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.
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.
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.
the new chamber is almost ready. I've attached three of four heaters, the fourth one has a fabrication error and has to be replaced. The chamber is currently pumped using the old varian ion pump. Current is already down to 100uA and one can watch it further decreasing. So my guess is that it will be clean beginning next week. I turned on two of the heaters to "bake" it a little bit. I baked the ion pump this afternoon while pumping with the turbo at 250C. The new insulated feet are ready and fit now to the chamber. Next task will be cutting the remaining holes into the foam and putting the temp sensors on the chamber.
Here a picture of the current status:
this afternoon the PSL RT DAQ equipment arrived and we moved all the stuff down to the PSL lab. We now have to expansion chassis, 6 A/D and 2 D/A cards available for the PSL stuff. Some of the stuff we have to share with Peter's power stabilization experiment, but some we can use for the cavity experiment. The current plan is to install a second front-end computer in the PSL lab which we can use for high-speed sampling of important signals of the stabilization setup.
i've made a cable to test the two Varian ion pumps we have. Before we move the cavity to the new chamber i would like to bake it and pump it using one of those pumps to see if everything is ok. Unfortunately we don't have a driver and a cable for those pumps. So i made a cable to use it with a Stanford high-voltage power supply. I hooked up the first pump to a test setup and it seems to be ok, but the valve i used is leaking so i gonna replace it by a spare one from 40m i got this afternoon. I will test the second one later. So as at least one is working we should buy a driver for that pump, maybe a used one or so. Right now we don't have any spare pump at all. Unfortunately the pumps used for the current chambers are different too ( totally different drivers and pumps), so we can't replace the failed part easily. I don't think that we have to worry but it might be handy to have one, e.g. if we want to use the third chamber to design and test the in-vacuum thermal shields.
we cleaned and assembled the new vaccum chamber. we installed an electical feedtrough (for sensors we might wanna have inside, e.g on the thermal shields or the stack), an electrical isolator part for the ion pump( to avoid ground loops with the high voltage source for the pump) and the first AR-coated window. We will replace the second one after we moved the cavity. Right now we only have to mount the ion pump and then we can bake the whole thing before we finaly move the cavity to its new home.
The mechanical workshop has to rework the new insulated feet we got today. The upper part where the cavity sits on was machined really badly. It didn't fit to the chamber at all...
We are still waiting for the additional heaters. They claim that they already shipped them last week so we don't know what happened to them. A week from San Diego to Pasadena? Who knows where they shipped it to (Antarctica?)
We've cut the last two parts of the insulating foam and glued everything together. Next we have to cut the holes for the flanges and feet of the chamber. The hole for the beam will be cut after we aligned the beam into the cavity
by now all parts are baked and ready for assembly. The only part we couldn't bake is the chamber itself. But Bob said that putting everything together and using the heaters on the chamber to "bake" the whole thing at lower temperature would be ok. So the plan is to assemble everything including an old 8l ion pump and bake the whole thing. As soon as all the insulation, temp sensors and stuff is ready we move the cavity into the (hopefully) clean chamber and replace the 8l pump by the 20l pump currently used on the other chamber.
we've cut the rest of the foam parts to fit on the smaller diameter (the main tube with the heaters). We already started to glue them together and have two halfs by now with a length of about 20". We have to make two thinner slices tomorrow to finish this inner part. We also have to glue the end caps together.
The current plan is to make 3 parts:
- one main section including the endcap parts divided into two parts which you put on the chamber from the sides
- one full endcap, which you put on the window from the end next to the edge of the table for easier access if we want to open the chamber
The current plan is to add one layer of aluminum foil and tie the three parts with aluminum tape together. We only need little force to put them together without any gap as we decided to use a 0.1" thick intermediate layer of foam which is typically used for wrapping stuff (its soft, flexible and Rod has plenty of it for packing things). This intermediate layer on top of the heater helps us to fill the gap between the heater, sensors and free space and the really stiff yellow foam. If we want to change things in vacuum we simple cut the aluminum tape along the junction and remove the endcap. This gives us full access to all the screws to open the chamber.
The new insulated legs should be finished by tomorrow morning...
over the weekend we baked the two AR-coated windows for the new chamber. Bob doesn't need the oven the next couple of days so i restarted the baking again and will continue to bake the remaining parts the next couple of days. We also set up peters old vacuum pump. We got lots of stuff from 40m and cleaned all parts today. The pump is now running and pumping the whole system including the hose to the chamber. We also wrapped some heaters around the parts and started heating the stuff to make it a bit cleaner as no one knows for what the parts have been used before. They all looked pretty clean and wiping everything didn't show any obvious contamination. We can't bake it to high as some parts are viton sealed.
The new, insulated feet should be finished by wed or so. Tomorrow we start cutting the remaining foam to size and glue the parts together. I ordered the remaining heaters, one is already attached to the chamber and it fits good except that the sticky back is not sticky enough to hold it in place. The bending force is too high, so i added some aluminum tape which holds it in place now (the corners didn't want to stick). We have plenty of space for temp sensors and we will add several AD590 and a couple of platinum sensors. If we find out that the AD590 is not good enough we can easily switch to the other sensors. We should discuss how many we want and especially where. My guess is that we should add some more on this first prototype to get a feeling for the gradients or so. We can then reduce the amount on the second chamber if we want. The platinum sensors are cheap. Typically one is about $8-10 each, but I bought a pack of 100 directly from the manufacturer and so its about $1 each only.
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,
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 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.
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.
40m entry on RIN induced thermo-optic noise here
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.
Check out the PSL directory from the SVN to see the details of the calculation.
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
last night one of the DAQ cards failed and the acav crate stopped working, so also the temp stabilization of the analyzer cavity stopped woking. I restarted everything this morning and the setpoint should be reached again by lunch time or so
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 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.
The calibration for ifr203 input for external frequency modulation is 0.714 MHz/Volt. For 160 MHz carrier, @7dBm, over +/- 1V range.
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 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.
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.
tuned the temperatures over night to be able to lock both cavities today. Cavities are now locked and Tara is optimizing the beat signal.
the laser stopped working while beeing in the lab but not touching the laser or table or anything. I was looking for an EOM mount and suddenly the chiller beeped. The power supply stopped working with a "PS error" message, but the more interesting fact is that the setpoint of the chiller was changed to 15degC (my guess is from the PS), but it should be at 26.5degC. So maybe the PS is also causing the freezing of the chiller, simply sending the wrong setpoint values when having a fault...
I finished aligning the beams from Ref cavity and Acavity, broke a connector on the PD we were going to used to measure the beat .
Frank borrowed another PD to use for now and ordered the replacement for the broken connector which should arrive next week.
Now I'm waiting for the temperature to settle, so both cavities can be locked and see the beat.
Peter gave me the hint that the default values are stored in an eeprom in the laser head. So connecting the head to a driver not used before shows the default values for the head. For this head it's 0.86A. So i measured the slope of the NPRO up to that value, reaching the 100mW at the default value stored in the head without tuning the diode temperature. So the head seems to be refurbished and not dead...
so please bring it back - we are waiting for it and can't go on without those... For the future: if you take it, bring it back and don't wait until someone is missing it...
does anyone know the typical operating current for the 100mW lightwave laser model ? (M126N-1064-100) It's typically ~1.1A for the 200mW model. I've set up everything and it starts to lase around 0.44A, so at least its not dead but i don't know how high up i can go. My guess is that it is something around 0.8A but i have no datasheet which tells me...
we had several shutdowns of the laser within the last days. A couple of times the well known "HT error", today we had an "PS error" for the first time. When does this happen? The other error is related to a malfunction of the chiller as we found out by luck. The chiller temp readout jumps from 26 down to 15 or so within a fraction of a second (so it's not real). This causes the PS to start heating even if the temp is high enough. This screws up the stabiliy of the laser and sometimes causes a chiller error as well. But the "PS error" ? Any idea?
I took a picture of the setup in PSL lab, and drew a line for laser path. I omit the mode cleaning part since it's not in use now.
Today I calibrated the QPD on the ref cavity. The armlength from AOM to the quad is 2.1 m.
The calibration for x and y channel on the QPD in radian unit
QPDX: radian = [dV]* 10^-3 / (3.36 * Vo)
QPDY: radian = [dV] * 10^-3 / (3.689 * Vo)
QPDY: radian = [dV]
/ (3.689 * Vo)
where [dV] is reaout voltage in Volt, Vo is the sum voltage from 4 quadrants in Volt.
If the prefer units are in micro radian and milli volt
QPDX: urad = [d_mV] / (3.36 * Vo)
QPDY: urad = [d_mV]/ (3.689 * Vo)
where [d_mV] is reaout voltage in milliVolt, Vo is the sum voltage from 4 quadrants in Volt.
Looking better. I'm curious about what the existing loop shapes are. The old FSS hardware is designed to drive an NPRO + 1 EOM. Is that the existing layout?
However, I think its not designed to drive an EOM. The PDH box should be able to drive the VCO if we replace the output OP27 with a TLE2027. The main point is that the refcav only provides a pole @ 40 kHz and we need the electronics to be 1/f below there. The old FSS board used to do this by the combination of the series resistor and the NPRO PZT capacitance. The VCO is not a capacitive load. I guess that Tara is working on a Simulink model of this whole setup?
I kind of doubt that we will have success without using resonant RFPDs. In most of the PSLs we use a pretty large modulation depth and its necessary to really tune the 2f trap to get rid of the J1^2 term.
Different idea: why not just use the VCO/AOM to lock to the ACAV? Then if we pick off the beam for the RCAV after the AOM, the feedback to the NPRO can be used as the differential cavity signal. In this way, we are not sensitive to the VCO calibration issues since its squashed by the ACAV loop.
Idea #2: Just use the transmitted light from the cavities and beat them. Its only phase detection, but in principle, this is easily good enough to detect what we want. Also RF sidebands are gone and all we need is an 1811 or such to detect the beat signal.
measured a couple of times today with everything re-aligned and different gain settings for the FSS stuff. Measured also to lower frequencies. The problem here is that the frequency band of interest can not be measured with epics (to slow) and the measurement using the SR785 takes so long that the operating point of the VCO and therefore the coefficient changes during the measurement and different spectra don't fit together quite well. so we have to measure a couple of times to get some measurements fitting together when the changes are so small that we don't see it with our eyes. this gives us a new upper limit of:
maximum gain settings:
CG : 24dB
FG : 13dB
everything is back, pointing is much much less, power modulation too, ugf of the other loop is much higher - but nevertheless the performance is not much better, only one order of magnitude. it turns out that the noise is limited by the existing FSS stuff. tuning the gains (common, fast) can produce almost any shape and level (see graph), except much lower levels are not possible, at least not for more than a couple of seconds before everything starts to oscillate. so i will to debug the old FSS stuff first to see whats going on there. will try to investigate the noise of the FSS loop and maybe replace the VCO by a function generator and probably the RF photodiode to see if that changes something. Those can be exchanged easily without changing to much...
After verifying that the Isomet AOM causes the power modulation and realigning it did not seem to help improving the pointing effect, we switched to a Crystal Tech AOM. The 1st order diffracted beam has 55% of the total power, the remaining power before entering the cavity is ~15mW. The pointing effects observed from QPD improved a little bit to 500 mV on X and Y directions.
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...
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.
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?
seems like a bad AOM and an impedance matching problem together. exchanging the semi-rigid cable by other cables produces a lot of different results in power modulation / pointing vs frequency. right now i found a combination where for frequencies 80MHz and higher it is almost flat, but below it drops a lot. the pointing is related to the absorbed power in the AOM. - you can really feel the heat of the crystal, no joke. will measure the temp on tomorrow for different frequencies where the pointing is worst case to see if it is a macroscopic change in temp. Aidan has this nice thermal image camera to do this. i think we should try the crystal technology AOM we have. i don't think that aligning helps a lot here. tried this the whole afternoon :-( the lowest power modulation over the entire frequency range is about 20%pp
added the following channels:
C3:PSL-RCAV_DIFFPWR : diffracted power (single pass) measured behind curved mirror
C3:PSL-126MOPA_PWRMON : laser output power monitor measured after PC
C3:PSL-RCAV_QPDSUM changed back to QPD sum signal
all signals available in both framebuilders
Seem like a screwy AOM. I would take the double-passed beam and beat it with the initial beam (no cavities). Make sure the frequency shift is appropriate and make sure there is no amplitude change in the beat over the whole VCO range.
in order to get an idea about the part/subsystem causing the RF power modulation while sweeping the VCO frequency i changed the length of the cable to the AOM. I decided to just extend the existing cable using a cheap SMA cable, not one of those semi-rigid ones. Now i thought due to more losses, low quality cable, longer, more connection etc. the shifted power should be less, but it seems to be the opposite, see screenshot. The purple curve is the transmitted power through the refcav, red the VCO input signal, the rest unimportant. On the left third the old cable, then a short break where i extended the cable, the center part with the longer cable, another short break and then with the original length. First i thought it's some aligment related thing, touching the cable, changing the alignment of the AOM etc. But it's not. The PD behind the curved mirror (not shown) shows exactly the same. Any ideas?
added a detector behind the curved mirror to measure the changes in deflected power (1.order) while changing the frequency of the VCO. PD is a large area Si diode to reduce pointing effects. Signal is connected to QPD-SUM instead of the QPD signal (QPD channels for x and y still connected and valid, only sum disconnected !!). We didn't trust the sum anymore as we can see slightly pointing related changes there as well.
values for VCO pwr 4.7V
RCTRANSPD signal: 5.17V
DC-level of RF-PD:
refcav unlocked: 590mV
refcav locked : 202mV
current cavity temp: 67.7degC
slow actuator value : 0.8966V
After adjusting the mirror for reflecting beam back to the AOM, the QPD signal shows the better alignment. Before the voltage different readout is about 1.2 V, now it's reduced to ~500 mV.
We'll try to add a translational stage for the mirror for better alignment.
we spent the whole night to re-align everything. By now everything is back online, both cavities were locked.for a short time. We still have the pointing problem. It's different now, seems to be smaller but is now almost the same for x and y. Will do this later today ....
The 10kHz peak in the old spectrum seems to be related to the crossover frequency of the fast actuator and the pc of the fss loop.
The PDH box is modified, all filter stages now have socket adapter boards to easily change the filter frequencies by changing the capacitor value within seconds to optimize everything. A zero for compensating the cavity pole is also installed. A modulation input too.