And how the voltages were supplied from the power supply port of the SR560? 7815 is the regulator meant for +15V and 7915 is for -15V. So the input voltages need to have at least 3V larger voltages than the target voltages (like +18V for 7815, -18V for 7915). If the +/-12V are naitvely applied, the regulators don't reach the operating point.
Check "Voltage Drop" descriptions in the data sheets of the regulator chips.

3) What's the purpose of these diodes? I believe they are for the regulator protection against the transient sign flip during power switching etc as well as over voltageprotection. The circuit of the 7915 has the larger potential difference (like -18V) while the output has -15V. This means the diode will always be on. If this is just a typo in the figure, it's not a big deal. If this is the real situation, it is a big problem.

4) Why were there such huge 60Hz lines? Was the SR560 properly operated with its battery?

3. You need to flip the direction of the diode.

1&2 OK, so the circuits were not fucntioning. Use a dual voltage supply (in a proper cascading setting) and give +/-18V.

4. When you use SR560 as a power supply, you need to disconnect the AC power supply. Otherwise, the AC power, which charges the +/-12V lead battery, contaminates the output voltage with the 60Hz lines.

1. Heat: Check the polarity of the electrolytic or tantalum caps. 

2. Add 0.1uF high-K ceramic caps in pararel to these electrolytic or tantalum caps.

3. Why does LM317 have only one volt drop? It requires minimum 3V mergin between the input and output voltages. (See the datasheet) 

InAsSb PDs were housed in the PD cages. The cages were engraved to indicate the batch (Sb3512 or 3513) and the serials (A1, A2, ...).

The PD legs does not have an indicator for the pin1. So, the tab of the PD case is directed "UP". Also the direction of the tab is marked on the cage. The tab of the short plug was also aligned to Pin1. However, the PD case is too thin and the PDs can rotate in the cases.
So the face photo was also taken so that it indicates how Pin 1 looks like from the PD face. (Attachment 4)

Also made the cable for the LaserComponents PD and the InAsSb PD. Pin n shows up as Pin n of DB9 Male connector.

Once we have the PD test is the bias circuit (with a monitor) and some patch panel kind of preparation, we can start working on the PD test.

DB9 switchable breakout box is ready. We are ready to do some PD test now.

The TF looks good. But the noise measurement is obviously limited by the SR785 noise. We need a preamp, which is only for the purpose of the measurement. It has to have the input reffered noise about a factor of a few better than the noise predicted by Zero. At high frequency, probably we will be able to use SR560. With this low noise level, probably we can just use the flat gain of 100 for the SR560 setting. This will give you the input referred noise (of the preamp) of  ~4nV/rtHz at kHz band. Note that the gain needs to be larger than 100 to have low noiseness of SR560.

You need to check the voltage noise of the regulator outputs with the opamps connected. Probably you did it. If so, it is a riddle why the 8kHz bump is not observed in the regulator outputs, but is in the opamp outputs...

Does the noise bump happen with the +/-15V supplied by 7815/7915? How about to change the capacitor values for LM317/337 to the ones recommended in the data sheet?

It is great to see the noise peaks were largely reduced by LT1792. This is what I found before although I can't explain why.

- Previously, your TIA was pretty much dominated by the thermal noise current of the 5K transimpedance resistor (=0.129/sqrt(5000) nA/rtHz ~2pA/rtHz).
So, I believe it's impossible to measure 1pA/rtHz. Please check if you had any saturation or anything along the chain.

- Do you need SR560? If you think you are limited by the input noise of SR785 when having no SR560, you can use your whitening filter, which is supposed to be sufficient and better in terms of the output voltage range.

- Please note the serial number of the PD under the test.

- And, try to isolate your box from the optical table.

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.

Fiber Collimator (Thorlabs F028APC-2000+AD11F+LMR1) and MIR sensor cards (Thorlabs VRC6S Qty2) were delivered.

The sensor card is liquid crystal and seems temperature sensitive. It's slow and diffused. But at least we can now see 2um beams in a certain condition.

The fiber collimator seems working fine, but this gave me another issue. Now because the beam is small (w<500um) everywhere, I can't focus it very well. To make a focused beam, one needs a large beam, of course. Previously, the beam was not well focused. Therefore the final focused beam with f=150mm was sufficiently small like w=50um.

It looks like some kind of  telescope is necessary.

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

From Cryo Cav setup

Borrowed ITC510 Laser Driver/TEC controller combo -> QIL

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.

Ah, I have designed the PD holder with the venting targeted for 1/4-20 holes with 1" grid...

The dog clamps to hold the PD units also need to be compatible with 4-40 screws.

How big the hole diameter should be? Can you find a suitable drill at the 40m?

The large cryostat has 1/4-20 holes on a 2 inch grid

The IR labs cryostat has 4-40 holes on a 1 cm grid

I'll check the 40m for a bit (5/8-3/4")

Here you are.

Salvaged an assort of vented/non-vented screws, washers, spring washers, and clamps for #4-40 & 1/4-20 from the 40m cleanroom stock. They are clean enough for the cryostat use.

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

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

Normal solder (Sn63 Pb37): with flux, wetting o

Pure Indium - In 99.995: no flux, wetting x, low melting temp, like paste

Pb93.5 Sn5 Ag1.5: with flux, wetting o, high melting temp (soldering iron setting 380~430F)

Cryo solder In97 Ag3: no flux, wetting x, low melting temp, like paste

The external Dsub cable is ready except for the 32pin connector to be plugged-in to the chamber. See QIL ELOG 2460 for the pin assignment.

While I'm still waiting for the proper connector for the vacuum feedthru of the IRLabs cryostat, I have connected to the Dsub9/15 split cable to another Dsub9 connector so that I can test the cooling of the InAsSb sensor in air. Also, the 2004nm laser, a fiber-coupled faraday isolator, and 90:10 beam splitter was moved to the cryostat table and fixed on a black al breadboard. [Attachment 1]

The InAsSb TEC was controlled by the TEC controller of ITC-50. I didn't change the PID parameters of the controller but the temperature nicely setteled to the setpoint. The sensor has a 2.2kOhm thermister. And the max current for the TEC was unknown. The TEC driver had the current limiter of 0.3A and it was not changed for now. With this current limit, the thermistor resistance of 10Kohm was realized. This corresponds to the temperature of about -20degC. According to the data sheet given by Alex, the resistance/temperature conversion is given by the formula

1/T = 7.755e-4 + 3.425e-4*log(R)+1.611e-13*log(R)^3

Questions:

• What is the max current for the TEC?
• What is the calibration formula of the thermister (TB04-222) at cryogenic temperature?
  -> Thermistor datasheet link (pdf)

To satisfy the curiosity, the dark current of a (500um)^2 element was measured between -250K and -300K. At -254K, the dark current went down to the level of 40uA (1/15 of the one at the room temp). For the measurement, the bias voltage was set to be 0.5 and 0.6V. However, it was dependent on the diode current. (Probably the bias circuit has the output impedance). This should be replaced by something else.

To Do

• 2um laser beam setup (w=100um beam)
• Bias circuit
• Quick check of the QE and dark noise at -20degC

[Raymond and Koji]

We dunked the PD socket test piece into LN2 and repeated heat cycle 8 times. No obvious change was observed. Then the wires were pulled to find any broken joint or etc.
None of the solder joints showed the sign of failure.

For cleanliness, we are going to use In-Ag solder (no flux) for the actual wiring.

Attachment 1: Frozen connector

Attachment 2-4: Inspection after thawing.

The quantities we want to measure as a function of the temperature:

- Temperature: 2.2k thermister resistance / 100ohm platinum RTD

- QE (Illuminating output / Dark output / Reference voltage / Reference dark output)

- Dark current (vs V_bias) -> Manual measurement or use a source meter

- Dark noise (PSD) 100kHz, 12.8k, 1.6kHz, 100Hz

I borrowed KEITHLEY 2450 source meter from Rich. The unit comes with special coaxial cables and banana clips. Most of the peripherals are evacuated in the OMC lab.

The dark current of A2P2, A2P3, A2P6 were measure with different temperatures (300K, 270K, 254K). The plot combined with the previous measurement ELOG QIL 2425.

== How to use the source meter ==

- Two-wire mode: Connect the wires to the diode

- Over voltage protection: [MENU] button -> SOURCE / SETTINGS->Over Voltage Protectiuon 2V

- Sweep setting: [MENU] button -> SOURCE / SWEEP -> e.g. Start -750mV, Stop +500mV, Step 10mV, Source Limit 1mA -> Select Generate

- Graph View: [MENU] button -> VIEWS / GRAPH

- Start measurement: Note: The response of [TRIGGER] button is not good. You need to push hard
  This starts the sweep, or a menu shows up if your push is too long -> Select "Initiate ..."

- Data Saving: [MENU] button -> MEASURE / READING BUFFERS -> Save to a USB stick

I can see some screws are not vented. You also need to use a vented screw for the additional temp sensor if the face screws of the PD mounts are not vented.

You can use a bunch of clean clamps and screws I brought. They are in a mylar bag.
If you need more vented screws, please specify the size and length. I can grab some from the 40m cleanroom.

Wow. This is great, thanks Chris.

[Raymond, Aidan, Chris, Koji]

P6 element (500um)^2

- We looked at the current amp (FEMTO) output. The amplifier saturated at the gain of 10^3 V/A. Looking at the output with a scope, we found that there is a huge 1.2MHz oscillation. Initially, we thought it is the amplifier oscillation. However, this oscillation is independent of the amplifier bandwidth when we tried the our-own made transimpedance amp.

- Shorting the cryostat chamber to the optical table made the 1.2MHz significantly reduced. Also, connecting the shield of the TEC/Laser controller made the oscillation almost invisible. This improvement allowed us to increase the amp-gain up to 10^7.

- Then the dominant RMS was 60Hz line. This was reduced by more grounding of the cable shields. The output was still dominated by the 60Hz line, but the gain could be increased to 10^8. This was sufficient for us to proceed to the careful measurements.

----

- The dark current was measured by the source meter, while the photocurrent (together with the dark current) was measured under the illumination of the ~1mW light on the PD.

- Attachment 1 shows the dependence of the dark current against the swept bias voltage. We had ~mA dark current at the room temp. So, this is ~10^5 improvement.

- Attachment 2 shows the dependence of the apparent QE against the swept bias voltage. The dark current was subtracted from the total current, to estimate the contribution of the photocurrent in the measurement.

- Attachment 3 shows the dark noise measurement at the reverse bias of ~0.6V. Up to 1kHz, the noise level was below the equivalent shotnoise level of 1mA photocurrent.

---------

All the data and python notebook in the attached zip file.

== Currently, A2P6 is aligned ==

1) I've brought another TEC driver fro the PD temp control. This unit was borrowed from the 2um ECDL setup. Eventually, we need to return this to ECDL. (Attachment 1)
The PID loop of the TEC control works. But it is not well optimized yet. If you change the target temp too quickly, the TEC out seemed oscillating. Watch the TEC out carefully and change the temp setpoint slowly.
So far I have tried to cool the thermister up to 30kOhm (~232K) and I_TEC was 0.33A. I did not try further. I felt it was better to cool the PD base for further trial.

2) A part of the alignment study, the beam is aligned to A2P6. Also, the lens position was investigated, and I decided to move the lens ~1 inch away from the window.  (Attachment 2)
    In fact, this allowed us to insert the power meter between the lens and the window. 

The QEs were measured at 293K, 239K, 232K, and 293K again. The cooling was provided by the PD TEC.  At each temperature, the incident power was changed from 30uW to 1mW to see the dependence of the QE on the incident power to check the possible saturation.

The QE was 79~81% (the window T=96.6% was already compensated). I'm not 100% sure this 1% variation in the plateau is real or due to insufficient calibration of the REF PD.
The REF PD was calibrated at 1mW at 100mA injection current to the laser.

No obvious saturation was observed.

We can cool the PD with LN2 and we should make a careful alignment of the beam at each temperature.

Item lending as per Ian's request: Particle Counter from OMC Lab to QIL

The current particle class of the room was measured to be 800.

The particle counter went back to the OMC lab on Aug 10, 2020.