I brought in the instrument and components for 2um ECDL:

1. SAF Gain chip / SAF1900S / Qty2
2. Grating / GR25-0616 / Qty2
3. 3axis piezo mount / POLARIS-K1S3P / Qty2
4. Lens / 390093-D / Qty2
5/6 Thorlabs small components / F3ES20, F3ESN1P / Qty2 ea
7 N/A
8~13 Machines Metal Components / D1900435, D1900429, D1900433, D1900432, D1900430, D1900434 / Qty 2ea
14~17 McMaster Carr fastners / 92196a192, 92196a110, 92196a079, 92196a081 / Qty 100 ea
18 Temp Controller / TED200C / Qty 2 Note One unit temporary used by 2um PD test setup
19 Laser Current Driver / LDC220C / Qty 2
20 Piezo Driver MDT694B / Qty 2

I entered Crackle lab circa ~11:15. I started some basic lab inventory and started cleaning/organizing stuff. We will use the first optical table (as you enter the lab) because it's the easiest to clear (see below before and after clearing). Some of the cleared items on the table include:

- UHV foil (moved to top left cabinet above the work bench)
- OSEM components for Crackle (?) (moved to top left cabinet above the work bench)
- Various metallic parts/components (moved some in a plastic container to the second drawer from the bottom of the second red tool storage, and others to the second optical table)
- Various screw/screwdriver kits (moved some to work bench right by the electronic storage area and others to the second optical table)
- Power supply and laser diode driver (moved to control/acquisition rack)

I then moved the 1064nm pump Innolight Mephisto 800NE to the clear table, clamped it down, and cleaned/organized the lab a little, which involved:

- Shelve orphan/incomplete PCBs and electronic components from the work bench up to the cabinets.
- Organized some cables by the fume hood.
- Organized other random hardware on the work benches. 

I found the Emergency STOP (OMRON STI #A22EM02B) button buried on the fume hood, so I gave it a quick test, and after confirming it worked I wired it to the interlock on the back of the laser controller. Then tested it along with the interlock and verified it's working, but I have yet to solder it nicely (I didn't commit to the wire lengths yet).

Left at ~ 14:45. Noted that we had more cockroaches in the floor at the beginning of the day than 2 um laser sources. Now we are tied.

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;

a=1.129241e-3

b=2.341077e-4

c=8.775468e-8

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.

Attachment 1: Black Diamond (GeSbSe) Lens was mounted on the flexure mount. The flat surface should face to the gain chip. It was aligned on the wipe to be flush with the protrusion.

Attachment 2: Applied glue on the four grooves of each flexure mount.

Attachment 3: The grating was bonded on the mount. The arrow marks were arranged as Paco directed. The mount could not stand by itself. And the screws were placed to stop the grating skating on the mount.

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

2: Thermistor

3: Thermistor

4: TEC+

5: TEC-

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.

The gratings and aspheric lenses glued on the mounts were delivered to the lab on Thu.

The powermeter controler + S401C head was lent from OMC Lab. Returned to OMC Jul 15, 2020 KA

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.

Log of the output power vs current in the 1064 nm (Innolight) pump laser. The crystal temperature was set to 45.5 C, and the current limit is set to 2.1 A

I placed the two beam profilers with the two laptops and chargers right inside the Crackle lab, as requested by Paco.

Note: Please don't try to connect these old Windows to the network. We just extract the data via USB etc, and that's all the connection we allow.

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.

> Temp Controller / TED200C / Qty 2 Note One unit temporary used by 2um PD test setup

I brought the brand new TED200C from QIL to Crackle (Permanent move).

The unit used for 2um PD test setup will stay in QIL (Permanent)

Shruti took back the beam profilers today AM to Cryo.

Shruti: returned to Gabriele's office

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

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.

Borrowed 1 (new focus) broadband EOM from CTN for temporary use in Crackle (2 um OPO exp)

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).

Measurement details:

• Connected New Focus 4004 broadband IR phase modulator to the output of the SN06 EOM driver.
• Splitted R output of AG4395A. One end went to the input of SN06 EOM driver.
• The other end goes to input R of Ag4395A.
• The RFMon of the driver is fed to input A of AG4395A.
• Used a tuning stick for the coilcraft inductor to tune the resonance peak to 36 MHz.
• Took measurements with the configuration files in this directory.

Data

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.

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.

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.

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.

Received one Marconi 2023A (#539) from CTN and an SRS FS725 Rb clock. (See CTN/2605)

• Took a delay line box DB64 from QIL from the WOPO table. The box was marked Crackle on it.
• Took the compressed nitrogen cylinder for optics cleaning which was stored in Adaptive Optics lab.
• Took some lens from the cabinet in Adaptive optics lab.
• Took some other optics parts like pedestals, posts, lens mount etc.

QIL elog entry: QIL/2524

Photos, please, because we don't allow a free-rolling cylinder in a lab.

Borrow both beam profilers and laptops from WB 264A.

Covesion order arrived, containing 2x

• Crystal oven (20 mm long) (below)
• Clips (for mounting crystals) (below)
• Blank crystal (to press on the ppktp crystal) (below)
• OC2 oven controller
• Controller cable and power cable

• This is a repetition of SUS/1864.
• Used Data Ray Beam'R2-DD.
• Took 50 averages and recorded beam "diameters" at 10 different points after the laser head.
• Configuration file is BeamProfileConfiguration2um.ojf.
• Used fitBeamWidth function of ala mode to fit X and Y beams separately and then their geometric mean.
• We'll use the geometric mean as the seed profile for future calculations.

Data

• After installing a 400mm focal length plano-convex lens at 24" from the laser head at (20, 12), we found that higher-order modes are present in the beam.
• We installed an iris at 34" from laser head at (17, 5)
• Configuration file is BeamProfileConfiguration2um.ojf.
• Used fitBeamWidth function of ala mode to fit X and Y beams separately and then their geometric mean.
• We'll use the geometric mean as the seed profile for future calculations.
• Found a beam waist of 306 um at 58" from the laser head.
• Installing the EOM between 49" and 52" from the laser head where the beam waist is between 1 mm and 740 um.

Data

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).

Goals and restrictions:

• Use the fewest lenses as possible.
• The beam widths in both onward and reflection direction should be such that there is a 5-inch space somewhere where we can put in the faraday isolator which has an aperture size of 3 mm and intensity limit of 500 W/cm².
• The lens should not be closer than 1.5 inches from each other or to the EOM mount or cavity edge.
• Choose a lens from a list of focal lengths available in west bridge labs.
• Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.

Analysis & Results

• CavityLens.m is run to try all possible lens combinations for 2-lens or 3-lens solutions using ../20201222_BeamProfileNeatEOM/SeedBeam.mat as the seed beam.
• Then save solutions with more than 70% overlap in CavityModeMatchingSolutions.mat.
• findBestSolutions.m increases the overlap threshold to 0.9, calculates reflected beam profile for the sideband reflection from the cavity (blue curves in the figures), and discards solution which does not have a 5-inch long area where we can place a faraday isolator with aperture of 3 mm.
• Black lines show the region where Thorlabs IO-3-1064-HP can be placed safely without clipping or exceeding the intensity limit with 1W power.
• All solutions in order of sensitivity are plotted here with details of lens choice and positions. In total 7 solutions were found which are stored in BestSolutions.mat.

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:

• Oven clip
• Oven
• ITO crystal spacer
• PPKTP crystal

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.

Goals and restrictions:

• Use the fewest lenses as possible.
• The beam widths in both onward and reflection direction should be such that there is a 5-inch space somewhere where we can put in the faraday isolator which has an aperture size of 3 mm and intensity limit of 500 W/cm^2.
• The lens should not be closer than 1.5 inches from each other or to the EOM mount or cavity edge.
• Choose a lens from a list of focal lengths available in west bridge labs.
• Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.

Analysis & Results

• CavityLens.m is run to try all possible lens combinations for 2-lens or 3-lens solutions using …/20201222_BeamProfileNeatEOM/SeedBeam.mat as the seed beam.
• CavityLens4.m is run to try all possible lens combinations for 4-lens solutions using …/20201222_BeamProfileNeatEOM/SeedBeam.mat as the seed beam.
• Then save all possible solutions (in which placing a Faraday isolator is possible) with more than 90% overlap in AllPossibleSolutionsAbove90.mat.
• findBestSolutions.m increases the overlap threshold to 0.97 and plots the best solutions in order of positional sensitivity of the lens. These are stored in BestSolutions.mat.
• Black lines show the region where Thorlabs IO-3-1064-HP can be placed safely without clipping or exceeding the intensity limit with 1W power.

Analysis and Data

Wed Jan 6 10:00:35 2021: This analysis was wrong. See SUS_Lab/1887.

Errors in the previous analysis

• Previously, we wrongly assumed. that the reflected light from the cavity would be as if a reflection is happening from a flat mirror. It actually follows the same paths as incident light in the reflection path.
• Lens were restricted to non-overlapping regions but that meant that solutions where lens are close to each other can only happen near the boundaries of these regions. Removing this condition widens the search for a good solution.
• We collimated the beam to near 0.5 mm radius with a 229.1 mm focal length lens at 67" from laser head and put faraday isolator in front. So now the problem only remained to match the mode after this point to the cavity mode.

Goals and restrictions:

• Use the fewest lenses as possible after having used a fixed lens at 67" point before the faraday isolator.
• Choose a lens from a list of focal lengths available in west bridge labs.
• Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.
• The lens should not be closer than 1.5 inches from each other or to the EOM mount or cavity edge.
• The beam widths should not exceed 4mm in diameter anywhere to ensure small areas of lenses are used.

Analysis & Results

• CavityLens.m is run to try all possible lens combinations for 1-lens or 2-lens solutions using ../20201222_BeamProfileNeatEOM/SeedBeam.mat as the seed beam.
• CavityLens3.m and other versions of it are run to try all possible lens combinations for 3-lens solutions using ../20201222_BeamProfileNeatEOM/SeedBeam.mat as the seed beam.
• Then save all possible solutions with more than 90% overlap and where lenses are atleast 1.5" away from each other and the cavity in AllPossibleSolutionsAbove90.mat using findPossibleSolutions.m.
• findBestSolutions.m increases the overlap threshold to 0.995, allows maximum beam radius of 2mm anywhere and plots the best solutions in order of positional sensitivity of the lens. These are stored in BestSolutions.mat.

Analysis & 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,

1. Installed the oven, plugged it to the controller and went to the nominal temperature setpoint (40 C) to match the expected path length inside the NL crystal
2. Placed the output coupler (roc = 15 mm) and roughly align so that the retroreflection is overlapped with the input beam.
3. Set up a PD (Newfocus 2001) and scope, operating the laser at relatively low power (current ~ 840 mA), and optimize the FI rejected power.
4. The output coupler is mounted on a three axis mirror mount (Polaris, hoping to get low drift) such that we have some knobs to tune the mode matching initially.

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.

Error in previous calculations:

• We did not take into account the effect of cavity input mirror on the coupled light. It would act as a thick concave lens for the coupled light into the cavity.
• We did not take into account the divergence due to refraction at the crystal surface.

Goals and restrictions:

• Use the fewest lenses as possible after having used a fixed lens at 67" point before the faraday isolator.
• Choose a lens from a list of focal lengths available in west bridge labs.
• Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.
• The lens should not be closer than 1.5 inches from each other.
• The beam widths should not exceed 4mm in diameter anywhere to ensure small areas of lenses are used.
• Take into account the concave lens due to the input mirror.
• Take into account the refraction due to crystal surface.

Analysis & Results

• CavityLens.m is run to try all possible lens combinations for 1-lens or 2-lens solutions using ../20201222_BeamProfileNeatEOM/SeedBeam.mat as the seed beam.
• The cavity input mirror is modeled as two refracting surfaces separated by 6.5mm. The first surface is flat while the second has ROC of -25 mm.
• The crystal is modeled as two refracting flat surfaces separated by 20 mm.
• The target beam waist is kept at the center of the crystal with 35.578 um diameter.
• Then save all possible solutions with more than 90% overlap and where lenses are atleast 1.5" away from each other in AllPossibleSolutionsAbove90.mat using findPossibleSolutions.m.
• findBestSolutions.m increases the overlap threshold to 0.995, allows maximum beam radius of 2mm anywhere and plots the best solutions in order of positional sensitivity of the lens. These are stored in BestSolutions.mat.

Analysis & Data

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.

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).

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 -->

1. Line noise (hunting for ground loops or equipment, e.g. power supplies, analyze LO spectrum before/after splitters, mixers, etc...)
2. Mode matching (this was the first reaction, as improving the cav refl SNR by means of mode matching makes a better pdh err signal)

Other things attempted so far -->

• Switched mixers, splitter, and RF cables
• Bypass the phase shifter completely
• Scan LO phase
• Floated RFPD power supply
• Floated PDH box power supply (really only affecting the phase shifter if anything, though unlikely to matter at this point)

Methods

• Use the actual measured beam profile in X and Y directions.
• Propagate them with the current position of lens.
• Assume the position of cavity mirror and crystal as given by the second solution in BestSolutions.mat in the Jan 8th analysis which is implemented currently.
• Calculate the overlap with target and position of waists in X and Y direction.

Conclusions

• Astigmatism should not be hurting us significantly.
• The mode matching in principle can be improved in the experiment.

Analysis

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.

Demodulation stage

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).

Lock

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.