I ran the script for measuring arm-loss and calculated rough Y-arm round trip loss temporally. The result was 89.6ppm. (The error should be considered later.)
The measurement was done as follows:
('AS_DARK =', '0.0019517200000000003') #dark noise at ASDC
('MC_DARK =', '0.02792') #dark noise at MC2 trans
('AS_LOCKED =', '2.04293') #beam power at ASDC when the cavity was locked
('MC_LOCKED =', '2.6951620000000003')
('AS_MISALIGNED =', '2.0445439999999997') #beam power at ASDC when the cavity was misaligned
('MC_MISALIGNED =', '2.665312')
#normalized beam power
the script "armloss_AS_calc.py",
Some changes were made in the script for getting the signals of beam power:
In the yesterday measurement the beam power of ASDC is higher when locked than when misaligned and I wrote it maybe caused by over-coupled cavity. Then I did a calculation as following to explain this:
I used these values for measuring armloss:
then the uncertainties reported by the individual measurements are on the order of 6 ppm (~6.2 for the XARM, ~6.3 for the YARM). This accounts for fluctuations of the data read from the scope and uncertainties in mode-matching and modulation depths in the EOM. I made histograms for the 20 datapoints taken for each arm: the standard deviation of the spread is over 6ppm. We end up with something like:
XARM: 123 +/- 50 ppm
YARM: 152+/- 50 ppm
This result has about 40% of uncertaintities in XARM and 33% in YARM (so big... ).
In the previous measurement, the fluctuation of each power was 0.1% and the fluctuation of P(Locked)/P(misaligned) was also 0.1%. Then the uncertainty was small. On the other hand in my measurement, the fluctuation of power is about 2% and the fluctuation of P(Locked)/P(misaligned) is 2%. That's why the uncertainty became big.
We want to measure tiny value of loss (~100ppm). So the fluctuation of P(Locked)/P(misaligned) must be smaller than 1.6%.
(Edit on 10/23)
I think the error is dominated by systematic error in scope. The data of beam power had only 3 degits. If P(Locked) and P(misaligned) have 2% error, then
You have to check the configuration of scope.
The scripts for measuring armloss are in the directory "/opt/rtcds/caltech/c1/scripts/lossmap_scripts/armloss_scope".
Some facts which should be considered when doing this measurement and the associated uncertainty:
sstop using the ssscope, and just put the ssssignal into the DAQ with sssssome whitening. You'll get 16 bitsśšß.
I increased the resolution on the scope by selecting Average (512) mode. I was a bit confused by this, since Yuki was correct that I had only 4 digits recorded over ethernet, which made me think this was an i/o setting. However the sample acquisition setting was the only thing I could find on the tektronix scope or in its manual about improving vertical resolution. This didn't change the saved file, but I found the more extensive programming manual for the scope, which confirms that using average mode does increase the resolution... from 9 to 14 bits! I'm not even getting that many.
The UL signal of the shadow sensor on ETMY went to zero this evening.
This was due to a loose connection on the cross connection board on the 1Y4 rack.
In order to make them tighten, a combination of stand-offs and screws were installed on the connectors. They won't be loose any more.
I pushed the "closed loop" button on PZT2 YAW around 3:40 pm today, then roughly recentered it using the DC Offset knob on the PiezoJena controller and the IP ANG QPD readbacks. There was a large DC shift. We'll watch and see how much it drifts in this state.
Here's the trend.
The transient at ~22:40 is Rob switching to 'Open Loop' on the Piezo Jena PZTs. I don't see any qualitative change in the drift after this event.
At 05:55 UTC, I removed an iris that was blocking the IP POS beam (the sum goes up from 2 to 6.5) without disturbing the mirrors who's oplev beam are on that table. Steve has conceded one sugar Napoleon after betting against my ninja-like iris skills.
We should recenter the beam on IP POS now that its unclipped - I'll let it sit this way overnight just to get more drift data.
I wanted to see how long our IP POS beam has been badly clipped - turns out its since April 1, 2007.
Steve's April Fool's joke is chronicled then. The attached trend shows that the drop in IP POS is coincident with that event.
In trying to align IPPOS, I noticed that someone has placed a ND2.0 filter (factor of 100 attenuation) in front of it. This is kind of a waste - I have removed IPPOS to fix its resistors and avoid this bad optic. Also the beam coming onto the table is too big for the 1" diameter optics being used; we need to replace it with a 2" diamter optic (Y1-2037-45P).
IP ANG dropped by a factor of 2 back in early August of '08.
We need this guy on the investigation:
[Valera / Kiwamu]
The pointing of the incident beam to the interferometer has been jumping frequently.
Due to this jump the lock of the Y arm didn't stay for more than 2 min.
We turned off the strain gauge loop of PZT2-YAW and PZT2-PITCH, then the spot motion became solid and the Y arm locking became much more robust.
To aid in lock-loss studies, I made a new program called 'lookback', similar to 'getdata', to look at past data.
When called with channel name arguments, it runs continuously, storing all channel data in a ring buffer. When the user hits Ctrl-C, all the data in the ring buffer is displayed. There is an option to store the data in the ring buffer to disk as well.
controls@rosalba:/opt/rtcds/caltech/c1/scripts/general 0$ ./lookback -h
usage: lookback [-h] [-l LENGTH] [-o OUTDIR] channel [channel ...]
Lookback on testpoint data. The specified amount of data is stored in a ring
buffer. When Ctrl-C is hit, all data in the ring buffer is plotted. Both 'DQ'
and 'online' test point data is available. Use NDSSERVER environment variable
to specify host:port.
channel Acquisition channel. Multiple channels may be
specified and acquired at once.
-h, --help show this help message and exit
-l LENGTH, --lookback LENGTH
Lookback time in seconds. This amount of data will be
stored in a ring buffer, and plotted on Ctrl-C.
Default is 10 seconds
-o OUTDIR, --outdir OUTDIR
Output directory to write data (will be created if it
doesn't exist). Data from each channel stored as
'<channel>.txt'. Any existing data files will be
For some purposes I looked back the data of some channels which don't exist any more. Here I explain how to do it.
If this method is not listed on the wiki, I will put this instruction on a wiki page.
(1) Edit an "ini" file which is not associated to the real-time control (e.g. IOP_SLOW.ini)
(2) In the file, write a channel name which you are interested in. The channel name should be bracketed like the other existing channels.
(3) Define the data rate. If you want to look at the full data, write
datarate = 2048
just blow each channel name.
Or if you want to look at only the trends, don't write anything.
(4) Save the ini file and restart fb. If necessary hit "DAQ Reload" button on a C1:AAA_GDS_TP.adl screen to make the indicators green.
(5) Now you should be able to look at the data for example by dataviewer.
(6) After you finish the job, don't forget to clean up the sentences that you put in the ini file because it will always show up on the channel list on dtt and is just confusing.
Also don't forget to restart fb to reflect the change.
This first plot shows the RC temperature channels' performance from 40 days ago, before we disabled the MINCO PID controller. Although RCTEMP is supposed to be the out of loop sensor, what we really care about is the cavity length and so I've plotted the SLOW. To get the SLOW on the same scale, I've multiplied the channel by 10 and then adjusted the offset to get it on the same scale.
The second plot shows a period after that where there is no temperature control of the can at all. Same gain scaling has been applied to SLOW as above, so that instead of the usual 1 GHz/V this plot shows it in 0.1 GHz/V.
The third plot shows it after the new PID was setup.
Summary: Even though the PID loop has more gain, the true limit to the daily fluctuations in the cavity temperature and the laser frequency are due to the in-loop sensors measuring the wrong thing. i.e. the out-of-loop temperature is too different from the in-loop sensor. This can possibly be cured with better foam and better placement of the temperature sensors. Its possible that we're now just limited by the temperature gradients on the can.
Here is a 7 years plot of of the 40m temperature variations.
I checked out the cable that I took from you, and all of the connections looked right. The only thing I did notice was that some of the soldered wires on the 37-pin connector had gotten hot enough to melt their insulation, and potentially short together. I cut off that connector, and left it on your desk to check out. I put on a new connector, and checked the pinout. If the Guralps still doesn't work, we'll have to check out other possibilities.
The lock of DRMI wasn't stable enough to measure the sensing matrix. Failed.
PRMI and SRMI were okay and in fact they could stay locked robustly for a long time.
I added a new option in the C1IFO_CONFIGURE screen so that one can choose Signal-Recycled Michelson in carrier resonant condition.
Additionally the orthogonalization of the I-Q signals on REFL55 should be done because it hasn't been done.
Since REFL11 has gone I tried locking the PRMI with combination of REFL55 and AS55.
Without any pain the lock of PRMI was achieved successfully. AS55 was used to sense MICH and REFL55 was used for PRC.
Additionally I was modifying several scripts which are invoked from C1IFO_CONFIGURE.adl. Some details about the scripts will be uploaded on the wiki later.
An important thing is that now we are able to use the "restore" commands for the Y arm, X arm, Michelson and PRM locking.
The scripts will automatically acquire the lock of each DOF. The image below is just a screen shot of the medm screen where you can call the scripts.
( Still to do)
* PRM actuator response measurement
* PRC noise budget
* MICH-PRC actuator decoupling
Through some locking exercise I found that several things are degrading.
Remember the interferometer is like a cat, so we have to feed and take care of her everyday. (Otherwise the cat will be dead !)
Locking of the Arms :
Locking of PRM :
I had to realign PSL beam into the MC in order to reobtain the MC lock. We lost lock at sometime around 8:30 AM on Tuesday. See attached trend data for MC_RFPD_DCMON.
The is the second time this week that I had to do this when we were unable to obtain the MC lock. On both occassions the zig-zag at the end of the PSL table was tweaked to minimise the MC_RFPD_DCMON.
We have been using the MC as a Beam Axis Reference. And therefore we are adjusting the PSL beam to maximise coupling into MC. However if MC's beam axis has shifted, then would it not be best to use the pzt's to re-obtain coupling into the arm cavities?
80 days: PMC is drifting
There was no shaking (that disturbed the locking) tonight!
The problem is hard to debug because we are feeding back on the ETMs, BS and PRM, and at the low CARM offset (= high PRG), all the DoFs are cross coupled strongly so just by looking at error/control signals, I can't directly determine where the noise is originating. The fact that the ALS CARM spectrum shows a noise increase suggests that the problem has to do with the test masses, PSL, IMC, or end green PDH setups.
My plan is to do a systematic campaign and eliminate some of these possibilities - e.g. install some baffling around the fiber coupler and the end green PDH photodiodes and see if there is any improvement in the situation.
* In attachment #1, the "Ref" traces are when the CARM offset is 0, and the arms are buzzing in and out of resonance. The non-reference traces are for when the CARM offset is ~28kHz (so several linewidths away from resonance).
Last night I was trying to calibrate the MICH error signal and the actuators on BS and ITMs.
However I gave up taking the data because the MC locking was unstable. MC3 drifted a lot.
I was trying to make the DRMI lock more robust.
Increasing the gains of the oplev on SRM helped a lot, but the lock is still not solid enough for measurements.
According to some line injection tests, the SRCL and MICH signals show up in AS55Q with almost the same amplitudes.
I tried to diagonalize the input matrix (particularly MICH-SRCL in AS55) based on the result of the line injection tests, but I ran out the time.
Now that the updated ALS is stable, and the PRC angular FF is revived, I've been working on relocking PRFPMI. While the RMS arm fluctuations are surely smaller than they used to be, there is no noticible difference to the ears when buzzing around resonance, but this doesn't really mean much.
Frustratingly, I am not able to stably blend in any RF CARM error signal into the slow length control path (i.e. CARM_B). Bringing AS55 Q into DARM with the 20:0 integrator is working fine, but we really need to supress CARM to get anywhere. I'm not sure why this isn't working; poking around into the settings that were used when we were regularly locking didn't turn up any differences as far as I could tell. Investigations continue...
Some minor changes to the locking script were made, to account for the increased ALS displacement sensitivity from the longer delay line.
Since the ALS is now in a fairly stable state, I've updated the calibrated PSD template at /users/Templates/ALS/ALS_outOfLoop_Ref.xml, and added some coherence plots for some commonly coupled quantities (beat signal amplitude, IR error signal, green PDH error signal and green transmission).
Here is a plan in my mind and these are basically the details of the gantt chart (#6143):
Keiko, Anamaria, Koji
We were not able to establish the stable DRMI tonight. We could lock MICH and PRCL quite OK, and lock the three degrees of freedom at somewhere strange for several seconds quite easily, but the proper DRMI lock was not obtained.
When MICH and PRC are locked to the carrier, REFL DC PD reading dropps from ~3000 counts to 2600~2700 counts as REFL beam is absorbed to PRC. We'll try to lock PRC to sidebands - but flipping gain sign didn't work today, although it worked a few days ago.
POP beam (monitor) is useful to align PRM.
No real progress.
Probably I spent a bit too much time realigning the beat-note optical path.
(what I did)
Status update on the LSC activity:
Today we worked on PRM angular servos and Y-arm ALS stabilization.
In the current PRMI angular control configuration two servos simultaneously drive PRM - oplev and POP ASC. We considered 2 ways to redesign this topology:
The first option requires model rewiring so we started from the second one. We had to redesign POP ASC pitch and yaw servos for this because PRM TF has changed. Attached is servo OLTF.
This method worked out well and once PRMI is locked we turned off oplev servo with ramp of 0.5 sec and enable ASC POP servo with ramp of 1 sec.
Once PRMI was locked and ASC running we have turned off PRM angular local damping that presumably prevents us from bringing arms into resonance due to IR coupling to shadow sensors.
PRMI was stable using only ASC POP servo and we moved on to ALS. We found Y-arm beatnote and enabled control to ETMY.
Cavity was stabilized but not robust - we were loosing IR in a minute because green relocked to 01 mode with transmission equal to more than half of 00 mode. This is probably due to angle to length coupling of ETMY.
We were also loosing IMC during cavity stabilization. We made MCL servo and will tune it tomorrow looking at the arm spectrum as an OOL sensor.
Tonight we worked on tweaking up the PRCL new ASC, and then PRMI+1 arm locking. We were unable to get the Xarm to stay locked on a TEM00 mode for very long, and after an hour or two of using the PZTs to try to align the beam to the cavity, we gave up and just used Yarm green.
NB: We haven't done anything to MCL, although it is not in use. Den is still going to get around to elogging what servo shaping he changed on that last night.
I wrote a script that will handle the transitions between the new PRCL ASC and the PRM oplev and local damping. The script is accessible from the PRC ASC screen, and will detect when the PRMI is locked or not. When it is locked, it will turn down the PRM oplev gains and turn on the ASC, and then it will turn off the local shadow sensor damping for PRM pitch and yaw. When the PRMI unlocks, the script will turn off the ASC and restore oplev and local shadow sensor damping.
We saw that the bounce mode of the PRM was getting rung up with our new ASC, so we included a band stop in the ASC, and also turned on the triggering for the PRCL LSC FM6, which has the resonant gain for the bounce mode (as well as roll, and the stack mode). This made the PRMI spot very stable.
We then moved on to green arm locking. The Yarm is behaving perfectly nicely (as nice as it has been lately - it's alignment and mode matching could also use some work), but Xarm was giving us a bit of trouble. As always (since the PZTs were installed?), the mode matching isn't excellent for the green to the arm, so it can be hard to catch a TEM00 mode. Also, even if we did catch a good mode, it would often not stay locked for more than a few tens of seconds. We tried several alignment tweakings, and several different end laser temperatures (within the confines of seeing the beatnote under 100MHz), and didn't have a lot of success. It looks like Eric had the slow servo engaged for the Xend laser, so the temperature offset was something like +300,000, which seemed totally crazy. I turned that off, and found the beatnote somewhere around output of -10,300. So, I haven't gone to the end to look at the temperature, but it's going to be different than when Eric was taking measurements this afternoon. It seems like the main problem with the Xarm is poor mode matching - the maximized input pointing for TEM00, when you unlock and relock the cavity, is just as likely to give you a TEM_9_0 mode, as TEM00.
So, we gave up on the Xarm for the evening, and tried PRMI+1arm, with the new PRCL ASC. This was successful! The Yarm beatnote was around laser slow servo output of +4450. Beatnote at 46.0MHz, -26dBm. We found the IR resonance, moved off, locked the PRMI, transitioned to the new ASC, and brought the Yarm back to IR resonance. What we see is that the power fluctuations in the PRC are much smaller than they were back around Halloween (elog 9338), however the arm power fluctuations still seem very, very large. This is certainly partly due to the fact that we haven't done a thorough Yarm alignment since before messing with the greens, so we will have drifted somewhat. Also, the ALS beatnote sensor isn't perfect, so won't be perfect at holding us near resonance.
Den is thinking about whether we can use the arm transmission QPD signals to feed back to the ETM ASC servos, to try to reduce the RIN in the arms. I feel like we should also see if this amount of power fluctuation can be explained by our ALS noise, because maybe we'll be fine once we transition to IR and turn off the ALS system. Attached is a plot showing that the arm's RIN is coherent with the spot motion seen by the transmission QPD, so we need to check the alignment of the cavity, as well as consider using the trans QPD in an ASC feedback loop.
Here is a plot of the PRC sideband power, as well as the Yarm transmission. The GPS time for this plot is approximately 1070963372.
According to the measurement by Eric, the X-arm green PDH UGF is too low. We still have some room to increase the gain.
The out of loop stability of the ALS for each arm should be measured everyday.
Otherwise we can't tell whether the arm is prepared for advanced locking activities or not.
We expect to see the arm stablity of ~50pm_rms for the Y arm and ~150pm_rms for the X arm.
I had a look on x,y arms stabilization using ALS. Input green beam was misaligned and I was loosing 00 every few minutes. I vent on the floor and realigned green beams.
YARM alignemt was smooth - transmission increased from 0.4 to 0.85 with PSL shutter off.
XARM was tough. Steering mirrors did not have any derivatives when transmission power was 0.5. I walked the beam with piezos but got only 0.55. It seems that the input beam is mismatched to the cavity. When the transmission was 1 last time? Does anyone have a model of the xend table to compute mode matching?
Input green alignent was improved and I could keep arms stabilized for periods of ~30min - 1 hour. Still not forever.
I noticed that ALS_XARM and ALS_YARM servos have limiters of 6000 and control signal had high frequency components that were not rolled off as shown on the plot "ETMY_DRIVE". I have added a low pass filter that reduced RMS by factor of 5 and took 7 degrees of phase at UGF=150 Hz. Now margin is 33 degrees.
Then I excited ETMY longitudinally at 100 Hz and measured first and second harmonics of the YARM RIN. I got total DC offset of 0.3 nm. This means significant length coupling to RIN. First of all, "scan arm" script does not tune the offset very precise. I guess it looks at DC power, checks when cavity passes through symmetrical points of the resonance and takes the average. It is also useful to look at POX/POY and confirm that average is 0. Plot "ALS_RIN" shows comparison of YARM power fluctuations when it is locked using IR and stabilized using ALS. By manually correcting the offset I could reduce length coupling into RIN, coherence was ~0.1.
Cavity RMS motion also couples length to RIN. Plot "ALS_IR" shows YARM error signal. I also looked at POY signal (LSC-YARM_IN1) as an OOL sensor. At low frequencies POY sees only IMC length fluctuations converted to frequency. I have engaged MCL path and ALS error and LSC error signals overlaped. Cavity RMS motion is measured to be 200 pm.
Some results and conclusions from tonight:
PRC macroscopic length is detuned. We measured REFL phases in carrier and sideband configurations - they are different by ~45 degrees for both 11 and 55 MHz sidebands. Additional measurement with phase locked lasers is required.
We got stable lock of PRMI+2arms with CARM offset of ~200 pm. We think this is the point when we should transition to 1/sqrt(TR) signals. We plan to rewire LSC model and also test CM servo with 1 arm during the day.
POP ASC OL shape changes when we reduce CARM offset probably due to normalization by sum inside the PD. Servo gets almost useless when PRMI power fluctuates by a factor of few.
SMA cables were made and installed for the REFL165 RF amplifier in lsc rack.
[ericq, Jenne, Zach]
We spent some time tonight trying to push our CARM locking further, to little avail. DARM/CARM loop oscillations kept sneaking up on us. We measured some MC2 motion -> REFL11 Transfer Functions to see if we could see CARM plant features; plots will come in the near future...
Probably things would have worked better if you would have gotten your hair done at the same place as me.
The goal tonight was to go through the locking scripts to see if I could recover the state from November 2019, when I could have the arm lengths controlled by ALS, and sit at zero CARM offset with the PRMI remaining locked and the arm powers fluctuating between 0-300. The IR-->ALS transitions went smoothly tonight, and the PRMI locking was also fairly robust when the CARM offset was large, but was less good when reduced to 0. Nevertheless, it is good to know that the system can be restored to the state it was late last year. Next step is to figure out how to keep the PRMI locked and get the AO path engaged, this was what I was struggling with before the new year.
- Routine alignment
Locked the arm cavties. Ran ASS. As this was not enough precise alignment for PRMI locking, Yarm alignment was re-adjusted by sliders.
Xarm was also aligned in the same way.
- OPLEV alignment
Once the arms were aligned, OPLEV spots were adjusted. For this adjustment, PRM had to be aligned and OPLEV servos needed to be turned off.
- LSC offset nulling
While Jenne was measuring the dark output of the POP PD, LSC offset nulling script was executed.
- Compensation of the POP spot size fix
As Jenne reported the POP path now has a lens and the denominator for the normalization got bigger.
To compensate this change, PRMI(sb) was locked by the same configuration as yesterday (i.e. AS55Q for MICH, REFL33I for PRCL).
After some try and error, configuration for stable locking was found.
Signal source: REFL33I / Normalization POP110I x 1.00 / Trigger POP110I 80up 10down
Servo: input matrix 1.00 -> PRCL Servo FM3/4/5/6 Always ON G=+8.00
Actuator: output matrix 1.00 -> PRM
Signal source: AS55Q / Normalization POP110I x 0.01 / Trigger POP110I 80up 10down
Servo: input matrix 1.00 -> MICH Servo FM4/5 Always On G=-30
Actuator output matrix -1.00 -> ITMX / +1.00 -> ITMY
This suggests that POP110I signal is 5~6 times more than before the lens was installed.
- SQRTing option for POP110I was implemented
The PRMI optical gain is derived from (Carrier)x(1st order Sideband) or (2nd order SB)x(1st order SB).
Here the carrier and the 2nd order sidebands are nonresonant.
Therefore the optical gain is proportional to the amplitude power recycling gain of the 1st order sidebands.
On the other hand, POP 2f signals are derived from the product of the 1st and -1st order sidebands.
This means that we should take a sqrt of the POP signals to compensate the recycling gain fluctuation.
- Locking with SQRT(POP110I)
Signal source: REFL33I / Normalization SQRT(POP110I) x 10 / Trigger POP110I 10up 3down
Servo: input matrix 1.00 -> PRCL Servo FM3/4/5/6 Always ON G=+8.00
Actuator: output matrix 1.00 -> PRM
Signal source: AS55Q / Normalization SQRT(POP110I) x 0.1 / Trigger POP110I 10up 3down
Servo: input matrix 1.00 -> MICH Servo FM4/5 Always On G=-30
Actuator output matrix -1.00 -> ITMX / +1.00 -> ITMY
The lock seems not so different from the ones without SQRTing.
The spot was still moving in yaw direction. If I chose a correct alignment, I could minimize the modulation of the internal power
by misalignment. As you can see in the following plot.
When the alignment was deviated from the optimum, the misalignment induced RIN was much worse although this was the longest lock I ever had with the PRMIsb. (more than 8 min)
- Locking with other signal sources
Demodulation phase was adjusted to make the difference of the peak heights for MICH maximized.
After the lock is acquired, I tried to swap the signal source at the input matrix. PRCL swapping was successful but
MICH swapping was not successfull.
It is much more hard to lock the interferometer with REFL55I compared with REFL33I.
As REFL165 PD never produced any useful signal, I tried to swap it with the BBPD used in the green setup.
- Borrowed the PD, power supply from the green setup.
- Put REFL165PD aside. Placed the BBPD in the path. The DC output was 0.8V. This corresponds to the input power of ~5mW.
- Checked the signal but it was very litte (several counts even at the maximum whitening gain).
- Decided to use the power reduction pick off to introduce much more light on the PD.
This PO mirror is 90% reflector. Therefore I had to be careful no to fry the diode.
Currently there are OD1.3 (x1/20) power attenuator to reduce the input power down to 6.5V (40mW).
- The resulting signal is very wiered suggesting the saturation of the PD at the RF stages.
- Probably I need to make a new PD circuit which has the high pass filter to reject other low frequency components.
I succeeded in locking the end green laser to X arm with the new ETM.
Though the lock is still not so stable compared to the previous locking with the old ETM. Also the beam centering is quite bad now.
So I will keep working on the end green lock a little bit more.
Once the lock gets improved and becomes reasonably stiff, we will move onto the corner PLL experiment.
- beam centering on ITMX
- check the mode matching
- revise the control servo
We eventually succeeded in locking X arm with the infrared beam.
The PDH signal is taken at MCL's ADC instead of c1lsc's, and fedback to MC2_POS through the MCL path.
Right now the lock is not so stable for some reasons, so we need to investigate it more.
(what we did)
- strung a long BNC cable to connect the demodulated signal and the ADC of c1ioo.
We didn't touch anything on the demodulation system, so the setup for the demodulation is exactly the same as that of yesterday (see here).
- disconnected the actual MCL cable from the ADC breakout board at 1X2 rack. And put the demodulated signal onto it.
- checked the analog dewhitening filter state for the MC2 coil driver, found the analog filter are always off.
So we just made simDW and invDW always on.
- changed the gain of the MCL loop to have a stable lock for the X arm.
right now a reasonable setup in the MCL filters are:
FM1:ON, FM10:ON, G=0.1
- In fact the lock of the MC is not so stable compared to before, frequently an attempt of locking the X arm leads to the unlock of the MC.
I locked MI while both arm length are stabilized at IR resonance. This could be done using DC READOUT, in other words, use AS_DC as MICH error signal.
Lock using RF signals are still not successful.
The IFO is locked, at the operating point (zero CARM offset). The problem with reducing the residual CARM offset in the last stage appears to have been because the common mode gain was getting too high, and so the loop was going unstable at high frequencies.
The cm_step script is currently a confusing mess, with all the debugging statements. I'll clean it up this afternoon and check that it still works.
I guess I succeeded in locking of the cavity with the green beam
Strictly speaking, the laser frequency of the end NPRO is locked to the 40 meter arm cavity.
Pictures, some more quantitative numbers and some plots are going to be posted later.
After the alignment of the cavity I could see DC fringes in its reflection. Also I could see the cavity flashing on the monitor of ETMY_CCD.
I drove the pzt of the NPRO with f=200kHz, and then the spectrum analyzer showed 200kHz beat note in the reflection signal. This means it's ready to PDH technique.
And then I made a servo loop with two SR560s, one for a filter and the other for a sum amp.
After playing with the value of the gain and the sign of the feedback signal, the laser successfully got lock.
To make sure it is really locked, I measured the open loop transfer function of the PDH servo while it stayed locked. The result is shown in the attached figure.
The measured data almost agrees with the expected curve below 1kHz, so I conclude it is really locked.
However the plot looks very noisy because I could not inject a big excitation signal into the loop. If I put a big excitation, the servo was unlocked.
The current servo is obviously too naive and it only has f-1 shape, so the filter should be replaced by a dedicated PDH box as we planed.
Congratulation! Probably you are right, but I could not get this is a real lock or something else.
1) How much was the fringe amplitude (DC) of the reflected beam? (Vref_max=XXX [V] and Vref_min=YYY [V])
Does this agree with the expectation?
2) Do you have the time series? (V_ref and V_error)
What did you use to filter the 2f components from your error signal? A homemade low pass or what?
I am using a homemade low pass filter.
It's just a RC passive LPF with the input impedance of 50 Ohm.
A progress on the end PDH locking :
by using a modified PDH box the green laser on the X-end station is locked to the arm cavity.
So far the end PDH locking had been achieved by using SR560s, but it was not sophisticated filter.
To have a sophisticated filter and make the control loop more stable, the PDH box labeled "#G1" was installed instead of the SR560s.
After the installation the loop looks more stable than the before.
Some details about the modification of the PDH box will be posted later.
Although, sometimes the loop was unlocked because the sum-amp (still SR560) which mixes the modulation and the feedback signal going to the NPRO PZT was saturated sometimes.
Thus as we expected a temperature control for the laser crystal is definitely needed in order to reduce such big low frequency drive signal to the PZT.