Brief Summary: I am currently looking at the acoustic noise around both arms to see if there are any frequencies from machinery around the lab that stand out and to see what we can remove/change. I am using a Bluebird microphone suspended with surgical tubing from the cable trays to isolate it from vibrations. I am also using a preamp and the SR875 spectrum analyzer taking 6 sets of data every 1.5 meters (0 to 200Hz, 200Hz to 400Hz, 400z to 800Hz, 800Hz to 3200Hz, 3.2kHz to 12kHz, 12kHz to 100kHz).
· Attachment 1 is a PSD of the first 3 measurements (from 0 to 12kHz) that I took every 1.5 meters along the x arm with the preamp and spectrum analyzer
· Attachment 2 is a blrms color map of the first 6 sets of data I took (from 2.4m to 9.9m)
· Attachmetn 3 is a picture of the microphone set up with the surgical tubing
Problems that occurred: settings on the preamp made the first set of data I took significantly smaller than the data I took with the 0dB button off and the last problem I had was the spectrum analyzer reading only from -50 to -50 dBVpk
PMC and IMC re-aligned and re-locked. Both cavities are staying locked. Arm cavities are also locked.
[Annalisa, Terra, Koji, Gautam]
Summary: We find a configuration for arm scans which significantly reduces phase noise. We run several arm scans and we were able to resolve several HOM peaks; analysis to come.
As first, we made a measurement with the already established setup and, as Jon already pointed out, we found lots of phase noise. We hypothesized that it could either come from the PLL or from the motion of the optics between the AUX injection point (AS port) and the Y arm.
In this configuration, we were able to do arm scans where the phase variation at each peak was pretty clear and well defined. We took several 10MHz scan, we also zoomed around some specific HOM peak, and we were able to resolve some frequency split.
We add some pictures of the setup and of the scan.
The data are saved in users/OLD/annalisa/Yscans. More analysis and plots will follow tomorrow.
We installed two heaters setup on the ETMY bench in order to try inducing some radius of curvature change and therefore HOMs frequency shift.
We installed two heaters setup.
Elliptic reflector setup (H1): heater put in the focus of the elliptical reflector: this will make a heat pattern as descirbed in the elogs #14043 and #14050.
Lenses setup (H2): heater put in a cylndrical reflector (made up with aluminum foil) 1'' diameter, and 2 ZnSe lenses telescope, composed by a 1.5'' and a 1'' diameter respectively, both 3.5'' focal length. The telescope is designed in such a way to focus the heat map on the mirror HR surface. For this latter the schematic was supposed to be the following:
This setup will project on the mirror a heat pattern like this:
which is very convenient if we want to see a different radius of curvature for different HOMs. However, the power that we are supposed to have absorbed by the mirror with this setup is very low (order of 40-ish mW with 18V, 1.2A) which is probably not enough to see an effect. Moreover, mostly for space reasons (post base too big), the distances were not fully kept, and we ended up with the following setup:
In this configuration we won't probably have a perfect focusing of the heat pattern on the mirror.
See Koji's elog #14077 for the final pin connection details. In summary, in vacuum the pins are:
13 to 8 --> cable bunch 0
7 to 2 --> cable bunch 2
25 to 20 --> cable bunch 1
19 to 14 --> cable bunch 3
where Elliptic reflector setup (H1) is connected to cables 0 and 1, and the lenses setup is connected to cables 2 and 3.
This is the installed setup as seen from above:
Annalisa, Gautum, Koji, Terra
Summary: with the reflector setup, we measured a frequency shift of the first and second order modes! First looks of shifts show 1st HOM shift ~-10 kHz, 2nd HOM shift ~-18 kHz (carrier ~4 kHz). We saw no shift with the cylinder/lenses set up.
- - - - -
Tonight we modified the cavity scan setup: the LO is provided by the Marconi which, at the same time, is also used to scan the AUX laser frequency instead of the Agilent. In order to get rid of the free running noise between Marconi and Agilent, the Marconi frequency was scanned and, point by point, the Agilent center frequency was changed accordingly. In order to speed up the process, the whole procedure was automated. The script is called AGfast.py and can be found in /users/annalisa/postVent.
One thing that helped in improving the data quality of the phase information was to set the Agilent IF bandwidth @1kHz. Not yet clear why, but it was better than having a lower bandwidth. To be further investigated.
With this setup, we made some coarse scan of the full FSR and then we "zoomed" around the main peaks in order to increase the resolution and get a more precise information about the peak frequency.
Here are the frequency ranges that we scanned:
We powered the heater of the lenses setup @4:55 am at 14.4V and 0.9A. Then we slightly increased the power @5:05am and the final "hot state" configuration is with heater powered at 16V and 0.9A.
With this setup we couldn't see any frequency shift
Then, at around 6:30 am we turned on the reflector setup and we measured a frequency shift of the first and second order modes. First scans show 1st HOM shift ~10 kHz, 2nd HOM shift ~18 kHz. First attachment shows carrier hot/cold, second attachment shows HOM2 hot/cold. We started to get plauged by high seismic noise. Heaters turned off at 7:45 am. Lots of scans and actual analysis to go.
gautam: about the questionable plotting -
My personal favourite plot is Attachment #3, which is a 5 MHz scan (cold) to identify positions of the various peaks. The power of including phase information in the analysis is clear. The second FSR on the right edge of the plot is not as prominent as the first is because the arm transmission was degrading throughout the measurement. For future measurements, we should consider locking the IMC length to the arm cavity - this would eliminate such alignment drifts, and maybe also make the PLL control signal RMS smaller.
- The new REFL165 PD was installed on the AP table
- The REFL165I/Q signals are now showing sensible and robust PRCL/MICH signals
- PRMIsb was locked only with these REFL165 signals
- Installation of the REFL165 PD
We prepared the REFL165 PD for the 4" optical height. The actual issue was the power supply for the PD.
We soldered wires between the PD and the RF PD interface break-out board. Then the PD interface
cable for the old REFL165 (iLIGO style) was connected.
At the REFL port, most of the light is rejected by the first beam splitter (R=90%?). We attenuated the beam by a factor of 10
using an ND filter. The new PD showed the DC output of ~10V. This corresponds to the photocurrent of 5mA.
(cf. the shot-noise intercept current is ~1mA)
The output of the REFL165 PD was checked with the RF spectrum analyzer. It was a bit surprising but we had a forest of
RF signals betwen 11MHz and 178MHz. We tried to use a high-pass filter with fc=100MHz (SPH-100) but still the rejection
was not enough. We ended up with using SPH-150 (fc=150MHz).
- Whitening / Demodulation phase
Then we connected the RF output to the SMA cable to the LSC rack. We immediately saw the nice signals from REFL165I/Q
channels, namely sensible structure of pendulum resonances (1/3/16Hz peaks) and floor level.
The whitening level was changed from 21dB to 45dB (max). The DC offsets in the I/Q channels (of the order of 2000~4000)
were removed by the ./LSC/LSCoffset script.
Firstly we locked the PRMI with the usual signals (REFL33I and AS55Q).
The demodulation phase was roughtly tuned (1deg precision) such that the Q phase signal is minimized,
assuming most of the signal is coming from PRCL. Our choise is 74deg.
In this configuration, PRCL shows same quality of signal as our prefered PRCL (i.e. REFL33I) in the amplitude and the sign.
We switched to the REFL165 signal by handing off at the input matrix. The input matrix element for REFL165_I was gradually
increasded up to 0.8 while the element for REFL33I was gradually reduced to 0. We did the same for REFL165_Q with the element of 0.2.
Now we tried locking with REFL165I/Q from the beginning. Once the alignment is adjusted, the lock was immediately obtained
only with REFL165I/Q. Today we did not adjusted the ASC stuff (OPLEVs and PRM ASC) so the lock was not long (<1min). Particularly
ITMX poiting kept drifting and it made the lock difficult. We should check the oplev setup carefully.
- LSC summary
Signal source: REFL165I (74deg) / Whitening gain 45dB
Normalization sqrt(POP110I x 0.1) / Trigger POP110I 100up 3down
Servo: input matrix 0.80 -> PRCL Servo FM3/4/5 Always ON G=+2.50
Actuator: output matrix 1.00 -> PRM
Signal source: REFL165Q (74deg) / Whitening gain 45dB
Normalization sqrt(POP110I x 10.0) / Trigger POP110I 100up 3down
Servo: input matrix 0.20 -> MICH Servo FM4/5 Always On G=-40
Actuator output matrix -1.00 -> ITMX / +1.00 -> ITMY
- Refine the PRM asc servo (AC coupled)
- Align oplevs
- ITMX oplev is drifting quickly (~1min time scale)
[Annalisa, Jenne, Rana, Steve]
We installed the fibres on cable trays the 1Y2 and the Control Room.
Still to do: find a power supply for the Fiboxes and plug everything in.
I studied the eigenfrequencies of a mirror support using COMSOL.
I studied the eigenfrequencies of a mirror mount designed with COMSOL.
I imposed fixed constraints for the base screws and for the screw connecting the base with the pedestal. Note that the central screw is connected to the base only for a small thickness, and the pedestal touches the base only with a thin annulus. This is in way to make a better model of the actual stress.
Shown in fig. 2 is the lowest eigenfrequency of the mount.
I' going to change the base and study the way the eigenfrequency vary, in way to find the configuration which minimizes the lowest eigenfrequency.
We want to measure the g-factor of the PRC using the beat note of the main laser with an auxiliary NPRO laser.
We are going to phase lock the NPRO to the main laser (taking it from POY) and then we will inject the NPRO through the AS edge of the ITMY.
Today Sendhil and I installed the auxiliary laser on the ITMY table moving it from the AS table.
We also installed the beam steering optics, except the BS which will launch the beam through the AR edge of the ITMY.
To do: install the BS, take the POY beam and mix it with the auxiliary laser on a photodiode to phase lock the two lasers, do better calculations for the mode matching optics to be used for the auxiliary laser beam.
Sendhil and I installed the S polarized BS on the ITMY table to steer the NPRO beam through the AR wedge and align it to the POY beam.
We took a shutter from the BSPRM table (which was not used) and a beam dump from the AS table (which was used by the auxiliary laser already removed and installed on the ITMY).
To do: do better alignment of the NPRO beam, maybe installing some iris after the BS and before the AS wedge, phase lock the two beams.
Zach has just replied, and said that we should feel free to take the laser from his iodine setup in the West Bridge subbasement, in the ATF lab.
Annalisa, please ask Koji or Tara to show you where it is, and help you bring it to the 40m. You should install it (temporarily) on the PSL table, measure the waist, and find the beat in IR. Elog 3755 and elog 3759 have some of the details on how it has been done in the past.
Ok, I'm going to contact Koji.
1) Annalisa is going to start working on mode profiling and beat note search for the old MOPA NPRO.
2) In the meantime, Manasa is working on the end table items. This will be reviewed by KA in the afternoon.
The laser at ATF is moved to the 40m when the status of 1) and 2) is determined by KA to be reasonable.
We also make the beat note measurement for the ATF laser too.
Today I installed mirrors to steer the pick-off from the main laser beam in a more free part of the PSL table and make the beat note measurement between it and the NPRO.
At the beginning I took the beam from the harmonic separator after the doubling crystal, and I was going to bring it in a less full part of the table . At the end I realized that there was already a beam steered up to a more free part of the table, and the beam is taken from the transmission of the PMC.
Tomorrow I'm going to use that beam to find the beat note with the NPRO.
I also removed almost all the steering optics that I used on the ITMY table to send the auxiliary beam for ABSL through the window parallel to the POY beam. The most important thing is that I removed the BS, which was on the same path of the POY beam (see elog 8257).
I moved the auxiliary laser from the ITMY table to the PSL table and installed all the optics (mirrors and lenses) to steer the beam up to a PDA55 photodiode, where also the pick-off of the PSL is sent.
Tomorrow I'm going to measure the beat note between the two.
Yesterday I tried to find the beat note between the main PSL and the auxiliary NPRO, but I didn't :(
Today I will do a better alignment of the two beams in the PD and try again.
Give us more info on the elog:
What PD are you using? How much power the beams on the recombining BS are? What kind of BS is it?
How are you looking for the beat note? (on the scope? or spectrum analyzer?)
What was the scanned temp range?
Three points to be checked:
I'm using a 1611 New Focus PD (1 GHZ, with maximum input power 1mW), and the total power hitting on the PD is of about 0.650 mW.
The current of the NPRO laser is set to 1.38 A, so that the input power is 19 mW. The beam is initially damped by a 10% reflection BS and then it hits a 33% reflection BS (where it recombines with the PSL pick-off beam) with 2 mW power.
After this second BS the power is reduced to 0.592 mW.
The PSL pick-off hits on the 33% reflection BS with 65.5 uW power, and it exit with a 47 uW power.
I connected a power supply to apply a Voltage to the slow frequency BNC, in way to tune the laser frequency.
I'm using the AGILENT 4395A Spectrum analyzer to make the measurement. I tried to use the HEWELETT PACKARD 8591E spectrum analyzer, but the monitor didn't turn on.
The temperature spanned until now in only of about 10 deg C, because I realized that I needed a better alignment, so I added a lens in front of the PD and I did a better alignment.
Moreover, the current of the laser is too low, so I need to increase it and add more beam splitters in the beam path to dump the beam, in way to don't reach the PD threshold.
I knew that both the beams are s-polarized, but maybe I can check it again.
Is there a reason to use non-45 degree incident angles on the steering mirrors between the laser and the PD? I would always use 45 degree incident angles unless there is a really good reason not to.
Actually not, it is a mistake! It is one of the things I'm going to modify, in addition to add more BS to reduce the power.
[Annalisa, Manasa, Koji]
I updated the setup for the beat note. The main reason is that I needed to keep the ADJ to 0 in way to operate at the nominal laser power.
Now the input power of the laser is increased (about 315 mW) and needs to be dumped so as not to exceed the PD threshold of 1mW.
Moreover, a lens has been added to match the two beams size.
A BS has been removed from the PSL pick-off beam path, so the PSL power hitting the BS is now about 100 uW, and the total power on the PD is 0.7mW.
I also verified that both the beams are S polarized.
To find the beat note, the laser temperature has been varied through the laser controller and not adding a Voltage with the power supply.
A range of temperature of 30 degC has been spanned, but we suppose there should be some calibration problem with the controller, since set temperature is not the same as Laser temperature on the display.
Anyway, no beat note has been found up to now.
An external monitor has to be added to check the real temperature of the crystal.
The next possible plan is to vary the PSL temperature and try to find the beat note.
P.S.: The HEWELETT PACKARD 8591E spectrum analyzer works! The monitor only took some time to turn on!
The beat note between the main PSL and the auxiliarly NPRO has been found!
The setup didn't change with respect to the one described on the previous note on the elog. A multimeter has been connected to the laser controller diagnostic pin to read out the voltage that indicated the laser crystal temperature.
The connector has been taken from the Yend table laser controller.
The voltage on the multimeter gave the same temperature shown by "Laser temperature" on the display of the controller, while "set temperature" was wrong.
The temperature has been varied using the laser controller with reference to the voltage read on the multimeter display.
Starting from 35.2 °C, the temperature has been first lowered until 20 °C and no beat note has been found, then temperature has been increased up to 35.2 °C and the first beat note has been found at 38.0 °C.
It has been detected at a frequency of about 80 MHz with an RF power of -27 dBm and a frequency fluctuation of about +/- 4 MHz.
I made more measurements slowly varying the laser temperature, to see how the beat note frequency changes with it. I'll make the plot and post it as soon.
After measuring the beat note, the "Alberto" NPRO auxiliary laser has been moved from the PSL table to the POY table. Its beam profile is going to be measured. It's going to be used as green laser on the END table, in place of the broken one.
The auxiliary laser borrowed form ATF lab (which will be used for the ABSL measurement) has been set on the PSL table to make a measurement of the beat note between it and the main laser.
The setup is mostly the same of the previous beat note measurement . In this case, laser input power is 326 mW, so I needed to replace one of the mirrors of the steering optics with a BS 50% reflecting in order to have less than 1 mW on the PD.
Now, the total power on the PD is less than 0.5 mW.
I didn't measure the beat note yet to leave the PSL table as quite as possible for the locking procedures.
Measure the beat note, fiber coupling the NPRO laser to bring it to the POY table.
I plot the variation of the beat note frequency as a function of "Alberto" NPRO laser's temperature.
After some discussion, now I'm going to vary the PSL temperature and find the auxiliary NPRO temperature matching to have the beat note between the two.
The beat note for the ATF lab laser has been found.
The measurement has been carried out in the same way as described in elog 8368.
The only difference is that in this case I started from a temperature of 35.2 degC, and I reduced it until the minimum which was 30.71 degC. No beat note in this range.
Then I rised on the temperature and I found the first beat note at 41.46 degC. It has been detected at a frequency of about 120 MHz with an RF power of -53 dBm and a frequency fluctuation of about +/- 5 MHz.
I tried to improve the alignment to have a stronger beat, but it was the maximum I could reach. Maybe I could increase the power hitting the photodiode, which was 0.453 mW.
"Alberto"NPRO laser has been moved again on PSL table in order to make a measurement of the beat note varying also the PSL temperature.
It is useful because if the PSL temperature would drift we have to know which is the NPRO temperature that returns the beat.
I'm going to measure it tomorrow.
I measured the beat note between the "Alberto" NPRO laser and the PSL varying the PSL temperature and find the matching NPRO temperature that gave the beat.
I first switched off the FSS loop for the PSL, then I varied its temperature and switched on the loop back.
PSL temperature has been varied starting from 31.88 °C (its starting temperature) down to 23.88 by 1°C step, and then from 31.88 °C up to 36.92 °C, always with a 1°C step.
For each PSL temperature, the NPRO temperature was varied as well, in way to find the temperature to have a beat note between the two.
The trend of the NPRO laser temperature reminds the frequency change of the laser as a function of the crystal temperature continuous tuning.
I made measurements only for the first temperature of the NPRO laser which gave me the beat note. Tomorrow I'm going to find the beat note also for higher frequencies of the NPRO laser.
The beat note between the PSL laser and the "Alberto" NPRO laser has been measured. In particular, for each PSL temperature, more than one Aux laser frequency has been found.
The second of the three curves seems to be more stable than the other two, even if a "step" trend can be found in all of them (maybe due to the frequency change of the NPRO laser as a function of the crystal temperature continuous tuning, as mentioned in the previous elog). This is the reason why the points are not perfectly aligned, and the errors on the fit parameters are so big.
I made a Simulation with COMSOL for the Yend table. Mainly, I tried to see how the lower eigenmode changes with the number and the size of the posts inside.
The lateral frame is just sitting on the table, it is fixed by its weight. I also put a couple of screws to fix it better, but the resulting eigenfrequency didn't change so much (less than 1 Hz).
In Fig. 1 I didn't put any post. Of course, the lowest eigenfrequency is very low (around 80 Hz).
Then I added 2 posts, one per side (Fig. 2 and Fig. 3), with different diameter.
In some cases posts don't have a base, but they are fixed to the table only by a screw. It is just a condition to keep them fixed to the table
Eventually I put 4 posts, 2 per side.
The lowest eigenfrequency is always increasing.
At the end I also put a simulation for 4 1.6 inch diameter posts without base, and the eigenfrequency is slightly higher. I want to check it again, because I would expect that the configuration shown in Fig.5a could be more stable.
P.S.: All the post are stainless steel.
The ATF NPRO auxiliary laser has been moved on the PSL table. All the optics for beat note measurement are in place and alignment has been done.
The setup for this measurement is the same as described in elog 8333.
I rotated some mounts along the green beam path, and I started aligning the beam again.
The beam is aligned up to the waveplate just before the doubler crystal, even if I couldn't reach more than 88% transmission for the Faraday. Next week I will finish the alignment and I'll put the lenses that Manasa already ordered.
Yend table - Current status
Today the 2m focal length lens along the oplev path (just after the laser) has been added. In Manasa's layout it allows to have a beam waist of 3.8mm on the OPLEV QPD, even if it seems to be smaller.
The laser is closer to the box wall than the layout shows (it's on the line n.1 instead of line n.9), so maybe it has to be moved in the position shown in the layout, as Steve suggests, to leave empty space just before the window.
Rana suggests a 2mm diameter beam on the QPD, so a new calculation has to be done to add a second lens.
The beam has been aligned until the doubler, but after the crystal it it has a small tilt, so a better alignment has to be done.
Moreover, the beam waist has to be measured after the Faraday for the green, in way to choose the focal length of the lenses necessary for the mode matching.
Then the three steering mirrors to send the beam into the arm have to be put.
A lens which has to be put on the Transmon path (already ordered) has to be added, and the beam alignment on the QPD-y and on the PDA520 has to be done.
The new lenses arrived, and I put the right 250mm before the doubler. I'm still not so confident with the alignment, because I cannot get more than 11-12 uW out from the "green" Faraday, with more than 200uW going in.
I replaced the Y1 mirror with an HR1064-HT532. The alignment has to be done. Today the 50cm focal length lens arrived, and I'm going to put in tomorrow.
I still have problems in maximizing the power out from the doubler. I realized that the real green power I obtain is about 30 uW, and it is the power which really enters the Faraday.
Before I was measuring it just after the Harmonic separator, and there was some residual IR beam which increased the power on the power meter, that's why I obtained about 200 uW.
I also tried to slightly vary the position of the mode matching lens, but I was not able to get more than 30 uW on the power meter.
The 50 cm focal length lens has been added in the position shown on Manasa's layout, and the beam has been focused on the PD.
The alignment for the green has been improved, so that we have much more green power.
The first lens position along the IR path has been changed in way to have the beam waist at the center of the first Faraday. In this way we had about 91% of the input power out from it.
The two cylindrical lenses which were used to correct the ellipticity of the beam have been replaced by a single lens. Its focal length is intermediate between the focal lengths of the two cylindrical.
Moving the position of the lens before the doubler crystal and improving the alignment we got about 1mW of green light (0.35% of the incoming IR beam).
After aligning the green beam through the second Faraday, the beam waist of the outgoing beam has to be measured and the mode matching calculation has to be done to choose the two MM lenses. Then the steering mirrors will be placed to send the beam into the arm.
I aligned the green beam into the Faraday. I needed an HWP to have the right polarization for the light entering the Faraday itself.
I tried to dump as much beams as possible with razor dumps, but eventually I had to use some "temporary solutions" for higher beams, because I didn't find the right mounts for razor dumps.
I measured the beam waist after the Faraday with the beam scan. Analysis and MM calculation to follow.
I aligned back the beam (we lost part of the alignment after we put back the box and after the posts were installed). The green beam out from the crystal is still low, but anyway I get about 1.2 mW of green out from the Faraday.
Mode Matching calculation (tomorrow)
Fix the dumping situation
Replace some of the mounts with more solid ones (in the future)
QPD, PD and Camera have been rotated as Rana suggested last Wednesday. A 1m focal length lens is on the main beam transmitted path (before the harmonic separator), and the beam diameter on the QPD is about 5mm. We put another lens with a shorter focal length to put the PD very close to the beam waist and in way to have a reasonable beam size on the camera. Tomorrow I will write down all the correct sizes of the beams.
(for Steve) I marked a possible beam path for the Oplev (the laser is not in the right place in the picture, but I left it in the correct place on the table). I also put the QPD for the IP-ANG, so we know in which part of the table the beam can be steered.
The space in the red rectangle (right corner) has to be left empty to put a PD for the rejected beam from the green Faraday.
Mode Matching calculation for green beam - Yarm
After measuring the beam radius out from the Faraday for the green, I made the calculation to match the green beam mode with the IR mode inside the arm.
The beam waist after the Faraday is elliptical, and I found the following value for the waist:
w0x = 3.55e-5 m @ z0x = -0.042 m
w0y = 2.44e-5 m @ z0y = -0.036 m
(the origin of the z axis is the output of the Faraday, so the waist is inside the Faraday itself)
I did the calculation using a la mode, using as beam waist and its position the following values:
w0 = sqrt(w0x*w0y) = 2.943e-5 m @ z0 = (z0x+z0y)/2 = -0.039 m
The results are shown in the attached plots.
Focal length (m) position (m)
lens1 0.125 0.1416
lens2 0.100 0.5225
L 1.000 1.5748 (fixed lens used to focus transmitted beam)
As the first plot shows, the green beam size on the ETMY is about 6mm. My concern is that it could be too big.
The third plot shows the X and Y section of the beam. It is strongly elliptical, but nevertheless the coupling factor calculated with Koji's formula gives C=0.936 for the astigmatic beam, and C=0.985 for the non astigmatic beam, so it seems to be still ok.
I got confused. Is the mode calculation in the cavity correct?
Are you sure the wavelength in the code is 532nm?
The first plot says "the waist radius at ITMY is 2.15mm". This number is already very close to
the waist size of the cavity mode (2.1mm@ITM, 3.7mm@ETM), but the spot radius at ETMY is 6mm.
They are inconsistent.
Jenne and I just realized that a la mode has 1064e-9 m as default value. I'll change it and make the calculation again.
Mode matching calculation for green - Yarm
I did again the mode matching calculation. The previous one was using 1064nm as wavelength, so it was wrong.
The seed beam waist and its position are the same as in elog 8637. The new results are shown in the attached graphs.
I got the following values for focal lengths and positions of the two Mode Matching lenses:
Focal length (m) Distance from the Faraday output (m)
lens1 0.125 0.1829
lens2 -0.200 0.4398
L 1.000 1.4986 (fixed)
The position of the lens L has changed because the path lengh has been slightly reduced.
The Coupling factor for he astigmatic beam is C = 0.959 (it is C = 0.9974 if we consider the beam as non astigmatic).
I put the lenses and aligned the beam up to the shutter, which has been moved from its initial position because the beam size on it was too large.
The green beam needs to be aligned and sent into the arm cavity.
Polarization has to be checked.
Many beams still have to be dumped, both in IR and Green paths.
I put many razor dumps along the IR/green path. The rejected beam from the IR Faraday needs to be dumped (about 1.5 mW). I used all the new razor blade I had, so I need one more for that beam.
The IR reflection of the Harmonic separator right after the doubler needs to be dumped in a better way. At the moment there is a black screen, but we need something suitable to dump more than 300 mW.
After the second steering mirror along the green beam path there is a very small transmission (about 6 uW), which is difficult to dump because there is no space enough. Can it be dumped with a black screen?
The Oplev has a lot of reflection hitting the central BS (The BS for the transmitted beam). It is very difficult to dump them without intercepting the main beam path. Maybe we have to slightly change the Oplev beam angle to avoid so many reflections.
For some strange reason the Yarm shutter cable runs up to the POY table, where it is connected to another cable going to the rack. It has to be put off from the table, at least. It would be better to have only one cable going directly to the rack.
I roughly aligned the green into the Yarm and I've seen the green beam flashing on the PSL table, but the mode matching is not so good and I get an higher order mode, so I'm going to fix the mode matching tomorrow.
Temporary oplev in place. The spot on the qpd is still big. My two lens solution did not work.
I will finalize optical component position of the oplev after the the arm transmitted and green beam optics in place. They have priority.
Oplev spot size on qpd ~ 1 mm
PS: I realized it later that the returning beam is going through a lens for TRY. This is a nono.
This beam path will be relayed again as the TRY, green beam and IP-ang get there place.
Oplev is disabled. I removed one of the steering mirrors because it was on the green beam path.
Since the beam waist after the Faraday had changed since the last time I measured it (maybe alignment changed a bit), I made a new mode matching calculation for green. I attached the results.
I'm going to align the beam into the Yarm.
RXA: JPG images deleted - replace with PDF please.
After working some more on the EY table, we are getting some TEM00 flashes for the Y arm green. We have had to raise the height of one of the MM lenses to prevent clipping.
We used a function generator to apply a ~300 mV 10 Hz triangle wave to scan the laser frequency while aligning.
We tried to use the C1:ALS-TRY_OUT channel to help us in our alignment but there are a couple problems:
1) It seems that there is an uncompensated whitening filter before the ADC - Annalisa is making a compensation filter now.
2) The data delay is too much to use this for fast alignment. We might need to get a coax cable down there or mount a wired ethernet computer on the wall.
3) We need to make DQ channels for the TRY and TRX OUT. We need long term data of these, not just test points.
I made the anti-whitening filter for the C1:ALS-TRY_OUT channel. But then I forgot to make an ELOG because I am bad.
[Annalisa, Gautam, Rana]
I made the anti-whitening filter for the C1:ALS-TRY_OUT channel.
zpk [,,1] Hz
Now we can look at the picks of this signal to align the green into the cavity.
We already had some 00 flash, but a better alignment has to be done.
- put the shutter along the beam path
- check the polarization (we have a new PBS for visible)
The green beam alignment has been improved, so we see much more 00 bright flashing. We checked the polarization and the Ygreen shutter is back in place.
A mirror is already in place to steer the rejected beam from the green Faraday into a PD, tomorrow morning we'll put a lens and the PD to take the signal for PDH locking.
DQ channels have been created in the C1ALS model for TRX and TRY. They are called TRX_OUT and TRY_OUT and the sampling rate is 2048 Hz.
The rejected beam from this Faraday comes out at a tiny, tiny angle and so its tough to pick it off without clipping the main beam.
Some care must be taken in setting this up - Steve may have some good ideas on what kind of mount can be placed so close to the beam.
Why did we ever order this terrible Faraday? Let's never get a Faraday with a tiny angle between the beams again.
The rejected beam from the Faraday is steered with a mirror into the PDA32A PD and a 75mm fl lens is used to focus the beam into it.
The main beam is a few millimeters away from the mirror mount (maybe 2mm), and I think it should be fine as long as the main beam is not supposed to move.
After connecting the PD with the reflection from the arm to the PDH box, theY arm has been locked on the 01 mode. Maximizing the alignment, we obtained a 00 mode locking, but we couldn't maximize the power.
The size of the reflected beam was different with respect to the size of the incoming beam, so probably a bad mode matching was one of the issues.
Moreover, the reflected beam is very low power. We need to figure out why it is so (bad alignment? related to mode matching?)
After measuring better all the distances, I did a new mode matching calculation. I put the lenses after measuring the beam waist, so the size of the beam on the lenses was the same as expected from the calculation. Nevertheless, the beam size on the beam splitter looks bigger than expected, and also in this case green flashes into the cavity at some HOM (again 01).
I also tried to lock again the cavity and maximize the alignment, but I didn't get any improvement with respect to the previous mode matching.