I measured TF of PDH box, D0901351, (The one we have was modified). This box sends the signal to VCO.

SR785 measures at low frequency ( 1 Hz to 100kHz)

4935A measures at high frequency (10Hz to 1Mhz)

The integrator switch of the PDH box is turned off. This will be calculate later. The gain is set at 10.

The magnitude as mesured by 4935A is corrected for impedence match by x1.2.( 4935A and the PDH box have 50 ohm impedence for both inputs and outputs.)

This data will be used for control loop model later.

Blue, data from sr785

Green, Data from 4395A, I didnot use the power splitter to split the signal from source.

Red, Data from 4395A, with power splitter to divide power from source. (The power has to be increased to -30dB)

The first plot is magnitude of the TF, the second plot is phase shift, as usual Bode plot.

This is the control loop for the current PSL setup.

There are still components to be added.

1) TF of the PDH box, the one we have is a modified D0901351, so I measured the TF of this box when the integrator is off (April13,2010 entry.)

  This will be added in the model later. It is set to 1 for now.

2) TF of the photodiodes, I assume they are  Newfocus 1811 and choose the same value as used in linfss6.m.

3) I will verified the value of TF of the RefCav path (both Fast and PC paths are calculated from D980536) to see if they agree.

4) The TF of actuators will be added later.

I calculated the TF of the modified PDH box and fit it with the measurement. The comparison does not match perfectly. I'll take a look and check if all Rs and Cs in the circuit are actually the same as those in the box.

The circuit can be found at:

https://dcc.ligo.org/DocDB/0003/D0901351/002/pdh_b_v2.pdf

I checked only U7 and U4

R28 is 360 ohms

C18 is 3300 pF

C6 is 0.66 (2x0.33uF) uF

R30,R31, R23,R16,R24 have the same resistance as specified

C20,C28, C29, C14,C15, R25, C11 are removed.

 The calculation assumes that the integrator switch is off (R16 is connected parallel to R24 and C6)

If this works, the TF for PDH will be used in the simulink model.

Use LISO for circuit simulation.

    The values of some r and c in the circuit are corrected, I used wrong values last time ( details will be added later.)

   The measured TF and calculated TF using LISO are plotted below. The measurement and calculated data agree well from 1 to 10^5 Hz using SR785.

 The correction factor due to mismatch impedance when using 4395A will be checked again.

   The weird TF result from PMC seems to be the result of the insufficient voltage input. When I increased the swept sine voltage from 2mV to 500 mV, the result of the TF becomes as expected. See fig 1.

    Before the signal is fed back to PMC, there is a PMC notch box. It is a low pass filter. It's TF looks fine (fig.2.)

However, when the TF of the servo and the notch is measured together, they look shaky.

I just read Frank's comment. I'll check the schematic of the PMC servo again.

that's not the TF of the PMC servo, it's something else. look at the gain level: -100dB. that's not more than some crosscoupling. Never trust a flat response, always think if the measured form and values make sense at all !

Take a minute and think about the form and values of the TF you expect from a servo like this. Have a look into the schematic and draw the TF shape of the individual gain stages and add them to an overall TF or use LISO to simulate it.  Then measure parts of the servo step by step in order to verify that the individual parts are working as expected.

From the bode plot, something is not quite right. I'll debug the PMC servo. My plan is

1) Measure the TF from FP1 test to FP4 (output mon), change gain setting and see if the TF change as expected.

 *note the real TF is 20log (Vpzt/ Vin) but Vpzt ~ 50 Vmon.  Vmon is connected to Vpzt with divider circuit. To get the real TF, 20log(Vpzt/Vin), the magnitude from out TF between FP1 and out mon will be added by 20log50 = 34 dB.

2) Compare it with the calculated TF from PMC schematic 

Today I aligned the AOM so that I could align the analyzer cavity to lock the cavity. We already aligned and locked the PMC last week so we had a beam to work with. The AOM is now aligned and the beam coming from it is aligned into the analyzer cavity. We scanned the cavity to find the mode we want and currently the output of the photocell is being mixed with the local oscillator to find the error signal. It's not quite how it should be right now, so we will look at it tomorrow to figure out what is wrong. Then we can connect it to the PDH servo and lock the cavity!

I also did some cable-management. They are now neatly organized and held in place by a large number of zip ties so they don't get in the way of everything else! The cameras are now connected so that we can see the beam from the PMC, analyzer cavity, and reference cavity.

The PMC is still locked (except when we do something to unlock it) and both the analyzer cavities and reference cavities have the ability to be locked. We have not locked them simultaneously yet but will be working on that so we can get a beat. We connected the VCO driver to the AOM for the analyzer cavity and got it locked.

A preliminary calibration for the photodetectors in the path of the transmitted light from the reference cavity has been calculated, although changes in the next day or two might change it, so it has not been put into the computer.

The table by the cavities is now very pretty . Until we fill all the space we just cleared with everything we need to beat the two beams together!

Preparations for that will be started tomorrow by putting the correct optics in the path of the transmitted light of the analyzer cavity and better aligning those in the path of the reference cavity's transmitted light.

Also, as an extra note, last week we calibrated some inputs, so we no longer have anti-energy reflected from the PMC! In fact, at this moment, we have 1.4mW. Success is still 100 and strength is down to 44! Guess it's time for dinner.

For future people searching in the elog, calibration for the slow actuator is 1182 MHz/V. Or for the temperature actuator. Whichever one decides to search in that moment.

The model I made of the VCO is now on the svn, in the filter folder in liso, listed as VCODriverNoise.fil

Both the analyzer cavity and reference cavity are able to be locked, and were simultaneously locked for a while yesterday . However, the temperature control loop is not working because the computer is not seeing the temperature data, so it didn't stay locked . But now we know it's possible !

The mirrors, beam splitter, and photodetectors are set up and aligned to measure the beat of the two signals and I am currently working on mode matching to get the beams the same size at the photodetector.

FSS_RCTRANSPD and ACAV_TRANSPD have been calibrated:

FSS_RCTRANSPD = 1.98 mW/V

ACAV_TRANSPD = 2.18 mW/V

All of the optics were set up in the path of the transmitted light from both cavities. Each path needed both a quarter wave plate and a half wave plate to be able to get the s polarization needed by the beam splitter. This solved problems we had earlier with the 50/50 beam splitter doing more like 25/75!

For mode matching, 2 lenses were placed in the path of the RCAV, one with a focal length of 175mm at a distance of roughly 7.5 inches from the first mirror and the other with a focal length of 200mm roughly 5.5 inches before the second mirror. This gave a waist of 224 µm 7.5 inches after the beam splitter.

The ACAV got only 1 lens, with a focal length of 200mm, 2.5 inches before the second mirror, with a waist of 208 µm at the same distance (7.5 inches from the beam splitter).

The first photodetector was placed with a mirror (for better alignment) above the beam splitter, and the other was placed after 2 mirrors and another lens in the other path. The second photodetector has a smaller detector area, so we ideally would like the waist at the detector to be under 100 µm. To make the beam smaller, we used another lens with a focal length of 30mm roughly 2 inches after the second mirror. This still leaves the beam a little larger than we'd like (it's very sensitive to alignment), but it will be fine for the moment.

The two cavities have still not been intentionally locked simultaneously. Yesterday, they differed by roughly 17 MHz (if I remember correctly) and so they were being heated overnight so their temperature can stabilize and we can see if they are closer today.

The cavities were still too far off to lock simultaneously, but they should hopefully be better today.

Instead, we played with the Wenzel BluePhase 1000 phase noise test system.

Following the directions in the manual, we locked an IFR/Marconi 2023A to an IFR/Marconi 2023B, calibrated according to the manual, and got a curve very similar to what we had before . This means it's probably working correctly!

So then, just to be adventurous, we decided to connect the VCO in place of the 2023B to measure its phase noise. Apparently that was too adventurous for the BluePhase 1000 . We couldn't lock it (with the feedback loop going to the 2023A) with any input range below roughly 100 kHz and the output (after calibration as per the manual) was a flat line. After readjusting cables and reconnecting cables and finally reverting back to the 2023A vs 2023B to make sure the machine still worked, we decided the unity gain frequency was probably pretty high and the manual calibration did not take that into account. So we used a swept sine measurement to find the transfer function of the system with the VCO connected and locked. The UGF was around 5.3 kHz with the 100 kHz input range . This means that the calibration doesn't account for everything when the UGF is high . But it also means we may have found the problem with our data when the VCO is connected ! So I have to take the data we have, apply a zero at 5.3 kHz, and see if that gets it to line up correctly.

Meanwhile, we installed a program on the computer that, when connected to the BluePhase 1000, can control all the knobs and buttons and locking remotely. And we discovered you can do more on the computer than with the switches on the front! Like change the capacitor value.

So the summary of yesterday's activities is: don't ever completely trust the calibration the manufacturers tell you to use. They might not be taking something (UGF) into account.

And the calibration as per the manufacturers:

1) Adjust the offset until exactly 1 period is displayed on the oscilloscope

2) Divide the time divisions on the oscilloscope to 1/100th the original (this gives 0.02Pi radians)

3) Measure the voltage difference across the two ends of the line

4) Calculate your slope! (gives V/rad)

5) Noise [dBc/Hz] = [PSD]-[20log(slope)]-[amp gain]-[correction for SSB measurement]

Yesterday we set up channels to record the noise of the locked signal generators. After a while playing with channels and filters and everything, the results are that the missing factor might be somewhere in the calibration of the data in 40m because this data fits where we would expect it to, slightly above the electronic noise converted to phase noise at low frequencies and much higher at high frequencies.

I tried to check the calibration in 40m this morning, but Jenna had the rubidium clocks hooked up and was collecting data with that, but I might be able to go back this afternoon and get something to see if it's in the calibration of rad/count.

We got both cavities locked today!

They stayed locked for at least an hour and were still locked when we left. Hopefully they'll still be locked tomorrow morning!

We aligned both beams into the photodiode (they were already pre-aligned) and maximized the alignment to get the largest signal out of the photodiode. We locked the signal from the photodiode with the IFR/Marconi 2023B signal generator and took some measurements of noise and transfer functions to determine UGFs, which I will post graphs of when I put them together. The plan is to let the temperatures really settle down (they were still settling a bit when we left) and take a good measurement of the noise and measure the transfer functions to determine UGF tomorrow. Then we will pull everything apart (not really) to replace the two cables that have extensions on them to see if something in the connection can be fixed to give better stabilization.

*I added a graph of the noise we measured yesterday at 3 different times*

We were going to work on building a metal box around the cavities, but they didn't have enough sheet metal and would have to order more, so Frank is working on a design for the box and will order the metal.

Instead, we got free foam! Someone ordered too much, so we took a sheet to build a new box around the cavities to see if it would help with stabilization.

After a while spent cutting the foam, we built a box!

And it is very happy to stabilize the cavities. We placed some foam bits on top to make a makeshift lid, and will use another foam sheet today to cut a real lid that fits on the box.

The new foam box has a hat! Or lid... Whichever you prefer.

 - personal notes -

• tiny, periodic spikes in acav RF PD
• ~36MHz oscillation (without light too) at RCAV RF pd dc output
• PMC transPD signal into DAQ broken, changes periodic from real value to some random number. However real signal at input of cross-connect panel is constant (on scope & multimeter)
• kicking the table introduces 5mK jumps in temp signals (random channels)

to-do-list (other stuff)

• switch temp readout to VME based system (3123, channel 9-16, J5 and remaining channels from J4)
• improve PID parameters (so I and D so far)
• exchange linux workstation (freezes from time to time)
• update boot values for all channels
• change channel name for VCO input signal
• change channel names for all PID controllers
• measure TF for all loops
• ...

i tried a couple of things today to figure out what determines the low frequency noise level of the beat measurement.
I did the following steps, nevertheless the noise level didn't change:

• re-aligned everything starting with the PMC
• coupling into the cavities is >90%
• matched power levels in both paths to match power at RF photodetector
• re-positioned the RF photodiodes for locking the caviries
• added beam dumps for beams reflected from RF photodiodes
• optimized overlap of both beams on RF photodiode (beat)
• changed mixer from 7dbm model to 13dbm model
• removed cable going to DAQ from DC output of RCAV RF photodiode as it is causing ~36MHz oscillation of RF PD (even with no light)
• changed gains for both loops, ACAV and RCAV
• added first QPD at pick-off right in front of periscope into chamber for RCAV to check pointing

things to be checked tomorrow:

• power fluctuations of beams into both cavities
• power fluctuations of transmitted beam
• pointing of both beams (should also show up in power noise spectrum in transmission)
• measure TF of all loops with current setting for noise estimation

other things to change:

• move temp readout to VME based stuff
• rename VCO input monitor signal channel name
• remove PSL RT stuff from fb0 to see if networking problems are caused by that
• replace current cables for temp readout due to loose connection somewhere on table
• check ACAV RF photodiode
• change PID controller variable names in order to add other software loops like feedback from VCO input signal to RCAV temp setpoint. Right now we have already 4 software loops:

1. feedback to laser for RCAV using FAST actuator signal
2. temp ctrl for RCAV
3. temp ctrl for ACAV
4. feedback to laser temp for ACAV using VCO input signal (used to track ACAV temp tuning to match RCAV resonance)

The panorama is a good idea. We should make it a mandatory step whenever we make any change to the setup.

The asymmetry in the paths of the transmitted beams is dis-satisfying though. I would have tried to just take the transmitted beam and interfere them via short path lengths and no lenses. The cavity eigenmodes of each cavity should already be matched to ~1%. If the path length from the cavities to the BS can be made equal, the overlap should also be good. In general, it is hard to make a low phase noise setup with a long path length.

To ensure low backscatter from the transmission RFPDs back into the cavity, the beat signal PD should be placed slightly ahead of the focus of the final lens.

And since the Z=40 kOhms for the 1811, the light power from each cavity should be made ~200 mV / 40000 ~ 5 uW. I know its tiny, but otherwise the 1811 is not going to be very linear. And the RF signal going into a level 13 mixer ought to be less than 300 mVrms.

The better option is to use a RFPD with only ~1 kOhm of RF transimpedance...

i know the the asymmetry is not nice but we made the first beat setup symmetric and suffered a lot from the (former) little space behind the cavities.
Also one of the cavities is tilted and so the beam height is about .5in different.
Now as me moved both cavities a bit further away from the end of the table we should re-think about changing it to a symmetric setup. I think it's a good idea.
Anyway, shouldn't we just see all the resonances of the mounts in the spectrum, not a broadband noise hump? or do you expect lots of scatter from all the unnecessary optics?

you are right with the power, i reduced it by a factor of 10 and 100 but no change in noise spectrum (almost absolutely the same). But if i reduce the power some pickup of the 35.5MHz shows up in the signal (which is tiny), which i didn't see before. But it's ground loop related as you can change it by connecting/disconnecting some more channels to the scope e.g.. About the gain: i didn't measure the TIA gain of the PD, but the manual says 24e3 in the text, and 40e3 in the table at the end. Which one is right?

Another problem is that the bandwidth is only 125MHz, but we are looking at 160MHz. That shouldn't be much of a problem, only the noise of the PD increases and we are more sensitive to changes in the TF of the PD due to almost anything like power fluctuations, power supply fluctuations, temperature etc... So that probably the best argument right now to add the second AOM to reduce the beat frequency to something we can handle much better

 Frank showed me that the RIN level from PMC is too high and caused by back reflection. 

PMC's psd is reduced by blocking the beam going to ACAV and RCAV.  This can be seen

on SR785, by blocking the beam.

So I removed PBS in front of the laser, and replaced it with a Faraday isolator. 

The isolator is temporarily mounted on a post, we will use a more rigid  V-shape block holder later.

The isolator shifts the beam path a bit, and the beam is needed to be re-align to the PMC. 

I couldn't finished it by Friday night, I think Frank re-aligned it already.

I'll measure RIN  and compare them again.

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. ()

A perl script/ an medm screen are created for SLOWDC PID control. The gain is not optimized yet. Some debugging might be needed.

The script for SLOW_PID.pl is similar to rcav_PID_2011_01_25.pl. I just changed the channel names for SLOWDC.

The process is C3:PSL-FSS_FAST

The actuator is C3:PSL-FSS_SLOWDC

Setpoint is C3:PSL-FSS_SLOWPID_SETPOINT (set to 0)

and other parameters follow the form of C3:PSL-FSS_SLOWPID_...

The perl scripts can be executed on the sun machine, but the result is abnormal. I tried to adjust P gain,

but for either signs I chose for Kp, SLOWDC seems to rail, instead of being steady at a certain value.

I'll check this later, for now it's not very important, since another loop for SLOWDC is also working, and

we can lock the cavity for a long time without SLOWDC feedback.

note for thermal PID

      RCAV   ACAV

P        -0.9         -0.9

I       -0.006      -0.0028

D       0                0

 I listed some issues for FSS experiment that we should discuss within this week and other small details.

              ==important issues==

• ***Design Suspension of both cavities in the same vacuum chamber ***
• ** Decide if we want to active control the cavities' temperature or not, so we can choose

           1) heater/insulation for the cavities, If we don't want to actively control the temperature, insulation inside the chamber might not be necessary.

           2) the appropriate AOM type, and

          3) Local oscillator for beat measurement

•   Experiment's Goal, do we want to develop a new TNI? if so, should we redesign the cavity so that the spot size is smaller, and make the cavity shorter in order to bring up the thermal noise level and become less sensitive to seismic noise?
  

              ==smaller topics==

•  Design for RFPD, do we want to change the sideband frequency?
• Servo for ACAV path, where do we get one, will we use the previous FSS?
• New periscope, do we want fixed or adjustable mirror mounts?
•  Beam splitter for the beat path, is it ok to use a cube bs with standard bs mount?

As Koji suggested,  I list out the plans which will be implemented in the setup and why do we want it. Comments and suggestions are very welcome.

The target is to measure the coating thermal noise from the beat signal to get 

 Mechanics part

•  new design for Seismic stack : so that we can keep both cavities in the same chamber (M1)
  

                       keeping both cavities in the same chamber will reduce the drift of the beat frequency. The design for the stack will be done and submitted to the machine     shop before this Friday by Frank.

•  thermal shield and DC heater (no feedback control for temperature) (M2)
  

                    Since both cavities do not have the exact same length, we need to be able to heat the cavity individually to obtain the required beat frequency (~10kHz). The heater will operate at DC level with no feedback control, so the noise from heater should be small. The stack, heater, and shield should be ready in 2 weeks. Once we have them we will just clean them without baking, since we are not worrying about residual gas.

•  Order Periscope set from Thorslab. (M3)
  

                     The periscope still has high Q resonance, we might need to think about damping it. 

Electronics part

•  Modifying 35.5 MHz RFPD for low power (1mW)  (E1)
  

                 We want RFPDs for lower power (1mW or less). Raphael is still working on it.

• beat RFPD  (E2)
  

                 The current RFPD is 120 MHz, so the area of the PD is very small. The beam has to be collimated down to ~100 um (the diameter of 1811 is 0.3 mm). The small spot size might cause extra scattering noise in the detection. So if we can reduce the beat frequency to ~ 10kHz, the beat RFPD can be a bit larger and scattering light may be lower.

• RF summing box (E3)
  

                    The RF summing box is for adding 35.5 MHz signal and high voltage feedback from the servo to the broadband EOM. It's not completely fixed yet (the ideal goal is to have resonant frequency at 35.5 MHz with 50 ohms impedance).  Once we fix this, we can check if the loop performance is getting better or not.

• SERVO for RCAV (TTFSS) (E4)
  

                     Once the RF summing box is fixed, we can measure the TF again. The measured bandwidth of the new TTFSS is about 200kHz, but we expect ~500kHz. We want to make sure that it works as designed and have enough gain. The current gain that we measured might be too low to suppress NPRO noise below thermal noise.

• servo for ACAV (E5)
  

                    For test, we will use the previous FSS for RCAV (from iLIGO). The gain for this servo just has to be large enough to suppress VCO noise. If we need a better servo, we can switch to the UPDH 2.0.

Optics part

• Opimize RCAV optics (EOM, faraday isolator, mode matching) (O1)
• Installed optics for ACAV path  (O2)
  
• design layout for beat path        (O3)
  
• RFAM from EOM (common mode effect)  (O4)
• Installed optics for beat (O5)
  

               I have to minimize the RFAM from EOM, but it will be a common mode effect. I have to think about what noise level it will show up in the beat detection if the two cavities are not exactly identical.

Misc

• correct the seismic noise model in the noise budget, since the transfer function of the stack is not what we have, and the suspension will be removed.
  

Time line(week start with)

Aug 1   :  submit design for seismic stack/thermal insulation (M1,M2), optimize the setup for RCAV loop (O1, O4), order periscope (M3)

Aug 8   :  Fix RF summing box ( E3), determine TTFSS loop performance (E4) (2011/08/09), if we are lucky, we can test the modified 35.5 MHz RFPD as well (E1)

Aug 15 :  Start install ACAV optics (O2), finish lay out for beat (O3)

Aug 22 :  Have seismic stack ready and put two cavity in the same chamber.   work on ACAV servo (E5)  ( Do and pass candidacy exam)

Aug 29:   install beat setup (O5), Try to get beat signal & configuration, try new beat RFPD (E2).

Sep 5: If we are lucky, we should see beat signal by the end of this week

Sep 12: done with poster. for Sep LVC meeting.

   We put two cavities in the same chamber. Next, we will measure the frequency difference between the two cavities when they have the same temperature to determine which frequency we need for the second AOM.

Before we put the stack back in to the chamber, we measured Q from two modes of the stack(with cavities on it).

The modes are longtitudinal motion (translation along the beam line) and horizontal transverse motion [see this entry]. Other modes i.e. pitch, yaw die away too fast to be measured with ring down measurement.

The cavities' mirrors were checked and cleaned. ACAV's mirrors had a trace of smudge which is now cleaned.

Since the pendulum suspensions were removed, the cavity's height is reduced from 7" to 6" above the table. The periscope's height were adjusted accordingly.

The beam is aligned to RCAV, the visibility is ~85%, it decreases because the position of the cavity's height changes.

Now I'm trying to align the beam to ACAV. Once the beam aligned, we can lock the beam and measure what is the differential length of the two cavities, hence the frequency.

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.

Evan, Kate

We made a few minor modifications to the optical breadboard in transmission of the cavities.

For one, we removed the QWPs which were the first optics in the transmission paths. These had been necessary for the prior cavities where the Silica Tantala mirror coatings were not birefringent. The circular polarization which was transmitted needed to be turned into linear polarization to get the beat note on the PD. Now, because the cavities with AlGaAs coatings are birefringent, the resonant and transmitted light is already linearly polarized and the QWPs unnecessary. Before removing them, the power on the main readout PD, a PD1811, was 208 mV. Afterwards, it was 194 mV. 

Second, we started to set up the fiber coupler to send some light to the ATF lab where it will later be used in a PLL to stabilize the laser used for the seismometer sensing. There's a 29.5 uW pick-off of the North cavity transmitted light which had been dumped. I found a high reflector mirror to put in its place to direct light to the fiber. I also made sure the fiber coupler at the other end is secured to the table and the output dumped. A first attempt to couple the light did not work, but I need to find a way either to monito remotely the power transmitted or just temporarily feed the fiber back into the CTN lab. 

On the south path, we have placed a HWP so that the transmitted beams can have their polarizations matched. It is on a 1" post and held down with a fork.

In the longer term, this should probably be replaced with the solid metal blocks that were used to hold the QWPs. If these blocks are reinstalled, the waveplate mount should be twisted slightly in yaw in order to reduce the amount of backscatter into the cavities.

With cavities mode matched and PDH locked on reflection we are now looking to find the beat notes between the two separate cavity-laser systems from the transmitted refcavity beams.

The transmitted beams were realigned through the combining beam splitter using the existing iris positions.  It appears that the the beam alignments, after various adjustments, were coincident on the 125 MHZ beat note detector but at different angles.  One would expect to see a beat note but reduced by the fringe across the beam front. As it is best practice to have overlapping and co-propagating beams, the south path and then the north path were walked to correspond with iris located on the unused port of the PLL (phase locked loop) combining beam splitter. 

We searched for a beat note over a selection of laser temperature offsets.  The South laser was locked to its refcavity at 0.8,1.68,2.53, 3.50 and 4.36 V slow control offset* and then the North cavity was locked at all combinations of 0.475, 1.359,2.252,3.147,4.129,5.042,5.965,6.976 V.  We did not observe a beat note when looking at the PD RF output on a spectrum analyzer. It is very possible that the corresponding FSRs of the two cavities just were not within the bandwidth of the detectors.  We should look around for a RF detector with >=1 GHz bandwidth and place at a pick off somewhere further up the laser path as a way of quickly checking the laser offset without the need to also align the cavity resonance.  This would save us a lot of time and pain carefully walking the refcavity temperature until we had corresponding overlap.  Once we have known operating point this process will be much easier, but a second RF PD would be a very useful diagnostic tool to have; we should generally try to do things the easy way.

*Note that there is a mode hopping region towards the top of the south cavity's range wich prevents going any higher in temperature offset (using the front panel voltage) for now.

How about this procedure?

- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.

- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.

- Lock one of the cavities.

- Scan the laser temp of the other laser to find the beat again.

- Lock the second cavity with several temp settings.

- Figure out how much heating of which cavity you need.

Hi, I'm an undergrad and I'll be helping to upgrade the lab data acquisition system. I'm starting off with getting data from fb4, making plots of lab temperature, laser power etc which would lead to posting them into html pages.

Link to Github repo: https://github.com/shufay/LIGO-plots

- Shu Fay

quickplot.py makes quick plots of data from desired channels. See: https://github.com/shufay/LIGO-plots.

On ws1 cd to ~/Git/LIGO-plots. In Ipython:  %run quickplot.py <channel 1> <channel 2> ... <(optional) gpsLength> < (optional) gpsStop>

To see usage: %run quickplot.py usage

Arguments:

<channel 1> <channel 2> ... Channels that you want to make plots of

<gpsLength> Length of time to fetch data. Default is 3600s.

<gpsStop> GPS time to fetch data until. Default is now. So the default parameters would fetch data from (now-3600, now).

took some images with the thermal imaging camera of the AOM installed in the PSL so far. The first three pictures show the AOM driven with 75MHz, 80MHz and 85MHz.

85MHz:

80MHz:

75MHz:

it is interesting that the hottest zone is at the end of the crystal, not where the pzt is mounted. It looks like the crystal is not proper mounted. Here a normal image for comparison...

so i took some pictures of a different AOM, but same model. Here are the pictures for different power levels @ 80MHz:

this looks normal...

*** Rana: I've replaced Frank's AOM picture with a zoomed in one. This circuit looks a little sloppy to me - why are the coils so loose?

measured the impedance from 70MHz to 90MHz for three different AOMs we had in the lab - two Isomet 1205C-843 and one Crystal Technologies 3080-194.
It turns out that the tiny power modulation on top of the big power modulation and pointing effects is related to some resonance effects in the AOM.
The Isomet AOMs show both this feature, the other one not. So we decided to install the one from Crystal Technology...

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

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

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

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

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

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

[koji, frank, tara]

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

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

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

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

I'm optimizing the AOM to make sure that we have max power, and to minimize pointing  due to frequency change, and minimize ellipticity of the transmitted beam.  With the current status, the ellipticity of the beam is 0.59. I'll check what happen to the visibility of ACAV with this beam shape.

  Today I decided to optimize the AOM since I removed it for inspection (I thought it was broken, but it's not). Before I put the AOM back to its place, I made sure that the double-passed beam path overlaps with itself nicely, and parallel to the table plane. Then I inserted the AOM in the beam path, and adjusted the position(tilt/yaw) until the first order beam's power was maximized and got the beam with ellipticity = 0.59. 

   With the non gaussian beam profile we will have less power in ACAV, more noise on RFPD. In addition,  the visibility of the cavity will be reduce and we have to measure it so that we can estimate the power input to the cavity accurately. This number will effect the calculation of our photothermal noise.

In order to debug the intensity noise that I found after the installation of the EAOM in the South path I have removed it from the path. The ASD measured at the ISS photodiode

located in transmition of the South cavity is anyway higher than what we have in transmition at the north cavity. Tomorrow I will try to optimize the other two EOMs alignment located

in the South path and then implement the EAOM again. However I see a very high drifting in the beate note.

The EAOM is again in place but not the lambda/4 after it yet.

I found out that the intensity noise issue that I found after the implementation of the EAOM

is due to the lambda/4 implemented after it. I did not figured out what it is happening there.

At moment with the only EAOM there is no intensity noise issue.

(As reminder the lambda/4 is supposed to be rotated to produce circular light, splitting the light

in s and p at the output of a PBS located after it)

However I have optimized the setup of the lambda/4 soon after the laser. The light coming 

out from the laser is optimized to be linear and the lambda/2 after it is rotated to produce 1mW

after the Faraday Isolator (for the moment). This setup will be changed with the use of PBSO to

dump the light.

sandwich structures using plexiglas : "Damped Windows for Aircraft Interior Noise Control" http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.79.9176

bare plexiglas: http://www.eplastics.com/Plastic/Plastics_Library/Plexiglass-Noise-Reduction

We started making the acoustic enclosure around the input beam area (second half of the table, before the chamber). The frame is done. We haven't received all items for the panels yet, so we just tried to use the aluminum bubble wrap as test panels. And we just used a piece of plastic planes to cover the top. There is no improvement in acoustic coupling yet.

We added the lid on top of the enclosure. More work is needed to complete the box.

    We made the closing lids by cutting a 1/8" acrylic panel. A strip of soft foam was added between the frame and the lids to form a seal.

    We did a qualitative test by placing a white noise source inside the box and listening. The aluminum bubble wrap we used did not provide good noise reduction. So we replaced one side by a plastic piece (~1/8" thick) with a damping pad on. It could damp the noise pretty well. I'll borrow a blue bird microphone from Den tomorrow, so we can measure the TF or just the relative noise signal to check how much attenuation we get from our structure.