I'm making a new layout for the 2 laser setup. It is in progress. I just want to make a note about some features I want to have in the setup.
There is a pool of water in the lab. It is localized only around the floor near the fume hood. I'm not sure where it comes from,but it drips down from the edge of the sink where there is a small pool of water on it. I'll keep an eye on it. If there is still more leakage, I'll contact pma to take a look.
I did not see anything yesterday, so I guess the leak just started last night.
I updated the new task sheet for thermal noise probe experiment:
The goal is to have the first beat measurement:
The plan for the upcoming week (Jan28) will be
I don't think the second PMC will be ready yet, I'll try to lock the second laser w/o the PMC first, and add PMC later.
For Feb4 week, I will
I ordered some clean supplies which will be used when I open the vacuum chamber for installing the short cavities.
Some of the items that might be needed
The EOM driver is working. For the same modulation depth, it can drive a broadband EOM using less power.
==measurement and result==
I used PMC setup to test this EOM driver because its frequency range is only ~21 - 24 MHz, and the sideband for locking PMC is 21.5 MHz. So what I did:
From the schematic, the board is supposed to have 30V output from 0.15 V output (x200). In dBm that will be 20log(200)~ 23dBm. So It is roughly ok.
fig1: BB EOM with the driver. One of the unused output is for output mon.
fig2: Error signal from scanning the laser, with BB eom and the driver, measured at mixer out.
The board is definitely working and will benefit us, definitely for locking PMC. If we use a marconi to drive a BB EOM, the max output is 13dBm. The power is halved (one to the EOM, one to the mixer). That means ~ 10dBm to the EOM (we will probably split more some where for RFAM pickup, but we can do that on the line that goes to the mixer), so assuming we have ~ 10dBm for the EOM. With the board it will be ~ 10+18.5 dBm = 28.5dBm (~6V) . It should give modulation depth of 0.09, see psl:745, This might not be enough for locking the refcav( see,psl:929. where we have beta of 0.18), but we can add another RF amplifer, or use the board for PMC servo . I'll check what are the appropriate modulation depth for locking PMC and refcav.
I clean the vented screws and peek pieces for cavity mount with ultra sonic bath, I'll check if I need to bake them or not.
I got an EOM driver from Rich Abbott, I'm checking if this thing works well or not.
The EOM driver is just an amplified resonant circuit with +/- 18V input. With the driver connected to a broadband (BB) EOM, we can use the combination to add sideband to the laser. This is better than a resonant EOM because we can pick a range of frequencies, instead of having a fixed one from the manufacturer.
I checked the TF between input and the RF mon, the resonant peak can be moved between 20.7 MHz and 24.5 MHz by adjusting an inductor on the board. Since the RFPD for the PMC is 21.5 MHz, I'll use it to check the modulation index of a BB EOM equipped with this board.
The plan is
Once I verify this I will check the frequencies for refcavs and pmcs, so that I can decide the value of L and C on the board.
We are interested in the longitudinal mode along the Y direction. That is the only one which is problematic for the servo. Please remeasure so that you excite Y and measure Y and then model the first longitudinal mode.
The other modes are interesting, but they're not the main thing we care about.
The PMC was tested & lowest resonant frequency was 330 Hz; FEA model was adjusted to new frequency of 441 Hz
Results from December 18, 2012
The PMC in 058B W. Bridge was secured with several dog clamps to the laser table. This table is not as stiff as the table in the Modal Lab in Downs, but was thought to be sufficient for this test. Testing was done with the B&K system, using a laser vibrometer for the accelerometer and the small 8206 B&K hammer for excitation. Below is a representation of the axis for this test, to understand where the PMC was excited and measured.
Measurements and excitations were approximately at the center of the corresponding face, as indicated in the image.
Below is a graph of the results of these measurements. You can see that the lowest resonant frequency is at 330 Hz.
Next I will update the ANSYS model to be more accurate, hopefully showing about 330 Hz as the lowest mode.
FEA Model in ANSYS:
Briefly: the previous model reported in the elog was changed by refining the mesh on the slots/cones and the bearings. This would allow for that portion to behave more accurately. The contacts were left as Program-controlled (any other control seemed to overestimate the contact, raising the predicted resonant frequencies).
Below is an image of the lowest mode, at 441 Hz. The arrow indicates the motion - the mode is roughly a transverse flagpole mode.
Now that the model has been made more accurate, steps can be taken to raise the resonant frequency. While the initial goal that was mentioned to me of 1kHz is improbable, there are certainly ways to raise the frequency and damp the modes that are problematic.
Kristen and Norna came to ATF for impact-hammering of the metal PMC in the gyro setup
Above you can see the first mode shape of the PMC. The colors represent the displacement - deep blue indicates no motion, while red indicates the gr
This does not look like a longitudinal mode. Do you have the frequency for the first longitudinal mode(along cavity length)? the first longitudinal mode should look like this ( this model has no fixed boundary condition, just a block in space).
[Peter, Tara], we assembled 2 short reference cavities today. The bonding between the spacers and mirrors are strong and holding the mirrors nicely.
I got the cavity fixtures (made from delrin) from the machine shop today, so I asked Peter to help me assembling the cavities. All picture can be found here
I tested the bond by lifting the whole cavity by handling at the mirror on top only, and wiggling it a little bit. The bonds weren't broken. The hardest part was cleaning all surfaces to make sure that there was no dust.
From hindsight, I don't really need to see the fringes to do the bond. If the surface is clean, the pieces will be bonded instantly after a light pressure. If there are particles on the surface that cause fringes, the bond will not form anyway. So for Si cavity, Dmass can try to do optical contact without a setup to see the fringes.
fig1: the mirror is placed in position by the fixture. The mirror is not pressed on the spacer yet. Fringes can be seen on the polished ring on the mirror. See the video to see how the fringes vanish after applied pressure.
I made a simulation for PMC body mode, and found out that for Al PMC, the first body mode is 1kHz. And 780 Hz for stainless steel pmc.
November 27, 2012
It is desirable for the first body mode of the PMC to be at or above 1000 Hz in order to provide consistent length for the cavity.
Above you can see the first mode shape of the PMC. The colors represent the displacement - deep blue indicates no motion, while red indicates the greatest amount of motion. The animation of this mode shape shows the PMC spacer rocking transversely on the PMC base. The PMC base does not move at all.
One question that came up is whether ANSYS is importing the geometry file at the correct size. According to the scale on the screen, it is the right size. However, when the material is changed to resemble fused silica, the lowest body mode is 998 Hz, which is about an order of magnitude lower than expected. This indicates some other error, possibly in importing the structure into ANSYS.
/more to come
I calculated brownian noise in AlAs/GaAs coatings, brownian noise and thermoelastic noise in fused silica substrate for different beam sizes. From the plot, we can see that a smaller spotsize might be better for us.
This is a quick study to see the how spotsize on a mirror affects Brownian noise and thermoelastic noise in coatings and substrate. The radii of the beam (where the beam intensity drops by 1/e^2) used in the calculation are 91, 182, 364 um. Loss in coatings is 10^-5, loss in substrate is 10^-7. Note for 1.45" cavity with 0.5m RoC mirrors, the beam radius is 182 um.
The Brownian noise in the coatings is more comparable to TE noise in substrate with smaller beam size although the crossing between the two noises are at higher frequency. So it should be able to see the total noise from both effects. However, to get smaller beamsize, we probably have to use even shorter cavities, or smaller RoC mirrros. So it might not be practical for us.. Nevertheless, going to smaller beam size should be a good idea.
If the Cerdonio paper is correct, then just use those equations instead of the Thorne ones.
I usually use Cerdonio's in the noise budget. But I used Thorne's to compare with COMSOL result because of the same adiabatic assumption.
What is the reason for the COMSOL TE noise to diverge from the analytic one at low frequencies?
Don't you have to consider the coherent TO noise between the coatings and the substrate?
At low frequency, the adiabatic approximation (used by BGV, Liu & Thorne) breaks down. Basically, it assumes that heat inside the material is not diffusive. The only place that heat flow must be considered is in the volume integral where gradient of expansion is non zero for the dissipation. (see Liu & Torne explanation just above eq 8). This is easier for COMSOL simulation. Since I just press the substrate with static force, and calculate the deformation of the body which, under adiabatic approximation, gives me the temperature change of the body.
If I want to take into account the heat flow, I have to (see Cerdonio)
Then for COMSOL simulation I have to use different setting and I'm not exactly sure how to do this yet.
The PMC round trip is ~ 0.32m. The end mirror has ROC = 1.0m. The spotsize is 384 micron. The end mirror has radius ~2.5 mm. The clipping loss will be ~ 1*10^-43 on the curve mirror, and much smaller at the flat mirrors. The number seems very small but I think it is correct.
This is just a simple integration for power of the beam P(a) where a = radius of the mirror (2.5mm). The total loss on all three mirrors per one trip is definitely way below 1ppm.
Kriten sent me solidwork part files for the steel pmc. I'm checking all the parts and will decide what material we want to use.
She reported that a ss pmc will have the first body mode at 780 Hz, while an aluminum one will have the first body mode at 1kHz. But we have to take thermal expansion, stiffness into account. here are some material properties
I think the thermal expansion will be a problem, but their thermal expansion coefficients are not that different. I don't know about stiffness of the body. I'll ask someone about this. Otherwise Al might be a better material if we look for higher resonant frequency.
Found the problem. My noise budget code was wrong. So after I fixed it, the TE noise in substrate result from COMSOL agrees pretty well with the analytical result (within 20%).
The result from COMSOL is plotted in dashed-black line. The result from Cerdonio is plotted in dashed pink. Since my simulation uses the adiabatic assumption (used in BGV and Liu&Thorne paper), the results agree at high frequency. So I think the calculation is correct. I'll check some options (changing spot size, changing material) to see if TE noise can be made lower for AlAs/GaAs samples.
I attached my COMSOL file below. This is done in 3D model. It could have been done in 2-D axis symmetric setting, but I used 3-D for spacer sagging before, so I just used the same geometry I had.
I realized that the mesh size was too large, even with the finest mesh for default setting. So I reduced the mesh size around the beam area and the results got closer to the analytical prediction. It is still a factor of 2 below the prediction. I'll see if I can hunt down this problem . I think it will be a good idea to verify my model by using my model to calculate Brownian noise and comparing with the result reported by Braunschwig group.
When I defined mesh size in COMSOL, I used the predefined value provided by COMSOL. The finest mesh has maximum element size ~500 um, and minimum ~5um. Since the beam size is ~ 180 um, the maximum element size should be ~10 um. So I changed the values around, defined new area for smaller mesh until the results did not change much. I ran the simulation a few time to make sure that the solution converges. Right now my substrate has 3 regions
I tried to change the mesh size/boundary size a bit to get the result accurate enough without taking too much time. The TE estimation still a factor of 2 below the analytical estimate.
I used COMSOL model to calculated Brownian noise in substrate. This was done for cross checking my model simulation. The result from model is within 2% compared to half infinite model calculation.
I followed Levin's Direct approach to calculate Brownian noise in substrate, basically, to calculate the elastic energy inside the substrate under the applied test force. This can be done using COMSOL and analytical calculation. The comparison between the two is shown below.
U is the stored energy in substrate.
Note: I used the same COMSOL model for TE noise calculation. I just asked it to produce the strain energy in the substrate (no spacer).
The simulation is very close to the analytical result. So I think my spacer-cavity model and all the factors in the calculation are correct. The TE calculation is a little more complicated, since I have to calculate the gradient of expansion in COMSOL and it might be wrong somewhere. I'll check that.
I'm checking the calculation for TE noise in substrate and spacer. I'm comparing the results from analytic calculation and simulation. The results still do not agree. Comsol gives a result ~ a factor of 4 lower than its analytical counterpart.
Since the TE noise in substrate will be significant in AlAs/GaAs mirrors, the TE noise estimation should be correct. The TE calculation in substrate was done by (BGV, Liu and Thorne, and Cerdonio). The correction was noted in Numata 03 and Black 04 papers. I think the calculation is well established because the calculations from all of the papers agree (with all corrections taken into account). So It will be nice if an FEA simulation predicts the similar result as well.
I followed the calculation done by Kessler etal paper where they calculated the Brownian noise in spacer. The mirror-spacer assembly is pushed by a static force with Gaussian profile, P = 2 F0 / (pi*w0^2) * exp(-2r^2/w0^2), where w0 is the spot radius = 182 um for 1.45" cavity with 0.5 mRoC mirrors, at r = w0, intensity drops by 1/e^2, F0 is the magnitude of the force (1N for my simulation) .
Note about COMSOL: I used extra fine mesh in a small volume where the beam hits the mirror, fine mesh in the rest of the substrate, and normal mesh in the spacer. This reduced the memory used in the calculation a lot and should not introduce a lot of error, since all the deformation will concentrate near the beam spot.
I have been practicing to optically contact two flat mirrors together. I think now I get it.
Since I need to build my refcav by optically contacting mirrors to a spacer. I tried the procedure by contacting two flat blank mirrors together. I bought blank fused silica mirrors from Thorlabs, pf10-03. Here are the instructions:
fig1: On the right, fringe patter can be seen when two surfaces are put close together. On the left, after applying pressure the fringe should go away. The fringe pattern(purple-yellowing) on the left side indicates that these surface are not properly contacted.
fig2: These mirrors are optically contacted somehow, except the center. I don't know what happen here that cause white area in the center. I might be that the mirrors are not flat enough. But the rim seems to have a nice optical contact. I tried to remove the mirrors by hands but they are well stuck. I'll ask peter for more about what causes the white area here.
As I tried to do this, I got an idea of a fixture for optical contacting the spacer to mirrors. It will be a cap with a center hole for the mirror position. Here is a solidwork drawing. The part can be aluminum. I have to think about the tolerance of the piece, but from the calculation it can be an order of a cm (to keep the beam to go through the window). So the hole can be ~ .01 inch larger than the spacer and the mirror.
I finished the drawing for refcav mount and the top plate. Everything fits together, so I'll submit the drawing tomorrow.
New things I added:
The refcav mount is a bit wider than the top seismic stack plate, but it fits inside the chamber, see the assembly. So I don't think it will be a problem.
To measure the laser output profile is not actually that easy, although measured values are not so terrible.
Make sure the PBS you used is not BK7, but Fused Silica.
It would be nicer if you don't need to use transmissive optics for the measurement.
i.e. Put a fused silica AR-coated window and use the reflection.
BK7 windows may not work. BK7 has more than x10 CTE compared with Fused Silica. (7.5 ppm/K vs 0.55 ppm/K).
The absorption may also be higher.
I scanned the profile of the laser borrowed from 40m. The avg beam radius is 220um ~ 1 cm in front of the laser opening. This number will be used for a new table layout.
The laser was operated at full power (~700mW as expected). I used a mirror to attenuated the beam and use WINCAM to measure the beam profile (power incident on WINCAM was ~0.7mW). To measure the full power and avoid burning the power meter, I used a polarizing beam splitter with 1/2 wave plate to reduce the beam power by half then measured and summed the power from two sides of the PBS.
The beam shape is looking more like a blob than an oval. This might explain why the fitting does not match the measurement well.
I finished the design for dual cavity mount. The assembly looks fine, all parts fit together. I'll make sure that the mount can be screwed down to the current seismic stack before I submit the drawing.
After finding the optimum support points using COMSOL, I redid my cavity support design. The picture below shows the assembly of a metal base, peek pieces for support points, and copper shield. The picture shows only half of the mount.
The current design, the beam height is 1.32 inch above the top seismic stack, ~5.5 inch measured from the table.
With the new cavity mount design, the beam height will be 1.5 inch above the top seismic stack.
I looked into the model a bit more to make sure that I included all the effects and get the coupling right [more to come]
fig1: displacement(beam line direction) per unit acceleration on the mirror surface. X-axis represents the position on the mirror along vertical line. Each plot represents result from different support positions. For optimum point (1.17"), the sensitivity to vertical seismic is around 2.1x10^-12 m/(m/s^2).
fig2: This plot shows the result as in fig1, with the means removed. Typically we want the tangent line at the center to be zero for minimum tilt.
I used COMSOL to estimated thermoelastic noise in 1.45" spacer. The noise is not significant for our coating Brownian measurement. I still need to verify the model with some analytical estimation.
As Rana and Jan suggested, I thought about the effect of mirror's radius of curvature and DC tilt effect to cavity length noise. I ran a few simulation tests and the results were not changing much.
==cavity length noise==
The cavity length mainly changes from two effect
1) Actual position change:
2) Tilting of the mirror: If both mirrors tilt up or down together by theta, the cavity length will be longer by R*theta^2, see the attached picture. The calculation takes mirror's ROC and optical axis shifting into account.
So, to find the best place to support the cavity, the contribution from both effects should be minimize.
==COMSOL Simulation: effects from different boundary conditions==
We have been discussing about different boundary conditions for support points whether to use fixed in all direction, fixed in only vertical direction, point support, or finite area. So I decided to check the effect from the following condition
There are no significant differences among the chosen boundary conditions. I varied the angle from 30, 45 and 60 degree, all the boundary conditions resulted in the same sagging behavior, so the best choice will be 30 degree as discussed in PSL:xx .The plot below shows the displacement per [m/s^2] of the mirror center along the beam line and tilt angle per [m/s^2] of the mirror, (the support angle is 30degree). With any boundary conditions, the optimum position will be quite the same (30 degree, 1.2" apart).
==where is the optimum spot? what is the coupling from seismic to cavity length?==
From the simulation, with our restriction on support position (angle between 30 to 60 degree, 0.5 - 1.2" apart), the mirror positions always extend the cavity length. Since tilt will always increase the cavity length, we cannot cancel the effect between translation and tilt. The best way is to minimize translation and choose zero tilt as before.
About the coupling, at the optimum spot where tilting is zero, there will be no tilting motion. Displacement noise will only come from translation of the mirrors.
I talked to Jan about how to calculate thermoelastic (TE) noise in a spacer. I will use comsol to estimate the thermoelastic noise in our cavity.
Thermoelastic noise has not been estimated for our setup. I think it will not be that high. As the previous result with 8" cavity, the measured signal was very close to the estimated coating Brownian noise. However, our noise budget will be more comprehensive if we include TE noise in the spacer.
Basically to do that, we have to apply force on the spacer , then calculate the gradient of the strain inside the spacer [Liu and Thorne, 2000]. I think this can be done by COMSOL. I am working on it. I'll add more details on CTN wiki page later.
I checked the cavity length sensitivity to horizontal acceleration ( normal to the beam line axis). Unlike the result from vertical acceleration, the cavity length did not change smoothly with the position. For the optimum point(30 degree, 1.18 inch apart), the displacement sensitivity due to horizontal acceleration is about a factor of 2 larger than that of vertical acceleration.
Since both horizontal(H) and vertical(V) seismic noise on the table are comparable [psl:xxx], I want to make sure that there will be no serious displacement noise due to acceleration on vertical direciton.
On the COMSOL model, I fixed only one side of the support to push against horizontal acceleration (see pic). As the support can only push against the cavity, not pull. It should make more sense to use this boundary condition.
For the effect from H acceleration, I varied the angle from 12 to 42 degree, and distance from 0.8 to 1.2 inch. The displacement did not change smoothly with the support positions. So I could not tell which way I should choose for the optimum support. However, the displacement seems to be around 2x10^-10 inch for most of the positions. [the unit on the y axis of the plot should be inch per acceleration].
For the optimum support (1.18 Inch, 30 degree) the senstivity is about 1x10^-10 inch/ (m/s^2).
After checking displacement and tilt of the mirrors from various support points, the tentative support positions will be 1.2" and 30 degree, (see entry 1060 for their definitions). I'll check the sensitivity along other directions (horizontal), and see if the noise budget will be acceptable or not.
After running more simulations, I got the cavity's length sensitivity due to vertical acceleration. The angle varied between 12 to 45 degree. And the nice point seems to be 30 degree at 1.2". I will check the length and tilt sensitivity on horizontal acceleration, and compute the noise budget to make sure that seismic noise will be acceptable. After that I'll finish the drawing for the cavity mount.
Note: I just realized that I should have used strain/acceleration unit in the displacement plot. I'll fix that later.
I ran a few more simulations to see how support area would affect the displacement. It turned out that it was not significant, for area = 0.056 x 10-3, 0.9 x 10-3 and 5.6 x 10-3 [inch2]. This is good because we don't have to worry too much about the effective area of the contact points in the simulation. The errors will probably be dominated by other parameters (mostly, support positions). Judging from all the requirements, I think I'm close to making the decision for the support position.
The plot below shows results from different support angles, (theta = [30,45,60]).
==a few comments about the plot==
I think possible choices (considering loss, machining, safety) for the support positions will be some where around 0.7-1.2 inch along the beam line, and the angle will be ~ 12 - 30 degree. I'll run more simulation to see if I can find it.
I used COMSOL with MATLAB to run the simulation. I tried to vary support position and checked the mirror displacement along the beam line axis and tilt angle.
With Aidan help, I am finally able to run matlab with comsol to get the results (displacement of the mirror surface and tilt).
We are not planning to cut the cavities for support points, so we will choose the support positions on the spacer's surface, with parameter X and theta, see the figure below for their definitions.
As a start, I chose theta = 30 and 60 degree. The displacement and tilt (due to cavity sagging under its weight at 1g)as a function of support position are plotted below.
It is possible to minimize the tllt, but the displacement is still a bit bad. The result from 8" spacer, the sensitivity to acceleration is (dL/L) / (m/s^2) = 2x10^-10, while the current result will be about 1x10^-10. Since the cavity is shorten by ~ a factor 4, I expect a better sensitivity to vibration.
I'll try to change the area of the support points to check its effect on the displacment.
I have to check if I can constrain the support points in one direction or not.
I'm still working on COMSOL, now my model has the following features:
Milo etal. 2009 Phys Rev A 79.053829.
Faraday Isolator, for 700mW 1064 laser. This will be installed after the laser (and waveplates for polarization adjustment).
I looked at Thorlabs website and found one that meets our requirement.
IO-3-1064-HP, the max power is 15W, and the max intensity is 500 W/cm^2. If the 700mW beam has 300 um radius, the avg intensity is~ 250 W/m^2.
I'll ask around to check if this is a suitable one.
Just a list for something we need to buy.
Optics& opto-mech parts
The table is not floated. Either the legs are broken, or there is a leak in the tube system. I think it is likely that one (or more) of the legs is broken. Since it happened before with the older legs. Their rubber part in the leg gradually failed over time. We might need to reconsider buying brand new legs again. The pump connected to the table could not keep up with the leaking rate, so I turned it off.
I'm using COMSOL to simulate the effect of cavity sagging to find the optimum suspension points. The answer is not yet ready, I'm still working around COMSOL.
fig1: cavity sagging, on 4 point suspension. The cavity is not symmetric on left and right.
So, as a start, I switched to half line support. As my cavity support will be rods placed perpendicular to the refcav, the simulation might not be off by much. Then I checked the displacement at the center of the mirrors. The result was, the further to the ends of the spacer, the less displacement of the mirrors. I think this is strange. I also remember a paper about this and their cavity dimension is similar to what we have, and their result is slightly away from the ends [ref]. I'll have to double check the result again.
Note: I think what is wrong is how I use the displacement of the mirrors along the beamline as differential length of the cavity, I have not taken into account tilting of the mirrors yet. Also, I'll try to position the venting hole downward to see if there is any differences in the result or not.
I checked the temperature control servo for the vacuum chamber and found out that it was off. I could not turn it back on yet since there was some problems with the channel. I'll ask Peter to help me on this.
About a week ago, I tried to add another channel for controlling the second PMC servo card. I did not write an elog since it was not done yet.
I created PMC2.db in /usr1/epics/psl/db. It had only one channel for C3:PSL-PMC2_GAIN. I used #C6 S2. This channel might already be used for temp control. I'll try to remove the channel and see if the problem can be solved.
modification of the previous mount, work in progress.
[with Zach and Dmass] We discussed about the stainless steel pmc design and here are the list of what should be modified.
The drawing can be found, on svn.
PZT can be ordered from www.pi.ws.
The requirements for PZT from (LIGO-xx), are (A) pzt range = 2.7 FSR, for 0 - 375 V, (B) resonant frequency at 10kHz or above.
Zach is using model P-016.10H. The displacement is 15 um (with 1000V), OD = 16mm, ID = 8mm, L = 15mm, resonant frequency = 67kHz. Assuming the pzt is linear, the displacement will be 5.6 um for 375V, this corresponds to 11 FSR for our cavity (FSR = 454 MHz). I don't know if this will cause some locking problem or not, or it might just give us an extra gain in the pmc loop.
If I follow the requirement, the displacment of 5um @ 1000V will be enough for us (model P-016.00H), but the length of the PZT will be 7 mm, and I think we have to fix the drawing accordingly.
Above, an excerpt from the pzt catalog, the full one can be found HERE.
I'm working on the new mount for 1.45" refcav. I 'll discuss the design with Eric G and some mech engineers (Mike Smith, Ken Mailand) later.
Here is the assembly of the mount, with only one cavity shown.
copper thermal shield around the cavity is 1.75" OD, 1.686" ID, wll thickness = 0.064", 3" long. (I'll order the tube from McMaster-Carr.
bottom mount will be a single piece, holding both cavities together.
I'll add a top plate to hold the shield and cavity later.
Note: I'm thinking about using teflon to make all the mounting pieces (top and bottom) so the mount will act as heat insulation between the shield and the platform.
I got the mirror blanks for optical contact practicing. I tried to contact them together, but I have not succeeded yet.
The mirrors are not transparent on the back, but we can still see the fringe due to the gap between the two surfaces clearly with just room light, see the picture below. I might not clean it well enough. I'll try to do it again later.
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.
I got PMC drawing from Dmass, this will be similar to gyro's steel PMC. I'll submit the work to machine shop soon.
The drawing is on svn full PMC assemble can be found at ATF:1543.There are spare mirrors in PSL that can be used. I still have to look for a PZT.
The round trip length is 0.33 cm. this corresponds to FSR = 454.45 MHz. If I want to be able to scan through 2 FSR, the displacement range of the PZT will be dL = 2*FSR * L / f, where L = 0.33m, f = c/lambda. dL ~ 1um.
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.