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 Cryo Lab eLog, Page 36 of 59 Not logged in
ID Date Author Type Category Subject
338   Fri Nov 4 17:24:06 2011 DmassLaserLaserPLANEX retesting

After some unintentional (possibly mild) abuse I retested the diodes.

PLANEX 102068:

• 4/11/11
• Rth = 8.998 k
• Iset = 104.9 mA
• Pmeas = 12.90 mW
• 10/21/11 (elog:313)
• Rth = 8.997 k
• Iset = 105.1 mA
• Pmeas = 12.82 mW

PLANEX 102085:

• 4/11/11
• Rth = 8.109 k
• Iset = 135.1 mA
• Pmeas = 13.4 mW
• 10/21/11 (elog:313)
• Rth = 8.114k
• Iset = 105.1 mA
• Pmeas = 13.04

I contacted an engineer at RIO and he said if the diodes had roughly the same power output and linewidth, then it was reasonable to assume they were undamaged.

I also got the beat back on the PD at similar settings to what I had before, and it showed similar coherence time (plot 1 in elog:320)

EDIT BY DYM: The LM14S2 has a reverse bias protection diode in it, so it was not possible for me to screw them up like this. Everything is fine, move along.

340   Tue Nov 8 02:24:40 2011 DmassLaserLaserPLANEX retesting

Further update in clif note form:

• Diodes are fine according to the LM14S2 manual, there is a reverse bias protection diode which should kick in if you plug it in wrong
• Transistors for the slow start still shipping
• Talked to Koji and came up with a slow start mod to the board with a switch on the front (and a ~12 second pole for on / off)
• Tested slow start switch and I get maximum slew rates of ~30mA/sec (on and off, though slightly different for each)
• Manual for diodes says 10 mA/sec - emailed RIO engineer
• Redid the cables which were wrong for diode driver
• Sleeping on it for a day before I plug them in to help no stupididty happen

Ready to plug in tomorrow and see if diodes get all quiet like!

342   Thu Nov 10 01:37:53 2011 DmassLaserLaserPLANEX retesting

Fired up diodes.

Slew rate limit seems to work nicely.

Fast measurement linewidth time seems small.

Linewidth still dominated by low frequency fluctuations (over a few seconds i get a broadening to ~500 kHz on a 4395 via averaging)

Low frequency fluctuations likely environmental - I think I need to:

• A) give the temperature feedback a little more love
• B) Build some enclosures for the little diodes

The PLL *sort of* works - I don't have enough bandwidth currently. I can get the signal to follow my twiddles on the marconi carrier frequency for ~300 kHz, but the range should be 3.2 MHz. When I try to increase the gain of my loop via 560, the PLL doesn't lock. I do not yet understand this.

I am changing my PD tomorrow to something slightly less unintelligent, then will plug everything back in and see what I get. If I still can't get the PLL to lock, b/c of the low frequency noise, I will grab a frequency counter and see if my low frequency noise seems consistent with the RIO data.

WILL MAKE THESE PLOTS:

• Temperature noise spectra for each diode
• Linewidth of laser at a few time scales / averaging factors (HP4395)
• Scope trace of *new* beat
• Frequency noise calculated from the scope trace (easy)
• Pictures of PLL discriminator getting *barely locked*

347   Mon Nov 14 13:53:42 2011 DmassLaserLab WorkPLL details

Some details on the phase lock loop I set up in 050 for the PLANEX lasers.

The Setup:

I lock PLANEX 102085 to PLANEX 102068 with a ~40 MHz offset in frequency by modulating the current on the 102085

Notes:

• PLANEX 102068
• Iset = 104.9 mA
• Rth = 9.384k
• PLANEX 102085
• Iset = 122.3 mA
• Rth = 8.145k
• I initially tried to lock the PLL with just proportional feedback to the frequency actuator (1/f loop) but did not have enough low frequency gain before my servo oscillated
• I added another 560 as low frequency boost to increase the low frequency gain (and therefore range) of my servo
• Was able to see better low frequency suppression of error signal using the low frequency boost
• With just proportional gain (no boost), my loop oscillates around 157 kHz when I turn the gain up too high (measured on HP4395)
• When I turn the gain down so that the oscillation disappears, the loop can not hold lock for very long (~seconds)

355   Tue Nov 15 02:11:28 2011 DmassLaserNoise BudgetPLANEX FREQ NOISE MEASURED

As promised in elog:350, I made a comparison of the frequency noise measured by my PLL and the frequency noise RIO specifies for their PLANEX (elog:331).

It seems I'm sitting on the PLANEX noise as measured by RIO below 1kHz and the measurement is heavily limited by gain peaking of the UGF starting around 10 kHz.

The blue trace if my calibrated PLL control signal.

The red trace is a "by eye" fit to the plot in based on:

• 10 Hz/rtHz at high freq
• 100 Hz/rtHz at 200Hz with a slope of 1/sqrt(f) => 1400/sqrt(f)
• red trace = sqrt(1.96e6 / f + 100) [Hz/rtHz]

358   Tue Nov 15 15:04:46 2011 DmassLaserLaserLaser Sidebands

I found some sidebands on the beat which I don't recall existing before today. They are ~1 MHz out, and ~30 dB down from the carrier as measured by the HP4395 (spectrum mode), with a 30kHz VBW/RBW.

Turn on procedure:

• Turn on TEC loops (controlled by Thorlabs driver still)
• Set Rth for each laser, wait for it to stabilize:
• 102085:
• 102068:
• Power up each laser with the "slow start" switch
• Tune beat slowly via temperature knob

I may be experiencing mode hopping, which happens for the diodes at:

• 102085:
• 102068:

I will try powering them up a few times, with slight variations on this

I restarted the driver box, and did a "anti-mode hop" start to the diodes:

• Turn on TEC loops
• Set Rth a little higher than its operating point for each diode
• Rth = 9.5k => T = ____
• Turn on diode current
• Slowly tune Rth down (T up) to desired operating point

The sidebands went away......and then they came back on their own.

477   Fri May 11 15:45:29 2012 DmassLaserGeneralNumbers Needed

I need to get the time constant for the laser diode TEC. Going to do this with a simple step response (step current to TEC, look at resistance of 10k thermistor) - I may be able to use the Thorlabs ITC510's to do this.

Rich is thinking about how to do the temperature feedback to the Diodes in some "not completely mickeymouse" way while he is in FLA this week.

577   Tue Sep 11 16:25:38 2012 DmassLaserstuff happensLaser On, TEC off?

When I went to move one of the lasers to the PSL lab bench from teh cryo lab, and noticed an error on one of the temperature controllers.

The current switch had been hit to "on" by someone working around the rack at some point. I do not know how long it was like this. I promptly turned them off, and am now going to test to make sure both lasers still lase. In retrospect, I probably should have checked to see if a beam was coming out first.

The diode which was set to "on" was 102068

I turned them on, let them warm up for 15 minutes, and measured the output power.

Testing (compare with elog:338)

Diode 102068

• R_therm = 9.002 k
• I_set = 105 mA
• P_meas = 12.98 mW

Diode 102085

• R_therm = 8.106 k
• I_set = 135 mA
• P_meas = 9.96 mW
• Old recorded value at these parameters is P_old ~ 13mW

The above is not good. It should be higher.

Investigating further in the next elog.

578   Wed Sep 12 03:14:35 2012 DmassLaserstuff happensLaser On, TEC off?

Measured the hysteresis curves for the diodes as a function of temperature. This will be my basic benchmark for monitoring state change for now.

Manual data entry, so the rest of the plots following tomorrow.

I will also turn R_thermistor into temperature.

Attachment 1: toelog1.png
Attachment 2: toelog2.png
605   Mon Nov 19 02:26:01 2012 DmassLaserCavitySi Cavity flashes

I managed to align close enough to the Si cavity to see flashes:

video

More details to follow!

606   Tue Nov 20 08:10:18 2012 nicolasLaserCavitySi Cavity flashes

sweet

608   Tue Nov 20 14:36:48 2012 DmassLaserCavitySi Cavity flashes

Below is a picture (+ beams) of what I put on the table to do cavity scans. The mode matching lenses are what Nic called out in elog:603.

Cavity flashes can be seen in the video in elog:605

I spent a little while trying to get a clean cavity scan in reflection (I want to see the dip in power from the 00), but got nothing which resembled a lorentzish dip. (Noisy crap)

I scanned frequency via the laser current, using the ITC510 unit to control the diode.

Scan:

• Triangle wave ~10 mA from 1 Hz -> 1 kHz
• 40 MHz / mA at diode
• => ~400MHz sweep
• Tuned temperature so that 00 flashes on camera were near middle of scan
• Super Duper noisy, with the refl PD oscillating / spiking as I sweep through 00

I will post sweeps when I can get them off this fine floppy disk format onto my computer and matlab them into readable plots.

The relevant time constant for filling the cavity with light is:

• Finesse ~20k
• Length = 8"
• FSR = 1.5GHz
• LW = 1.5GHz/20k = 75kHz
• 20cm * 20e3 / 3e10 cm/s  = 13 us
• 75kHz/13us = 5MHz/ms

so if we pass through the cavity at 5MHz/ms, we are sweeping through the cavity on the order of its filling time

Sweeping at 400MHz:   400MHz * ms / 5MHz = 80 ms. Since it's a triangle wave, period = 160 ms, or f _sweep > 6Hz

I do not understand the behavior, I am sure there is something simple I am missing.

Attachment 1: cav_scans.png
613   Wed Nov 28 16:22:27 2012 DmassLaserCavitySi Cavity flashes

I am naming the cavities by their mirror serial numbers. We have:

• Cavity1934 (mirrors 0019 and 0034)
• Cavity1621 (mirrors 0016 and 0021)

I have convinced myself that Cavity1934 is good, but think that 1621 might need a redo. The story:

=============================

Assuming zero loss on the mirrors, I recall we designed the cavities to have ~20k finesse. As mentioned before, this corresponds to a ~13us fill time (just effective path length).

1. Aligned to Cavity1934 so that I saw mostly 00 and 01 modes on the camera
2. Put a beamsplitter on the transmission (I borrow an unused 1064 nm one from the ATF and checked that it transmitted a reasonable amount of 1550, ~25%)
3. Aligned onto a PD (PDA50B from thorlabs - 400kHz BW // ~800ns rise time)
4. I played around with sweep speed and amplitude
• SR DS335 function generator to make a triangle wave
• this into the "analog sweep" input of the ITC510 with a voltage divider on the input
• Calibration of the setup is ~6mA / Vpp
5. I tuned the alignment by making the crappy forest of transmitted 00 flashes "higher" on average
6. I couldn't get a clean transmitted peak with slow sweeps (frequency+length noise presumably a bit high), so I cranked the sweep frequency until I saw something smooth
7. I got a smooth (repeatable) asymmetric tranmission by sweeping through the 00 a bit faster than the decay time of the cavity.
1. 2Vpp // 500 Hz triangle wave (12 mA @ 500 Hz)
2. ~7us time constant on the smooth part of the ramp up
3. ~15 us tau for cavity decay
8. I tested at a slowed sweep speed (200 Hz), and saw the same dynamics
9. For some (possibly naive) reason, I think that the transient decay of the cavity can give me a number for the cavity time constant
1. This number was 15.3 +/- 1.1us
10. If this is stupid for some reason, I think that I can *at least* average the ramp up and ramp down to get a rough idea of finesse
1. ramp up tau ~ 7us => 7us < tau_cav < 15 us
2. If this IS just the time constant of the cavity, we get finesse by:
• 1/tau = f_pole = 90 kHz ==> Finesse ~ 1.5GHz / (2 x 90kHz) = 8300
3. This does not seem like a completely ridiculous number.

I repeated the process for cavity1621, and was able to get flashes / some sort of "ok" alignment, and when I sweep VERY FAST and average like the Dickens, I get a cavity pole of ~22 MHz, or a finesse of 33.

I tried:

1. Realigning
2. Using the same sweep parameters from cavity1934's measurement - was unable to get anything resembling that measurement
3. Slow and fast sweeps (up to 12mA at 4kHz with a triangle wave)
4. Restarting the laser multiple times in hopes that I had found a "bad operating region" and mode hopped accidentally

Either I am missing something, or I need pop off the mirrors, clean everything, and reassemble cavity1621. I will crowdsource ideas for what I could be doing wrong shortly.

[EDIT: I no longer trust any of the red text - I was using the ITC510 to do the sweeps, and now believe that it was responsible for the crappy ungrokkable transient behavior I saw. I moved to using the ITC510 *just* for temperature control, and Rich's nice current driver to do the current supply / sweeps, and was rewarded with things that looked like transmission peaks]

614   Wed Nov 28 20:33:21 2012 ranaLaserCavitySi Cavity flashes

IF we have measured the reflectivity of the mirrors, there's no reason for the Finesse to be anomolous; the amount of unforseen losses that we get from dirty surfaces is not large compared to the transmission of the mirrors in a F=20k cavity.

Take a look at the 40m measurements of the PMC finesse or the measurements of the RefCav finesse which Yoichi did a while ago.

616   Fri Nov 30 03:16:41 2012 DmassLaserCavitySi Cavity scans

I redid the scans for both cavities today with a brief break for the crazy air leak.

They look non-flaky now: I blame the ITC510 sweep mechanism / current noise for the previous mickeymousery. I will fit and post them tomorrow.

617   Sun Dec 2 23:51:51 2012 DmassLaserCavitySi Cavity scans

Attached are the cavity scans.

I used a function generator to make a 100Hz 1Vpp triangle wave, and drive the modulation input of the current drivers.

The calibration of the sweep is:

(Sweep speed) x (current modulation input) x (laser diode current to frequency)

(1 Vpp / 5e-3 s) x (1 mA / V) x (0.31 pm / mA) x (1.94e14 Hz / 1550nm) = 7.76e9 Hz / second

I took a few traces for each cavity, and fit an Airy function to each one using fminsearch. Relevant MATLAB code:

xvalz=linspace(0,200e-6,1e4); %time
[XXX2,FVAL2]=fminsearch(@(X) sum(abs(...
transpose(TEK00002(:,2))-X(1)./(1+X(2).*sin(16.2.*xvalz-X(3)).^2))),...
[3 1e8 1.1e-4*16]);

The X(2) in the above is the coefficient of Finesse, its just 1/sqrt(F)*FSR to get the cavity HWHM.

The 16.2 is: pi x 7.76e9 Hz / sec x 1 / (FSR = 1.5GHz)

For cavity 1621, the four measurements of HWHM that this gives us are [1.5519e5  1.5570e5  1.6110e5  1.5525e5] Hz

I will use the mean of these as my 1st measurement of the cavity pole.

For cavity1621: f_pole = 1.57e5 +/- 2.9e3 Hz

===============================================================

I repeated the process with cavity1934

The five HWHM measurements for cavity1934 are  [1.6388e5  1.4913e5  1.4889e5  1.5713e5  1.5277e5] Hz

For cavity1934: f_pole = 1.54e5 +/- 6.3e3 Hz

The variance in the 2nd set of measurements was a bit bigger.

I have no idea what the systematics are here, or why the sweeps are asymmetric. I do not believe that these are actually 2-6% numbers, but I think "good enough" is the word of the day.

I will put them in the cryostat and close up today

Attachment 1: sweepz.png
Attachment 2: sweep1934a.png
Attachment 3: sweep1934b.png
676   Tue Feb 5 00:02:25 2013 DmassLaserLaserDIrect FM modulation sidebands

A slightly more detailed summary of problems and solutions that came up while trying to directly modulating the laser current.

The Marconi and the Current driver shared a ground (drawn in blue) - the "ground isolation" (2) addition was not initially present. When I plugged everything in without this addition, the current from 5 was shorted straight to the ground of the marconi, so the diode drew no current.

I decided to put a capacitor in the return path to block this short. I chose the value 1 nF based on:

1. Make the LC crossover to be relatively high compared to the bandwidth we want the current driver to operate over (up to ~MHz)
2. Make the impedance of the capacitor relatively small compared to 50 Ohms at the sideband frequency (33.5 MHz)

I am also making an LC resonance with the ground connection, but I seriously doubt this has any appreciable Q

If I ignore the real part of the L and C, the capacitor only effects the amplitude of the current drivers response and not the phase.

a 1nF capacitor has an impedance of 4.5 Ohms at 33.4 MHz, so it only has a small (~10%) effect

I also worried slightly about shooting the current from the current driver into the Marconi (~100 Ohms + a 3nF cap vs a 4 uH inductor) -> this crosses over around 4 MHz. To be careful about this (not shown), I put a 20 dB attenuator in between the marconi and the diode (as well as a splitter for the PDH locking); the response (gamma / RF power) was large enough that I only needed -12 dBm to get beefy sidebands, and I had some headroom from the Marconi (13 dBm - 6 dB - 20 dB = -12 dBm)

Here is a picasa album with pictures of parts 2, 3, and 4 from the schematic below, as well as how it all fits together

Attachment 1: imodsidebands.png
739   Thu Apr 25 14:47:23 2013 DmassLaserLaserLaser Tuning Coefficients

Because I have no elog containing all the tuning coefficients for both the lasers in one place in the right units. See elog:313

Converting:

• laser m->Hz  = (1.934e14Hz) / (1550nm) = [1.25e20 Hz/m]
• Thermistor (elog:48) : kOhms -> Kelvin = (1.25K / 500 Ohms) | (@8.5k-9k)
• Thermistor (elog:48) : kOhms -> Kelvin = (1.50K / 500 Ohms) | (@7.5k-8k)

102068 (operating at 9k = 27.8C, 105 mA)

• FSR ~1.51 GHz
• dLambda/dT = 25 pm/K
• df/dT = (25 pm/K) * (1.25e20 Hz/m) = 3.125G Hz/K
• Kelvin/FSR = (1.51GHz/FSR)/(3.125 GHz/K) = 0.483 K/FSR
• Ohm/FSR ~ (0.483 K/FSR) * (500 Ohm / 1.25 K) = 193 Ohms/FSR
• dLambda/dI = 0.31 pm/mA
• df/dI = (0.31 pm/mA) * (1.25e20 Hz/m) = 38.75 MHz/mA
• mA/FSR = (1.51 GHz/FSR) / (38.75MHz/mA) = 39 mA/FSR

102085 (operating at 8.11k = 30C, 125 mA)

• FSR ~1.51 GHz
• dLambda/dT = 27 pm/K
• df/dT = (27 pm/K) * (1.25e20 Hz/m) = 3.3375 GHz/K
• Kelvin/FSR = (1.51GHz/FSR)/(3.3375 GHz/K) = 0.452 K/FSR
• Ohm/FSR ~ (0.452 K/FSR) * (500 Ohm / 1.25 K) = 180 Ohms/FSR
• dLambda/dI =  0.19 pm/mA
• df/dI = (0.19 pm/mA) * (1.25e20) = 23.75 MHz/mA
• mA/FSR = (1.51 GHz/FSR) / (23.75 MHz/mA) = 64 mA/FSR
742   Fri Apr 26 19:38:39 2013 DmassLaserLaserLaser Tuning Coefficients

Got gamma~0.15 back after playing with the sideband setup for a bit (one bad cable, 1-4 cracked capacitors). Was better / more solid after.

Rebuilt beat readout, realigned onto beat PD, found beat frequency:

f_beat = 117.8 MHz @ -14dBm

Rth_102068 = 9.237k

Rth_102085 = 8.018k

The viewer + card worked OK to see the dimmer spots. It was easy to see the ~2mW beam right after the window (~9 inches from cavity waist). It was very hard to see a single beam after the beamsplitter (18 inches after the cavity waist, and 1/4 the power because of beamsplitters) sans lenses. With an IR card and the IR viewer from the ATF I was able to see the dim spots without issue

806   Thu Jul 4 15:22:19 2013 nicolasLaserControl SystemRough tuning of laser temperature offset

We've noticed that sometimes the cavities prefer to have their PDH control signal to be offset slightly from zero mean, by where the temperature knob is set. I don't really know the reason for this, it doesn't seem like the actuator is hitting the rail. One known difference between the two cavities is that the west is still using the blue thor labs servo box.

The PDH control offload servo in the Cymac allows for an offset setpoint. I tuned the setpoints of the two cavities roughly to produce the lowest transmitted RIN of the cavities. I was able to reduce the fluctuations by an order of a few for the west cavity, while the easy cavity already seemed to be doing well at zero offset. The transmission of the west cavity also increased by about 8% with the new setpoint.

The attachment shows the cavity trans spectrum with and without the offsets. There is also a difference in the RIN of a factor of 3 between the cavities, this should be investigated. (Alignment?)

Here are the current settings:

controls@gaston:~$ezcaread X1:CRY-W_LASER_SETPOINT X1:CRY-W_LASER_SETPOINT = 400 controls@gaston:~$ ezcaread X1:CRY-E_LASER_SETPOINT
X1:CRY-E_LASER_SETPOINT = 0


Attachment 1: cavtrans.png
807   Mon Jul 8 13:02:17 2013 DmassLaserTransfer FunctionsLaser Transfer Function

DMASS NEEDS TO LOOK AT THESE PLOT TITLES AND FIX THEM (10/1/13)

Add G(f) = the driver transfer function as well + link the data on teh svn somewhere

We took some measurements with Rich a while ago in an effort to answer the question "where is all our phase going?"

We locked the laser to the cavity, an added in a drive signal (swept sine) at the PDH input, using an HP4395. We took the B/A transfer function, where B was taken from the PDH mixer before the low pass (using RF electronics), and A was taken at the input to the laser diode driver.

This gives us the: "diode driver + laser + cavity + RFPD + mixer + time delay" transfer function (call this H(f))

We also took a transfer function straight through the diode driver using a 50 ohm terminator as a load (100 mA into 50 Ohms = 0.5 Watts), and putting the beefiest capacitor Rich could find across it as an AC coupling (call this G(f)). The pole from that (Z_in = 50 ohms + capacitor) was below the start frequency of the measurement.

I divided the diode driver transfer function out of the first transfer function (H(f)/G(f)). This yields:

"laser + cavity + RFPD + mixer + time delay",

so long as we assume that loading the diode driver with a 50 Ohm resistor didn't change its transfer function in any meaningful way (Rich assured us that because the diode driver was a Howland current source, it should not matter if we load it with a diode or a 50 Ohm terminator)

The first attached plot is the transfer function H/G, with a pole + time delay overplotted. The data points to there being another pole around 140 kHz (roughly the 3 dB point between the green and blue traces)

The 2nd plot is cavity pole + mystery pole @ 140 kHz + time delay. This happens to fall too fast. Can whatever looks like zeros up above 1MHz (if they are in fact zeros) pull the magnitude response up fast enough to make the green and blue overlap, or is this a clue that A) the transfer function is somehow not described well by simple poles/zeros or B) something is screwy with the measurement?

Attachment 1: laserTF1.png
Attachment 2: laserTF2.png
809   Fri Jul 12 20:01:17 2013 Steve Maloney, Nicholas Smith-LefebvreLaserCavityAbsorbance of 532 nm on cavity mirror surface

see attachment

Attachment 1: ELog.docx
810   Fri Jul 12 23:57:45 2013 nicolasLaserCavityAbsorbance of 532 nm on cavity mirror surface (pdf)

 Quote: see attachment

PDF for posterity

Attachment 1: ELog.pdf
817   Thu Aug 8 17:52:30 2013 DmassLaserLab WorkCavity scans + mode matching

We didn't have a recorded quantitative measure of the mode mismatch of the cavities (though we had claimed that it was as bad as 40% based on transmitted power in the East cavity.

I plugged the transmitted and reflected PD signals into the scope (these have been realigned since we opened up the cryostat and realigned/relocked in air).

I drove the current driver (1mA/V) with a function generator (3Hz, 2 Vpp) T-ed into the scope (Zin = 1M).

Turned down the total power at the refl PD by adjusting the HWP at the laser output before the isolator to stop saturating the DC path of the PD (P_incident = 2mW)

I tweaked the alignment to maximize the transmission / reflection dips

I turned down the speed / voltage range while looking at the dips in reflection on resonance the peaks (transmission and reflection) stopped getting bigger (because at high enough speeds, I don't fill the cavity).

Sweeps:

East:

Are we sweeping slow enough?:

• Sweep speed: [ 2 V / (1 / 6 s) ]  x  [ 1 mA / V ]  x  [ 23.7 MHz / mA ]   =   280 MHz / sec
• Cavity pole ~ 30kHz
• t_fill ~ 1 / f_pole = 33 us
• t_sweep = 30kHz / [ 280 MHz / sec ] = 107 us
• t_sweep ~ 3 x t_fill (so we can treat the sweeps like steady state measurements)

Reflection:

• Vmax = 223 mV
• Vmin = 45 mV
• Vdark = - 20.6 mV (* yes this is large, idk why)
• % not reflected (trans + loss) = ( ( Vmax - Vdark ) - ( Vmin- Vdark ) ) / ( Vmax - Vdark ) * 100 = (Vmax - Vmin) / (Vmax - Vdark)*100 = (223 - 45) / (223 + 20.6)*100
• West trans + loss = (223 - 45) / (223 + 20.6)*100 = 73%***

West:

** I noticed that while sweeping with the same magnitude sweep that I used for aligning the East cavity (10Vpp and 10Hz), the dips in reflection/transmission seem to jump around a lot more than they did with the East cavity. I turned off the laser current and temperature, waited for 5 minutes, and then turned back on temp, waited 5 minutes, and turned on the current. It's possible that one of the temperature loops is less stable than the other (since they are little plastic screw knobs for PID tuning with no readback on two different ITC510 controllers. The oscillations were ~1s timescale and the thermal pole for the TEC/laser is ~6Hz, so this might not be crazy.

Are we sweeping slow enough?:

• Sweep speed: [ 2 V / (1 / 6 s) ]  x  [ 1 mA / V ]  x  [ 38.8 MHz / mA ]   =   466 MHz / sec
• Cavity pole ~ 30kHz
• t_fill ~ 1 / f_pole = 33 us
• t_sweep = 30kHz / [ 466 MHz / sec ] = 65 us
• t_sweep ~ 2 x t_fill (so we can treat the sweeps like steady state measurements)

Reflection:

• Vmax = 200 mV
• Vmin = 32.4 mV
• Vdark = - 1.6 mV
• % not reflected (trans + loss) = (Vmax - Vmin) / (Vmax - Vdark)*100 = (200 - 32.4) / (200 + 1.6)*100
• East trans + loss = (200 - 32.4)/(200 + 1.6) = 83%***
818   Fri Aug 9 15:50:27 2013 EvanLaserCavityCavity skewness relative to outer rad shields

Last week Dmass and I measured the positions of the cavity axes relative to the outer radiation shield axes. We aligned the beams to the cavity axes. At each aperture, Dmass held a card flush with the face of the shield to make the spot visible, and I took some photographs.

For each of the back (output) apertures, the camera was positioned looking dead-on into the aperture. For each of the front (input) apertures, the camera was placed in two different positions: vertically above the beam axis, and horizontally to the side of the beam axis. For each camera position I took two photos.

I opened each photo in Inkscape and drew an ellipse which (by eye) coincided with the edge of the aperture (representative photo attached; the full set is in the ligo.wbridge picasa album). The major and minor axes of the ellipse are constrained to lie horizontally and vertically, so here we're exploiting the assumption that (a) the roll of the camera was negligible, and (b) the oblique viewing angle of the camera was either entirely horizontal or entirely vertical for each photograph. I read off the coordinates of its four vertices (top, bottom, left, and right) in terms of pixels, as well as the center of the beam spot (determined by eye). Since the aperture is a circle with 1/2-inch diameter, I used the horizontal axis of the ellipse (in pixels) to convert the horizontal coordinate of the beam spot in pixels to the horizontal coordinate in inches from the center of the aperture, and likewise for the vertical coordinate. I also assigned uncertainties by eye and propagated them forward.

To maintain a consistent, right-handed 3D coordinate system, the horizontal coordinates for the back face measurements are given a sign flip. Then +z is normal to and directed outward from the front face, +x points from west to east, and +y points upward. The shield apertures are separated by z = 6.5 inches. To get the displacement of the cavity axis relative to the shield axis, we take the average (xFront + xBack, yFront + yBack) / 2. To get the pitch and yaw of the cavity axis, we take the (x, y, z) coordinates of the two spots, subtract the back coordinates from the front coordinates, normalize the resulting vector, and then read off (pitch, yaw, 1). The resulting angle convention is that positive pitch means the back of the cavity is tilted up relative to the front of the cavity (positive rotation about the x axis), and positive yaw means that the cavity is rotated counterclockwise when looking downward (positive rotation about the y axis).

 West cavity East cavity x displacement −14±2 mil −13±3 mil y displacement −83±2 mil −84±3 mil yaw −0.39±0.04 deg −0.77±0.05 deg pitch -0.03±0.04 deg +0.28±0.05 deg

[dym (adding data for completeness, and complained about convention for pitch being backwards so that we fixed it).

In order to be careful while we were doing these measurements, I used a small metal ruler to measure the beam spot location with respect to the outer radiation shield aperture, and generate the same numbers that Evan did using the pictures we took. All errors were set to be 1/128", which is 1/4 of the 1/32 scale I was using to do the measurements (a.k.a. "this is how well I think I could identify the spot location w.r.t. the edges of the radiation shield using the ruler and my eye")

 West Cavity East Cavity x displacement -12 ± 11 mil -31 ± 11 mil y displacement -78 ± 11 mil -86 ± 11 mil yaw -0.41 ± 0.1 deg -0.83 ± 0.1 deg pitch 0.0 ± 0.1 deg 0.41 ± 0.1 deg

Once we use conventions for pitch that are consistent with the coordinate system we agreed upon, then our numbers agree in sign, and are close in value w.r.t. the error bars. Only the East Cavity pitch seems to disagree, but since the numbers for the pictures seemed better all around (by error estimates), we are using these for the shield redesign]

Attachment 1: crop_east_back_1_annot.jpg
Attachment 2: skew_code.zip
821   Tue Aug 13 14:24:11 2013 EvanLaserCavityCavity skewness and spot locations on windows

I've taken the above displacements and angles and used them to project the cavity spots onto the cryostat windows, assuming the radiation shields can be perfectly aligned to the window axes.

In both the Solidworks model and the actual cryostat, the radiation shields are displaced inwards relative to the window axes. Dmass has measured the distance of closest approach of the rad shields to be 1/2 inch. In the Solidworks model, this means that the west rad shield is displaced 0.31 inches toward the east relative to the west windows, and the east rad shield is displaced 0.31 inches toward the west relative to the east windows. I've included these displacements in the following calculations.

I used two different methods to project the spot locations: (1) Solidworks, and (2) python code. For Solidworks (see first attachment for picture), I first aligned the radiation shields to the window axes (plus the aforementioned displacements), and then used the move/rotate tool to skew the cavities. I then projected the cavity axes onto the windows and used the measure tool to get the displacement between the spot and the window center. The python code (attached) is fairly simple; it's just takes the displacements and angles and propagates them using the small angle approximation. Since the windows are at 19° relative to the cavity faces, there's an obliquity factor of cos(19°) that I correct for in the horizontal coordinates.

The spot locations as determined from the Solidworks drawing are as follows:

 West cavity East cavity frontX +256 mil −468 mil frontY −75 mil −124 mil backX −387 mil +217 mil backY −85 mil −41 mil

The spot locations as determined from my python code are as follows:

 West cavity East cavity frontX +253±7 mil −462±8 mil frontY −79±6 mil −125±8 mil backX −374±7 mil +222±8 mil backY −87±6 mil −43±8 mil

The conclusion that Dmass and I have drawn from this is that it is not necessary to engineer any pitch correction into the supports for the radiation shields.

Attachment 1: dewar_and_cavity_stripped_skewed.jpg
Attachment 2: windowSpots_code.zip
885   Tue Oct 1 17:51:26 2013 nicolasLaserControl SystemLaser response according to RIO paper

Fig. 3

I don't understand how we can even get a 100kHz loop with this transfer function of the laser. The amplitude starts rolling off already at 1kHz and there is 90deg of phase as 10kHz.

In any case, it seems this laser is slow. So we should try to compensate it in the PDH control box, or perhaps with a phase correcting EOM.

886   Tue Oct 1 17:54:01 2013 DmassLaserControl SystemLaser response according to RIO paper

 Quote: Fig. 3 I don't understand how we can even get a 100kHz loop with this transfer function of the laser. The amplitude starts rolling off already at 1kHz and there is 90deg of phase as 10kHz. In any case, it seems this laser is slow. So we should try to compensate it in the PDH control box, or perhaps with a phase correcting EOM.

This might be the transfer function through the RIO supplied ORION driver (which is a big part of what motivated us to make our own) - Disclaimer: I haven't checked the paper yet, so this is just a suspicion

950   Tue Nov 26 16:30:06 2013 DmassLaserTransfer FunctionsTemperature Plant Measured

[Dmass, Nic]

We measured the temperature plant of the laser using the Cymac and the ITC510.

We measured the following transfer functions:

• [Hz / V_510]
• [K / V_510]

where V_510 is the temperature tuning BNC input on the back of the ITC510 (this is, in principle, mA to the TEC, but we need to calibrate it)

What we did:

Locked laser (east/west) with PDH loop

• Used Cymac to excite laser temperature via BNC on back of ITC510 (e.g. X1:CRY-E_LASER_CONTROL_OUT)
• This injects a signal into the error point of the ITC510 PID loop
• Take transfer function between temp error point and ITC510 temp readback (thermistor) for each laser
• Take transfer function between temp error point and PDH control signal for each laser

East cavity looked healthy

West laser looked like there was gain peaking in the temperature loop

We re-mounted the West laser with thermal paste (arctic silver 5) [pictures are on the wbridge picasa album]

The bottom of the west laser had a lot of surface area not covered by thermal paste

We cleaned the old thermal grease off, and re-applied the new AS5 (same paste as before)

The transfer function measurements after we re-thermal-greased the West cavity looked much nicer

We redid the temperature loops that the Cymac is responsible for, and both temperature loops worked fine

951   Tue Nov 26 18:19:27 2013 nicolasLaserTransfer FunctionsTemperature Plant Measured

Attached are the transfer functions for both lasers. We drove the temp control inputs on the back of the thorlabs laser controllers (LASER_CONTROL_OUT), and we read back from both the PDH control signal (LASER_CONTROL_IN1) as well as the laser temperature (LASER_TEMP_OUT).

The thermistor resistances (R_TH) were 8.6kO (west) and 7.4kO (east).

These are the final states of the TFs, after pasting the west laser. Units are ADC counts over DAC counts.

954   Tue Dec 3 03:09:24 2013 DmassLaserTransfer FunctionsTemperature Plant Measured

[Dmass, Nic]

The temperature loops have gain peaking around 10 Hz - we had to decrease the gain in the East loop from 0.5 to 0.005 in order to make the oscillations disappear.

Dmass thinks we can lower the UGF of these loops a lot without actually hurting their functionality.

It is important to remember than any noise we inject at the "control point" of the PDH box is only suppressed by the PDH box gain (this is ~20 with no integrator, and more with the boosts o

We may need to look into redistributing gain in a more intelligent fashion (this has not been optimized; the gain setting of the current driver, 1 mA/V, was set semi-arbitrarily by Rich when he had Sam stuff this, and we simply have not changed it)

971   Wed Dec 11 01:09:30 2013 DmassLaserLab Work

I measured some of the things outlined in elog:969

Noteworthy things along the way:

West:

• The West temperature was oscillating at ~3.3 Hz (close to the pitch freq);
• I discovered that the SR560 used to buffer it had been changed from (gain = 1, pole @ 300 Hz) to (gain = 10, no pole) - fixed this
• I turned the gain of the temperature loop way down, and the oscillations went away (it was high from a burtrestore)
• Aligned West cavity (got close with 00 trans, optimized by minimizing 01 and 10 trans)
• Checked Temp trans funxn at new gain levels - seemed fine
• N.B. Need to adjust offset at error point b/c PDH board integrates now, so setpoints do nothing in terms of the offset of the PDH error point
• Played with offset on the PDH board - couldn't see any significant difference in transmission spectrum or DC level for the available offset range
• Adjusted the gain on the PDH board while looking at the error signal until we were barely gain peaking around 100 kHz (why is the gain peaking so low?)

East:

• Swept East cavity to realign (it was fairly misaligned, ~50% of max transmission)
• East fringe visibility = 1 - 11.2mV/115mV = 90%
• Max trans voltage on sweep = 1.37V (dark to bright)
• Locked East cavity
• Boosts wouldn't engage - LED indicators on front panel not illuminated. LEDs flickered if I wiggled cable
• Found another bad BNC cable (the yellow ones, as usual) - replaced it and threw the faulty broken cable away. We should never ever ever ever ever buy those again.
• Tuned gain to make gain peaking around 100 kHz barely disappear
• Rth = 7.48 kOhms
• Noticed input RIN spectrum was significantly jumpier on East
• Tuned up waveplates on input to cavity to maximize refl PD signal - got barely noticeable gain
• No temp loop offset
• Temp loop oscillating slightly around 6 Hz - tuned it - left it relatively high

The measurements I made for each cavity:

1. PDH error signal
2. PDH control signal
3. Input RIN
4. Transmitted RIN
972   Thu Dec 12 03:14:22 2013 DmassLaserLab Work

WEST

• West visibility = 1- 17mV/153mV = 89%
• RFPD DC level while locked = 9 mV +/- 1mV
• DC power on RFPD while unlocked = 1.450 mW
• Power on Trans mon PD  = 0.635 mW
• Power at output of cavity = 1.173
• Input power = 1.874
• Rth = 8.573k
• Temperature loop setpoint offset = 0
•  all measurements ASD AC coupled + float
• Input noise of SR785:
• PDH err = -32 dBVpk
• DPH ctrl = -32 dBVpk
• Trans RIN = -24 dBVpk
• Input RIN = -50 dBVpk

EAST

• East fringe visibility = 1 - 11.2mV/115mV = 90%
• RFPD DC level while locked = 10.6 mV
• RFPD DC level while unlocked = 105 mV
• DC power on RFPD while unlocked = 1.840 mW
• Power on Trans mon PD  = 0.967 W
• Power at output of cavity = 1.810 W
• Input power = 2.020 mW
• Rth =7.480k
• Temperature loop setpoint offset = 0
• Input noise of SR785:
• PDH err = -30 dBVpk
• DPH ctrl = -30 dBVpk
• Trans RIN = -30 dBVpk
• Input RIN = -46 dBVpk

The RIN levels are worth checking out.

The West transmitted PD is the signal we have seen up to a Coherence of 0.5

Attachment 1: RINcompareEW.png
Attachment 2: PDHcompareEW.png
Attachment 3: RawMeasDataE.png
Attachment 4: RawMeasDataW.png
973   Thu Dec 12 11:32:44 2013 nicolasLaserLab Work

Attachment #2 of Dmass' log shows the PDH control falling like ~1/f^(1/2) with a level of 1e-5V/sqrt(Hz) @ 100Hz. (above 10^4 it ticks up, but this is probably due to the pole in the calibration of Hz/V, which supports the hypothesis that the 18kHz pole is in the laser actuator, not the cavity)

If I'm right that this was measured at the control output of the PDH2 board, then assuming 38MHz/V, this gives a noise level of

380*(100Hz/f)^1/2 Hz/rt(Hz)

this is about a factor of 2 higher than the level given in the Kenji Numada paper. So not too crazy.

976   Mon Dec 16 02:35:12 2013 DmassLaserLab WorkGold PDs calibrated; AM/PM measured

I calibrated the Gold PDs against the PDA10CF diodes I put in to monitor RIN/RAM, and measured the AM/PM ratio.
West:

• Gamma_PM = 0.13
• Gamma_AM = 7.1e-5
• Gamma_AM / Gamma_PM = 5.3e-4
• Z_rf x eff x resp = 18.2e3V/W (see cryo:1069 for correct numbers)
• Z_dc x eff x resp = 109 V/W

East:

• Gamma_PM = 0.08
• Gamma_AM = 7.7e-5
• Gamma_AM / Gamma_PM = 9.6e-4
• Z_rf x eff x resp = 6.45e3 V/W (see cryo:1069 for correct numbers)
• Z_dc x eff x resp = 57 V/W

PM levels were measured by sideband transmission peaks at the time of this measurement

The difference in modulation depth was just due to different drive levels, and is consistent with old measurements. I finally got the new RF amp for the East path in hand, so I will increase the modulation depth when I swap it in

The RAM levels had not been optimized for some time, so we can probably do better than this with waveplates to match the axes (and probably active or passive temperature stabilization for the EOM so that its axis doesn't drift around)

Need to investigate discrepancy in Gold PD transZ gains. Make sure we don't have a QE ~ 50% diode in the East path.

983   Fri Dec 20 12:57:31 2013 DmassLaserLab WorkPDH Error signal cailbration

I calibrated the error signal to reflect what is actually on the table more accurately

[kHz / Volt] Error signal calibration:

• East = 42.5 kHz / Volt
• West = 11.7 kHz / Volt

What numbers went into this calibration:

[Watts / Hz] at the PDH RF PD:

D = 2 P_0 \Gamma / f_c

• East = 7.3e-9 [Watts/Hz]
• \Gamma = 0.08
• P_0 = 1.84 mW
• f_c = 40 kHz
• West = 9.4e-9 [Watts/Hz]
• \Gamma = 0.13
• P_0 = 1.45 mW
• f_c = 40 kHz (assumed to be equal to measured East pole)

[Volts / Watts] from the gold PD to the RF mixer

(QE x responsivity x transimpedance)

• East = 6.45e3 [Volts/Watt]
• West = 18.2e3 [Volts/Watt]

[Volts / Volt] through PDH RF electronics

East = West = 1/2 (there might be another factor of 1/2 here because of the 50 ohm terminator T-ed into the IF output of the mixer before the low pass - these should be replaced by the actual transfer function as soon as we measure it)

What should we do with these numbers?

This calibration can be applied to the input referred voltage noise of both PDHv2 boards, and added to the electronics portion of the noise budget

1102   Fri Jun 20 01:05:14 2014 DmassLaserTransfer FunctionsMeasurements

Measured:

Cryo 002 input referred noise - there was a weird 10 Hz comb that seems to be new, and possibly to do with the way we are bypassing the third op amp, which Evan is investigating

Cryo 002 transfer function (to MHz) - no boosts engaged

West and East transfer function through current drivers, including cabling to the laser diode mount - out to several MHz

Plots incoming - some nontrivial calibration to do first.

1103   Mon Jun 23 23:49:49 2014 ZachLaserSiFiCavity axis angle shift vs g-factor

To decide whether or not we can go with 1" windows (easier and cheaper than 2"), here is a conservative calculation of the expected cavity axis shift as a function of the (symmetric) g-factor we choose.

The mirror deflection angle is chosen to be a (rather high) 10 mrad, and the displacement is calculated at 20 cm from the cavity center, which is probably farther than the windows will be.

The calculation is made with one line from the formula in Siegman p. 769.

As you can see, the displacement for even this large angle should be on the 1-2 mm level for us, so we can use 1" windows.

1182   Wed Dec 17 13:54:19 2014 ZachLaserSiFiLasers mounted, energized, beat set up

On Monday, after I did some inventory of all the parts we have received from various companies, Dmass helped me mount the RIO lasers into their mounts so that I could get started with the optical setup. We cleaned the surfaces with methanol, applied a small layer of silver thermal compound, and then screwed them in.

I then borrowed the following to run the lasers:

• The (separate) ThorLabs diode driver and temperature controller from Haixing's maglev setup
• An integrated ThorLabs diode driver / temperature controller from the TCS lab

After finding the right cables, I powered up the lasers and verified the P-I curve for each as listed on the spec sheets.

I then built a quick (temporary) optical beat setup, combining the two beams on an 1811. I had the temperatures (actually, thermistor resistances) set to what was listed as the testing set point on the datasheet, and as soon as I overlapped the beams and focused them onto the PD, there was already a strong ~50 MHz optical beat.

I have spent some time since then trying to lock various kinds of PLLs, both to interrogate the free-running frequency noise and to get used to controlling the lasers. Some things I've tried:

• Locking a Marconi to the free-running beat, which I think might be an exercise in futility due to the relatively small range of the Marconi FM
• Locking one laser to the other directly using a PLL, which I think might be an exercise in futility due to the bandwidth of the current actuation from the ThorLabs driver
• With Dmass's help, locking a Zurich PLL to the free-running beat. This appeared to work, and we saw a preliminary frequency noise spectrum that looked about right, but I'm skeptical because the control signal doesn't seem to respond to my slewing one laser's frequency.
• Briefly, locking one laser to the other at low frequencies using the Zurich PLL control signal as a frequency discriminator. This didn't work, adding to my suspicion.

The first two were not helped by the fairly basic loop shaping afforded by attenuators and an SR560.

I think my next step will be to simply use the I-Q demodulation method (like I did to measure the no-FM Marconi noise in ATF:1877) to measure the frequency noise. I'll compare that to what I get with the Zurich PLL.

1183   Wed Dec 17 14:40:15 2014 DmassLaserSiFiLasers mounted, energized, beat set up

• With Dmass's help, locking a Zurich PLL to the free-running beat. This appeared to work, and we saw a preliminary frequency noise spectrum that looked about right, but I'm skeptical because the control signal doesn't seem to respond to my slewing one laser's frequency
• Briefly, locking one laser to the other at low frequencies using the Zurich PLL control signal as a frequency discriminator. This didn't work, adding to my suspicion.

If the "locked indicator" light is not green on the Zurich (first tab, under "Reference", then what you get out is junk (e.g. you have unlocked the lock in, and i hasn't re-acquired yet) - you can do this by kicking it too hard with a frequency shift, which would be easy to do if you were slewing laser frequency, as the coefficients of the laser [Hz/mA] is so big. When the lock in loses the signal, you have to manually re-lock it (toggle off and on the button which has the mouseover text: "enable the fixed center frequency mode of the PLL"). You can get  something which sort of looks like a PLL signal which has terrible noise and weird glitchy response when the lock in isn't locked in.

Your instinct to look for slewing at the PLL control point is correct, and a sign that the state of the PLL is healthy/unhealthy

1184   Wed Dec 17 18:11:38 2014 ZachLaserSiFiLasers mounted, energized, beat set up

 Quote: If the "locked indicator" light is not green on the Zurich (first tab, under "Reference", then what you get out is junk (e.g. you have unlocked the lock in, and i hasn't re-acquired yet) - you can do this by kicking it too hard with a frequency shift, which would be easy to do if you were slewing laser frequency, as the coefficients of the laser [Hz/mA] is so big. When the lock in loses the signal, you have to manually re-lock it (toggle off and on the button which has the mouseover text: "enable the fixed center frequency mode of the PLL"). You can get  something which sort of looks like a PLL signal which has terrible noise and weird glitchy response when the lock in isn't locked in. Your instinct to look for slewing at the PLL control point is correct, and a sign that the state of the PLL is healthy/unhealthy

Yes, I noticed this effect. I'm talking about immediately after acquiring---or re-aquiring---PLL lock. I did this several times at different beat frequencies to see what effect it had on the noise (the spectrum changed considerably, which is another bad sign).

1185   Thu Dec 18 03:39:32 2014 ZachLaserSiFifree-running laser frequency noise

I spent some time tonight measuring the free-running laser beat noise in various ways. Recall that, as of yesterday, I had tried setting up a couple analog PLLs to no avail and I didn't trust the spectrum I was getting from the Zurich PLL. So, I wanted to measure it another way to see if I could corroborate.

First, eye candy:

Now, an explanation of the various measurements.

I-Q demodulation method

This is a method I have used with some success in measuring the Marconi noise in its quietest state (with no modulation and therefore no means of feedback---see ATF:1877). It is done in the following way:

1. Split the beat PD output and send it to the RF input of two mixers (I used level-7 ZAD-1-1s), using equal path lengths.
2. Set Marconi to a frequency close to the beat (~50 MHz in this case) and an amplitude of +10 dBm
3. Split the Marconi output, send one splitter output to each mixer from (1), but with 90º rotation between them.
4. The outputs of the mixers are now at the difference frequency between the beat and the Marconi, but maintain their I-Q separation. (This is the reason for using the Marconi rather than beating the lasers at a lower frequency in the first place---the I-Q separation is maintained regardless of the differential laser drift, and it also only requires a short cable length.)
5. Acquire both I and Q signals and perform the I-Q analysis:
1. Normalize the signals and atan2(I,Q) to get phi, then unwrap(phi) to get continuous phase evolution vs time
2. diff(detrend(phi))/diff(t)/2/pi to get instantaneous frequency as a function of time
3. pwelch

The main complication here is that, as you can see in the plot, the high-frequency RMS of the beat is several tens of kHz, which means you still have to sample at a high rate to get what you need. The best acquisition scheme I could think of was the Zurich box, which can do 460 kS/s. Still, to take meaningful data, I had to very carefully tune the laser beat to the Marconi LO and then quickly engage acquisition before the (wildly fluctuating) IF signals drifted above the Nyquist frequency (around one second of data was used to make this trace).

That said, the result doesn't look that crazy, and in fact it agrees very well with the DFD measurement that was carried out in a completely different way (see below).

Delay-line frequency discriminator (DFD) method

This is the usual scheme where one mixes a signal with a time-delayed version of itself to create dispersion. What I did:

1. Split the PD signal
2. Using one splitter output, find the appropriate combination of attenuators and amplifiers needed to obtain a LO-worthy +7-dBm signal (I needed -7 dB and then ~+25 from a ZFL-500-LN) and send it to a mixer LO input via a long (several-meter) cable.
3. Send the other output to the mixer RF input via a short cable (attenuate if necessary---wasn't in my case).
4. Verify that the DC level of the IF output varies sinusoidally with the beat frequency
5. Null the output and measure the frequency resolution. I measured 5.5 nV/Hz.
6. Amplify with SR560 and measure spectrum on spectrum analyzer
7. Divide spectrum by SR560 gain and the number in (5) to get frequency noise

This method worked swimmingly and reproduced exactly the result I found using the I-Q scheme. The noise floor (cyan in the plot) was measured by sending a quiet Marconi sine wave of the same amplitude and frequency as the beat through the pipeline.

Zurich PLL method

This method is incredibly straightforward. Simply plug the beat (ensuring it's < 1 Vrms and under 50 MHz) into the Zurich box and lock the internal PLL by pressing "ON" on the screen. Route the PLL control signal to one of the front panel outputs and choose the scale factor in V/Hz. I chose the same number as I measured for the DFD (including the SR560 gain) for ease of comparison on the spectrum analyzer.

Results

• All methods agree below ~50 Hz
• The I-Q and DFD methods agree everywhere, but they are higher than the PLL result by ~2 from 50 Hz to around 10 kHz, above which they re-converge somewhat
• All traces (save for the PLL in a narrow band from ~50-500 Hz) are higher than those on the spec sheets sent with the laser (shown in black on the plot---note that the West laser is everywhere noisier than the East one).

I'm not sure what to believe. One would think the Zurich PLL is the most trustworthy, but a) I still am bothered by the time-domain behavior I see in the PLL control signal when I adjust the laser beat while watching it, and b) I've generated two nearly identical spectra that differ from it using completely different schemes from measurement to FFT.

All that said, I think the excess noise (and thanks to Dmass for saving me time by pointing this out) is just coming from the ThorLabs drivers, so this should be redone when we have our low-noise ones.

1187   Fri Dec 19 21:37:12 2014 ZachLaserSiFiAmplitude modulator characterization

Tonight, I did some characterization of the Photline fiber-coupled amplitude modulators we will use for our experiment (MXAN-LN-10 --- datasheet attached nope google it yourself). These are electro-optic devices that work by using an internal mach-zehnder to convert phase modulation into amplitude modulation.

The test setup for all measurements was the same. I used the exact configuration that I have been using for the beat (see CRYO:1182), but I simply blocked one laser, so that only one beam was hitting the 1811 PD. The amplitude modulators were inserted (one at a time) between the East laser and its output coupler.

Insertion loss

The first thing I did was to investigate the insertion loss of the modulators. We chose the low-loss option, which just meant that the company hand-selected modulators with loss of < 3dB (= 50% power transmission).

I didn't go crazy with precision here, because systematics with fiber coupling can easily prevent a measurement to better than a few percent (an example of this: I installed a 1-meter patch fiber between the laser and the output coupler, instead of the modulator, and I actually saw a slight increase in output power vs. the case with the laser going straight to the output coupler… go figure).

In both cases, I measured very nearly 50% reduction in power (at the top of the MZ fringe---see below) vs. the case with no modulator. So, these things have a loss very close to 3 dB, as advertised. An important thing to point out is that we will need to bias these away from maximum transmission to get a linear PM -> AM coupling, so the real power reduction in our setup will be more than 50%.

DC response

These modulators have an SMA-connectorized "RF" input, as well as two bare pins connected to a separate set of "DC" electrodes (they also have two more pins connected to the cathode and anode of an internal PD, presumably at the other MZ output port, which is kind of cool). As far as I can tell, the RF input is also DC coupled, only it is 50-ohm terminated.

I did a DC sweep of both electrodes from 0-10 V while measuring the output power:

(The RF applied voltage range is lower due to sagging from the 50-ohm load).

Fitting these curves, I determined the following Vpis:

• S/N 03
• DC: 6.46 V
• RF: 4.19 V
• S/N 17
• DC: 6.39 V
• RF: 4.91 V

These are consistent with the numbers listed on the datasheet.

Transfer functions

Next I measured the actuation transfer functions ([RIN/V]) from 1 Hz to 100 MHz, driving the RF input while applying a mid-fringe bias to the DC input, and using

• Agilent 35670A FFT analyzer and the 1811 DC output for 1 Hz - 50 kHz, and
• Agilent 4395A RF analyzer and the 1811 AC output for 500 kHz - 100 MHz

Note the dead zone from 50-500 kHz---this was by accident, as I forgot to check the low-frequency resolution of the RF measurement. I will redo this sometime.

Here are the results:

Notes:

• The jump from 50 kHz - 500 kHz is from the measurement dead zone and carries no information
• The lag beginning around 10 kHz is from the stated ~50 kHz bandwidth of the DC output of the 1811. The AC output has a low end at ~25 kHz, so there isn't really a good way to make a measurement in this region with that detector. We could use a DC-coupled version to make a continuous spectrum.
• The slow rollup at low frequencies is well-sampled and repeatable. I'm not sure what causes it, but it appears to be real. In any case, it's pretty small.
• The delay at high frequencies is consistent with the optical path length from the modulator to the PD. I calibrated the cables' transfer function out, and what is left is this delay which has a 4.13-m free-space equivalent. There is ~64 cm of free-space travel on the table, plus well over a meter from the output fiber of the modulator.

The response very flat, and roughly what is expected from the DC sweep:

(1/P0) * dP/dV|mid-fringe = pi/Vpi ~ 0.5 ( = -6 dB).

1190   Wed Jan 14 02:38:43 2015 ZachLaserSiFiPMC set up as test cavity

To continue with the laser/modulator testing, I have added Dmass's old PMC to the temporary characterization setup. I have used the other output of the 50/50 BS that combines the two laser diode outputs, so that we can keep the beat setup intact while also being able to send either of the two beams into the PMC.

To do this, I:

• Made a cursory razor beam scan of the beam emerging from the BS
• Calculated a MMT solution to the PMC mode using some of our new lenses
• Installed the telescope and directed the beam towards the PMC
• Macropositioned the PMC by hand to rougly center it on the transmission of the single-pass beam, as measured using a power meter
• Scanned the PZT using a 0-10 V triangle from an SRS function generator, then used the diode temperature as a coarse adjustment to look for modes
• Maximized the first found mode (a horizontal HOM)
• Looked for nearby lower-order modes, then maximized them and iterated to get to TEM00
• Installed HWP upstream and then maximized visibility by rotating polarization

The coupling isn't stellar yet, at roughly ~66%, but the MMT is fairly tight and I'm sure I can improve easily. The laser and cavity are stable to well within a linewidth at high frequencies, and only drift apart over many seconds.

Some things I plan to do with this setup:

1. Dither lock the PMC to the laser(s)
2. Characterize the phase modulators
3. Set up reflection PDH lock and feed back to lasers
4. More stuff
1195   Fri Feb 6 04:46:26 2015 ZachLaserSiFiBeam profile remeasured, test PMC aligned well and locked

I was having some issues with the beam(s) I had previously mode matched into the PMC. Apart from not having gotten great coupling to begin with, the alignment seemed to have drifted over a few days (I noticed this last week). I attributed this to 2 things: 1) the MMT I had was a pretty sensitive one, owing partly to the fact that I had to work with the beam far outside the Rayleigh zone due to the beam beat recombination being upstream, and 2) having the recombining BS in the way, I was susceptible to clipping in the output path I was using for the PMC. I don't really need the beat setup at the moment, and I can do the modulator characterization using a single laser, so I decided to rebuild the PMC test setup using a single laser.

As a first step, I simply remeasured the output beam profile of the West laser using the razor blade technique. The beam seems very circular and not astigmatic, so I only profiled in the horizontal direction. The result:

Using this, I recalculated a better MMT:

------------------------------------------------------------
Other solution:

mismatch: 0.00011786
w0x = 303.7849 um
w0y = 303.7849 um

lens 1: f = 103.2118 mm
lens 2: f = 206.4236 mm
Distances:
d1 = 6.161 cm
d2 = 14.3007 cm
d3 = 29.5383 cm
(Total distance = 50 cm)

I then installed this, aligned the PMC and was able to get ~96% coupling with little trouble. By locally optimizing the second lens, I pushed this to about 97.5%. While a bullseye was faintly evident on the card in the first case, it was very hard to tell what was reflected after the reoptimization.

I borrowed the RF electronics from the steel gyro PMC temporarily (splitter, mixer, bias tee and filters). For some reason, the 1-MHz dither I used with that PMC did not work with this one, but I was able to derive a nice error signal using a 300-kHz dither at 3 Vpp. I wanted to use the uPDH box I used to use before I had the digital servo for the gyro PMC, but I forgot that Eric Q had borrowed it for the 40m. Instead, I was actually able to lock robustly and stably with just an SR560 and a single pole at 10 Hz. The control signal stays within its output range over ~10 min+ time scales. (I didn't bother measuring the loop---all I needed for my phase modulator characterization is essentially a DC lock, and the bandwith was easily 10s-100s of Hz).

The transmission dither lock leaves the REFL port open so that I can measure the rejected sideband light pumped by the modulator as planned.

1196   Fri Feb 6 05:31:43 2015 ZachLaserSiFiPhase modulator characterization

After rebuilding the PMC setup (see CRYO:1195), I was finally able to move on to characterizing the Photline fiber-coupled phase modulators we will be using (MPX-LN-0.1 --- datasheet attached nope google it yourself). I measured a couple things:

## Insertion loss

As with the amplitude modulators (see CRYO:1187), I determined this simply by measusing the power straight out of the laser, then quickly connecting each phase modulator (one at a time) between the laser and the output coupler and measuring again. As I mentioned in the linked post, this is not an exact science due to the somewhat unpredictable behavior from connector to connector. Nevertheless, one can be confident at the one-to-few-percent level.

### S/N 10:

2.66 mW out / 5.00 mW in --> loss ~ 2.74 dB

### S/N 2:

2.88 mW out / 5.38 mW in --> loss ~ 2.71 dB

Supposedly, we had these two units hand selected for loss < 2.5 dB (for free, after we paid for the \$500 low-loss selection of the amplitude modulators), while the standard typical loss from the datasheet is closer to what we have at 2.7 dB. An extra 0.2 dB isn't going to break the bank, but it's a bit disappointing that they didn't give us what they said. Probably too late to say anything anyway...

## Response

My plan was to use the modulators to pump light into RF sidebands, then use the frequency selectivity of the PMC to measure the SB power and back out the actuation strength (Vpi). I was able to do this, to a degree, but I was thwarted by an unexpected issue: the modulators and the fibers coupling to/from them appear to change the output mode emerging from the collimator. What's worse, the mode seems highly sensitive to any touching of the fiber whatsoever. This was most egregious with S/N 10, with which my new cavity coupling maxed out at 83%(!), even after slight empirical MMT tweaking. S/N 2 wasn't as nasty; I got ~91.5% with it.

Given this, my new plan was to make a quick-and-dirty measurement in the following way:

1. Optimize the mode matching and record the contrast defect (17% and 8.5%, for S/N 10 and S/N 2, respectively, as mentioned above)
2. Drive the modulator at a chosen RF frequency (I chose 30 MHz since this is near where we'll be using them), and determine the amplitude necessary to double the reflected power.
3. The measured amplitude is associated with the modulation depth necessary to pump the same fractional power as the contrast defect out of the carrier (really, you could use any SB power level additively distinguishable from the contrast defect, but doubling it seemed the easiest thing)
4. Use the bessel function to infer that modulation depth, then scale the measured amplitude up to infer Vpi.

### S/N 10:

Measured amplitude to double REFL power: 0.78 Vpp --> 0.39 Vpk.

2*J1^2 = 17% --> gamma = 0.611

Vpi = 0.39 * (pi / 0.611) ~ 2.00 V

### S/N 2:

Measured amplitude to double REFL power: 0.52 Vpp --> 0.26 Vpk.

2*J1^2 = 8.5% --> gamma = 0.422

Vpi = 0.26 * (pi / 0.422) ~ 1.93 V

The datasheet claims 3.5 V typical, so this seems pretty good (though the spec is only officially at 50 kHz drive). Holding the amplitudes constant, I also swept the frequency down from 30 MHz to 10 MHz, and the reflected power was stable to around 5%.

Again, this is only really a quick-and-dirty measurement. Unfortunately, the only real way to get a good measurement is to reprofile the beam again with each modulator in place. Then, the contrast defect can presumably be brought down closer to 2% or better again, and the measurement can be made more cleanly. I'm hesitant to waste time doing so, though, given the observed mode dependence on the fiber resting position.

1203   Wed Feb 11 03:58:52 2015 ZachLaserTransfer FunctionsWipf nonlinear temperature actuation proof of principle

Nic elucidated to me today Chris W.'s idea for getting truly wideband (~500 MHz) actuation out of our diode lasers. In case the reader isn't familiar, the lasers have two parallel linear actuation pathways converting current into frequency: one from current modulating the temperature, which is the strongest effect at DC and then dies off above ~1 MHz due most likely to the thermal response, and another, weaker but much wider-band, flat pathway arising from solid state effects that did not survive the elucidating. At some frequency (around 50 MHz, I believe?), there is a crossover between these paths, but there is a differing sign, which creates a "non-minimal-phase zero", leaving the phase at -180° and making the overall system a difficult actuator to deal with at high frequencies.

As I understand it, Chris's idea involves using the full, nonlinear current-to-temperature response to effectively circumvent the direct linear response at low frequencies. This can be done, for example, by pumping a strong RF carrier current (say, around 1 GHz) into the diode, and then using amplitude modulation on this carrier to produce baseband frequency actuation from the temperature beating. By choosing the phase of the AM correctly, one can make it so this pathway (now dominant at low frequencies) results in a nicer crossover with linear pathway #2 from above.

I performed a very simple proof-of-principle test today by doing the following:

• Dither lock my temporary diagnostic PMC to one laser using the setup described in CRYO:1195.
• Set the UGF fairly low (a few 100 Hz)
• Drive the laser current with a 1-kHz sine wave, strong enough to be clearly present above the noise in the error signal. I found that 200 uVpp (= 2 uApp) gave me a nice SNR around 20.
• Using a Marconi into the SMA bias tee adapter directly on the diode, inject a fairly strong RF carrier current. I used 600 MHz at ~200 uArms, though the amplitude was determined empirically over the course of the test to see an effect.
• Engage amplitude modulation at 1 kHz and a pretty strong modulation (I chose "50%").
• (As I mentioned a couple bullets above, in reality, I removed the direct 1-kHz injection and pumped this RF-with-AM current up until I saw an effect in the error signal)
• With these two signals on, and adjusting the AM phase, I was clearly able to see modulation of the line in the error signal, indicating that the two drives were interfering as desired.

Trimming the RF amplitude and phase a bit to get a nice result, I was able to take the two spectra shown below. In the first trace, only the direct current line is present at 1 kHz. In the second one, the RF source is engaged and you can see an exact cancellation of the line in the error signal. Increasing or decreasing the RF (or audio) amplitudes led to the reemergence of the line (assuredly with 180º relative phase from one case to the other). To do the wideband actuation, one would simply make sure that the RF power is strong enough that the nonlinear path dominates.

## So, it should work!We'll have to change the measurement setup to make a full transfer function showing clean actuation to very high frequencies, but it should be pretty straightforward.

1204   Thu Feb 12 15:34:12 2015 Nic, ChrisLaserTransfer Functionshigher bandwidth frequency readout

In order to better measure the effect of this nonlinear current to frequency modulation, we'll need to do Zach's measurement but with much higher drive frequencies. (His measurement was 1kHz).

We'd like to do a full TF of the nonlinear current amplitude modulation path to the laser frequency. There are two effects in Zach's setup that limit the bandwidth of the measurement.

First, is the modulation input of the Marconi, which only reaches 30kHz. We plan to use a mixer to do higher frequency AM of the RF carrier.

The second is the frequency readout. We potentially could PLL the two lasers together and have a pretty high bandwidth readout. or, instead we decided to add some additional PDH sidebands to the light using the fiber modulator. This was then sensed in reflection of the PMC and demodulated. We used 30MHz at 0.5Vpp into the fiber modulator.

With this setup, we were able to measure some amount of nonlinear current to frequency modulation, and when we unlocked the cavity the transfer function was reduced by at least 20dB, which rules out some other coupling path.

Next step is to set up high bandwidth AM of the 500MHz marconi output (driving the current).

1207   Mon Feb 23 14:57:23 2015 ZachLaserM2 ISS3-mm diode initial noise measurement

I was preparing to do an initial test of the M2 ISS readout board with the 3-mm diodes on the SiFi test setup when I noticed some anomalously high noise on one of the diodes. So, I decided to make a more careful measurement and test all 4 diodes. I found that only one (S/N 7845) exhibits this very bad excess 1/f noise, but all four have it present at some level.

For this test, I had the transimpedance fairly high at Z = 2.7 k$\Omega$ since I am only working with < 5 mW of power, and the diodes were completely blocked for this measurement and put in a dark box. The bias was 10 V at first, but then reduced to 5 V in an attempt to reduce the excess noise after I read on the datasheet that 10 V was an absolute maximum for some reason. I did not record the difference in noise from 10 V to 5 V, but this is a test I will likely try (though perhaps not up to 10 V anymore).

While 7845 is clearly bad, the others are probably OK for now; they are not acceptable for low-power/high-Z operation, but are likely just fine for our high-power testing since we will expect shot noise levels of >100 pA/rtHz, with SNR with respect to PD noise increasing as $\sqrt{P}$.

1208   Mon Feb 23 17:57:49 2015 ranaLaserM2 ISS3-mm diode initial noise measurement

IF the DC dark current is out of spec, we might be able to get a replacement. Might be specs on the website. I think Frank had a Keithley instrument to measure dark currents that are low - probably in his diode destruction elogs or DCC docs.

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