Fully board successful with modification. The saturation issue has been fixed on all the four channels.
I finished testing S1900294 and S1900297 and plan to ship them to the sites.
I got stuck at a couple things. I think it is good to make a note of these stuff.
1. Diaggui gives "unable to start excitation".
Solution: restart diaggui
2. Drive signal does not go in
Solution: check the status of the digital system - it crashed and needs restart.
3. FASTIMON gives too much noise that the transfer function looks like junk.
Solution: I use 50ohm resistors to replace the coils and 500 counts noise to measure the transfer function. If the resistance is large and the input signal is small, the current will be too small.
The noisemon board has been modified according to 1835. We do not expect any change in the transfer function but we see an increase in the gain above 20Hz.
Attachment 1 shows the comparison between modified board transfer function and the original board transfer function.
Attachment 3 shows the difference of the transfer functions in attachment 1. There is about 8-10dB increase at 100Hz after the modifications.
Attachment 4 is a comparison of LISO simulations. I calculated the transfer functions of the whole noisemon circuits before and after the modifications. I substracted them and found the difference.
Without putting attachment 3 and 4 in the same picture we can see that they are very different. LISO basically says there will not be any significant change in the transfer function but actually our measurement shows that there is.
I did some documentation work these days.
- Noisemon Test Plan: https://dcc.ligo.org/LIGO-E2000007
- Updated DCC, new schematics, PCB layout and BOM (due to changes logged in 1835)
- Two more boards are modified and will be tested.
I am testing another two boards to be shipped to Hanford today. I found and fixed some bad soldering. However, the frontend crashed and I needed to restart it. I could not log into cymac somehow. It says "no route to host" but the internet is still working. I cannot go on testing anymore. Let's wait and try again tomorrow.
Chris fixed the problem - I went on finishing the tests and the boards are ready to go now. They will be shipped tomorrow.
We have sent version 2 noisemon boards (modifications from version 1 noted in 1835) to Livingston and Hanford. Chris noticed that there might be some upconversion problems under 20Hz (attachment 1 and 2). These plots from Chris are noisemon output with drive subtracted. Attachment 1 is the spectrum after replacement of noisemon board (version 2 board used), compared to attachment 2 which is before the replacement (version 1 board used). There is something going on near 15Hz. We think it might be due to upconversions under 15Hz.
We still have a spare board here. I modified it, tested it and looked at this issue, trying to reproduce. I sent a 10Hz 100 count amplitude signal to the board and compared it to the output with no input. This produces attachment 3. We see that once the 10Hz sine signal is sent, there are lines above 10Hz, which is a sign of distortion.
I changed the input frequency to 5Hz and got attachment 4.
Attachment 5 is a linear plot to see where exactly those lines are. It seems they are indeed harmonics of 5Hz when the input signal is 5Hz.
I also tried higher frequencies up to 100Hz and saw similar harmonics.
it might be the low input impedance of the board which the coil driver cannot drive..
I suggest you use probes to see where in the noisemon circuit the distortion is starting
Trying to figure out the nature of the distortion found in noisemon (1843), I made a piece of DSUB9 breakout like attachment 1. I just used two wires and two clippers wires to break the DSUB connector into something I can clip into the board.
I sent a 10Hz drive to the coil driver and measured the spectrum at all the test points (including one pair on the coil driver board).
First, I compared the distortion situation at the input and output of noisemon (attachment 2). I measured the spectrum at TP10 and TP12 of coil driver (this is where noisemon picks up the differential input, reference DCC: D070483), and subtracted them quadratically, which gives us the input signal of noisemon. Then I measured the spectrum at the output of noisemon. Looking at attachment 2, it seems the coil driver does not contribute to the distortion.
Then I used the probe and measured all the test points individually. Attachment 3 also shows TP10 and TP12 but instead of measuring them individually and subtracting them, I just put one probe on TP10 and another on TP12. It looks like the coil driver does have a little distortion, contrast to the conclusion above.
Attachment 4 shows the spectrum at the noisemon input. There is a buffer between TP10/TP12 on coil driver and the input of noisemon (reference D070483). It looks like the distortion is less, somehow.
Attachment 5 is the spectrum after the passive filters, between TP3 and TP4. It seems the distortion comes back a little under 60Hz.
Attachment 6 shows the spectrum after the instrument amplifier.
Attachment 7 and 8 shows the output of two stages of high pass filters. The distortions gets much worse after two HP filters.
Attachment 9 shows the output of the low pass filter. We can see the high frequency harmonics are gone.
Attachment 10 is very confusing. Between TP8 and TP9 is just a buffer (LT1128 buffer unfortunately) - why does it give so many lines even without any drive? I tried to see them in the oscilloscope but there wasn't anything significant. Maybe it was the clippers/wires making the noise? I did check the signal at the conjunction point between the clipper and the wire using oscilloscope (attachment 11) - it is indeed bad (attachment 12) - but why these lines only show up in TP9?
I am very confused and need to think about what is going on now. I think a couple immediate questions are:
1. Why do I see so many lines at the output of noisemon even when there is no input? I should only see noisemon noise when there is no drive, but I did not short the inputs like when I measure the noise. I did shorted the inputs and checked the noise after the measurement - it is normal. Thus, the lines could be caused by not shorting the inputs of the coil driver, but I did not short the inputs in other measurements either - I just unchecked the excitation on diaggui.
2. Besides the TP9 confusion, we still see a lot of harmonics in uppersteam testpoints. Considering the TP9 situation, I think we should first ask - are they real? Then, if they are real, how bad are they? Considerations could include the functionality of noisemon board at the sites? This potentially include risk of saturation, increase of the noisemon noise and errors in measuring the DAC noise.
I measured the spectrum with the digital system at all test points on noisemon with 10Hz sine drive (1845), but I saw a lot of distortion harmonics and other confusing stuff. At the end, I realized it could be caused by the clipper wire I used to breakout the DSUB9 connector. After talking to Chris today, I also realized that the low input impedance of the digital system could be invasive to the circuit. Considering these potential issues, I repeated yesterday's measurements with SR785 (attachment 3). There is not much difference - instead of using the digital system to send the drive and do the measurements, I used SR785 to drive and measure. SR785 has 1MOhm of input impedance while the ADC has only 10-20kOhm, according to Chris, so it could resolve the input impedance issue. Also, there is no DSUB connectors used; it also let me get rid of the clipper wires.
I sent a 10Hz, 10mV p-p sine signal to the coil driver and measured all the test points from the output of the coil driver to the output of the noisemon. The results are in attachment 2, plotted in attachment 1. The test points can be referenced in the schematics in attachment 4. The plots with two numbers after 'TP' means the measurement is between the two test points - with postive clipper on one, negative on the other. Others are between the test point and ground.
We can see there is a little bit distortion lines in the spectrum under 100Hz, but they are very small compared to the response to the drive. I think maybe they could explain the low frequency bump logged in 1843. However, it seems unlikely that they will be a serious issues in the passband, but still needs a little more rigorous justification.
I calculated the DAC noise for L1. Attachment 1 has all the plots and data. Attachment 2 is the result in strain.
We have noisemon at all four stations: ITMX, ITMY, ETMX, ETMY. DTT gives me the CSD, the coherences and the ASD of all the channels at all the four stations. I use this to calculate the transfer functions of the coil driver and the noisemon.
where D(f) is the drive and CSD is the CSD between the drive and the output of noisemon. The absolute value of H(f) will be the gain of the circuit, in ADC counts / DAC counts. Then I use the coherence to calculate the total noise
This noise has DAC noise, ADC noise, noisemon noise in it. This noise is in DAC counts. I will call this "DriveNoise".
Then I picked another time when the interferomter is not running and the drive is zero. I measured the noisemon spectrum N(f) at that time. The plots and data of these spectrum can be found in attachment 1. The plots are considered to be a result of the noisemon noise and ADC noise, which I will call "NoDriveNoise" (in ADC cts). Since the drive is zero, there is no DAC noise in it - just ADC noise and noisemon noise. I use the transfer function to covert it to DAC counts
Then I subtract the noise drive noise from the drive noise to get the DAC noise to get the DAC noise, which is then converted to DAC volts
Then I find the current on the coil using the transfer functions of the coil driver, assuming the coil driver is in LP OFF and ACQ OFF state. The transfer function can be found in attachment 1.
where the transfer function H is a voltage-to-current transfer function.
Then we have the force
Lastly we have the displacement and strain
For each station I summed all the four channels, LL, LR, UL, UR and then I calculated for all the four stations and summed as
I tried to compare this with the GWINC model - it is much higher. I do not have real L1 noise at the moment. I will see once we have real noise data.
for some reason the DAC noise estimate is too high, it can't really be so large compared to the real DARM curve (see the noise budget curves from LLO - there are other noise sources besides DAC noise)
I hve modified the code to plot nicer and also to remove some divide by zero problems. There is also still some warnings about other divide by zero - those should probably be fixed by examining how better to handle it when the coherence goes to zero.
Today; entered lab at ~09:08. I verified the orientations of the aspheric lenses and blaze gratings relative to the flextures, packaged and then dropped the parts for epoxying to Koji in 40m ~ 11:00. Spent some time between 12:00 and 12:45 finishing the ECDL connections. Everything looked good so I hooked it up to the TED200C controller. After a bit of research, I found out the Steinhart constants for the 10k thermistor;
Plugging these into the Steinhart equation give the actual temperatures from the Tact output on the TED200C (otherwise read as kOhm). According to the spec sheet, the TEC was tested at 250 mA (0.40 V), so not knowing a bunch more, set I_TEC on the TED200C to this limit and inspect the actual TEC current by scanning the Tset (setpoint) and recording the current in the ~ 15 - 25 deg C (attached plot, horizontal line marks room temperature). The diode current driver is hooked up, and everything is on the table as is. Left Crackle ~ 18:30.
Here is a summary for how to connect the SAF1900S gain chip to TED200C temperature controller and LDC220C diode current driver. The chip itself lacked substantial documentation, so this comes after requesting tech support from the manufacturer. The SAF1900S pinout is
1 - TEC+
6 - TEC-
2 - Thermistor
3 - Thremistor
4 - Anode
5 - Cathode
The TED200C has a DSub15 output, but the cable provides a DSUB9 adapter. Then, only the following pins are connected to the SAF1900S
The LDC220C has a DSUB9 output, and its bipolar nature allows it to drive either anode-grounded or cathode-grounded diodes, so the question was wether the SAF1900S is AG/CG? In a first attempt, I assumed the diode was meant to be driven with a floating source (and that the LDC220C could do that), but the driver remained in "LD OPEN" state. Then, I revised the documentation for TLK1900 (an old, discontinued laser kit using the same gain chips). There, the bottom line seemed to suggest CG, but to be sure I asked a technician in thorlabs. They say most of their 14 pin butterfly chips are AG, but the 6pin ones seem to be CG. Anyways, the relevant pins (for either connection) are:
3: Ground (for AG/CG)
7: LD Cathode (for floating / AG)
8: LD Anode (for floating / CG)
After some communication with ANU's Disha, I found the diode pins are floating from the case (personally confirmed this), and an additional connection between pins 1 and 5 of the LDC220C needs to be established to override the interlock. The suggested connections are three: shortcut, resistance < 430 Ohm, or LED || 1 kOhm resistor (to match the Laser ON status in the front panel). I opted for this last one, made the connections and was able to correctly feed the SAF gain chip.
Entered Crackle ~ 8:47 AM.
Briefly fixed the LDC220C connection to the SAF1900 as described previously, and then installed the aspheric flexture and shoulder to the assembly (pictures below). Then, I used the thermal power meter head borrowed from OMC to check for emission as a function of laser diode current at a fixed temperature of 25 C (to match testing conditions). The result is below, where I seem to be getting slightly better amplified spontaneous emission (ASE) power than the attached test sheet. It may as well be that I am not measuring the ASE power alone, but I cannot presently determine this.
I added the grating and moved the power meter to the correct output aperture, but failed to detect any power. This suggests a wrong grating orientation, although I will try to verify this more carefully.
Very exciting to see the gain chip curve!
Grating orientation: Whaaat... If you already have the 1um laser SOP approved, you can use that laser to check the grating orientation.
Set grating in front of 1064 nm beam (current set to 1.058 A for a beam visible on the IR card). After testing both orientations, it becomes clear the grating is misoriented. The difference is very clear, there is only specular reflection in the current configuration, whereas the m=0, and +- 1 orders are visible in the 180 deg flipped configuration.
Attempted two methods to soften EP30-2, the results are summarized below.
(a) Heat -- Used the heat gun set to 200 F (~ 93 C) and held it near the back of the part so that the grating surface was never in direct exposure. The airflow was kept constant for a period of ~ 10 min, while I periodically checked to see if there were any signs of bond softening. After no signs of softening, I stopped and moved to method (b).
(b) Solvent -- After brief investigation and referring to T1400711, I got some acetone from CTN, and set up a ~ 50 ml bath. The part was not completely submerged and was arranged such that the grating face was always exposed to air, which I left for ~14 hours. The drawback of this method is that some of the acetone evaporated and at some point the EP30 bond stopped being in contact with the solvent. A picture for reference is attached, with the light blue line indicating the highest acetone level, and the red line indicating the EP30 bond level at the beginning of the bath.
Used BeamR and WinCamD to profile the two light sources (ECDL and OPO pump)
(1) ECDL; profile 19** nm beam after the aspheric lens. I guess we want this beam to be nominally collimated for optimal feedback with the Littrow-configured grating, so I recorded the 1/e^2 waists (x, y) as a function of longitudinal displacement. The result is attached below. Linear fits provide rough estimates for the beam divergences, giving 2.0 mrad along x (parallel to the table) and 1.2 mrad along y (normal to the table) suggesting some astigmatism which is common in high NA aspheric lenses. I inspected the distance from the aspheric lens to the SAF gain chip and measured ~ 2.0 mm (compared to the 1.99 mm working distance specified for this lens with NA=0.71). The SAF1900S specifies a beam divergence angle of 35 deg (corresponding to NA=0.57), so there is room for improvement by tweaking the aspheric flexure alignment.
(2) NPRO; profile 1064 nm beam at low power (~10 mW) right after the head output. Having 10x more power made things way easier for this as compared to the ECDL, but the method was the same (record 1/e^2 waists as a function of longitudinal displacement). The result is attached below. Linear fits provide rough estimates for the beam divergences, giving 2.1 mrad along x, and 2.1 mrad along y. Here I grabbed the specified divergence of 2.3 mrad from a relatively old manual, and even drew the displaced waist profile (w0 = 160 um) which seemingly fit the profile, but the actual values may be different.
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).
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.
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.
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).
Analysis & 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.
Analysis and Data
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.
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.
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.
Record TF for RFPD SN09, resonant at 36 MHz, using the exact procedure as with EOM Resonant Driver.
would be good if you could find a solution that is not very sensitive to precise lens placement
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 -->
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.
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.
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?
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.
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.
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
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:
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.
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).
In order to transition the ECDL laser noise characterization to a heterodyne setup, we needed to test the AOM (acousto-optic modulator). We wanted to drive the AOM at 80MHz using the Marconi signal generator. Since the AOM has a max driving power of 600 mW, we determined that if we run the Marconi at max output power (13dBm), we saturate the AOM through a variable attenuator and a 5W amplifier. The detailed setup is in Attachment 1.
As we scanned the AOM RF input power, we monitored the mean of the 0th and 1st order power outputs using 2 amplified photodiodes on the scope. Attachment 2 plots the results of the scan; although we noticed the 0th order dropping, we did not see evidence of diffraction in the 1st order. Our suspected theory is that the lost power from the 0th order is due to thermally-driven attenuation inside the AOM (we do not know what is inside the AOM, so this is purely speculative). The next thing we want to try is to add a DC power level to the AOM RF input, but we will double check with Aidan.
We changed the setup to use a low power amplifier rather than the 5W amp from last time. The updated schematic is in Attachment 2. This is in part because 5W is an overkill to drive a fiber AOM which is known to saturate at 0.6 mW of RF input, but also because working with lower power active elements is easier and considerably safer. We dropped the 5W amp. in Rana's office last Friday, and got a ZHL-3A-sma. This little guy gives a max power output of 29.5 dBm (~890 mW) which should be more than enough while using the Marconi as our source (max output +13 dBm).
We hooked the amplifier to the load (AOM) without any couplers or attenuators in between, powered it with +24 VDC and quickly repeated a scan of the source power level while to see any sign of diffraction in the PDs. The result is in Attachment 2. We were a little bit disappointed that there appeared to be no diffraction, so next we tried scanning the RF frequency (it was nominally at 80 MHz) around and we finally succeeded in seeing some diffraction at 95 MHz! Paco thinks the internal fiber coupling made for the design wavelength (2004 nm) is suboptimal at 80 MHz and ~1.4 um wavelength. Therefore, to couple the 1st order back into the fiber, we need to shift the RF frequency to restore the diffraction angle at the cost of potentially not driving the optimal efficiency. An interesting observation made at the same time we saw 1st order light was that the power seemed to drift very slowly (-1%/min), which may have to do with some thermal drift inside the crystal... Our plan is to make a complete characterization of the diffraction efficiency at 1.4 um, and also investigate the slow intensity drifts as a function of RF input. The goal is not so much to understand and fix this last one, but to be able to operate the setup at a point where things are stable for a low frequency, frequency noise measurement.
We started rebuilding the DOPO in the lab even though the new optical table hasn't arrived. For this reason, we are using a 1 ft x 3 ft x 0.5 in anodized aluminum breadboard which we can then attach handles to move the setup. This also makes the prototype's footprint smaller. The first thing we did as usual was settle on a beam height. The beam height used before was ~ 3in (~ 75mm), and since the EOM, Faraday Isolator, and RFPD are nominally at that height from the breadboard, the only thing we had to fix was the pump laser head. The bare height is 55 mm, so we stacked two 9 mm thorlabs bases together, bolted them down to the breadboard and then mounted the NPRO laser head on the top. Finally, using a level we secured it to the breadboard using the three points and long 1/4-20 screws while being careful as we didn't want to flex the head too much.
Next up is aligning the laser to the EOM and Faraday Isolator. For this, we will use the beam profiles measured late last year. Another task ahead is to implement the new mount for the cavity.
When previously trying to characterize the AOM, we had noticed no 1st order diffraction when operating at 80 MHz, but significant diffraction at 95 MHz. This motivated us to take measurements while sweeping across both RF drive frequency and Marconi drive power. For frequency, we swept from 80-120 MHz in steps of 1 MHz. For power, we swept across [3, 0, -3] dBm (3 dBm is max power before saturating AOM). We took our measurements of 0th and 1st order signal using an oscilloscope.
Contour plots of the 0th and 1st order signals can be seen in Attachments 1 and 2, respectively. Peak 1st order diffraction seems to occur at ~106 MHz. Using this AOM for a beat note measurement, the frequency difference would be greater than intended, which could lead to a weaker beat note signal.
*Bonus: Today we moved the ECDL setup off the cryostat table and onto the other table. These measurements were taken after the move.