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  2522   Mon Jan 18 20:58:40 2010 AlbertoUpdateABSLMeasurement in progress

Quote:

I started a long measurement of the PRC's transmissivity. I'm leaving the lab and I'm going to be back at about 8 tonight. Please do not disturb the interferometer. it is important that the MC and the PRC stay locked all the time.

 That measurement is finished. I'm now going to start another one that will take another hour or so. I'm leaving it running for the night. If you want to work on the IFO, it should be definitely done by 11pm.

  2531   Tue Jan 19 12:54:39 2010 AlbertoUpdateABSLMeasurement in progress

A measurement will be running for the next hour. Please be careful.

  2532   Tue Jan 19 16:21:18 2010 AlbertoUpdateABSLWatchdogs not working and then fixed

This afternoon the watchdogs stopped working: they didn't trip when the suspension positions crossed the threshold values.

I rebooted c1susaux (aka c1dscl1epics0 in the 1Y5 rack), which is the computer that runs the watchdog processes.

The reboot fixed the problem.

  2534   Wed Jan 20 20:11:56 2010 AlbertoUpdateABSLSome preliminary results from measuring PRC's transmissivity for an amplitude modulated beam
Here I'm posting a plot showing a set of measurements that I made in the past few days to determine the absolute length of the PRC cavity.
As in my other AbsL measurements, I inject an auxiliary laser beam into the cavity and look at the transmission. In the PRC case, the beam is injected through the dark port and I look at a pick-off of the REFL beam. The aux laser is phase locked to the PSL beam and I control the differential frequency between the two. The PSL and the aux beam interfere and beat at their differential frequency.
 
The attached plot shows the transmitted power as a function of the beat frequency.
 
Fitting the data with the model would let me determine the cavity length. 
By now I can estimate the length of the PRC at about 2.257m, but it's still a rather approximate value.
I can't provide accurate error bars yet. I need to optimize the measurements to get a more precise value.
 
I will go more through the details of the measurement technique and of the fitting function as soon as I have more definitive results.
 
The data points shown here were taken at different times and not always in optimal alignment condition of the PRC. 
To get a good fit of the data I should have fewer frequency segments, taken in a shorter period of time, and in which the power circulating inside of the cavity (ie SPOB) fluctuates as little as possible.
 
For what regards the time needed for a measurements, I already significantly sped up the measurements (i.e. optimizing the scanning and acquisition GPIB scripts, and fixing a couple of problems with the PDH box used in the PLL), and finally now I can scan several tens of MHz in few minutes.
About the frequency segments, so far they have been determined by two factors
1) Tthe frequency generator in the PLL: the Marconi works as a continuous wave generator only in limited ranges. Switching from one to another brakes the wave in a way that causes the PLL to lose lock.
2) Getting below 18 MHz a series of other beats appear on the PLL photodiode and make the PLL lose lock.
 
For the first problem, I'm thinking of using two Marconis and to mix their signals. I would keep one at 300MHz and I would scan the other from 300MHz to 500MHz. In fat, in that frequency range the Marconi has not discontinuity.
 
To try to avoid the other beats at low frequency, I'm not entirely sure about what to do yet. 
 
To be continued...
Attachment 1: 2010-01-19_PRCtransmissivityVsModel.png
2010-01-19_PRCtransmissivityVsModel.png
  2541   Fri Jan 22 02:54:06 2010 AlbertoUpdateABSLOvernight measurement

I'm leaving a measurement running overnight. It should be done in about one hour.

Tomorrow morning, If you need to use the interferometer, and you don't want to have the auxiliary beam going onto the dark port, you can turn down the flipping mirror and close the NPRO's mechanical shutter.

  2543   Fri Jan 22 14:40:49 2010 AlbertoUpdateABSLOvernight measurement

Quote:

I'm leaving a measurement running overnight. It should be done in about one hour.

Tomorrow morning, If you need to use the interferometer, and you don't want to have the auxiliary beam going onto the dark port, you can turn down the flipping mirror and close the NPRO's mechanical shutter.

 This is what I measured last night:

2010-01-21_PRCtransmissivityVsModel.png

 This is not a fit. It's just a comparison of the model with the data.

  2545   Mon Jan 25 16:30:37 2010 AlbertoUpdateABSL166 MHz sideband turned off

I turned off the modulation at 166MHZ becasue I don't need it if I'm only locking the PRC.

It was introducing extra amplitude modulations of the beam which interfered with the AbsL's PLL photodiode.

I'm going to turn it back on later on.

  2546   Mon Jan 25 16:46:33 2010 AlbertoUpdateABSL166 MHz sideband turned off

Quote:

I turned off the modulation at 166MHZ becasue I don't need it if I'm only locking the PRC.

It was introducing extra amplitude modulations of the beam which interfered with the AbsL's PLL photodiode.

I'm going to turn it back on later on.

 I turned back on the 166MHz modulation just a bit. I set the slider at 4.156.

When it was totally off the MZ seemd quite unhappy.

  2550   Wed Jan 27 11:02:30 2010 AlbertoUpdateABSLPRC Cavity Length
 I fitted the data from scanning the PRC by changing the beat frequency of the auxiliary laser beam with the PSL beam.
The data points that I've taken so far over the entire frequency range (0-300 MHz) are not continuous. For several reasons the PLL was unable to maintain lock for such a large range and I had to break it into smaller segments. The measurements to acquire them stretched over a too long period of time during which the status of the PRC changed.
 
Because of that, before I get a continuous set of data points (perhaps normalized by the circulating power inside of the cavity), I restricted the fit to a 55MHz range around 100MHz. I obtained the following numbers for the fit parameters:
Length PRC = 2.169 +/- 0.007 m
Schnupp Asymmetry: 0.471+/- 0.006 m
 
The fit is shown in the attached plot:
2010-01-21_PRCtransmissivityVsFit.png
When I fit over the entire set of data I get this:
 
2010-01-21_PRCtransmissivity_EntireFreqRange_VsFit.png
 
Length PRC = 2.224 +/- 0.005 m
Schnupp Asymmetry: 0.457+/- 0.005 m
 
The results are different. Evidently I have to improve the measurement. I'm working on it.
 
For posterity:
The function I used to fit the transmitted beat power vs. frequency is the following:
 
E_trans = - t_prm * r_itm * exp(1i*2*wb*l_prc/c) .* sin(wb*l_/c) ./ ( 1 + r_prm * r_itm * exp(1i*2*wb*l_prc/c) .* cos(wb*l_/c)
 
Where wb is the angular frequency of the beat, l_prc and l_ are the length of the PRC and the Schnupp asymmetry, respectively; r_itm, t_itm, r_prm, t_prm are reflectances and transmittances of PRM and ITM; c is the speed of light.
 
  2551   Thu Jan 28 09:14:51 2010 AlbertoConfigurationComputersc1iscey, c1iscex, c1lsc, c1asc rebooted

This morning the LSC scripts wheren't running properly. I had to reboot c1iscey, c1iscex, c1lsc, c1asc .

I burtrestored to Monday January 25 at 12:00. 

  2552   Thu Jan 28 09:17:32 2010 AlbertoUpdateLSC166 Modulation turned off

I temporarily turned off the 166 modulation.

  2553   Fri Jan 29 12:06:58 2010 AlbertoUpdateABSLMeasurement running today at lunch time

I just started a measuremtn that will be running for the next hour or so. Please be careful with the interferometer.

  2554   Fri Jan 29 13:14:49 2010 AlbertoUpdateABSLMeasurement running today at lunch time

Quote:

I just started a measuremtn that will be running for the next hour or so. Please be careful with the interferometer.

Done. IFO available

  2560   Tue Feb 2 15:30:03 2010 AlbertoFrogsTreasureWild Oats
FYI. Sitting on the top shelf of George I found an opened jar of raspberry jam and an opened jar of creamy peanut butter. Both are branded Wild Oats Market.
 
Wikipedia:
"Wild Oats Markets was an operator of natural foods stores and farmers markets in North America... Whole Foods officially completed their buyout of Wild Oats on August 27, 2007 [...]"
  2562   Tue Feb 2 18:15:47 2010 AlbertoUpdateelogElog restarted it

 Zach made me notice that the elog had crashed earlier on this afternoon. 

I just restarted it with the restarting script.

Instructions on how to run the last one are now in the wiki page. Look on the "How To" section, under "How to restart the elog".

  2567   Wed Feb 3 10:46:12 2010 AlbertoUpdateelogelog restarted

 Again, this morning Zach told me that the elog had crashed while he was trying to post an entry.

I just restarted it.

  2576   Mon Feb 8 14:13:03 2010 AlbertoUpdateABSLPLL Characterization

Lately I've been trying to improve the PLL for the AbsL experiment so that it could handle larger frequency steps and thus speed up the cavity scan.

The maximum frequency step that the PLL could handle withouth losing lock is given by the DC gain of the PLL. This is the product of the mixer's gain factor K [rad/V ], of the laser's calibration C [Hz/V] and of the PLL filter DC gain F(0).

I measured these quantities: K=0.226 V/rad; C=8.3e6 Hz/V and F(0)=28.7dB=21.5. The max frequency step should be Delta_f_max = 6.4MHz.

Although in reality the PLL can't handle more than a 10 KHz step. There's probably some other effect that I'm not.

I'm attaching here plots of the PLL Open Loop Gain, of the PLL filter and of a spectra of the error point measured in different circumstances.

I don't have much time to explain here how I took all those measurements. After I fix the problem, I'm going to go go through those details in an elog entry.

Does anyone have any suggestion about what, in principle, might be limiting the frequency step?

I already made sure that both cables going to the mixer (the cable with the beat signal coming from the photodiode and the cable with the LO signal coming from the Marconi) had the same length. Although ideally, for phase locking, I would still need 90 degrees of phase shift between the mixing signals, over the entire frequency range for which I do the cavity scan. By now the 90 degrees are not guaranteed.

Also, I have a boost that adds another 20 dB at DC to the PLL's filter. Although it doesn't change anything. In fact, as said above calculating the frequency step, the PLL should be able to handle 100KHz steps, as I would want the PLL to do.

Attachment 1: 2010-02-08_Old_PDH_Box_Filter_TF_gain_knob_0_Boost_OFF.png
2010-02-08_Old_PDH_Box_Filter_TF_gain_knob_0_Boost_OFF.png
Attachment 2: 2010-02-08_PLL_OLG_gain_knob_0_Boost_OFF.png
2010-02-08_PLL_OLG_gain_knob_0_Boost_OFF.png
Attachment 3: 2010-02-08_PLL_Noise_Budget.png
2010-02-08_PLL_Noise_Budget.png
  2577   Mon Feb 8 14:56:17 2010 AlbertoUpdateABSLSuddenly a much better alignment of PRC

I just aligned PRM and locked PRC and I noticed that SPOB is much higehr than it used to be. It's now about 1800, vs 1200 than it used to be last week.

Isn't anyone related to that? If so, may I please know how he/she did it?

  2579   Mon Feb 8 15:41:51 2010 AlbertoUpdateABSLSuddenly a much better alignment of PRC

Quote:

Quote:

I just aligned PRM and locked PRC and I noticed that SPOB is much higehr than it used to be. It's now about 1800, vs 1200 than it used to be last week.

Isn't anyone related to that? If so, may I please know how he/she did it?

 oops, my bad.  I cranked the 33MHz modulation depth and forgot to put it back.  The slider should go back to around 3. 

 I was actually hoping that the alignment got better.

  2581   Tue Feb 9 09:07:06 2010 AlbertoUpdateABSLPLL Characterization

Quote:

Lately I've been trying to improve the PLL for the AbsL experiment so that it could handle larger frequency steps and thus speed up the cavity scan.

The maximum frequency step that the PLL could handle withouth losing lock is given by the DC gain of the PLL. This is the product of the mixer's gain factor K [rad/V ], of the laser's calibration C [Hz/V] and of the PLL filter DC gain F(0).

I measured these quantities: K=0.226 V/rad; C=8.3e6 Hz/V and F(0)=28.7dB=21.5. The max frequency step should be Delta_f_max = 6.4MHz.

Although in reality the PLL can't handle more than a 10 KHz step. There's probably some other effect that I'm not.

I'm attaching here plots of the PLL Open Loop Gain, of the PLL filter and of a spectra of the error point measured in different circumstances.

I don't have much time to explain here how I took all those measurements. After I fix the problem, I'm going to go go through those details in an elog entry.

Does anyone have any suggestion about what, in principle, might be limiting the frequency step?

I already made sure that both cables going to the mixer (the cable with the beat signal coming from the photodiode and the cable with the LO signal coming from the Marconi) had the same length. Although ideally, for phase locking, I would still need 90 degrees of phase shift between the mixing signals, over the entire frequency range for which I do the cavity scan. By now the 90 degrees are not guaranteed.

Also, I have a boost that adds another 20 dB at DC to the PLL's filter. Although it doesn't change anything. In fact, as said above calculating the frequency step, the PLL should be able to handle 100KHz steps, as I would want the PLL to do.

I might have found the problem with the PLL that was preventing me from scanning the frequencies by 100KHz steps. A dumb flimsy soldering in the circuit was making the PLL unstable.

After I fixed that problem and also after writing a cleverer data acquisition script in Python,  I was able to scan continuosly the range 10-200MHz in about 20min (versus the almost 1.5-2 hrs that I could do previously). I'm attaching the results to this entry.

The 'smears' on the right side of the resonance at ~33MHz, are due to the PSL's sideband. I think I know how to fix that.

As you can see, the problem is that the model for the cavity transmission still does not match very well the data. As a result, the error on the cavity length is too big (~> 10 cm - I'd like to have 1mm).

Anyway, that was only my first attempt of scanning. I'm going to repeat the measurement today too and see if I can come out better. If not, than I have to rethink the model I've been using to fit.

Attachment 1: 2010-02-08_PRCtransmissivity_EntireFreqRange_VsFit.png
2010-02-08_PRCtransmissivity_EntireFreqRange_VsFit.png
  2582   Tue Feb 9 10:10:58 2010 AlbertoUpdateABSLback to analog

I want to try to do the measurement with the network analyzer used as local oscillator, instead of the Marconis that I'm using now. Tha could give me better noise rejection. It would also give me information about the phase.

Also I wouldn't dislike abandoning the GPIB interfaces to acquire data.

  2604   Tue Feb 16 09:51:22 2010 AlbertoUpdateGreen Lockingtake some optics away from the ETM end table

Quote:

In the last two days Steve and I took some optics away from the both ETM end table.

This is because we need an enough space to set up the green locking stuff into the end table, and also need to know how much space is available.

Optics we took away are : Alberto's RF stuff, fiber stuff and some optics obviously not in used.

The picture taken after the removing is attached. Attachment1:ETMX, Attachment2:ETMY

And the pictures taken before the removing are on the wiki, so you can check how they are changed.

http://lhocds.ligo-wa.caltech.edu:8000/40m/Optical_Tables

The PD Kiwamu removed from the Y table was TRY, which we still need.

My bad if he took that. By mistake I told him that was the one I installed on the table for the length measurement and we didn't need it anymore.

I'm going to ask Kiwamu if he can kindly put it back.

  2605   Tue Feb 16 10:01:16 2010 AlbertoConfigurationLSCArms and PRC not locking

Since last Friday either the arms or the PRC can't lock.

The montors show the beam flashing on the end mirrors, but the cavity can't get locked. The error signal looks fine. I suspect a computer problem.

Also PRC can't lock. SPOB is suspiciously stuck at about -95. Although that's not a fixed number, but covering the by hand the SPOB PD on the ITMY table doesn't change the number. I check the DC output of the photodetector and it is actually seen the beam.

Suspecting computer problems started after last Thursday's IP switch, I rebooted the frame builder, c1dcuepics, c1daqctrl and all the front ends. I then burtrestored to February 1st at 1:00 am.

Before I burtrestored c1iscepics, SPOB had gone back to more typical numbers around 0, as it usually read when PRC wasn't locked.

But burtrestoring c1iscepics, return it to the -95 of earlier.

Burterestoring to other times or dates didn't solve the problems.

  2608   Tue Feb 16 15:25:00 2010 AlbertoConfigurationLSCArms and PRC not locking

 

 shock.jpg

  2640   Thu Feb 25 15:49:05 2010 AlbertoAoGCDSNew IO Chassis for the new CDS
Yesterday Kiwamu and I went to Downs to take all the available parts of the IO chassis that Gary and I had put together over there.
 
We've got only 3 of the 5 that we need for the Upgrade. The other 2 are currently being used for some other purpose in Downs labs.
 
I'm not sure about what each chassis has supposed to contain. They all also look different from each other.
Anyway, it looks like there should be a sort of motherboard and an IO Chassis Interface Board (DCC# D0902029) in each of them. The IO Chassis Interface Board is just a board with a bunch of PCI slots.
 
This is what the 3 chassis that we've got yesterday have:
Chassis 1
- 1 very big "motherboard"
- power supply
Chassis 2
- small motherboard
- IO Interface Board (DCC# D0902029)
- power supply
Chassis n.3
- "Dolpjin" motherboard
- IO Interface Board
- power supply
 
Apparently 2 of these 3 chassis are still missing their IO interface boards,
 
Also all chassis are still missing all the connections to powering, fans, LEDs, power and reset buttons. It's not clear how these connections should be. Gary didn't know it either.
  2653   Wed Mar 3 18:32:25 2010 AlbertoUpdate40m Upgrading11 MHz RFPD elctronics
** Please add LISO file w/ component values.
 
I designed the circuit for one of the 11 MHz photodiodes that we're going to install in the 40m Upgrade.

This is a simple representation of the schematic:

          gnd
#          |
#          Cw2
#          |
#          n23
#          |
#          Lw2
#          |
#           n22
#          |
#          Rw2                
#                 |                   |\            
#           n2- - - C2 - n3 -  - -  - |  \          
#            |    |      |   |        |4106>-- n5 - Rs -- no
# iinput    Rd   L1     L2 R24    n6- |  /     |           |
#      nin - |    |      |   |    |   |/       |         Rload    
#           Cd   n7     R22 gnd   |            |           |          
#            |    |      |        | - - - R8 - -          gnd              
#           gnd  R1     gnd      R7 
#                 |               |
#         gnd               gnd
#                 
#
#

I chose the values of the components in a realistic way, that is using part available from Coilcraft or Digikey.

Using LISO I simulated the Tranfer Function and the noise of the circuit.

I'm attaching the results.

I'll post the 55MHz rfpd later.

Attachment 1: rfpd11_v2_TF.pdf
rfpd11_v2_TF.pdf
Attachment 2: rfpd11_v2_Noise.pdf
rfpd11_v2_Noise.pdf
  2655   Thu Mar 4 08:43:35 2010 AlbertoUpdate40m Upgrading11 MHz RFPD elctronics

Quote:
** Please add LISO file w/ component values.

oops, forgotten the third attachment...

here it is

Attachment 1: rfpd11_v2.fil
# Resonant RF diode front end
#
#		  gnd
#		  |
#		  Cw2
#		  |
#		  n23
#		  |
#		  Lw2
#		  |
... 60 more lines ...
  2656   Thu Mar 4 19:53:56 2010 AlbertoUpdate40m Upgrading11MHz PD designed adjusted for diode's resistance; 55 MHz RFPD designed
After reading this study done at LIGO MIT in 1998 I understood why it is difficult to define an effective impedance for a photodiode.

I read a few datasheets of the C30642GH photodiode that we're going to use for the 11 and 55 MHz. Considering the  values listed for the resistance and the capacitance in what they define "typical conditions" (that is, specific values of bias voltage and DC photocurrent) I fixed Rd=25Ohms and Cd=175pF.

Then I picked the tunable components in the circuit so that we could adjust for the variability of those parameters.

Finally with LISO I simulated transfer functions and noise curves for both the 11 and the 55MHz photodiodes.

I'm attaching the results and the LISO source files.

 

Attachment 1: rfpd55_Noise.pdf
rfpd55_Noise.pdf
Attachment 2: rfpd55_TF.pdf
rfpd55_TF.pdf
Attachment 3: rfpd11_v2_TF.pdf
rfpd11_v2_TF.pdf
Attachment 4: rfpd11_v2_Noise.pdf
rfpd11_v2_Noise.pdf
Attachment 5: rfpd11_v2.fil
Attachment 6: rfpd55.fil
  2680   Thu Mar 18 12:27:56 2010 AlbertoUpdateABSLPLL reconstructed

Quote:

Last night (Mar 17) I checked the PLL setup as Mott had some difficulty to get a clean lock of the PLL setting.

  • I firstly found that the NPRO beam is not going through the Faraday isolator well. This was fixed by aligning the steering mirrors before the Faraday.
     
  • The signal from the RF PD was send to the RF spectrum analyzer through a power splitter. This is a waist of the signal. It was replaced to a directional coupler.
  • Tee-ing the PZT feedback to the oscilloscope was producing the noise in the laser frequency. I put the oscilloscope to the 600Ohm output of the SR560, while connectiong the PZT output to the 50Ohm output.
  • In addition, 6dB+6dB attenuators have been added to the PZT feedback signal.

Now the beating signal is much cleaner and behave straight forward. I will add some numbers such as the PD DC output, RF levels, SR560 settings...

Now I am feeling that we definitely need the development of really clean PLL system as we use PLL everywhere! (i.e. wideband PD, nice electronics, summing amplifiers, stop poking SR560, customize/specialize PDH box, ...etc)

I also had noticed the progressive change of the aux NPRO alignment to the Farady.

I strongly agree about the need of a good and robust PLL.

By modifying the old PDH box (version 2008) eventually I was able to get a PLL robust enough for my purposes. At some point that wasn't good enough for me either.

I then decided to redisign it from scratch. I'm going to work on it. Also because of my other commitments, I'd need a few days/1 week for that. But I'd still like to take care of it. Is it more urgent than that?

  2702   Tue Mar 23 15:38:26 2010 AlbertoUpdateelogelog just restarted

I found the elog down and I restarted it.

Then, after few seconds it was down again. Maybe someone else was messing with it. I restarted an other 5 times and eventually it came back up.

  2704   Tue Mar 23 22:46:43 2010 AlbertoUpdate40m UpgradingREFL11 upgraded
I modified REFL11 according to the changes lsited in this schematic (see wiki  / Upgrade 09 / RF System / Upgraded RF Photodiodes ).
I tuned it to be resonant at 11.06MHz and to have a notch at 22.12MHz.
These are the transfer functions that I measured compared with what I expected from the LISO model.

2010-03-23_REFL11_model_to_meas_comparison.png

The electronics transfer function is measured directily between the "Test Input" and the "RF Out" connector of the box. the optical transfer function is measured by means of a AM laser (the "Jenne laser") modulated by the network analyzer.
The AM laser's current was set at 20.0mA and the DC output of the photodiode box read about 40mV.
The LISO model has a different overall gain compared to the measured one, probably because it does not include the rest of the parts of the circuit other than the RF out path.

I spent some time trying to understand how touching the metal cage inside or bending the PCB board affected the photodiode response. It turned out that there was some weak soldering of one of the inductors.

  2710   Wed Mar 24 14:52:02 2010 AlbertoUpdateGreen Lockingtwo NPRO PLL

Quote:

Now some pedestals, mirrors and lenses are left on the PSL table, since we are on the middle way to construct a PLL setup which employs two NPROs instead of use of PSL laser.

So Please Don't steal any of them.

 Can I please get the network analyzer back?

  2711   Wed Mar 24 14:57:21 2010 AlbertoUpdate40m UpgradingREFL11 upgraded

 

 Hartmut suggested a possible explanation for the way the electronics transfer function starts picking up at ~50MHz. He said that the 10KOhm resistance in series with the Test Input connector of the box might have some parasitic capacitance that at high frequency lowers the input impedance.

Although Hartmut also admitted that considering the high frequency at which the effect is observed, anything can be happening with the electronics inside of the box.

  2715   Thu Mar 25 17:32:42 2010 AlbertoUpdate40m UpgradingREFL55 Upgraded

I upgraded the old REFL199 to the new REFL55.

To do that I had to replace the old photodiode inside, switching to a 2mm one.

Electronics and optical transfer functions, non normalized are shown in the attached plot.

2010-03-25_REFL55_model_to_meas_comparison.png

The details about the modifications are contained in this dedicated wiki page (Upgrade_09 / RF System / Upgraded RF Photodiodes)

Attachment 1: 2010-03-25_REFL55_model_to_meas_comparison.png
2010-03-25_REFL55_model_to_meas_comparison.png
  2732   Mon Mar 29 21:43:27 2010 AlbertoConfigurationPSLReference Cavity PD Noise Spectrum

[Rana, Alberto]

This evening we measured the noise spectrum of the reference cavity PD used in the FSS loop. From that we estimated the transimpedance and found that the PD is shot-noise limited. We also found a big AM oscillation in correspondence of the FSS modulation sideband which we later attenuated at least in part.

This plot shows the spectrum noise from the RF output of the photodetector.
 
 (here you should be able to see an attached figure, if not it's probably becasue imagemagic has having problems with displaying png files)
2010-03-29_FSS_PD_shotnoise_and_darknoise.png
 
The tall peak at 21.5 MHz is the AM modulation introduced by the EOM. It seems to be caused by a misalignment of the EOM which might be somehow modulating the polarization.
The mount in which the EOM sits is not very solid. We should change it with something similar to that of the other two EOMs in the Mach Zehnder.
By tightening down the plastic screws of the mount Rana reduced the amplitude of the AM modulation by 20dB.
 
The bump in both the dark and shot noise are in corrispondence of the resonance of the PD's electronics. As it appears, the electronics is not well tuned: the bump should coincide with the AM peak.
 
In the case of the dark noise spectrum, the bump is due to the thermal noise of the electronics. It's a good sign: it means that the electronics is good enough to be sensitive to it.
 
Transimpedance Estimate
As a "sanity check" we made an approximate estimate of the transimpedance to make sure that the PD is dominated by shot noise rather than other noises, ie electronic's noise.
 
  1. Supposing that the laser beam hitting the PD was shot noise limited, we measured 1.1V at the DC output. That let us estimate the photocurrent at DC of 20mA, for a 50Ohm output impedance.
  2. The shot noise for 20mA is 80 pA/rtHz
  3. From the nosie spectrum, we measured 3e-7 v/rtHz at 21.5MHz
  4. The impedance at RF is then Z_rf = 3e-7 V/rtHz / 80e-12 pA ~ 4000 Ohm
  5. Since the RF path inside the PD has a gain of 10, the transimpedance is ~400Ohm, which is about as we (ie Rana) remembered it to be.
  6. The PD seems to be working fine.
Attachment 2: 2010-03-29_FSS_PD_shotnoise_and_darknoise.png
2010-03-29_FSS_PD_shotnoise_and_darknoise.png
  2758   Fri Apr 2 08:52:21 2010 AlbertoUpdateelogelog restarted

i just restarted the elog for the third time in the past 12 hours.

I checked the elog.log file to debug the problem. It doesn't contain eveidence of any particular cause, except for png/jpg file uploads happened last night.

I'm not sure we can blame Image Magic again because the last crash seems to be occurred just after an entry with e jpg picture was included in the body of the message. I think Image Magic is used only for previews of attachments like pdfs or ps.

Maybe we should totally disable image magic.

  2760   Sat Apr 3 16:07:40 2010 AlbertoConfigurationPSLReference Cavity PD Noise Spectrum

 I was aware of a problem on those units since I acquired the data. Then it wasn't totally clear to me which were the units of the data as downloaded from the Agilent 4395A, and, in part, still isn't.

It's clear that the data was in units of spectrum, an not spectral density: in between the two there is a division by the bandwidth (100KHz, in this case). Correcting for that, one gets the following plot for the FSS PD:

2010-03-29_FSS_PD_shotnoise_and_darknoise.png

Although the reason why I was hesitating to elog this other plot is that it looks like there's still a discrepancy of about 0.5dBm between what one reads on the display of the spectrum analyzer and the data values downloaded from it.

However I well know that, I should have just posted it, including my reserves about that possible offset (as I'm doing now).

Quote:

The units on this plot are completely bogus - we know that the thermal noise from the resonant part of the circuit is just V = sqrt(4*k*T*Z) ~ 3nV/rHz. Then the gain of the MAX4107 stage is 10. The output resistor is 50 Ohms, which forms a divide by 2 with the input impedance of the spectrum analyzer and so the bump in the dark noise should only be 15 nV/rHz and not microVolts.

Quote:

[Rana, Alberto]

This evening we measured the noise spectrum of the reference cavity PD used in the FSS loop. From that we estimated the transimpedance and found that the PD is shot-noise limited. We also found a big AM oscillation in correspondence of the FSS modulation sideband which we later attenuated at least in part.

This plot shows the spectrum noise from the RF output of the photodetector.

  2761   Sat Apr 3 19:54:19 2010 AlbertoUpdate40m UpgradingREFL11 and REFL55 PDs Noise Spectrum

These are the dark noise spectrum that I measured on the 11MHz and 55MHz PD prototypes I modified.

The plots take into account the 50Ohm input impedance of the spectrum analyzer (that is, the nosie is divided by 2).

2010-04-03_REFL11_darknoise.png 2010-04-03_REFL55_darknoise.png

With an estimated transimpedance of about 300Ohm, I would expect to have 2-3nV/rtHz at all frequencies except for the resonant frequencies of each PD. At those resonances I would expect to have ~15nV/rtHz (cfr elog entry 2760).

Problems:

  1. For the 55MHz PD the resonance peak is too small
  2. In the 55 MHz: noise is present at about 7MHz
  3. In the 11MHz PD there's a lot of noise below 10 MHz.

I have to figure out what are the sources of such noises.

Suggestions?

  2763   Sun Apr 4 17:32:07 2010 AlbertoMetaphysicsGeneralnew y-arm?

Quote:

There's several more of the this vintage in one of the last cabinets down the new Y-arm.

 Hold on, did the arms get re-baptized?

  2767   Mon Apr 5 10:23:40 2010 AlbertoUpdate40m UpgradingREFL11 Low Frequency Oscilaltion Reduced

After adding an inductor L=100uH and a resistor R=10Ohm in parallel after the OP547A opamp that provide the bias for the photodiode of REFL11, the noise at low frequency that I had observed, was significantly reduced.

See this plot:

 2010_04_05_REFL11_darknoise_with_100uH_coil_10ohm_res.png

A closer inspection of the should at 11MHz in the noise spectrum, showed some harmonics on it, spaced with about 200KHz. Closing the RF cage and the box lid made them disappear. See next plot:

 2010_04_05_REFL11_darknoise_wide_freq_window_lid_open-closed.png

The full noise spectrum looks like this:

2010_04_05_REFL11_darknoise_wide_freq_window_lid_open-closed.png

A big bump is present at ~275MHz. it could important if it also shows up on the shot noise spectrum.

  2768   Mon Apr 5 10:33:12 2010 AlbertoOmnistructureElectronicssoldering iron broken

This morning the pencil soldering iron of our Weller WD2000M Soldering Station suddenly stopped working and got cold after I turned the station on. The unit's display is showing a message that says "TIP". i checked out the manual, but it doesn't say anything about that. I don't know what it means. Perhaps burned tip?

Before asking Steve to buy a new one, I emailed Weller about the problem.

  2772   Mon Apr 5 13:52:45 2010 AlbertoUpdateComputersFront-ends down. Rebooted

This morning, at about 12 Koji found all the front-ends down.

At 1:45pm rebooted ISCEX, ISCEY, SOSVME, SUSVME1, SUSVME2, LSC, ASC, ISCAUX

Then I burtestored ISCEX, ISCEY, ISCAUX to April 2nd, 23:07.

The front-ends are now up and running again.

  2775   Tue Apr 6 11:27:11 2010 AlbertoUpdateComputer Scripts / ProgramsData formats in the Agilent AG4395a Spectrum Analyzer

Lately I've been trying to sort out the problem of the discrepancy that I noticed between the values read on the spectrum analyzer's display and what we get with the GPIB interface.

It turns out that the discrepancy originates from the two data vector that the display and the GPIB interface acquire. Whereas the display shows data in "RAW" format, the GPIB interface, for the way the netgpibdata script is written, acquires the so called "error-corrected data". That is the GPIB downloaded data is postprocessed and corrected for some internal calibration factors of the instrument.

Another problem that I noticed in the GPIB downloaded data when I was measuring noise spectrum, is an unwanted factor of 2 in the amplitude spectral density.
For example, measuring the amplitude spectral density of the FSS RF PD's dark noise at its resonant frequency (~21.5 MHz), I would expect ~15nV/rtHz from the thermal noise - as Rana pointed out in the elog entry 2759). However, the spectrum analyzer reads 30nV/rtHz, in both the display and the GPIB downloaded data, except for the above mentioned little discrepancy between the two. (The discrepancy is about 0.5dBm/Hz in the power spectrum density).
 
My measurement, as I showed it in the elog entry 2760) is of ~15nV/rtHz, but only becasue I divided by 2. Now I realize that that division was unjustified.
 
I'm trying to figure out the reason for that. By now I'm not sure we can trust the netgpib package for spectrum measurements with the AG4395.
  2776   Tue Apr 6 16:55:28 2010 AlbertoUpdateComputer Scripts / ProgramsData formats in the Agilent AG4395a Spectrum Analyzer

Quote:

Lately I've been trying to sort out the problem of the discrepancy that I noticed between the values read on the spectrum analyzer's display and what we get with the GPIB interface.

It turns out that the discrepancy originates from the two data vector that the display and the GPIB interface acquire. Whereas the display shows data in "RAW" format, the GPIB interface, for the way the netgpibdata script is written, acquires the so called "error-corrected data". That is the GPIB downloaded data is postprocessed and corrected for some internal calibration factors of the instrument.

Another problem that I noticed in the GPIB downloaded data when I was measuring noise spectrum, is an unwanted factor of 2 in the amplitude spectral density.
For example, measuring the amplitude spectral density of the FSS RF PD's dark noise at its resonant frequency (~21.5 MHz), I would expect ~15nV/rtHz from the thermal noise - as Rana pointed out in the elog entry 2759). However, the spectrum analyzer reads 30nV/rtHz, in both the display and the GPIB downloaded data, except for the above mentioned little discrepancy between the two. (The discrepancy is about 0.5dBm/Hz in the power spectrum density).
 
My measurement, as I showed it in the elog entry 2760) is of ~15nV/rtHz, but only becasue I divided by 2. Now I realize that that division was unjustified.
 
I'm trying to figure out the reason for that. By now I'm not sure we can trust the netgpib package for spectrum measurements with the AG4395.

 I noticed that someone, that wasn't me, has edited the wiki page about the netgpibdata under my name saying:

 " [...]

* A4395 Spectrum Units
Independetly by which unites are displayed by the A4395 spectrum analyzer on the screen, the data is saved in Watts/rtHz
"

That is not correct. The spectrum is just in Watts, since it gives the power over the bandwidth. The correspondent power spectral density is showed under the "Noise" measurement format and it's in Watts/Hz.
Watts/rtHz is not a correct unit.
  2779   Wed Apr 7 10:48:04 2010 AlbertoUpdateElectronicsREFL11 Noise Simulation
LISO simulations confirm the estimate of ~15nV for the noise of REFL11.
The largest contribution comes from the 50Ohm output resistor (Rs in the schematic below), the 450Ohm feedback resistor of the max4107 opamp stage; the 10KOhm resistor at the Test Input connector.
 
See attached plot.
 
(It's also all in the SVN, under https://nodus.ligo.caltech.edu:30889/svn/trunk/alberto/40mUpgrade/RFsystem/RFPDs/)
#
#                 gnd
#                 |
#                 Cw2
#                 |
#                 n23
#                 |
#                 Lw2
#                 |
#   gnd           n22
#   |             |
#   Rip           Rw2
#   |             |                   |\
#   nt- Rsi-n2- - - C2 - n3 -  - -  - |  \
#            |    |      |   |        |4106>-- n5 - Rs -- no                                                            
# iinput    Rd   L1     L2 R24    n6- |  /     |           |
#    |- nin- |    |      |   |    |   |/       |         Rload
#           Cd   n7     R22 gnd   |            |           |
#            |    |      |        | - - - R8 - -          gnd
#           gnd  R1     gnd      R7
#                 |               |
#                gnd             gnd
#
#
#
Attachment 1: rfpd11_testinput_noiseplot.pdf
rfpd11_testinput_noiseplot.pdf
  2781   Wed Apr 7 11:11:19 2010 AlbertoUpdateElectronicsREFL11 Noise Simulation

Quote:

What??? I don't see any gray trace of Rs in the plot. What are you talking about?

Anyway, if you are true, the circuit is bad as the noise should only be dominated by the thermal noise of the resonant circuit.

Quote:
LISO simulations confirm the estimate of ~15nV for the noise of REFL11.
The largest contribution comes from the output resistor (Rs in the schematic below).
See attached plot.

 

The colors in the plot were misleading.
Here's hopefully a better plot.
The dominant sources of noise are the resonant of the photodiode (~10Ohm), the max4107, the resistor in series to ground at the - input of the max4107.
Attachment 1: rfpd11_testinput_noiseplot.pdf
rfpd11_testinput_noiseplot.pdf
  2782   Thu Apr 8 10:17:52 2010 AlbertoUpdate40m UpgradingREFL11 Noise Vs Photocurrent

From the measurements of the 11 MHz RFPD at 11Mhz I estimated a transimpedance of about 750 Ohms. (See attached plot.)

The fit shown in the plot is: Vn = Vdn + sqrt(2*e*Idc) ; Vn=noise; Vdn=darknoise; e=electron charge; Idc=dc photocurrent

The estimate from the fit is 3-4 times off from my analsys of the circuit and from any LISO simulation. Likely at RF the contributions of the parassitic components of each element make a big difference. I'm going to improve the LISO model to account for that.

2010_04_05_REFL11_ShotnoiseVsPhotocurrent.png

The problem of the factor of 2 in the data turned out to be not a real one. Assuming that the dark noise at resonance is just Johnson's noise from the resonant circuit transimpedance underestimates the dark noise by 100%.

  2783   Thu Apr 8 10:24:33 2010 AlbertoUpdate40m UpgradingREFL11 Noise Vs Photocurrent

Quote:

From the measurements of the 11 MHz RFPD at 11Mhz I estimated a transimpedance of about 750 Ohms. (See attached plot.)

Putting my hands ahead, I know I could have taken more measurements around the 3dB point, but the 40m needs the PDs soon.

  2785   Fri Apr 9 06:45:28 2010 AlbertoUpdate40m UpgradingREFL11 Noise Vs Photocurrent

Quote:

Something must be wrong. 

1. Physical Unit is wrong for the second term of "Vn = Vdn + Sqrt(2 e Idc)"

2. Why does the fit go below the dark noise?

3. "Dark noise 4 +/- NaN nV/rtHz"   I can not accept this fitting.

Also apparently the data points are not enough.

 1) True. My bad. In my elog entry (but not in my fit code) I forgot the impedance Z= 750Ohm (as in the fit) of the resonant circuit in front of the square root: Vn = Vdn + Z * sqrt( 2 e Idc )

2) That is exactly the point I was raising! The measured dark noise at resonance is 2x what I expect.

3) I don't have uncertainties for the fit offset (that is, for the Dark Noise). The quick fit that I used (Matlab's Non Linear Least Squares method) doesn't provide 95% confidence bounds when I constrain the offset parameter the way I did (I forced it to be strictly positive).
Sure. It's not a very good fit. I just wanted to see how the data was going.

I also admitted that the data points were few, especially around the 3dB point.

Today I'm going to repeat the measurement with a new setup that lets me tune the light intensity more finely.

  2786   Sun Apr 11 13:51:04 2010 AlbertoOmnistructureComputersWhere are the laptops?

I can't find the DELL laptop anywhere in the lab. Does anyone know where it is?

Also one of the two netbooks is missing.

ELOG V3.1.3-