I checked the boost stage on U7 of LIGO-D040105-C on the south path box (37 MHz). It appears that Craig had changed its value to 6.8 nF to match the North path.
The resistor was actually 5.088 kΩ, instead of 5.6 kΩ as listed. The resistor was changed from the radial thick film (5.088 kΩ) to a thin film surface mount type (5.6 kΩ). I moved the capacitor from the switch (at the end of the wires) to immediately at the board. The switch on the side of the box now only has two wires to short the capacitor when we want to disable boost. This should mean that there is an integrator with a pole at DC and a zero at 4.2 kHz and won't have any funny feedback impedance due to long wires and switch. This now matches the North path.
Anchal is going to figure out how to increase gain of south FSS RFPD from ~1.5 kΩ to 2.5 kΩ of the north path. In the meantime I've added a mini circuits low noise amplifier to the output of the RFPD. This has a noise figure of about 3 dB and a gain of +24 dB. I put a 20 dB attenuator after the amplifier. I think the extra gain stage will only add a few mHz/rtHz to the input referred noise, so should be fine for now. Once Anchal has solved the crossover TF optimization he'll have time to fix this.
And that is the quick guide on how to twist wires with a drill.
temperature time series for the last couple of days measured right above the table between both cavities.
i checked the resonant frequency of both cavities in order to see if we can lock both using the existing frequency actuator (AOM) for the first one. Used the slow frequency actuator of the NPRO (temp) to scan the frequency.
refcav1 is resonant @ 0.7578V
refcav2 is resonant @ 0.5068V
assuming about 1GHz/V the resonances are about 250MHz different. So we have to use the thermal actuator on one of the cavities in order to tune it. I started a calibration scan for the heater on friday in order to set the correct heating power as the time constant is more than an hour and trial and error method would take too long...
the channels used for the refcav temp scan are:
locked the laser to cavity2 using the PZT and scanning the temp of cavity1
I don't know if its worth the trouble, but we do have a ~200 MHz AOM. Sam Waldman had us buy one of these for doing the OMC g-factor measurements.
i think the problem is that we don't have the VCO for the FSS for 200MHz. so i think it"s easier to heat one of the cavities. the temperature required to match them should be only a couple of Kelvin difference. and by heating one of those cavities the frequency noise due to ambient temperature fluctuations might be uncorrelated as well.
i had a problem taking the data from one of the photodiodes over the weekend. We had a loose connection for the cavity1 transmitted light PD. i only checked on a scope and assumed that the (already) connected cable to the DAQ is OK as i saw some fluctuations while scanning. so i have no data from that long scan over the weekend. we repaired this today and also made the FSS for the first cavity work. now everything is working and i started a new measurement...
sweep frequency is 50uHz which is almost 3h for the full ramp from min to max
C:PSL-FSS_RMTEMP is used for the heating voltage set point (not the heater voltage!). calibration is [-273-(voltage*100)] (is still calibrated for temperature sensors)
we will add the temp sensors on the cavity tank tomorrow...
currently the setting for the NPRO temp slider is:
changes for refcav2 are large due to the missing insulation and heater on that chamber. The LIGO refcav heaters don"t fit as the position of the connections to the main chamber tube are different (and in quantity too). changes over the last couple of hours are .7350 to .7570, which is abou 20MHz and so more than the tuning range of the VCO
current settings for the slow actuator to bring the cavities on resonance are:
this morning the laser was off with the error "HT error". As the chiller was off too i think that means high temperature error, right?
So i checked everything and started the laser again. So far everything is fine and working. Any idea?
added the four Minco heaters and a first layer of 1" thick foam insulation to the second refcav chamber. Pressure is down to 4e-8 torr. started heating with ~20W to bring it above room temperature.
slow actuator settings for being resonant:
heater refcav1: programming voltage 1.95V
heater refcav2: power supply voltage 16V
added a single temp sensor on refcav2 (AD590) last week in order to monitor the temp of refcav2. We tried to change the heater power on both cavities to match the frequencies. So far we have no tempctrl. Changes of the room temperature have too large (and different) influence on the individual cavities. So it"s almost impossible to predict the room temp changes and change the heater power to match both cavities. We need a real temp stabilization of the chambers...
the name of this sensor is C:PSL-FSS_RCTEMP
room temp next to both chambers is measured with C:PSL-FSS_RMTEMP
there is a full LIGO RefCav available here at LHO. They don"t use it anymore and its just sitting here on the table with the pump running. Rick offered me to ship the whole thing to Caltech in order to replace the second cavity in the PSL lab. The advantage would be that we have two identical setups, a good heater and a fitting insulation. In addition we could get analog control.electronics for temperature stabilization.
we repaired the sensors on the first refcav and renamed and added channels to the DAQ. The channels are now:
C:PSL-FSS_MINCOMEAS -> temperature of first refcav (average of 4 AD590 sensors), cal. in degC
C:PSL-FSS_HEATER -> heater power supply control voltage (heater voltage is x10)
C3:PSL-GEN_DAQ1 -> temperature of second refcav (one AD590), cal in degC
C3:PSL-GEN_D2A1 -> heater power supply control voltage (heater voltage is ~ x2.5)
settings for the FSS:
common gain : 1dB
fast gain : 15dB
phase : 1.5V
VCO power : 4.5V
RF amplifier gain : 7V
we added two endcaps for the first refcav and additional insulation for the ion pump and valve.
here a plot of the temperatute stability in peter's lab (upper graph) if nobody is working in the lab (christmas) and if someone opens the door to the lab (the large spikes in room temp). lower graph shows the temp of the refcav (in-loop, no out-of-loop sensor so far. Except the large spikes stability is about 4mK/pp. I will add a single ool-sensor today (will be C:PSL-FSS_RCTEMP). We used this channel so far for the second cavity but have a second temp box since christmas...
the last couple of days we fixed a couple of thinks:
We found a wrong calibration of the temp readout of the first refcav, which gave us a wrong residual noise level compared to the other one. The absolute value was almost correct so we didn't realize this before. Now we are limited by the DAQ noise so the next step will be improving the gain settings in the temp sensor readout-box.
A big problem is the changing room temperature as soon someone opens the door even for only a couple of seconds. The temp control of the lab goes crazy and changes a couple of degrees and is oscillating half a day after that. The delay to the cavities is about 30min and a couple of deg changes of the room temp also change the cavity temperature, only a couple of tens of mK but enough to shift both cavities away from each other as the room temp couples different into both due to the different thermal insulation (insulation and time constants are different). We opened the room temp sensor and adjusted the temperature to i higher level, removed surrounding parts to get a better air flow to the sensor. We don't understand why opening the door has such a large effect on the room temp as 10 seconds of open door don't change the room temp by 2 degC or so. So if anybody has an idea plz let us know even if it seems to be stupid.
In order to improve the temp stability of the analyzer cavity we added a second layer of insulation and wrapped the whole thing in aluminum foil, see picture below. The insulation is 2" thick, except a small part at the large flanges at the end where its only 1" thick. We also changed the gain of the temp sensor readout in order to reduce the influence of DAQ noise. If nobody is working in the lab we have a stability of about 6mKpp within hours. As soon someone is working in one of the labs this changes to tens of mK for working in other labs to about a hundred mK if working in the PSL lab. We are currently working on improved servo settings...
in order to build a custom fit insulation for the cavities i've built some hot wire foam cutting machines to cut the 2" thick polyurethane thermal insulation. The wire i use is a .010 piano wire typically used for suspensions (thats what we had i the lab). Nominal current is about 1.5A at a couple of volts, so any simple power supply does the job.
Here are some pictures of the large foam cutter to cut the 48" x 48" boards into smaller peaces and the circle cutting device:
original panel size
cutting into smaller segments:
surface comparison before and after hot wire cutting:
some cut parts:
ready to cut the rest...
refcav: 0.9512 @ 70.02 degC
acav: 0.9252 @ 39.996 degC
with 0.858V/GHz -> ~30MHz difference
1pm: changed refcav temp to 69.9 degC -> SLOWDC=0.9488
4pm: changed refcav temp to 69.5 degC -> SLOWDC=0.928 (but not final)
7pm: acav now at 0.923, refcav at 0.928 -> difference now ~6MHz
--> good temp values are ~69.5degC for the refcav and ~40degC for the acav
heater value refcav: ~2.983
heater value acav: ~6.245
both @21.6 degC room temp
after some trouble with fluctuating temperatures and a brocken cable to the vco i could finally lock both cavities. So now we can take first data.
temperatures are still fluctuating and the software loops have to be optimized for the new thermal insulation. Currently only a p-servo is running and the channel names have to be finalized...
slow actuator signal: C:PSL-FSS_SLOWDC
room temp: C3:PEM-SENS1_TEMP
actuator signal: C:PSL-FSS_HEATER
temp signal: C:PSL-FSS_MINCOMEAS
actuator signal: C3:PSL-GEN_D2A1
temp signal: C3:PSL-GEN_DAQ1
- more insulation to the refcav chamber added
actual slow actuator values for both cavities:
refcav : 0.4385
acav : 0.2539
-> ~215MHz difference
with ~6mK/Mhz -> refcav temp reduced by 1.3K
refcav temp now 67.7, slow actuator 0.304 -> reduced temp further to 67.5
refcav : 0.243
acav : 0.251
refcav: 0.2456 temp:67.4
acav: 0.2582 temp: 40.0
01/27/10 9 pm
refcav: 0.2423 temp:67.55
acav: 0.2578 temp: 40.0
refcav: 0.2525 temp:67.65
acav: 0.2578 temp: 40.0
01/29/10 4 pm
refcav: 0.2485 temp:67.65
acav: 0.2335 temp: 40.0
I guess if you have a frequency counter with a GPIB interface or a simple flip-flop XOR based phase/frequency discriminator, you can feed the output to a 3113 and use at as an input to a slow EPICS PID to bring the beat frequency to within range.
Actually, we need a frequency discriminator for the Green Locking so it might be good to brainstorm about this with Aidan and Koji.
good idea - we now know where we have to be +- acouple of 100mK as the impact of changes in room temp is not equal for both systems so we have to slightly adjust the temperature of one cavity.
Right now we have a much larger problem with the AOM. if we lock both cavities and the AOM frequency changes slowly over time (both cavities drift a bit) we get a huge amplitude modulation for the refcav, meaning a drift in power from almost 0 to 2.5V (the full range) topped by a sine-like modulation, looks like an etalon with about 10% modulation depth, every 1.4Mhz. It's not the power of the VCO, that changes only by about .2%, but you see this modulation there as well, but almost covered by the noise of the DAQ already. It's a power modulation of the diffracted beam as you can see it in the reflected and transmitted light with the same sign, so it's not pointing or so. I think it could be an impedance matching problem causing some standing wave resonator. We try to investigate it today. I will post a graph later today...
measured the beam pointing caused by driving the AOM frequency modulation input. data is uncalibrated so far, just a screenshot of the dataviewer. PDHOUT is the VCO input signal...
The power at the AOM and the reflected beam from the AOM vs frequency are measured, but they seems to be unrelated to the problems.
When the VCO is modulated by a triangular function +/- 6V, a small fluctuation is observed in transmitted and reflected beam. There might be some small constant phase shift between the two.
at 78 MHz the modulation width is about 0.3 MHz, with +/- 2.4% amplitude
at 79.5 MHz, the width is 0.238 MHz, +/- 1.1 % in amplitude,
at 81 MHz, the width is 0.193 MHz, +/- 1.5% in amplitude.
The first plot shows the comparison between the transmitted beam and the reflected beam between 78 - 82 MHz. The second and the third plots are the whole data from the transmitted beam and the reflected beam respectilvely. The data is taken aroud 2010-02-03 20:00:00 UTC.
measured tf of the analyzer cavity including VCO (wideband input, around 80MHz center frequency), double-passed AOM, cavity, pd (Thorlabs PDA10CS) and mixer for different source levels (to see if there are nonlinearities).
exchanged the old mirror (T330-HR, T331-AR) by a simple Y1-1025-45P to get more power.
measured laser power : 7.17W
downstream of the new output coupler : 134.6mW
added waveplates & pbs to make the power adjustable. current power through the EOM is 8mW which gives about 4.33V on the RF-PD (Thorlabs PDA10CS, 0dB-setting, 17MHz)
acav is now re-aligned. As Tara stopped aligning the AOM i will use the NPRO pzt to lock it (instead of the VCO). Then i will also see where the shit of the other tf comes from (hopefully).
we spent the whole night to re-align everything. By now everything is back online, both cavities were locked.for a short time. We still have the pointing problem. It's different now, seems to be smaller but is now almost the same for x and y. Will do this later today ....
The 10kHz peak in the old spectrum seems to be related to the crossover frequency of the fast actuator and the pc of the fss loop.
The PDH box is modified, all filter stages now have socket adapter boards to easily change the filter frequencies by changing the capacitor value within seconds to optimize everything. A zero for compensating the cavity pole is also installed. A modulation input too.
After adjusting the mirror for reflecting beam back to the AOM, the QPD signal shows the better alignment. Before the voltage different readout is about 1.2 V, now it's reduced to ~500 mV.
We'll try to add a translational stage for the mirror for better alignment.
values for VCO pwr 4.7V
RCTRANSPD signal: 5.17V
DC-level of RF-PD:
refcav unlocked: 590mV
refcav locked : 202mV
current cavity temp: 67.7degC
slow actuator value : 0.8966V
added a detector behind the curved mirror to measure the changes in deflected power (1.order) while changing the frequency of the VCO. PD is a large area Si diode to reduce pointing effects. Signal is connected to QPD-SUM instead of the QPD signal (QPD channels for x and y still connected and valid, only sum disconnected !!). We didn't trust the sum anymore as we can see slightly pointing related changes there as well.
in order to get an idea about the part/subsystem causing the RF power modulation while sweeping the VCO frequency i changed the length of the cable to the AOM. I decided to just extend the existing cable using a cheap SMA cable, not one of those semi-rigid ones. Now i thought due to more losses, low quality cable, longer, more connection etc. the shifted power should be less, but it seems to be the opposite, see screenshot. The purple curve is the transmitted power through the refcav, red the VCO input signal, the rest unimportant. On the left third the old cable, then a short break where i extended the cable, the center part with the longer cable, another short break and then with the original length. First i thought it's some aligment related thing, touching the cable, changing the alignment of the AOM etc. But it's not. The PD behind the curved mirror (not shown) shows exactly the same. Any ideas?
Seem like a screwy AOM. I would take the double-passed beam and beat it with the initial beam (no cavities). Make sure the frequency shift is appropriate and make sure there is no amplitude change in the beat over the whole VCO range.
added the following channels:
C3:PSL-RCAV_DIFFPWR : diffracted power (single pass) measured behind curved mirror
C3:PSL-126MOPA_PWRMON : laser output power monitor measured after PC
C3:PSL-RCAV_QPDSUM changed back to QPD sum signal
all signals available in both framebuilders
seems like a bad AOM and an impedance matching problem together. exchanging the semi-rigid cable by other cables produces a lot of different results in power modulation / pointing vs frequency. right now i found a combination where for frequencies 80MHz and higher it is almost flat, but below it drops a lot. the pointing is related to the absorbed power in the AOM. - you can really feel the heat of the crystal, no joke. will measure the temp on tomorrow for different frequencies where the pointing is worst case to see if it is a macroscopic change in temp. Aidan has this nice thermal image camera to do this. i think we should try the crystal technology AOM we have. i don't think that aligning helps a lot here. tried this the whole afternoon :-( the lowest power modulation over the entire frequency range is about 20%pp
After verifying that the Isomet AOM causes the power modulation and realigning it did not seem to help improving the pointing effect, we switched to a Crystal Tech AOM. The 1st order diffracted beam has 55% of the total power, the remaining power before entering the cavity is ~15mW. The pointing effects observed from QPD improved a little bit to 500 mV on X and Y directions.
everything is back, pointing is much much less, power modulation too, ugf of the other loop is much higher - but nevertheless the performance is not much better, only one order of magnitude. it turns out that the noise is limited by the existing FSS stuff. tuning the gains (common, fast) can produce almost any shape and level (see graph), except much lower levels are not possible, at least not for more than a couple of seconds before everything starts to oscillate. so i will to debug the old FSS stuff first to see whats going on there. will try to investigate the noise of the FSS loop and maybe replace the VCO by a function generator and probably the RF photodiode to see if that changes something. Those can be exchanged easily without changing to much...
measured a couple of times today with everything re-aligned and different gain settings for the FSS stuff. Measured also to lower frequencies. The problem here is that the frequency band of interest can not be measured with epics (to slow) and the measurement using the SR785 takes so long that the operating point of the VCO and therefore the coefficient changes during the measurement and different spectra don't fit together quite well. so we have to measure a couple of times to get some measurements fitting together when the changes are so small that we don't see it with our eyes. this gives us a new upper limit of:
maximum gain settings:
CG : 24dB
FG : 13dB
Looking better. I'm curious about what the existing loop shapes are. The old FSS hardware is designed to drive an NPRO + 1 EOM. Is that the existing layout?
However, I think its not designed to drive an EOM. The PDH box should be able to drive the VCO if we replace the output OP27 with a TLE2027. The main point is that the refcav only provides a pole @ 40 kHz and we need the electronics to be 1/f below there. The old FSS board used to do this by the combination of the series resistor and the NPRO PZT capacitance. The VCO is not a capacitive load. I guess that Tara is working on a Simulink model of this whole setup?
I kind of doubt that we will have success without using resonant RFPDs. In most of the PSLs we use a pretty large modulation depth and its necessary to really tune the 2f trap to get rid of the J1^2 term.
Different idea: why not just use the VCO/AOM to lock to the ACAV? Then if we pick off the beam for the RCAV after the AOM, the feedback to the NPRO can be used as the differential cavity signal. In this way, we are not sensitive to the VCO calibration issues since its squashed by the ACAV loop.
Idea #2: Just use the transmitted light from the cavities and beat them. Its only phase detection, but in principle, this is easily good enough to detect what we want. Also RF sidebands are gone and all we need is an 1811 or such to detect the beat signal.
Today I calibrated the QPD on the ref cavity. The armlength from AOM to the quad is 2.1 m.
The calibration for x and y channel on the QPD in radian unit
QPDX: radian = [dV]* 10^-3 / (3.36 * Vo)
QPDY: radian = [dV] * 10^-3 / (3.689 * Vo)
QPDY: radian = [dV]
/ (3.689 * Vo)
where [dV] is reaout voltage in Volt, Vo is the sum voltage from 4 quadrants in Volt.
If the prefer units are in micro radian and milli volt
QPDX: urad = [d_mV] / (3.36 * Vo)
QPDY: urad = [d_mV]/ (3.689 * Vo)
where [d_mV] is reaout voltage in milliVolt, Vo is the sum voltage from 4 quadrants in Volt.
I took a picture of the setup in PSL lab, and drew a line for laser path. I omit the mode cleaning part since it's not in use now.
we had several shutdowns of the laser within the last days. A couple of times the well known "HT error", today we had an "PS error" for the first time. When does this happen? The other error is related to a malfunction of the chiller as we found out by luck. The chiller temp readout jumps from 26 down to 15 or so within a fraction of a second (so it's not real). This causes the PS to start heating even if the temp is high enough. This screws up the stabiliy of the laser and sometimes causes a chiller error as well. But the "PS error" ? Any idea?
does anyone know the typical operating current for the 100mW lightwave laser model ? (M126N-1064-100) It's typically ~1.1A for the 200mW model. I've set up everything and it starts to lase around 0.44A, so at least its not dead but i don't know how high up i can go. My guess is that it is something around 0.8A but i have no datasheet which tells me...
Peter gave me the hint that the default values are stored in an eeprom in the laser head. So connecting the head to a driver not used before shows the default values for the head. For this head it's 0.86A. So i measured the slope of the NPRO up to that value, reaching the 100mW at the default value stored in the head without tuning the diode temperature. So the head seems to be refurbished and not dead...
I finished aligning the beams from Ref cavity and Acavity, broke a connector on the PD we were going to used to measure the beat .
Frank borrowed another PD to use for now and ordered the replacement for the broken connector which should arrive next week.
Now I'm waiting for the temperature to settle, so both cavities can be locked and see the beat.
the laser stopped working while beeing in the lab but not touching the laser or table or anything. I was looking for an EOM mount and suddenly the chiller beeped. The power supply stopped working with a "PS error" message, but the more interesting fact is that the setpoint of the chiller was changed to 15degC (my guess is from the PS), but it should be at 26.5degC. So maybe the PS is also causing the freezing of the chiller, simply sending the wrong setpoint values when having a fault...