This measurement was done already about a week ago, in elog 7984. Can you please describe why the numbers for the last measurement were not believable, and what was done differently this time?
The comment itself was added by me.
The difference between the previous and new measurements should be described by Riju.
In the entry 7984, the description has several PDs mixed up. The measurement was done with the MCREFL PD.
But the DC transimpedance of the thorlabs PD (5e3) was used, according to the text.
I first wonder if this is only a mistake not in the calculation but only in the elog due to a sloppy copy-and-paste.
But the resulting shot-noise-intercept current was 50uA, which is way too small
compared with a realistic value of 0.1~1mA. I have never seen such a good value with
C30642 at the resonant freq ~30MHz. That's why I said "hard to believe". I guessed this wrong
DC transimpedance was actually used for the calculation.
You may wonder why this 50uA is unreasonable number.
Basically this is just my feeling and probably is same as Rana's feeling.
But "my feeling" can't be a scientific explanation. Here is some estimation.
Looking at my note in 2010:
https://wiki-40m.ligo.caltech.edu/40m_Library (Comparisons of the PD circuits by KA)
The expected shot noise intercept current (idc) is
idc = 2 kB T / (e Rres),
where Rres is the impedance of the resonant circuit at the resonant freq.
This Rres is expressed as
Rres = 1/(4 pi^2 fres^2 Cd^2 Rs),
where Cd and Rs are the capacitance and series resistance of the diode.
If we input realistic numbers,
Cd = 100pF
Rs = 10 Ohm
fres = 30MHz
We obtain, Rres ~ 300Ohm, and idc = 0.2mA
In other words, Rs needs to be 2~3Ohm in order to have idc = 50uA.
This is too small from the previous measurements.
Test Results for C30642 LSC Diode Elements by Rich Abbott
I measured openloop transfer function of the phase tracking loop for the first characterization of phase tracker.
What is phase tracker:
See elog #6832.
For ALS, we use delay-line frequency discriminator, but it has trade-off between sensitivity and linear range. We solved this trade-off by tacking the phase of I/Q signals.
Figure below is the current diagram of the frequency discriminator using phase tracker.
OLTF of phase tracking loop:
Below. UGF at 1.2 kHz, phase margin 63 deg for both BEATX and BEATY. Phase delay can be clearly explained by 61 usec delay. This delay is 1 step in 16 KHz system.
Note that UGF depends on the amplitude of the RF input. I think this should be fixed by calculating the amplitude from I/Q signals.
BEAT(X|Y)_PHASE_GAIN were set to 300, and I put -3dBm 100 MHz RF signal to the beatbox during the measurement.
Other measurements needed:
- Linear range: By sweeping the RF input frequency and see sensitivity dependence.
- Bandwidth: By measuring transfer function from the modulation frequency of the RF input to phase tracker output.
- Maximum sensitivity: Sensitivity dependence on delay-line length (see PSL_Lab #825).
- Noise: Lock oscillator frequency with phase tracker and measure out-of-loop frequency noise with phase tracker.
- Sensitivity to amplitude fluctuation: Modulate RF input amplitude and measure the sensitivity.
I swept the frequency of RF input to the beatbox to calibrate and check linearity range of phase tracker.
Calibration factors are;
C1:ALS-BEATX_FINE_PHASE_OUT 52.1643 +/- 0.0003 deg/MHz
C1:ALS-BEATY_FINE_PHASE_OUT 51.4788 +/- 0.0003 deg/MHz
There was systematic error to the linearity check, but at least, calibration factor changes less than 50 % in the frequency range of 10 MHz to more than 500 MHz.
What I did:
Used network analyzer(Aligent 4395A) to sweep the frequency RF input to the beatbox and getdata of phase tracker signal. I swept from 10 Hz to 500 MHz with 501 points in 50 sec. This sweep is slow enough considering we could lock the 40m arms (typical speed of a mirror is 1 um/s, so bandwidth of the phase tracker should be more than 1 um/sec / 40 m * 3e14 Hz = 75 MHz/s).
RF amplitude was set to be -3 dBm and splitted into BEATX and BEATY.
Plots for BEATX and BEATY are below;
- Considering delay line length is ~30m, expected calibration factor is;
2*pi*l/v = 2*pi * 30 m / (2e8 m/s) = 0.94 rad/MHz = 54 deg/MHz
so, this calibration is reasonable.
- Since frequency sweep of network analyzer is not continuous, phase tracker output is like steps with some ringdown. This makes some systematic error for checking linearity. I'm planning to do slower sweep or continuous sweep. Also, the phase tracker seems like he can exceed 500 MHz.
I measured noise level of the phase tracker by inputting constant frequency RF signal from marconi.
Measured frequency noise was ~2 Hz/rtHz @ 100 Hz. It's not so good.
What I did:
1. Unplugged 11MHz marconi and put RF signal to the beatbox from this. Frequency and amplitude I put are 100 MHz and -3 dBm.
2. Measured spectra of phase tracker outputs, C1:ALS-BEATX_FINE_PHASE_OUT, C1:ALS-BEATY_FINE_PHASE_OUT.
3. Calibrated using the factor I measured (elog #8199).
4. Put marconi back to orignal settings.
- According to Schilt et al., this noise level is not so good.
- By changing the delay-line cable length or optimizing whitening filter etc., we can improve this.
We found that our phase tracker noise is currently limited by the noise introduced in DAQ.
We confirmed that the frequency noise was improved from 2 Hz/rtHz to 0.4 Hz/rtHz by increasing the gain of the whitening filter.
The whitening filters should definitely be refined.
What we did:
1. Put constant frequency RF input to the beatbox from Marconi and measured noise spectrum of the beatbox output(BEATX I) after the whitening filter with a spectrum analyzer. Noise floor level was ~0.2 Hz/rtHz at carrier frequency range of 15-100 MHz. Calibration factor of the beatbox output was ~380 mV/MHz.
2. Measured noise spectrum of C1:ALS-BEATX_FINE_I_OUTPUT(figure below). The noise floor didn't change when there was RF input of 100 MHz from Marconi(blue) and DAQ input was terminated (green). Also, C1:ALS-BEATX_FINE_I_IN1(which is before unwhitening filter) showed a flat spectrum. These show our spectrum is limited by DAQ noise, which is introduced after the whitening filter.
3. We increased the gain of whitening filter by x20 to show frequency noise performance can be improved by better whitening filter(red). But we can not use this setup as the other quadrature will be saturated by a too much gain at DC. Thus we need to carefully consider the signal level and the gain distribution of the whitening filters.
- Better whitening filters. The current one consists of zero 1 Hz and pole 10 Hz with DC gain of 5 using SR560.
- Better beatbox. We can increase the RF input power to the mixer and unify the preamplifier and the whitening filter in the box.
I think you have the splitter that splits the RF signal from the network analyzer in the wrong place.
Usually you split the signal immediately after the RF Out, so that half of the signal goes to the A-input of the Analyzer, and the other half goes to your controller (here, the laser diode controller). Then you would take the output of your controller and go straight to the actual laser diode, with no splitting in this path.
Here our device under test is the photodiode. So for the reference I wanted to retain the response of the laser diode controller. Otherwise I have to consider the transfer function of that LDC too. I may check both the options at the time of experiment.
I was sad to see that there wasn't a photo of the POX situation after the fiber work was done on Thursday.
Also, I was out looking at something else, and noticed that the fibers aren't in a very good/safe place from the POX table over to your splitter. Getting to the POX table is certainly more tricky than the AP table, since the fiber splitter is right next to the AP table, but we should go back and try to make sure the fibers to the more distant tables are laid in a nice, safe way.
Is there a reason that we're not using the clear plastic tubing that Eric bought to put the fibers into? It seems like that would help a lot in keeping the fibers safe.
I took a few photos of the things that I'm sad about:
1. We should not be keeping fibers on the floor in an area where they can be stepped on. This will be fixed (I hope) as part of putting the extra coiled length over by the splitter.
2. Again, in an area where we semi-regularly walk, the fibers should not be a tripping hazard. Behind the table legs (rather than under the middle of the table) is safer, and will help tuck them out of the way.
3. It's not obvious when we're pumped down, but we remove the access connector (top right side of this photo), and need to walk in this area. I can pretty much guarantee that within 1 day of the next time we vent, these fibers will be stepped on, tripped over, and broken if they are not moved to a different location. I'm not yet sure what the best way to route these fibers is, but this is not it.
Riju, since Eric will be away next week, please let one of us "40m Regulars" know when you plan to come over (at least a few hours ahead of time), and we can give you a hand in protecting these fibers a little bit better. Thanks!
Today I have rerouted the fibers on AP table to remove the fiber rolls out of the AP table. I removed the fibers one by one from both ends - from the 1x16 splitter and from the AP table - keeping the fiber roll intact, and then connected it in reverse way, i.e. the fiber end which was on AP table now is connected to the splitter (since length of the outside the roll is shorter that side) and the fiber end connected to splitter is now rerouted on AP table.
We need to keep the fibers in such a fashion so that no sharp bending occurs anywhere, and also it does not get strained due to its weight, particularly near the 1x16 splitter. Jenne suggested to use a plastic box over the splitter rack to keep the fiber rolls for time-being. We discussed a lot how this can be done nicely; in future we may use array of hooks, Koji suggested to use cable hangers and to tie the rolls using more than one hanging point, Jenne suggested to use the bottom shelf of the rack or to use one plastic box with holes. We tried to make holes on the plastic box using drill, but it developed crack on the box. So ultimately I used the opened box only and put it over the rack.
The corresponding photographs are attached herewith.
Tomorrow we will reroute the fibers for POX table.
Today I have rerouted the fibers on POX table. The aim was to lay it overhead through the plastic pipe. A pipe ~50ft (~15.5m) long was taken for this purpose. I disconnected the two 25m long fibers for POP55 and POX11 PD (those had been already routed) from both of their ends - i.e. from the POX table and also from 1x16 splitter. Jenne and Koji suggested that we may have another two PDs ( POP22 and POP110) on POX table in future. So we used another 25m long fiber for these two (POP22/POP110). We could not use two fibers for these two since we have only four 25m long fibers and one of them we need for POY11 PD on POY table. Jenne and me put the three fibers inside the pipe using a copper tube. The tube then was put on the overhead rack, Manasa helped me to do it. The fiber ends were finally laid on the POX table at one end and connected to the 1x16 splitter at the other end.
The corresponding photos are attached herewith.
Nice work. That was a lot of effort, but having done it so nicely will definitely pay off, since it is now much harder to break the fibers.
2 small issues: In your attachment 3, I see a coil of fiber just outside the POX table. I thought Koji had asked that all spare coiled-up length of fiber always be at the splitter side. Also, in securing the plastic tubing as it comes down near the PSL table, you have zip-tied the tubing to the PSL table. Since that is a space that we need to access to align the Xgreen beatnote stuff, please disconnect that zip tie, and secure the tubing on the north side somewhere, underneath the AP table, rather than the PSL table (when you look closer, you may notice that no cables in that bundle are attached to the PSL side at the bottom, for this same reason).
I have found in the depths of the elog the (original?) list of fibers and lengths that were decided upon: elog 6535.
In Suresh's elog, we were assuming that POP22 & POP110 would be served by a single PD. This is still the nominal plan, although we (Rana is maybe still thinking about this in the back of his head?) think that it might not be feasible. Riju and I were hoping to put a 4th fiber in the tubing so that we wouldn't have to add it later if POP22 & POP110 are eventually 2 separate PDs. Anyhow, for now, all we have available are 3 fibers for the POX table, so that is what was installed this afternoon.
Today I routed fiber from 1x16 splitter to POY table. Manasa helped me doing that. The fiber(25m) was laid on overhead rack through plastic pipe of length ~76ft. We put the fiber inside the pipe using one copper tube, and then tied the plastic pipe on the overhead rack. Finally one end of the fiber was laid on POY table and the other end was connected to the 1x16 splitter. The photographs corresponding are attached. There is no picture of splitter end, cause it was dark that time.
Koji asked me to look at what the ideal RF modulation frequency is, for just the PRMI case (no arms). If we had a perfect interferometer, with the sidebands exactly antiresonant in the arms when the arms resonate with the carrier, this wouldn't be an issue. However, due to vacuum envelope constraints, we do not have perfect antiresonance of the sidebands in the arm cavities. Rather, the sideband frequencies (and arm lengths) were chosen such that they pick up a minimum amount of extra phase on reflection from the arms. But, when the arms are off resonance (ex, the ETMs are misaligned), the sideband frequencies see a different amount of phase.
We want to know what a rough guess (since we don't have a precise number for the length of the PRC since our last vent) is for the ideal RF modulation frequency in just the PRMI.
If I take (from Manasa's kind measurements from the CAD drawing yesterday) the relevant distances to be:
L_P[meters] = 1.9045 + 2.1155 + 0.4067
L_X[meters] = 2.3070 + 0.0254*n
L_Y[meters] = 2.2372 + 0.0359*n + 0.0254*n
L_PRCL = L_P + (L_X + L_Y)/2 = 6.7616 meters.
The *n factors (n=1.44963) are due to travel through glass of the BS, and the substrate of the ITMs.
I find the FSR of the PRC to be 22.1686 MHz. For the sidebands to be antiresonant, we want them to be 11.0843 MHz. This would correspond to a mode cleaner length of 27.0466 meters. Our current modulation frequency of 11.066134 MHz corresponds to a MC length of 27.091 meters. So, if we want to use this 'ideal' modulation frequency for the PRC, we need to shorten the mode cleaner by 4.4cm! That's kind of a lot.
To change the MC length is not the point.
If we can improve the length sensing by the intentional shift of the modulation frequency from the MC FSR, that's worth to try, I thought.
But that is tough as the freq difference is 18kHz that is ~x4 of the line width of the MC.
Not only the 55MHz sidebands, but also the 11MHz sidebands will just be rejected.
Nevertheless: Is there any possibility that we can improve anything by shifting the modulation frequency by ~1kHz?
[Eric, Riju, Annalisa]
Today we have cleared up the fiber spool near AP table. We have put the 1x16 fiber splitter and a box (we made two openings on it) for fiber spool on a different part of the rack. Also put a plastic tubing or the fibers coming out of AP table. Now the fibers coming out from AP table and also from POX table first enter the box through one opening and the end of the fibers come out of the other opening to get connected to to splitter. Photographs of the work are attached. I don't think enough fiber is there to make a similar loop for fiber coming from POY table.
I positioned the fiber loaded protecting tubing and anchored them so they can do their job.
However, the area needs a good clean up.
Let's see if the ripples observed in the MC ringdown can be due to tilt motion of the mirrors.
The time it takes to produce a phase shift corresponding to N multiples of 2*pi is given by:
t = sqrt(2*N*lambda/(L*omega_T^2*(alpha_1+alpha_2)))
L is the length of the MC (something like 13m), and alpha_1, alpha_2 are the DC tilt angles of the two mirrors "shooting into the long arms of the MC" produced by the MC control with respect to the mechanical equilibrium position. omega_T is the tilt eigenfrequency of the three mirrors (assumed to be identical). lambda = 1.064e-6m;
The time it takes from N=1 to N=2 (the first observable ripple) is given by: tau1 = 0.6/omega_T*sqrt(lambda/L/(alpha_1+alpha_2))
The time it takes from N=2 to N=3 is given by: tau2 = 0.77*tau1
First, we also see in the measurement that later ripples are shorter than early ripples consistent with some accelerated effect. The predicted ripple durations tau seem to be a bit too high though. The measurements show something like a first 14us and a late 8us ripple. It depends somewhat on the initial tilt angles that I don't know really.
In any case, the short ripple times could also be explained if the tilt motions start a little earlier than the ringdown, or the tilt motion starts with some small initial velocity. The next step will be to program a little ringdown simulation that includes mirror tilts and see what kind of tilt motion would produce the ripples exactly as we observe them (or maybe tilt motion cannot produce ripples as observed).
Isn't it just a ringing of the intracavity power as you shifted the laser frequency abruptly?
Hmm. I don't know what ringing really is. Ok, let's assume it has to do with the pump... I don't see how the pump laser could produce these ripples. They have large amplitudes and so I always suspected something happening to the intracavity field. Therefore I was looking for effects that would change resonance conditions of the intracavity field during ringdown. Tilt motion seemed to be one explanation to me, but it may be a bit too slow (not sure yet). Longitudinal mirror motion is certainly too slow. What else could there be?
Laser frequency shift = longitudinal motion of the mirrors
Ok, so the whole idea that mirror motion can explain the ripples is nonsense. At least, when you think off the ringdown with "pump off". The phase shifts that I tried to estimate from longitudinal and tilt mirror motion are defined against a non-existing reference. So I guess that I have to click on the link that Koji posted...
Just to mention, for the tilt phase shift (yes, there is one, but the exact expression has two more factors in the equation I posted), it does not matter, which mirror tilts. So even for a lower bound on the ripple time, my equation was incorrect. It should have the sum over all three initial tilt angles not only the two "shooting into the long arms" of the MC.
It is essential we take a look at the ringdown data for all measurements made so far to figure out what must be done to track the source of these notorious ripples. I've attached the plot for the same showing the decay time to be the same in all cases. About the ripples; it seems unlikely to both Jan and me that the ripples are some electronic noise because the ripples do not follow any common pattern or time constant. We have discussed with Koji about monitoring the frequency shift, the input power to the MC and also try other methods of shutting down the pump to track their source as the next steps.
The fire alarm test was completed at 13:30 yesterday.
I updated the 40M Emergency Calling List by replacing Rob by Yoichy. The calling order: Vass, Aso and Taylor.
Rana and Yoichy were added to the "Registered PSL Operator List" and posted it in the lab. (not in the document, that should be up dated)
We are getting ready for the annual safety audit. It will be held next week at 14:00 Friday, Feb 13, 2009
Please participate in the preparation by correcting it or just tell me.
Rod Luna is organizing the pick up of the following old equipment:
HP laser jet 5000n, 3 of 19" 10 base-T network ports and 4 small hubs,
2 Sun monitors, 1 Viewsonic monitor, 7 keypads and
Hitachi scopes 2 of V-355, 2 of V-422, 1 of V-202 and 1 of V-6165
Osamu and Kiwamu received 40m safety training on Thursday, Feb 19, 2009
The 40m lab specific safety training is done. The participants were
Stephanie Erickson, Clara Bennett, Chris Zimmerman, Zach Commings, Michelle Stephen surfs and Drew Cappel postock.
They have already went through the Caltech Safety Office laser and general safety training.
They still have to read, understand and sign the the SOP for the laser & lab