I'm setting up fiber optic so that I can send frequency stabilized laser to ATF. Right now the power coming out is small (0.8 mW from 20mW input). I'm working on better mode match for better efficiency.
Note: due to the space limitation, I cannot pick the beam after the broadband EOM used for frequency stabilized the laser. The beam is taken after the PMC, where it was used to dumped the excess power.
optical fiber: nufern pm980
modematching: The focal length of the collimator is 2.0 mm, MFD of the fiber is 8 um. The beam diameter at the lens is then ~360 um.
The coupled power is quite small. I'll check the mode matching again to get more power coming out.
I adjusted the mode matching a bit ( changing lenses positions and rotate the lens on the collimator). The coupling efficiency was up to 66%. This should be enough for now.
The power input can go up to ~20mW, so the output is ~12mW which should be enough for gyro. I also adjusted the polarization, so that the polarization of the input beam matched the fast axis of the cable. I tested the polarization of the output beam with a PBS and got the extinction ratio of ~ 670.
The fiber is polarization maintaining fiber, nufern pm980.
It has fast and slow axes, and we have to match the polarization of the input beam to the fast axis. To do that
If the beam polarization matched the fiber axis, the output beam will have linear polarization which gives the maximum extinct ratio. Meanwhile if the beam polarization does not match the fiber axis, the output beam will have elliptic polarization and the extinction ratio will be lower, since certain amount of power will be transmitted and reflected no matter how you rotate the beam.
Tara showed me the lab, along with the needed booties and safety glasses. We took off the plastic covering the table. I took inventory of lenses in the lab so I know what I have to work with in order to mode match the laser to the collimater/fiber optic. Mode matching will prevent the laser from scattering and allow the most power through.
We measured the distance from the PMC to the lens (8.2 in) and the total length from the PMC to the beam splitter that would come right before the lenses used to mode match (45 in).
I will try to catch up on understanding the parameters needed to mode match and will copy the programs Beam Profile Simulator and Mode Matcher onto my laptop in order to determine which lenses are needed. I also need to read up on various optical components used in the lab.
I read more about various optical components used in the PSL lab, such as the half and quarter wave plate, electro-optic modulator, etc.
Over lunch, Dr. Weinstein gave another talk for LIGO, this time focusing on the specific components on the interferometers, such as the mirrors and seismic isolators. Also of interest, he explained why we have certain elements in our bodies, namely due to neutron star mergers and star core implosions.
Another measurement was taken of the setup because I missed a lens in the beam path yesterday. I copied the program Beam Profile Simulator and Mode Matcher onto my computer from one of the lab computers and spent the afternoon learning how to use it and what different parameters meant. The programs allow you to enter certain parameters (beam waist, waist positions, lenses' focal lengths, total length, etc) and from those, determine needed ones, such as position of the lens. For some reason, when I input the setup into both programs, it did not work in MM. Then Tara and I tried entering the data into another program (I will ask for the name later) and that agreed with the BPS. Even when I changed the overall length of the whole system to be at the beam waist, MM still did not work.
I better understand how the collimator is set up, so I will obtain the needed information from Thorlabs, listed in the following link.
The two collimators we are considering to use have focal lengths, f=2mm and f=4.6mm.
Product numbers: CFC-2X-C, CFC-5X-C
We want the beam waist to be located at the lens in the collimator.
For mode matching, one needs to match the spot size (beam radius, w) and position.
Note: On the BPS, beam size means the beam radius, spot size, or w, as named in the paper by Kogelnik and Li, 1966.
for June 24, 2013
I used MatLab and JamMT to mode match a laser beam coming from the PMC to the cavity for Tara (see diagram at post 1100). It is for the lower or left PMC. Used various equations from Kogelnik and Li, 1966, to determine the q value and beam waist at various points, particularly equations 16, 18,10 and 41. The revised setup is in attachment 1.
I went to the LIGO laser safety training session led by Peter King. He discussed the importance of safety training, various biological effects lasers can have on the body, particularly the body, and how to work around lasers safely. Safety glasses have an OD number for different wavelengths and should be checked at the door before entering a lab. We have two forms that we need to fill out.
Then I went on to mode matching the fiber optic to the laser with the previously mentioned programs and will try coupling the fiber optic tomorrow.
June 25, 2013
Read some papers on noise due to fiber optics as well as cancellation of phase noise.
We borrowed a fiber optic cable from 40m, that maintains polarization. Realized that I did not include the collimator in calculations for mode matching, so I redid it. The collimator I chose has f = 2mm, with the distance between the fiber and lens 3.5 um. The lenses used to mode match have focal lengths f = 143.2mm and f= 74.9 mm.
I have set this up on the table, but have yet to do alignment with the cable. Tomorrow (June 26), I'll try to get the laser beam into the cable.
To clean lenses:
June 26, 2013
Spent the day trying to mode match.
First I used the power meter to measure the output of the fiber but its response was very slow so it was hard to align the beam and make adjustments so I hooked up a camera instead. Then when the light could be seen on the camera, I would switch to the power meter to see if I could incrase the power and have a quantity for the power output. The whole process took a long time because there were many parameters and I trouble at times aligning the beam.
Higher up in the beam path the power is 7mW. The highest power output I have obtaned is about 850 uW, which is improvement from the intitial 7 nW.
Note: Optimize one parameter before changing another (1 lens).
I turned off the hepa fans over the table over the night. I came back this morning and the temperature (measured on the vacuum tank) was very stable(within 2mK) over 2 hrs.
above:BLUE Temperature measured on the can, the Y scale is in degree C. The temperature variation is within 2mK over 150 mins.
So I looked at the PD for Erica's fringe measurement, the fringe wrapping was slow, so with better temperature insulation, we should be able to hold the fringe for at least a minute.
above: The fringe signal from PD, the cursors show the max/min signal from the fringe. The signal drifts from min to max over ~ 60 seconds compared to ~10seconds as before.
So the drift we saw before was very likely to be from the temperature drift (1mK per second for 20second fringe wrap). More thermal insulation on the optic should reduce the temperature drift.
1064 nm PM FC/APC Patch Cable: Panda Style
Tara improved the alignment so we got a little over 1V coming from the fiber alone. We took another run of data of the recombined beam:
laser not locked to cavity:
deltaV = 3.64V
Vmin = 880mV
Vmax = 4.52V
laser locked to cavity:
deltaV = 0.17V
This matches up with the original data taken.
We also took data for the noise of the spectrum analyzer.
This can be approximated at a straight line. I took an average of the points so the noise level is at 1.9*10^-6 Hz/rtHz.
Then we took data with the laser locked to the cavity. The power is much lower because we accidentally misaligned the beam.
As seen in the graph below, the fiber seems to be okay; the plots of the laser unlocked to the cavity match up.
We took data for the error signal from the servo. The slope of the error signal for ACAV path is 200kHz/V (see elog entry 1920) . We are using this to convert from voltage noise to frequency noise. The shape of the spectrum from the recombined beam follows the shape of the error signal.
I removed the 90% reflector from the north transmission path on the ISS breadboard and then installed the fiber launcher.
The ThorLabs power meter says 440 uW going into the fiber on the CTN side; the ThorLabs fiber power meter says 260 uW coming out on the ATF side.
I used the 633 nm fiber illuminator and the ThorLabs power meter (set to 633 nm) to test the 60 m polarization-maintaining fiber that we have.
Power right out of the illuminator was 1.25(2) mW, and the power out of the fiber was 0.45(1) mW. Since this fiber is only specked to work above 980 nm, I'm not sure how to interpret this number.
I'd like to compare to the 35 m PX980-XP fiber we have strung from CTN to Crackle.
I performed the same test with the 1060XP fiber (50 m, not polarization maintaining). I got 0.12(1) mW transmission.
To couple CTN light into the fiber, we decided to pick off using the reflected port of the PBS directly after the south EOAM. In order to mode-match into the fiber, we installed two lenses (and a steering mirror) between the PBS and the fiber.
Mode matching details are as follows: the round trip length of the PMC is 42 cm and the radius of curvature of the concave mirror is 1 m; this gives a waist of 370 microns. From there, we calculated the proper lenses needed: PLCX-25.4-64.4-UV-1064 (lens 1, focal length 124 mm) and a PLCX-25.4-128.8-UV-1064 (lens 2, focal length 250 mm). Between the two lenses is a mirror, Y1-1037-45-S, which is tilted at a ~45 degree angle to guide the light from lens 1 to lens 2. Lens 1 and Lens 2 are roughly 2 inches away from each other. There is a fiber coupler placed 4 inches away from the second lens.
Currently, is about 1.4 mW going into the fiber, and about 150 uW coming out.
Edit: We did some more aligning and found that there is 2.2 mW going into the fiber and .7 mW coming out.
Coupling through the PMC was very bad today; we saw 12 mW incident and ~1 mW transmitted. I (Evan) touched the three steering mirrors before the PMC and brought the transmission up to 5 mW.
In order to have more power incident on the fiber, we changed the angle of the HWP immediately after the PMC from 306.5 degrees to 338.0 degrees.
I switched the post to V-block for Faraday Isolator mount, for better stability, and adjusted the Faraday isolator to minimize back reflection to the laser.
I also measure the TF from ACAV path, The UGF is ~65 kHz.
The faraday isolator was installed on a standard pillar post, so I use a V block to mount it instead.
After adjusting the FI, I remeasured the beat note frequency, and the signal did not change from yesterday measurement.
(no differences between red and green plots)
blue: beat signal before fixing the ACAV opening
red: beat signal after fixing the ACAV opening
green : beat signal after re installing the FI
ACAV TF: I connect the signal output after the PDH servo box to SR560 A and SR785 B (resp)
Then source out from SR785 is connected to SR560 B
The output of SR560 is connected to SR785 B (ref) and to the VCO
The setup for SR560 is DC coupling for A and B, select A - B, gain 1, no filter.
I designed the layout for optics behind the cavities for beat measurement, and calculated the mode matching for the beam.
Since the current optics height for beat is quite high (7 inches), we want to lower it to 3 inches, make it more symmetric, and more compact.
The PD's diameter is 300 mm, so the beam spot on it will be ~50um.
All the lenses I need are prepared.
Beat measurement optics' height is changed to 3". I cleaned all optics already, but I couldn't really clean 1/2 and 1/4 wave plates, one of the f =200 mm lens is quite hard to clean.
I'll wait and ask someone before trying to clean again. I cannot lock both cavities at the same time, once I can, I'll align the beam on the PD.
Also ACAV's PD for ACAV_trans_PD is broken. It gives out 11 V regardless of the beam falling on the PD, so I replace it with a PD that is used for NPRO_PWRMON.
Both cavities are locked at the same time. The temperature setting are, RCAV = 34.95, ACAV = 37.2.
I realigned the beam onto the PD to get maximum contrast. I'll readjust the setting back to the original value
and see if the beat noise is improved.
I just notice that one of the beam on the mirror on ACAV's path behind the cavity is almost clipped. I'll readjust it tomorrow.
I measured the beat noise after I realigned all optics behind the cavities. The power has not been reduced to 1 mW yet.
This is just a quick measurement to see where we stand (red curve). The noise gets worse compared to the best measurement (green) before the optics behind the
cavities are rearranged, but the mechanical peaks around 1kHz are suppressed significantly.
I minimized the RFAM by aligning the 35.5 MHz EOM and remeasured the RIN coupling coefficient.
The upper limit is 5 [Hz/uW (fluctuation of input power = RIN x Pin) ]@ 10 Hz
(This entry is approved by Kiwamu and is written in his style)Tue May 10 19:20:22 2011
As pointed out in the LIGO-X meeting that my setup might suffer a lot from RFAM, so I came back to:
The power input was 1mW as usual.
The frequency of Vmod is 10Hz. The amplitude of Vmod to EAOM for amplitude modulation was varied from 2 to 10 Vpkpk. Common/Fast gain was 500/900. I had to reduce it so the signal is not too large. I measured the spectrum of FASTMON and tried to observe the peak at 10Hz with 12.5 mHz linewidth. The background level was ~10mV.
I do this to determine what is the maximum driving voltage where the effect from RFAM is still small compared to the background.
Drive Vpkpk FASTMON peak(Vrms/rtHz)
2 ~comparable to BG level ~ 10mV/rtHz
[ 1. aligning EOM ]
I picked up the beam after EOM on RCAV path and sent it to a PD (Thorlabs PDA10A.) There were 35.5 MHz pick up on the table, so I had to choose where the peak from pickup was minimum. Then I adjusted the half wave plate before the EOM and EOM's pitch/yaw position to minimize the peak.
[ 2. determine max Vmod ]
Although we want to modulate the power as small as possible to have a good linear approximation, we also need the signal to be large enough to be able to see the effect. However, the alignment of the EOM is not perfect, there will be RFAM effect adds into the signal. If the modulation is too large, the RFAM will mask the real signal. I need to determine what is the maximum Vmod I can use without having the RFAM effect excited above the background.
To see the effect of RFAM, I kept the setup similar to what I did with RIN coupling coefficient measurement, but without locking the cavity, and the laser frequency off from the resonance. This will tell us how much "fake signal" is produced by RFAM.
When the cavity is not locked, all the carrier and sidebands will be incident on the RFPD. The signal should be flat (beat between the carrier and both sidebands cancel each another,) and after it is demodulated by 35.5 MHz from LO, the level should be zero. However, if the amplitude is modulated at 35.5 MHz due to misalignment of the EOM, this will appear as DC signal at the error point. Hence, any power modulation at f0 (for this case, 10 Hz) will multiply up the error signal and cause offset fluctuation and slope change at f0. Slope change is not a problem, but the offset is. It will change the point where the laser will be locked, as the error signal moves up and down. Thus the system will interpret it as frequency noise of the laser and try to fight against it. This will appear as a peak in the FASTOUT spectrum at the modulation frequency, f0.
I measured the spectrum of FASTOUT (MIXER OUT is another option) to see the effect of RFAM
[ 3. remeasure RIN coupling coefficient ]
So I used 2Vpkpk drive, locked the cavity, and measured FASTMON again to see if I can measured the RIN coupling or not. The gain was set back to optimum value (common = 970 fast= 900.) However, there was no observable peak at 10Hz from FASTMON signal. It was quite flat ~100 uVrms/rtHz.
I made sure that the amplitude was really modulated by checking RCTRANSPD. It had a 5.37 Vrms/rtHz peak at 10Hz with 200mV DC level. Therefore, the laser noise is higher than the thermo-optic effect at this modulation level. I cannot increase modulation depth because the RFAM will mask the signal.
If I use this number to calculate the coupling coefficient, (flat level of FASTOUT, and peak from RCTRANSPD)
it will be ~ 8 [Hz/ uW of fluctuation of the input power into the cavity] still larger than 1[Hz/uW] as measured at 40m, but it's getting smaller than the last entry (60 [Hz/uW] of input power)
I still can change the power input, but I think the RFAM will scale up by the same amount and mask the signal again. I'll try that later.
Let's check what does this value give us in the noise budget @10Hz. The input power is 1mW, RIN = 10^-4. Frequency noise will be
8 [Hz/uW] x 1000 [uW] x 10^-4 [RIN] = 0.8 [Hz/rtHz] which is higher than coating noise (10 [mHz/rtHz]@10Hz) So we still cannot ignore the effect.
I tried 16 mW input power, there was signal from RFAM when I measured FASTOUT with unlocked cavity, the peak was 46 m[Vrms/rtHz] above 10m[Vrms/rtHz] background. Vmod = 1Vpkpk.
When I lock the cavity and measure the coupling:
FASTMON peak = 278.6 uVrms/rtHz
RCTRANSPD peak = 30.82 mV, DC level = 2.57 V, Pin = 16mW. Linewidth = 12.5mHz.
Common/Fast gain = 480/906
Use the calculation from here.
The upper limit for coupling coeffiicient is ~ 5 [Hz/uW]. It is only the upper limit because RFAM effect is still present.
I planned the layout for new fss setup.
The new setup has 1) both cavities placed in the same vacuum chamber, 2) two AOMs used in both RCAV and ACAV paths, 3) more compact beat path.
In the layout, I assumed that
This is just a plan, no mode matching has been calculated yet.
I am concerned that the mode matching lens might block the beam in ACAV path where the incoming beam and reflected beam cross, but this can be adjust later.
The outer foam box will be smaller, but it should have enough space to keep some electronics inside like we have now.
I should find two similar sets of beam splitters/ mirrors for beams in the beat path behind the cavity. So the pick up beams from two cavities can have same power.
Right now the power going into two PDs for RCTRANSPD are not the same because the splitter are not the similar.
Note that we might install a platform behind the cavities so that we don't need the periscopes to lower the beam, and get rid of their associated mechanical peaks.
I added more details on the layout, and necessary half wave plates in the beam path.
I ordered a few opto mechanical components to replace the current shaky periscopes.
The new periscope is shown here, elog:574. Currently we have only one set, so I ordered a post clamp to complete another set.
I also ordered 4 mirror mounts that can be mounted on 45 degree mounting adapters. The thickness of these mounts are thinner than a regular mirror mount, so it can be fit on the adapter. I plan to use these in Crackle experiment as well.
The periscopes for the refcav ought to be made custom. None of the store bought type are stiff enough. Koji has a design from the 40m green that Daisuke made.
I looked up 40m elog and found Daisuke's design for periscope. I'll make a sketch FSS' periscopes.
The design for 40m pericopes by Daisuke can be found here .
They are found in DCC. Some comments
- You can not steer the beam. The beam should be steered before or after the periscope.
- The side plates were too thick. It can be 1/2" thickness to reduce the total weight.
The design for 40m pericopes by Daisuke can be found here .
we have to design our own. The 40m one has 2" mirrors (too large, we don't have the space), wrong height for incoming/outgoing beam and is clamped to the table, which i think is bad in terms of stability.
The design principle does not look much different compared to the original refcav periscope design, except for the mirror holder itself. That was bad designed for the old one.
The mode matching for new FSS is calculated. The plan is shown below.
Note for the setup:
1) the spotsize in the AOM is 200um, the specsheet says 550 um (I might have to correct this).
2) Two AOMs are of the same model.
3) For mode matching to the AOM in acav path, I used only a single lens.
4) focal lengths of the lenses are in mm, We have to order the one with * (f = 57.4 mm)
5) Both cavities are 1" apart (3" from center to center)
6) Mistake in the drawing: the x2 QWPs just before the beams enter the vacuum chamber should be placed before the periscopes, not after.
The new mode matching for optics in front of the cavities is done. The rest (for beat measurement) will be finished soon.
A few changes in this layout are:
1) spotsize for AOM is 500 um, as specified by the datasheet.
2) Mirrors behind the AOMs will be changed to R= 2.0 m instead of 0.3 m.
3) Spot size in the 35.5 MHz EOM is ~300um which is good for the model.
4) More mirrors (for steering the beam) for the AOMs are added.
I'm a bit worried about using f=57.4mm lenses because they are quite sensitive when we have to move the lenses around, but the space is very limited this time.
I'll let Raphael double check my calculation so he can learn how to do mode matching.
There's no need to use such a large spot size on either the AOM or the EOM.
When using high power this could be an issue, but you can use a beam radius of more like 100-200 microns for the AOM to get fast response time.
I edited the layout so that the spots in both AOMs are 200 um. I'll list what optics we might have to buy.
Most of the optics are already used on the table. I need to find:
The optics on ACAV path have been removed, I left the optics on RCAV path for now because Raphael might want to remeasure EOM TF.
Once the measurement is done, all optics will be removed. We will clean the table, clean the optics before put them back on the table.
the lens and mirror are in the ATF, a second VCO is in the left cabinet.