tuned the temperatures over night to be able to lock both cavities today. Cavities are now locked and Tara is optimizing the beat signal.

 Today  we finally see the beat between transmited beams from Ref Cavity and A Cavity.  Now we are trying to use a local oscillator [IFR2023b] to demodulate the signal.

The beat signal will be fed to a low pass amplifier and  sent to IFR2023 as external  frequency modulation.

  The drift in the demodulated signal (beat frequency x local oscillator) can be tracked by using ifr2023 and sr560. We successfully set the control loop, but

the detail about how ifr2023 works will be reviewed for clarity. The SR560 is set at gain -100, low pass at f = 10Hz. The power spectrum of the drift can be seen from the attachment.

Volts? What are Volts???

This plot should be converted into radians/rHz or Hz/rHz in order to be used.

Plots without physical units should almost never be used. Always Calibrate.

The calibration for ifr203 input for external frequency modulation is 0.714 MHz/Volt. For 160 MHz carrier, @7dBm, over +/- 1V range.

 I changed the gain setting on SR560 to find how it will effect the noise floor of the RC noise.

It seems that the gain and the cutoff frequency do not alter anything at lower frequency (below 100Hz), but they

change the position of the peak around 1kHz.

I'll find out noise from SR560 and the PD to see if their noises are dominating in this f.

Noise floor from PD and SR560 are measured, then converted from V/rHz to frequncy/rHz by the calibration from IFR2023 (0.7 MHz/Volt).

 SR560 has the same setting (gain invert 100, low pass filter at 1 Hz) as it did yesterday (when I measured the power spectrum

of the feed back signal.) The power spectrum of the feedback signal (from yesterday) is plotted in gray,  noise floor from PD( mixed with 160MHz signal from IFR2023) is plotted in pink, and noise from SR560 is in blue. I'll get the RC noise data on 2010Feb09, so I can plot them on the same graph.

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

Check out the PSL directory from the SVN to see the details of the calculation.

I got a quarter wave plate from Greg Ogin this afternoon. The attached plots show:

black-> RC noise when there are no quarter wave plates. It's the beat of circularly pol beams.

green-> when one wave plate is intalled.(from last week)

Blue-> two wave plates are in used. I'm surprised that nothing changed much from green.

Red-> I use the Buzby box to amplifie the demodulated signal and connect it to SR560 for filtering. The SR560 complains about overloading signal when I

set gain at 1, but it's ok when gain is 2.

It seems still far from noise budget Rana gave me. The peak around 1000 Hz might to be investigated.

40m entry on RIN induced thermo-optic noise here

This link points to a measurement of the frequency noise at the 40m.

'MC_F' means the feedback to the VCO used to keep the MC locked. Below ~100 Hz, there is also feedback to the MC length and so you cannot assume its frequency noise.

From 100-1000 Hz its all acoustic pickup on the PSL table. Above 1 kHz, I believe its all VCO phase noise.

I also suggest that from now we all decide to post the actual data along with the plots we post here. Also the .m matlab file which generates the plot as done at GEO. This makes it much easier to reproduce the result a year down the line.

 I took the data of frequency noise of the functiongenerator("Marconi") and spectrum analyzer's noise from Mott's elog on  Nov 13 14:37:11 2009,  AdhikariLab.  Thanks, Mott

 To measure the noise @160Hz, two Functiongenerators are set at 160MHz, then mix two signals togather to get phase noise. Multplying the phase noise by corresponding frequencies to get frequency noise. 

Assuming two ideal function generators, the freq noise is divided by sqrt(2) to get noise contributed by one function generator.

The attached graph shows frequency noise from Marconi noise, detector noise(spectrum analyzer that measured Marconi noise) , beat noise( noise from beating two beams after the cavities), and estimated noise.

VCO noise will be updated soon.

VCO frequency noise is measured. V input is 4.7 Volt. The signal output from the VCO (which controls the AOM) is mixed with signal from ifr2023 at ~80MHz. The demodulated signal

is then fed to SR560. Gain setting is 5 x10^3, low pass at 1Hz. The output signal is split into two. One is sent back to ifr2023 for freq modulation, another one is fed to the spectrum analyzer. The voltage output is

then converted to frequency by calibration from last week (0.714 MHz/V).

The plot shows frequncy noise from:

1: function generator (ifr2023) in brown

2: VCO,  in pink

3&4 Frank's RC noise data from FEB09, in red and blue

5:  RCnoise from beat measurment, in green

6: estimated noise

VCO seems to limit our noise at f=100Hz and higher/

 you have to measure a new calibration coeff. If you change the center frequency of the marconi the maximum FM modulation range changes too and so the coeff. If i remember right your range was 500kHz compared to 1Mhz last week, right? So it should change about a factor of 2 or so, so the actual measured noise is a little bit less but nevertheless much to high

 Yes, I completely forgot about that. I calibrated it again, at 80Mhz, 7.0 dBm, freq devn 500khz. and it is 0.35655 MHz/volt. About a factor of 2 smaller.

I'll put up the corrected plot soon.

I measured the VCO noise again.  2 methods that I tried

1) Measuring noise from a) suppressed signal (signal from pre-amp which is fed back to IFR2023) and b) error signal ((beat signal coming out of the mixer)) and match them together

Setup for 1)

ifr2023b,  carrier f = 79.994620 Hz, power =    7.0 dBm,     FMdevn = 500kHz

mixer     Mini Circuit ZFM-3-S+   (Lo input = 7dBm, RF input = -1.05 dBm)

pre amp, SR560, Gain inv10, low pass at 1Hz

2) Lowering the unity gain frequency and measuring only the error signal (beat signal coming out of the mixer)

Setup for 2)

ifr2023b,  carrier f = 79.995 316 Hz, power =    7.0 dBm,     FMdevn = 1kHz

mixer     Mini Circuit ZFM-3-S+   (Lo input = 7dBm, RF input = -1.05 dBm)

pre amp, SR560, Gain inv20, low pass at 0.03Hz

Converting V/rHz to f/ rHz

1)The feed back signal can be convert to freq noise by using the calibration from last week, which is 0.35 MHz/V, since two setups on IFR2023 are similar.

We can obtain the calibration by giving input voltage to the LO and see how the carrier freq changes.

2) For the  error signal out of the mixer, disconnect the feedback signal and measure the slope of the signal output. That will be [V/rad] calibration.

Then we can convert V/rHz -> rad / rHz , multiply by the corresponding f to get f/rHz

 Red and Green plot are from the 1st method,

Blue plot if from the 2nd method.

The results between two methods are not even close to each other, I'll check tomorrow to see if I did something wrong,

 I calculated the frequency for other higher Hermite-Gaussian modes, (n+m up to 20) to make sure that there is no overlap between these frequencies

and our choice of sideband frequency.

The beam frequency for higher order modes will be different from that of fundamental mode because of the phase shift. The frequency shift can be found in

Siegman, p 762 (Thanks Zach for pointing me to this). The phase shift from TEM00 will be multiples of 258.29 MHz. To find 20 possible lines, we  use

  N* 258.9  mod 737   , N = 1,2,3....20 (the free spectral range is 737 MHz), This will be the contribution from left side of the interested peak. To find the contribution from the right  side we use,

 737 -  N*258.9 mod 737.  

The plot below show the allowed frequency for higher order TEM (from the closest fundamental mode at f=0 and 737 MHz)  on x axis, the FWHM is exaggerated for clarity. They are smaller in real life.  The side bands at 21.5 and 35.5 MHz (our EOMs) are plotted in red.

I only plot from -100 to 100 MHz.

I deleted your plot, since it contained no axes labels. We have a strict rule against plots like those. You must have physical units and labels in all plots. See the gyro HOM plots for an example.

There's a correction

1) the beam waist inside the cavity, it seems that the 237 um(which corresponds to frequency shift = 258.29)  I got does not agree with the ROC and the cavity length.

 I calculated the beam waist based on the same ROC and the cavity length, and got 261 um (which correspond to frequency shift = 219.32 MHz)

This plot based on the frequency shift of 219.32 MHz step,

And now I plot from -100MHz to 100 MHz, the 35.5 MHz peak coincides with a peak from one of higher order modes( n+m = 20)

The exact number of that peak is 35.6 MHz.

Last time, the calculated beam waist I got was based on two identical mirrors with no reflecting coating layers on the back mirror. When that layers are taken into account, I got the same waist size.

The new plot is attached below. Two sidebands are 21.5 and 35.5 MHz. The peaks, from left to right, are 9th, 6th, 3rd, 28th, 25th, 22nd, and 19th order. Choosing 35.5 MHz for frequency modulation should be fine.

The layout for the new PSL setup ( lenses, and their positions are to be calculated.)

A Lightwave 100mW NPRO laser will be the source. AOM will be in the ACav path.

Two cavities will be covered by a box of insulation/ heater.

1) From the laser to the PMC,

    1/4 waveplate, to linearly polarize the elliptical polarized beam from NPRO

    1/2 wave plates and PBS, to adj the power of the beam

    lens, to focus the beam to the EOM

   two lens, two mirrors, to mode match the beam to the PMC

   a photodiode, a lens, two mirrors, (one for steering the beam, another one for attenuating the beam), for PDH locking

 a photodiode and a ccd camera, for the beam behind the PMC

* there will be a Faraday Isolator somewhere here. I forgot to add it.

2) From PMC to PBS

  a lens to focus the beam to 35.5 Mhz EOM

  1/2 wave plate, to adjust the power between two beams for  ACav and RefCav

 PBS, to split the beam into two paths

3) AOM path

 1 PBS, for reflected beam from the AOM

 a lens, an AOM, 1/4 waveplate, two irises, 1 curve mirror; to double pass the beam and shift the frequency

another iris and a mirror, to select only the 1st order beam and send it to  ACav.

4) RefCav/ACav path

 1/2 wave plate, to correct the polatization

two lens and a set of periscope, to mode match the beam to the cavity

 a pbs with 1/4 wave plate, a lens, a mirror, a photodiode, to PDH lock the beam

5) Transmitted beam

1/4 waveplate, to linearly polarize the transmitted beam

a photodiode/ a ccd camera to monitor the transmitted beam with necessary mirrors

lenses, to focus the beam to the PD that measures the beat signal

 a beam splitter, to mix two beams together

HOM frequency shift for RefCav is plotted below. The second one has clearer dots.

Y axis is the frequency shift in MHz

X axis is the (n+m)th order of the Hermite Gauss mode

The waist of the beam inside the cavity is 261 um* (symmetric cavity, R =0.5m, L = 0.2032 m.) 
Thus, the frequency shift between n+m+1 and n+m mode is 219.763 MHz. (see Lasers, p 762 for details)

Blue line represents the 0th order of the carrier's frequency (thus =0) The purple and the brown lines, at y= 35.5 and -35.5 MHz, are the 0th of + and - sidebands respectively.

The color dots represent the frequency shift from Higher order mode which is specified on x-axis, blue for HOM from the carrier's frequency, purple and brown for HOM from +/- sidebands.

 Choosing 35.5 sideband seems to be ok, the 27th order should be small and negligible. 

*The number 237 um for waist size is the effective beam waist size of the "emerging beam," not the real waist size in the cavity. The beam passes through the mirror which acts as a negative thin lens and changes the

beam parameter. 

 code is in SVN

I measured the beam waist of Lightwave NPRO 1064nm 100mW with WinCamD.

The nominal beam waist are 380 um and 500 um, 5cm from the center. the number I got from the measurement are 237 um (major) and 187.3 um (minor) which are quite different from the nominal values.

I'll check it again tomorrow to see if the data are still the same.

I measured the beam waist again. The laser was operated at full power ~100mW. A mirror attenuated the beam to 60 uW and ND  4.0 was on the CCD.

The fits give

 Wx = 155 um, 3.45 cm in front of the opening.

Wy = 201 um, 2.8 cm in front of the opening.

Details for Mode Matching

1)   Laser to PMC

The laser has

              Wx = 155 um, 3.45 cm in front of the opening.

            Wy = 201 um, 2.8 cm in front of the opening.

The average number for calculation is w = 180 um, 3cm  in front of the opening.

First, we focus the beam to the EOM, w can be  250 – 500 um.

We pick 350 um.

F= 200 mm,

W1= 180 um

W2=350 um

Distance from w1 to the lens, d1, = 9.36”.

Distance from w1 to w2, L, = 22.8 “.

Then we mode match this beam to PMC

F1= 50.2mm

F2= 63mm

W1=350 um

W2= 370um

Distance from w1 to first lens, d1,         =157mm =6.2”

Distance from first lens to 2^nd lens, d2 =120mm = 4.7”

Distance from 2^nd lens to PMC, d3         = 358mm

Distance from w1 to w2, L,                      =  63.5 cm = 25”

I'll check if I can find f =200mm, 63mm, 50mm in the lab or not.

we still have serious problems with the 100mW laser head. I traced it down to the connection between the PCB and the hermetically sealed optical part. There is a really loose connection somewhere. The connector seems to be OK, the PCB and all solder points are OK too (visually). But if you slightly touch the PCB you can see the yellow LED flickering, if you touch it a bit more the laser goes off and on. This happens too if you touch the D-SUB cable on the back. I added some little stress to the PCB when putting the thing back together, now the situation seems to be better, but is far away from being gone. In order to get started we are using it now as long as we can and think about a solution in the meantime. One option would be to fix the busted NPRO Peter has. This would probably take about a week or so. We have to align the laser diode and focusing lens and solder (!)  the focusing lens in place. The problem is that you can't align in vertical direction, so you have to remove the laser diode, put some more or less indium foil below and start again from zero. But it's an option. Peter had it already back to 550mW (out of 700mW) or so (for a couple of minutes, simply holding all parts in place. So the difficult part is to keep it permanent in the right place...

I'm trying to align the PMC.  The transducer is connected to the PMC servo card's HV out.

HV in is driven by a function generator with triangular function. The output signal looks weird. It is not a nice triangular form, it's more like a u shape waveform connecting to each others with a plateau on top.

I'll check if the transducer on the PMC and the HV out from the card are working correctly or not. Right now, there is not a glimpse of signal coming out of the PMC on CCD.

don't the words HV IN ring a bell? look into the schematic. there are only a couple of inputs at the front so it's not too hard to figure out why there was already a BNC cable connected to that input. Did you check the monitor signal? If you would you would have seen that your HV supply is missing now

After checking the PMC servo card,

ramp signal goes to ext DC 

HV in is applied properly,

HV out is fixed.

Now the PMC is scanning and working fine.

The alignment is done. Although optimization is still needed,  I can work on the rest of the setup.

*note on PMC servo card

Ratio between Voltage input (Vin), V monitor (Vmon), and High Voltage output (HVout),

HVout = 24 Vin

Vmon = 1/50 HVout

Vmon = 24/50 Vin

 see the attachment

 We still need two SMA cables. One connects between 21.5Mhz EOM and PMC servo card, another one connects between PMC PD and the servo card.

The 21.5 and 35.5 local oscillator were in the wrong slots, we fixed them.

The 21.5 MHz  photo diode that detects the reflected beam from PMC saturates at 15 mW.

Now I'm trying to optimize the PMC setup so that we have maximum transmittedd power.

You should make sure not to blow up the PMC PD: i.e. the total power on the PD out of lock should be less than 100 mW. The beam size on the PD should also be ~0.5 mm diameter with the actual beam waist being more like 0.3 mm dia.

PD should also be tilted by ~30 deg from normal incidence and the reflection dumped. Its OK if the RF output saturates somewhat at the peak of the PDH signal, but the in-lock RF level should be below ~50 mVrms into 50 Ohms.

 The power is small, our laser is 100 mW. The power on the PD is not saturated. Right now it's receiving ~10mW. It's tilted a bit and the reflected beam is blocked appropriately.

Dear elog

  I don't know why I can't lock the PMC. After changing the 21.5 MHz card because of the loosen SMA connector, the 21.5 MHz EOM is working.

The error signal looks good. I adjusted the gain, flipped the phase by 180 degree, and still cannot lock the cavity. 

(The medm was frozen this morning, Peter helped reset it back to work this after noon.) It might be insufficient amount of power coupling into the cavity.

The minimum reflected beam I could get is only ~1/2 of the total DC power, I'll try to align the beam and move the lens a little bit more to see if I can optimize it better.

So I skip this PMC part for now, and pre align the path to mode cleaner. The mode matching is ok. Two lens, f1=114.5 and f2=286.3 mm are good. A CCD behind the cavity is set in place.

      After adding another 1/2 plate to have P wave into the cavity, I can lock the PMC cavity. It's been 30 minutes so far.

There is only one transmitted beam now (there were two when I used S wave.) 

     When I work or knock on the table, sometime the beam switches to another mode (might be it's side band.)

It's very close to the main TEM00 mode. I need to adjust the DC offset a little bit to get back.

 The incoming power is 9 mW, and the Transmitted beam has ~ 6 mW.

Current setting:

RF Amp: 7V

Phase shift: 2.87 + 180

Gain: 27.91 dB

DC offset: -2.3 V

     I tried to pre align the ref cavity. It's harder than I thought, can't see the beam that well. I'll have to check the manual for the laser controller, so that I can

scan the RefCav when I align the beam into the cavity.

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.

This is the schematic of PSL fss servo.

I have to make sure that the modulation voltage will not exceed the controller's limit (0-100V.)

I switched the cable from the 10W controller to the original controller for 100 mW laser. It is working well now, the cables are tied properly.

For now, I don't need to use the FSS servo card to scan the laser frequency.

I'm using a function generator for fast channel (PZT), and a voltage calibrator for slow channel (thermal control.)

The alignment is in progress. With the aid of a CCD camera and a macroscopic lens, looking for the beam position on the mirror is getting easier.

Currently I see some light at the back of the cavity.

 Since the frequency of the laser going into RefCav is determined by PMC ,I decided to temporarily remove the PMC for now, so I can scan the laser frequency while aligning RefCav.

It might not be a good idea since the PMC might alter the beam path a little bit, but I just want to align the cavity first.

The plan is after  RefCav is aligned, I'll bring back the PMC and fine tune the beam going to RefCav again.

I still got many higher order modes coming out of the cavity.

 the PMC is locked to the laser, so it follows the NPRO frequency when you scan the frequency

I see TEM00 transmitted beam out of RefCav. I think I have to fine tune the FSS gain a bit more becasue the spot still oscillates a bit.

The left monitor shows the spot from  PMC. The right monitor shows the spot from RefCav. it looks distorted becasue of the filter.

The common gain is 16.1 dB

slow actuator is 9 V

Fast Gain 15dB

Optimization is yet to be done. There is still plenty of reflected power.

 nice

This is the schematic for PSL setup.

http://131.215.115.52:8080/PSL_Lab/117

At this point, Pre Mode Cleaner (PMC) and Reference Cavity (RefCav) are locked. The rest will be locking Analyzer Cavity (ACav) and setting up for beat noise measurement.

ACav's beam path will have double pass AOM [Crystaltech 3080 194]. We'll use +1st order beam. When hook up the VCO, make sure that the power is on only when the VCO and the AOM are connected, otherwise the VCO dies.

Next is aligning the AOM. A good alignment will maximize the power of the +1st order beam. The beam should get close to the AOM's transducer as much as possible to minimize time delay.

The beam at the AOM will be focused to 75 um.

The mirror that reflects the beam back to the AOM is a 0.3m concave mirror, which will be placed 0.3 m away from the AOM. The reflected beam should completely overlap on itself. This will neutralize

the pointing instability when the modulating frequency shifts.

Then we can align ACav, this time I'll try not to remove the PMC when I scan the beam frequency (at~3-10Hz.) If the PMC cannot catch up with the laser, increase the gain of the PMC, sideband power. 

 ACav should be locked before Monday June 8.

I'm aligning the AOM. The R=0.5m mirror's position crosses the insulation border by 1.5" (see attached picture.) The black line on the table shows the border of the insulator. The mirror is on a translational stage.

I'm thinking of 2 choices to solve this,

 1)using a mirror to turn the beam to the side of the table. The mirror will be placed after the AOM, around the edge the border.

2) using 2 mirrors (after the beam is split to RefCav and ACav's paths) to shift the beam path to the side of the table.

The first choice will be better, since I won't have to recalculate the mode matching, but there might be unexpected problems.

The VCO is working fine, I can see +/- 1st order beams coming out.

 I'm aligning the AOM and maximizing the diffracted beam's power by positioning the AOM and adjusting the beam size by moving the lens.

For single pass, the maximum efficiency I could get is only ~60%, so for double pass, the power will be down to 36%, but for now I'll settle with this number.

I could not find the manual for Crystal technology AOM 3080-194. The closest one is model 3080-197 which is attached below.

I'm not sure what is the difference between the two model, but 3080-197 has 70% diffraction efficiency.

Because of adjusting the lens, the RefCav's beam path also changes, now I have to realign RefCav again.

  Another step for AOM alignment is adjusting the mirror that reflects the transmitted beam back to the AOM again.

The distance between the mirror and the center of the AOM should be the same as ROC of the mirror.

After this I should be able to start locking ACav.

 did you measure the power of the vco? How much is it if you tune it to maximum?

Here a copy of a general datasheet for the 3080-194. maximum efficiency is ~80% @2W RF power. You should ask peter about the detailed datasheet which comes with each AOM and contains measured values for the one you are using. Measured values depend on the beam size and RF power. Typical values are 87% in reality.

 Oh, I see, the beam diameter is 1100 um, I use 150um. I'll try changing the beam size and see what happens. Thanks Frank. I'll measure the power of the VCO too.

 have a look into the datasheet which came with the AOM. Don't make it too large. Clear aperture is about 1.7mm max. You can also have a look into the manual of the 35W laser (ATF lab). It contains a copy of one of these datasheets as well (with the graph of efficiency vs beam size). You don't need more than 60%, but you should try to get around 50% for the double-passed beam as we don't have so much laser power in total available. Assuming the original 15mW on the RF detector you need about 45mW for the acav now and 15mW for the refcav, so 60mW total after the PMC. With the current 95mW out of the laser it should be no problem( in principle). After the isolator and EOM you might have something about 85mW upstream of the PMC which means you need 70% transmission through the PMC. Anyway, a larger beam size gives you better eff.  If you make it 500um or so you should get 50% in the double-passed configuration.

 I manage to get 70% efficiency from P wave. When I try S wave, I get 78% which is close to the specified value. So for double pass, the efficiency should be upto 50%. The beam size is ~550 um.  I redo the mode matching calculation for the AOM (and also RefCav and ACav) and move the beam a bit to the side of the table so that the insulation box won't get in the way.