It was fine when I came in earlier today, but I just got back from dinner, and it's not good. I looked in dataviewer, and it seems to have been sliding out for the past couple of hours... Here is a picture:

I swear I am not responsible this time... all I've been doing is working in the control room.

 Mode cleaner bounced back on its own about 2 hours ago.

locking work proceeding apace tonight.

diagonalized DRM with setDDphases & senseDRM. 

initial locks are fairly quick, aqstep script succeeds reliably.

first part of cm_step (handoff CARM-> MCL) usually works.

tuning up later parts of cm_step (presumably due to optical gain changes resulting from MOPA decline). 

got to arm powers ~60.

Tried to lock the interferometer but arm power didn't get over 65.

Tonight, after the weekend, I resumed the work on locking.

When I started the Mode Cleaner was unlocked because the MZ was also unlocked.

I aligned the MZ and the transmitted power reached about 2.5

Initially the interferometer lost lock at arm power of about 3-4. It looked like the alignment wasn't good enough. So I ran the alignment scripts a few times, first the scripts for the single parts and in the end the one for the full IFO.

Then I also locked again the MZ and this time the transmitted power got to about 4.

In the following locking attempts the the arm power reached 65 but then the lock got lost during the handing of CARM to C1:LSC-PD11_I

I'll keep working on that tomorrow night.

While trying to make the OAF work, I found that the XYCOM switches for MC1/3 have been set in the bad way for awhile. This means that the hardware filters were bypassed and that MC1 & MC3 were moving around too much at high frequency and possibly causing trouble with the locking. I have put them back into the default position.

On Friday, Jenne and I were playing around with turning the dewhitening off/on to see if it efffected the OAF stability. At the time, I didn't pay too much attention to what the state was. Looks like it was in the wrong state (hardware bypassed) when we found it. For the OAF work, we generally want it in that bypassed state, but its bad because it makes noise in the interferometer. The bits in question are bits 16-23 on the XYCOM screen.

I have updated the snapshot and set the screen in the appropriate settings. I used a swept sine measurement to verify the filter state. In the attached plot, green corresponds to XYCOM green and red corresponds to red.

It's been a miserable week for lock acquisition, with each night worst than the last.  The nadir was around Sunday night, when I couldn't even get a PRM to lock stably, which meant that the auto-alignment scripts could not finish successfully.  It now appears that was due to some XYCOM mis-settings. 

We've also been having problems with timing for c1susvme2.  Attached is a one-hour plot of timing data for this cpu, known as SRM.  Each spike is an instance of lateness, and a potential cause of lock loss.  This has been going on for a quite a while.

Tonight we also encountered a large peak in the frequency noise around 485 Hz.  Changing the MZ lock point (the spot in the PZT range) solved this.

Good progress in IFO locking tonight, with the arm powers reaching about half the full resonant maximum. 

Still to do is check out some weirdness with the OMC DAC, fix the wireless network, and look at c1susvme2 timing.

Attached is a 3 day trend of SRM CPU timing info.  It clearly gets better (though still problematic) at some point, but I don't know why as it doesn't correspond with any work done.  I've labeled a reboot, which was done to try to clear out the timing issues.  It can also be seen that it gets worse during locking work, but maybe that's a coincidence.

More progress with locking tonight, with initial acquisition and power ramps working.  The final handoff to RF CARM still needs work.

I found the wireless router was unplugged from the network--just plugging in the cable solved the problem.  For some reason that RJ45 connector doesn't actually latch, so the cable is prone to slipping out of the jack.

Late last night after the ETMY settled down from the quake I made some more progress in locking, with the handoff to RF CARM succeeding once.  The final reduction of the CARM offset to zero didn't work, however.

Working well tonight: the handoff of CARM to RF (REFL2I), successful reduction of CARM offset to zero, and transition control of MCL path to the OUT1 from the common mode board.  All that's left in lock acquisition is to try and get the common mode bandwidth up and the boost on.

Lock acquisition working well tonight.  Was able to engage CM boost (not superboost) with bandwidth of ~10kHz.  Also succeeded once in handing off DARM to DC readout.

Last night, 2+ hour lock, probably broken by me driving too hard (DARM_EXC).

The IFO can now be locked during the daytime.  Well, it's locked now.

This is huge.    Five hours of lock only interrupted by intentional break from transfer function abuse.

Awesome 5 hrs of locking  Rob!

Lock acquisition has gone bad tonight. 

The initial stage works fine, up through handing off control of CARM to MCL.  However, when increasing the AO path (analog gain), there are large DC shifts in the C1:IOO-MC_F signal.  Eventually this causes the pockels cell in the FSS loop to saturate, and lock is lost. 

 This problem has disappeared.  I don't know what it was. 

The first plot shows one of the symptoms.  The second plot is a similar section taken from a more normal acquisition sequence the night before.

All is not perfect, however, as now the handoff to RF CARM is not working.

LockAcq is back on track, with the full script working well.  Measurements in progress.

The problem has returned.  I still don't know what it is, but it's making me angry. 

I never actually figured out exactly what was wrong in entry 2162, but I managed to circumvent by changing the time sequence of events in the up script, moving the big gain increases in the common mode servo to the end of the script.  So the IFO can be locked again.

This afternoon, I tried to lock the interferometer again after a few days.

After a couple of failed attempts, and relocks of the MZ, the interferometer stayed locked continuously for about 50 minutes, with arm power of about 92.

I just wanted to check the status of the interferometer so I didn't do any particular measurement in the meantime.

Locking has gone sour.  The CARM to MCL handoff, which is fairly early in the full procedure and usally robust, is failing reliably. 

As soon as the SUS-MC2_MCL gain is reduced, lock is broken.  There appears to be an instability around 10Hz.  Not sure if it's related.

 Five day minute trend.  FAST_F doesn't appear to have gone crazy.

 Whatever the locking problem was, the power of magical thinking has forced it to retreat for now.  The IFO is currently locked, having completed the full up script.  One more thing for which to be thankful.

I wrote down the settings according to which I tuned the optickle model of the 40m Upgrade.

Basically I set it so that:

1. PRC alone anti-resonant for the carrier and resonant for both sidebands
2. SRC alone resonant for the carrier and resonant for the f2 sideband

In this way when the carrier becomes resonant in the arms we have:

1. carrier resonant in PRC and anti-resonant in SRC
2. f1 resonant in PRC and non resonant in SRC
3. f2 resonant in SRC

The DARM offset for DC readout is optional, and doesn't change those conditions.

I also plotted the carrier and the sideband's circulating power for both recycling cavities.

I'm attaching a file containing more detailed explanations of what I said above. It also contains the plots of field powers, and transfer functions from DARM to the dark port. I think they don't look quite right. There seems to be something wrong.

Valera thought of fixing the problem, removing the 180 degree offset on the SRM, which is what makes the sideband rather than the carrier resonant in SRC. In his model the carrier becomes resonant and the sideband anti-resonant. I don't think that is correct.

The resonant-carrier case is also included in the attachment (the plots with SRMoff=0 deg). In the plots the DARM offset is always zero.

I'm not sure why the settings are not producing the expected transfer functions.

 In my calculation of the digital filters of the optical transfer functions the carrier light is resonant in coupled cavities and the sidebands are resonant in recycling cavities (provided that macroscopic lengths are chosen correctly which I assumed).

Carrier and SB (f2) shouldn't be resonant at the same time in the SRC-arms coupled cavity. No additional filtering of the GW signal is wanted.

The SRC macroscopic length is chosen to be = c / f2 - rather than = [ (n+1/2) c / (2*f2) ] - accordingly to that purpose.

I calculated the frequency of the double cavity pole for the 40m SRC-arm coupled cavity.

w_cc = (1 + r_srm)/(1- r_srm) * w_c

where w_c is the arm cavity pole angular frequency [w_c = w_fsr * (1-r_itm * r_etm)/sqrt(r_itm*r_etm) ]

I found the pole at about 160KHz. This number coincides with what I got earlier with my optickle model configured and tuned as I said in my previous entry. See attachments for plots of transfer functions with 0 and 10pm DARM offsets, respectively.

I think  the resonance at about 20 Hz that you can see in the case with non-zero DARM offset, is due to radiation pressure. Koji suggested that I could check the hypothesis by changing either the mirrors' masses or the input power to the interferometer. When I did it frequency and qualty factor of the resonance changed, as you would expect for a radiation pressure effect.

This gave me more confidence about my optickle model of the 40m. This is quite comforting since I used that model other times in the past to calculate several things (i.e. effects of higher unwanted harmonics from the oscillator, or, recently, the power at the ports due to the SB resonating in the arms).

 Still I didn't see any beat note signals..

 With a help from Suresh, Yuta and Rana, I tried searching for the green beat note by changing the temperature of the X end NPRO.

The noise level after it goes through an RF amplifier (G=23dB) was about -70dBm at 50MHz.

This noise may cover the beat note signal.

I am going to post some details later.

Last night I tried searching for a beat note signal with two different PD trans impedance gains.

Although I didn't find a  beat note signal.

- (1. trans impedance gain = 2400)

 I started with a trans impedance resistance of R=2.4k, which is 10 times bigger resistance than the original.

The total PD gain should be about 960 [V/W] theoretically if we assume the responsibility of the PD is 0.4 [A/W].

Then I checked the bandwidth of the RFPD using Jenne laser.

The bandwidth was about 30MHz, which is 3 times narrower than the original. And it agrees with our expectation.

As Koji and I mentioned at the last weekly meeting, the cut off frequency of an RFPD follows inverse square root of the trans impedance resistance R.

where C is a capacitance of the photo diode. (See this)

I was expecting the signal level of  -50 dBm / rtHz with a 23dB RF amplifier, assuming the line width of the signal is 10kHz.

- (2. trans impedance gain = 240) 

 I also tried it with the original trans impedance gain (see this entry). 

 R = 240 [Ohm]

 G = 96 [V/W]

 BW = 100 [MHz]  (I didn't measure it in this time)

 expected signal level = -70 dBm/rtHz

finally we found it !

Nice work!

(notes on signal level)

 The signal level of the observed peak was -48dBm.

However I was expecting it is like -28dBm with some ideal assumptions.

There may be a 20dB unknown loss which sounds big to me.

The expectation was calculated by using the following simple math.

                       SIGNAL = A x Z x G_RF x sqrt(P1 / 2) x sqrt (P2 / 2)

 where A is the responsibility of the PD and Z is the trans impedance gain. G_RF is a gain of the RF amplifier.

The laser powers of green beams, P1 and P2, are divided by 2 due to a beam splitter. 

I was assuming the parameters are like:

           A = 0.39 [A/W]   (assuming 90% quantum efficiency at 532nm)

           Z = 240 [V/A]  

           P1 = 17 uW  (measured by Newport power meter)

           P2 = 30 uW (measured by Newport power meter)

           G_RF = 23 dB

- What is the actual photocurrent for the beam1 and beam2? We don't care how much power do you have before the BS, but care how much current do you have on the PD.

- How much is the visibility? There is mismatching of the beams. i.e. The beam diameter looked quite different. Also the beams are not TEM00 but have fringes probably comes from the TT mirrors. You maybe able to measure the visibility by the DC output, making the beat freq go through df=0 slowly.

- What is the measured gain of the RF amp? Does it include the voltage division by the output/input impedance?

---------------------

 The signal level of the observed peak was -48dBm.

However I was expecting it is like -28dBm with some ideal assumptions.

There may be a 20dB unknown loss which sounds big to me.

I was assuming the parameters are like:

           A = 0.39 [A/W]   (assuming 90% quantum efficiency at 532nm)

           Z = 240 [V/A]  

           P1 = 17 uW  (measured by Newport power meter)

           P2 = 30 uW (measured by Newport power meter)

           G_RF = 23 dB

The mirror which was moved during the mode matching of PSL light to the MC (ref elog #3791) has been repositioned.  We once again have the green light from the NPRO on the X (south) arm available on the PSL table. 

This light was supposed to be collimated by the two plano convex lenses (f=200mm and f=50mm  ref to elog #3771) but it was converging.  So I moved the f=50mm lens backwards to make the beam collimated.  I checked the beam collimation by introducing an Al coated mirror infront of th PD and diverting the beam temporarily in a free direction.  I could then check the collinearity and collimation of both the green beams over a meter.  After alignment the mirror was removed and the light is now incident on the PD once again.  We can now proceed to look for green beats.

The power from the PSL NPRO was attenuated for the MC locking work of yesterday.  It has now been increased to the maximum by rotating the Half Wave Plate (HWP).  The power after the PSL is now about 450mW  (500mW - 10% picked off for the doubling).

Yuta and Suresh

The MC2 transmission is seen on the QPD

Once the laser was locked to the cavity, and the PMC was able to follow the laser (ref: elogs by Yuta and Rana today)  we had a steady TEMoo mode in the MC.  This gave us sufficient transmission through MC2 to be easily visible with an IR viewer and we were able to guide the beam on to the QPD.  The beam however seemed to over fill the QPD, a lens (f=180mm) was placed before the beam folding mirror and the spot sized reduced.   The the QPD was found to be not fixed to the table and this was also recitified.  The signal level is still low: total signal is just about 7 DAQ steps amounting to about 5mV.  Tomorrow we plan to work on the alignment of the PSL and MC and thus increase this signal.

A new channel to observe the length variations in the MC.

A long BNC cable was laid from the MC locking electronics next (southwards) to the PSL table to the DAQ cards picking up the signals from the PRM OSEMS.  This is to pick up one of the MC locking signals and collect it on a DAQ channel.  However as there are no spare DAQ channels currently available one of the PRM OSEM (UL and LL) photodiode channels was unplugged and replaced with the signal from the long BNC cable.   For identifying the correct DAQ channel we put in a 2 Vpp 10Hz signal with a function generator into this BNC.  Tow signals can be picked up in this fashion and they are available on PRM_LLSEN_IN1 and PRM_ULSEN_IN1. We plan to use this for improving the alignment of the MC tomorrow. 

The algorithm for this alignment is that if the beam from the PSL is not centered on the MC1 then tilting MC1 would result in a change in the length of the cavity as seen by the light.  Using this as feedback the spot could be precisely centered on the MC1 and then the MC alignment could be completed by moving MC2 and MC3 to reobtain TEM_oo within the cavity.

Fiber coupling 1064 nm light at the end of X arm

This is 'work in progress'.  The attempt is to bring a few milliwatts of the 1064 nm light from the NPRO at the end of the South(X) Arm to the PSL table through an single mode optical fiber.  This would enable us to tune the two NPRO's to be less than 15 MHz apart by looking at their beat frequency before doubling.  Because we have a 1GHz bandwidth PD at 1064 nm, while the photodiode for green has a BW of about 30MHz.

A PBS (P-type) cube has been introduced into the beam of the X arm NPRO  (between the lamda/2 plate and the input lens of the doubling crystal).  By rotating the face of the PBS slightly away from normal incidence, I have diverted away 1.5mW of the 1064 light for coupling into the fiber. The beam has shifted slightly because of this and the green beam from the south arm has to be realigned to reach the PSL table.

A single mode fiber (Thorlabs SM980-5.8-125),  which was already laid half way, has been extended all the way to the PSL table. It runs along the  South arm in the cable tray. 

A pair of mirrors have been arranged in a zig-zag to steer the beam into a fiber coupler.  There was some hope that this coupler had been aligned at some point in the past and that attaching a fiber might result in some transmission.  But this is not the case and fiber coupler needs to be readjusted.

In order to see the light transmitted through the fiber, a camera has been set up on the PSL table.  Its output has been routed into the 'Ref Cavity reflected' video signal.  A video cable running from the ETMX to the Video-MUX used to be connected to the input channel 9 of the Video MUX.  This has now been shifted to output channel 25 of the MUX and disconnected from the camera at the ETMX.   The 'Ref Cav Refl.' video signal has been routed to the output channel 25.  The camera looking at the fiber output can now be seen on a local monitor at the end of the X arm and on the video monitor in the control room.

With the fiber disconnected, the 1064 nm beam was steered into the fiber coupler and its transmission maximised by observing  with an IR viewer.  The fiber was then connected and then the transmission at the PSL table was sought.  There was no transmission seen after a searching around this region for a few mins.

The plan is to purchase a Visual fault locator which would enable us to quickly get a rough alignment of the fiber coupler. A local vendor is listed as a distributor for this product from JDSU.  Contact info:

DuVac Electronics (EDGE)
Tel: 626-796-3291
Email: jack@duvac.com
1759 E Colorado Blvd
Pasadena, CA 91106

[Kiwamu, Suresh]

We decided to use the 1064nm beam reflected from the Y1-1037-45-P mirror after the collimation lens following the doubling crystal for coupling into the optical fiber (ref 3843 and 3847 ).

We replaced a beam dump which was blocking this beam with a Y1-1037-45-P mirror and directed the beam into the fiber coupler with another Y1-1037-45-P.  The power in this beam was about 1W.  This has been stepped down to 10mW by introducing a reflective ND filter of OD=2.  The reflected power has been dumped into a blade-stack beam dump.

Steve has ordered the The Visual Fault Locator from Fluke.  It is expected to arrive within a day or two.

The Fluke Visual Fault locator (Visifault) arrived and I used it to couple 1064nm light into the single mode fibre at the south-end-table.

Procedure used:

When the output end of the fiber is plugged into the Visifault the light emerges from at the south end (input side for 1064nm).  This light is collimated with the fiber coupler at that end and serves as a reference for the optical axis along which the 1064 light must be directed.  Once the two beams (red and 1064) are overlapped with the beam steering mirrors, the Visifault was disconnected from the fiber and the  fibre output ( 1064 at the PSL table) is maximized by walking the beam at the input end and adjusting the collimation at the input.

The output of the fiber has been collimated with a fiber coupler.

7.5mW are incident on the input end and 1.3mW have been measured at the output.    This output power is adequate for the observing the beats with PSL NPRO.

All the temporary changes to the video cables and the video MUX ( 3843 ) have been reversed and the system returned to its original state.

[Koji, Suresh]

We wanted to check and make sure that the relative phase of the two inputs ( local oscillator and photodiode signal ) to the demod board is such that the Q output is maximised.   We displayed the I and  Q signals on the oscilloscope in XYmode with I along the X direction.  If Q is maximised (and therefore I is minimised) the oscillocope trace would be perfectly vertical since all the signal would be in Q and none in I. Initially we noted that the trace was slightly rotated to the CCW of the vertical and that a short cable was present in the PD input line.  Removing this rotated the trace CW and made it pretty much vertical.  The screen shot of the oscilloscope is below.

.

These are the next steps for a better operation of the arm locking. They are suitable for the day time activities

Reconfiguration of the X-End table

- End transmission power monitor (CDS model exists, need to configure the PD)

- IR steering mirror for the transmon

- Restore/align end green beam

- Relocate the end trans CCD

- Connect the video output cable for the X-end CRT monitor

LSC Whitening

- LSC Whitening binary IO connection

They are not urgent but also good things to do

MC servo characterization

- Error signal: frequency noise

- Loop characterization

Arm cavity characterization with cavity sweep

- Arm finesse for 1064nm and 532nm

- Arm FSR measurement with 1064 (and optionally with 532nm simultaneously)

- Arm g-factor for 1064nm and 532nm

Objective:

      We wish to study the coherence of the two NPROs i.e. PSL and the X-end-NPRO by locking both of them to the X-arm and then observing the green beat frequency fluctuations. 

What we did:

   a) locked the PSL to the X-arm as described in 4153

   b) locked the x-end-NPRO to the X-arm with a PDH lock to the reflected green from the ETMX

   c) Obtained the green beat signal with a spectrum analyser as described in 3771

Observations:

   Please see the attached screen shots from the spectrum analyser.   They are taken with different BW and sweep range settings.  They give a estimate of the width of the green beat signal and the range of the frequency fluctuations of the beat-note.

Estimates:

   a) width of the beat note is less than 6KHz if measured over time scales of a few milli seconds

   b) the frequency fluctuations of the beat note are about 100KHz over time scales longer than 100ms

Next Step:

    We wish to record the beat note frequency as a function of time in order to establish the stability over time scale of a day.