by tuning the servo Tara unlocked both cavities and they are out of range right now, so plz no more temp servo tuning until further notice

- personal notes -

New Focus PD:

power from ACAV: 0.958mW
power from RCAV: 0.967mW

DC-OUT: 1.94V
AC-OUT: 1.27Vpp @160MHz in 50R

laser power is still drifting with the ISS turned on, but much less. Primary suspects are

1. sensing PD: is a Silicon PD and so the change in Responsivity caused by thermal fluctuations could be the reason. Sensitivity is typically 1.5%/K @1064nm for Si.
2. Input offset of SR560

Replaced the PD with a InGaAs diode and re-started a measurement. Will also add sampling of the laser power in the VI. I can't easily switch to the lock-in amplifier as it does not have a GPIB interface to read it.

pictures taken from the existing power supply.

.

The light transmitted from both the cavities has been monitored while the cavities where locked (Vacav = 1.369 V, Vrcav = 5.7909 V) and beats on RF photodiodes where visible. Power on the three photodiodes PD-Rcav (North cavity), PD-Acav (South cavity) and PD-RF decreases of about 10% from its maximum value on PD-RF and  about 5% on the others photodiodes periodically every ~7 minutes. 

I also notice that:

1. The PD-Rcav trace is noisier than PD-Acav trace;
2. The mean voltage values of the two photodiodes are way different:

PD-Rcav = ~60mV (+- 2/3%);

PD-Acav = ~170mV (+-2/3%);

       3. Enabling/disabling the boost switch on the FSS box does not give any improvement;

       4. Pressing the red botton (gain) on the FSS box neither;

In the same condition of locking the control signals of both PDH loop have been monitored too.

Here we can see that:

1. the PDH-North loop is noisier;
2. A step occurred in the cian trace (at the second division) without causing variation in the transmitted light;
3. Enabling/disabling the boost switch on the FSS box does not give any improvement;
4. Pressing the red botton (gain) on the FSS box neither aside from the fact that when the botton is kept pushed it slightly "cleans" the trace (not quantified);

It is worth also monitoring the error signal while the cavities are locked and when they are not (with a triangular wave applied at the laser PZT).

pressure for RCAV is OK. Ion pump current is currently at 52uA and falling. Turned turbo off.

Today we changed the suspension on RCAV back to the original one (spring + wire) and put the cavity back in the chamber.

The beam to RCAV is realigned, but the beam position changes a bit and might clipped on the insulation's opening.

The cavity is being pumped down now.

We decided to switch back to original suspension for RCAV for now before designing a new one.

However, a stronger eddy damping system (thicker metal plate, larger magnet) we added last time is still kept.

This thicker plate might add extra weight to the cavity and cause the spring to extend more, sothe cavity hangs ~5mm lower from the original setup.

I had to adjust the height of the input periscope, but there was no other problem.

The mode matching efficiency is more than 95%.

The exiting beam seems ok, see below figure.

However, the incoming beam is off by almost 5mm and migh clip at the edge.

 I'll fix the opening tomorrow, and check if the vacuum works or not.

I borrow a torque wrench from 40m and used it to tighten the screws on the flange.

There should be no problem this 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 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. ()

The maximum power after AOM double pass is 37%, worse than the expected 50% efficiency, but it should be enough.

The good news is, a new mode matching (RefCav and ACav) is calculated, and all positions for the lenses are clear.

I got all the lenses, and borrow one plcx-24.5-51.5-c-1064 from 58C

The problem about the position of the PBS is solved. It will be at the original place, since the clipped beam is the 0th order of the reflected beam which we do not use.

I'll put the lenses in their places and try to lock RefCav again.

 Groovy.                   

I add the broad band EOM in the beam path. After adjusting the periscope, I can steer the beam into the RefCav and see the reflected light. It's not aligned yet.

The 35.5 MHz is set on the table with a lens to focus the beam on the PD.

I'm not sure if I have to use the laser controller for 126 model or I could use the 10W laser to scan the beam, I'll consult Peter tomorrow.

Right now we are using the 10W laser controller to power the 100mW laser. The connector had been unstable, but now it's working fine.

It will be better if I can use the 10W controller to dither the laser frequency because I won't have to switch the cable, and avoid the risk of having to deal with the cable again.

Forgot to log this yesterday:

The PMC servo in medm's command window is correct.

I need to make SMA cables (properly insulated kind) too.

From 21.5 MHz PD to servo, 25 feet,

from 35.5 MHz PD to servo,   25 feet,

from 35.5 MHz EOM to signal box, 5 feet,

from 35.5 MHz LO to signal box, 20 feet.

Mixer will be driven very hard to saturate it. To operate the mixer in the required saturated mode, the RF signal level should be at least:

• +1 dBm for Standard Level (+7 dBm LO) mixers
• +7 dBm for Standard Level (+13 dBm LO) mixers
• +10 dBm for High Level (+17 dBm LO) mixers

so if we use the right (optimum) cable we would have ~8dBm, which should be perfect for a level 13 mixer.

Let's see if we can confirm the calculations...

here is a list of all channels of the psl subsystem. We changed the generic channel names to final names now.
Refcav channels are now C3:PSL-RCAV and analyzer cavity channels are C3:PSL-ACAV. Rest see below...

#####################
# 10W MOPA channels #
#####################
[C3:PSL-126MOPA_AMPMON]        # internal laser power monitor
[C3:PSL-126MOPA_126MON]        # internal NPRO power monitor
[C3:PSL-126MOPA_DS1]            # diode sensor 1
[C3:PSL-126MOPA_DS2]            # diode sensor 2
[C3:PSL-126MOPA_DS3]            # diode sensor 3
[C3:PSL-126MOPA_DS4]            # diode sensor 4
[C3:PSL-126MOPA_DS5]            # diode sensor 5
[C3:PSL-126MOPA_DS6]            # diode sensor 6
[C3:PSL-126MOPA_DS7]            # diode sensor 7
[C3:PSL-126MOPA_DS8]            # diode sensor 8
[C3:PSL-126MOPA_126PWR]        # NPRO power monitor
[C3:PSL-126MOPA_DTMP]        # diode temperature
[C3:PSL-126MOPA_LTMP]        # pump diode temperature
[C3:PSL-126MOPA_DMON]        # diode output monitor
[C3:PSL-126MOPA_LMON]        # pump diode output monitor
[C3:PSL-126MOPA_CURMON]        # pump diode current monitor
[C3:PSL-126MOPA_DTEC]        # diode heater voltage
[C3:PSL-126MOPA_LTEC]        # pump diode heater voltage
[C3:PSL-126MOPA_CURMON2]        # pump diode current monitor
[C3:PSL-126MOPA_HTEMP]        # head temperature
[C3:PSL-126MOPA_HTEMPSET]        # head temperature set point
[C3:PSL-126MOPA_FAULT]        # laser fault indicator
[C3:PSL-126MOPA_INTERLOCK]        # interlock control
[C3:PSL-126MOPA_SHUTTER]        # shutter control
[C3:PSL-126MOPA_126LASE]        # NPRO lase status
[C3:PSL-126MOPA_AMPON]        # power amplifier lase status
[C3:PSL-126MOPA_SHUTOPENEX]        #
[C3:PSL-126MOPA_STANDBY]        #
[C3:PSL-126MOPA_126NE]        # NPRO noise eater
[C3:PSL-126MOPA_126STANDBY]        # NPRO standby
[C3:PSL-126MOPA_DCAMP]        #
[C3:PSL-126MOPA_126CURADJ]        #
[C3:PSL-126MOPA_126SLOW]        #
[C3:PSL-126MOPA_BEAMON]        # beam on logical

#######################
# 80 MHz VCO channels #
#######################
[C3:PSL-FSS_VCODETPWR]        # 80 MHz VCO PWR
[C3:PSL-FSS_VCOTESTSW]        # enable/disable test input
[C3:PSL-FSS_VCOWIDESW]        # enable/disable wideband input

######################
# other FSS channels #
######################
[C3:PSL-FSS_SW1]            # frequency servo front panel switch
[C3:PSL-FSS_SW2]            # frequency servo front panel switch
[C3:PSL-FSS_INOFFSET]        # 21.5 MHz mixer input offset adjust
[C3:PSL-FSS_MGAIN]            # frequency servo common gain
[C3:PSL-FSS_FASTGAIN]        # phase correcting EOM gain
[C3:PSL-FSS_PHCON]            # 21.5 MHz phase control
[C3:PSL-FSS_RFADJ]            # 21.5 MHz oscillator output
[C3:PSL-FSS_SLOWDC]            # slow actuator voltage
[C3:PSL-FSS_MODET]            #
[C3:PSL-FSS_PHFLIP]            # 21.5 MHz 180 degree phase flip
[C3:PSL-FSS_MIXERM]            # 21.5 MHz mixer monitor
[C3:PSL-FSS_SLOWM]            # slow actuator voltage monitor
[C3:PSL-FSS_TIDALINPUT]        #
[C3:PSL-FSS_RFPDDC]            # 21.5 MHz photodetector DC output
[C3:PSL-FSS_LODET]            # detected 21.5 MHz output
[C3:PSL-FSS_PCDET]            #
[C3:PSL-FSS_FAST]            # fast actuator voltage
[C3:PSL-FSS_PCDRIVE]            # drive to the phase correcting EOM
[C3:PSL-FSS_RCTRANSPD]        # reference cavity transmission
[C3:PSL-FSS_RMTEMP]            # room temperature
[C3:PSL-FSS_RCTEMP]            # reference cavity temperature
[C3:PSL-FSS_HEATER]            # reference cavity heater power
[C3:PSL-FSS_TIDALOUT]        #
[C3:PSL-FSS_RCTLL]            # reference cavity transmitted light level
[C3:PSL-FSS_RAMP]            # slow actuator ramp, used in lock acquisition

################
# PMC channels #
################
[C3:PSL-PMC_SW1]            # PMC servo front panel switch
[C3:PSL-PMC_SW2]            # PMC servo front panel switch
[C3:PSL-PMC_MODET]
[C3:PSL-PMC_PHFLIP]            # 35.5 MHz 180 degree phase flip
[C3:PSL-PMC_PHCON]            # 35.5 MHz phase control
[C3:PSL-PMC_RFADJ]            # 35.5 MHz oscillator output
[C3:PSL-PMC_PMCERR]            # PMC error point
[C3:PSL-PMC_RFPDDC]            # 35.5 MHz photodetector DC output
[C3:PSL-PMC_LODET]            # detected 35.5 MHz output
[C3:PSL-PMC_PMCTRANSPD]        # PMC transmission
[C3:PSL-PMC_PCDRIVE]            #
[C3:PSL-PMC_PZT]            # PMC PZT voltage
[C3:PSL-PMC_INOFFSET]            # 35.5 MHz mixer input offset adjust
[C3:PSL-PMC_GAIN]            # PMC loop gain
[C3:PSL-PMC_RAMP]            # PMC PZT ramp, used in lock acquisition
[C3:PSL-PMC_BLANK]            # blanking input to the PMC PZT
[C3:PSL-PMC_PMCTLL]            # PMC transmitted light level

################
# ISS channels #
################
[C3:PSL-ISS_SW1]            # intensity servo front panel switch
[C3:PSL-ISS_SW2]            # intensity servo front panel switch
[C3:PSL-ISS_AOMRF]            # rf drive for intensity stabilization
[C3:PSL-ISS_ISERR]            # intensity servo error point
[C3:PSL-ISS_GAIN]            # intensity servo gain
[C3:PSL-ISS_ISET]            # intensity servo set point

#############################
# 16bit D/A channels - ACAV #
# 4116-card               #
#############################
[C3:PSL-ACAV_HEATER]            # analyzer cavity heater power
[C3:PSL-ACAV_SLOWDC]            # feedback to tidal input of other cavity

#############################
# 16bit A/D channels - ACAV #
# 3123-card                    #
#############################
[C3:PSL-ACAV_RCTEMP]            # reference cavity temperature
[C3:PSL-ACAV_RMTEMP]            # room temperature
[C3:PSL-ACAV_RCTRANSPD]         # analyzer cavity transmission
[C3:PSL-ACAV_RFPDDC]            # RF photodetector DC output
[C3:PSL-ACAV_PDHOUT]            # PDH servo output signal

#############################
# software channels - ACAV  #
#############################
[C3:PSL-ACAV_KP]                # pid loop p-gain
[C3:PSL-ACAV_KI]                # pid loop i-gain
[C3:PSL-ACAV_KD]                # pid loop d-gain
[C3:PSL-ACAV_LOCKEDLEVEL]       # threshold level below which pid does nothing
[C3:PSL-ACAV_TIMEOUT]           # pid loop sample time
[C3:PSL-ACAV_VERSION]           # pid loop software version
[C3:PSL-ACAV_DEBUG]             # pid loop debug messages on/off
[C3:PSL-ACAV_ENABLE]            # pid loop on/off
[C3:PSL-ACAV_SETPT]             # temperature setpoint
[C3:PSL-ACAV_SCALE]             # scaling factor

##############################
# software channels - REFCAV #
##############################
[C3:PSL-RCAV_KP]                # pid loop p-gain
[C3:PSL-RCAV_KI]                # pid loop i-gain
[C3:PSL-RCAV_KD]                # pid loop d-gain
[C3:PSL-RCAV_LOCKEDLEVEL]       # threshold level below which pid does nothing
[C3:PSL-RCAV_TIMEOUT]           # pid loop sample time
[C3:PSL-RCAV_VERSION]           # pid loop software version
[C3:PSL-RCAV_DEBUG]             # pid loop debug messages on/off
[C3:PSL-RCAV_ENABLE]            # pid loop on/off
[C3:PSL-RCAV_SETPT]             # temperature setpoint
[C3:PSL-RCAV_SCALE]             # scaling factor

##############################
# software channels - TIDAL  #
##############################
[C3:PSL-TIDAL_KP]               # pid loop p-gain
[C3:PSL-TIDAL_KI]               # pid loop i-gain
[C3:PSL-TIDAL_KD]               # pid loop d-gain
[C3:PSL-TIDAL_LOCKEDLEVEL]      # threshold level below which pid does nothing
[C3:PSL-TIDAL_TIMEOUT]          # pid loop sample time
[C3:PSL-TIDAL_VERSION]          # pid loop software version
[C3:PSL-TIDAL_DEBUG]            # pid loop debug messages on/off
[C3:PSL-TIDAL_ENABLE]           # pid loop on/off
[C3:PSL-TIDAL_SETPT]            # temperature setpoint
[C3:PSL-TIDAL_SCALE]            # scaling factor

I closed the chamber. The turbo pump is on and pumping down.

 I realigned the beams so the visibilities for both cavities were 80% or more. This made sure that the beams' path would be close to the optimized path.

Now, the window reflection won't overlap with the cavity reflection, and can be dumped properly.

Note about a few things to do:

• The beam holes on the foam might have to be fixed, the beams slightly clip at the openings. I have to check if the beams are clipped at the periscope or not.
• modification of the seismic stack as suggested by Koji. The teflon pieces at the bottom plate are not screwed down to the stack making it hard to put the stack in the chamber. I think this should be fixed after the SiO2/Ta2O5 measurement is done and we have to reopen/ installed AlGaAs cavities.
• There is some strayed beam from the PBS in RCAV path. This is from left over beam in S-light that reflected off, and bounced back at the PBS surface before going to the PD. This might have to be fixed too.

FYI for torque wrench setting for CTN cavity:

 The CTN cavity is 10" OD,  the Torque required is 190 InchPound.

The turbo pump is removed, and the ion pump is on. The initial value is ~7mA.

I removed the turbo pump and turn on the ion pump, see the procedure on wiki page. The initial value on the ion pump is ~ 7mA, similar to the last time we opened the chamber although this time I left the turbo pump on 4 days instead of 2 days. So I think this is the limit of this turbo pump.

I ordered a few opto mechanical components to replace the current shaky periscopes.

The new  periscope is shown here, elog:574. Currently we have only one set, so I ordered a post clamp to complete another set. 

I also ordered 4 mirror mounts that can be mounted on 45 degree mounting adapters. The thickness of these mounts are thinner than a regular mirror mount, so it can be fit on the adapter. I plan to use these in Crackle experiment as well. 

The periscopes for the refcav ought to be made custom. None of the store bought type are stiff enough. Koji has a design from the 40m green that Daisuke made.

I looked up 40m elog and found Daisuke's design for periscope. I'll make a sketch FSS' periscopes.

The design for 40m pericopes by Daisuke can be found here .

They are found in DCC. Some comments

- You can not steer the beam. The beam should be steered before or after the periscope.

- The side plates were too thick. It can be 1/2" thickness to reduce the total weight.

we have to design our own. The 40m one has 2" mirrors (too large, we don't have the space), wrong height for incoming/outgoing beam and is clamped to the table, which i think is bad in terms of stability.
The design principle does not look much different compared to the original refcav periscope design, except for the mirror holder itself. That was bad designed for the old one.

C3:PSL-NPRO_PWRMON channel, and npro.db file are added

for monitoring power output of the NPRO (reflected beam from Faraday Isolator)

I haven't reset the crate yet, so the channel may not appear in DAQ yet.

It's connected to VMIVME-3113 at #C0 S60.

The photo diode is Thorlab PDA55 with RG1000 filter.

Calibration for power is 1.81 mW/V, now it reads 5.56V.

I borrowed one of the quarter wave plate and added it after the beam from the Ref Cav. One more QWP is needed for the ACav.

The power after the Ref cav is measured:

1) just after the cavity : 5.4 mW

2)First BS: Reflected beam (to the PD): 3.8 mW, Transmitted beam: 1.4 mW

3) 2nd BS: Reflected beam (for beat measurement): 0.56 mW,  Transmitted beam(to the camera): 0.62 mW

The waveplate is set at 32 degree, for max transmitted beam from a PBS( horizontal polarization in this setup)

Power after the A cav:

1) just after the cavity: 19.3 mW

2) First BS: Reflected beam(to PD):19 mW, Transmitted beam: 0.21 mW

3) 2nd BS: Reflected beam(for beat msmt): 0.255 mW

I got the quote for substrates from ATF (AR coated on flat-wedge, blank on concave, R=0.5m side) quantity of 10 and 20. They also offer the cavity assembly as well, so I sent them the drawing of our cavity to ask for the quote.

For the spacer, I asked two other companies,cidra, and tecoptics, for the quote, but they have not replied back yet. So, I just asked another company,aiceramics.com, in hope of getting it done soon.

I called Coastline about the lead time for the substrates and asked if we could speed up the process by changing the specs to be less demanding (coarser roughness/ sphericity). The answer is no, all the main features of the substrate (curve surface with polished annulus, wedge surface) are the main driving factors, not the roughness or how flat it is. Additionally, they told me that, from their experience, the width of the annulus of 2-3 mm is enough for optical contact. 5mm width would be too large and it causes the surface near the annulus to deform, and it takes longer time (he could not tell exactly how longer). I put the quotes on CTN wiki.

the shields/heaters for the two cavities will be delayed by an unknown amount at this point. I'm still waiting for the two polished copper tubes and mounting parts to be finished. Also the Kapton heaters still didn't arrive.Even if we would get everything beginning next week at will take at least a week to get it cleaned, heaters wired, glued, cleaned and baked. Same for temp sensors etc. So we will try to use the cavities without it for a first shot. We might be lucky and the difference in frequency is not that big. (370MHz max possible). If we are lucky it's small (and we can slightly tune it by choosing the absolute temperature if they are not equal in length)). We can still work on the alignment and locking and upgrade next week or so. But even if we have a beat at 300MHz we can check the performance of seismic isolation and scattered light and see if we made a big mistake...

Situation: The heater wire should not go all the way to the vacuum feedthrough connector. Instead the connection from the heater to the connector should be made with some low conductivity material to only add heat where we want it.

So i tried to solder copper wires to the end of the Nichrome wire and failed. Whatever people suggest on the web does not work properly. i tried 6 different kind of tin with different fluxes, 3 separate fluxes incl rosin and acid flux for brazing steel. Oxidized, scratched, etched the surface - nothing worked. you can get a blob of tin at the end but not a clean solder joint.

will go for crimped connections unless someone has an idea....

went to Downs to get the right crimp tool for the d-sub connectors. It's a DMC crimper for D-sub contacts for UHV connectors (crimp tool). Turns out that the DMC tool does not properly fit the contacts we have, (female pins).. The contacts we have are NASA approved, space qualified, low outgassing crimp contacts (link) and are the same as delivered with the aLIGO, in-vacuum PEEK D-sub connectors. The crimp location seems to be too high for those contacts, but is still working. The insert is the correct one (description), (pdf).

Interesting is that the pins which come with the UHV connector have little colored markings on them as can be seen on the picture (see above link to female pins). Marking can not be removed with methanol, acetone etc.. My guess is that it's OK and actually might be not simply paint.

Will use d-sub contacts to crimp on the nicrome wire and run kapton insulated copper wire to the main vacuum feedthrough. Having the contacts right next to the copper tube makes it a lot easier to work on the whole cavity assembly.

• wire can be stripped using an old rotary sanding tool where only the matrix is left (no abrasive stuff left).
• Crimp tool has to be set to setting 1(!) to crimp correctly, default is 5.
• clamped the wire using #2-56 screws and washers facing round side down to not have sharp edges which could cut the insulation of the wire.
• added tiny amounts of the super thin kapton tape to further protect the wire
• total amount of turns is 21
• total resistance is 154ohms - calculated value was 147ohms
• crimped femaile contacts to both ends

As the cavities' height changed, I adjusted the lenses to fine tune the mode matching for RCAV, the visibility is ~85%

We might want to use a bigger beam splitter (the current one is 10mm cube) where the beam split to ACAV and RCAV paths, the spot radius is ~ 3mm. It might cause some diffraction problem.

The next step is to check the beat signal with the previous FSS. We suspect that something might be wrong with the TTFSS. If the old FSS can give us with better signal,

we will use the old one.

  I readjusted the lenses for mode matching a bit more and the visibility for both cavities are now ~93%. We will check beat measurement with floated table tomorrow.

as we only did a quick-and-dirty measurement of the spring constant last time we re-measured it a couple of times. The resonance frequency varies by a few Hz from measurement to measurement and it also depends on over how many periods (and where, beginning or more to the end) of the ring down we measure. So we will analyze some ring down measurements using the same setup (Al-block with accelerometer resting on one RTV spring sitting on the optical table) and take the average for a new calculation.

trying to verify our FE model for both cavities. In order to measure the coupling i tried different versions of attaching a "shaker" or PZT  to the table to excite the table in vertical direction. As i'm worried about attaching something to the floating table as it could easily destroy the attached PZT or shaker i try to excite the non-floating table. The best solution i found is to put a PZT right below the vacuum chamber underneath the optical table. It does not look very sturdy but the performance is very good. Driving the PZT with only a few volts using our HV piezo amplifier (old, modified PMC servo) i can easily get an SNR of 100 above the seismic noise. And it's making a lot of noise if you drive it even harder. Turns out that the optical table is a good loudspeaker. Also driving it a low frequencies one can feel the whole lab vibrating!

Measurements of the coupling are currently performed. Check back later...

trying again to measure the coupling from seismic to cavity length for the individual cavities. Measuring the coupling to differential length (beat signal) is not a problem at all, but to the individual length.

The problem arises from the small coupling coefficient from vertical acceleration to changes in length. to not measure any effect from the filtering from the stack we have to measure at low frequencies. The first mechanical resonances occur at around 6Hz, the stack itself has it's first resonance at 16Hz. So we have to measure below 5Hz. The coupling to changes in length is small, about 1e-9 * Length of the cavity  [units: m/(m/s²)], so about 2e-10 m/(m/s²). The signal from shaking the table around 1Hz is estimated to be a few hundreds of Hz/rtHz with maximum modulation. However the laser frequency noise is 10kHz/rtHz @1Hz, so we have to integrate very long to get a reasonable SNR. We cant use anything on the table to pre-stabilize the laser to reduce it's noise as this would be shaked as well and we don't really know which one we actually measure.

For the first cavity we have to look at the feedback to the laser pzt as this tells us how much the laser frequency has to be corrected. We assume that shaking the table at 1Hz does not mechanically modulate the laser frequency in any other way. For the second cavity we can't simply lock it the usual way as we would have two coupled cavities (that's what we measure using the beat signal already). So we have to lock the laser to the second cavity instead without using the FSS path by feeding back to the laser (fast actuator) instead of the VCO.

A first measurement showed that we have an additional mechanical resonance around 6Hz which we currently don't have in our current stack model (and actually don't know exactly where it's coming from).
We measured the Eigenmodes of the stack some time ago and have two candidates for it (see here)

1. beam line, translational motion, f = 6.96 Hz, Q = 21.5 
2. horizontal transverse motion, f = 6.35Hz, Q = 25.9.

So i will re-measure the TF below 10Hz to clearly identify which one it is.

resonance frequencies of the stack : 16.1Hz and 55.6Hz
other resonance frequency: 6.5Hz

coupling to cavity length: COMSOL model: 53 kHz / (m/s²)
                                               measured: ~100 kHz / (m/s²)

All data and plots on the svn in /measurements/2012_02_12.

Notes for calibration:

TF to beat signal:

• accelerometer sensitivity: 1023mV/g
• range of PLL VCO: 10kHz -> 7kHz/V

TF to fast actuator:

• accelerometer sensitivity: 1023mV/g
• FAST MON output:  3.07 MHz/V

started realigning everything from scratch and calibrate all channels right. PMC optical power channels need re-calibration.

input beam to PMC: 28.9mW
PMC transmitted beam: 23.4mw
transmission trough curved mirror: 1.63mW

--> only 81% visibility (87% including back mirror leakage), but should be enough for what we want to do. Don't know the loss mechanism (didn't investigate)

A reminder for Frank about the setup, 

• The AOM's case on RCAV beam path is not screwed down to the body, so it will be blocking the beam.
• The height for Faraday isolator behind the PMC is not correct, i think only ~80% of the power is transmitted. I haven't had it fixed yet
• When I measured the visibility of RCAV, I scanned the cavity and minimize the dip as seen on the REFL RFPD. I got sth ~90% transmission, but when I measured the actual tranmitted power, the visibility is lower than that, maybe only~ 75%.
• There is some weird reflection on ACAV RFPD, I'm not sure if it comes from the window or not.

thanks, for the info.

I only did the PMC so far as i has to fix the daq and add the new channels. What's the problem with beam height of the isolator? Is the beam too low or the mount too high? Do you know?

The V-block's height is a bit too high. The beam height is very close to 3".

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.

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.

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.

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.

I'm re-arranging the optics in PMC path a bit. The work is in progress, so ACAV path is still down.

I'm investigating why ACAV TTFSS performance is worse than that of RCAV. One thing is that ACAV has the PMC. This area has not been optimized for awhile, so I'm checking everything.

PMC path is back, I aligned the polarization of the input beam to the BB EOM for TTFSS. The visibility of PMC is now ~ 80%.

reduced our RG58C/U cable length to optimum value (134.2ft) and characterized it. Below the confirmation that it is what it should be.

loss is 8.6dB, delay 206.4ns

    After PBS is rotated to the right position (yesterday I made a mistake by minimizing the split beam.

The split beam is minimized. This guarantees that the beam passing through is highly linearly polarized.

The reflection back to PMC is reduced from 1.9 mW to ~1.1 mW.

The cross correlation between the beat measurement and RIN is measured before and after the reduction of back reflection.

The plot shows certain correlation between RCAV's RIN and beat measurement when there is back reflection to PMC.

• A message has popped up on the oscilloscope that I've been using saying it needs to be recalibrated.
  
• The wire connecting the probe to the power meter (PM100D, the red one) is coming out of the insulator.
  
• Rana had me bring the SR620 to 40m so that's where it is.
  

Thermal shields and caps are ready. I cleaned them in ultra sonic bath with Isopropanol. They fit nicely to the mount.

The caps are teflon.

==Wire Heater==

I'm think about how to wire the heater around the cavity. I'm reading Frank's entries in PSL:765,768 ,776,786, . Seems like I might need to drill a few holes on the shields for the wires.  There are still more similar wire left. I'll calculate how long the wire has to be for heating the cavity up by 20 K.