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  588   Sat Apr 23 22:15:39 2011 ranaDailyProgressNoiseBudgetRIN coupling to frequency noise

 

 The measurement of the RIN -> frequency coupling of the 40m Reference Cavity is here. Its an upper limit of 1 Hz/uW.

What are the equivalent numbers for the ACAV and RCAV here?

  589   Sun Apr 24 22:02:07 2011 taraDailyProgressFSSPreparing to install TTFSS

 I can lock the laser frequency to RCAV with TTFSS, but the noise in the beat is higher than before.

 I'm trying to bring the beat back. This is the to do list

1) check the Power from LO input to match the designed value for TTFSS (10dB)

   The LO power available is 20 dBm

  The mixer, JMS-1H, requires 10dBm input. I used 10 dB Attenuator to attenuate the LO signal before plugging in the servo.

   I forgot to use the attenuator before. I'm not sure if the 20dBm signal will affect the RF amplifier and the others components or not. I might have to check the box again**.

 

2) check the error signal, make sure that the phase is right.

     To make sure that the box is working properly, I checked the error signal, the details are here. Since we changed the setup the phase adjustment between LO and PD needs to be changed. It is set to 5.3659 V to make the error signal symmetric.

 

3) optimize the gain, between common gain and fast gain. To minimize the noise from MIXER MON

     I adjusted common gain and fast gain while monitored the noise spectrum from MIXER MON from dc - 10 MHz to make sure that the noise was minimized and there was no oscillation at any frequency.

     COMMON GAIN knob is set to 455. FAST GAIN knob is set to 800.

 

4) Then I checked the beat signal. It is an order of magnitude higher than before.

 beat_2011_04_24.png

The setup:

1mW power into both cavities

 RCAV gain: FAST 800, COMMON 455.

ACAV gain: 7

 

Attachment 1: beat_2011_04_24.png
beat_2011_04_24.png
  590   Mon Apr 25 10:40:00 2011 FrankDailyProgressNoiseBudgetRIN coupling to frequency noise

at which modulation frequency? The text doesn't say.

Quote:

 

 The measurement of the RIN -> frequency coupling of the 40m Reference Cavity is here. Its an upper limit of 1 Hz/uW.

What are the equivalent numbers for the ACAV and RCAV here?

 

  591   Mon Apr 25 17:53:20 2011 taraDailyProgressFSSPreparing to install TTFSS

I switched the polarity for EOM actuator on RCAV loop. Now the beat signal is comparable to what we had before.

          The beat measurement from the previous entry (with new TTFSS) was higher than what we had with the old FSS. It was because of the wrong polarity of the EOM actuator. The error signal can be flipped at 2 points in RCAV servo. The first point is the phase flip button controlled by the 35.5 MHz LO card. This switch can be activated on the medm screen. The second point is the switch on the FAST path on TTFSS (a square box with +/- in the cartoon diagram). This one can be switched manually. To choose the right polarity for FAST actuator and EOM actuator, we pick certain polarities at point 1 and 2 that allow us to lock the cavity with FAST actuator. Then try to invert both switches, and see if we can lock it better (higher bandwidth.)

                                           

     LO signal ---> [pol switch 1]  ---------------> Fast act---> [pol switch 2]

                                                  |----------> EOM act

     I tried 10kHz and 1kHz input range on the LO for PLL for beat measurement. I measured the calibration factor for 10kHz to be 7.1e3 Hz/V, but I did not measure the calibration factor for  1kHz input range, I just estimated it to be 0.71e3 Hz/V. The results look fine.

beat_2011_04_25.png

 

The setup is similar to what we have had before.

I use 14dBm attenuator, so the power to LO is ~ 8 dBm.

Power to the periscope: 1mW

Fast gain: 600

Common gain: 383

Offset:     455

C3:PSL-FSS_PHCON(ADJ phase):  5.3659

C3:PSL-FSS_PHFLIP(phase flip): 0

FAST polarity switch on TTFSS: (-)

 

Currently I use C3:PSL-FSS_SLOWDC to control SLOWDC value on the laser. The knob is a bit sensitive, so it is hard to control.

       Next, I will check the UGF of RCAV servo, we expect to see UGF around 0.5 MHz. Once I verify the UGF, I'll measure the RIN-> frequency noise coefficient.

 

 

  595   Wed Apr 27 22:09:10 2011 taraDailyProgressNoiseBudgetRIN coupling to frequency noise

 I list the plan for measuring the effect of RIN to Frequency noise.

 

     1) Measure the TF between FASTMON / RCTRANSPD. FASTMON [V]will be converted to frequency [Hz]. RCTRANSPD will be converted to power of the laser [W]. This will be useful once we know the coupling coefficient and want to project the noise in the noise budget.

      1.1) TF between FASTOUT and FAST MON will be measured. We observe the signal from FASTMON, but the signal from FASTOUT will drive the laser, so they will not be exactly the same because of other electronic components along the paths, for example, low pass filter at 10Hz for FASTOUT.  We already know that the calibration at FAST modulation at the laser input is 3.09 [MHz/V].

     2) Amplitude modulate the laser power by modulating the polarization of the laser via EAOM. The beam is sent to a PBS which allows only one polarization out. Thus the amplitude of the transmitted beam through the PBS will be modulated.  I choose to modulate at 10 Hz, because the frequency is not too high so the effect is small, and the frequency is not too low that it might be masked by seismic noise.The modulating voltage is 2Vpkpk, I want the Vmod to be large enough, so that I can see the signal, but not too large because we want the system to have linear response.

     3) Measure the PSD of FASTOUT, RCTRANSPD, make sure that the peak at 10 Hz is visible.

     The input power is 1mW, I will increase more power later. I want to see if  the signal is measurable at this power level. If so, the higher power should increase the signal by the same factor.

    

    To sum up, I'll repeat the measurement with

    1) changing the laser power

    2) changing the modulation depth

   3) changing the modulating frequency

    and see if the effect behaves like what we expect. It should be smaller as the laser power or modulation depth decrease, or at higher frequency.

  596   Sun May 1 22:39:56 2011 taraDailyProgressNoiseBudgetRIN coupling to frequency noise

I measured RIN -> frequency noise coupling coefficient at 10Hz from RCAV. The result is 0.02 Hz/uW (frequency shift/power fluctuation built up in the cavity*)

*by power fluctuation built up in the cavity, I mean [RIN x power input as measured at the input periscope x Finesse/ pi ]

 

Following the list I wrote down here

1) TF between FASTOUT/FASTMON is a lowpass with a pole at 10Hz. FASTOUT is fed to the laser for controlling its frequency. FASTMON is a channel for monitoring. This is as expected from the RC low pass filter at FASTOUT.

TF.png

 2) I modulated the beam's polarization and used a PBS so the polarized transmitted beam is amplitude modulated. I used sine wave out from SR785, 2Vpp @ 10Hz connected to the EAOM. I measured the RCTRANSPD signal with an oscilloscope to check the signal is 211 mV +/- 7mV. The power is modualted by ~ 4%.

Then measure the peak at the modulating frequency (10Hz) from RCTRANSPD and FASTMON spectral density with SR785. The linewidth is 0.976 mHz. FASTMON was connected to chA, RCTRANSPD to chB. The data were averaged over 4 samples.

  The DC level of RCTRANSPD was 220 mV.

 PSD.png

 The peak from FASTMON and RCTRANSPD are 6.75 mV/rtHz and 55.2 mV/rtHz. To convert this to the coupling coefficient we have to:

1) convert the voltage output from FASTMON to FASTOUT, the TF is measured and shown above. 

    V_fastout = V_fastmon x sqrt(  1 /   1 + (f/10Hz)^2  ), so  for 10Hz,

    V_fastout = V_fastmon x sqrt (1/2)

2) convert the Voltage out to frequency change by 3.09 MHz/V factor.

3) convert RCTRANSPD to RIN by dividing the PSD by DC level (220mV)

4) convert RIN to power fluctuation by x power input (1mW) x Finesse (9710) / pi

 

or

Freq/Hz   =  Vmon   x TF  x 3.09 [MHz/V]   

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

                 V_RCtranspd / rctranspd_DC     x Pin  x Finesse/pi

 

  For 10Hz, 1mW power input, rctranspd_dc = 220 mV, we get

  RIN coupling coeff at 10Hz  =        0.0188 Hz/uW   = 18.8 mHz/uW.

 

I repeated the measurement with:

1)half modulating power, 1Vpp. the coupling coefficient decreases to  0.015 Hz/uW, so it seems that the modulation range I chose might be too large so the effect was not linear.  The coefficient should remain constant!!

2) 20Hz modulating power, 2Vpp (but the line width was 7.8 mHz, not 1mHz for quick measurement)

, the coupling coefficient is  9.35  mHz/uW. The result is smaller than that of 10Hz, which is expected from 1/f effect, but I think I should have used the same line width and look at a few more frequencies.

 

I'll have to check the coupling coefficient from ACAV, and check the beat signal to see if they are canceled or not.

 


Comparison to the calculation I did

The calculation I did here gives the noise at 10Hz from 10mW input, RIN = 10^-4, Finesse 10^4, to be 1.4 [mHz/rt Hz] which is already lower than the coating noise (10mHz/rtHz at 10Hz)

I can convert it to the estimated coupling coefficient to compare with what I measured.

1.4 mHz/rtHz       =   coupling coeff [Hz/W] x  Pin [W] x RIN [1/rtHz] xFinesse/pi,  or

coupling [Hz/W]    =   1.4 mHz/rtHz  / Pin [W]  / RIN [1/rtHz] / (Finesse/pi)

                            =   0.44 Hz/W

This is 5 order of magnitude lower than what I measured !!! and the calculated noise is only one order of magnitude lower than the coating. It means that the calculation is not correct and we cannot ignore the effect. we will run into it before reaching coating thermal noise if the effects on both cavities aren't canceled.

 

  597   Mon May 2 10:32:58 2011 FrankDailyProgressNoiseBudgetRIN coupling to frequency noise

did you try to optimie/minimze the coupling before the measurement? If not you should check how you can make it worse/better (e.g. alignment to cavity, RFPD, polarization, RF-AM etc) and then measure it. It might be that you are sitting on kind of a maximum right now, who knows.

How about measuring a complete TF? Your signal looks large enough to do that.

Quote:

I measured RIN -> frequency noise coupling coefficient at 10Hz from RCAV. The result is 0.02 Hz/uW (frequency shift/power fluctuation built up in the cavity*)

*by power fluctuation built up in the cavity, I mean [RIN x power input as measured at the input periscope x Finesse/ pi ]

 

Following the list I wrote down here

1) TF between FASTOUT/FASTMON is a lowpass with a pole at 10Hz. FASTOUT is fed to the laser for controlling its frequency. FASTMON is a channel for monitoring. This is as expected from the RC low pass filter at FASTOUT.

TF.png

 2) I modulated the beam's polarization and used a PBS so the polarized transmitted beam is amplitude modulated. I used sine wave out from SR785, 2Vpp @ 10Hz connected to the EAOM. I measured the RCTRANSPD signal with an oscilloscope to check the signal is 211 mV +/- 7mV. The power is modualted by ~ 4%.

Then measure the peak at the modulating frequency (10Hz) from RCTRANSPD and FASTMON spectral density with SR785. The linewidth is 0.976 mHz. FASTMON was connected to chA, RCTRANSPD to chB. The data were averaged over 4 samples.

  The DC level of RCTRANSPD was 220 mV.

 PSD.png

 The peak from FASTMON and RCTRANSPD are 6.75 mV/rtHz and 55.2 mV/rtHz. To convert this to the coupling coefficient we have to:

1) convert the voltage output from FASTMON to FASTOUT, the TF is measured and shown above. 

    V_fastout = V_fastmon x sqrt(  1 /   1 + (f/10Hz)^2  ), so  for 10Hz,

    V_fastout = V_fastmon x sqrt (1/2)

2) convert the Voltage out to frequency change by 3.09 MHz/V factor.

3) convert RCTRANSPD to RIN by dividing the PSD by DC level (220mV)

4) convert RIN to power fluctuation by x power input (1mW) x Finesse (9710) / pi

 

or

Freq/Hz   =  Vmon   x TF  x 3.09 [MHz/V]   

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

                 V_RCtranspd / rctranspd_DC     x Pin  x Finesse/pi

 

  For 10Hz, 1mW power input, rctranspd_dc = 220 mV, we get

  RIN coupling coeff at 10Hz  =        0.0188 Hz/uW   = 18.8 mHz/uW.

 

I repeated the measurement with:

1)half modulating power, 1Vpp. the coupling coefficient decreases to  0.015 Hz/uW, so it seems that the modulation range I chose might be too large so the effect was not linear.  The coefficient should remain constant!!

2) 20Hz modulating power, 2Vpp (but the line width was 7.8 mHz, not 1mHz for quick measurement)

, the coupling coefficient is  9.35  mHz/uW. The result is smaller than that of 10Hz, which is expected from 1/f effect, but I think I should have used the same line width and look at a few more frequencies.

 

I'll have to check the coupling coefficient from ACAV, and check the beat signal to see if they are canceled or not.

 


Comparison to the calculation I did

The calculation I did here gives the noise at 10Hz from 10mW input, RIN = 10^-4, Finesse 10^4, to be 1.4 [mHz/rt Hz] which is already lower than the coating noise (10mHz/rtHz at 10Hz)

I can convert it to the estimated coupling coefficient to compare with what I measured.

1.4 mHz/rtHz       =   coupling coeff [Hz/W] x  Pin [W] x RIN [1/rtHz] xFinesse/pi,  or

coupling [Hz/W]    =   1.4 mHz/rtHz  / Pin [W]  / RIN [1/rtHz] / (Finesse/pi)

                            =   0.44 Hz/W

This is 5 order of magnitude lower than what I measured !!! and the calculated noise is only one order of magnitude lower than the coating. It means that the calculation is not correct and we cannot ignore the effect. we will run into it before reaching coating thermal noise if the effects on both cavities aren't canceled.

 

 

  598   Tue May 3 11:59:55 2011 taraDailyProgressNoiseBudgetRIN coupling to frequency noise

The coupling coefficient I measured before changed with input power/ alignment of the beam. What I measured before might be the RF-AM effect. [figures and more details will be added]

 

I read Frank's entry, and tried to see the effect of other parameters on coupling coefficient. I tried

      1) misaligning  the  beam into the cavity, and

        The power level is still 1mW input. I turned the knob of the periscope to misaligned the beam. RCTRANSPD DC value dropped from 250 mV to 207 mV, the peak at 10Hz in FASTMON disappeared. 

      [add fig]

 

 

      2) changing power input to the cavity. I tried 0.5 mW, 1 mW, 2mW, 5mW

 As I changed the power, I adjusted the gain so that the noise from MIXEROUT remained the same, but at certain power I could not see the peak in FASTMON signal.

[add fig]

 

 

 

  599   Wed May 4 01:20:00 2011 taraDailyProgressopticminimizing RFAM/ aligning 35.5 MHz EOM

I minimized the RFAM by aligning the 35.5 MHz EOM and remeasured the RIN coupling coefficient.

The upper limit is 5 [Hz/uW (fluctuation of input power = RIN x Pin) ]@ 10 Hz

(This entry is approved by Kiwamu and is written in his style)Tue May 10 19:20:22 2011

  As pointed out in the LIGO-X meeting that my setup might suffer a lot from RFAM, so I came back to:

  • 1. minimize the RFAM by aligning the 35.5 MHz EOM,
  • 2. determine how much Vmod I can apply to amplitude modulation without exciting the RFAM noise above the background, and
  • 3. remeasure RIN coupling coefficient again with the allowed maximum Vmod.

 


[Setup]

The power input was 1mW as usual.

The frequency of Vmod is 10Hz. The amplitude of Vmod to EAOM for amplitude modulation was varied from 2 to 10 Vpkpk. Common/Fast gain was 500/900. I had to reduce it so the signal is not too large. I measured the spectrum of FASTMON and tried to observe the peak at 10Hz  with 12.5 mHz linewidth. The background level was ~10mV.

I do this to determine what is the maximum driving voltage where the effect from RFAM is still small compared to the background.

____________________________________

Drive Vpkpk     FASTMON peak(Vrms/rtHz)

10                   74.7

8                     48.17

5                     29.6

3                    23.02

2                  ~comparable to BG level ~ 10mV/rtHz

____________________________________

[ 1. aligning EOM ]

I picked up the beam after EOM on RCAV path and sent it to a PD (Thorlabs PDA10A.)  There were 35.5 MHz pick up on the table, so I had to choose where the peak from pickup was minimum. Then I adjusted the half wave plate before the EOM and EOM's pitch/yaw position to minimize the peak.

[ 2. determine max Vmod ]

 Although we want to modulate the power as small as possible to have a good linear approximation, we also need the signal to be large enough to be able to see the effect. However, the alignment of the EOM is not perfect, there will be RFAM effect adds into the signal. If the modulation is too large, the RFAM will mask the real signal.  I need to determine what is the maximum Vmod I can use without having the RFAM effect excited above the background.

     To see the effect of RFAM, I kept the setup similar to what I did with RIN coupling coefficient measurement, but without locking the cavity, and the laser frequency off from the resonance. This will tell us how much "fake signal" is produced by RFAM.

     When the cavity is not locked, all the carrier and sidebands will be incident on the RFPD. The signal should be flat (beat between the carrier and both sidebands cancel each another,) and after it is demodulated by 35.5 MHz from LO, the level should be zero. However, if the amplitude is modulated at 35.5 MHz due to misalignment of the EOM, this will appear as DC signal at the error point. Hence, any power modulation at f0 (for this case, 10 Hz) will multiply up the error signal and cause offset fluctuation and slope change at f0. Slope change is not a problem, but the offset is. It will change the point where the laser will be locked, as the error signal moves up and down. Thus the system will interpret it as frequency noise of the laser and try to fight against it. This will appear as a peak in the FASTOUT spectrum at the modulation frequency, f0.

     I measured the spectrum of FASTOUT (MIXER OUT is another option) to see the effect of RFAM 

[ 3. remeasure RIN coupling coefficient ]

So I used 2Vpkpk drive, locked the cavity, and measured FASTMON again to see if I can measured the RIN coupling or not. The gain was set back to optimum value (common = 970 fast= 900.)  However, there was no observable peak at 10Hz from FASTMON signal. It was quite flat ~100 uVrms/rtHz.

 I made sure that the amplitude was really modulated by checking RCTRANSPD. It had a 5.37 Vrms/rtHz peak at 10Hz with 200mV DC level. Therefore, the laser noise is higher than the thermo-optic effect at this modulation level. I cannot increase modulation depth because the RFAM will mask the signal. 

If I use this number to calculate the coupling coefficient, (flat level of FASTOUT, and peak from RCTRANSPD)

it will be ~ 8 [Hz/ uW of fluctuation of the input power into the cavity] still larger than 1[Hz/uW] as measured at 40m, but it's getting smaller than the last entry (60 [Hz/uW] of input power)

       I still can change the power input, but I think the RFAM will scale up by the same amount and mask the signal again. I'll try that later.

 

 Let's check what does this value give us in the noise budget @10Hz. The input power is 1mW, RIN = 10^-4. Frequency noise will be

8 [Hz/uW] x 1000 [uW] x 10^-4 [RIN] = 0.8 [Hz/rtHz] which is higher than coating noise (10 [mHz/rtHz]@10Hz) So we still cannot ignore the effect.


[Take II]

 I tried 16 mW input power, there was signal from RFAM when I measured FASTOUT with unlocked cavity, the peak was 46 m[Vrms/rtHz] above 10m[Vrms/rtHz] background. Vmod = 1Vpkpk.

When I lock the cavity and measure the coupling:

FASTMON peak = 278.6 uVrms/rtHz

RCTRANSPD peak = 30.82 mV, DC level = 2.57 V, Pin = 16mW. Linewidth = 12.5mHz.

Common/Fast gain = 480/906

Use the calculation from here.

The upper limit for coupling coeffiicient is ~ 5 [Hz/uW]. It is only the upper limit because RFAM effect is still present.

  600   Mon May 16 22:59:45 2011 frank, taraDailyProgressNoiseBudgetRIN coupling coefficient

I tried to measure coherence TF between RCTRANSPD and FASTMON, but I could not. After I adjusted gain setting, RFAM adjustment,electronic offset , I could not observe frequency shift due to amplitude modulation.

      From this entry, I measure the power spectrum of transmitted power(RCTRANSPD) and laser frequency change(FASTMON). But it is only one peak at 10Hz. This does not give much information about how much the frequency shift comes from RFAM or the real RIN coupling effect.  So, I want to measure the TF between FASTMON and RCTRANSPD, then misaligned the polarization of the laser into the EOM to increase the RFAM effect ,and see how much TF will change.

     Since I already aligned the EOM to minimize the RFAM effect, and adjusted the electronic offset. I checked if I could see the peak in FASTMON due to amplitude modulation or not. If there is no peak, it means I cannot measure TF. It turned out that I could not see the peak at amplitude modulating frequency anymore.

 I tried:

  • changing power input to be 1mW, 5mW and 10mW and Vmod was varied from 2 Vpkpk to 10 Vpkpk.
  • changing amplitude modulating frequency to 5Hz, 10Hz, 20Hz
  • changing modulating function from sine wave to square wave, with various frequencies and amplitudes.

     compare.png

      The above plot shows the PSD of FASTMON (top) and RCTRANSPD (bottom) when the system is amplitude modulated, and no amp modulated. The power input is 10mW, the modulating function is sine wave, 10Vpkpk, the linewidth is 125mHz. Common/Fast gain = 300/430. There is no sign of any peak at 10Hz in FASTMON at all.

    

     I think the peak I saw before came from :

1) electronic offset: it was not optimized before. To adjust it, block the beam on the RFPD, monitor mixer out (via "common out2" channel), and adjust the offset so  that mixerout becomes zero.

2) RFAM: this is explained in here.

3) gain setting: I made a mistake before, as Frank pointed out, when I optimized the gain. I only looked at noise level from mixer out at low frequency without      checking the oscillation at high frequency. Now I adjusted it back so there is no oscillation, and the system is more stable.

       So, to sum up, the coherent TF measurement could not be done for now, because there is no peak in FASTMON. It means that the laser noise is noisier than the RIN coupling effect and any possible residual RFAM effect.

I'll work on the TF of the servo before continuing on the RIN coupling coefficient, but the next test might be trying larger modulation depth.

  602   Wed May 18 20:51:35 2011 taraDailyProgressopticoptic layout for new fss setup

I planned the layout for new fss setup.

The new setup has 1) both cavities placed in the same vacuum chamber, 2) two AOMs used in both RCAV and ACAV paths, 3) more compact beat path.

 In the layout, I assumed that

  • Two cavities in the chamber are 3 inches apart.
  • Two AOMs are of the same model, have the same setup
  • There is no change of plan for the layout between PMC and the laser

This is just a plan, no mode matching has been calculated yet.

I am concerned  that the mode matching lens might block the beam in ACAV path where the incoming beam and reflected beam cross, but this can be adjust later.

The outer foam box will be smaller, but it should have enough space to keep some electronics inside like we have now.

I should find two similar sets of beam splitters/ mirrors for beams in the beat path behind the cavity. So the pick up beams from two cavities can have same power.

Right now the power going into two PDs for RCTRANSPD are not the same because the splitter are not the similar.

Note that we might install a platform  behind the cavities so that we don't need the periscopes to lower the beam, and get rid of their associated mechanical peaks.

fss_layout_2011_05_18.png

  603   Thu May 19 11:32:46 2011 taraDailyProgressopticoptic layout for new fss setup

I added more details on the layout, and necessary half wave plates in the beam path.

fss_2011_05_18.png

  614   Tue Jun 14 23:07:03 2011 taraDailyProgressopticoptic layout for new fss setup

The mode matching for new FSS is calculated. The plan is shown below.

 

Note for the setup:

1) the spotsize in the AOM is 200um, the specsheet says 550 um (I might have to correct this).

2) Two AOMs are of the same model.

3) For mode matching to the AOM in acav path, I used only a single lens.

4) focal lengths of the lenses are in mm, We have to order the one with * (f = 57.4 mm)

5) Both cavities are 1" apart (3" from center to center)

6) Mistake in the drawing: the x2 QWPs just before the beams enter the vacuum chamber should be placed before the periscopes, not after.

2aom_fss.png

  615   Thu Jun 16 00:03:11 2011 taraDailyProgressFSSTF measurement on FAST path for TTFSS

I measured TF of Fast path from TTFSS and compared with LISO model. The results agree well.

      == Motivation ==

     This work is a part of loop characterization. The goal is to understand why the UGF of the open loop gain is only ~200kHz instead of ~500 kHz. So I check if the TF of the loop match the design or not. FAST path is working as designed, see Bode plot of the TF below. I'll measure EOM path next.

      == setup ==

       I used SR785 to measure the TF. The source from RFout was split by a T, one for Ref (chA), another one for excitation at Test in. The Response was taken from Out2 to ChB. I used an oscilloscope to monitor Out2 to make sure that the signal is not saturated. The gain setup (common/fast) of TTFSS was set to 350/100. The power input of the laser was 1mW.

TF_fast.pdf

FIGURE1: Setup for fast path measurement. The ports are shown on the corresponding schematic by green arrows.


     

fastpath.png

FIGURE2: Comparison between LISO model and TF measurement of Fast path. The model is offset to compensate for common gain/ fast gain setup in the servo.

 

Next is to measure EOM path and compared with LISO, then the whole OLG TF of the setup. 

  617   Mon Jun 27 23:55:43 2011 Raphael CervantesDailyProgressFSSFSS measurements progress

Update: Numbers are wrong. Will talk to Tara tonight. Open Loop transfer function is too low and I probably made a mistake with the suppressed noise plot.

 

Common Gain Knob and Fast Gain Knob Calibrations

Objective: The knobs on the FSS servo boards adjust the gain of the AD602JR variable gain amplifiers between Out 2 Common and Out 1 Fast. However, the knobs are simply numbered between 1 and 1000 and do not tell us what the actual gain of these amplifiers are. We aim to measure calibrate these knob numbers to actual gain numbers in dB.

Procedure: We measure the open loop gain from Out 2 Common to Out 1 Fast. The path includes two variable gain amplifiers, AD602JR, one of which is controlled by the Fast Gain knob and the other which is controlled by the Common Gain knob.

  1. We start off with both knobs at 0.
  2. We use a spectrum analyzer to inject a signal into RF test. We use Out 2 Common as the reference and Out 1 Fast as the response.
  3. We measure the transfer function of this path. We pick a frequency to monitor it's change as we vary the Fast and Common Gain. In our case we picked 1.05 kHz and measured it's gain to be 12.4dB when both knobs are 0.
  4. We  vary the common gain while we keep the Fast Gain constant and measure the subsequent gain at 1.05kHz. We than fit a line through the measured points to obtain an equation for the response of the transfer function as a function of the Common Knob number.
  5. We repeat the same measurement by setting the Common Gain knob to 0 and measuring the gain at 1.05kHz as we vary the Fast Gain Knob.

Results:

These graphs show the measured transfer function from Out 2 Common to Out 1 Fast.

Gain_Knobs_Calibration_Magnitude_Transfer_Function.jpgGain_Knobs_Calibration_Phase_Transfer_Function.jpg

We fit a line into both measurements and obtain the following:

Total Gain (dB) = 0.04* Common Knob # + 0.04*Fast Knob # +12.2

Graph for this fitting will come later if necessary.

 Measuring the Suppressed NPRO Free Running Noise

 Objective:

Procedure: We recognize that the suppressed NPRO free running noise is calculated by the expression: NPRO suppressed free running noise = NPRO free running noise/(1 + FSS Open Loop Gain). We noite that the FSS Open Loop gain is in absolute units and not in dB.

Measuring the Open Loop Gain:

To get the FSS Open Loop Gain, we use a spectrum analyzer to measure the open loop gain from Common Out 2 to Common Out 1 and the open loop gain from Common Out 1 to Common Out 2. Since the transfer function is in dB, calculating the entire FSS Open Loop Gain is just a matter of adding the two transfer functions together. To measure the transfer function of from Common Out 2 to Common Out 1, we use the spectrum analyzer to inject a signal into the Common Excitation and take Common Out 2 as the reference and Common Out 1 as the response. To measure the transfer function from Common Out 1 to Common Out 2, we inject a signal into RF Test and take Common Out 1 as the reference and Common Out 2 as the response.

Results:

We add both transfer functions to get the FSS Open Loop Gain and obtain the following result:

FSS_Open_Loop_Gain.jpg

NPRO Free Running Noise:

Frank gave me his data for his NPRO free running noise and I fit the data into a line. I am justified in excluding certain points because of the line harmonics in the lower frequency and the equipment noise floor in the higher frequency. log(free running noise) = -1.08 * log(frequency) +4.53

NPRO_noise.jpg

Suppressed Free Running Noise:

We simply calculate NPRO suppressed free running noise = NPRO free running noise/(1 + FSS Open Loop Gain) where the FSS Open Loop Gain is in absolute units insted of dB to get the following result.

Suppressed_NPRO_noise.jpg

 

Attachment 1: Gain_Knobs_Calibration_Magnitude_Transfer_Function.jpg
Gain_Knobs_Calibration_Magnitude_Transfer_Function.jpg
Attachment 2: Gain_Knobs_Calibration_Phase_Transfer_Function.jpg
Gain_Knobs_Calibration_Phase_Transfer_Function.jpg
Attachment 3: Suppressed_NPRO_noise.jpg
Suppressed_NPRO_noise.jpg
Attachment 4: FSS_Open_Loop_Gain.jpg
FSS_Open_Loop_Gain.jpg
Attachment 5: NPRO_noise.jpg
NPRO_noise.jpg
  618   Wed Jun 29 15:35:06 2011 Raphael CervantesDailyProgressFSSneed help improving LISO models

I've tried making LISO models for the PZT path, EOM path, the EOM HV path, and the notch filter right next to TP2 on the RF path. The files for these are entitled pzt.fil, eom.fil, eomHV.fil and filter1.fil. From there, I tried to fit poles and zeroes to the LISO models. These files are named pztfit.fil, eomfit.fil, eomHVfit.fil, and filter1fit.fil. The results of these fits are displayed on the attached graphs. As you can you, I've had trouble fitting poles and zeroes at higher frequencies. If someone can help me with that, that would be fantastic.

The purpose of these fits is so that I can obtain all the poles and zeroes necessary to put into a simulink file simulating the entire FSS setup. We plan to compare the entire FSS setup to the measured transfer function to see how they are different.

Attachment 1: eom.fil
op u4 ad797 gnd n3 n4
c c23 1u n1 n2
r r22 1.1k n2 n3
r r19 24.9k n4 n3
c c15 3.3n n4 n3
c c24 1u n4 n5
r r24 1.3k n5 n6
op u5 ad797 gnd n6 n7
r r20 47k n7 n6
c c18 33p n7 n6
... 19 more lines ...
Attachment 2: eomfit.fil
zero 921n #zero0
zero 84.5m 1k #zero1
pole 700 #pole0
pole 399 #pole1
pole 112 #pole2
pole 146 #pole3
pole 1.9k #pole4
zero 191k #zero2
#zero 101k #zero3
#zero 506k #zero4
... 14 more lines ...
Attachment 3: eomHV.fil
#bottom half of schematic
c c21 1n n1 n2
c c22 220n n1 n2
r r42 3.3k n2 n3
op u6 pa85 gnd n3 n4
r r47 100k n4 n3
c c25 47p n4 n5
r r45 1k n5 n3

#top half of schematic
... 16 more lines ...
Attachment 4: eomHVfit.fil
zero 60.2n
pole 218.7
pole 29.3k
pole 2.1M
#pole 7.04M
zero 1.4M
delay 5.5n
factor -139n

param factor 1f 1M
... 9 more lines ...
Attachment 5: filter1.fil
l l3 1.2u n1 n2
c c12 47p n1 n2
c c13 220p n1 gnd
c c14 220p gnd n2
uinput n1
uoutput n2
freq log 1n 1000M 10000
Attachment 6: filter1fit.fil
pole 8.35M 1k
zero 21.14M 1k
factor 1

param factor 1n 1M
#param pole0:f 1M 20M
#param zero0:f 1M 20M
fit filter1.out dbdeg rel

Attachment 7: pzt.fil
#stage 1
op u8 op27 n3 n2 n4
r r35 560 n1 n2
r r38 499 gnd n3
r r30 4.87k n4 n2
c c36 3.3n n4 n2

#stage 2
r r36 750 n4 n5
op u9 ad797 gnd n5 n6
... 29 more lines ...
Attachment 8: pztfit.fil
pole 9.6k #pole0
pole 33k #pole1
pole 222k 3 #pole 2
zero 233k 50 #zero0
pole 4M #pole3
pole 8M
pole 8M
pole 7M
pole 5M
factor 1
... 14 more lines ...
Attachment 9: EOM_HV_Magnitude_Circuit_Model_vs_Pole_Zero_Model.jpg
EOM_HV_Magnitude_Circuit_Model_vs_Pole_Zero_Model.jpg
Attachment 10: EOM_HV_Phase_Circuit_Model_vs_Pole_Zero_Model.jpg
EOM_HV_Phase_Circuit_Model_vs_Pole_Zero_Model.jpg
Attachment 11: EOM_Magnitude_Circuit_Model_vs_Pole_Zero_Model.jpg
EOM_Magnitude_Circuit_Model_vs_Pole_Zero_Model.jpg
Attachment 12: EOM_Phase_Circuit_Model_vs_Pole_Zero_Model.jpg
EOM_Phase_Circuit_Model_vs_Pole_Zero_Model.jpg
Attachment 13: Filter1_Magnitude_Circuit_Model_vs_Pole_Zero_Model.jpg
Filter1_Magnitude_Circuit_Model_vs_Pole_Zero_Model.jpg
Attachment 14: Filter1_Phase_Circuit_Model_vs_Pole_Zero_Model.jpg
Filter1_Phase_Circuit_Model_vs_Pole_Zero_Model.jpg
Attachment 15: PZT_Magnitude_Circuit_Model_vs_Pole_Zero_Model.jpg
PZT_Magnitude_Circuit_Model_vs_Pole_Zero_Model.jpg
Attachment 16: PZT_Phase_Circuit_Model_vs_Pole_Zero_Model.jpg
PZT_Phase_Circuit_Model_vs_Pole_Zero_Model.jpg
Attachment 17: D040105-C.pdf
D040105-C.pdf D040105-C.pdf D040105-C.pdf
Attachment 18: d0901893-a.pdf
d0901893-a.pdf d0901893-a.pdf d0901893-a.pdf
Attachment 19: D0901894-A.pdf
D0901894-A.pdf
  619   Wed Jun 29 15:47:40 2011 JenneDailyProgressPMCLISO models

Just a thought....

If you're going to post LISO models (the .fil files), it might be handy to also include a sketch of the circuit, or a link to the circuit.

  620   Wed Jun 29 15:59:51 2011 JenneDailyProgressPMCLISO models

Quote:

Just a thought....

If you're going to post LISO models (the .fil files), it might be handy to also include a sketch of the circuit, or a link to the circuit.

 Already done. Sorry about that.

  621   Wed Jun 29 18:01:30 2011 Raphael CervantesDailyProgressFSSSuppressed Free Running Noise

The suppressed NPRO free running noise is: suppressed NPRO free running noise=NPRO free running noise / (1+FSS Open Loop Gain)

I explained the measurement procedure in a previous elog. I made a mistake when I converted the FSS Open Loop Gain from dB to absolute units.

Frank mentioned that he believed the FSS Open Loop Gain is too low.

Attachment 1: FSS_Open_Loop_Gain.jpg
FSS_Open_Loop_Gain.jpg
Attachment 2: NPRO_noise.jpg
NPRO_noise.jpg
Attachment 3: Suppressed_NPRO_noise.jpg
Suppressed_NPRO_noise.jpg
  622   Thu Jun 30 03:25:13 2011 KojiDailyProgressPMCneed help improving LISO models

For EOM HV:

You need a single zero (or double zero) in order to make the bend of the magnitude curve at high freq.
However, this adds the phase advance at around the cut off freq although you actually have the phase delay.
This means that you need additional double pole just above the measurement freq. This pole may have relatively
high Q so that it can cancel the phase advance by the zero.

 

For EOM circuit:

A zero is missing at around 10^5 Hz. Also a pole or several poles are necessary to realize the magnitude curve and the phase delay at the high freq edge.

 

Filter1:

Limit the freq range from 10^5 instead of rediculous 10^-9.

 

PZT:

Replace one or two single poles by a double pole.

  623   Tue Jul 5 14:30:35 2011 Raphael CervantesDailyProgressFSSneed help improving LISO models

Hey, I tried to improve the models by Koji's suggestions, but I still can't quite get them right. Can someone give me some suggestions? The files are attached.

Attachment 1: eomfit.fil
zero 21n #zero0
zero 84.5m 1M #zero1
pole 1k #pole0
pole 124.8 #pole1
pole 141.3 #pole2
pole 319.8 #pole3
pole 1.9k #pole4
zero 28k #zero2
zero 218k #zero3
zero 218k #zero4
... 37 more lines ...
Attachment 2: D040105-C.pdf
D040105-C.pdf D040105-C.pdf D040105-C.pdf
Attachment 3: eom.fil
op u4 ad797 gnd n3 n4
c c23 1u n1 n2
r r22 1.1k n2 n3
r r19 24.9k n4 n3
c c15 3.3n n4 n3
c c24 1u n4 n5
r r24 1.3k n5 n6
op u5 ad797 gnd n6 n7
r r20 47k n7 n6
c c18 33p n7 n6
... 19 more lines ...
Attachment 4: d0901893-a.pdf
d0901893-a.pdf d0901893-a.pdf d0901893-a.pdf
Attachment 5: eomHV.fil
#bottom half of schematic
c c21 1n n1 n2
c c22 220n n1 n2
r r42 3.3k n2 n3
op u6 pa85 gnd n3 n4
r r47 100k n4 n3
c c25 47p n4 n5
r r45 1k n5 n3

#top half of schematic
... 16 more lines ...
Attachment 6: eomHVfit.fil
zero 215.7n #zero0
pole 218.4 #pole0
pole 29.1k #pole1
pole 30M #pole2
zero 2.7M #zero1
delay 32.4n 
zero 15M 1k #zero2
pole 168.5M 1M #pole2
factor -139n

... 12 more lines ...
Attachment 7: pzt.fil
#stage 1
op u8 op27 n3 n2 n4
r r35 560 n1 n2
r r38 499 gnd n3
r r30 4.87k n4 n2
c c36 3.3n n4 n2

#stage 2
r r36 750 n4 n5
op u9 ad797 gnd n5 n6
... 29 more lines ...
Attachment 8: pztfit.fil
pole 10k #pole0
pole 32.5k #pole1
pole 240.6k 3 #pole 2
zero 239.1k 20 #zero0
pole 14.2M #pole3
pole 5.4M 1.2 #pole4
pole 14.2M #pole5
pole 14.2M #pole6
delay 6.8n
factor 200
... 15 more lines ...
Attachment 9: EOM.jpg
EOM.jpg
Attachment 10: EOM_HV.jpg
EOM_HV.jpg
Attachment 11: PZT.jpg
PZT.jpg
  624   Mon Jul 11 15:52:36 2011 RaphaelDailyProgressFSSneed to modify RF summing box

Last Friday, we attempted to measure the transfer function of the EOM and the EOM actuator gain. However, we were unsuccessful due to problems with impedence matching.

What we did notice was that the RF summing box, which takes the TTFFS: EOM HV signal and the 35.5MHz RF signal as inputs and outputs to the EOM, that we are using for the current setup is different from the RF summing box that was designed to work with our setup.

Here is the current RF summing box:

119.JPG

Here is the RF summing box schematic that we are supposed to be using for the current setup:

D040469-B.pdf

If we modeled the signal going from the HV Actuator input to PC out on the RF summing box designed for this experiment to be:

IMG_0086.JPG

we expect the impedance to be:

RF_Summing_Box_Notch_Impedence.jpg

where the notch is designed to prevent signals with frequencies near 35.5MHz from going into the HV actuator input and interfering with the TTFSS.

We measured the transfer function of the summing box from the HV Actuator input to PC out:

Measured_RF_Summing.jpg

As you can see, the notch is at 21.5MHz, whereas the RF signal is at 35.5MHz. This is problematic because the RF signal can go into the HV actuator input and interfere with the TTFSS. Such a phenomenon may explain why the whole FSS hasn't performed as expected. We now need to modify the RF summing box so that the notch will be at 34.4MHz. We need to also be careful with impedance matching.

Attachment 3: Measured_RF_Summing.jpg
Measured_RF_Summing.jpg
  626   Thu Jul 14 16:14:09 2011 RaphaelDailyProgressFSSSumming Box Calculations

Tara, Frank, and I were unsure of the values that we read off the summing box board in the PSL lab.

IMG_1771.JPG

We took the transfer of the current box from the servo input to the PC output and then used LISO to modify the values that we read off the board. I plotted the physical transfer function versus the LISO plot using the parameters I think we should have.

ServoInputToPCOutputSummingBoxTF.jpg

We see a notch at 21.5MHz, which is problematic because we are using a 35.5MHz local oscillator RF input. Thus, the RF input will not be attenuated by the filter in front of the Servo input, so the RF signal will go into the servo. This is not what we want, so we need to redesign the box so that the notch is at 35.5MHz

So I believe the current summing box looks something like this:

IMG_0089.JPG

The LISO file is attached: sumbox1.fil

I then plotted the transfer function from RF in to PC out and from RF in to HV in using LISO:

CurrentSummingBoxRFtoPCTF.jpg

We see resonance at around 21.5MHz

CurrentSummingBoxRFtoHVTF.jpg

We see attenuation at around 21.5MHz. I'm not exactly sure why the box is designed to have resonance at a slightly high frequency.

I also plotted the impedance as seen from the RF input, which also shows resonance to be at 21.5 MHz.

CurrentSummingBoxRFImpedance.jpg

Perhaps measuring these transfer functions would let me be more sure of the values I picked for the LISO model.

We must redesign the box so that the filter in front of the servo input has a notch at 35.5 MHz and the path from RF in to PC out resonates at 35.5MHz. I propose the following schematic:

IMG_0090.JPG

The LISO model for this schematic is attached: sumbox2.fil

I plotted the transfer function going from the servo input to PC out. The notch frequency is at 35.8MHz.

NewSummingBoxHVtoPCTF.jpg

I also plotted the transfer function from RF in to PC out. We have resonance at about 35.8MHz.

NewSummingBoxRFtoPCTF.jpg

 We also plotted the transfer function going from RF in to servo in.

NewSummingBoxRFtoHVTF.jpg

And then I plotted the impedance as seen from RF in. Unfortunately, it is not impedance matched to 50 ohms at 35.5MHz. I couldn't figure out a way to do it and Frank suggested that I just move on and that anything that I do would be better than what we currently have.

NewSummingBoxRFImpedance.jpg

  627   Fri Jul 15 18:37:51 2011 RaphaelDailyProgressFSSPhotodiode modification progress

Currently, I am working on modifying a photodiode so that will be used to lock the laser to the reference cavity. We want to modify it so that it resonates at 35.5MHz and so that it is optimized for a low powered laser (~100microWatts). Here is the schematic for the photodiode circuit.

D980454-00.pdf

Using LISO, I modeled the RF part of the circuit using the values they suggested in the table to make it resonant at 35.5MHz. Here is the file that I used to plot the RF part

pd.fil

And here is the obtained transfer function.

PDRF.jpg

Frank is unsatisfied with the resonance at 35.5 MHz.

We then try to optimize the circuit for a smaller laser power. We want to adjust the resistors connected to U1 to change the gain at that stage. I plotted on LISO the transfer function of that single op amp for various amplifications to see if 35.5MHz would fall in the nonlinear region of the op amp if we increased the gain.

PDOpAmpGainStability.jpg

I found that we can't really increase the gain much without it becoming nonlinear. Frank said it wasn't worth trying.

I plan to adjust the DC path for low power on Monday.

So the plan is to test the functionality of the PD at it's current state, which is to resonate at 54MHz. Once we determine that the PD is working properly, we will adjust it to resonate at 35.5MHz, and see how else we can optimize it from there.

Today, we just did a smoke test of the PD and everything seems ok so far.

P.S. I soldered for the first time today. I made my first cable today, and it only took my an entire afternoon! :p

 

  628   Mon Jul 18 02:20:01 2011 taraDailyProgressopticoptic layout for new fss setup

  The new mode matching for optics in front of the cavities is done.  The rest (for beat measurement) will be finished soon.

A few changes in this layout are:

1) spotsize for AOM is 500 um, as specified by the datasheet.

2) Mirrors behind the AOMs will be changed to R= 2.0 m instead of 0.3 m.

3) Spot size in the 35.5 MHz EOM is ~300um which is good for the model.

4) More mirrors (for steering the beam) for the AOMs are added.

 

I'm a bit worried about using f=57.4mm lenses because they are quite sensitive when we have to move the lenses around, but the space is very limited this time.

I'll let Raphael double check my calculation so he can learn how to do mode matching.

2aom_fss_2011-07-15.png

 

 

  629   Mon Jul 18 11:41:22 2011 ranaDailyProgressopticoptic layout for new fss setup

There's no need to use such a large spot size on either the AOM or the EOM.

When using high power this could be an issue, but you can use a beam radius of more like 100-200 microns for the AOM to get fast response time.

  630   Mon Jul 18 19:58:05 2011 RaphaelDailyProgressFSSPhotodiode modification progress

Today I learned how to solder an inductor onto a board.

We measured the transfer function of the photodiode board from the test input to the RF output. I changed parts on the board to have resonance at 35.5 MHz and a notch at 71 MHz. We changed L2 to 330nH and and L8 to 330nH. Frank also changed parts on the board, but I'm not exactly sure what he did.

Here is the schematic of the board along with the recommended values for a 35.5MHz resonant frequency.

D980454-00.pdf

Even when we changed L2 to 330nH instead of the recommended 81nH, resonance is at around 40MHz. However, the response still looks good and we do indeed see a notch and a resonance. Frank suspects that there is something wrong with the capacitance of the photodiode (might be broken)

  631   Wed Jul 20 15:54:21 2011 taraDailyProgressopticoptic layout for new fss setup

  I edited the layout so that the spots in both AOMs are 200 um. I'll list what optics we might have to buy.

 

Most of the optics are already used on the table. I need to find:

  •  a lens with f = 343.6 m (plcx R =154.5mm)
  •  one more curve mirror with R = 0.3m for the second AOM.
  • aom adaptor plate (need to submit this to the work shop to have it done
  • periscope sets for both ACAV and RCAV (we need 4 in total, but we have only 2 sets)
  • second VCO

The optics on ACAV path have been removed, I left the optics on RCAV path for now because Raphael might want to remeasure EOM TF.

Once the measurement is done, all optics will be removed. We will clean the table, clean the optics before put them back on the table.

 

 

 

2011_07_20_layout.png

  632   Wed Jul 20 23:00:30 2011 FrankDailyProgressopticoptic layout for new fss setup

the lens and mirror are in the ATF, a second VCO is in the left cabinet.

 

 

  633   Thu Jul 21 18:03:20 2011 taraDailyProgressAOMAOM adaptor plate,

I made drawings for aom block and adaptor plate. The assembly is for 3" beam height.

 

The assembly is consisted of two parts. The bottom aluminum part is for mounting new focus 9071 4 axis stage on the table.

The top plastic part, is for mounting an AOM to the 4-axis stage.

 

The bottom part actually is designed for a standard EOM, so the height is  3". With a plastic adaptor plate, it can be used for an AOM as well, so I'll order a few of the alumnium parts.

 

There are 2 designs for AOM adaptor parts, because we have two AOM models. They have different screw size and mounting positions, I 'll order a couple for each design. 

 

Attachment 1: aom_base1.PDF
aom_base1.PDF
Attachment 2: aom_base2.PDF
aom_base2.PDF
Attachment 3: eom_block1.PDF
eom_block1.PDF
Attachment 4: eom_block2.PDF
eom_block2.PDF
  634   Fri Jul 22 00:17:03 2011 RaphaelDailyProgressFSSPhotodiode progress

Today I spent the whole day wondering why I obtained a bad optical transfer function from the photodiode. It turned out that the power supply to the reference diode in the Jenny laser setup was unplugged. Seriously, a certain person told me that the power supply to the reference was already setup, and another certain person told me that the power supply looked like it was on when I asked for help. I was also getting a readout from the DC output. I still don't understand why I got a DC readout if the power supply was turned off!

Frank also replaced the photodiode in the circuit (perhaps unnecessarily).

The transfer function seems to look good. The notch frequency is at 71.5MHz, which is where we want it to be. I need to modify the board so that the resonance is at 35.5MHz. Currently it is around 11MHz. 

I also need to double check my calculations for the quantum efficiency with Frank. We both suspect that the dc voltage out is too low given the responsivity and the measured input power of 0.39mW

Tomorrow, I plan on finishing modifying the photodiode board to be resonant at 35.5MHz and to optimize the dc output for a low powered laser. I also plan on modifying the summing box.

-Raphael

  635   Fri Jul 22 16:31:12 2011 taraDailyProgressAOMAOM adaptor plate

 

 As Frank suggested, I edited the drawing, so that the adaptor plate can accommodate both types of AOM.

 

 

Attachment 1: aom_base.PDF
aom_base.PDF
Attachment 2: eom_block1.PDF
eom_block1.PDF
Attachment 3: eom_block2.PDF
eom_block2.PDF
  636   Fri Jul 22 20:51:13 2011 frank, taraDailyProgressopticcleaning opto mechanical parts

As we removed some optics on the table, we use pressurized air to blow away dust/dirt on the mechanical parts (mount/ post/ lens holder) Optics have not been cleaned yet. We will clean it before we put everything back on the table. The cleaned parts are kept in a plastic box.

 

IMG_1862.JPGIMG_1863.JPG

  638   Mon Jul 25 23:59:56 2011 RaphaelDailyProgressElectronics EquipmentPhotodiode Progress

Hey, so I readjusted the resonant frequency of the photodiode so that resonance would be at ~35.5MHz frequency. Here is the photodiode circuit: D980454-00.pdf. I chose L2 to be 100uH. Here is the optical transfer function:Photodiode_Optical_TF.jpg

Here is the Jenny Laser setup to measure the optical transfer function:

 jennyLaserSetup.png

 

The power coming into the photodiode is 0.41mW. I recalculated the theoretical voltage that we are supposed to get from the quantum efficiency to be 12.1mV. I measured 7.09mV. I calculated the quantum efficiency from the equation QE= responsivity *hc/(lambda*e). From there, I multiplied power by QE to get the photocurrent. The voltage coming into the first detector would just be V=I*R, where R is 20 ohms. The next amplifier multiplies the voltage by about 1.9.

One problem that I know of is that the spot size of the laser hitting the  photodiode isn't the smallest it could be. The spot size hitting the diode could be smaller if I moved the photodiode back. However, I ran into problems trying to do this. After I positioned the photodiode to the point where the spot size was smallest, the beam ended up clipping off the lens whenever I tried to readjust the reflector. It took me forever to adjust the Jenny laser so that the beam was collimated and the laser hit decent parts of the 50/50 beam splitter and reflector. So I think in the near future, I will try to raise the lens by some amount so it doesn't clip.

  640   Tue Jul 26 18:42:16 2011 raphael, taraDailyProgressopticcleaning opto mechanical parts

Today we removed the optics behind the PMC, ACAV, and cleaned the table.

  •   Optic mounts and posts are cleaned by pressurized air, and kept in a plastic box.
  •   Lens and mirrors are kept in optic cases.
  •   ACAV is moved to the end of the table, ion pump is unplugged.
  • Table is cleaned with methanol, but some grease( under acav) is still on it.

NOTE: I just realized that the HEPA filter above the table close to the entrance ( the one that has the laser) is unplugged.

I could not find any available outlet to plug it back yet. We should turn it on soon.

IMG_1867.JPG

 

 

IMG_1866.JPG

  642   Wed Jul 27 00:23:12 2011 RaphaelDailyProgressElectronics EquipmentPhotodiode Progress

I took the transfer function of the photodiode (D980454-00.pdf) from the test input to rf out and found that the resonance and notch where at the same frequency and that the biggest difference is in the gain. Thus, I conclude that it is safe to use the test input to assist someone in readjusting the notch and resonance.

Test_Input_vs_Optical.jpg

We also want to adjust the dc output gain so that a photocurrent of 5mA will produce 10V at DC output. Before the buffer amplifier, the 5mA produces a voltage of 0.1V. So we would like the last amplifier to amplify this voltage by 100. Since it is a non-inverting amplifier, we would like that R16/R11=99. However, I can't think of a combination of resistors in our possession that will produce this. I therefore chose R11=10ohms and R16=1kohms.The board has been modified accordingly.

  643   Thu Jul 28 01:24:40 2011 ranaDailyProgressElectronics EquipmentPhotodiode Progress

This is sort of OK, but check out the 40m elog for the right way to characterize RFPDs. You really need more resolution to resolve the notch and the y-axis should be in units of Ohms.

Also, the test input is a fine rough guide, but its not accurate to get the high frequency characteristics right or the 2f notch with any accuracy.

  644   Thu Jul 28 01:54:21 2011 frank, taraDailyProgressopticcleaning opto mechanical parts

  Today we started working on the layout. There is one mistake in the layout, the mirror behind AOM for REFCAV is too close to the insulation box, so we have to fix the layout.

 

 The oil on the table actually comes from holes on the table. About 5-6 screw holes had lot of oil, so I flushed them with methanol a few times.

IMG_1869.JPG

The HEPA filter is plugged in and turned on. I unplugged one of the monitors and used the outlet for the filter.

 

 The window is measured to be ~ 6 inches in diameter. Thus, the assumption in the design that the centers between two cavities are 3 inches is ok. If necessary, it can go up to 4 or 5 inches.

IMG_1875.JPG

IMG_1874.JPG

The layout is updated. The spot size in both AOMs are adjusted to 220 um.

2011_07_28.png

  645   Thu Jul 28 16:42:26 2011 RaphaelDailyProgressFSSfitted LISO models for EOM and PZT

I fitted the measured transfer functions for the EOM and PZT using LISO. Here are the results:

pzt_measured_vs_fitted.jpg

And here is the LISO file that fitted the data to the measured data: measuredPZT_TF_fit.fil

Likewise, for the PZT actuator

pzt_act_measured_vs_fitted.jpg

measuredPZT_Actuator_TF_fit.fil

Likewise for the EOM

eom_measured_vs_fitted.jpg

measuredEOM_TF_fit.fil

I would really appreciate help in improving these models. I particularly need help improving the EOM fit, as LISO didn't even perform the fit. I just eyeballed it since LISO would do something crazy whenever I told it to fit the parameters. I think the chosen quality factors need to be improved before I can get a decent fit.

  647   Fri Jul 29 00:42:19 2011 frank, taraDailyProgressopticcleaning opto mechanical parts

 We preparing optics for the new layout. To reduce scattering noise, most of the Y1-1064 mirrors we have been using will be replaced by super polished mirrors. 

 

We think Y1-1064 mirrors can cause scattering noise in the setup because the coating surfaces look very milky.

IMG_1880.JPG

fig1: Y1-1064 mirror.

     We have ~ 10-20 super polished mirrors. Some of them are good, some of them are rejected from the site. The good one will be used for periscope/ beat setup.  I tested a couples of the rejected mirrors, but they can reflect both p and s beams with high efficiency. We will ask Peter to find out what is wrong with them.

IMG_1883.JPGIMG_1889.JPG

fig2: Left and right, super polished mirror.

IMG_1886.JPGIMG_1878.JPG

fig3: left, mirrors' case, right, certificate.

 

    I have cleaned about half of the required optics, I think we should be able to lock the first cavity before next Wednesday.

  648   Tue Aug 2 23:58:40 2011 taraDailyProgressopticNew RCAV setup is locked

  RCAV is locked, I have not optimized the mode matching yet, the coupling efficiency is ~ 67%.

This new setup has a double passed AOM. The frequency is shifted by 160 MHz.

I will try to optimize the mode matching tomorrow, then I can check the loop performance that it works as before.

IMG_1911.JPG

 

rcav_2011_08_02.jpg

  649   Wed Aug 3 18:06:32 2011 RaphaelDailyProgressElectronics Equipmentpdrf progress

Hey,

I measured the optical transfer function with more points (again after the floppy didn't properly save the data from yesterday) and I made some shot noise measurements (after finding out that the shot noise measurements I made yesterday weren't as good as I thought they were). Matlab is acting stupid on me right now, so I will post the plots tomorrow.

  651   Thu Aug 4 22:45:10 2011 RaphaelDailyProgressElectronics Equipmentrfpd progress

I measured the shot noise again because the data I got yesterday was kind of garbage. I tried to get an error estimate by taking average of at least 10 measurements for each dc output. However, Frank said that the error wasn't so important. I was happy to know that I could just let the spectrum analyzer do 100 averages and not worry about getting a standard deviation. The data that I got looks pretty decent now. I'll post the graphs after I analyze the data.

  652   Tue Aug 9 00:01:08 2011 RaphaelDailyProgressFSSsimulink model

Hey,

So I'm done-ish with the simulink file that models the loop that locks the laser to the reference cavity. I will ask Frank/Tara for a second opinion to see if I am missing any details or if I need to change something.

  653   Tue Aug 9 00:39:30 2011 taraDailyProgressopticRCAV modematching optimized

 I optimized mode matching for RCAV. The coupling is ~75%.  I also minimized RFAM from the 35.5MHz EOM.

            

          For RCAV mode matching, I moved only two lenses in front of RCAV (R=51.5 and 20.6 mm) to optimize the mode matching.

I have not tried moving other lenses or the mirror behind AOM yet, because I think 75% is enough for now.

 The reflected beam will cause the shot noise level to be higher, but it should not be critical for our current situation.

 

 

  654   Tue Aug 9 21:12:22 2011 taraDailyProgressFSSOpen loop gain TF measurement of the new setup

I checked the RCAV loop performance with the new layout (double pass AOM + semi fixed RF summing box). The TF looks better, but it seems that the gain is still not enough. The suppressed laser noise (measured at mixer out) is still higher than the coating thermal noise .

          Since the cavity can be locked, I checked the loop performance of the new setup. A few changes in this setup are

  •  double passed AOM
  •  RF summing box is modified so that it has a notch at 35.5 MHz for high voltage input for feedback (previous value was 21.5 Mhz) but the impedance as seen from 35.5 MHz LO input is ~5-10 ohm instead of 50.  

==Setup==

  I kept most of the setup similar to what we have used before. The power input = 1mW, RF amplitude adjust = 10, RF Phase adjustment = 5.06V (so that the error signal appears symmetric), with phase flip = 180. The phase flip for FAST actuator on the TTFSS is at (-). Common/Fast gain =544/773.

The measurement follows the procedure in this entry:PSL 592.

==Result==

OLGTF.png

fig1: Transfer function of RCAV loop. UGF is at 33kHz. (this will be compared with the calculated TF later)

The result looks much better compared to the previous measurement entry:PSL 594. It is very likely that the modified RF summing box which now has a notch at 35.5 MHz improves the loop performance. The RF summing box does not have 50 ohm impedence match for RF input yet. 

 

       With the current gain setup, the suppressed laser noise is still higher than coating thermal noise (this is bad because to be able to measure the coating noise, the noise of the laser must be lower than that of the coating). The suppressed laser noise can be measured at mixer out. Use the error signal slope (0.675 MHz/V) to convert to frequency noise.  By increasing more gain (common/fast), we can suppress more laser noise but oscillation occurs. I'm not sure yet what causes the oscillation. The phase margin seems to be enough, so we may have to investigate more.

 suppressed_noise.png

fig2: Lasers' suppressed noise measured from mixer out (red) is still higher than coating noise level. The beat plot was taken before I switched the RCAV servo to the new TTFSS

 

   The peak to peak value of the error signal is only ~ 80mV (used to be up to 200mV). If we fix the impedance of the RF summing box so that more power from LO can couple into the EOM, we might get larger error signal.

 

 

  655   Wed Aug 10 20:54:58 2011 taraDailyProgressAOMChanging Crystal Tech AOM to Isomet AOM

[koji, frank, tara]

Today we changed the AOM in RCAV setup from Crystal Tech AOM to Isomet AOM. The beam shape distorted and cause the reduction in cavity visibility from 75% to 63%

 

For RCAV, we will use ISOMET AOM because it will operate at a fixed frequency.  ISOMET AOMs have weird f dependent impedance, while Crystal Tech, see elog PSL 59 , so we don't want to use it for keeping laser locked to a cavity.

As the beam radius is quite large ~350 um for the ISOMET, the spot shape becomes more elliptic causing the visibility to reduce.

 I will check if I can find an easy solution for mode matching to make the waist smaller at the AOM (~150um in radius) or not. If this is not possible, I'll see how shot noise level will increase due to residue reflected light on RFPD.

  656   Thu Aug 11 15:18:28 2011 RaphaelDailyProgressElectronics Equipmentshot noise characterization

Here is the characterization of the shot noise.

shotnoise2.jpg

I ended up with two different answers for the shot noise intercept. When I fitted it to Z(2e(i_{dc}+i_{int})^{1/2}, where Z is the transimpedance, i_dc is the measured data, and i_int is the shot noise intercept, I got 0.27mA.  When I fit two lines for the input referred noise level and the shot noise, the shot noise intercept is at 0.17mA.

Also, everytime I get more data, I end up with a different answer :(

While I'm at it, I will post the transfer function of the PD.

PDOTF.jpg

  658   Mon Aug 15 23:13:53 2011 taraDailyProgressopticRCAV modematching optimized

 I recalculated the mode matching so that the spot radius in AOMs is 100 um. Now the visibility of RCAV is 90%.

     From the previous mode matching calculation, the spot radius in AOM is 220 um. This was too large for ISOMET AOM and caused beam distortion. The AOM was designed for much smaller spot radius (50 - 110 um). So I recalculated to make the spot radius inside the AOM to be 100 um. This spotsize is small enough for ISOMET and not too small for Crystal Tech AOM.

Rise time is 35 ns (28.5MHz) for 100 um radius in ISOMET AOM, diffraction eff ~80%. This should be sufficient for our less than 1MHz bandwidth loop.

 

For the new layout, I have to remove the Faraday isolator behind the EOM for another lens. I'll try to intall it back later.

 

 

 

Attachment 1: 2011_08_15.png
2011_08_15.png
  659   Wed Aug 17 20:41:13 2011 taraDailyProgressopticACAV path is up

I put most optics on ACAV path. I have not tried to lock the cavity yet. I'll install ACAV RFPD next.

layout_2011_08_17.jpg

 

 

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