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?
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...
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
- 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.
With Aidan's assistance, I borrowed
for ~ 2 um imaging in the Crackle lab.
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.
- 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.
- 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.
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.
[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?
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.
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.
Should measure the S-matrix using a bi-directional coupler.
Today we tried to pick up from  by repeating the sweep measurements across RF frequency, at 3 dBm (max power). We noticed that the 0th order signal would dip around the expected value, consistent with the plot in . However, there was no signal from the 1st order. Clearly diffraction was occurring as seen by the dip in 0th order, but nothing was coming out of the 1st order port. We spent some time debugging by swapping the photodetector inputs / playing with the PD gains / performing power cycles, but got no insight into the issue.
We suspected the 1st order fiber coming out of the AOM might be damaged, since it loops around fairly tightly. After giving it more slack, we still saw no signal. We wanted to test the fiber, so we took an unused output of the 50-50 beamsplitter and fed it into the 1st order port, effectively running the AOM in reverse. We hooked up the input and 0th order ports to the photodiodes and did not observe any signal. From here we were more convinced that the 1st order fiber may have seen some damage.
For next steps, we can still use the existing fiber setup to take measurements of relative intensity noise (RIN), using the 0th order output of the AOM. I plan to do this in the next few days. Meanwhile, Paco is looking into ordering parts for a free space setup. We found a free-space AOM at 1064nm that seems promising, and we will work to transition the setup accordingly.
I took a relative intensity noise (RIN) measurement of the ECDL, by feeding the 0th order output of the AOM to the SR785. The RF power driving the AOM was set to 0 dBm. The RIN at 1 Hz is about 3x10-5, which is consistent with informal measurements we took on 08/13. From my understanding this noise looks pretty low, which is good. I will consult with Paco and add more discussion or conclusions, if any.
Uninstalled the fiber AOM and temporarily removed the third fiber 2x2 port beamsplitter. We are now using this free-space AOM. Then, I managed to launch one of the outputs of the second fiber beamsplitter into free space using a F220APC-1550 fixed collimator. The beam clears the AOM aperture nicely and lands on the other side.
This AOM operates at a RF frequency of 35 MHz, so we set up a sweep on the Marconi to cover a window of 35 MHz +- 15 MHz. Using an IR detector card, we looked for evidence of 1st-order diffraction (from the setup geometry, the 1st order beam should have been visibly discernable). We first scanned the AOM across yaw but did not notice diffraction. Then, Paco lowered the height of the fixed collimator and we repeated scanning across yaw. We eventually saw the beam "jump" - diffraction! We adjusted yaw until we recovered both 0th and 1st order beams, at 50/50 intensity.
In summary, the free-space AOM works and we have managed to see 1st order diffraction. Next steps will be to quantitatively measure this diffraction while sweeping across RF frequency and power.
We had previously noticed that the ECDL laser power seemed weaker compared to when we originally set it up and tested it. Today Paco opened it up and tweaked the grating inside to obtain a max power of 3 mW. This way, we could better resolve the 0th and 1st order beams coming out of the AOM.
Since we don't yet have a lens to send the collimated 1st-order beam to fiber, we connected a power meter to detect the beam and hooked it up to the oscilloscope. We noted peak diffraction at around 38.5 MHz (rough estimate). Using the inverse relationship between laser wavelength and the RF frequency , and the fact that the AOM is designed to operate at 1550 nm at 35 MHz, we calculated that the ECDL wavelength should be ~1409 nm. Of course this is a rough estimation, but it is a quick validation that we are indeed operating near 1418 nm.
Last Friday we received a new lens to direct the AOM 1st-order beam from free space into a fiber cable. We mounted the lens and connected a fiber cable into the photodiode, and tried to align the lens and see a jump in the oscilloscope. We were not able to do so and wrapped up for the day.
Today we continued aligning the lens with the fine adjustment on the mount, and eventually saw signal on the scope! Hooray, done with free space. We then prepared for eventually taking a heterodyne beat note measurement and hooked up the appropriate inputs/outputs to the beamsplitters. We added in the 50-50 beamsplitter that takes in the 1st order diffracted beam along with the beam from the delay line as inputs. We passed one of the outputs to the photodiode and had to retweak the freespace-to-fiber lens until we recovered signal on the scope, and we saw the beatnote signal.
Next, while Paco is out of town I will continue to work towards making a frequency noise measurement. We made a roadmap today:
I will demodulate the beat note using a mixer and a 35 MHz LO sourced from the Marconi. The result will be a 2f cosine term, along with a much lower frequency term which encloses the frequency noise information. This will be passed through a low-pass filter to get rid of the first high-frequency term. The remaining time-domain signal will be passed to the SR785 to obtain a spectra of the frequency noise. Calibration will need to be performed to obtain the right units for the spectra, Hz2/Hz (or Hz/rtHz).
I took spectra of the resulting signal using the SR785 (Attachment 2). Note that these units are still in V/rtHz, since the signal has not been calibrated to the appropriate units for frequency noise, Hz/rtHz. Finding the calibration term will involve study of delay line frequency discrimination.
Restarted ECDL characterization last Friday. After some lab cleanup, and beatnote amplitude optimization we borrowed Moku Lab from Cryo lab to fast-track phase noise measurements. Attachment #1 shows a sketch of our delayed self-heterodyne interferometer. The Marconi 2023A feeds +7 dBm to a ZHA-3A amplfier which shifts the frequency of the laser in one of the arms using a free space AOM. The first order is coupled back into a fiber beamsplitter to interfere with a 10 m delay line beam.
The 38.5 MHz beatnote was barely detectable before when using PDA20CS2 because at unity (lowest) gain stage, the bandwidth was only 11 MHz... We instead switched to an FPD310-FC-NIR type which has a more adequate high-frequency response. Attachment #2 shows the beatnote power spectrum taken with Moku Lab spectrum analyzer. The two vertical lines indicate (1) the heterodyne beatnote frequency and (2) the "free spectral range" indicating the actual delay in the MZ arms, which is calibrated to = 9.73 m (using 1.46 for n, the fused silica fiber index).
We then tried using the phase meter application on the Moku. The internal PLL automatically detected the 38.499 MHz center frequency and produced an unwrapped RF phase timeseries (e.g. shown in Attachment #3). The MZ interferometer gives an AC signal
oscillating at , i.e. the angular beatnote frequency. The delay (calibrated above) characterizes the response of the MZ relating the RF phase noise spectrum to the optical phase noise spectrum. The RF phase obtained through the phase meter has a fourier transform
So the optical phase spectral density is related to the rf phase spectral density by a transfer function Then, the RF & optical phase power spectral densities are related by or
Then, because the instantaneous laser frequency is , in fourier domain the frequency and phase PSDs are related by the magnitude square of this transfer function like
Following this prescription, we compute an estimate for the frequency noise ASD (square root of the PSD) shown in Attachment #4. The frequency noise estimated by this method has several contributions and *does not* necessarily represent the free-running ECDL frequency noise.
Following Koji's request, I took some time to clear the area surrounding the crackle chamber so it can be migrated to the former TCS lab.
I moved the DOPO setup which was sitting on a breadboard for easy transportation (Attachment #1) and placed into the other table in the lab. Attachments #2-3 shows the cleared area. Several instruments from the DOPO experiment still remain around the other side of the crackle chamber, if they need to be relocated I can move them as well.