I don't know why it would be easier to have higher finesse with longer arms. Something about beam size???

The NPRO PZT TF's are all in the 40m elog - there are many measurements of TF made over the past 10 years. Its like Raiders of the Lost Ark - you have to believe its there while searching.

Following up on the last post, here I presented a near back of the envelope calculation of how different choices of AUX cavity finesse and laser source for mariner would affect the prospects of calibration scheme.

Laser sources considered:

As mentioned in the last elog post, here I considered using an NPRO seeded auxiliary laser source (converted to 1418nm by whatever method), ECDL based on ANU design with a modified PDH loop and same ECDl with a digital compensation of PZT resonances. I have taken the residual frequnecy noise of these lasers as the dominant noise source in the calibration scheme. Craig and Gautam in their proposal for SoCal wanted the AUX laser to be locked to the arm cavity in a PDH shot noise limited way. That would be necessary for 4km interferometers and would be easier to achieve there with higher laser powers and higher cavity finesse.

Finesse of 40m Arm Cavity for 1418nm:

Here I considered three cases. First assumes about 3% transmittance of 1418nm in ITM and ETM HR coatings for mariner. This gives a finesse of about 100 and a cavity pole of 18.9 kHz. I believe this is the existing case at 40m. Next we consider transmittance of 0.5% and 0.05% (500 ppm) of 1418nm in ITM and ETM HR coatings for mairner. These cases give finesse of 625 and 6.28k respectively with cavity poles at 3 kHz and 299 Hz respectively.

Page 1: Consideres the case of finesse of 100. The green dashed line shows the amount of drive strength (in m) required at different frequencies if we use ECDL with PZT resonance compensation, to get an SNR of 1000 in 100s of integration time.

Page 2: Same as above but for Finesse of 625.

Page 3: Same as bove but for Finesse of 6280.

Page 4: Comparison of different finesse cases for the ECDL with PZT compensation option. Dashed curves represent requried drive strength (in m) for different cases.

Page 5: Same as above but for NPRO seeded auxiliary laser.

Note: For the NPRO seeded auxiliary laser, we have assumed that the noise of conversion to 1418 nm is similar to noise due to SHG process which is not dominant. There would be an effect of multiplying with a factor ranging form 1-1.5 due to frequency conversion but I have ignored it here for simiplicity. Also, NPRO case is limited in bandwidth due to PZT resonances. We might be able to get away with them using digital compensation like the case study for ECDL. But I haven't attempted that here as we do not know our NPRO PZT's resonance features yet.

We can use Thorlabs SAF1450S2 gain chip to generate 1418 nm light using an ECDL design similar to the one described in Kapasi et al. Optics Express Vol. 28, Issue 3, pp. 3280-3288 (2020) (ANU 2um ECDL design).

PZT Transfer function

I have contacted Disha and Johannes to get the actual measured data for the PZT transfer function of this ECDL design. Fig.5b in their paper plots the transfer function of the PZT. Since, in ECDL PZT directly changes the cavity length, it has a more powerful actuation strength (2 orders of magnitude more) with actuation of 560 MHz.V upto 1 kHz. It however had a very low pole at 1 kHz and two mechanical resonance-antiresonance pairs near 1 kHz and 2 kHz. I modeled a transfer function by eye using Fig.5b of the paper. Page 1 in the attached pdf shows this modelled transfer function.

AUX PDH Loop

Next, we need to change the PDH loop for the auxiliary laser lock with the 40m cavity since the PZT has changed. I modelled one from scratch. This simple analog loop's performance is shown in orange in pages 2-5. This loop seemed stable from all the metrics I know, viz: phase margin of about 55 degrees (Page 2), no strong peak in close loop transfer function (page 3), and no remanant oscillations in time domain response (page 4).

I also modeled a similar loop but with digital compensation of the resonance-antiresonance features. This loop is plotted in green on pages 2-5. Both these loops have 300 kHz of bandwidth just by using PZT. I beleive this could be increased but I have not taken into account any saturation of PZT.

ECDL Frequency Noise

From Fig.4. of the paper gives a frequency noise estimate for free running ECDL. They mentioned that a roll-off below 10 Hz was due to their thermal feedback to remain in linear range of their frequency noise emasruement method. I modeled the noise of ECDL hence by

where the flicker noise contribution is similar to NPRO noise but ECDL has a white noise of 15 Hz/rtHz due to natural linewidth of spontaneous emission or Schawlow-Townes linewidth (with several broadening factors). I think this is an inherent limitation of ECDLs.

Page 5 shows both unsuppressed and suppressed frequency noise estimate for ECDL with the loops mentioned above and current values of NPRO noise are also plotted for comparison.

Code

Came into lab today, with two main goals:

1) Bring Downs equipment for vibration measurements of cryo cooler during next operation

--> DONE, see photo

2) Assemble shields, with two layer scheme using electropolished and mill finish panels.

--> issue: the electropolished shield set was only partially shipped out in Ticket 15101 - unfortunately there was a misinterpretation by Logistics personnel regarding which parts to ship, due to the formatting of the ticket page. I will coordinate shipment of the balance of the panels to the vendor Able Electropolishing for completion of PO S479514

I entered QIL just before Wed Sep 23 00:27:51 2020 to check out and photograph the sprinklers, spent about 20 min looking around the lab and drawing inspiration for in cryo. Wore shoe covers and gloves, touched nothing, sanitized doors after.

HEPA filters on top of the WOPO table have been turned to High (earlier were at Low).

2020 Sep 01, StephenA with remote assistance from RaymondR

Highlights

• Silicon Mass - Rana had dropped off the Silicon mass in the first room, so I found it when I arrived - thanks!
• Organization - It was my first time accessing the QIL lab, but everything was pretty well organized and easy to find. All tools for modifications to parts were used in the EE lab which was also well organized. Raymond helped me to figure out where to access things on a few occaisions.
• Packages - received from Downs Logistics room the Electropolished shield set, a Grainger order with deburring tool and step drill bit, and a McMaster order with a range of bolts - these have all been transported to the lab. Also transported the QIL machined parts that I had received from Machining Solutions to the lab.
  • Koji's Photodiode holders are in the QIL lab ready for pickup.
• Summary of progress
  • Assembled frame (using torque values from T1100066- #8-32 used 20 in*lb)
  • Assembled brackets to frame
  • Hung silicon mass from music wire
    • Wire is captured under #4-40 SHCS with washers ((using torque values from T1100066- #4-40 used 5 in*lb)
• Outstanding tasks and questions
  • Did the hang hold?
  • Do we want to have the layered Electropolished and Plain shields during the first installation? Or some sanded state?
  • Assembly requires oversized #8-32 washers which I wasn't able to track down from inventory - these are now on order, along with some more supplies for roughening the surface.

Full Details

• Assembly work
  • Refer to the DCC - T2000538 - for the videos capturing this assembly effort. I've snagged some screenshots which I've dropped into the attachments.
    • There is a tree catching procedures and other experiment documentation for the QIL Setup at T2000539
• Issues - there were four issues with the fabricated parts, three of which required small modifications;
  • D2000299-01 small angle rails had threaded holes where there should have been clearance holes for the interface (no issue on big angle rails)
    • modified by drill press to drill out clearance holes at same location
  • D2000308 interface cubes all were threaded only partially through.
    • No action taken, just paid attention and made sure the threads I needed were adequate. Seemed like an offset of only a turn or two, suggesting the CAM program was just a little off (this can happen with tapping, the tap is tapered and the machinist needs to thread deep enough to have the thread major diameter realized through the hole.)
  • D2000307-04 frame upper spacer had threaded holes that were not tapped all the way through.
    • I ran a tap through all of these threads.
  • D2000299-02 large angle rails had threaded holes that didn't pass all of the way through, and we happened to be inserting screws into the wrong side.
    • I ran a screw through these threads, which required a little bit more force than I would have liked, and forcing the screw provided an adequate thread.
      • Note that I anticipate that there will be an issue similar to this, with similar resolution, on the D2000299-01 small angle rails. The shield panels installed on the sides are to be installed from the inside. This can be resolved with a screw coming in from the outside.
  • There was also some inability to access certain screws with the long torque driver, especially if loosening/tightening after putting the frame together.
    • This was managed by use of an L allen key, which of course meant those joints were not torqued to spec. I'm not worried about this compromise.

Cryopump is up and running. Initial attempts to run the cooler were stymied by an open circuit in the cold head to compressor connection caused by one of the two accessory port fuses (right, circled in attachment 1). The compressor would run but the valve motor wouldn't start in the cooler itself. I extended the spring in the fuse housing (attachment 2) and it seems to have fixed the problem, as now the valve motor starts at the same time the compressor is turned on. Attachment 1 also shows the highly technical cord management procedure done to reduce the trip hazard caused by the compressor power plug. 

User manual recommendations*:

• Equalization pressure (when not operating): 270-275psi
  • Currently sits at 270psi
• ​Operating pressure: 290-330psi
  • Operating at 300psi
• ​Insulating vacuum pressure: 1 x 10^-3 Torr
  • Vacuum pressure stabilized at 8.5 x 10^-1 Torr
    • ​Edwards diaphragm pump listed ultimate pressure is 1.5 mbar ≈ 1.1 Torr, so either the multirange gauge is malfunctioning or we're getting better backing pressure than expected from the diaphragm pump. A Pirani gauge will be attached to the vacuum space going forward so we'll see how it compares; either way we're above the recommended insulation pressure. The 11" nipple surrounding the coldhead does become cool to the touch during operation, but it does not get cold enough to create condensation.

*Manuals for both the compressor and the cryocooler are linked on the West Bridge wiki manuals page

The diaphragm pump was turned on earlier this week after finally closing up this external adapter tank. Out of an overabundance of caution the tank and cryocooler are supported by the skycrane and a number of posts to prevent it walking off the foam resting pad once the cryocooler is switched on.

All temperature sensors agree with each other within 0.1 K at room temperature 

Used the 19-pin MIL feedthrough to run 4 platinum RTD's and a 25 Ω 100 W resistive heater to the cold head. Attachment 1 is the wiring diagram for the feedthrough and the D-sub connector to the CTC-100 temperature sensor. Attachment 2 shows the three RTDs placed on the cold head. It also shows the thermal anchoring of all lead wires. Attachment 3 shows the RTD attached to the cooler below the cold head using cigarette paper and cryo varnish (stored in the flammables cabinet in QIL).

 The Al block is a premade PT-RTD integrated mounting setup, which was placed on some indium sheet bits and clamped down with a screw and belleville washer. The other two cold head sensors are pressure fit to the cold head by a spring loaded mini dog clamps, and one of the two has some indium underneath the RTD to see if there is any value in doing so going forward with these mounting springs. The glued sensor was attached by painting a thin layer of cryo varnish on the cooler, adding a strip of cig paper, layer of varnish, press in sensor, another strip of paper, paint over all of it with a last thin layer of varnish that reaches beyond the bounds of the paper strips. 

Picked up the prototype shield panels from Hamilton Metalcraft 7/22 and brought them to QIL. All of the parts are wrapped by part number and in a bin (see attached photo). There are 6 sets of shield panels, but 2 full sets were removed for coating vendors. One full set is as follows (20 parts total):

All component #'s are preceded by 'D2000298-'. 031, 032, and 033 are 03 panels but with hole variations, same goes for 131, 132, and 133 with respect to panel 13  

Alex dropped off the new round of 2um PD's, they're on the north table accompanied by his data sheet.  

First day back (7/15) found the particle board trim w/ powerstrip on the QIL workbench had collapsed. Re-glued and added 4 screws to the middle board where vertical boards from the shelves extended low enough. See attached photos for before, during, and after looks. 

Copper parts picked up July 23rd and brought to QIL, now only waiting on PO# S477874 and the pirani gauge from Koji's bulk JPL order

Torque driver set for QIL setup bolted joints, with range 15 in*oz - 50 in*lb, p/n WIHA 5HYL9, is on order from Grainger, with anticipated delivery in the week of July 20th. Refer to  PO S477925. *update* Tracking Number UPS 1Z19W9330321365493

Cryo connection copper parts PO S475316 will be finished early next week by the machine shop in Torrance, I'll bring them to campus or to Raymond's place (TBD).

I've updated the PSOMA optical layout. I still have some questions on locking, and there are a few additional configurations that we could try. In particular:

1. Amplifier cavity as a 2-port device (signal and pump mixed before reaching an overcoupled amplifier cavity)
2. Amplifier cavity as a 4-port device (signal and pump enter the cavity through different ports)
   1. Overcoupled case -- 'signal out' at the same mirror as 'signal in'
   2. Critically coupled case -- 'signal out' through the 'pump in' port

Each of these configurations also has a couple different ways to pickoff an LO for homodyne readout. Shruti and I enumerated these configurations on a zoom whiteboard a couple weeks ago, and I've attached them (the zip contains png).

Chris also mentioned last week that we may run into a frequency-dependent loss in the critically coupled cavity configurations. The pdf I've attached shows a configuration that I think is a minimal modification of the Mach-Zehnder amplifier described in PSOMA. One of the ring cavities is replaced with a tunable steering mirror, and the LO is picked off before the pump reaches the MZ.

In the new diagram, I'm thinking about controlling the following degrees of freedom:

• Signal phase relative to pump phase
• Pump frequency relative to amplifier cavity resonance
• pump intensity
• MZ relative path length
• Homodyne mixing angle
• Signal and pump spatial mode

Some things I'm unsure about:

• I am not currently controlling signal laser intensity -- can this be done by sending the first order AOM beam to a PD rather than dumping it as shown?
• Not sure about the right place to place several of the EOM
• If we have transmission through the cavity's curved mirror, is the associated loss acceptable?

Shruti and I are now tracking our work on git issues in the PSOMA repo.

Attachment 1 contains the SR785 dark noise measurements at number of PD reverse bias voltages from 77-295K with filenaming convention:

[PD]_drkspec_[date]_[temp]_[input V]_[scan freq]_[FEMTO gain]_[date]_.txt

It also contains the keithley sweeps for QE calculations.

I'm still working out what is wrong with the QE data and how to effectively process the dark noise versus temperature. 

I put together two PSOMA layouts, one for a bowtie cavity and one for a ring cavity configuration.

I expect there are a number of problems with the layout as I've drawn it, and I note a number of these in the bowtie diagram. Among these

• Should I put PSOMA updates in the QIL elog, or in Cryo, SUS, or elsewhere? I think QIL has some 1550nm light, as does cryo. Cryo is pretty crowded, although cryo Q has the possibility of moving to the QIL cryostat.
• What's the best way to get pump and signal -- signal picked off from the pump, or two separate lasers?
• I expect this layout is rife with errors in how the locking should go. Shruti and I should probably just talk with someone (and each other) about this rather than listing my various uncertainties.
• Probably also picked off the homodyne LO from a nonideal place
• I haven't fully considered a number of things like: mode matching, scattered light, LO phase noise and homodyne angle, etc.

Additional notes:

I did not see anyone in the building.

Attachment 1/2: Our labs have no sticker/paper to indicate any disinfection of the room. (Make sense)

Attachment 3: Most of the basement offices have the notes to indicate disinfection.

Attachment 4/5: Our offices have no notes.

To check the status of all the labs, I went to WB. There was no ongoing water leakage in the labs.

Attachment 1: The subbasement was completely dry.

Attachment 2: Upon the lab inspection, I took PPE from the OMC lab. This was intended to prevent me to pick up anyone's anything and you to pick up my anything.

Attachment 3: The EE shop has no problem

Attachment 4: Cryo Lab. No problem.

Attachment 5: Crackle Lab. No problem, but a lot of dead cockroaches on the floor!

Attachment 6: OMC Lab. No problem.

Attachment 7: C.Ri.Me Lab. Gabriele has already checked the status in the morning. And I found no problem. Didn't bother to turn on the light.

Attachment 8: CTN Lab. No problem.

Attachment 9: QIL Lab. The floor was mostly dry. Did someone wipe the floor?

Attachment 10: Some water drip was found in front of the workbench.

Attachment 11: It comes from the ceiling.

Attachment 12: Left a trash box to catch future possible leak.

Attachment 13/14: TCS Lab. No problem found.

Attachment 15: As per Aidan's request, the instruments were moved to the North-East area of the room to avoid future possible leak.

West Bridge flooding Apr 6th due to rain in the night

Looks like the first responder was Calum. The attached photos were sent from him.

Last Friday, I found the HEPA units on the squeezer table were not on. I turned them on at "SLOW".

Still trying to figure out how to set up the particle counter remotely. The current particle count is 576.

Note: the particle count is the number of particles detected over 0.3um size.

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.

Facilities workers replumbed the water lines feeding the air handler units in the QIL. Tomorrow they plan to come back for about an hour to insulate the lines. We'll keep the tables wrapped up until that's complete.

[Raymond, Chris, Koji, Chub, Aidan, Duo]

Both tables were surrounded with plastic shielding in preparation for the HVAC maintenance/repairs to be performed tomorrow, Tuesday 24/02/2020. The IR Lab cryostat roughing and turbo pumps were turned off to avoid overheating while inside their new plastic cocoon. Sticky floor mats were placed in the oil-slick area in front of the flammables cabinet to improve safety for all foot traffic in the area, as well as to mitigate the urge to boogie while working in the lab. 

Here is the data for the last week. The temperature for the Northside is noticeably higher than the Southside. This is probably a calibration error in the North sensor because there is not a noticeable temperature difference when walking across the room. I would guess the south sensor is more accurate in its overall temperature reading because it feels more like 22 degrees than 24.

Got data from the HOBO temperature recorders. The temperature has been fluctuating between about 80 degrees F to 60 degrees F. The Temperature has been fluctuating wildly in the last month that the recorder was on. I will restart the HOBO monitors to get more current data.

*Note the HOBO app changed how it reports the time data. The updated code is attached which uses the AM and PM 12 hour .csv file generated by the app.

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.

== 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 QE measurements from the first couple of photodiodes are attached below.

• plot_JPL_diode_results.m - A2P6 analysis
• plot_JPL_A2P2_diode_results.m - A2P2 analysis

QE = [I_photocurrent]/[P_PD] * h *nu/e

P_PD = Power incident on photodetector = 0.966*power_incident on cryo window

Power incident on cryo window = F(voltage on reference PD)

Opened the cryostat to resolder the heater and re-wrap the thermal anchor for the sample RTD and PD connection. All connections are working as expected at room temperature. A2 is still in the sample mount. 

• Monitored an autorun of the photocurrent for A2P6 from about 77K-300K and adjusted the laser alignment as necessary. I've not yet looked through the data, as I was keen on fixing the assorted wiring issues in time to pump down for tomorrow. Chris found the laser power issue rooted in my sub-optimal diode-collimator mating connection, ie I didn't fully plug back in the laser. I've learned my lesson and will simply shield the fiber with Al foil for future nitrogen pouring endeavours .
• Opened the cryostat and confirmed the pin assignments (see attached list).
• Cathode 2 was shorting on the thermal anchors for the PD plug. I re-wrapped the quad twist wire around the bobbins and this has fixed the issue, though I did not see any portion of the quad twist wires that was missing the Formvar insulation. 
• The DT-670 silicon diode thermometer was busted, this was likely the case from the start and should have been checked (by me) prior to wiring. I've replaced this diode with another 4-lead platinum resistor, it is plugged into input 3 on the CTC100 (still labeled 'PDdiode'). 
• The thermistor on A2P6 is still intact, showing about 1.9 kΩ at room temp

Still to do:

• Move the radiation shield PT RTD to the outer shield
• Re-wire the baseplate pins in the DSUB9 connection to the CTC100 temp controller (switched V+ and V-)
• Check feasibility of adding 25Ω heater to the sample mount to stabilize the temp of the PD rather than measuring while sweeping 

(Raymond, Chris)

• Baseplate and shield RTD temperature sensors are working, and digitally recorded.  The temperature sensing diode on the PD mount seems to be unconnected.
• We made dark current vs bias measurements on the P6 and P2 elements at room temperature.  We also checked P3 but it appears to be unconnected.  Then, after realigning the P6 element, we measured its photocurrent vs bias at room temperature.  The photocurrent and dark current data looks much like what Koji previously reported at room temperature.
• Going from 77 K to room temperature, the beam had to be moved downward to correct the alignment.  The magnitude was about 1 clockwise turn of the vertical alignment knob on the steering mirror.  The horizontal shift was very small.
• Armed with this knowledge, we re-wetted the cryostat with LN2, bringing the baseplate and shield temperatures down into the 80 K range.  Our plan was to let it warm up overnight while running an automated test series of dark current and photocurrent vs bias and temperature.  We offset the vertical alignment by a half turn, in hopes of having the PD well aligned near the expected QE sweet spot of 150-200 K.
• The procedure for adding LN2 to the cryostat involves temporarily disconnecting the fiber coupler.  Sadly, it appears in our haste we did not fully recover the alignment after this step, or else the calibration somehow shifted by a lot.  The photocurrent is at the 0.1 mA level instead of the ~1 mA we expect.  Accordingly, this run will primarily be useful as a test of the automation, as an indication of dark current vs temperature, and as a very rough, qualitative indication of QE vs temperature.
• The script is running on qil-ws1 in a screen session.  It can be interrupted with screen -RAad autorun to connect to the session, followed by Ctrl-C to kill the script.
• In the morning, we need to decide whether to open the cryostat and fix the wiring, or to repeat this run with tighter control over the alignment.

don't we also want to record the dark noise spectrum as a function of T and V_Bias ? I would guess that the dark noise doesn't always scale with dark current at low frequencies since its probably more like a random walk than shot noise.

[Aidan, Chris, Ray]

1. Add LN2 to cryo-chamber
2. Turn on heater to 25W
3. Wait for LN2 to boil off
4. Turn on the REFPD bias switch, and enable a 200 mV bias on the JPL PD (set C4:TST-FM13_OFFSET to 200)
5. Switch the readout to the transimpedance amplifier (relay control output from C4:TST-FM15 at 30k ct)
6. Turn on the laser
   1. Power up the LDC201C and set it for half-power output (50 mA), with the modulation output from C4:TST-FM12 at zero
   2. Set C4:TST-FM12_OFFSET to 1 to go up to full power
7. Monitor the PD output on oscilloscope and adjust horizontal and vertical alignment of laser beam so that the output of the PD is maximized (also adjust TIA gain if needed so the output is not saturated)
8. Switch the readout to the Keithley (relay control output from C4:TST-FM15 at zero)
9. Record bright PD response
   1. Turn on laser (set C4:TST-FM12_OFFSET to 1)
   2. Scan bias voltage and record the PD response using the source meter (scripts are located in $HOME/JPL_PD/scripts on qil-ws1)
      1. ./runsweep.py triggers the Keithley to sweep the bias (range of sweep is defined in the script)
      2. ./getdata.py FILENAME downloads data from the Keithley and writes it to FILENAME
   3. Record REF PD reading
   4. Record RTD resistance
10. Record dark PD response
    1. Turn off laser (set C4:TST-FM12_OFFSET to -1)
    2. Repeat steps 9-2 through 9-4
11. Switch the readout to the transimpedance amplifier (relay control output from C4:TST-FM15 at 30k ct)
12. Measure dark noise vs bias (using either the SR785 or the cymac, TBD)
13. Wait two minutes and repeat steps 6 through 12 (there is a script ./autorun.sh which continuously repeats steps 9 and 10)
14. Continue until RTD reaches room temperature (approximately 60-90 minutes).

[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.

We placed a power meter after the fiber collimator, 75mm focal length lens and HR mirror at 45 degrees - basically, we placed the power meter immediately before the input window to the cryo chamber after all the intervening optics from the fiber output.

For a series of laser diode current levels, we measured the power on the power meter and the corresponding voltage on the reference photodetector that is monitoring a 10% pick-off from the laser. The calibration is as follows:

Wow. This is great, thanks Chris.

• A Keithley 2450 source meter will be used to measure QE and dark current vs bias.  It is on the QIL network as 10.0.1.130.  A command interface is available via telnet for running measurements and grabbing data.
• A FEMTO transimpedance amp and SR785 will be used to take dark noise spectra, with bias supplied by a DAC channel filtered by an SR560.  The 785 is on the QIL network as 10.0.1.66, and the usual GPIB scripts should be sufficient.
• A relay has been installed to control the switching between the Keithley and SR785 readouts.
• BNC cables and breakouts connect the electronics on the table with the cymac (see table below for the channel assignments at the moment).

Note: in the preceding table, channel numbers use the digital convention (numbered starting from zero), which is not the convention used by the AA/AI chassis front panel (numbered starting from one).

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.

The IR Labs cryostat has its internals wired and attached to the baseplate. PD A2 was clamped and the vacuum pumps turned on for the first cooling test.

[in the morning I will update with a detailed pin-out and label the attached photo (labeled 12/13, pin out in separate post)]

Recently Duo wanted to make an arbitrary waveform excitation using the QIL cymac, but it wasn't working.  An excitation would die after 10 seconds or so, with awgtpman reporting that the data was too far in the future.

It turns out this was caused by a missing leap second in the RTS software.  It is now fixed upstream, and we're running a patched version of awgtpman on fb4, until the change propagates to the packaged version.
 

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

Update on the 32 pin female connector:

Mouser's overnight delivery was rejected on 12/3 by mail services for being soaked in an "unkown liquid" and was therefore taken away by the courier for return to Mouser. This was the last one in stock, so I ordered a replacement through Digikey for express delivery this morning 12/4, but it has not yet arrived. I've called Fedex and the package was sorted at the LA facility but not given to the courier for morning delivery. It is now estimated for delivery this afternoon. 

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

[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.

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

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.

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