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ID Date Authordown Type Category Subject
  365   Sat Sep 18 01:12:03 2010 taraSummaryPMCLISO model of the PMC servo

I started work on a LISO model of the PMC servo - it does not yet agree with reality.

Yesterday, I measured the open loop gain (OLG) of the PMC loop.

It consists of two parts, which are the PMC servo's OLG and the rest, i.e. OLG of photodiode, PMC, PZT actuator.

Knowing each part's OLG is useful for modification.

Since we are going to do most modification on the PMC's servo, we want to know what is its TF. 

 

This is where LISO comes in. I use it to simulate the TF of the PMC servo.

I don't know how to model AD602, because it is not actually an opamp and therefore not in the LISO opamp library.

The datasheet says it has -3db at 35MHz. Thus, for our region of interest, it probably has a flat response. It is just an

adjustable gain amplifier.

 

I'm not sure how to use LISO to calculate poles and zeros of my model yet. I'm reading the manual.

This simulation will be compared with the measurement.

Once we verify that all the parts behave the way they should, we can think about the modification.

We want to modify the TF because even though we maximize the gain slider, the system is still stable

( no sign of oscillaltion from too much gain.) It means we can still optimize our TF for better stability.


 Lastly, knowing the servo's OLG and the whole loop OLG,

we can compute what is the OLG of the rest of the system by simple subtraction.

 

I hope the quality of this elog entry is improved, however slightly it might be. It has motivation why I do what I do, details, and people who want to reproduce my work should be able to follow it.

Thanks Koji for a useful discussion on how to elog properly.

 

The attached plot shows modeled transfer function of the PMC Servo card + PZT capacitance.

Components' names in LISO code are taken from the schematic

  366   Mon Sep 20 13:53:12 2010 taraNotesPMCcapacitance of PMC's PZT is measured

I measured the capacitance of PMC's PZT to be 0.23 uF.

The PZT actuator attached to PMC middle mirror is used to change the total length of the PMC cavity.

It has internal capacitance which effects the TF of the system. The value is used in LISO model to calculate the PMC loop TF.

 

To measure that, I removed high voltage output from PMC card from the PZT connector on the table,

discharged the PZT with 50 ohm resistor for a minute. Then I used an L-C meter to measure the capacitance.

  367   Mon Sep 20 19:48:13 2010 taraDailyProgressPMCPMC servo TF is fitted by LISO

I fitted the TF of PMC servo card simulated from LISO, and found poles at 2.01 Hz, 59.7 Hz, 13.4 MHz, and zero at 479.4 Hz.

 From last week, I used LISO to model the TF but it did not provide the values of poles and zeroes.

Thanks Koji who taught me how to use LISO fitting feature. Now I can find poles and zeroes of the simulation.

 

The frequency range spans from 1 Hz -  1 MHz, covering out region of interest (~1 Hz up to a few hundred kHz).

  For higher frequency, I couldn't make it work yet.

pole 2.0139158941  ### fitted (name = pole0)
zero 479.4339553158  ### fitted (name = zero0)
pole 59.7091341574k  ### fitted (name = pole1)

factor 9.5703858437

pole 13.4116105660M 99.7575256877m  

 

The plot shows simulated data and fit data, we have a nice fit that we can hardly see red and green plots.

 The code for simulating TF is pmc.fil, which can be found on previous entry, the code for fitting, tffit.fil, is attached below.

 

  368   Mon Sep 20 20:57:40 2010 taraDailyProgressPMCa part of PMC servo is verified

 

I measured PMC's TF between MIXER OUT and PCMON, the measurement agrees well with LISO model.

 

The source is 10mV swept sine. It is sent through FP2. MIXER OUT and PCMON are connected to chA and chB respectively.

The measurement was done twice, with loop closed (PMC is locked to the laser )and loop opened (PMC's enable switch is off.)

The data from LISO model is added by 30dB to match the gain slider.

 

The result from closed loop looks weird at low frequency, I'm not sure why so I'm reading Application note 243 to find out about coherence.

 

I did this to make sure that the schematic and the board are matching, all parts in the schematic are labeled correctly.

Please ignore fig1, I did not have phase plot in it.

  370   Tue Sep 21 19:24:37 2010 taraNotesPMCPMC's FWHM

Summary: The PMC's FWHM was measured to be 3.8 MHz.


- Motivation

We are characterizing the PMC loop TF. We needed to measure the cavity's FWHM in order to know the cutoff frequency of the cavity.
This number will be used in the Simulink model to simulate the TF of PMC.

- Method

The length of the PZT was scanned, while the PDH error signal was also recorded.

The time span between the maximum and minimum peaks of the error signal was measured in order to obtain the width of the cavity resonance. The time-to-frequency conversion [second to Hz] will give us the FWHM. The conversion between the time and the frequency was obtained by looking at the zero crossings of the error signal which are separated by the modulation frequency.

- Measurement setup

  • A function generator provides a triangular waveform at 200 Hz, 10V pk-pk, which is split by a T connector. One goes to an oscilloscope for trigger, another goes to EXT DC channel on PMC card.
     
  • The signal going to EXT DC ch is used for scanning the PMC.
     
  • The sideband is 21.5 MHz away from the carrier.
     
  • Another ch on the oscilloscope is connected to MIX OUT ch on PMC card.
    This measures the error signal after the mixer.

- Result

  • The time span between the carrier and the sideband: 768 us.

  • The sideband frequency: 21.5 MHz

  • ==> the conversion factor: 21.5 MHz/ 768 us = 28 GHz/s

 

  • The time span between pk-pk of the error signal at the carrier resonance: 136 us
     
  • ==> The FWHM is 136 us x 27.5 GHz/sec = 3.8 MHz

- Discussion

1) The cavity pole frequency is obtained from the measured FWHM. It is FWHM/2 = 2 MHz.
FSR is  c/Lroundtrip = c/(0.42m) = 714 MHz.
Thus we can compute the finesse F = FSR/FWHM = 188.

2) Another way to measure the FWHM is by measuring the transmission peak of the transmitted light while scanning the cavity. I’ll try this and see if two results agree.

Many thanks to Koji for useful discussion on the measurement and how to improve elog quality.

  371   Mon Oct 4 21:05:21 2010 taraDailyProgressPMCPMC open loop TF

 I checked the PSL setup today, and the PMC gain setup has to be changed from 30 dB (maximum on gain slider) to 22.5 for best transmission signal.

 

From the previous  setup, see quote below, the maximum gain of 30 dB on the PMC gain slider was not high enough,

This means that even though the gain is set to maximum, the signal from the transmitted light does not oscillate.

But today it did oscillate, and I had to reduce the gain to 22.5 dB. When I checked the PMC gain, I turned off the FSS loop

to make sure that the FSS loop won't actuate on the NPRO, and the signal are purely from PMC loop.


There are no other changes of parameters. The power input is 30.7 mW, RF power, phase shift are the same

as before.



After adjusting the PMC's gain, I also roughly adjust FSS's loop gain. I haven't optimized it yet, just determined it by looking

at C3:PSL-RCAV_RCTRANSPD signal on the oscilloscope.

common gain is changed from 5 to 8 dB

Fast gain is changed from 8.5 to 13 dB.


I saved change the values for PMC gain, common gain and fast gain in STARTUP.CMD file.

I don't know what causes the gain changes here, I will check the TF of PMC loop again. If it is real, it means

we might not have to modify the PDH servo card.

NOTE: the slowDC for locking both cavities is ~ -0.103 V

 

Quote:

Today I measured open loop transfer function of PMC loop.

The measurement has two parts.

First the swept sine signal is sent to FP2 test point, TP4 is connected to A, TP3 is connected to B,

the magnitude, B/A, gives us [C][D][E] .

For the second part, the swept sine is sent to ext DC channel,

TP3 is connected to A, and TP4 is connected to B.

this is the TF of [F][A][B]

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

 [A]--<FP2>-----[B]-----{TP4} ------[C]-----[D]---<Ext DC> ----[E] ---- {TP3}----[F]-----> back to [A]

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

 

 

 

The magnitudes and phase from both measurements are added up

to get the whole open loop TF of PMC loop [A][B][C][D][E][F].

UGF is ~1k Hz.

PMC setup

Gain 30dB (mzx)

LO PWR 0.585

Power input 30.9 mW

PMC_PHCON 2.5 + 180 flip

PMC_RFADJ 4.0

I'll verify that the schematic matches up with the real circuit we are using.

 

  372   Mon Oct 25 21:46:26 2010 taraNotesNoiseBudgetthermal expansion noise due to RIN
I write up the calculation for thermo elastic noise due to RIN. The result from pure thermo elastic noise is 0.012 Hz at 10Hz.

We are concerned with extra noise due to absorption from RIN in the cavity. Because, currently,
it seems to be the limiting noise source in our experiment.
This extra noise comes in two forms, thermo-elastic (TE) and thermo-refractive (TR), and they
are supposed to cancel each other, although not entirely.

As a starting point, I calculate the effect from TE only.

I use a half-infinite model, with coating thickness d ~4.4 microns.
I treat the multiple-layer coatings as a single layer coating with average thermal properties
between Ta2O5 and SiO2.

*I use the results from this paper,
<http://prd.aps.org/abstract/PRD/v78/i10/e102003>,
to calculate thermal expansion coefficients of thin film, and the average
thermal properties of the coating.

I analytically calculate the transfer function of the heat response inside the mirror.
Then I use MATLAB code to plot and calculate the effect numerically.

The result turns out to be very small. It can be either,
I might make some mistake in the code, or
TE effect is small, and we are doomed by TR.

Thank you Greg Ogin for his insight about heat equation.

I'll think about TR next.

DYM: We should strive to make the elog a beautiful easily parsable wonder of the interwebs: it automatically dumps the contents of your .m file, and pdf's should be thumbnailed only when there is a reason to thumbnail them (plots: yes, reports / text docs: no)
  374   Wed Oct 27 13:12:12 2010 taraNotesEnvironmentschedule for pipes installation

 At 8:30 am, tomorrow, a workman will come in and install two pipes in the lab.

The pipes will be brazed, so no smoke or dust. 

The working area will only above the fume hood near the entrance.

I'll be in the lab during the installing process.

  375   Thu Oct 28 10:49:53 2010 taraNotesEnvironmentschedule for pipes installation

The pipes are installed. The insulation for the pipes will be installed on Nov 2, Tuesday 8:30am.

The work area will be the same, they just wrap insulation around the pipe, there should not be a lot of dust.

Quote:

 At 8:30 am, tomorrow, a workman will come in and install two pipes in the lab.

The pipes will be brazed, so no smoke or dust. 

The working area will only above the fume hood near the entrance.

I'll be in the lab during the installing process.

 

  376   Mon Nov 1 20:24:40 2010 taraNotesRefCavSpotsize calculation on different cavity length

I calculate the spotsize on mirrors of different cavity length.

 

The setup for FSS experiment can be modified to measure coating thermal noise, (providing we can push down to coating noise limit).

The possible setup will have two cavities with different length. The short one will be more sensitive to coating than the long one.

(I'm not sure yet why should we have two short cavities.)

 

  I used R=0.5 m mirrors in the calculation, since that what we have. the length starts from 1 cm to 20 cm.

Results from shorter length might be added later.

The blue plot shows the spotsize on the mirror for symmetric cavity,

the red and green plots show the spotsize on flat and curve mirror for curve-flat cavity.

 

From [ref1], the minimum spot size that the adiabatic approximation still holds (with 1% accuracy) is

R_heat/ w ~0.01

R_heat is sqrt(k / Cf ), so for SiO2, k = 1.38 W/mK, C = 1.6 J / Km^3 R_heat is ~10 um at 10Hz,

Thus, the minimum spotsize is ~ 1 mm. The cavity length can go down to 5 mm.

 


  377   Tue Nov 2 17:50:33 2010 taraNotesEnvironmentschedule for pipes installation

The insulation work is done. 

Quote:

The pipes are installed. The insulation for the pipes will be installed on Nov 2, Tuesday 8:30am.

The work area will be the same, they just wrap insulation around the pipe, there should not be a lot of dust.

Quote:

 At 8:30 am, tomorrow, a workman will come in and install two pipes in the lab.

The pipes will be brazed, so no smoke or dust. 

The working area will only above the fume hood near the entrance.

I'll be in the lab during the installing process.

 

 

  379   Wed Nov 3 01:22:57 2010 taraNotesPMCTF from PMC servo

I determined the OLG TF of the whole PMC loop, and TFs from servo paths and optical path.

 

We want to modify the PMC servo to optimize the PMC loop, so we have to know what are the TFs from part where we can modify,

and where we can't (optical path).

 

The whole TF is measured before, but I remeasured again just to make sure that there won't be any problem from the laser.

How I measure the whole TF is [here].

 

 I measured the OLG TF from the PMC servo.

The results agree well with the LISO model, see fig 1.

The pole (in LISO model)around 100kHz comes from non ideal behavior of PA85.

When I switch to ideal opamp model, the response is flat.

 

Then,

 Optical TF = Whole TF - Servo TF.

The Optical TF won't be modified. It will be used to compute the whole TF after the PMC servo modification. 

The measurement at low frequency does not look nice because the signal was suppressed by the gain.

But the TF around UGF still looks fine to work with.

  381   Wed Nov 3 15:38:05 2010 taraNotesPMCTF from PMC servo

Sorry for the confusion, PZT actuator is included in the optical TF. 

The plot on fig2 below shows the TF of PZT part, offset by 1 dB to match the misnomer optical path TF.

Thus, the real optical TF is rather flat with magnitude~ 1 dB, the phase shift is 180 degree,

 and the modifiable TF (LISO model) is plot on fig1. This plot has not taken the gain from the slider into account yet.

Quote:

Incomprehensible.

Why is the optical TF not (kinda) flat?

Why does the PZT actuator completely ignored?

You need to talk to me tomorrow afternoon when I am in ATF.

Quote:

I determined the OLG TF of the whole PMC loop, and TFs from servo paths and optical path.

 

We want to modify the PMC servo to optimize the PMC loop, so we have to know what are the TFs from part where we can modify,

and where we can't (optical path).

 

The whole TF is measured before, but I remeasured again just to make sure that there won't be any problem from the laser.

How I measure the whole TF is [here].

 

 I measured the OLG TF from the PMC servo. The results agree well with the LISO model, see fig 1.

Then,

 Optical TF = Whole TF - Servo TF.

The Optical TF won't be modified. It will be used to compute the whole TF after the PMC servo modification. 

The measurement at low frequency does not look nice because the signal was suppressed by the gain.

But the TF around UGF still looks fine to work with.

 

 

  383   Thu Nov 4 21:13:32 2010 taraNotesPMCTF from PMC servo

I got the calibration from [here]

1) DC ext channel on PMC servo: 32.82 MHz/ V

The DC gain between DC ext channel and the voltage at PZT is 27.65 dB (x24.13),

so the Actuator gain will be 32.82/24.13 = 1.36 MHz/ V;

 The plot on fig1 is the Transfer function of the PZT actuator in MHz/ Volt.

 

The liso plot, [fig1] offset by 30.5 dB, match the result from the measurement.

This means that the gain from AD602 is 30.5 dB, even though the gain slider says 30dB.

 

Assuming that from DC to 100kHz, the TF from optic is flat.

The OLG TF measurement must equal The TF from servo(From LISO) + gain slider(30.5 dB) + PZT(LISO) + optics(flat offset)

The offset in the plot is 25.5 dB. With the 30.5 dB from gain slider, TF from optics is -5dB flat, with 180 degree phase shift see fig2. [add calibration from Hz -> V] [plot2]

The result from previous entry which gives the optic's TF to be flat at 1 dB is wrong because I did not use the whole TF from the servo

when I compare the model and the measurement, so I missed -6 dB from AD797.

 

 

 

Quote:

What are the units of the vert axes?

Separate the open loop gain into three part:

- Optical Gain, Unit [V/m] or [V/Hz], usually flat or simple low path shape

- Servo Filter Gain, Unit [V/V], various shape

- Actuator Gain, Unit [m/V] or [Hz/V], flat or low path filter like up to kHz~100kHz (depending on the time constant of the RC filter),
mechanical resonances above that freq region, which usually determin the highest UGF.

You can change the servo gain by modifying the circuit.

You can change the optical gain by changing the amount of the light in the cavity / on the PD as well as changing the cavity finesse etc.

You can change the actuator gain by replacing the actuator.

Quote:

Sorry for the confusion, PZT actuator is included in the optical TF. 

The plot on fig2 below shows the TF of PZT part, offset by 1 dB to match the misnomer optical path TF.

Thus, the real optical TF is rather flat with magnitude~ 1 dB, the phase shift is 180 degree,

 and the modifiable TF (LISO model) is plot on fig1. This plot has not taken the gain from the slider into account yet.

Quote:

Incomprehensible.

Why is the optical TF not (kinda) flat?

Why does the PZT actuator completely ignored?

You need to talk to me tomorrow afternoon when I am in ATF.

Quote:

I determined the OLG TF of the whole PMC loop, and TFs from servo paths and optical path.

 

We want to modify the PMC servo to optimize the PMC loop, so we have to know what are the TFs from part where we can modify,

and where we can't (optical path).

 

The whole TF is measured before, but I remeasured again just to make sure that there won't be any problem from the laser.

How I measure the whole TF is [here].

 

 I measured the OLG TF from the PMC servo. The results agree well with the LISO model, see fig 1.

Then,

 Optical TF = Whole TF - Servo TF.

The Optical TF won't be modified. It will be used to compute the whole TF after the PMC servo modification. 

The measurement at low frequency does not look nice because the signal was suppressed by the gain.

But the TF around UGF still looks fine to work with.

 

 

 

 

  384   Fri Nov 5 21:23:27 2010 taraNotesPMCSlope of error signal from PMC loop vs RF adjustment

I measured the slope of the error signal vs RF voltage, the result is plotted below

 To increase the overall gain of the PMC loop, one thing we can do is changing the slope of the error signal.

This will increase the gain on the optical path of the loop.

So I measured the slope of the error signal, this information will allow me to know how much

gain I would get from each RF setting. The slope increases as the RF voltage increases, until V_RF ~ 8 V.

The error signal does not change at all when V_RF on the slider is between 8 to 10 [Max] V, and

there is no saturation in the signal.

 

Note: I use the oscilloscope to measure the slope around the center of the error signal, by

measureing dt and dV to get dV/dt around the center, (this can be converted to dV/dHz by the sideband)

but the result has large deviation, so I measure the pk-pk in stead, and divide that by the

cavity FWHM = 3.8 MHz which corresponds to the peak-peak of the signal. to get the average slope.

It will be lower than the actual value

but I'll keep it for now.

 

  385   Tue Nov 9 15:05:00 2010 taraSummaryPMCTF plot for each stage in PMC loop

I plot the TF from each stage in the PMC loop and plot below.

 

1)  Servo (+ gain slider)[V/V], From the mixer output to the output of PA85. The amplitude can be added upto 30.5 dB by the gain slider setup

 

2) PZT [V/V]. From PA85 V output to V at PZT. This includes the last R in the servo (R44 = 64.3k Ohm) and C_pzt (0.23 uF). 

 

 3) Opt [V/V]. This includes the PMC and the frequency discriminator part up to the signal to the mixer.

The PMC converts V -> Hz [1.36 MHz/V]. The PMC pole is 1.9 MHz, so I

assume that it is flat at the region of interest (1-100kHz). The frequency discriminator convert Hz->V, and assuming flat response for now.

Thus the total unit of this part is [V/V] too. I'll separate this part into PMC and+ RFPD later.

 

 

  386   Tue Nov 9 22:10:01 2010 taraNotesElectronics Equipment 
  387   Wed Nov 10 01:06:40 2010 taraSummaryPMCPMC OLG TF with different RF/gain settings

I measure the OLG TF of the PMC with 3 different RF and gain slider settings. I plot the OLG TF of each setup and identify their UGF.

 

 I increase the RF as a first step to optimize PMC loop w/o modifying the circuit. This will increase the TF of the optical path.

The setting are

      

set RF V Gain slider (read out/acutal) UGF [Hz]
b 5.6 15 / 17.5 775
c 5.7 11 / 11.5 520
d 5.7 13 / 14 630

 

 

 

First I  adjust the RF power to reach where I can adjust the stability by changing the gain slider.

RF V above (6 or 7) the gain is too large, even with smallest gain slider, the signal is not stable and the PMC_RCTRANSPD drops from maximum.

So RF V ends up around 5.5 V. this makes the gain slider sit around 10 -15 where I obtain maximum stability. 

The gain slider should not to be set too low because of stability problem. The voltage supply for the opamp should be > +10V.

The gain slider setups are chosen to obtain the maximum stability and maximum power out put (PMC_RCTRANSPD.)

\

c and d have the same RF V, I change the gain to see if there would be any significant change in the performance, and

the data will be used for RF V calibration (how much gain we got from RF V adj).

 

Now we have some room to increase the gain once we lower the power, but

I have to understand why increasing the gain slider makes signal unstable.

The phase margin seems to be ok.  It might be the slope of the TF at UGF that causes instability.

  388   Wed Nov 10 20:04:13 2010 taraSummaryPMCoptical gain vs Vrf for side bands

 I checked that the optical gain in PMC loop increases as the power in the sideband increases. The result is 10.7 dB/V.

 

This measurement is for checking how much gain (in optical path) will we get from changing power in the side bands.

The excitation is sent to EXT DC channel on PMC. Reference signal is at HV mon, response is picked up at Mix mon.

This TF includes PZT and OPT paths, PZT TF should remain the same independent from the side band power.

 

I vary the RF voltage, and adjust the gain slider for maximum stability.  The gain setup should not matter

in the TF part we are measuring as long as the loop is stable.

 

I measured the gain at 3 different frequencies, 290.8 Hz, 1.035 kHz, 5.09 kHz where the TF look reasonable and smooth.

(The loop UGF is ~ 500-900 Hz, Thus the data at 1k and 5 kHz are nicer than that of 290 Hz)

 the slopes from each fit are

 

290 Hz 10.3 dB/V
1.035 kHz 10.72 dB/V
5.09 kHz 10.84 dB/V

 

The results are fairly linear in our region (RF between 4.8 to 5.9 V). The gain slider for this voltage range is between 13 - 20 dB.

At higher RF voltage, PMC_RCTRANSPD starts to drop significantly.

At lower RF voltage, the gain is too low.

 

This means we can increase the gain in OPT TF up to 10 dB by adjusting RF voltage (increase side band power)

  389   Wed Nov 10 22:36:59 2010 taraSummaryPMCcurrent PMC's OLG TF

The current plot for PMC's OLG TF is plotted below. The RF V is 6V, Gain slider is 14 dB.

 

The UGF is 820 Hz with phase margin (PM) = 180 - 53 = 127 degree.

 

At higher gain slider setup, the system starts to oscillate. One possible cause is the peak near 10^4 Hz which

might be the PZT's resonance frequency.

Without the notch the total gain we can increase will be limited by the peak.

I'll make a notch to damp it down.

The current spec will be ~20dB notch at 12.5 kHz, FWHM ~1kHz

 

From current setup, the optical TF should be + 16.5 dB flat, and the gain added by the gain slider is +14 dB.

previous setup we have opt TF = -5 dB and +30 dB from gain slider.

So we have improved the overall gain by ~5 dB and to UGF increased from 530 to 830 Hz.

 

  390   Thu Nov 11 14:52:06 2010 taraNotesElectronics EquipmentSR560

Three newly repaired SR560s have arrived. I put two in EE lab, and one in PSL.

 

  391   Thu Nov 11 22:50:14 2010 taraDailyProgressBEATbeat noise measurement

I measured the beat note signal from two different setup (f modulation) and plot the result below

 

We want to see where our beat signal is, and compare it to the noise budget, and improve the sensitivity.

I'm using the same noise budget for now, because

my noise model from RIN has not been finished yet. I'll try to finish it soon.

 

For beat measurement, I measured the feedback signal of the PLL loop,

since the UGF of the loop, which is 53 kHz, covers the region of our interest.

 

I used two frequency span on marconi, 20 kHz and 100 kHz.

I checked the calibration for each frequency spans which are 14.31 kHz/V and 70.8 kHz/V respectively.

The results are the same at low f to ~ 200 Hz, at higher f, 20 kHz span(brown line) has better sensitivity.

 

*b100 and b20 in the data.mat files are the result of beat at fmod 100kHz and 20 kHz in the format of

frequency, V/rtHz , f / rtHz

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

Setting

 

PMC

power input: 16mW

V_RF: 5.7 V

Phase ADj: 2.23 V

Gain: 13 dB

 

RCAV

power input: 2.86 mW

V_RF: 10

Phase ADJ: 4.2 V

Common gain: 18.9 dB

Fast Gain: 20 dB

 

ACAV

power input: 2.0 mW

  392   Mon Nov 15 18:05:34 2010 taraDailyProgressBEATcoherence <fbeat|RCAV>, <fbeat|ACAV>

 I realigned the beam to PMC, ACAV, RCAV, optimized gain, and find a significant coherence between ACAV_RCTRASPD and RCAV_TRANSPD.

 

I haven't re-aligned the beam to each cavities for awhile, and the alignment was quite bad.

PMC_RCTRANSPD: 9.4 - > 10.9 V

RCAV_RCTRANSPD: 1.7 ->2.07 V

ACAV_RCTRANSPD: 0.8 -> 1.3 V

So I need to optimize the gain setup again, see detail below. 

I measured beat signal and coherence before and after realigning. 

For coherence, I see nothing significant except <beat | ACAV>, see fig1, so I did not save the rest of the measurement.

After I realigned the beam, there is a big coherence between ACAV andRCAV see fig 2.

the coherence between PMC and RCAV follow the same trait as that of PMC and ACAV, but slightly less, so I show only PMC and ACAV.

However,  the beat note before and after realigning the beam are still the same, see fig3.

 

I'll add RIN from  RCAV/ACAV/PMC .

 

 

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

gain setup

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

PMC: Gain 16

         RF_ADJ 5.7 V

 

FSS: Common gain 21.5

         FAST gain 21.2

        RF_ADJ  10

These values allow maximum Transmission power and stable lock (I checked this by unlock and lock the cavity and see if the signal is stable after loss lock)

  394   Tue Nov 16 03:36:55 2010 taraDailyProgressNoiseBudgetPreparation to suppress intensity noise.

I modulate the laser power @ 100Hz and measure RIN from PMC,RCAV,ACAV trans and beat note at 100 Hz, t

and then find The conversion dp/df for RIN to frequency noise to be 1.01 MHz/W. I also measure the TF from the power adj to PMC TRANSPD.

 

High coherence between RCAV and ACAV trans seems suspicious, and it  changes with FSS gain setting.

My gain setup might not be optimized. I'll check it tomorrow.

 

I use a function generator to send sine wave signal at 100 Hz, 0.2 Vpk-pk to modulate the laser power.

The peaks at100 Hz for RIN from PMC/RCAV/ACAV  and fbeat are

                Vrm/rtHz at 100 Hz [V]          DC [V]             RIN 100Hz (Vrms/DC) [1/rt Hz]

 PMC          12.49 e-3                             1.5                  8.327 e-3

ACAV          3.27 e-3                              0.313            10.45 e-3

RCAV        11.57 e-3                              0.59              19.61 e-3 

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

 

 fbeat         984 e-3      with 70.8 kHz/V covertion ->     70Hz/rtHz 

 

The coherence between beat note and RCAV/ACAV rin is ~1. So if we assume that the mechanism

that converts RIN to noise in beat note are the same in both cavities, we have

 

fnoise ^2 = (RIN_AC x Pin_AC x conversion)^2 + (RIN_RC x Pin_RC x conversion)^2 .

(This might not be accurate, since the peak from f beat is not that relatively high)

 

 

Pin for Acav and Rcav are 2.3 and 3.3 mW.

This gives conversion = 1.01 MHZ / V @ 100HZ

This conversion will be applied to RIN level to see how much it would be convert to fbeat noise.

 (assuming the conversion is flat at all freq, we will find this conversion at different frequencies later)

 

 TF from  power mod input to PMC trans readout is plotted below. The magnitude at 100 Hz is -17dB

 The source from SR785 is split by a T, one goes to ref channel A, another is sent to laser power modulation.

Response B is

 

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

NOTE: (from the yesterday)

 DC level for

ACAV_RCTRANSPD  = 285 mV 

RCAV_RCTRANSPD = 612 mV

PMC_RCTRANSPD = 1.48 V

Quote:

We need to see a plot of the coherence between the PMC trans and Rcav trans and the Acav trans. Also the RIN of all 3 plotted on the same plot.

Then, inject a small line into the current adjust at 100 Hz and measure the height of the peak on all three trans and the beat signal. Use this peak to scale them all and put them on the same plot in physical units.

Then measure the TF from the current adjust actuator to the PMC trans and close an AC coupled feedback loop on it.

 

  395   Tue Nov 16 21:38:24 2010 taraNotesNoiseBudgetnew NoiseBudget

After getting converting factor (RIN-> freq noise) = 1.01 MHz/W. I plot the result below.

 The contribution to frequency noise in beat note from RCAV and ACAV due to RIN is calculated by,

fnoise = RIN x power input x df/dp factor. 

The beat note freq noise is plot on the noise budget.

Our beat frequency noise is not limited by RIN. I'll have to think what would be the current limiting source.

I'll check Megan's report about VCO and Marconi phase noise.

 

 

  398   Thu Nov 18 14:17:45 2010 taraDailyProgressNoiseBudgetNoise from scattering light near the cavity's foam cap opening

The Voltage vs Frequency conversion for VCO is here.

It's not linear over the range. I have to check what is the average value of the VCO during our measurement.

Then, if it's small enough, linear approximation can be used to convert the data from VCO to Hz/rt Hz.

I think It's better to use linear fit because the psd of the voltage does not contain information about the sign of the signal.

Quote:
  • Even after the removal of the Al tip, the scattered light noise looks still exist.
  • Particularly I still could see apparent fringe wrapping when I shook the table or touch the foam construction.
     
  • I still could not reject the possibility of the backscattering towards the PMC.
  • We had the fringe wrapping up to ~200Hz. This corresponds to the motion of the scattered body or the optical
    path for the scattered light by ~100um. Is that possible?
  • The VCO feedback and the beat note PLL feedback seemed to have the same information so far.
  • The lollipops at the trans mission ports are terrible. They are mechanically incorrect.
  • What is the correct conversion between the VCO feedback and the beat note PLL feedback??
    Both are VCO feedback signals but the slope looks different. Need precise investigation.

 

 

 

  400   Thu Nov 18 21:42:33 2010 taraDailyProgressNoiseBudgetNoise from scattering light near the cavity's foam cap opening

I measured the calibration df/dV for VCO to be ~48 kHz/V. Then I convert the Vnoise fro VCO to frequency noise.

 

I injected the signal at 90 Hz and 1 kHz and measure the corresponding peaks from beat noise and VCO.

Then I rescale both data to match the peaks to get the conversion factor.

                               

                        f=90 Hz, pk/floor               f =1kHz pk/floor

VCO                   1.43mV/20uV                   2.95 mV/20 uV

 

BEAT                 2.18mV/ 25uV                     4.11 mV / 25uV

conversion fac  

 beat =  VCO x K  =  1.53                                   1.37

let's use 1.4 for average

Since df/dV for beat is 71kHz/ V, df/dV for VCO is 71/1.4 kHz/V =  51kHz/ V.

The plot below shows RC noise as measured by VCO and beat note.

Quote:

What about the frequency response. Why did we see the different shapes between the spectra even with the coherence ~1.
You can measure the transfer function between those two VCO feedback and should try to explain it.

Is there any transfer function in the AOM VCO, for example?
If I can believe D980401-B.pdf, the VCO freq control path seems to have pole-zero pair at 1.5Hz and 41Hz.

Quote:

The Voltage vs Frequency conversion for VCO is here.

It's not linear over the range. I have to check what is the average value of the VCO during our measurement.

Then, if it's small enough, linear approximation can be used to convert the data from VCO to Hz/rt Hz.

I think It's better to use linear fit because the psd of the voltage does not contain information about the sign of the signal.

Quote:
  • Even after the removal of the Al tip, the scattered light noise looks still exist.
  • Particularly I still could see apparent fringe wrapping when I shook the table or touch the foam construction.
     
  • I still could not reject the possibility of the backscattering towards the PMC.
  • We had the fringe wrapping up to ~200Hz. This corresponds to the motion of the scattered body or the optical
    path for the scattered light by ~100um. Is that possible?
  • The VCO feedback and the beat note PLL feedback seemed to have the same information so far.
  • The lollipops at the trans mission ports are terrible. They are mechanically incorrect.
  • What is the correct conversion between the VCO feedback and the beat note PLL feedback??
    Both are VCO feedback signals but the slope looks different. Need precise investigation.

 

 

 

  402   Fri Nov 19 01:21:53 2010 taraDailyProgressNoiseBudgetNoise from scattering light near the cavity's foam cap opening

Here is the TF of LIGO's VCO. I measured the TF at 3 different RF output voltage levels (C3:PSL-FSS_VCOMODLEVEL) and plotted them together.

 

To measure the TF of the VCO box, I reduced the VCO RF before disconnecting any cables.

Source out from SR785 is split by a splitter. One goes to Ref ch A on SR785, another goes to VCO wide band input.

RF out to AOM is connected to Response ch B on SR785. 

The TF looks bad when I turned the RF output V to 5 which is normally used during the lock. I'm not sure if there will be

reflection of signal or not, so I decide to lower the RF output V.

The magnitude from each measurements on the plot are offset for comparison purpose.

 

As we can see, VCO does not have flat freq response. This should be able to explain the different shape between

Beat note freq and VCO feedback signal.

Quote:

TRANSFER FUNCTION!

Quote:

I measured the calibration df/dV for VCO to be ~48 kHz/V. Then I convert the Vnoise fro VCO to frequency noise.

 

I injected the signal at 90 Hz and 1 kHz and measure the corresponding peaks from beat noise and VCO.

Then I rescale both data to match the peaks to get the conversion factor.

                               

                        f=90 Hz, pk/floor               f =1kHz pk/floor

VCO                   1.43mV/20uV                   2.95 mV/20 uV

 

BEAT                 2.18mV/ 25uV                     4.11 mV / 25uV

conversion fac  

 beat =  VCO x K  =  1.53                                   1.37

let's use 1.4 for average

Since df/dV for beat is 71kHz/ V, df/dV for VCO is 71/1.4 kHz/V =  51kHz/ V.

The plot below shows RC noise as measured by VCO and beat note.

Quote:

What about the frequency response. Why did we see the different shapes between the spectra even with the coherence ~1.
You can measure the transfer function between those two VCO feedback and should try to explain it.

Is there any transfer function in the AOM VCO, for example?
If I can believe D980401-B.pdf, the VCO freq control path seems to have pole-zero pair at 1.5Hz and 41Hz.

Quote:

The Voltage vs Frequency conversion for VCO is here.

It's not linear over the range. I have to check what is the average value of the VCO during our measurement.

Then, if it's small enough, linear approximation can be used to convert the data from VCO to Hz/rt Hz.

I think It's better to use linear fit because the psd of the voltage does not contain information about the sign of the signal.

Quote:
  • Even after the removal of the Al tip, the scattered light noise looks still exist.
  • Particularly I still could see apparent fringe wrapping when I shook the table or touch the foam construction.
     
  • I still could not reject the possibility of the backscattering towards the PMC.
  • We had the fringe wrapping up to ~200Hz. This corresponds to the motion of the scattered body or the optical
    path for the scattered light by ~100um. Is that possible?
  • The VCO feedback and the beat note PLL feedback seemed to have the same information so far.
  • The lollipops at the trans mission ports are terrible. They are mechanically incorrect.
  • What is the correct conversion between the VCO feedback and the beat note PLL feedback??
    Both are VCO feedback signals but the slope looks different. Need precise investigation.

 

 

 

 

 

  403   Fri Nov 19 01:24:03 2010 taraDailyProgressopticchanging Faraday Isolator's mount/ TF from ACAV path

I switched the post to V-block for Faraday Isolator mount, for better stability, and adjusted the Faraday isolator to minimize back reflection to the laser.

I also measure the TF from ACAV path,  The UGF is ~65 kHz.

 

  The faraday isolator was installed on a standard pillar post, so I use a V block to mount it instead.

After adjusting the FI, I remeasured the beat note frequency, and the signal did not change from yesterday measurement.

(no differences between red and green plots)

blue: beat signal before fixing the ACAV opening

red: beat signal after fixing the ACAV opening

green : beat signal after re installing the FI

 

 

ACAV TF: I connect the signal output after the PDH servo box to SR560 A and SR785 B (resp)

                 Then source out from SR785 is connected to SR560 B

                 The output of SR560 is connected to SR785 B (ref) and to the VCO

 

The setup for SR560 is DC coupling for A and B, select A - B, gain 1, no filter.

 

  404   Fri Nov 19 14:28:28 2010 taraDailyProgressBEATTF comparison between PLL feedback and VCO feedback

I measured the TF between VCO feedback and PLL feedback. The result agrees with the TF of the VCO.

 

  The frequency noise measured from PLL feedback and VCO feedback do not have the same shape, even though

the coherence is ~1. There might be some devices that do not have flat frequency response and change the shape

of the frequency noise. 

    So I measure the TF between the feedback signal to VCO (ref, chA) and the feedback signal to Marconi (resp, chB).

The excitation is sent to In test 2 ch on FSS servo input which modulates the frequency of the laser to RCAV.

Then I measure the TF of the LIGO's VCO box.

The shape of the transfer functions agree well except a resonance peak around 100 Hz and a pole at 7 kHz.

 This TF can be used to calibrated the feedback signal to the VCO to real frequency noise between two cavities.

  406   Fri Nov 19 22:35:10 2010 taraDailyProgressNoiseBudgetNoise due to RIN

I measured the RIN of PMC/ACAV/RCAV and noise in beat note when 250mV white noise was injected in the laser current actuator.

The result is note quite reasonable because the noise grows with f^1.5 instad of 1/f  .

The intensity servo is working now and reduce the noise from beat measurement a bit.

 

I did this to find out how RIN couples to frequency noise in beat note measurement, thus I can refine my noise budget.

 

I used SR785 as a source for white noise  @100mV and sent it to OUTPUT ADJ CH on the laser driver. It is the current acutator

which adjusts both intensity and frequency of the laser.

RIN from three cavities change almost at all frequency, but quite alot around 100 Hz -10 kHz, see fig1.

The noise from beat measurement also increases around that frequency range as well, see fig2.

and the coherence between RIN from each cav and the beat measurement are similar, so I plot only the coherence between RCAV_RCTRANSPD and beat note (fig3).

 

The sum of noise is
 

S_beat^2 = S_rin^2 + S_other^2     ----(A) , so

S_beat_with noise^2 = S_rin with noise^2 + s_other^2    -----(B)

 (B) - (A)  will give

 (extra frequency  noise due to RIN)^2 = (RIN)^2 * (conversion factor)

should tell us how RIN couples into S_beat

 

 The residue from white noise injection is plotted on fig4.

The input power to ACAV is 2.2 mW, RCAV is 2.5 mW.

I match the frequency noise residue with RIN by   quadrature sum of RIN * Pin * 2e3 * f^1.5, see fig 5.

This is very unreasonable. We expect noise due to RIN to be 1/f. 

 

 Meanwhile, suppressing intensity noise is still a good idea. I used SR560 to stabilize laser intensity.

The PD used for monitor the laser power is AC coupled to SR560, and the output is fed back

to the laser driver OUTPUT ADJ ch,

Gain setting 2 INV with 30dB attenuator at the output.

Low pass, with a pole at 100 kHz

 

 To my surprise, the noise of the beat note is reduced a bit, despite verey small coherence  between RIN and fbeat

we observed before. I compare with the data from this morning and I measure the beat frequency noise again

right after I finished with the data with intensity servo (2am), see fig 6.

I'll verify the data again by comparing RIN from each cavities, with and with out intensity servo.

 

 

 

 

 

  408   Mon Nov 22 10:32:33 2010 taraDailyProgressNoiseBudgetNoise due to RIN

setting up the gain for intensity servo

1) TF of current actuator and PMC trans PD:

    The TF goes down at 40 dB per decade(1/f2). It seems to be a good idea to have the UGF around 103 where the phase margin will be compensated a bit and the slope will go with 1/f.

     - We want to build a servo that looks like a band pass. Our servos can be separated into three parts

      TF (whole servo) = TF of current actuator + TF of SR560 + TF of our intensity servo box, or
     H(s)   = A(s) * B(s) * C(s), we can write H(s) in polynomial terms. TF of current actuator can be measured and fitted to determine its Laplace Transform.

 TF of SR560 is a band pass. Then we can calculate the TF of our intensity servo and bulid it.

The fit and its parameters of current actuator is plotted below.

The poles and zeroes are

pole 1, simple:  14.7 kHz;   1/(s + f_pole *2pi)

pole 2, complex: 6.39 kHz, Q = 0.227;    w2 / (s2 + w*s/Q + w2)  ;w  = 2 pi *f_pole

pole 3, complex: 7.25 kHz, Q = 0.642;

pole 4, complex: 89.14kHz ,Q = 1.143; (This one is less important, since the expected UGF is ~ 30 kHz)

zero:  390.4 Hz, Q=0.433, (I'm going to treat it as a simple zero)  s - f_zero *2pi

 

The calculation details are in the matlab code, file bandpass.m.

The TF of the servo will be fitted by LISO.

2) CLG of the whole thing

3) PMC trans PD noise spectrum, (before and after the intensity servo)

Quote:

You have to post here the TF between the laser current actuator and the PMC trans PD. Then, you should think about how to make an effective servo using a single SR560 (i.e. what poles).

Then when you hook it up, measure the CLG of the whole thing and see if the PMC trans PD noise spectrum goes down as expected.

 

  409   Wed Nov 24 01:53:10 2010 taraDailyProgressNoiseBudgetIntensity Servo

I modified Dmass' intensity servo, but the servo is not working yet.

 

We want an intensity stabilizing servo, such that the UGF of the whole OLG TF is ~ 30kHz. So I need to build a servo to meet this requirement.

The OLG  TF of the current actuator to PMC trans PD was plotted in the previous entry. The slope around 30kHz goes like 1/f^3

so I need a servo with a slope of f^2 around 30 kHz to compensate, and get the total slope of 1/f for stability.

 

The designed servo consists of double stages of simple one zero- one pole TFs, where the corners frequency are 20 kHz (zero) and 70 kHz (pole) for both stages. 

See fig 1 for (a) simulated TF of the designed ISS servo, (b)TF from current actuator and the servo, and (c) sum of (a) and (b).

If we increase the gain so that the UGF is 25 kHz, PM is 40 degree. and if UGF is 30 kHz, PM is 27 degree.

PM is too small. I might need to modify it later.

The ISS filter I need is fairly simple, see fig 2.

The schematic for the ISS I got from Dmass is here. I modified the top part of the schematic for my use.

After the modification, I measrued the TF of the intensity servo part. The TF looks nonsense. I have to check what did I do wrong.

 

 

 

 

  411   Mon Nov 29 14:34:35 2010 taraNotesNoiseBudgetnoise budget update

I updated the noise budget, see the plot below. LO phase noise and RIN are updated in this version

 The code is also attached below.

1) LO noise

I used Megan's LO phase noise measurement, converted it to freq noise and added it to the nb.

Note that the plot is from 160MHz carrier and 100Hz range.

She has data for 160 MHz, 100Hz range and 160MHz, 10Hz range. There is no measurement at 100kHz range which I used for my measurement.

At larger range the LO noise is higher, and the shape of LO noise between two ranges are different at high frequency (1k and above).

 My beat measurement was taken at 160MHz, 100kHz range. So the actual LO noise in the measurement will be higher,

but should not be more than an order of magnitude. 

I decide to keep it as it is,and that should give us the lower limit of the current LO phase noise.

 

2)RIN

Second, I injected intensity modulation at 100Hz and measure the beat to get the conversion from RIN -> fbeat.

This is not accurate, since the output adjustment that I used for intensity modulation also changes the frequency as well, but

it should give us some limit how intensity couples to fbeat.

Right now we cannot purely modulate the intensity, and AOM will be added to modulate/stabilize the intensity soon.

  412   Tue Nov 30 21:48:13 2010 taraDailyProgressRefCavCavities egienmode

I measured the beat signal and VCO feedback signal to see a peak from cavities' mode of vibration, there are 4 peaks from 1kHz to 20kHz.

 

We expect to see  the cavities' resonance around 10k Hz with high Q, say 10^6. I measured the beat signal and VCO feed back to double check my results.

There are 4 peaks, 5.98 kHz, 10.28 kHz, 12.8 kHz, and 13.85 kHz, that coincide. However, the peaks do not look sharp (Q is too low), and the FWHM

of peaks at 5.98kHz and 13.85kHz might be too broad to be the cavities' mode. So 10.28 and 12.8 seem to be good candidates.

  414   Wed Dec 1 00:19:20 2010 taraDailyProgressLaserTF between Laser Current Actuator and PZT on NPRO

I measured the TF between the current actuator of the laser and PZT on NPRO to see how much the current actuator changes the frequency.

The result, if the current act on the laser mostly adj Freq or Intensity, is yet to be determined

 

The current actuator on the laser driver changes both frequency and intensity of the out going beam.

This experiment aims to learn how much the current actuator drives the frequency of the laser compared to the intensity.

 

1) TF between current actuator (Vact) and NPRO PZT (Vpzt)

The source is split and sent to 1)the current actuator, 2) ref ch A on SR785

The response is picked up at fast mon on FSS loop, the voltage between Vmon is the actual voltage sent to the PZT, see the schematic.

The calibration for NPRO PZT is 3.07 MHz/V

Thus response/ref is chB/chA = Vpzt/Vact. I correct the unit to be Hz/Vact by multiplying the Vpzt by the calibration, 3.07MHz/V. or add the original result (in dB) by 20log(3.07 e6) dB.

 

 

 2)The TF between the current actuator (ref) and PMC trans PD (resp) was plotted in this entry.

The magnitude on the Y axis is Vpmc_transPD / Vactuator. To correct it to RIN, divide Vpmc by DC value of PMC_trans PD [1.37 V] or subtract 20log[1.37] from the result in (dB).

 

3) TF between  pmc trans PD (ref chA) and PZT (resp chB), see fig 2.

   The unit after the measurement is [Vpzt / Vpmc_pd]. To correct it to Hz/RIN, multiply Vpzt by the calibration and divided Vpmc_pd by its DC level,

or add 20log(3.07e6) + 20log(1.37) 

 

 To sum up,

1) TF between current act and PZT tells us how much  frequency changes when we modulate through current actuator,

   2) TF between current act and PMC trans PD tells us how much RIN changes when we modulate through current actuator,

3) TF between PMC trans and PZT tells us what changes more after current act is modulated.

This will be compared with frequency change (calculation) due to RIN-> thermo optic.

Then we can decide if current adj mainly change frequency or intensity.

 

 

  415   Wed Dec 1 22:07:05 2010 taraDailyProgressBEATbeat vs Power Input

I measured the beat signal from 3 different power input levels, the signal goes down with the power mostly at higher frequency.

When HEPA filters above the table are off, the noise also goes down, this might suggest that we still have scattering light problem.

 

 We are hunting for the noise source for our experiment. We want to see if our setup is limited by intensity noise or not

so, we try to change the power level and see the beat signal.

 

 I used 3 levels of power (measured after PMC) to be 10 mW (original value), 20 mW , 2mW, (the settings are listed below.)

The reason that the level at 10 mW decreases from previous beat msmt is that the gain for FSS and AOM loops are optimized

PDH box for AOM loop is also modified (R12, 1k -> 10k), the power into both cavities are adjusted to be about the same.

 The noise level at 10 and 20 mW are almost the same except a hill at 60 kHz on 20mW, and

  the noise from 2mW setup is lower than other settings at high frequency, starting ~ 20kHz.

 

 At 1 mW, I can't lock the PMC (the transmitted beam on the PD is so faint), so I increase to 2 mW to make locking easier.

At 2mW power, the range for feedback signal for PLL loop becomes smaller.

If it exceeds approximately +/- 50 mV [3.5kHz], the lock loses, but I still keep the frequency range at 100kHz.

I tried to increase the gain on SR560 which is a servo for PLL loop, but

the beat signal was worse at high frequency, starting at 2kHz.  Fortunately, the temperature drift at night (~10mV/min)is much smaller than

the drift during the day(~5mV/sec), so I can wait long enough to measure the beat at low frequency, with the gain level of 1, as usual.

 

At 2mW setting, I have the HEPA filter on and off in comparison. The noise level decreases a bit when the fan off, this might suggest that

scattering light is still a problem.

 

Even though the noise from beat measurement change with the power, the amplitude does not go with the power change.

i.e, the noise in the beat note does not go up with the same factor of the power change.

So the setup is not limited by intensity noise.  

 

setting for the measurement ["-" means the same as previous value]

                                      10 mW              20 mW             2mW

PMC: common gain         16 dB                 -                     30dB                       

PMC:RF                            5.7V                 -                       -

FSS:FAST gain               18.5 dB              -                      -

FSS:common                  22 dB               18 dB             20dB   

FSS:RF                           10 V                  -                      -

PDH for AOM:GAIN        4.45 (knob)       3.2                 7.0

Pin RCAV/ACAV            2.1/2.1 mW        4.8/3.9 mW    0.4/0.4 mW

  416   Fri Dec 3 01:34:02 2010 taraDailyProgressBEATFaraday isolator added behind the PMC

I added a Faraday isolator after PMC and 35.5 MHz broadband EOM, now the noise becomes less susceptible to the

input power to the cavity.

 

The isolator is installed between the 35.5 MHz EOM and a lens just before the beam splitter that splits the beam into

ACAV and RCAV path. I measured the beat signal at 10mW (measured in front of the BS, as before).

The beams going into both cavities have the same power level ~0.4 V, see blue plot.

  Then I realized that I had not re-aligned the beams into both cavities. The isolator significantly

alters the beam paths, I aligned the input beam, then measured the beat noise again, at 4mW and 2 mW, see green and red plots.

*I'm very surprised to see no significant difference between before and after alignment.

All new three results are very comparable to 2 mW measurement before the isolator was installed, see pink plot.

However, if we compare 10mW from today and yesterday measurements, there is a significant broad band effect at high frequency.

 

By adding the isolator, we reduce the back reflected power to the PMC, and the laser. The power dependent noise we saw before

might come from this back reflected beam to the laser.  Tomorrow, I'll try adding an EAOM.

 

 

  417   Fri Dec 3 02:00:36 2010 taraNotesElectronics Equipment35.5 MHz pick up

There is a 35 MHz pick up from cables to the crate. Right now there are ACAV and RCAV_RCTRANSPD that cause the pick up.

When I unplug the cable to the crate and just measure directly at the PD output, the signal is fine.

  421   Tue Dec 7 16:15:38 2010 taraNotesBEATPLL OLG TF

I measured the open loop gain TF of the phase lock loop for beat measurement, at the current setup, the UGF is 40 kHz.

setup 

Gain on SR560 is x5 , power input for each cavity is 1mW.

  423   Thu Dec 9 00:20:33 2010 taraDailyProgressElectronics Equipmentswitching ACAV/RCAV RFPDs

 I switched the RFPD between RCAV and ACAV, now the gain for FSS loop is set to 26 dB, but the beat signal does not change.

 

From previous elog entry, RCAV's RFPD does not have a peak at 35.5 MHz, and ACAV's RFPD has a peak ~36 MHz. And the FSS loop did not have enough gain,

so I switched RFPDs between both cavities. Attached pictures below show the error signal from RFPD when the cavities are scanned, before and after I switched them.

 

The power into the cavities are ~ 1mW for both cavities. 

The phase is adjusted to produce the best error signal.

Phase adj were 4.16 then changed to 5.67 V

I also inverted the phase on PDH box for ACAV. 

FSS gain can be reduced from 30(max) to 26 dB. 

 

Then I measured the beat noise, with 100 kHz input range, gain 5. There are no change compared to before.

 

  424   Fri Dec 10 01:14:08 2010 taraSummaryRefCavRCAV/ACAV poles

I analyzed the pole for RCAV and ACAV from 2010_12_06 entry. ACAV has a pole at  54 kHz, RCAV pole is at 38kHz.

 

Cavity pole = FWHM/2. Knowing cavity pole and FSR, we can calculate a cavity's finesse (= FSR/ pole). These values

will be used when we simulate the TF of the system

We amplitude modulated the laser intensity via EAOM, and measured the TF between RCAV_trans_PD and PMC for RCAV pole.

Since PMC's pole is ~2 MHz and ACAV/RCAV poles should be around 35 MHz, PMC won't effect much on our measurement.

 

Cavity is 0.2032 m long -> FSR = c/2L = 738 MHz, then

 

ACAV's Finesse = FSR/FWHM = 6835

RCAV's Finesse =  9710

 

 

  426   Fri Dec 10 18:14:47 2010 taraDailyProgressFoamwiden in-out holes on the foam box

I widen the beam holes on the outer foam box.  Now all the holes are ~ 0.75" in diameter.

 

We are concerned about the scattering on the insulation, so we decided to increase the holes' size.

I used a heat gun to heat up a steel rod to melt the foam panel. 

 

As the insulation box was opened, I checked for scattering light inside the box. I did not see any scattering ligth except

on the ACAV inner insulation opening.

Also, there is a little scattering light at RCAV's insulation cap, so I measured the beat noise when I opened RCAV's inner cap.

There is no significant change, see fig 1. The trace with higher noise was taken when I opened the cap, probably because of too few average.

The scattering on RCAV cap might be negligible for now.

 

  427   Sun Dec 12 22:11:11 2010 taraNotesBEATnb with VCO noise at 10khz input range

I got Marconi noise data from Frank, and plotted it on the noise spectrum to show that we are limited by LO noise at high f.

Note that I use old RIN approximation as Frank suggested that  RIN noise and beat noise have a same feature.

 

I plot marconi noise with 100kHz input range and compare it with the bea noise with 100kHz input range, on fig2.

 

 

  436   Mon Dec 20 14:35:23 2010 taraNotesElectronics Equipmentadjusting polarization for 35.5 MHz EOM

Last week I accidentally changed the polarization of the beam to the 35.5 MHz EOM. So I optimize it again to minimize an RFAM effect.

I used the signal of the transmitted beam behind RCAV on the PD for beat signal. Since the cavity pole is around 37 MHz, I should be able to see the

signal at 35.5 MHz easily. I connected the RF signal from the PD to a spectrum analyzer and adjusted the 1/2 wave plate to  minimize the peak at 35.5 MHz.

However I also notice two peaks at 35.29 and 35.71 MHz (35.5 +/- 0.21 MHz) which are approximately the same size as the 35.5 MHz peak.

I'm not sure where they come from.

 

  437   Mon Dec 20 19:55:11 2010 taraDailyProgressopticrearraging optics for beat measurement

I designed the layout for optics behind the cavities for beat measurement, and calculated the mode matching for the beam.

Since the current optics height for beat is quite high (7 inches), we want to lower it to 3 inches, make it more symmetric, and more compact.

The PD's diameter is 300 mm, so the beam spot on it will be ~50um.

All the lenses I need are prepared.

  440   Thu Dec 23 22:41:28 2010 taraDailyProgressopticrearraging optics for beat measurement

Beat measurement optics' height is changed to 3". I cleaned all optics already, but I couldn't really clean 1/2 and 1/4 wave plates, one of the f =200 mm lens is quite hard to clean.

I'll wait and ask someone before trying to clean again. I cannot lock both cavities at the same time, once I can, I'll align the beam on the PD.

Also ACAV's PD for ACAV_trans_PD is broken. It gives out 11 V regardless of the beam falling on the PD, so I replace it with a PD that is used for NPRO_PWRMON.

 

Quote:

I designed the layout for optics behind the cavities for beat measurement, and calculated the mode matching for the beam.

Since the current optics height for beat is quite high (7 inches), we want to lower it to 3 inches, make it more symmetric, and more compact.

The PD's diameter is 300 mm, so the beam spot on it will be ~50um.

All the lenses I need are prepared.

 

  441   Sun Dec 26 02:42:47 2010 taraDailyProgressopticrearraging optics for beat measurement

Both cavities are locked at the same time. The temperature setting are, RCAV = 34.95, ACAV = 37.2.

I realigned the beam onto the PD to get maximum contrast. I'll readjust the setting back to the original value

and see if the beat noise is improved.

I just notice that one of the beam on the mirror on ACAV's path behind the cavity is almost clipped. I'll readjust it tomorrow.

Quote:

Beat measurement optics' height is changed to 3". I cleaned all optics already, but I couldn't really clean 1/2 and 1/4 wave plates, one of the f =200 mm lens is quite hard to clean.

I'll wait and ask someone before trying to clean again. I cannot lock both cavities at the same time, once I can, I'll align the beam on the PD.

Also ACAV's PD for ACAV_trans_PD is broken. It gives out 11 V regardless of the beam falling on the PD, so I replace it with a PD that is used for NPRO_PWRMON.

 

Quote:

I designed the layout for optics behind the cavities for beat measurement, and calculated the mode matching for the beam.

Since the current optics height for beat is quite high (7 inches), we want to lower it to 3 inches, make it more symmetric, and more compact.

The PD's diameter is 300 mm, so the beam spot on it will be ~50um.

All the lenses I need are prepared.

 

 

  442   Mon Dec 27 02:51:33 2010 taraDailyProgressopticrearraging optics for beat measurement

I measured the beat noise after I realigned all optics behind the cavities. The power has not been reduced to 1 mW yet.

This is just a quick measurement to see where we stand (red curve). The noise gets worse compared to the best measurement (green) before the optics behind the

cavities are rearranged, but the mechanical peaks around 1kHz are suppressed significantly.

  444   Mon Dec 27 23:11:32 2010 taraDailyProgressNoiseBudgetRIN coupling to frequency noise

I modulated laser power to see how it causes frequency noise in the cavity, no sign of RIN induced frequency noise has been seen yet.

 

We have been thinking about how RIN will cause random absorption on the mirror surface and turn into thermo elastic/ thermo refractive noise in the cavity.

So we try to observe it, by modulating the laser power with an EAOM. I used white noise, generated by SR785 at 2.5V, as a source to modulate the intensity.

  The power input to both cavities are 3mW. This corresponds to ~15mW total power before the BS (3 to RCAV, 12 to ACAV path).

 

1)The intensity changes as we can see from ACAV_trans_PD (fig1).

The lower curve is the power fluctuation of the laser without the modulation. The upper curve is the power fluctuation with the added white noise.

So we are sure that intensity does change from our modulation.

 

2)Then I measured the feedback signal to PZT and to AOM, with and without external white noise.  These signals represent the frequency change of the laser.

However no change observed in both signals. I plotted only the feedback signal to VCO(to AOM), since this signal should be more sensitive to change than feedback to PZT, as the

beam to ACAV path is stabilized to the RCAV already.

 

3) There is no observed effect from RIN on beat measurement yet, see fig 3.

 

The laser power output is ~54 mW, so the maximum power we can get to each cavity equally is ~10 mW.

I will try to increase more power to the cavity, as the effect should be proportional to the input power

Actually, we don't need both cavities to have the same power in order to measure the feedback signal. I can try

1) lock the laser to RCAV only with power of 10mW or upto 40mW and measure the feedback signal to PZT

2) lock both cavities, have maximum power to ACAV (could be upto 12,13 mW, the efficiency is ~25%) and measure the feedback to VCO

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