ELOG 40m

Attempt to replace the DC-DC converter (aborted)

Rich, Steve, Yoichi

We opened the MOPA box and inspected our NPRO.
We concluded that this NPRO is different from the ones at the sites.
At the sites, the NPROs have a connector on the board which accepts the output of the DC-DC converter.
Rich's replacement DC-DC converter has a matching connector to it. So replacement of the DC-DC converter is easy.
In our NPRO, there is no such a connector found. The cables coming from the external power supply are directly soldered
on to the PCB (see attm1).

We have to take out the PCB in order to work on it.
As shown in the second picture, there is a D-SUB connector sticking out of the box through the rear panel.
In addition, the PCB is connected to the metal box containing the crystal with an IDE style connector.
This means the PCB is tightly constrained.
To take out the PCB without applying too much stress on it, we have to take off the rear panel.
To do so, we have to remove the screws on the bottom of the NPRO box. That means we have to move the NPRO.
We did not want to do so, because it will screw up the alignment to the amplifier.

The model number of the DC-DC converter looks like NMH0512-something.
According to the datasheet of NMH0512S, the switching frequency is typically 95kHz. We saw 77kHz harmonics in the FSS error signal.
I'm not sure if this is the culprit. I will try to measure the EMI from this guy later.

Attachment 1: DCDC.JPG

Attachment 2: NPRO.JPG

1037

Wed Oct 8 23:18:23 2008

Correlation between the Sorensen switching noise and the FSS error signal

I took some spectra and coherence function of the FSS error signal and the +24V Sorensen power line.
The first plot shows spectra of the two signals. Looks like Sorensen is not responsible for most of the lines
in the FSS error signal.
The coherence function between the two signals supports it (second plot).
Slight coherence can be seen at 23kHz and 98.4kHz but not significant.

I will check the coherence of the power line with the ISS signal next.

Attachment 1: PowerLineSpe.png

Attachment 2: Coherence.png

1042

Mon Oct 13 11:32:50 2008

Yoixhi

MOPA is in trouble now

Steve, Alberto, Yoichi

A quick update.
The MOPA output went down to zero on Sunday early morning (00:28 AM).
We found that the NPRO beam is mis-aligned on the power monitoring PD (126MON).
We don't know yet if it is also mis-aligned to the power amplifier (PA) because the mechanical shutter is not working (always closed).
Most likely the beam is not aligned to the PA.
A mystery is that although the beam is terribly (more than a half inch) missing the monitor PD, the beam still goes through two faradays.
Another mystery is that the NPRO output power is now increased to 600mW.

The power drop was a very fast phenomenon (less than 1/16 sec).
We are trying to figure out what happened.
The first step is to fix the mechanical shutter. We have a spare in our hand.

Attachment 1: powerdrop.png

1043

Mon Oct 13 13:51:49 2008

attempt to measure FSS ref phase

On Friday I began a measurement of the FSS reference phase. The setup involves the following:
+ turn off the 166 MHz LO (top signal generator on 1Y2 rack)
+ bring FSS LO 21.5 MHz to the 166 MHz delay line phase shifter, and back out the phase shifter with a second length of cable
+ add a length of cable to the RF 21.5 MHz in preparation for measuring FSS IN2 as a function of delay

Trouble locking the FSS, and ran out of time before the measurement could be performed.

1044

Mon Oct 13 13:56:03 2008

MOPA is not that much in trouble now

The problem was found to be all to do with the shutter.
The shutter started to work again, after a while, apparently for no clear reason.
The alignment to the PA was actually not screwed, and the MOPA output is now slowly increasing.
We figured that the 126MON PD has been mis-aligned for a long time. It was just picking the
scattered light from the output of the PA. So when the shutter is closed, it is natural that 126MON also goes down to zero.
It is a bit difficult to center the beam on the PD because there is not much room for moving the PD.
However, Alberto came up with a configuration (flip the PD and reflect back the beam with a mirror to the PD), which seems to
be feasible. We will do this modification when the MOPA is confirmed to be ok.

Here is more detail about the shutter problem:
The shutter is controlled by the MOPA power supply. There are three ways to command the power supply.
The switch on the front panel of the power supply, the EPICS switch (through a XYCOM XY220), and the interlock.
The ribbon cable from the power supply's back is connected to J1 of the cross connect. The pin 59 of the cable is the one
controlling the shutter. It is then routed to J12 pin 36. The interlock and a XYCOM switch are both connected to this
pin.
Now what happened was, on the way tracking down those cables, I pushed some connectors, especially the ones on the XYCOM.
After that, I was able to open the shutter from the EPICS button.
Steve and Alberto tried the EPICS button many times in the morning without success.
My guess is that it was some malfunctioning of the XY220 accidentally fixed by my pushing of the cables.
But I cannot exclude the possibility of the interlock malfunctioning.

Quote:

1045

Mon Oct 13 18:59:39 2008

NPRO EMI and FSS error signal correlation

I made a simple loop antenna to measure the electro-magnetic inteference (EMI) around the master oscillator NPRO.

The first plot shows the comparison of the FSS error signal with the EMI measured when the antenna was put next to the NPRO (the MOPA box was opened).
There are harmonics of 78.1kHz which are present in both spectra. It is probably coming from the DC-DC converter in the NPRO board.

The second plot is the same spectra when the antenna was put far from the NPRO (just outside of the PSL enclosure).
The 78.1kHz harmonics are gone. So these are very likely to be coming from the NPRO.

The third plot shows the coherence functions between the signal from the antenna and the FSS error signal.
When the antenna was put near the NPRO, there is a strong coherence seen around 78.2kHz, whereas there is no strong coherence
when the antenna is far away from the NPRO.
This is a strong evidence that the 78.2(or 78.1)kHz harmonics is coming from the NPRO itself.

There are many peaks other than 78.1kHz harmonics in the FSS error signal spectrum. For most of them you can also find corresponding peaks in the EMI spectrum.
We have to hunt down those peaks to avoid the slew-rate saturation of the FSS.

Attachment 1: IMG_1692.JPG

Attachment 2: Spectrum.png

Attachment 3: SpectrumFar.png

Attachment 4: Coherence.png

1046

Tue Oct 14 14:19:36 2008

Today I made several measurements which should yield the optimized phase for the FSS 21.5 MHz reference. I made two sets of measurements, using the 166 MHz phase delay shifter. For each phase value I made 5 measurements of a 500 kHz injection into test2 at 1 Vpp, with the 4195 spectrum analyzer on in1 with the high impedance probe, 51 points, a 10 kHz range. It was surprisingly noisy. I will make plots using matlab to find the maximum, and hope for consistent results between the two sets of measurements. If it is too noisy or inconsistent I will repeat the measurement at ~800 kHz.

Once I find the phase which yields peak amplitude in in1, I will measure the relative phase between LO and RF going in to the FSS, measure the speed of light in RG58 cable, and construct a new cable which will implement the desired relative phase.

1048

Tue Oct 14 19:24:34 2008

FSS light power reduced

Rana, Yoichi

To change the gain distribution in the FSS, Rana reduced the VCO power for the AOM to reduce the light incident to the reference cavity.
Now the transmitted power of the RC is 2.3V compared to 6.5V before.
The FSS common gain can be increased to 5dB. I haven't changed the normal gain for this slider, so the mcup script will still set
the common gain to 1.5dB after an MC lock.
With this change, we take some gain from the optical part and give more gain in the electronics.
This might relieve the slew rate limit problem if it is happening in an early stage of the electronics.

1049

Wed Oct 15 17:40:50 2008

I set the PMC servo input offset: closed the MOPA shutter, zeroed the mixer output with the offset slider,
relocked everything, and set the nominal to the new value of -6 V.

1050

Wed Oct 15 22:07:52 2008

FSS ref phase measurements

Optimizing the FSS LO/RF phase at 500 kHz, above the servo band, proved to be noisy and those measurements were useless. Today I repeated
the measurement at 35 kHz and got good signal to noise. I've attached a plot of the 35 kHz peak in dBm as measured at IN2 by SR785, with
an injection into TEST2 at 35 kHz with 0.2 Vpp, as a function of delay in ns given by the delay phase shifter normally used by the 166 MHz.
I fit the bottom (quadratic) portion of this curve, and found an optimum delay of 25.8 ns, which can be implemented as 25.81 ns on the phase
shift box (25 + 1/2 + 1/4 + 1/16). This is an uncalibrated number and meaningless.. For all these measurements a very long SMA cable
(did not measure) was inserted on the RF output of the 21.5 MHz reference box. The actual phase difference depends on these cable lengths
which I didn't measure.

To determine the actual phase difference I compared the LO and RF input points with the 25.81 ns delay, using a scope with poor man's
averaging (33 manual triggers and recording the phase measurements). The phase difference was 8.24 degrees with an error on the mean of 3.4%,
with the LO having the longer effective cable (cable plus delay from the phase delay box). As a sanity check I set the phase delay box
to 20 ns and re-measured, and found 49.5 degrees. (1/21.5 MHz) * (49.5-8.24)/360 = 5.3 ns, which is about the difference between 20 ns
and 25.81 ns. I did the same with 0 ns dialed in, and found a difference of 21.5 ns (I expected 25.8 ns). So it is possible that the
phase delay box isn't too precise.

Finally, to determine the length of cable needed to implement 8.24 degrees of phase at 21.5 MHz with RG58 cable, I made some phase measurements
using the FSS reference box and mismatched cables. I used three cable lengths (93 cm, 140.5 cm, and 169.5 cm ) and two mismatched pairs with
dL of 29 and 76.5 cm. For each pair I took average of 20 measurements, finding 22.54 degrees mean for the dL=29 cm pair (0.78 degrees/cm, or
a speed of light of 1.0e10 cm/s, or 10.6 cm of cable length on the LO) and 43.57 degrees mean for dL=76.5 cm pair (0.57 degrees/cm, or a speed
of light of 1.4e10 cm/s, or 14.5 cm of cable length on the LO). I expected more precise agreement.

Maybe the 21.5 MHz reference box is not zero phase between the outputs. This could be easily tested. It might be interesting to repeat this
measurement with a few more dL values.

Attachment 1: phasedelay.png

1051

Thu Oct 16 09:44:49 2008

Yesterday arount 1:30PM, we lost the LO signal for the FSS.
I found it was caused by a bad cable connecting from the peter's RF oscillator box to the LO of the FSS.
I temporarily replaced it with a BNC cable of comparable length.

1052

Thu Oct 16 09:47:49 2008

FSS ref phase measurements

Quote:

I fit the bottom (quadratic) portion of this curve, and found an optimum delay of 25.8 ns, which can be implemented as 25.81 ns on the phase shift box (25 + 1/2 + 1/4 + 1/16).

The gain of the loop is sinusoidally dependent on the phase delay. So the fit will be better with a function like this: 1/(1+G*sin(dphi + const)).

1053

Thu Oct 16 13:12:58 2008

phase between FSS reference outputs

I verified the phase between the FSS reference outputs (used for LO and RF) using matched BNC cables. I measured 0.95 degree (average of 12 scope measurements).

1054

Thu Oct 16 16:26:26 2008

FSS phase matching cable installed

RG 405 cable has a solid teflon dielectric, and a velocity factor of 0.69. To get the 8.2 degrees of additional phase on the LO output at 21.5 MHz then requires 22 cm of cable. I made a cable that ended up being 21 cm long after I'd gained some experience putting on the connector. It gives a phase difference between LO and RF of about 10 degrees. It is currently installed.

1061

Mon Oct 20 20:50:09 2008

FSS board chip replacement

A quick update.

I changed two AD797s on the FSS board to AD829s to mitigate the 50MHz oscillation, which I plan to report later.
For some reason, the PA85 was broken after the replacement. So I had to replace it with a spare one too.
Right now the FSS is back and working. The oscillation is gone.
However, the maximum achievable gain is still about the same as before.

1063

Tue Oct 21 16:17:45 2008

AD797 Oscillation in the FSS board

I checked each op-amp's output in the FSS board to see if any indication of slew-rate saturation can be found.
PA85, which was the most suspicious one, actually has a very large slew rate limit (1000V/usec).
Its output swing was about 5V/usec. So PA85 was ok in terms of slew rate.
However, I found that an AD797 used at the first stage of the PC path was oscillating by itself, i.e. even without the loop closed.
The frequency was about 50MHz and the amplitude was large enough to reach the slew rate limit of this chip (the steepest slope was 30V/usec whereas the slew
rate limit of AD797 is 20V/usec).

I replaced it and another AD797 right after the oscillating one with AD829s. Just replacing the chips caused oscillation of AD829.
It was because there were no phase compensation capacitors connected to the pin 5 of AD829s.
Since the PCB was designed for AD797, there is no pattern for compensation caps. So I ended up putting Mica capacitors (47pF) across the pin 5 and the nearest ground point.
It worked and the oscillation stopped.

As I reported in an earlier elog, stopping the oscillation did not solve the problem of low FSS bandwidth.

1064

Tue Oct 21 17:52:30 2008

rana

Summary

FSS Photo: early October

This is a photo of the FSS board before Yoichi did his surgery - it was taken with the D40 in macro mode, sitting on the big Gorilla pod.

Attachment 1: fss.jpg

1078

Thu Oct 23 20:47:28 2008

FSS LO calibration for MEDM

Today I took a quick series of measurements to calibrate the FSS LO power measurement in the MEDM. This was done by using the spec.an. to measure the 21.5 MHz peak in dBm at the LO input to the FSS box on the PSL table, and recording the MEDM value, for attenuations applied at the FSS REF box output ranging from -5 dBm to -30 dBm.

I measured the loss due to the BNC cable I used, which was (19.66-19.50) dBm. I accounted for this and plotted ln(MEDM) vs. dBm on the attached plot. A linear fit of this gives the CALC field of a calc record for the IOC db:
6.29*LOGE(A)+5.36

Since no one knew how to do this nonlinear conversion in EPICS I will describe how to do it in detail tomorrow. It is simple, although it requires power cycling the scipe3 bunch (typing "reboot" or "ctl-x" at the command prompt took it down, but it did not come back). I did power cycle those computers a few times today.

Attachment 1: fss_lo_calibration.png

1079

Thu Oct 23 21:52:27 2008

I measured the open loop transfer function of the FSS, for the first time after I mitigated the oscillation.
The attached plot shows the comparison of the OPLTF before and after the oscillation was mitigated.
Blue curves are when AD797 was oscillating, and the red ones are after AD797s were replaced by AD829s.
The FSS gain slider values are the same for the both measurements.
There is a notable difference in the shape of the TF.
Right now the UGF is around 450kHz with the phase margin of 50deg.
When the gain is increased by a few dBs in the common gain slider, the PC path becomes saturated.
This might be caused by the peak in the OPLTF at 1.7MHz sticking out of the 0dB line.
Another peak at 770kHz is also annoying.
Too bad that I did not take the TF above 1MHz before the oscillation was mitigated.
Also at 100kHz, the new TF has a lower gain than the old one, although it looks like the slope of the red curve is getting steeper and
it is catching up the blue one at lower frequencies.
I will measure the TF below 100kHz later.

With this bandwidth, I was able to increase the MC gain further.
I will report on the MC open loop measurements soon.

Attachment 1: FSS_OPLTF.png

1080

Thu Oct 23 23:09:18 2008

I measured three loop transfer functions of the MC servo.
The blue curve in the first attachment is the overall open loop gain of the servo measured using
the sum-amp A of the MC board (it is the sum-amp in the common part).
The red curve is the transfer function measured by the sum-amp B of the MC board, which is in the VCO path.
Mathematically the measured transfer function is G_vco/(1+G_L), where G_L is the loop gain of the length path
and G_vco is the loop gain of the VCO path.
The green curve is G_L/(1+G_vco) which was measured from dtt by using C1:SUS-MC2_MCL_EXC.
The UGF of the MC loop is 75kHz with the phase margin of 27deg.
The cross over frequency of the two loops is 43Hz. The phase margin there seems OK.

The second attachment is the comparison of the MC open loop TF measured on Sep. 4 (old) and today (new).
The increased bandwidth of the FSS gave us a slight gain in the phase margin and the elimination of
the slight bump in the gain around 150kHz existed in the blue curve.

Attachment 1: MC_Loop.png

Attachment 2: OldAndNew.png

1083

Fri Oct 24 11:21:26 2008

FSS LO input calibrated in dBm

Based on the measurements described in my previous elog, I created a new calc record in the file /cvs/cds/caltech/target/c1psl/psl.db

grecord(calc, "C1:PSL-FSS_LOCALC")
{
        field(INPA,"C1:PSL-FSS_LODET")
        field(SCAN,".1 second")
        field(PREC,"4")
        field(CALC,"6.29*LOGE(A)+5.36")
}

After restarting scipe3 to load this change, I told C1PSL_FSS.adl to look at this record instead of *LODET. That MEDM screen now shows LO input calibrated in dBm.

For reference, the operators available for use in the CALC field are listed in the EPICS Record ref manual, Chapter 9. The manual can be found here:
http://www.aps.anl.gov/epics/EpicsDocumentation/AppDevManuals/RecordRef/Recordref-3.html

Yoichi said he was fixing an SVN problem, so I have not yet committed the two files I changed: /cvs/cds/caltech/target/c1psl/psl.db and /cvs/cds/caltech/medm/c1/psl/C1PSL_FSS.adl.

1095

Mon Oct 27 14:48:27 2008

EO shutter installed to the reference cavity

I'm now preparing for cavity ring down measurements of the reference cavity.
An EOM for polarization rotation is installed between the two steering mirrors for the reference cavity.
The location is before the polarized beam splitter (used to pick-up the reflected light from the cavity) and
after the half-wave plate. So we should be able to use the PBS as a polarizer.
While setting up the high voltage pulse generator, I realized that we don't have enough cables for it.
It uses special kind of connectors (Kings 1065-N) for HV connections. We need three of those but I could find
only two. I asked Bob to order a new connector.

For the moment, the EOM is left in the beam path of the reference cavity until the connectors arrive (Wed. or Thu. this week)
and the measurements are done.
The EOM distorts the beam and degrades the mode matching to the reference cavity.
I optimized the alignment of the crystal so that the RC transmission is maximum.
Even though, the transmission of the reference cavity is down from 2.8 (without EOM) to 1.7 (with EOM).
I increased the common gain of the FSS from 7dB to 10dB to compensate for this.
The mode clearner locks with this configuration.

If the EOM is really disturbing, one can just take it out.
Since I did not touch the steering mirrors, the alignment to the reference cavity should be recovered immediately.

1099

Wed Oct 29 12:23:04 2008

MZ alignment touched and the alarm level changed

Since the MZ reflection is alarming all the time, I tried to improve the MZ alignment by touching the folding mirror.
I locked the X-arm and monitored the transmitted light power while tweaking the mirror alignment to ensure that the output beam pointing is not changed.
I changed the alignment only a little, almost like just touching the knob.
The reflected power monitor was around 0.6 this morning and now it is about 0.525. Still large.
I changed the alarm level (HIGH) from 0.5 to 0.55.

1107

Mon Nov 3 09:59:47 2008

The psl enclosure HEPAs were tuned on.

Loose paper drawing was found on the psl inside shelf.
This can fall down into the beam and ignite a tragedy.

Thanks for the color coded correction. My spell checker is not reliable

1114

Tue Nov 4 17:58:42 2008

Alberto

MC temperature sensor

I added a channel for the temperature sensor on the MC1/MC3 chamber: C1:PSL-MC_TEMP_SEN.
To do that I had to reboot the frame builder. The slow servo of the FSS had to get restarted, the reference cavity locked and so the PMC and MZ.

1124

Fri Nov 7 18:38:19 2008

Alberto

MC temperature sensor hooked up

Alberto, Rana,
we found that the computer handling the signals from ICS-110B was C1IOVME so we restarted it. We changed the name of the channel to C1:PEM_TEMPS and the number to 16349. We tracked it up to the J14 connector of the DAQ.
We also observed the strange thing that both of the differential pairs on J13 are read by the channle. Also, if you connect a 50 Ohm terminator to one of the pairs, the signal even get amplified.

(The name of the channel is PEM-MC1_TEMPS)

1130

Wed Nov 12 11:14:59 2008

Caryn

MC temp sensor hooked up incorrectly

MC Temperature sensor was not hooked up correctly. It turns out that for the 4 pin LEMO connections on the DAQ like J13, J14, etc. the channels correspond to horizontal pairs on the 4 pin LEMO. The connector we used for the temp sensor had vertical pairs connected to each BNC which resulted in both the differential pairs on J13 being read by the channel.
To check that a horizontal pair 4 pin LEMO2BNC connector actually worked correctly we unlocked the mode cleaner, and borrowed a connector that was hooked up to the MC servo (J8a). We applied a sine wave to each of the BNCs on the connector, checked the J13 signal and only one of the differential pairs on J13 was being read by the channel. So, horizontal pairs worked.

1136

Fri Nov 14 19:20:42 2008

Thanks to Bob making the high-voltage BNC cables for the HV pulse generator, I was able to operate the EOM in front of
the reference cavity.

The conceptual setup is the following:

[HV pulse] ----+           +-->-- [PD2]
               V           |
->--[HWP]->-- [EOM] -->-- [PBS] --<->-- [QWP] --<->-- [Reference Cavity] -->-- [PD1]
                           |
                [PD3] --<--+

The high voltage pulse rotates the polarization of the light after the EOM. When the HV is applied, the PBS reflects most of the light
into PD2 (Thorlabs PDA255), shutting down the incident light into the cavity.
The transmitted light power of the reference cavity is monitored by PD1 (PDA255). The reflected light from the reference cavity
is monitored by the DC output of the RF PD (PD3). PD3 is low-passed so the response is not fast.
Thorlabs says PDA255 has 50MHz bandwidth.

The attached plot shows the time series of the above PD signals when the HV was applied.
Input Pulse (blue curve) is the input to the HV pulse generator. When it is high, the HV is applied.
"PBS reflection" (red) is PD2. "Reflection" (green) is PD3. "Transmission" (light blue) is PD1.

The red curve shows huge ringing. At first I thought this was caused by the bad response of the PD.
However, the same ringing can be seen in the PD3 and the peaks match very well.
When red curve goes down the green curve goes up, which is consistent with the energy conservation.
So it looks like the light power is actually exhibiting this ringing.
May be the HV pulse is distorted and the voltage across the EOM is showing this ringing.
I will check the input voltage shape to the EOM using a high impedance probe, if possible.

The green curve shows a slow decay because it has a long time constant. It is not an actual
trend of the reflected light power.

The RC transmission power shows some peaks, probably due to the ringing in the input power.
So just fitting with an exponential would not give a good estimate of the cavity pole.
Even though, we should be able to de-convolute the frequency response of the reference cavity
from the input (red curve) and output (light blue curve) signals.

Attachment 1: RingDown.png

1137

Fri Nov 14 20:35:47 2008

rana

To make the DEI pulser make a fast pulse on the EO shutter EOMs, we had to make sure:

1) the cable had a high voltage rated dielectric. cheap dielectrics show the 'corona'
effect, especially when there is a bend in the cable.

2) the EO has to have a resistor on it to prevent ringing due to the impedance mismatch.

3) We needed ~3.5 kV to get the EO shutter crystal to flip the light by 90 deg.

1138

Fri Nov 14 22:40:51 2008

Quote:

I'll check it with Bob.

Quote:

2) the EO has to have a resistor on it to prevent ringing due to the impedance mismatch.

Did you use a shunt or series resistor ?
If shunt, I guess it has to have a huge heat sink.
Actually, DEI says the pulser does not require any external shunt/series resistors or impedance-matching network.
Looks like it is not true ...

Quote:

3) We needed ~3.5 kV to get the EO shutter crystal to flip the light by 90 deg.

Yes, I adjusted the voltage to maximize the power change and it was about 3.5kV.

1140

Mon Nov 17 15:07:06 2008

I used MATLAB's system identification tool box to estimate the response of the reference cavity, i.e. cavity pole.
What I did was basically to estimate a model of the RC using the time series of the measured input and output power.

First, I prepared the input and output time series for model estimation.
The input is the input power to the RC, which I produced by inverting the PBS reflected light power and adding an offset
so that the signal is zero at t=0. Offset removal was necessary to make sure that the input time series does not give an
unintentional step at t=0.
The output time series is the transmission power of the RC. I also added an offset to make it zero at t=0.
Then I commanded MATLAB to compute the response of a first order low-pass filter to the input and try to fit
the computed response to the measured output by iteratively changing the gain and the cut-off frequency.
("pem" is the name of the command to use if you are interested in).

The result is shown in the attachment.
Blue curve is the input signal (I added a vertical offset to show it separately from the output).
The green curve is the measured output (RC transmission). The red curve is the response of the estimated model.
The estimated cut-off frequency was about 45kHz.

You can see that the red curve deviates a lot from the green curve after t=15usec.
By looking at this, I realized that the bandwidth of the RC cavity servo was too high.
The time scale we are looking at is about 50kHz whereas the FSS bandwidth is about 400kHz.
So when the input light was cut off, the error signal of the FSS becomes meaning less and the
input laser frequency was quickly moved away from the resonance. This is why the green curve does not
respond to the large peaks in the blue curve (input). The cavity was already off-resonance when the input power
showed bumps.

Since the red curve matches nicely with the green curve at the very beginning of the ring down, the estimated 45kHz
cavity pole is probably not that a bad estimate.

To make a better measurement, I will try to reduce the bandwidth of the RC servo by using only the PZT actuator.
If there were no ringing in the input light power, we wouldn't have to worry about the bandwidth of the servo because our
feedback is all made to the laser, not the cavity length.
In order to reduce the ringing in the input power, I asked Bob to make new HV cables using HV grade coax cables.

Attachment 1: Fit.png

1149

Thu Nov 20 09:37:39 2008

This 12 days plot shows that it can hold lock if the daily temp variation is not more than 3/4 of a degree C
The MZ is happy if it's HV is above 100V

Attachment 1: mztemp.jpg

1151

Fri Nov 21 16:11:26 2008

rana, alberto, rob

Mach Zender alignment

The Mach Zender's dark port DC voltage had gone up too high (~0.5 V)and was turning yellow
on the screen. I re-aligned by touching the knobs on the 166 MHz path. Doing alignment after the
166 EOM had very little effect. The main improvement came from doing yaw on the turning mirror
just ahead of the 166 MHz EOM; this is the one which as no adjustment knobs (duh).

During this procedure, I had the MC off, the ISS off, and the MC autolocker off. After finishing
the alignment, the power on the ISS PDs had railed and the dark port power was ~0.29 V. So we
put in a ND0.2 on the ISS path and now the voltages are OK (~2 V on each PD). We have to remove the
ND filters and change the first ISS turning mirror into a ~10-20% reflector.

So now the MZ alignment seems good; Alberto is on the MC periscope alignment like a cheap suit.

Attachment 1: PB210051-1.JPG

1155

Fri Nov 21 20:29:43 2008

rana, alberto, rob

Mach Zender alignment

Quote:

So now the MZ alignment seems good; Alberto is on the MC periscope alignment like a cheap suit.

And alignment is now mostly done - MC locks on the TEM00.

                 REFL_DC

Unlocked           4.50  V
Locked noWFS       1.30  V
Locked + WFS       0.42  V

and the 29.5 MHz modulation depth is really small.

We should be able to rerun the Wiener analysis on this weekends MC data.

I don't know what our nominal StochMon numbers now are, but after Alberto tweaks up the alignment he can tell us if the RFAM has gotten any better.

1160

Mon Nov 24 17:14:44 2008

It looks like the MZ has gotten less drifty after the alignment on Friday.

Attachment 1: Untitled.png

1162

Tue Nov 25 18:38:03 2008

Alberto, Rob

MC Periscope Alignment

This morning when I came in I found the MC cleaner unlocked and the autolocker script could not lock it. The reflected beam was quite off and showed in the bottom left corner of the IMCR camera. After turning off the WFS locking, I started slightly changing the alignment of the steering mirrors on the MC periscope, waiting for the LSC servo to lock the cavity. It didn't work. At some point I lost the beam from the IMCR camera and that is how someone might have found it when I left it for about one hour.

When I came back and tried again adjusting the steering mirrors, I noticed that the autolocker was working and was trying to lock the cavity. After just a bit of adjustment, the MC got easily locked.

After that, I spent a couple of hours trying to improve the alignment of the periscope to minimize the reflection and maximize the transmission. I started with a transmission of 0.4 V but, despite all the tweaking (I used the technique of turning both yaw knobs at the same time), I couldn't get more than 1.2 V (and 2.4 V at the reflection) if only the LSC servo was on. Looking at the camera, I moved the beam around to look for a more favorable spot but the MC wouldn't lock with the beam in other places. Maybe I could do better or maybe not because the cavity is not aligned. I'm going to try again tomorrow.

1166

Tue Dec 2 17:56:56 2008

In the attempt to maximize the Mode Cleaner transmission and minimize the reflection from the steering mirrors of the MC periscope, we could not get more ~2 V at the MC Trans PD and ~ 0.5 V at MC REFL_DC. As it turned out from the SUS Drift Monitor, the reason was that the MC optics had been somehow displaced from the optimal position.

After restoring the reference position values for the mirrors and tweaking again the periscope, we got ~3V at the MC TransPD and 0.5V at the reflection.
The beam was then probably clipped at the REFL PD so that we had to adjust the alignment of one of the BS in the transmitted beam path on the AS table.
We also zeroed the WFS PDs, but not before reducing the power from the MZ, for their QPDs not to saturate.

After relocking, the transmission was 3V and the reflection ~0.3V.

The beam isnow centered on the Trans PD and REFL PD and the Mode Cleaner locked. More details on the procedure will follow.

1169

Wed Dec 3 11:58:10 2008

Rana, Alberto,

more details on the MC alignment we did yesterday.

Last week Rana re-aligned the Mach Zender (MZ) on the PSL table to reduce the power at the dark port (see elog entry #1151). After that, the beam was aligned to the MZ but not properly aligned to the Mode Cleaner (MC) anymore. As a result the MC could not lock or did it on unwanted transverse modes. To fix that we decided to change the alignment of the MC input periscope on the PSL table.

The ultimate goal of the operation was to align the MC transmitted beam to the IFO and to maximize the power. Such a condition depends on:
a) a good cavity alignment and
b) input beam matching to the cavity TEM00 mode.

Since the MZ alignment had only affected the input beam, we assumed the cavity alignment was still good, or at least it had not changed, and we focused on the input beam.

The IOO computer, by the MC autolocker script, is able to change the cavity alignment and the length to match the input beam and lock the cavity. Although both the length servo (LSC) and the alignment servo (WFS) have a limited effective operating range. So for the script to work properly and at best, input beam and cavity matching have to be not far from that range.

The MC periscope has two mirrors which control the pitch and yaw input angles. By changing either yaw or pitch of both mirrors together (�two-knob" technique) one can change the input angle without moving the injection point on the cavity input mirror (MC1). So this is the procedure that we followed:

1) turned of the autolocker running the MC-down script
2) brought the reflected beam spot back on the MC-reflection camera and on the reflection photodiode (REFL-PD)
3) turned on the LSC servo
4) tweaked the periscope's mirrors until the cavity got locked on a TEM00 mode
5) tweaked the periscope aiming at ~0.3V from the REFL-PD and ~3V on the transmission photodiode (TRANS-PD).

Following the steps above we got ~0.5 V on the REFL-PD and ~2V on the TRANS-PD but no better than that.

Looking at the Drift Monitor MEDM screen, we found that the cavity was not in the reference optimal position, as we initially assumed, thus limiting the matching of the beam to the MC.

We restored the optics reference position and repeated the alignment procedure as above. This time we got ~3V on the TRANS-PD and ~0.5 on the REFL-PD. We thought that the reason for still such a relatively high reflection was that the beam was not well centered on the REFL-PD (high order modes pick-up?).

On the AS table we centered the REFL-PD by aligning a beam splitter in the optical path followed by the light to reach the photodiode.

We also centered the beam on the reflection Wave Front Sensors (WFS). To do that we halved the power on the MZ to reduce the sidebands power and prevent the WFS QPD from saturating. We then aligned the beam splitters on the QPD by balancing the power among the quadrants. Finally we restored the power on the MZ.

As a last thing, we also centered the transmitted beam on the TRANS-QPD.

The MC is now aligned and happily locked with 3V at the TRANS-PD and 0.3V at REFL-PD.

1190

Fri Dec 12 22:51:23 2008

Reference cavity ring down measurement again

Bob made new HV-cables with HV compatible coaxes. The coax cable is rated for 2kV, which was as high as Bob
could found. I used it with 3kV hoping it was ok.
I also put a series resistor to the pockels cell to tame down the ripples I saw in elog:1136.

Despite those efforts, I still observed large ringings.
I tried several resistor values (2.5k, 1k, 330ohm), and found that 330ohm gives a slightly better result.
(When the resistance is larger, the edge of the PBS Refl. becomes dull).
Since the shape of the ringing does not change at all even when the pulse voltage is lowered to less than 1kV,
I'm now suspicious of the DEI pulser.

Anyway, I estimated the cavity pole using the MATLAB's system identification toolbox again.
This time, I locked the reference cavity using only the PZT feedback, which makes the UGF about a few kHz.
So, within the time scale shown in the plot below, the servo does not have enough time to respond, thus the laser
frequency stays tuned with the cavity. This was necessary to avoid non-linear behavior of the transmitted power
caused by the servo disturbing the laser frequency. With this treatment, I was able to approximate the response of
the cavity with a simple linear model (one pole low-pass filter).

MATLAB estimated the cavity pole to be 47.5kHz.
The blue curve in the plot is the measured RC transmitted power.
The incident power to the cavity can be inferred from the inverse of the red curve (the PBS reflection power).
The brown curve is the response of the first order low-pass filter with fc=47.5kHz to the input power variation.
The blue and brown curves match well for the first 10usec. Even after that the phases match well.
So the estimated 47.5kHz is probably a reasonable number. I don't know yet how to estimate the error of this measurement.

According to http://www.ligo.caltech.edu/~ajw/PSLFRC.png the designed transmission of the reference cavity mirrors is 300ppm (i.e.
the round trip loss (RTL) is 600ppm).
This number yields fc=35kHz. In the same picture, it was stated that fc=38.74kHz (I guess this is a measured number at some point).
The current fc=47.5kHz means, the RTL has increased by 200ppm from the design and 150ppm from the time fc=38.74kHz was measured.

Attachment 1: RC-Ringdown.png

1191

Tue Dec 16 19:06:01 2008

Reference cavity ring down repeated many times

Today, I repeated the reference cavity ring down measurement many times to see how much the results vary.

I repeated the ring down for 20 times and the first attachment shows the comparison of the measured and estimated cavity transmission power.
The blue curve is the measured one, and the red curve is the estimated one. There are only 10 plots because I made a mistake when transferring data
from the oscilloscope to the PC, and one measurement data was lost.

The second attachment shows the histogram of the histogram of the estimated cavity pole frequencies.
I admit that there are not enough samples to treat it statistically.
Anyway, the mean and the standard deviation of the estimated frequencies are 47.6kHz and 2.4kHz.
Assuming a Gaussian distribution and zero systematic error, both of which are bold assumptions though, the result is 47.6(+/-0.6)kHz.

Now I removed the Pockels Cell from the RC input beam path.
I maximized the transmission by tweaking the steering mirrors and rotating the HWP.
Since the transmission PD was saturated without an ND filter on it, I reduced the VCO RF power slider to 2.85.
Accordingly, I changed the nominal common gain of the FSS servo to 10.5dB.

Attachment 1: RC_Ringdown_Estimates.png

Attachment 2: Cavity_Pole_Histogram.png

1228

Wed Jan 14 15:53:32 2009

steve

MC temperature sensor

Quote:

Where is this channel?

1244

Thu Jan 22 11:54:09 2009

Alberto

Mach Zehnder Output Beam QPD

I rotated by 180 degrees the 10% beam splitter that it is used to fold the beam coming from the Mach Zehnder (directed to the MC) on to the QPD.

The alignment of the beam with that QPD has so been lost. I'll adjust it later on.

The rotation of the BS had the (surprising) effect of amplifying the Absolute Length experiment's beat by 9 times. Maybe because of a polarizing effect of the Beam Splitter which could have increased the beating efficiency between the PSL and the NPRO beams?

1246

Thu Jan 22 14:38:41 2009

caryn

MC temperature sensor

Quote:

Where is this channel?

That's not the name of the channel anymore. The channel name is PEM-MC1_TEMPS. It's written in a later entry.

1248

Fri Jan 23 10:00:21 2009

steve

PMC transmission is down

The PMC transmission is going down.
I have not relocked the PMC yet.

Attachment 1: pmc4d.jpg

1250

Fri Jan 23 14:00:02 2009

PMC transmission is down

Quote:

The PMC transmission is going down.
I have not relocked the PMC yet.

I tweaked the alignment to the PMC.
The transmission got back to 2.65. But it is still not as good as it was 3 days ago (more than 3).

It is interesting that the PMC transmission is inversely proportional to the NPRO output.
My theory is that the increased NPRO power changed the heat distribution inside the power amplifier.
Thus the output mode shape changed and the coupling into the PMC got worse.
MOPA output shows a peak around Jan-21, whereas the NPRO power was still climbing up.
This could also be caused by the thermal lensing decreasing the amplification efficiency.

Attachment 1: LaserPower.png

1254

Wed Jan 28 12:42:51 2009

Yoichi, Jenne, Peter

As most of you know, the MOPA output power has been declining rapidly since Jan 21. (See the attachment 1)
There was also an increase in the NPRO power observed in LMON, which is an internal power monitor of the NPRO.
Similar trend can be seen in 126MON, which picks up some scattered light from the NPRO but there may be some contributions from the PA output.

The drop in the AMPMON, LMON and CURMON (NPRO current) from the middle of Jan 26 to the end of Jan27 was caused by me.
I tried to decrease the NPRO current to put the NPRO power back to the level when the MOPA output was higher. But it did not bring back the MOPA power.
So I put back the current after an hour. This caused the sharp power drop on Jan26.
By mistake, I did not fully recover the current at that time and left it like that for a day. This accounts for the long power drop period continued until Jan27.

Shortly after I tweaked the current, the MOPA output power started to fluctuate a lot. This drives the ISS crazy.
To see if this was caused by the NPRO or power amplifier,
we decided to fix the 126MON to monitor the real NPRO power.
We opened the MOPA box and installed a mirror to direct a picked off NPRO beam to the outside of the box through an unused hole.
We set up a lens and a PD outside of the MOPA box to receive this beam. The output from the PD is connected to the 126MON cable.
So 126MON is now serving as the real monitor of the NPRO power. It has not yet been calibrated.

The second attachment shows a short time series of the MOPA power and NPRO power. When the beam is blocked, the 126MON goes to -22.
So the RIN of the NPRO is less than 1%, whereas the MOPA power fluctuates about 5%. There is also no clear correlation between the power fluctuation of the MOPA and the NPRO. So probably the MOPA power fluctuation is not caused by NPRO.

At this moment, all the feedback signals (current shunt, slow and fast actuators) are physically disconnected from MOPA box so that we can see the behavior of MOPA itself.

Attachment 1: Recent10Days.png

Attachment 2: 126_MOPA.png

1256

Wed Jan 28 19:08:50 2009

Laser is back (sort of)

Yoichi, Peter, Jenne

Summary:
We found that the chiller water is not going to the NPRO base. It was hot whereas it was cold when I touched it a few months ago.
I twisted the needle valve on the water line to the NPRO base. Then we heard gargling noise in the pipe and the water started to flow.
The laser power is now climbing up slowly. The noisiness of the MOPA output is reduced.

I will post more detailed entry explaining my theory of what actually happened later.

Attachment 1: Improving.png

1257

Thu Jan 29 13:52:34 2009