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ID Date Author Type Categorydown Subject
  1369   Mon Oct 21 14:04:04 2013 ranaDailyProgressBEATNew intensity-to-frequency TF

 Even though it shouldn't be there, there is probably a coupling directly from RIN to the reflected PDH signal on each cavity. So you should measure all 3 terms (RIN 2 PDF for each cavtiy, and RIN 2 Beat).

  1373   Sat Oct 26 15:35:42 2013 EvanDailyProgressBEATBeat measurement with ISS

Summary: No good so far. Engaging the ISS seems to have basically zero effect on the beat. The beat overall looks worse than it did a month ago, and the shape seems to mimic the shape of the north cavity RIN. More optimization of the north EOAM is necessary.

Details: Having set up the north EOAM on Thursday (PSL:1372), I spent most of yesterday trying to get a RIN-suppressed beat measurement.

The continual drift of the laser frequency control signals was irritating, so I spent some time getting the slow digital PID controls for the lasers back up and running. At first only KP seemed to have no effect on the laser control signals; it turns out this is because the PID Perl scripts that run on the Sun machine rescale the KI and KD coefficients by a timestep variable, which had been set to zero. I've set it to 1. I've chosen KP = KD = 0 and KI = 0.0002 (with appropriate choice of sign for the two loops). The system is probably overdamped, but it manages to integrate the control signals down to zero in a resonable amount of time (<30 s) and I don't think it's a high priority to optimize it right now.

The south PDH error signal has noticeable 250 kHz oscillations which get worse as the common TTFSS gain is increased. The north PDH error signal is much quieter. Are we perhaps hitting a mechanical resonance of the EOM crystal? Or (dare I say it) do we have the wrong sign for the common path of the PDH loop?

I took out the hand-soldered integrating board that I built for the ISS loops; it was railing too often. The ISS setup for each path is now as follows: each ISS PD goes into the A input of an SR560, and a programmable voltage reference (Calibrators Inc. DVC–350A) goes into the B input. The voltage is chosen to match the dc voltage from the ISS PD. The SR560 is dc coupled and set to take the difference A − B. The gain is set to 5×103 V/V, with a single-pole low-pass at 1 kHz. The output from the SR560 is fed into the EOAM.

The suppressed and unsuppressed RIN measurements are given in the first two plots. Evidently, these simple ISS loops are able to suppress the RIN by a factor of 50 or so. Also, the north RIN is much worse than the south RIN, and the hump from 100 Hz to 10 kHz is reminiscent of a poorly aligned EOAM (as seen in PSL:1311, for example). So I'd like to spend some more time fiddling with the north EOAM to see if I can improve the RIN suppression. Alternatively, perhaps we are suffering because the north path has no PMC to stabilize the pointing into the EOM, EOAM, etc.

Anyway, I pressed ahead and looked at the beat. To convince myself of the repeatability of the setup, I took a measurement with the ISS loops on, then a measurement with the ISS loops off, and then a measurement with the ISS loops on again. The result is given in the third plot. Below a few hertz, the ISS may have a positive effect. Above this, there is either no effect or a small worsening effect.

Note that the shape of the beat follows the shape of the north cavity RIN. I think we should spend a little time noise hunting and optimizing on the north path to see if we can make this go away. Note also that the beat is worse than it was back in September (PSL:1321). Two immediate culprits that I can think of are (a) the installation of the EOAM or (b) the fact that the vacuum can is no longer floated. But it could just as well be that there's something else (e.g., PDH offsets) that I neglected to optimize.

Attachment 1: north_iss.pdf
north_iss.pdf
Attachment 2: south_iss.pdf
south_iss.pdf
Attachment 3: beat.pdf
beat.pdf
  1375   Mon Oct 28 22:55:11 2013 EvanDailyProgressBEATBeat measurement with ISS

I've taken Tara's farsi.m and changed the values of finesse F and absorption α in order to fit the magnitude of the TF measurement in PSL:1368. I've chosen 7500 for the finesse and 5 ppm for the absorption, although for this calculation they are degenerate (entering into the TF as F/α).

Using this, I've taken the RIN measurements from Friday and used them to estimate the induced frequency fluctuation in the beat readout, assuming a transmitted power of 1 mW from each cavity.

In the case when the ISS is off, the estimated effect of RIN on the current beat is significant only below 10 Hz. When the ISS is on, the RIN is insignificant over the entire measurement range. This perhaps explains the observed reduction in the beat PSD below 10 Hz when the ISS is on.

Attachment 1: rin_to_beat.pdf
rin_to_beat.pdf
Attachment 2: expected_rin-to-beat.pdf
expected_rin-to-beat.pdf
Attachment 3: expected_rin-to-beat_iss.pdf
expected_rin-to-beat_iss.pdf
  1382   Tue Nov 5 10:34:09 2013 EvanDailyProgressBEATNew beat measurement

[Tara, Evan]

Yesterday, we did a few final bits of optimization and then re-measured the beat spetrum.

Specifically:

  • Tara tweaked the north RFPD cable to symmetrize the north PDH error signal.
  • For each path, we hooked the RF output of each RFPD directly into the HP4395A and minimized the RFAM at 14.75 MHz (the PDH frequency).
    • For south, the initial RFAM was −107 dBm, and by adjusting the HWP before the resonant EOM, Tara was able to get it down to −112 dBm. This resulted in no visible change to the error signal offset (10 mV, compared to 300 mV peak-to-peak).
    • For north, the initial RFAM was −105 dBm, and by adjusting the HWP before the broadband EOM + resonant EOM (there is no intervening waveplate), Tara was able to get it below the noise floor of the spectrum analyzer (i.e., < −125 dBm). This resulted in a change to the error signal offset from 28 mV to 19 mV (compared to 300 mV peak-to-peak).
  • We measured the slopes of the error signals in order to get the calibration from voltage to frequency.
  • We locked the cavities with the slow digital control engaged. The south TTFSS gain was 712 slow and 796 fast, and the north TTFSS gain was 900 common and 900 fast. The south TTFSS control signal still has strong (~ 2 V peak-to-peak) oscillations at hundreds of kilohertz.
  • We measured the beat, the PDH error signals, and the RIN spectra. The modulation setting on the Marconi was 1 kHz, so the conversion factor is the usual 710 Hz/V.
  • After measuring the beat, we swapped out the SHP–150 on the RF input of the mixer for an SHP–100. The beat is currently at 120 MHz, and with the SHP–150 we were throwing away something like 60% of our power. Attenuation from the SHP–100 appears negligible when viewed on a scope.

The beat spectrum is attached, along with the expected coating Brownian noise estimate. I will post the estimates of the PDH and RIN contributions later.

Attachment 1: beat_2013-11-04.pdf
beat_2013-11-04.pdf
  1438   Thu Jun 26 17:10:09 2014 EvanDailyProgressBEATBeat

South laser slow at 1.234 V, north laser slow at 5.558 V, beat is 120(1) MHz at +5.5(2) dBm. South and north alignment has not yet been tuned up.

SR785 appears to have broken screen.

  1447   Wed Jul 9 22:31:59 2014 Emily, EvanNotesBEATfiber phase noise measurement

Installation of optics for fiber phase noise measurement

 

After light passes through the AOM, it is reflected back through the AOM and into the fiber.  We installed a 50/50 beamsplitter, quarter wave plate, mirror, lens and photodiode to do the beat measurement.  It is required that the beam spot size is 1/3 the diameter of the photodetector.  We installed a lens at the appropriate distance to obtain a waist that is roughly 50 microns.  We hooked up the photodiode to an oscilloscope and found that the voltage fluctuates between 100-500 mV.  We are not sure why the voltage is fluctuating, but we will continue to investigate the cause.  
Labsetup_ELOG.eps

 

 

  1492   Fri Aug 29 12:35:54 2014 EvanDailyProgressBEATBeat breadboard in place

Beat breadboard is slid back into place. North transmission appears on north camera. Still need to do south transmission.

  1493   Fri Aug 29 15:35:55 2014 EvanDailyProgressBEATMode-matching for beat

I predict (via alm) that the spot size on the diode (z = 991 mm) is 79 µm in the current configuration.

Attachment 1: ctnbeat_algaas.pdf
ctnbeat_algaas.pdf
Attachment 2: ctnbeat_algaas_alm.zip
  1494   Fri Aug 29 19:56:06 2014 EvanDailyProgressBEATBeat breadboard in place

Quote:

Beat breadboard is slid back into place. North transmission appears on north camera. Still need to do south transmission.

Tara has found south transmission on camera. I steered the transmitted beams onto the beat PD and then made the k-vectors as parallel as I could as seen on an IR card.

The DC voltage on the PD is okay (ca. 50 mV from each beam), but I cannot see a beat note on the AC path using the HP4395. Tara will give a temperature kick which hopefully will bring the beat note within the range of the 1811.

  1496   Tue Sep 2 11:06:54 2014 EvanDailyProgressBEATNo beat

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

  1497   Tue Sep 2 15:04:05 2014 EvanDailyProgressBEATNo beat

Quote:

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

On Koji's suggestion, I set up a second 1811 to monitor the beat on the input side of the cavities, so that we can see what the lasers are doing independent of the cavity resonances. For each path, I am using the s-polarized light that is rejected from a PBS, so that we don't need to add extra optics to the beam paths.

For example, for south slow at 1.206 V and north slow at 5.565 V, I get a 69 MHz beat (with both cavities unlocked).

We should use this in conjunction with the cavity locking optics to figure out what the correct axial cavity modes are, and whether the cavities need any temperature adjustment.

  1498   Tue Sep 2 17:11:27 2014 EvanDailyProgressBEATNo beat

Quote:

Quote:

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

On Koji's suggestion, I set up a second 1811 to monitor the beat on the input side of the cavities, so that we can see what the lasers are doing independent of the cavity resonances. For each path, I am using the s-polarized light that is rejected from a PBS, so that we don't need to add extra optics to the beam paths.

For example, for south slow at 1.206 V and north slow at 5.565 V, I get a 69 MHz beat (with both cavities unlocked).

We should use this in conjunction with the cavity locking optics to figure out what the correct axial cavity modes are, and whether the cavities need any temperature adjustment.

With this setup, I find that a sub-100-MHz beat occurs for north slow at 5.015 V and south slow at 0.725 V. This south slow voltage corresponds to a south cavity TEM00 mode, but the nearest north slow voltage is at 5.298 V.

  1499   Wed Sep 3 12:02:19 2014 Tara, EvanDailyProgressBEATBeat found

Tara added some more juice to the north cavity heater last night. Now we can lock both cavities to TEM00 and get a beat within the bandwidth of the 1811.

  • North laser slow: 5.020 V
  • South laser slow: 0.722 V
  • Beat frequency: 49.3 MHz
  1500   Wed Sep 3 16:34:37 2014 Tara, EvanDailyProgressBEATBeat, mode-matching

Quote:

Tara added some more juice to the north cavity heater last night. Now we can lock both cavities to TEM00 and get a beat within the bandwidth of the 1811.

  • North laser slow: 5.020 V
  • South laser slow: 0.722 V
  • Beat frequency: 49.3 MHz

Beat frequency drifted to 61 MHz over the course of a few hours. We need to wait for the cavity temperatures to settle.

I improved the mode-matching a little bit on the south cavity; it's about 50% (the theoretical max is 71%). The south lenses are now on translation stages.

I've attached a beat spectrum. Nothing is floated, RAM is not optimized, etc.; this is just a rough indicator of where things stand.

Here is what I think should happen next, in rough order of importance:

  1. Float chamber
  2. Measure RIN
  3. Measure photothermal TF (I also need to recheck my photothermal code — I don't believe the coating TE part)
  4. Put photothermal noise on noise budget
  5. Reduce RAM.
  6. Measure residual frequency noise.
  7. Measure PLL noise. Use ATF DAQ and make spectral histogram.
  8. Measure seismic noise (with Guralp or T240), with table floated and unfloated. Use ATF DAQ and make spectral histogram.
Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1509   Tue Sep 9 12:22:31 2014 EvanDailyProgressBEATReduced south scatter shelf

Last night, I looked at the TTFSS OUT2 on the spectrum analyzer, with the cavity unlocked and the laser PZT and broadband EOM unplugged. I believe this should give the demodulated spectrum of the RAM.

For north, the spectrum was white. For south, the spectrum was white down to 10 Hz, and below 10 Hz there was an apparent scattering shelf that rose several orders of magnitude above the white spectrum.

Tara and I went through the south path and placed an OD0.3 ND filter between various optics. We tried twisting one of the mode-matching lenses to reduce the scatter shelf, but it didn't seem to work.

This morning I went through the south path again with an OD0.5 filter, and eventually focused on the lens right after the FI. I found that I could greatly reduce the height of the scattering shelf (relative to the height of the white noise) by placing the filter downstream of lens, but not upstream of it. So I twisted this lens slightly, re-modematched into the cavity (minimal adjustment was required), and I found that the scatter shelf was reduced to less than 1 order of magnitude above the white noise.

Then I took a beat spectrum (carrier at 11.5 MHz, seems stable). The scatter shelf in the beat is not reduced by much, so I'll have to think about where to look next.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1514   Sun Sep 14 10:37:57 2014 EvanDailyProgressBEATNew beat

Quote:

We turned on both ISS loops today.

Here is an in-loop characterization of the south RIN with and without the ISS.

Here is the beat measurement from Thursday, with the ISS loops on and the table floated.

I've fudged the photothermal noise slightly by just using twice the south cavity's PT measurement, rather than south and north. I need to take RIN data from north, and then I can add south + north PT.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1515   Sun Sep 14 14:46:44 2014 EvanDailyProgressBEATReinstalled aux 1811

I reinstalled the auxiliary 1811 on the input side of the table.

I tried to get the free-running noise of the north cavity by locking south and then using the PLL to read out the beat from the 1811. However, even on a FM deviation of 400 kHz / Vrms, I could not get the PLL to lock. So I suspect the free-running noise is just too high to use this method.

I also used this aux 1811 to optimize the PDH EOM alignments. I saw no change in the beat spectrum after doing this. I would like to demodulate the 1811 signal using the PDH LO, but this will require some reconfiguration of the RF distribution.

  1517   Mon Sep 15 04:32:21 2014 taraNotesBEATnote for tonight beat

 RCAV transPD_DC :0.54 V

ACAV transPD_DC: 0.16 V (loop might oscillate when DC level was measured, need to double check)

  1518   Mon Sep 15 07:18:38 2014 EvanNotesBEATnote for tonight beat

Quote:

 RCAV transPD_DC :0.54 V

ACAV transPD_DC: 0.16 V (loop might oscillate when DC level was measured, need to double check)

 In this noise budget I corrected the PDH shot noise level (incident power is 1 mW, visibility is 0.5).

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1519   Mon Sep 15 18:29:47 2014 EvanDailyProgressBEATAttempts at scatter reduction

I went through the table today looking for ghost beams. Most were already dumped. For those that weren't, I put down a dump or an iris.

I again looked at TTFSS OUT2 with the cavities unlocked (i.e., the open-loop error signals) and found that the low-frequency seismic/scatter wall appears only on south. So I hunted around south for a while. I found a series of ghost beams reflecting off the EOAM input and hitting dangerously close to the EOM output aperture. So I moved the EOAM forward a few inches, then adjusted its kinematic mount to offset these beams a bit. The EOAM should be realigned, and we should check to make sure the ghost beams are not entering the EOM again.

With the increased space between the EOM and EOAM, I installed a flipper mirror that takes the beam to the 1811. Then I minimized the RAM (from –54 dBm to –72 dBm with 85 mV dc).

FM dev: 10 kHz

Averages: 10, 50, 100, 500

  1521   Tue Sep 16 15:08:57 2014 EvanDailyProgressBEATAttempts at scatter reduction

Quote:

I went through the table today looking for ghost beams. Most were already dumped. For those that weren't, I put down a dump or an iris.

I again looked at TTFSS OUT2 with the cavities unlocked (i.e., the open-loop error signals) and found that the low-frequency seismic/scatter wall appears only on south. So I hunted around south for a while. I found a series of ghost beams reflecting off the EOAM input and hitting dangerously close to the EOM output aperture. So I moved the EOAM forward a few inches, then adjusted its kinematic mount to offset these beams a bit. The EOAM should be realigned, and we should check to make sure the ghost beams are not entering the EOM again.

With the increased space between the EOM and EOAM, I installed a flipper mirror that takes the beam to the 1811. Then I minimized the RAM (from –54 dBm to –72 dBm with 85 mV dc).

FM dev: 10 kHz

Averages: 10, 50, 100, 500

I added a flipper mirror before the north EOAM, as well as a HWP before the resonant EOM (so that we can independently control the polarization entering the two EOMs). I optimized the RAM, but saw no improvement in the beat.

  1558   Sat Aug 1 09:24:58 2015 Aidan, AntonioSummaryBEATLasers locked to cavities - no beat - polarization? Transmission issues

Both lasers have been locked to the cavities for 24 hours. The slow control of the frequency is handed off to the PID loop. Antonio and I observed strange behaviour on the DC value of the cavity transmission.

As Evan had noted before, there are two polarizations that will resonate and they're about 3MHz apart (if I remember correctly). We can see these on the DC photodiodes on transmission (the ISS PD and the RF DC output). One peak is large and the other much smaller. However, when we have large transmission onto the RF photodiode we have small transmission onto the ISS PD and vice versa. It's likely we have a pick-off optic with strong polarization selectivity.

We couldn't find the beat yesterday or Thursday.

  1569   Tue Aug 11 16:49:55 2015 Aidan, Rich, AntonioSummaryBEATDiagnosing the terrible beat signal. RCAV HV supply not showing current drawn

The beat signal looks awful. It has some amplitude modulation at 6.75MHz and looks like it has some strange saturation effects going on. This is too much noise for the PLL to lock to.

We thought, for a minute, that the reason for this may be related to one of the HV supplies for the RCAV locking. The needles on the front of the positive supply unit +150V and 0mA current drawn. The other 3 HV supplies in use all show around 20-25mA current draw when used with the TTFSS boards.

We popped the top of the RCAV TTFSS box on the table and looked at the TP4 output on the HV/Interface board (this looks at the signal coming out of the high voltage amplifier that feeds the EOM, but reduced a voltage divider to 1/10th the value). It was freely swinging between +/-4V, so the HV amplifier seems to be happily getting both +ve and -ve voltages. There might be a problem with the needle on the HV supply.

 

Attachment 1: photo_1.JPG
photo_1.JPG
Attachment 2: photo_2.JPG
photo_2.JPG
  1570   Thu Aug 13 09:07:04 2015 Aidan, Rich, AntonioSummaryBEATPLL locked on beat note - TTFSS boxes were being used incorrectly previously

We investigated the way we were locking the PDH loops using the TTFSS boxes. Here's what we previously did:

  • With the loop open and the TTFSS interface set to LOCAL & TEST, we adjusted the temperature of the laser until we were close to TEM00 resonance.
  • Then we set the first switch from LOCAL to REMOTE. 
  • This locked the loop.

We realized, yesterday, that this wasn't the correct way to lock the loop as box now expected the gain settings for the loop to be set remotely (and we were providing none of that to the unit). Still, the default gain in REMOTE was enough to provide a stable lock and we didn't understand exactly how that box worked (which is obvious in retrospect). So, yesterday, we pored over the schematics for the TTFSS boxes (Rich is drawing a very nice block diagram to show the loop structure), and realized our error. The correct way to lock is the following:

  • Set to LOCAL & TEST
  • Adjust the temperature so we were close to TEM00 resonance.
  • Scan through the two polarization TEM00 eigenmodes (separated by ~1MHz)
  • We can sit (a) outside the mode with big transmission (big mode), (b) in between the two modes or (c) outside the small mode. 
  • We sit outside the big mode and then switch TEST to OFF to turn the loop on (I have been cursing about the naming conventions on these boxes for the last two days).
  • This locks the loop.
  • With the gains set locally.

From here, we were able to play with the common mode gain settings reduce the noise of the beat note between the lasers. And we were able to lock the PLL. The main evidence for the latter is the fact that we can change the DC value of the control signal in the PLL by varying the carrier frequency of the Marconi.

  1577   Thu Aug 27 17:11:43 2015 Aidan, AntonioSummaryBEATTrouble locking - 140kHz notes are switching supplies?

We tried locking the PLL today but failed. Looking at the beat note on the network analyzer revealed some 70/140kHz harmonics. We thought this might be what is preventing us from locking.

Rana suggested that these are from the switching power supplies (which switch at 70kHz). If so, this may be a red-herring. It's possible we're not trying with high enough gain settings ...

The large peak that is offset by about 50kHz is from the Marconi ... we can make it move around by changing the Marconi frequency.

Attachment 1: network_analyzer_140kHz.JPG
network_analyzer_140kHz.JPG
  1578   Wed Sep 2 22:13:47 2015 AntonioSummaryBEATPLL locking AGAIN

Description

As described in the elog entry n. 1577 we were not able to lock the PLL as has been described in elog n. 1570. I have started by playing with the two PDH gains of both loops (North and South) as this could have been causes of non tolerable noise in the PLL loop. I have also monitored the peaks described in entry n.1577 as we were suspicious for their preventing the PLL locking.

Conclusion

The PLL loop has been locked repeatedly after locking the cavities multiple times. This result has been achieved by setting the PLL gain on the SR570 at 20 (PLEASE NOTE I was not able to lock the PLL with any other gains settings). 

The peakes of entry n.1577 are not preventing us to lock the PLL.

 

Some settings:

North FSS interface: Common gain =700; Fast gain = 450; PID = 4.451V;

South FSS interface: Common gain = 850; Fast gain =  250; PID = 0.5527V;

Beat frequency = 69.6 MHz;

 

However these are not the only allowable settings for the gain, but the PDH loop gains are crucial for the PLL locking. Later I am going to give a quantitative analysis for our PDH loops in order to have them in a more stable and/or less noisy locking point.

  1579   Sun Sep 13 10:40:29 2015 AntonioDailyProgressBEATPLL measured noise

Summary

========

The goal of the TCN experiment is to measure the TC noise. This requires to lower down the noise level that we have at

the PLL output. I have decided to take noise measurement today in order to have a reference from the level we are starting with.

We should start to implement changes in order to lower the noise at the output and keep monitoring it.

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

 

Description

=========

I report a plot of the PLL noise in Vrms/sqrt(Hz) as it is not clear yet how to convert the units in Hz/sqrt(Hz). From older Elogs ID 889,

I see that there is available a calibration for the IFR 2023a, and this depends on the input range. I am not sure on what the input range is.

The measurement has been taken in dirrent frequency range in order to have a better resolution. For now, when we are at this level

of noise it is not worth it.

 

Note:

I have tried to lock the PLL with a FM deviation which is less than 300kHz but it was not possible. There is too much noise.

 

Figure 1:

Target noise and noise taken sometimes in the past by Evan’s (in Hz/sqrt(Hz)).

 

Figure 2:

Current measurements in Vrms/sqrt(Hz);

Beat freq = ~ 65 MHz;

FM deviation = 400kHz

gain = 20;

note: something happened after the first measurement, but at this point is not so important.

 

Data:

Data are on the lab control/home/data/20150912_PLL_noise

 

 

/

Attachment 1: NoiseEvan.pdf
NoiseEvan.pdf
Attachment 2: Noise_20150912_Vrms.pdf
Noise_20150912_Vrms.pdf
  1580   Sun Sep 13 11:31:04 2015 AntonioDailyProgressBEATPLL noise comparison at different PDH gain settings

Summary

========

Measurement of the PLL noise have been taken at different PDH gains settings. Noise start to increase when the gain on the oh North loop

are below North: Common/Fast=500/300 and South: Common/Fast=420/420. Stays fairly the same with higher values.

 

The measurement are still in V/sqrt(Hz), but for now I use them only for comparison purpose.

 

Figure:

Noise comparison at different gains: In the legend are the "values" of the gains (the numbers on the knobs)

Current measurements in Vrms/sqrt(Hz)

Beat freq = ~ 65 MHz;

FM deviation = 400kHz

gain = 20;

 

Data

Data are on the lab control/home/data/20150912_PLL_noise/

 

Attachment 1: NoiseComparison_20150912_Vrms.pdf
NoiseComparison_20150912_Vrms.pdf
Attachment 2: NoiseComparison_20150912_Vrms.pdf
NoiseComparison_20150912_Vrms.pdf
  1590   Wed Oct 14 20:30:27 2015 AntonioDailyProgressBEATPLL noise

Summary

========

 

New PLL noise measurements have been made with  Fdev=1KHz and Fdev=10kHz at the input range of the Marconi.

There is an improvement of a factor ~10 for frequencies above 200Hz. However the two set of measurement show

the same noise in Hz/sqrt(Hz) between them. The lock of the PLL has been done keeping the same gain; when Fdev = 10KHz

the sr560 gain is 50, and when Fdev = 1kHz the SR560 gain is 500. The latter setting provokes an increase of the noise in V/sqrt(Hz)

which compensate for the reduction in the input range when Fdev goes from 10kHz to 1kHz. It is not clear to me what is happening.

 

Additionally it is not clear what is the “extra” noise that we see in the following noise budget as the sum does not match with the measurement.

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

 

 

 

Plots:

=====

1. The first plot shows the noise budget with the measurement taken in September 12 (2015). We see that the PLL noise is dominated by the

Marconi and photo thermal noise;

 

2. The second plot shows the noise budget with the PLL noise measured today with Fdev=1kHz; Here I do not understand

what is happening above ~200Hz; the total noise is off from what has been measured. I also should check the power on the photodiode and

see if it matches the level (2dB) I have used for the Marconi noise measurement.

 

3. The same as in point 2. but here we have Fdev=10kHz; here the total noise is closer to match the measurement,

 

4. The last plot is just the comparison between the PLL noise taken in the past September and the one measured today.

 

Please note:  ISS is off

 

Data

====

Data are stored in TCN lab computer: controlfb2/data/20151014_PLL_noise

  1592   Fri Oct 16 22:24:35 2015 AntonioSummaryBEATPLL noise and beat frequency at 50MHz

Summary

Today the beat frequency was very difficult to find. Something changed, and I am not sure yet how to drive

the frequency of the beat. However the beat frequency is at 50MHz while so far it was at 64MHz. The PLL lock

more difficult and noisier.

 

Note:

Sometimes today or Yesterday we have increased the temperature of the lab (~ 2 degrees F).

 

Data:

lab computer/control fb2/home/data/20151016_PLL_noise_50MHz

  1605   Wed Nov 4 11:28:37 2015 AntonioDailyProgressBEATBeat noise did not improve with the ISSs ON

Summary

========

I have measured the noise at the beat note with both the ISS servo activated —>

NO improvement compared to the past. It is actually a bit worse. However with the EAOM

in the South path I had to change sluggishly the PDH gain settings.

 

- I took two measurements with different Fdev at the Marconi (1kHz and 10kHz) and the noise

is the same mostly. It seems that the limitation at moment relies NOT in the Marconi noise.

 

Data

=====

controlfb2/home/data/20151104_PLL_noise

 

Attachment 1: PLL_noise_comparison_20151104.pdf
PLL_noise_comparison_20151104.pdf
  1606   Tue Nov 10 09:29:10 2015 AntonioDailyProgressBEATPLL and EAOM noise generated

The intensity noise that I have noticed after the EAOM installation disappeard with the use of a PBS before it.

Has to be mentioned that the light coming from the EAOM becomes eliptical with following power ratios at the PBS after theEAOM:

Pinc = 1.59mW (p-polarized);

Ptrans = 1.30mW (p);

Prefl = 0.25mW (s);

 

We were steering the beam transmitted from the cavities with use of lenses. I have realigned (not easy thing with such space constrains).

Intensity noise is way better, although the overall noise did not improved as the intensity noise is not the limiting noise. I think we need to suppress

more free-running noise. Actually I will go back to our FSS servo because I think we need different performances (not for now).

 

I have accidentally misaligned the north cavity, better not to say how :-);

  1607   Tue Nov 10 12:00:59 2015 AntonioDailyProgressBEATPLL and EAOM noise generated

The north cavity is now in place with 48% visibility! 

Quote:

The intensity noise that I have noticed after the EAOM installation disappeard with the use of a PBS before it.

Has to be mentioned that the light coming from the EAOM becomes eliptical with following power ratios at the PBS after theEAOM:

Pinc = 1.59mW (p-polarized);

Ptrans = 1.30mW (p);

Prefl = 0.25mW (s);

 

We were steering the beam transmitted from the cavities with use of lenses. I have realigned (not easy thing with such space constrains).

Intensity noise is way better, although the overall noise did not improved as the intensity noise is not the limiting noise. I think we need to suppress

more free-running noise. Actually I will go back to our FSS servo because I think we need different performances (not for now).

 

I have accidentally misaligned the north cavity, better not to say how :-);

 

  1653   Wed Jun 29 15:20:40 2016 AntonioNotesBEATPlan decided on June 28th 2016

In line with what we already started we decided:

1. Build the North and South optical paths by July 17th (12:59pm) (Antonio North path for now, Andi South path) 

  • Both the paths must to be able to host the PMC, bb-EOM, AOM, and 2 resonant EOM in each of them

2. Week of July 18th beat noise measurements with what we have;

 

A summary with a slight different timing (written before the meeting) is in the following PDF (sorry for the weird layout)

Attachment 1: timeline.pdf
timeline.pdf timeline.pdf timeline.pdf timeline.pdf timeline.pdf
  1688   Tue Jul 26 15:12:53 2016 awadeDailyProgressBEATInitial search for beat note refcavs' beatnote

 

Quote:

How about this procedure?

- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.

- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.

- Lock one of the cavities.

- Scan the laser temp of the other laser to find the beat again.

- Lock the second cavity with several temp settings.

- Figure out how much heating of which cavity you need.

I have just borrowed a 1 GHz InGaAs and HP spectrum analyzer from the 40 m.  While I was doing some other things I have set one cavity to lock and the other path's laser on a ~1 mHz temp ramp with the spectrum analyzer on max hold.  Its still doubtful this will catch a beat note.

I will configure pick off from both paths so at least we can know that we are within the bandwidth of the detector without having to deal with the tiny frequency windows of transmission of the two reference cavities.

You make a good point of figuring out the appropriate heating needed, at least we will know if we are even in the ballpark of getting a beat note.

  1689   Wed Jul 27 23:05:45 2016 awade and AntonioDailyProgressBEATInitial search for beat note refcavs' beatnote

We built a temporary beat note detector before the cavities. The configuration is illustrated in the attached schematic.

We used some of the unused light from PBSs to provide a direct beat note measurement on a 1 GHz NewFocus 1611. This was before any EOMs or any other components so there is only a beat between the fundamental frequency of both lasers. Coated window optics were used to pick off a small amount of power and a 50/50 beam splitter was used to combine the light from the two lasers.  Some f = ~200 mm lenses were used to keep the beams reasonably collimated, but no big effort was put into mode matching.  There was plenty of light from both lasers to work with, so even poor spacial overlap was sufficient to give a beat note spectrum.  We achieved approximately -53 dBm beat note signal with a clearance of 10 dB above the dark noise level: this enough for our diagnostic purposes.

Using this diagnostic detector, it was found that the closest pair of refinances between north and south cavities was 300 MHz apart. We began the process of gradually tuning the north cavity temperature but believe it is taking a while to actually settle.  There is no feedback PID control on the cavity shield and then we only have control over one cavity with the other affected by variations in the lab environment.  This is something we will need to work on.  For now it looks like we can maybe dial some constants into a Newport 3040 to get a temperature readout from the existing sensor while electronics are organized to interface with the EPICS/acromag system.

For now we think we will keep the 125 MHz detector. This is convenent and from the data sheets it seems like the 1811's have a slightly better NEP. I couldn't see an actual noise spectrum on the manufacture's website so we might want to actually make a measurement of the dark noise of the two RF detectors for a true comparison (with the same power supply rigged up of course).

We left the north cavity to settle with the voltage slightly increased overnight.

 

 

Quote:

 

Quote:

How about this procedure?

- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.

- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.

- Lock one of the cavities.

- Scan the laser temp of the other laser to find the beat again.

- Lock the second cavity with several temp settings.

- Figure out how much heating of which cavity you need.

I have just borrowed a 1 GHz InGaAs and HP spectrum analyzer from the 40 m.  While I was doing some other things I have set one cavity to lock and the other path's laser on a ~1 mHz temp ramp with the spectrum analyzer on max hold.  Its still doubtful this will catch a beat note.

I will configure pick off from both paths so at least we can know that we are within the bandwidth of the detector without having to deal with the tiny frequency windows of transmission of the two reference cavities.

You make a good point of figuring out the appropriate heating needed, at least we will know if we are even in the ballpark of getting a beat note.

 

  1690   Thu Jul 28 19:56:08 2016 Antonio, AndrewDailyProgressBEATSearching for beat note

Summary
In the last two days we have been searching for beat note, in order to get 
our "first" noise measurement of the PLL. While we monitor the beat note of the lasers
at the input side we search the same beat note at the transmission side.

Conclusions
We did not succeded yet. I start to believe that what we see at the input side are
not the beat note we are looking for. Some mode hopping could be happening around
the temperature region we have chosen.
 


Things done

1. We built a Mach Zehnder interferometer at the input side of the lasers, in order to fined the beat
directly from the two lasers; The beat note were found around 450MHz.

2. Once we found them we changed the temperature of the vacuum chamber and we locked the cavities
whit laser beat note of 78MHz.

3. Trasmitted beam have been checked for their spatial overlap and polarizations;

4. Photodiodes have been changed in order to see if some problem was coming from there;

5. Network analizer have been changed in order to check if there were wrong settings.

6. PLL rialigned from beginning;


NO RESULTS

PLAN 
We may want to know where mode hopping happen; We need to scan the temperature and  first measure
the power and see where they are. 

Looking for other beat note at the lasers.

  1691   Fri Jul 29 16:29:53 2016 awadeDailyProgressBEATInitial search for beat note refcavs' beatnote

I forgot the schematic.  Attached to this post.

Quote:

We built a temporary beat note detector before the cavities. The configuration is illustrated in the attached schematic.

We used some of the unused light from PBSs to provide a direct beat note measurement on a 1 GHz NewFocus 1611. This was before any EOMs or any other components so there is only a beat between the fundamental frequency of both lasers. Coated window optics were used to pick off a small amount of power and a 50/50 beam splitter was used to combine the light from the two lasers.  Some f = ~200 mm lenses were used to keep the beams reasonably collimated, but no big effort was put into mode matching.  There was plenty of light from both lasers to work with, so even poor spacial overlap was sufficient to give a beat note spectrum.  We achieved approximately -53 dBm beat note signal with a clearance of 10 dB above the dark noise level: this enough for our diagnostic purposes.

Using this diagnostic detector, it was found that the closest pair of refinances between north and south cavities was 300 MHz apart. We began the process of gradually tuning the north cavity temperature but believe it is taking a while to actually settle.  There is no feedback PID control on the cavity shield and then we only have control over one cavity with the other affected by variations in the lab environment.  This is something we will need to work on.  For now it looks like we can maybe dial some constants into a Newport 3040 to get a temperature readout from the existing sensor while electronics are organized to interface with the EPICS/acromag system.

For now we think we will keep the 125 MHz detector. This is convenent and from the data sheets it seems like the 1811's have a slightly better NEP. I couldn't see an actual noise spectrum on the manufacture's website so we might want to actually make a measurement of the dark noise of the two RF detectors for a true comparison (with the same power supply rigged up of course).

We left the north cavity to settle with the voltage slightly increased overnight.

 

 

Quote:

 

Quote:

How about this procedure?

- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.

- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.

- Lock one of the cavities.

- Scan the laser temp of the other laser to find the beat again.

- Lock the second cavity with several temp settings.

- Figure out how much heating of which cavity you need.

I have just borrowed a 1 GHz InGaAs and HP spectrum analyzer from the 40 m.  While I was doing some other things I have set one cavity to lock and the other path's laser on a ~1 mHz temp ramp with the spectrum analyzer on max hold.  Its still doubtful this will catch a beat note.

I will configure pick off from both paths so at least we can know that we are within the bandwidth of the detector without having to deal with the tiny frequency windows of transmission of the two reference cavities.

You make a good point of figuring out the appropriate heating needed, at least we will know if we are even in the ballpark of getting a beat note.

 

 

Attachment 1: 20160727_DirectLaserBeatNoteDetector.pdf
20160727_DirectLaserBeatNoteDetector.pdf
  1692   Fri Jul 29 16:52:06 2016 awade and AntonioDailyProgressBEATBeat note signal

We have a beat-note.

For the last few days we have been able to bring the two lasers to within the 125 MHz bandwidth of the Newfocus 1811 detector at the output of the vacuum tank. By tuning the length of the North cavity (turning the temperature up) we were able to also bring the resonance of the cavities to within this bandwidth.  

Using the beat note detector setup we assembled at the start of the laser path (PSL_Lab/1689), we can see that the frequency offset of the north and south cavity resonance settled at 70 MHz for a north heater voltage set point of 11.5 V (74 mA). We still don't know what temperature that is because we have no thermistor monitor setup yet. However, it seems that with enough settling time it is stable enough for these initial diagnostic tests.

However, until today we had trouble finding a beat note after the two ref cavities.  We checked polarization, co-alignment of the beams as well as switching the detector in and out with the NF-1611 (1 GHz) detector borrowed from the 40-m.  It turns out there are some neutral density filters installed right before the final focusing lens in the post refcav beatnote setup. When I checked the specs for the NF-1811 it looked like we were well below the RF saturation point with a lot of room to increase power.

For the NF-1811 saturation power @ 1550 nm is 55 uW. The responsively at 1064 nm looks like about 0.6 of this.  So saturation at our wavelength is about 91.7 uW. I measured the total power after the combining beamsplitter, it was 227 uW, so with 13 dB worth of ND filters this was brought down to about 11.3 uW.  We first switched the ND filters (ND1.0+ND0.3) for a single ND0.5 and a beat note could be see 3 dB above the dark noise (75 dBm) of the detector. We then removed the ND filter completely at got a beat note (at 70 MHz) that had a clearance of 17 dB above dark noise.  Not sure if this is enough to make a 'good' measurement but at least we can now see something at the combined transmission of the cavities. The damage threshold is of course way above this.

There is quite a bit of work to be done improving on this and getting the PLL loop setup.  We also need to start work on getting a calibration together. The PDH loops also need some optimizing.

 

On another side note:

We can see the 14.75 MHz  side bands at the beat note output (and one 2x harmonic of this). I'm not sure if this is ok as I would have through the ref cavities would filter this out and I'm not sure if maybe HOM or other would let this to pass the cavity. 

  1693   Sat Jul 30 19:08:08 2016 Antonio and AndrewDailyProgressBEATbeat note work and PLL

Summary

  • Today we have worked reying to improve the beat note signal in its amplitude.
  • We also connected all the parts for the PLL loop and tryied to lock the PLL but we did not succeded;
  • We noticed at some pint a noise at the input of the sr560 which pheraps prevents the PLL lock


    Conclusions

           We need to improve the lock of the cavities in order to get a more stable signal and solve the origin
           of the noise at the input of the sr560 in order to lock the PLL.

 

  1831   Wed Mar 29 18:27:27 2017 awadeDailyProgressBEATReconstructing free running laser beat note detector

We now have >150 kHz UGF on both loops. This should be enough for an initial beat note measurement. The improved UGF in the south path was because I switch the SRS DB64 delay line box in the PD RF electronic signal path to the FSS box for a fixed cable length.  I didn't really look further into why the delayer box was causing this, once it was solved I moved on. Maybe internal reflections or loss. No bode plots/spectra/traces here, we'll do these properly again when Craig is finished with his candidacy presentation.  

On Monday we locked both cavities with pre-cavity beat note tuned to within the 125 MHz bandwidth of the NF1811 (125 MHz, InGaAs) transmission BN detector. We saw nothing for transmitted detector.  

In the pre-cavity BN dector we installed the new Newfocus 1611 detector (the other was returned to the 40m) and Craig and I were able to just find a faint beat note on the pre-cavity BN detector.  It was about 5 dB above the noise floor of the spectrum analyzer, so not great.

Beat note show up (~160 MHz) with south laser slow voltage ~0.5 - 6 V and north laser slow voltage of 3.0 V.  

Originally I'd spend no more than two hours setting this up and finding a beat note to start with so beam lines were not squared with the table and it generally needed neating up: it needed a rebuild anyway.  I spend some of today rebuilding.  Even though the overlap of the beams looked good and there was ~0.5 mW of light from each laser I was unable to find the beat note again. Its not very well mode matched but we would still expect at least some overlap.  It could also be that I need to step more slowly again, its always initially hard to get a fix on it.  Will try again tomorrow.

 

Ordering:

Also, we some how ordered the metric mounted version of the Newfocus 1611, we only realized this Monday.  I've emailed them to find out if they can just sell us the new shoe, they suggested ordering a converter grub screw TA-8M4-10, might be the cheapest option.

  1832   Thu Mar 30 15:49:51 2017 awadeDailyProgressBEATReconstructing free running laser beat note detector

After some playing around with alignment and increasing the size of the focused beam on the pre-cavity BN detector (an NF1611) we now have a beat note of -57 dBm (dark noise was -88dBm).  

Laser slow input (temperature) settings were 0.530 V (south) and 3.500 V(north) for a beat note of approximately 120 MHz.  

Temperature tuning of the lasers moves the beat note in the same direction for both laser.

One full FSR of the reference cavities is FSR = c/(2L) = 3e8/(2x3.64cm) = 4.13 GHz which corresponds to ~1 V of slow control tuning on the lasers.

So the previous regime of locking the south at slow actuation ~0.8554 V and the north at ~2.7159 V places the two lasers about 4 GHz apart.  

The north laser temperature needs to be tuned up in temperature by about 1.1 V (or one FSR) for us to be in a workable regime.  

Alternatively the north cavity temperature could be altered, this will require ~3 hours settling time.

  1833   Mon Apr 3 21:52:50 2017 Craig and AndrewDailyProgressBEATTransmission Beat Note Achieved + Visibility and Power Measurements

Craig and Andrew

We got a transmission beat note  (Plot 1).  It's weak, probably because of terrible mode matching both into the cavities and beyond it.

Our beat note directly from the lasers is much stronger, around -53 dBm at ~59 MHz.  The beat note from transmission was about -100 dBm.

South Laser Temp (in Volts) = 0.8544 V

North Laser Temp (in Volts) = 3.8879 V

 

In an attempt to diagnose why our beat note was relatively weak, we took some power measurements everywhere in our setup to measure the visibility of our cavity.  Visibility = (maxRefl - minRefl)/(maxRefl + minRefl)

South Trans Power = 1.120 mW

South Incident Power = 2.327 mW

South Refl Power = Incident - Trans = 1.207 mW

South Visibility = 31.7%

North Trans Power = 0.428 mW

North Incident Power = 1.814 mW

North Refl Power = Incident - Trans = 1.386 mW

North Visibility = 13.4%

These visibilities are lame.  We will work on mode matching into the cavities to try and get ~80-90% visibilities tomorrow.

  1834   Tue Apr 4 17:32:40 2017 ranaDailyProgressBEATTransmission Beat Note Achieved + Visibility and Power Measurements

That plot was so bad, I had to delete it. No ticks or labels on the x-axis !  This is supposed to be science, not performance art.

If you have ~0.5 mW in each beam, and the RF transimpedance of the 1811 is 40 kOhms, you should be getting a HUGE beat signal. It should saturate the 1811.

  1847   Thu Aug 3 00:39:51 2017 awadeDailyProgressBEATRe-establishing beat note detectors

Edit awade  Tue Dec 5 18:47:05 2017: Correction to the BN equation, this is incorrect in the oringal post and the intermediate edit, this should be correct:

BN = 20 \log_{10}[2\eta RG\sqrt{P_N P_S}]) - 10\log_{10}[50\Omega] + 30dBm

 

Edit (awade, Fri Aug 4 17:27:26 2017): correction to BN estimate, I was taking optical powers and not accounting for AC stage gain of the detector.

Over the last day an a half I've been trying to set up both the pre-cavity BN detector and the one post the cavities.  

PLL board beat note detector

On the output path I removed the anodized cube beam splitter mirror mount (pictured below).

Maybe the rational for this has been lost in space and time. Its a real pain to align beams through because the mirror is fixed in there with a ~5 coils/in sping that cuts the beam path. It can be hard to find an alignment that works and the spring presents a scatter hazard very close to the critical point of detection. We can look to finding a more optimal solution later in the noise hunting season. I replaced with a much simpler lens mount which will do for now.

The beam splitter used is a Newport 10Q20HBS (high energy 50:50 beam splitter).  Not sure about this choice, we might want a super polished high quality optic here, but for the purposes of getting a quick beat note this should be fine.

---

Pre-cavity beat note detector

I've changed the lab temperature since the last BN search and also changed the north cavity heater up and down.  The north heater is back to is original position but its possible that the cavity resonances have drifted out of the 125 MHz bandwidth of the final BN detector.

To find the beat more easily I had installed a pre-cavity BN detector using the waste light from some PBSs in the north and south path.  I'm using a Newport 1611 (1 GHz) detector.  There is 353 µW from the south path and 287 µW from the north path, both are s-polarized.  The mode matching is a poor effort, but the purpose of this PD is not final science data by diagnosis.  There is one lens in each path to make the beams a reasonable size to work with.

The beat note should be roughlyBN = 10\log_{10}(\eta \times Resp.\times Gain\times2\sqrt{P_n P_s}) - 30 dBm   that with 1% assumed overlap from each beam should be on order of -55 dBm. It should be clear as day, even with very poor overlap. With perfect overlap best case would be -35.05 dBm.

After the combining beam splitter I walked the alignment of the north path to match that of south: basically I checked if they over lap close to the BS and then a long distance away.  It looks like they are well aligned, but I don't see a beat note as I step the laser frequency slow control voltage slowly.  I tested south path at 0.500 V temperature offset and north path 3.500 V and then stepped slowly around those regions.  Yesterday I also tested 0.6 V (south) and 2.6 V (north).  In the past a 60 MHz beat note was see with combinations where north path was +2.900-3.000 V above the south slow offset. The same identical setup worked in the past and nothing has been changed on the laser head config settings.

Another thing: the DC voltage doesn't seem to make sense for the total 640 µW of power on the Newport 1611. Responsivity is ~0.7 A/W @ 1064 nm and gain 10e3 kΩ on the DC stage: so should expect 4.48 V.  I see 1.22 V (and this is with 1MΩ input impedance on the oscilloscope). Something is not right here. This is the new PD Antonio purchased

Possibilities:

  • MM is not as good as I think for the pre-cavity BN detector and in reality I'm looking at no overlap
  • Not actually pointing properly onto the PD. There is a f=75 mm lens focusing down. It looks like its all getting in there (no scatter out) but maybe its just not all going in
  • Laser freq noise is blurring  it out, mode hopping? I scanned PZT when around these them slow temp regions and didn't see any crazy ness so unlikely
  • Checked polarization, both good
  • Spectrum analyzer (HP 8560E, 30Hz-2.9 GHz) settings: initially I used RBW=100kHz (VBW = 100kHz) looking 1 MHz to 1 GHz, later I switched to RBW=10 kHz (VBW=10kHz).  This should be enough to see peaks above the noise floor of ~-100 dBm.  I turned the attenuation to 0 dB and averaging to 1
  • Scanning laser frequency too quickly: yeah, maybe, its easy to miss
  • PD power supply, not sure if I checked this before. Its the newport official one and says +/- 15 wt 0.3 A, voltage looked fine unloaded, but didn't test while under operation

This took me 1/2 a day to setup before and gave a BN with very little effort.  I was using the 40m's HP spectrum analyzer last time, which seem be easier to use for this task.  I don't think I've configured anything wrong with the cryolab HP 8560E, maybe need to double check. I'm a bit at a loss as to why its so much hard work this time around.  More work tomorrow.

 

 

  1849   Thu Aug 3 18:40:57 2017 awadeDailyProgressBEATRe-establishing beat note detectors

Pre-cavity beat note detector

Lens before pre-cavity BN detector is PLCX- 25.4-25.6-C-1064 (f = 50 mm), this wasn't quite at the right distance.  I removed lens and PD (Newport 1611) and looked at the overlap at large distance and fine tuned the MM.  After reinstalling PD and lens I got a DC voltage of -2.8 V (equivalent to 200 µW of power when I account for 1 MΩ load of oscilloscope).  So maybe position of PD in the focus could be better still.

I see a beat note of -80 dBm on a SA  noise floor of ~100 dBm: enough for diagnostics now.  This detector would perform much better with proper mode matching of the two paths.  But science return on investment isn't worth it now.

 

We get a beat note of ~62 MHz for

South slow controls voltage =  0.8052 V

North  slow controls voltage =   3.9250 V

Currently The north is locking at 3.92 V and the south at around 0.66.  So some fine tuning of the north cavity temperature is needed to produce a BN in the 125 MHz band width of the 1811 detector on the transmission of the cavities. 

Laser temperature tuning goes 4 GHz/V applied at the laser head.  

---

Changing the north cavity tempurature

The current BN with both caivities locked on thier closest resonances is 456 MHz  (south slow = 0.6597 V, north slow = 3.9109 V).  I turned north heater voltage off from 11.5 V to 0 V to test which direction the heating needed to go.

This was the wrong direction. Going back the other way turned voltage back up to 12.0 V. This will take a while

  1852   Thu Aug 10 11:47:19 2017 awadeDailyProgressBEATImproving pre-cavity BN detector

The beat note out of the pre-cavity beat note detector was too small to be workable and the effort going into tweaking alignment was costing more time that it was worth. The problem was poor MM between north and south beams whose waists were offset by above 20 cm.

The setup of the BN detector is shown in the attached schematic.  The lens in the south path was placed to give a waist of approximately 300 µm. The beam was profiled and parameters were:

Fitted horz beam waist: 275.2 +/- 9.564 um
Fitted horz beam position: -68.86 +/- 5.584 mm
Fitted vert. beam waist: 367.8 +/- 3.561 um
Fitted vert. beam position: 48.46 +/- 11.23 mm
Mean waist size is 321.5 um at position -10.2 mm.

data attached as 20170809_profileSouthPreCavBND.csv

To match the north path, I first profiled it.  Parameters, referenced to the first steering mirror after the north PBS were:

Fitted horz beam waist: 186.2 +/- 1.054 um
Fitted horz beam position: -2.287 +/- 1.417 mm
Fitted vert. beam waist: 188.6 +/- 1.211 um
Fitted vert. beam position: 46.74 +/-  1.5 mm

data attached as: 20170809_profileNPreCavBNDFromPBS.csv

To fit the beams I passed a set of lenses available through a la mode (taking the average of the waists and positions as a la mode is only one axis) and it came up with a 0.986 overlap for a f=200 mm lens placed at 215 mm from the first steering mirror after the north PBS. I then re profiled the beam, getting beam parameters:

Fitted horz beam waist: 397.8 +/- 2.791 um
Fitted horz beam position: -47.48 +/- 13.3 mm
Fitted vert. beam waist: 346.5 +/- 2.717 um
Fitted vert. beam position: -171.2 +/- 3.256 mm
Mean waist size is 372.1 um at position -109.3 mm.

A la mode predicted 389 µm at -61 mm, so the lens positioning error is a bit off.

 

 

Attachment 1: 20170810_PreCavBNDetectorConfig.pdf
20170810_PreCavBNDetectorConfig.pdf
  1854   Fri Aug 11 16:58:42 2017 awade, kojiDailyProgressBEATImproving pre-cavity BN detector

Koji came down to the lab to have a look at the pre-cavity BN dector.  With a might tigher lens f = 37 mm he was able to get a DC voltage of - 2.59 V for an total input power of 577 µW.  We should expect about 4.00 V for this power, but this is what we get.  We couldn't see a beat note until Koji started changing the laser crystal temperature on the south laser controller.  The set point temperature was changed from 34.4961 C to 47.0749 C. Slow laser voltage offset channels were set to 0.6901 V for the south path and 3.8294 V for the north path.  With these settings we were able to see a BN at 216 MHz of -50 dBm. This is sufficient for the diagnostic purpose of this detector.

For reference, here is a configuration summary of the lasers and controls at present:

BN = -50 dBm @ 216 MHz

South laser configuration
  Setting
ADJ 0
DC 2.04 A
DPM 0.00V
NE ON
LD ON
Display 5
DT 28.8 C
DTEC +0.6V
LT 47.2 C
LTEC -0.1 V
T +47.0749
Pwr 160 (not calibrated)

South slow frequency voltage offset = 0.6901 V

 

North laser configuration
  Setting
ADJ 0
DC 2.08 A
DPM -0.00V
NE ON
LD ON
Display 1
DT 22.3 C
DTEC +0.6V
LT 40.2C
LTEC 0.0 V
T +26.4650 C
Pwr 68 (not calibrated)

South slow frequency voltage offset = 3.8294 V

Todo: make a BN laser north vs laser south map similar to what koji did in this this post 40m:3759.  This should make the good operating parameter regiem obvious in the future.

  1855   Fri Aug 11 17:34:16 2017 awade, CraigDailyProgressBEATMapping the beat notes north temp vs south temp

Its seems like the issue with finding the beat note has to do with us having a poor idea of where the ideal operating regimes of the two lasers are relative to each other.  The laser crystal offsets were set to temperatures that seemed to be a good operating point at the time but now are not so great for some reason.  Also the slow offset voltages are somewhat arbitrary, being ~0.69 V for south and 3.8 V for north. It would be better if these were set at the mid point of the tuning range (±10 V) for better range when operating only from EPICS channels controls.  

We would like a north temperature vs south temperature map, similar to what Koji did in 40m:3759, to find the best operating points and to remove the stab-in-the-dark guess work that has been the method till now.

---

Mapping BN as function of south and north laser temperatures

The slow frequency offset EPICS channels were set back to zero and the north/south temperatures were walked back to a point where we could see the beat note again.

We see a slope of -668 MHz/Celcius in the North laser temp Vs the precavity beatnote frequency.

Edit (awade: Sun Aug 13 19:02:29 2017): note that the slow laser frequency tuning is actually positive, it was likely that we were looking at the beat note reflected about DC so the above number has the opposite slope.

From the manual (found here on the ATF wiki) the manufacture says that the temperature tuning is negative as a function of frequency and the front panel BNC is positive.

Edit (awade Mon Aug 14 16:16:35 2017): Must have been tired.  Yes the gradient in the graph is negative, so plot makes sense. See my reply post to this one for details on the laser operation

Attachment 1: NorthLaserTempVsPrecavityBeatnoteFrequency.pdf
NorthLaserTempVsPrecavityBeatnoteFrequency.pdf
  1857   Sun Aug 13 19:58:49 2017 awade, CraigDailyProgressBEATFixing the calibration of North laser Lightwave series 125/126 set point temperature

When Craig and I looked and mapping out good beat notes on Friday it wasn't clear that we had enough range to find more that one pair of temperatures that would work.  The two lasers seemed to have vastly different operating set points for a corresponding frequency match and re centering the slow controls BNC inputs to zero seemed to go in the opposite direction to that expect.  

As noted in the previous post summarizing config of the lasers (PSL:1854) South laser was set at +47.0749 C and North laser was set at +26.4650 C.  We were reluctant to tune the North laser much lower than 24 C, lest we risk condensation of water within the head. This meant that once the slow frequency controls voltage was reset back to zero, there was very little range of the North laser temperature settings left to find a BN within.

After checking in the manual, it turns out two things were not what we expected.  Firstly, the slow tuning has opposite sign to the laser temperature: increasing temperature tunes laser frequency down and increasing slow frequency voltage tunes laser frequency up. Secondly the laser set point temperature is a calculated value and note a true value (the screen value LT gives the true value).  It also doesn't include the offset caused by voltage on the front panel Slow frequency BNC.  So for North laser set point temperature of +26.4650 C the true value measured at the crystal is 44.8 C (with zero voltage on slow freq input).  

The discrepancy was probably caused by a poor calibration on setting up the laser head with the controller box.  The setup procedure when configuring a new laser head is to center the temperature setting in the middle of the range, wait for it to come to equilibrium and then to trigger a recalibration of temperature to this value.  This is done by putting the laser into standby and pressing: Display x2 (will read C +xx.xxxx) -> Set (to recenter real temperature) -> wait for equilibrium -> Set (to recenter the calculated set point temperature). 

It is likely that set was pressed twice in rapid succession and that the calibration was triggered before the laser had time to reach its midpoint temperature.  This hasn't affect operation, but it does mean that the set point looked much lower than it should have been, we might have been worried about selecting temperatures that were too low to safely operate the laser at. 

I have recalibrate the laser 'Center Temperature Calibration' on both lasers (North was too low and south was too high) so that we can have more confidence that the laser ranges are in a safe range.  Calibrated setpoints are now 48.4 ± 15 C for the South laser and 44.8 C ± 15 C for the North laser.

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