Worked for a few hours to get the aspheric properly aligned. The procedure is quite finnicky, as the four 2-56 flexure screws have too much game and the fine thread setscrew that adds tension is too constrained. Anyways, it generally goes like this:
After this, I installed a second amplified InGaAs detector, hooked up the unbalanced MZ beamsplitter output into the two PDs, adjusted the gains to equalize the output voltages and then hooked the two signals to the A and B inputs of an SR560 in "A-B" mode. The output (gain 1) was good enough to feed back in the HV PZT amplifier input modulation which allowed the MZ to lock mid-fringe. The lock is rough, as the balanced homodyne signal retains a tiny offset due to imperfect balancing... Attachment 1 shows the setup, including a typical scope trace after coarse current tuning (Ch1 and Ch2 in yellow and blue represent the photocurrents in the two MZ ports in the absence of feedback).
Indeed, scanning the nominal PZT voltage broke the lock, potentially after crossing a mode hopping region.
Tasks to be done:
Next, as was suggested during yesterday's group meeting, we will transition into a self-heterodyne setup (with an AOM which I have yet to check out in the QIL).
We tweaked the flexure alignment until we had a nominally collimated beam (~2 mW @ 250 mA of diode current) through the output aperture in the ECDL housing. We noted that the collimated beam is off-centered on that circular aperture along the horizontal (yaw) angle. After this, Radhika installed the ECDL grating and we hooked up the fiber output onto a InGaAs PD to monitor the power output. We tweaked the alignment of the grating (mostly yaw) to try and see a change in the power output to indicate optical gain in the diode, but saw no changes. We observed a change in the PD photocurrent as a function of the diode current in the absence of the grating (no optical feedback) which is indicative of ASE. We measured this level to be ~ 140 mV at 200 mA of current; with no observed threshold. In conclusion, we still need to refine our grating alignment to provide gain on the diode and observe lasing at the nominal 1450 nm wavelength.
Today we fired up the 1418 nm ECDL and attempted initial adjustment of the aspheric lens. The design follows D2100115 which is a copy of the 2 um ECDL so we just changed the diode, the grating flexure angle, and the aspheric + flexure assembly and we are good to go. Radhika removed the 1900 nm aspheric flexure and we mounted the new collimating assembly which uses a f=3.1 mm (NA = 0.69) lens. At the beginning we had to feed over 300 mA of current to be able to see a beam (which was still diverging) so we had to free the flexure completely and align by hand to find the nominal positioning for a collimated beam. We lost a 2-56 screw in the process, but the final assembly is still in progress. The plan to follow is:
Today the DOPO v0 got disassembled to make way for the optical table swap. Most components have been stored in the white cabinet's bottom panel.
[Paco, Anchal, Ian, Yehonathan]
Today, in preparation for the optical table to come, we vented the big crackle jar using the vent valve near the gauge. We detached the roughing pump and covered the bellows and pump connections with clean aluminum foil. We then proceeded to move several instruments, including some other pumps, a compressor, a couple of power supplies, power cords, the HeNe laser, misc. material blocks, and boxes with bearings and springs into the cage. The next operation required for us to displace the table is to lift the jar from the top and carefully dismantle the Crackle experiment and store it away somewhere.
Questions: where to store mostly?
Another Heimann Sensor / Boston Electronics delivered to Paco.
This unit (purchased May 2020/ / Delivered Aug 5th, 2020) has a FZ-Si window on it.
We don't know how it is.
- Have been investigating 316 Hz noise in the control signal for the DOPO lock. Here is a list of some things that have been ruled out, mostly electrical:
- EOM power supply --> noise still present in DOPO transmission
- RFPD DC out --> no funky ground loops with scope (also looking at demod signal in different channel), noise still visible in transmission
- RFPD power supply --> noise still visible in transmission...
- Pump laser intensity (upstream pickoff) --> not a great test because pickoff optics are also on the optical table..
- 2 x SR560s --> No effect after bypassing
- Marconi --> same result as with anything in the loop after RFPD demod
- Things left to rule out:
- Fume hood exhaust fan ** highly suspected, my phone's own cheap-o microphone power spectrum shows peaks at 316.5 Hz (!) when near the exhaust fan
- NPRO temp controller fan --> phone audio spectrum shows line noise (60 Hz) mostly, and also 188 Hz... need to test further independently of the fume hood...
In ruling out the 6-axis translation mount on the DOPO cavity, I removed the PPKTP crystal + oven temporarily but still saw the noise. Since the resonator was no longer stable without the crystal, I needed to bring the mirrors closer and realign the output coupler from scratch.
Restored DOPO cavity with crystal, alignment. MM efficiency ~ 35%... still optimizable.
- First test to grab frames was done in my personal Win10 machine, with no success. Either I was unable to configure the server correctly, or the software "ArraySoft" is not supported in Win10. Upon contacting Heimann, I received instructions to update to a newer version but was warned that it's just a new GUI, nothing really changed from v1 --> v2. So didn't even bother.
- Instead, wrote a simple python-socket UDP server to catch the video stream. Most trouble happened when using temperature mode (command "K"). The client streams a bunch of zeros... My guess is that this unit does not have an internal temperature calibration, and one could in principle be uploaded but we probably don't care. Streaming in raw voltage mode (command "t") works well, as shown by the sample frame shown in Attachment 1.
- After recovering the CTN Win7 laptop from Radhika, I gave "ArraySoft" another change just to know the frames I was getting in python were not bogus. For this I pointed a 532 nm laser pointer straight to the sensor and got an image shown in Attachment 2. The key difference is the processing of the video stream. Attachment 1 is a single frame, while Attachment 2 is the average of 30 frames with no offsets present.
- Another issue present during this task was a faulty USB connection. Sometimes moving the sensor around would interrupt the stream (power lost). I carefully removed the case and exposed the TO-39 package and surrounding electronics to inspect and search possible failures but after seeing none, I swaped the USB power cable with my portable battery charger and had a more robust operation... So I dumped the old USB cable, and will get a new one.
- Since this one was borrowed from TCS lab, I placed an order for another one which will be set up permanently in the lab. Hopefully this will be enough for the OSA.
Drew some new mounting scheme for the DOPO cavity; main revisions with respect to the current mount are -->
Attachment 1 illustrates the design; shows three views of the same assembly.
Concerns: mechanical noise from side mounted mirrors ... for this, there could be a solid piece which makes a rigid connection between the two mirrors (that's why they are upside down) and perhaps between the two tall posts (so S-shaped as viewed from the top)? Still working on this.
With Aidan's assistance, I borrowed
for ~ 2 um imaging in the Crackle lab.
- Noticed that the cavity transmission peaks @ 1064 nm were much wider than originally estimated by the dopo cavity design notebook suggesting a lower Finesse. So using the PDH error signal, and knowing the EOM sidebands are at 36 MHz estimated the current DOPO cavity linewidth to be 19.5 MHz, well in excess of the target 10.4 MHz.
- Updated the crystal AR coating specs from Raicol (R < 0.3% @ 1064/2128), but more importantly, I included the absorption coefficient of KTP, alpha=0.005/cm (often quoted as < 0.01 / cm) into the roundtrip loss and the design now gives 17.97 MHz. So, given the uncertainty in the absorption coefficient of the NL crystal, and all the coatings in the experiment, this adjustment might be enough to explain this observation.
Test long term stability of the DOPO cavity lock; The cavity remained resonant overnight (start ~ 8 PM yesterday) and lost around 11 AM today. It might be good enough to approach lock point manually using laser temp. control and then engage the fast loop. In any case, today will set up an acromag channel for this. Configured "XT1541-2um-SlowDAC" to 10.0.1.47
The plan during these past few days has been to have fast control loop of the cavity (locked to laser using PZT, which succeeded using SR560s), and slow control loop where the laser temp. actuator is fed back the integrated PZT input to follow the long term cavity drift. For that, have been messing around with the high-level (GUI) API of PyRPL, with basically no success. In fact currently the RedPitaya cannot even replace the SR560 fast controls, which probably has to do with the +- 1 Volt limits on the RP input/output.
Another issue is that any loop gain depends on the REFL power, which will be at some point slowly ramped up to cross the OPO operating threshold, and while there is a (PBS + HWP) knob on how much light is hitting the RFPD, the lock is not yet good enough to keep up with the slow human action.
WIth the cavity locked, and under ~ 220 mW of pump (right before the cavity, i.e. 1.3 Amps of current on the driver), noticed a tiny green dot coming from within the crystal oven. This is pretty great news in terms of phase matching, but not necessarily so in terms of the right parametric conversion process (understanding is that SHG is easier to attain even with single pass). See tiny green spot as caught using phone camera in the attachment.
Upon closer inspection the error signal seems to vary quite significantly on the scope (scanning @ 2 Hz), sometimes completely flipping its sign even though it always triggers on the same side of the ramp (see attachment for video, along with some neck excersise).
This might be the same behaviour from before, whereby the demodulated signal might still be "riding" a low-freq componen which can't be compensated with the LO (Marconi's carrier resolution = 1 Hz). Using the 10 MHz external Rb reference doesn't change anything. It seems that even with the coupler, reflections may be entering the mixer...
Adding a LP filter (BLP-1.9+) right at the mixer output solves this for good. Even using 36 MHz LO vs anything else doesn't make a difference so this explains the previous issue. Moving back to lock using stable err signal.
For reference, the LO carrier is set to 36.000 MHz, +7 dBm (so the EOM is driven with an estimated +30 dBm well below the saturation or damage threshold +40 dBm).
Achieved a good lock for pretty much all of the afternoon today. The laser ran at 937 mA current, the optimal gain on SR560 was found to be 50, with a LP cutoff at 300 Hz (12 dB/oct rolldown). The 300 Hz cutoff supresses most of the nasty 8 kHz noise (and harmonics) which I can hear with enough gain. Source still to be determined.
UPDHv3 box (serial 17142) is bogus. While retrieving values of some of the components to plug into working zero model, saw the VGA stage is bypassed by a previously unnoticed hack. Verified this by taking TF and not seeing any changes with respect to the gain knob (shown below are zero's model TFs suggesting a tunable UGF from ~ 10 Hz to 1 kHz), so this box is not good for a standalone servo.
As suggested a few meetings ago, made a quick and dirty lock using a single SR560 and took measurement of something* CLTF (SR560 gain = 10) below. New goal is to find a decent replacement, for which decided to use RedPitaya's python API "pyRPL". Just using the GUI out of the box can also lock the cavity relatively quickly but neither method results in longer than 1 minute lock... so took one step back to polish the pdh error signal.
* Something = Use SR785 TF measurement with source on Ch1, and to B input in SR560. The SR560 in (A-B) mode, and demodulated signal connected to A. The loop was closed with the SR560 output driving the PZT, and Ch2 of SR785. Wouldn't call this CLTF...
For the splitting, I recommend not to use a splitter.
Instead, you can use a -10 or -20 dBm bi-directional coupler. You send the -10 dBm signal to the EOM amp, and you can fill up the needed power for the LO mixer. Also the "bi" nature of the coupler means that you can check for reflected power to diagnose if you are having impedance mis-match. Since you don't have an isolation amplifier in your setup, its important to make sure that reflections from one leg don't go back into the oscillator and disturb the other leg. Or maybe your oscillator box has an isolation amplifier between the oscillator and the splitter?
Update on demod. for OPO cavity lock. Last related elog entry described prevalence of <= -77 dBm of odd line noise harmonics (60, 180...) Hz, along with poor SNR PDH error signal. First attachment is a drawing of the current RF connections. Upon completing list of suggested actions from this post, the difference was mostly made by looking at RFPD RF out power before mixer < -40 dBm. This was no good, so after realizing that the OD = 3 nd filter before RFPD was only allowing 80 uW of a nominally reflected ~25 mW, swapped the ND filter with HWP + PBS for adjustable power splitting. Then, a healthier -10dBm made it into the mixer and SNR improved considerably (see second attachment). Upon closer examination of err signal, low freq. sinusoidal modulation sat on top of it suggesting slightly off-resonant demodulation so finely adjusted the (Marconi) LO frequency from 36.000 MHz --> 35.999828 MHz until the error signal had a good enough shape (see third attachment below).
First attempt at cavity lock was done with ~46% mode matching efficiency and max. modulation depth (estimated ~0.21) on the EOM. The loop is achieved using UPDH box (v3) which I stole from CTN lab. Upon connecting all the inputs, scanning the phase shifter without making much of a difference, and enabling the lock, saw a stabler higher order mode on the cavity transmission which is nice. The natural follow up of scanning the PZT driver (i.e. as an offset) and re-engaging the lock resulted in what I can only describe as a "visit to the dentist", where the cavity PZT (on the output coupler) was resonating quite loudly (!!). After looking at the output monitor of UPDH box with engaged lock on SR785 an ~ 8 kHz peak explains the noise as an audible mechanical resonance. Adjusting the servo gain finely tunes it out a bit, and adding an SR560 in line before the PZT driver unit greatly helps, but changes the overall loop gain and the lock becomes unstable... Current efforts are therefore geared towards improving the pdh loop, for which an option is to bypass the thorlabs MDT694 HV piezo driver and directly connect the UPDH output to PZT (which it may be meant to directly drive) and use slow temp. control on pump laser to approach the lock point. Another option, involving way more time, would be to *not* use UPDH box at all and implement a digital feedback loop + filter with the Red Pitaya. Perhaps the pragmatic action is to get the analog solution working and develop digital solution on the side.
Motivated in part by the conclusions below, improved estimated mode matching efficiency from a poor 13% at the beginning of day to 48% (estimated using the reflection signal levels from the rfpd). What helped was walking the beam with the last two mirrors, and then scanning the cavity output coupler around to center the resonant mode which at this point seems optimal. This process was tedious, but effective apparently.
The distance between the two mirrors is ~ 45 mm which slightly undershoots the planned 47.5 mm which could limit the achievable 100% in simulation-land, but I'm moving on for now, hoping the lock will bump it up enough for the OPO threshold to be within our pump power range.
After getting what looked like a decent cavity reflection signal, installed RFPD yesterday. For this, removed the lens that was right before the PD because the RFPD area is large enough, but keep ND filter in place. Powered with +- 18 VDC and monitor DC out on the scope, and RF out is sent to the IF of the mixer in the PDH box. For the LO, split the Marconi RF output and connected the demodulated signal into Ch2 of the scope in hopes that there was an error signal.
A hint of the error signal is present (blue trace below), although deeply buried in line noise (and harmonics up to ~180 Hz) so there really are two things to optimize now -->
Other things attempted so far -->
would be good if you could find a solution that is not very sensitive to precise lens placement
Analysis & Data
Record TF for RFPD SN09, resonant at 36 MHz, using the exact procedure as with EOM Resonant Driver.
See equipment borrowing note here.
Attempting TF measurement for resonant EOM driver, but not having luck reproducing the measurements done recently (Dec-03), so I started debugging the circuit. Both power supply connections (+- 18 VDC) seem nominal. The MAX2470 buffer regulated input is nominal at 5VDC. Looking at MMBT5551 HF transistor, base-emitter voltage is -0.60 VDC (nominal wrt -0.66 V). Using a scope, I feed a single tone (36 MHz, 190 mVpp) and look at the RFmon output and it looks ok (gain ~ 1). I changed the RFmon SMA cable and that seemed to do the trick... Bad cable (now in trash) stole my morning.
Tune EOM driver resonance to 35.993 MHz (shown below for reference).
Observed first resonant transmitted (& reflected) light from the DOPO cavity; the PZT scan was centered at 31 V, at 2 Hz, with an amp. of 1.5 Vpp. To get there, revisited the path's alignment upstream to the last mirror (before the last lens), removing, inspecting, and reinstalling each component. After this, I used the camera at the end of the optical path as a "pinhole" (beam center placeholder) and after inserting each element (mirrors / crystal) checked carefully that the beam was landing straight. Then, patiently scanned various knobs (mirror mounts X/Y/XY, crystal manually) until HOM started resonating. After a bit of further alignment managed to see transmission dips in the FI pickoff. Below are two photos illustrating the current state (way more optimization is needed), as well as the setup viewed from one side (for the scope picture, purple is the ramp, yellow is cavity reflection, green is cavity transmission). Will keep optimizing in the couple next days, all at low power first, and then start cranking the power up to factor in any thermal effects into the optimized cavity.
Fresh attempt at mode matching. For this,
After a couple of iterations moving the mirror X,Y and then scanning all knobs (X,Y, and XY) to effectively translate along Z, the optimized FI rejection is ~(2.15 mW /2.95 mW) 75% of the input beam power. Looking closely at the backreflection from the output coupler, I can clearly see multiple scattered spots, which could definitely account for the defficiency. The most likely culprit is the crystal itself, which is mounted between brass and glass surfaces with no respect for anti-reflection measures. The waist is small enough that no clipping should be happening, so it looks like the NL crystal placement may have to be revisited. Other than that, this procedure should be fine.
Summary of solution number 2 (from previous post).
After installing the lenses, mirrors and some minor alignment, took the beam profile around the expected minimum waist position (~102" from laser head). The beam profile is astigmatic as can be seen from the plot below (red / blue = x / y), so the mode matching will be suboptimal from the start.
Taking the geometric mean of the waists (w = sqrt(wx * wy)) we represent our nominal mode and find a min waist of 36.8 um (shaded region in the plot).
The OPO cavity model targets a min waist of 35.5 um (for an optimal Boyd--Klein parameter of ~2.7), but solutions exist with slightly shorter cavities and slightly larger waists which would only compromise the optimal Boyd--Klein parameter to ~2.55 for the sake of better mode matching. I think this is a good place to move out of calculation-land and see how well we can make the cavity work in reality.
Analysis and Data
Wed Jan 6 10:00:35 2021: This analysis was wrong. See SUS_Lab/1887.
Assembled first DOPO oven with the crystal. The components (shown below) are:
The NL crystal sits in the (brass?) clip directly, with the ITO (dummy) crystal pressing it uniformly down. There are no placement references to align the crystal with the oven axis, so this was done very carefully by hand. Once this is roughly straight, the copper arms are fastened in place tight enough to hold everything in place but without excess strain on the NL crystal. The assembly (shown below) is then mounted enclosed in the oven. I put some kapton in place to shield from dust until operation.
Laseroptik optics (4x pairs of cavity mirrors) arrived earlier this week, so I began assembling the input mirror with Noliac (NAC2124) PZT. The (15 mm OD) pzt will sit between a 1" post spacer and the mirror. I applied a thin layer of BT-120-50 (bondatherm) adhesive, which I found in EE shop. From what I gather this adhesive doesn't have softeners (almost doesn't smell) and is a good electrical insulator. The PZT + spacer is sitting below a metallic weight block on the left corner of the table (by the electronics test bench corner), and should finish hardening in a little over 24 hours at room temperature. The PZT was labeled 520 nF (spec. 510 nF).
Covesion order arrived, containing 2x
Borrow both beam profilers and laptops from WB 264A.
Photos, please, because we don't allow a free-rolling cylinder in a lab.
QIL elog entry: QIL/2524
Received one Marconi 2023A (#539) from CTN and an SRS FS725 Rb clock. (See CTN/2605)
Today, after struggling to find a 4-pin circular power supply cable for the UPDH box (still interested btw) punched a hole for connec power connector in the back panel and found an appropriate cable. See attached photo. Intended for +- 15 VDC.
Two crystals from Raicol arrived. Picked them up from Downs today and inspected them (see photos below). The lengths are nominal (20 mm), they are serialized as 123 and 124, and the ends look like they have the specified (AR) coating. I reached out for Covesion two days ago to track the ovens so we can mount these guys, but have yet to hear back from them.
In the process of adding a PC/controls, and other related instruments, reorganized items in the lab. Threw out some boxes and stored cabling and unused power dock. Moved the sticky mat and put out large trash bin. Organized electronics rack to which a Sorensen (DCS33-33) power supply was attached. For this, took a 14 AWG wire (should be fine up to 15 A at 115 VAC) and cut plug end. Then connect neutral and live as indicated by the rear of the panel and add chassis ground. Tested DC output voltage of 3 V and it works ok.
There are now two workstations in the lab attached to the same monitor (VGA and DVI ports), and it is ok to ssh from one to the other. They both now have fresh debian 10 installs.
On Monday, tested a 1998 (Rev. 0) RFPD originally found in Crackle (serial #010). Looks like it was first resonant at 24.493 MHz, but was later tuned for 14.75 MHz. I used the AG4395A network analyzer in CTN following the procedure in the previous ELOG post, splitting R output into the test input of the RFPD. Driving at up to -10dBm, couldn't see any resonant feature in the TF below 150 MHz. Tuning the inductor L1 made no difference. The regulator (U3 and U4 near bottom right in picture below) outputs were nominal.
I borrowed a flat response (DC to 125 MHz) PD from CTN lab (New Focus 1811) along with its power supply for short term use.
Below are some photos of the aformentioned RFPD. I added some kapton to keep dust off the PD.
Enter lab ~09:20. Today I spent a while looking at the broadband EOM drivers used in CTN (presently optimized for 37 MHz) and installed the preceding steering and power control (half waveplate + pbs) optics. The beam path for the OPO pump beam is now set to 3 inches (note the NPRO head is nominally 4 inch above the table).
Borrowed 1 (new focus) broadband EOM from CTN for temporary use in Crackle (2 um OPO exp)
Wow! This is really cool! I didn't realize that this small box has such many remote capabilities.
We have this piezo controller everywhere in the labs and your code gives us a lot of opportunities to implement process automation.
Today entered lab ~ 09:00. Over the weekend I coded a PySerial wrapper for the thorlabs MDT694B single channel piezo controller. I spent some time testing and debugging the code but it now works fine (tested on Linux, python=3.8.6 and PySerial=3.4-4). The wrapper refers to the manual available here. The code is available in the labutils repo