I use COMSOL to simulate the temperature distribution inside the mirror. 

The mirrors are separate into 3 regions for different meshing quality see attached figures.

This model assume 0.95 emissivity on the surface, and 310 K at the surface between the mirror and the spacer.

The temperature between 10 mW and 10mW + RIN*10mW is plotted and fitted with polynomial function of order 9.

*the unit of y axis on fig3 is Kelvin

The Temperature difference will be applied to the coating surface and calculated the upper limit for photo thermal noise. 

The function is

T = p1*z.^9 + p2*z.^8 + p3*z.^7 + p4*z.^6 + 

    p5*z.^5 + p6*z.^4 + p7*z.^3 +

    p8*z.^2 + p9*z + p10;

       p1 =  -5.335e-10 ; %(-5.806e-10, -4.864e-10)

       p2 =   1.602e-08 ;% (1.474e-08, 1.731e-08)

       p3 =  -2.041e-07 ;% (-2.188e-07, -1.894e-07)

       p4 =   1.438e-06 ;% (1.347e-06, 1.528e-06)

       p5 =  -6.144e-06  ;%(-6.47e-06, -5.818e-06)

       p6 =   1.645e-05  ;%(1.576e-05, 1.715e-05)

       p7 =  -2.773e-05  ;%(-2.858e-05, -2.687e-05)

       p8 =   2.912e-05  ;%(2.857e-05, 2.967e-05)

       p9 =  -1.893e-05  ;%(-1.908e-05, -1.877e-05)

       p10 =   7.548e-06 ;% (7.534e-06, 7.562e-06)

I was measuring the power fluctuation behind PMC and RCAV, during that the beam was not locked and the medm screens were frozen, so I restarted the crate.

  C3:PSL-ACAV_TEMPAVG that tells the average temperature from ACAV is dead. Frank told me to reboot the PSL crate.

                                       RCAV      ACAV

C3:PSL-XXXX_SETPT      35.0       36.75

C3:PSL-XXX_HEATER    1.47521   4.9

I rebooted the crate, reload the cavity.stp file, the channel is still inactive. (only this channel)

set temp for the ACAV should be 37.3, that was the value since about a week. see here : http://131.215.115.52:8080/PSL_Lab/307

 I think ants are now building their colony in the lab, even though there is no garbage, they can find some food around here.

That seems like a cricket's leg (see attached pic). A few of them walk around the optic table/ optics too.

I think we should let an aardvark roam around the lab for a day.

any results from the poison from 40m they tried in the TCS lab? Did it work?If yes we should get some for the other labs too....

Yesterday, I reset the PSL crate behind the SUN computer, but the channel C3:PSL-ACAV_TEMPAVG is stil inactive.

looks like some fault of the database. /usr1/epics/psl/db/acav.db does not contain the correct entry. check rcav.db and copy the record for "C3:PSL-RCAV_TEMPAVG" into acav.db. Then simply change "RCAV" into "ACAV" everywhere for this record. Also change the setpoint for the ACAV in the startup.cmd file to 37.3 and the ACAV-heater value to 4.653. Those are the latest values when both where locked for several hours. Reset the crate again.

 Will do in a moment, I'm taking data from ACAV for now just to compare with yesterday results.

made the changes a minute ago. simply reboot after changing the values in the startup.cmd (those i didn't change)

be carefull with the data you are taking right now. it's wrong for your projection as the power fluctuations are different when locking only the ACAV using the AOM. The largest contributor might be the pointing from the AOM itself, which is different if the laser isn't locked to the other cavityat the same time.

Why don't you use the new fast channels you have hooked up last week? And don't forget to change the names of those :-)

can you plz check this channel as it is constantly at -14.4797 since yesterday. Did you disconnect it? If yes, plz elog those things!! Everything must be eloged

if you wanna measure anything in the ACAV loop without the RCAV locked, don't forget to DISABLE the FSS loop.
If you don't, your measurements will be shit as the PC is fluctuating like hell making the laser noisy which can't be handled by the other loops due to limited bandwidth...

 I didn't do anything on that channel. I just removed the BNC cable that connect to a T behind the PD and connected it to an oscilloscope this afternoon. I'll take a look.

was it grey? If yes the cable already connected to the T is the cable going to the DAQ. That's why there is a T already. Don't disconnect any signals without checking in advance what they are and what they are used for. Some of them are required for locking, e.g. the transmitted light is required for the servo as a feedback if it is locked or not for the temp feedback. If you have to disconnect anything do it but put an entry in the elog.

 No, it's the black one that goes to a multimeter. The grey cable to DAQ is untouched.

 I didn't disabled the loop when I measured it, and yeah it looks bad. I think I'll just try to lock both cavities for now.

I'll connect the signals from PDs behind Rcav and Acav to the new fast channels(32k).

About changing the channels' names, I asked DMASS to help and he suggested to use them as they are for now because of the risk of screwing the system up from a typo.

 I checked the beam. It hit the PD and gives ~500 mV output when only ACAV is locked. when I disconnected the grey cable that connects the PD to DAQ, the

value is still the same (-14.5).

The connection at DAQ looks good.

The cable is good, the signal can go through.

I thought 0the setup in  RCAV.db and ACAV.db files were wrong so I changed it to the red values

                             rcav.db                     acav.db

field DTYP        VMIVME-3113          VMIVME-3123 ->VMIVME-3113

INP                   #c0 S61@                     #c0 S1@  -> #c0 S62@

 ,and now it's working.

I remember changing these before Frank went to Hanford, I might not save the file. Anyway, now I saved it.

The fluctuation in VCOMON goes down to less than +/- 1V.

I adjust SLOWDC to see what value to lock each cavity.

For ACAV I adjust SLOWDC so that VCOMON fluctuates around 0 and record the value of SLOWDC

I adjust ACAV_SETPT to see how the DC changes (in RED), it seems I have to reduce the ACAV_SETPT

                             ACAV                              RCAV

SLOWDC       -0.1118 ->-0.1151             -0.1051

SETPT           37.3      -> 37.35                       35

Now, ACAV_SETPT is changed to 37.15, but ACAV_TEMPAVG goes up (after 30mins.) to ~37.43 (from 37.3).

The variance from ACAV's 4 temp sensors are getting larger.

don't change anything until it is stable and you thought about it carefully. The system was totally out of control, check the dataviewer!.
So there is no way to make an estimate for the values as you have no equilibrium!! That takes several hours after ANY change to the system, like rebooting the crate or turning on a temp control loop.
Remember: time constants are several hours!

So be patient and wait until tomorrow or late tonight before you make estimates and changes. Both temperatures are NOT close to their original setpoint and largely fluctuating.
RCAV is at 34.8 instead of 35 and ACAV changed from 37.1 to 37.45 within the last hours.
So no way to make the right adjustments right now, as your laser SLOWDC value does not reflect the actual temperature of the chamber.

Changed it back to the original settings (37.3) as those were working over several days before the system stopped working properly...

 Frank, did you remotely change ACAV_SETPT back to 37.3? I came back and it was back to 37.3.

 A tentative layout for almost symmetric layout for beat measurement. The double 1/4 objects  in the beam paths should be 1/4 and 1/2 waveplates.

Lenses might be used to focus the beam somewhere.

   Temperature on both cavities are fairly constant over -8 hrs, still can't simultaneously lock both cavities.                        

                              ACAV                      RCAV

XXX_SETPT          37.3                       35.0

XXX_TEMPAVG        37.301                 34.997

SLOWDC              -0.11645 (VCOMON center @ 0),    -0.1114

XXX_HEATER      4.72971              (FSS)1.49154

Since everything is stable now, I'll lock RCAV and adjust ACAV temperature to lock them together withing AOM range.

I'll change ACAV_SETPT from 37.3->37.4, to see how SLOWDC has to change to lock ACAV.

Wed Sep 01 21:04:24 2010

After 6 hrs, ACAV_TEMPAVG is stable around 37.4 C.

SLOWDC for ACAV is -0.1200 (VCOMON center @ 0)

 This means For ACAV, dV(slow dc) per dT(ACAV) = (0.1203-0.11645)/0.1 = 0.039 V/K.

Since RCAV is locked when SLOWDC = -0.1114, we have to decrease the temperature from 37.4 C by (0.1203 - 0.1114)/0.039 = 0.2282 K.

That is 37.4 - 0.2282 = 37.1718.

Now I set ACAV_SETPT to 37.2 C.

ACAV_TEMPAVG still adjusting it self to 37.2 (SETPT)  three overshoots in the last 8 hrs (+/-0.01 C)

                     ACAV                   RCAV

SETPT             37.2                  35

TEMPAVG    37.215               35.005

SLOWDC       N/A            -0.10932

VCOMON              1.11 V

*RCAV_TEMPAVG is very noisy compared to ACAV_TEMPAVG

ACAV's PD for transmitted beam is connected to PSL1

RCAV's PD for transmitted beam is connected to PSL2

And now RCAV_RCTRANSPD dropped from ~5.3 to ~5.05

               ACAV_RCTRANSPD dropped from  ~2.5 to ~2.4

50 mins ago, medm on Sun machine is frozen, most channels are inactive (except  ACAV_HEATER, and it goes up to 7 which is the maximum value). 

I reboot the PSL crate, but ACAV_HEATER is still at 7 and heat up ACAV to 39 C.

So, I realized that I had to reboot another crate. Now ACAV_HEATER is at 4.6 and decreasing.

Now the system should try to bring the temperature back and might take many hours.

 SInce the Faraday isolator alter the beam path and beam profile a bit. The mode matching efficiency drops.

I only realigned the beam to PMC and RCAV. Lenses before PMC haven't been moved yet.

                    efficiency (P_in/P_out)              DC level (from PD behind the cavity)

PMC        18.2mW/27.9mW = 65%                   2.5V

RCAV       4.56/4.94 [mW]  = 92% (this drops from ~96%)    1.3V

pmc_n and rc_n (blue/green) were taken together with Faraday isolator installed .

pmc and rc with no isolator (purple/brown) are data from Aug 30, 2010.    

pmc blocked (red)  is PMC RIN when the beam after the PMC is blocked, RCAV loop disabled, SLOWDC control is disabled.

There is no significant change here, I have to think about it.

 Today I investigated which optics reflect most power back to the PMC and the laser.

The back reflection seems to limit our sensitivity.

the power meter is placed to measure the reflected light that comes back and reflects off the PMC's mirror.

The beam path to the ACAV is blocked and ignore for now, since AOM in the path will act as an optical isolator.

The reflected power from this path will be smaller than that of Rcav's path.

periscope [M]  1/4&PBS  [L]  PLCX     [K]    Mirror

                                                                        [J]

                                                                      1/4

                                                                        [I]

                                                                       1/2

                                                                       [H]

                                                                    PLCX

                                                                      [G]                                                                                                         

                                             Blocked           PBS  [F]  PLCX [E]   1/2 [D]  EOM [C] PLCX  [B]   1/2 [A]      \  pmc  / <---- beam

                                                                                                                                                           power meter

A razor blade beam dump was placed at each position (alphabet in square brackets).

The red entries corresponds to second set of data after realignment

A

B 52.3 uW           49.2 uW 

C 0.184 mW        0.184mW

D 0.748 mW        0.745 mW

E 0.748 mW        0.752 mW

F 0.749 mW         0.754 mW

G 0.831 mW

H 0.831 mW

I 0.831 mW

J  0.831 mW

K 0.831 mW

L 0.831 mW           0.88 mW

M 0.831 mW          0.88 mW

reflection from RCAV     1.53  mW     1.96mW

It seems that the reflection from the cavity is giving us a hard time.

The PBS with 1/4 plate might not well adjusted to minimize the back reflection.

After I minimized it by tilting/rotating the PBS, the PBS was tilted around 5 degree  (judging by my eyes) and it looked bad.

The beam is almost clipped on the edge, and Frank noticed that the beam height was not right.

I realigned the beam again (the efficiency is ~97%).

Unfortunately, I lost the beam so much I had to remove a few optics ( 2 mirrors adjacent to the RCAV) behind RCAV,

and used a CCD camera to see the beam while adjusting the knobs. I placed the optics back, but

I haven't adjusted the beam path for beat measurement yet.

Now the PBS is quite flat on the table.

I  measured the power reflected at each optic again (see red entries in the table above.)

I rotated the PBS to see if the power reflected from RCAV can be reduced. It turns out that

I couldn't do much, only from 1.96 - > 1.93 mW.

i have several channels, e.g "C3:PSL-RCAV_RCPID_SETPOINT", loaded on the VME system as software channels.

If i try to access those channels from the VME console everything is fine.

psl1> dbpr "C3:PSL-RCAV_RCPID_SETPOINT"
ASG :               DESC: SETPT- set point                  DISA: 0            
DISP: 0             DISV: 1             NAME: C3:PSL-RCAV_RCPID_SETPOINT       
OMOD: 0             OVAL: 0             RBV : 0x0           RVAL: 0x0                 
value = 0 = 0x0

If i try to use commands from any other machine, e.g. ezcaread for the same channels i get the message  "channel not accessible".
At the same time the VME system throws the following exception on the console.

psl1> task: 0Xca8f00 CA client
Illegal Lock Set Lock Set out of range:dbScanLock
task: 0Xf9f7f8 taskwd
task ca8f00 CA client suspended

This only happens to some channels. I don't see a system except that the channel names are long, but not longer than working ones. For R3.13 the name must be <= 28 characters, For R3.14 the name must be <= 60 characters.

What's the problem?

We have used an instant PBS/QWP which is a PBS optically contacted to an aligned QWP in front of a reference cavity

(for PDH locking and optical isolation.) It cannot properly block the reflected beam, so certain amount of power got reflected back to

the PMC and back to the laser. This causes higher PMC's RIN. (When the beam path behind the PMC is blocked, no back reflection, RIN decreases.)

The PBS/QWP is removed and replaced by a regular PBS and a QWP.

Now the reflected power is less than 0.15 mW, which is 1.11 mW from the cavity and 0.985 mW when the beam is blocked in front of the periscope.

( it's more than 1 mW before with a PBS/QWP), and I think it can be less. 

How we test it:

There are QWP and HWP(half wave plate) in front of the PBS/QWP in our RCAV path.

They are adjusted(rotated and tilted) and the PBS is rotated so that the beam split from the PBS is minimized.

This is done to make sure that we have a perfect linearly polarized wave going to the cavity.

 The reflected beam power is measured as it reflected off at PMC's outport.

With the instant coupled PBS/QWP set, there is considerable amount of power coming back to the laser,

about half of the power picked up at the power meter coming from the cavity(see yesterday entry here, how we measure power reflected at

each optic).

We tried to correct the polarization by adding another QWP behind the PBS/QWP, but this does not work.

After we change to a regular single PBS and QWP, we can reduce the power by at least a factor of 10.

Note: to replace the PBS/QPS, I have to move the PBS back a bit so there is enough space for a QWP.

When I re adjust the split power, the PBS is rotated so much and it's very disturbing. This did not happen

when I tried it at the first time.

We have used an instant PBS/QWP which is a PBS optically contacted to an aligned QWP in front of a reference cavity

(for PDH locking and optical isolation.) It cannot properly block the reflected beam, so certain amount of power got reflected back to

the PMC and back to the laser. This causes higher value of RIN. ()

I turned off the laser for people who will work on sprinkler in the lab tomorrow. The warning light is off.

the calibration value is 71.3 kHz/Volt

for the following setup

center freq = 159.294 MHz

FM DVN = 100 kHz

This calibration will be used to convert the V/rt Hz unit from spectrum analyzer to Hz / rt Hz unit for beat measurement.

workers finished the piping stuff for today but have to come back to connect it to the stuff one floor above. It's not the sprinkler stuff, it's heating water.
So some other guys will show up in the future to drill holes for the sprinkler pipes and installation of the sprinklers.

turned the laser back on

After I increased the pk-pk of the error signal for RCAV system (by increasing the RF power to maximum), 

 C3:PSL-FSS_FAST is still railing once it goes below ~ -2 V kicking the system out of lock.

I'll check again if the laser is lock to the center of the error signal or not.

When I showed Frank that the card did not work, I mentioned that I got electric shock when I connected high voltage input to the card.

Frank realized that it happens only when the connector is not grounded, and it was not. The ground connection from HVin to the card

was broken, It's hard to see unless I touched the wire that connected the board to the female BNC on the panel. I replaced that rigid wire by a flexible wire.

Now the PMC card is working fine, and the original PA85 might not be broken at all.

I'm working on Simulink model to calculate PMC's open loop gain. For now, all the parameters for frequency discriminator are copied from linfss6.m.

I'll work on correcting those parameters later.

The simulink model will allow us to learn how external noise will look like in our system.

The bode plot for PMC's TF is plotted in fig1.

I'm not sure why the measurement on both magnitude and phase looks bad at low frequency (f<10 Hz).

I think the model (fig2) is not correct because of the wrong units ( angular frequency,w, to frequency, f.) I'll check  this again.

The phase part seem to be completely wrong.

The parameters, (for example, cavitiy pole, frequency discriminator) for  the TF are not confirmed yet. I'll elog them once I verify them.

 *******************************

Yesterday, I had a chance to test U6(OPA 27) in the schematic.The measurement agrees very well with the computed result, see fig 3.

The signal from source out was sent through FP2 test. TP2 and TP4 were connected to channel A and B on SR785 respectively.

SR785 was set to swept sine mode, frequency span from 1Hz to 10^5 Hz.

So we know that at least one part of the servo works properly.

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.

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. 

The current 1064nm, 100 mW laser in our setup, NPRO lightwave 126 is broken. We are looking for a new one to replace it.

The laser stopped working when I tried to lock the cavity and saw that RCTRANSPD fluctuated a bit even after I adjust the gain setup.

So I turned the HEPA filter above the table off to see if the signal would be more stable, it was not. When I turned it back on

the laser was off. I don't think the laser and the HEPA filter are associated, but that's what happened.

The power output, as indicated on the laser driver is 8 mW. When I turned the laser off for 5 mins and turned it on.

The temperature of  the crystal started from ~40 C and there was power out, then, in ~10 seconds, the temperature went up to  94 C, and the power dropped to 8 mW again.

The voltage supply for TEC went up to 4V which is the maximum V for cooling.

I switched to the 10W laser driver, the same symptom happened again, so the problem might be the head, not the driver.

 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.

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.

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.

The insulation work is done. 

Frank opened up the laser to find any burnt mark, but found nothing and put it back, and now the laser is working.

We don't know for sure yet, what's wrong with the laser. But I'll use this opportunity to work on modification of 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.

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.

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.

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.