I recalculated the mode matching so that the spot radius in AOMs is 100 um. Now the visibility of RCAV is 90%.
From the previous mode matching calculation, the spot radius in AOM is 220 um. This was too large for ISOMET AOM and caused beam distortion. The AOM was designed for much smaller spot radius (50 - 110 um). So I recalculated to make the spot radius inside the AOM to be 100 um. This spotsize is small enough for ISOMET and not too small for Crystal Tech AOM.
Rise time is 35 ns (28.5MHz) for 100 um radius in ISOMET AOM, diffraction eff ~80%. This should be sufficient for our less than 1MHz bandwidth loop.
For the new layout, I have to remove the Faraday isolator behind the EOM for another lens. I'll try to intall it back later.
I put most optics on ACAV path. I have not tried to lock the cavity yet. I'll install ACAV RFPD next.
found the box with the beam splitters Dmass bought almost 2 years ago but never unpacked or used. They are super-polished 50:50 beam splitters for 532&1064nm but optimized for 1064nm.There are 16pcs total, so i don't see why we can't use 3 of them for our beat setup. We now have only SP optics in the critical beam paths except for the windows of the vacuum can, all lenses and wave plates where required. I hope this will reduce the amount of scattered light a little bit. The new setup only uses a minimum of components.
I made a drawing for faraday isolator's base. I'll submit the drawing tomorrow.
I added mirrors to pick up stray beams just before the cavities. These beams will be used for monitoring RFAM.
I arranged the optics so that stray beams at the beam splitters (just in front of the cavities) could be used. The power of the beam is ~ 9 uW, but it can be increased by changing the polarization of the input beam later.
Two photodiodes are needed, I haven't checked yet if I still have some spare PDs left.
Then the signal from PD will be demodulated with 35.5 MHz signal (modulation frequency). The cable length + PD position will be adjusted so that the phase is the same as the PDH signal.
I made some minor adjustment to the optics layout so that the reflected beam at the PBS before the cavity can be used to measure RFAM. Now RCAV's beam can be picked up for RFAM measurement.
The PBS just before RCAV was moved Eastward a bit so that the reflected beams from both PBSs are not blocked. I removed mirrors with soft mounts and use only rigid 1" posts only.
I used a spare 35.5MHz RFPD for the pickup beam from RCAV path (in red). The power cable for RFPD was made and checked. It works properly. There is a spare new focus 1811 RFPD, but the connector is broken, the pins are bent. I'll try to fix this and use it for ACAV's RFAM pickup.
The AC signal from RFPD will be demodulated with 35.5 MHz signal which is split from the LO signal for ACAV PDH's lock. I have not adjusted the phase by trying different cable lengths yet. This will be done later.
There is one thing I'm a bit concerned with. The RF signal from the RFPD has DC level ~ 120 mV, I'm not sure if it's unusual or not. I'll check with another RFPD.
I replaced the isolator mount with the V block I drew. The height is a bit to high. I'll send it back to the machine shop to reduce the height.
The entire lens kit (Newport wooden box, v-coating for 1064nm) is missing . Checked other labs but can't find it.
Can't continue work without it
SO PLZ RETURN IT !
Note: new EOM base
This one will have total height of 1.31" . THe height of the EOM (base to aperture) is 0.56". The height of the 4-axis stage (new focus 9071) is 1.06 - 1.18 " (min to max), I use 1.13" as operating height. So the total height is 1.31 + 0.56+ 1.13 = 3" .
The drawings of EOM and 4-axis stage can be found here:
Mon Jan 16 17:10:25 2012
Base for Dual periscope is added. This will allow us to mount the plate to the table with screws. Clamps can be used to provide additional support as well.
As we decided to use lower sideband frequency (14.75MHz, instead of 35.5MHz), I replaced the Broadband EOM with 14.75MHz EOM.
The current broadband EOM give only small modulation depth (~0.06 rad with maximum power from the LO, seepsl:745) With a resonant EOM, we can get higher modulation depth with the same amount of power.
Plus, in general, the RFPD's Q will be also higher at lower frequency, so we should get higher gain to suppress more frequency noise (the exact number of Q has not been measured yet).
==To Do/ Problems==
We no longer use LIGO's old LO cards. All of the spares in the lab are also broken. We will use a function generator and adjust the cable length to change phase between LO and PD.
After I added the resonant EOM to the setup the beam path changed quite a lot, I need to re-aligned the beam before I can see the error signal and lock the cavity.
The 14.75 MHz EOM we have is for visible light, so we went to TNI and borrowed a 14.75 MHz EOM for IR and an 14.75 Mhz resonant RFPD. I will re-aligned the beam and measure the error signal tomorrow.
The current function generator can provide power up to 23dBm. So the EOM can be driven around ~ 19 dBm(~2V@ 50Ohm) (-3 dBm for a splitter, -1 dBm for loss in the cable). So we can expect the modulation index to be 0.2*2 = 0.4 rad.
RCAV is locked using TTFSS.
It took awhile before I could lock the cavity because the 14.75 MHz EOM tilts the beam path, and I had to realign the beam. We don't know why the beam path was changed that much. We checked the EOM with impedance kit. It has 14.75 MHz peak and the crystal looks nice, so we use it anyway.
The error signal looks nice after cable length adjustment.
I locked the cavity with fast feedback only to measure the transmitted power through the cavity. P_side band is 0.16mW, P_carrier is 0.57mW. So Psideband/Pcarrier ~0.3, this corresponds to modulation depth ~ 0.95. This is close to the calculation Frank did.
I have not tried to measure noise at the error point yet, since I have to flip the phase by 180 for feedback to EOM (TTFSS has a phase flip switch for FAST feedback only). I used a long BNC cable to change the phase by 180 degree, I think making an adaptor for EOM connector to flip the signal might be a better idea to try.
Don't change the way the EOM is wired !! If you do so the case is not connected to GND/protective earth anymore and your high voltage is on the metal case!
With the new EOM bases, we can place 2 EOMs and the Faraday Isolator back in to the setup.
The half wave plate(HWP) between the two EOMs is temporarily mounted with two posts mounted together on a cross holder, because there is not enough space. We will make a special post, so that it can be mounted between the EOMs.
We tried to redo the mode matching to RCAV by adjusting the lense position using translational stages. However the result does not improve that much. The visibility is still roughly the same at 96.5%.
We will do the mode match for ACAV. Right now the visibility for ACAV is ~90%.
We also monitored the beam reflected from ACAV. TEM02 shows up (see below figure), but we could not get rid of it by beam alignment. It is probably the distortion from the AOM.
I fixed the drawing for periscope base. Will submit to the machine shop soon.
just for reference which part is/was where for later...
I'm trying to re-align the beams to the cavities. Due to the new RTV springs for the seismic stack, the cavities' natural axes shift by ~1/4 " with respect to the previous position.
I had to adjusted the height of the top mirror of the periscope before I could align and lock RCAV (visibility ~ 95%) again. The pictures below show the position of the current beam. With the previous setup, the beam position was almost at the center of the holes. Now, for RCAV, the axis shifts closer to the edge. RCAV might yaw with respect to the previous position. Left picture shows the incoming beam position, Right picture shows the outgoing beam position.
For ACAV, however, it seems that the position changes a lot and the beam clips on the outer edge of the top mirror before I can even find TEM00. I think I'll have to add a spacer between the mirror mount and the vertical plate in order to re align the beam.
I think we can keep the stack position as it is for now, if I can lock both cavities and the transmitted beams can be adjusted on the breadboard for beat path. We might also have to increase the hole size on the insulation cap as well depending on where the beam position of ACAV will be.
I realigned ACAV and found TEM00, but now the transmitted beam is completely missed the opening on the insulation, it is off from the center by ~ 1 cm.
We noticed wide angle scattered light behind the PBS in front of RCAV. The scattering source is probably the curved mirror behind RCAV AOM. We borrowed the similar mirror from 40m and will try to compare them.
The wide angle scattered light behind the PBS in front of RCAV might contribute to the noise in beat signal. The picture shows the scattered light with area larger than the half inch PBS cube. This picture was taken when the beam's polarization was changed to P-polarization so that most of the light was reflected from the PBS. With small transmitted light through the PBS, the scattered light can be seen clearly behind the PBS, see here.
After the inspection, it is very likely that the curve mirror behind RCAV AOM is the source. So we borrowed another R=0.3 mirror from 40m to see if it will be better or not, this will be done soon.
Note: during the inpsection, we also identified another bad PBS,pic. This is the one in front of RCAV AOM. Its center surface looks dirty, so we replaced it with a better one.
Update, beat measurement after several optics replacement. Peaks around 10 Hz, 35 Hz show up this time.
Optics that we replaced are:
The problem with the curve mirror from last entry has not been fixed yet. It turns out that the mirror we borrow from 40m is worse than the one we have (surface is more milky), so we leave the original mirror as it is.
Note: The beat measurement was done when the air springs were inactive. Noise at high frequency goes down a bit.
The power input to each cavity is 1mW, setup on PLL is 1kHz input range, with gain = 200.
We tried to damp mechanical peaks from each optics. For now, by putting a rubber piece on a mirror mount can suppress mechanical peaks effectively. We are still thinking about more robust way to damp the peaks.
Beat signal has a lot of acoustic peaks from 100Hz up to 1kHz, and they may mask any improvement we work on flat noise. Damping them is necessary before we can work on the flat noise hidden underneath.
By tapping each optic, we can see peaks raising up in beat signal or feedback signal to ACAV AOM. We used the feedback to ACAV AOM to identify peaks in ACAV path first. The curve mirror behind AOM has a strong peak which can be damped by a rubber cone placed on top of the mount, see fig1 below.
fig1: Mirror mount1, with a damping rubber on top.
We also tried using different mounts to see if the peak would be reduced. The original mount was an anodized aluminium mount. We switched to different two stainless steel mounts, mount1 and mount2. The spectrum of the feedback signal to AOM (not calibrated) between two mounts with and without damping rubber are shown below. From the spectrum, there are not much different between the current anodized Al mount (not shown) and the steel mount in fig1.
Note: We also tried to damp the mirror mount with small rubber pieces placed between the frame and the body of the mount, but it did not help at all. The springs of the mount are stronger than the rubber, so this method is not effective.
To sum up,
We are damping most of the optics with rubber cones. There are a few peaks that we still could not find their origins. We are thinking to build an acoustic insulation box to cover the setup.
[details will be added soon]
I measured beat signal, after damping most of the optics, realigning the beams to the cavities, measured the slope of error signals and applied it to the measured detection noise. Acoustics peaks around 200Hz to 1kHz still present.
Fig1: beat measurement, I added shot noise and electronic noise from both cavities to a single trace called detection noise (from measurement).
I turned off the HEPA fans on the table and on the clean bench before measured the beat signal (after I finished, I turned on the fans as usual).
The peak at 58 Hz shows up this time. I think this is the peak from beam line motion of the stacks, see PSL:716. (I think that was before we switched to the softer springs, I'll double check). Note that the air springs were not activated during the measurement, we can try using it and see if there is any improvement.
There is a good improvement on minimizing the acoustic peaks, although still not enough. Also, increasing the modulation depth seems to help with the flat noise part at high frequency, we may really sit on detection noise.
I'm checking the properties/prices/availability of window for the vacuum chamber.
Plan1: 10" diameter window (6" window opening)
Plan2: 10" diameter blank with 2 smaller windows (1.5"/2" diameter)
Most of the manufacturers do not have good window for laser with 10" flanges. Finding two smaller windows with good optics properties is probably easier.
I forgot to change the code to disable the air springs, now the seismic coupling makes more sense.
If we go with plan2,
1)window and flange
2) Two Half-Nipple will be welded to the blank on the 10" flange. They will be 3" apart, as the input beams are. We might need something smaller than 2.75" diameter for accessing all the screws.
3) blank 10" flange: I think Frank said that we have one in the lab. For another one, we can order it from N-C, blank. It is ~$ 300.
I'm not sure how to mount the window and the flange together. If we buy the window set from Thorlab, I think it can be directly assemble them similarly to the current 10" flange, see figure below. Or we might need to mount the windows like Zach does for Gyro, see ATF:1601.
I planned to measure the beat at night with the air springs activated, but the power went out around 11:45 pm. I think the temperature servo got a kick and it is drifting very fast. So I cannot keep the cavities locked long enough for the low frequency measurement. I'm just turning the systems back on for now.
The laser, 3 Marconis for 14.75MHz EOM, for ACAV AOM, for beat are set back to the original setup, PMC medm screen are back on, the air springs are up and working.
The linux machine is on but I forgot the password, will ask Frank tomorrow.
I'm searching DCC for window/viewport examples. The following drawings give me some ideas how to make a window for our setup.
TCS viewports details
septum window flange
For small window option, I can either have it made from scratch ( based on LIGO's drawing) or buy the commercial windows from Thorlabs. Here I listed down all pros and cons for each choice as I discussed it with Frank. I 'll ask Steve tomorrow for his opinions.
== Using Thorlabs 2.75" OD windows:==
==Making custom parts (like LIGO, see quote window)==
I asked Steve about the choices, he thought the Thorlabs window should be ok for us.
What Steve suggested are:
About the blank with two openings for beam access, he said a vacuum company could do it for us. I'll make a drawing and get a quote from Nor-Cal and MDC. I have to specify that the blank will be for ultra high vacuum system (UHV).
I don't know how you gonna make the knife edge on the 10" flange of centered and wedged! If you put the small CF flanges on the big one you have parasitic cavities between the window and the cavity even if the window is wedged (only the outside is tilted relative to the flange, the inside is parallel by design. I also suggest going for a metal seal, but not copper as getting those knife edges will be complicated and expensive i guess. So why not using indium or the other single-use metal seal replacement techniques for o-rings available and you only need a flat surface on the big flange and a few tapped blind holes?
Request 2 degrees off set the the 2.75"cf knife edge and tapped holes on the 10" flange. The location is custom anyhow. You can gain some space this way. Or can you tip your chamber?
Check how parallel you cavity is with your chamber
cavity mirrors are parallel to the end surface of the chamber (not completely, but pretty close; changes every time we touch the stack as we can't fully control the position after sliding the stack into the long chamber. However we should rethink our procedure how we align the stack once in the chamber)
I got the reply from Thorlab the flange can't accept the thicker optical windows. So I think we have to make our own custom small flanges. I'll check TCS small windows design and make a drawing and consult with Steve again.
Thank you very much for your response. It looks like our flanges can only fit
windows 0.1 mm thicker, with a tolerance of +0.0/-0.2 mm, so these flanges would not
be cross-compatible with existing windows. I apologize for any inconvenience this
may cause. Please let me know if you have additional inquiries, as I am very happy
Nice reference for O-ring + groove design. I'll put it on CTN wiki as well.
The o-ring I plan to use for 2" OD window is #223, 0.139" thickness, ID = 1.609", OD = 1.887". McMasterCarr.
I finished the drawing for new vacuum windows. The o-ring for the windows will be #223 (1/8" thickness). I'll consult with Steve one more time before I submit the drawings.
A few comments about this desing:
I edited the drawing for 10" flange. The wedge surfaces for 2" windows are tilted by 2 degrees sideway.
I tried to assemble the pieces with 2" OD window, 0.25" thickness (without Oring). I think the clearance for the window might be too tight. I'll fix it.
2" optics with 2 degrees of wedge will have 0.375" thickness as std - get optics specification now
I thought about the design after talking to you yesterday:
a, use standard 3 3/8" od flange for your windows
b, the 2 degrees of off- set into the 10" cf design will have to be assembled in horizontal position so the teflon gaskit would stay in place
c, the vertical assembly requires that you put the 2 degrees off-set into the 3.37" flange ( one side CF - the other o-ring groove) and delrin cover plate on top of it
Frank showed me where we keep the spare cavity mirrors. They are in a cardboard box labeled REO in the left cabinet. There are 7 substrates with the coatings similar to what we use in the current setup. They are specified as polished annulus, and wedge (details are added in the proposal). So, if we have short spacers, we can assemble the cavities asap. The coatings profile is not written anywhere(# of layers, transmissivity), I'll ask peter if he has the information about this.
Peter told me that the fused silica pmc currently used in the lab is bonded by Vac-seal epoxy. So we don't need to polish any surfaces for optical contact.
Traces of vac-seal can be seen between the mirror and the tip, the tip and the spacer bonded areas. Vac-Seal epoxy is chosen for its low out gasing, so that the mirrors won't be contaminated.
I'm thinking about the spec for AlAs/GaAs coatings. Here is the list of what I have:
==Coating diamter for 0.5m ROC mirror==
About the coatings diameter, Garrett said it depends on the aperture size/ coating diameter. So I made a plot to estimate the loss due to the finite size coating vs Coating diameter for our spot radius of 182 um. The loss is simply calculated by the ratio of the power not falling on the coating = Ploss/Pin = (exp(-2*r0.^2./w0.^2))*1e6*26000/pi
where r0 = coating radius, w0 = spot radius, a factor of 1e6 for showing the result in ppm, 26000/pi is the total loss due to the light bouncing in the cavity.
fig1: Loss vs coating diameter (in meter)
It seems we can go to 2mm coating diameter, and the loss is still much less than 1ppm (the expected loss from absorption and scatter is ~ 10ppm). However, we have to consider about how well they can center the film, how well we can assemble the cavity. So larger coating diameter is always better. If we assume that 1mm error is limiting us, coating diameter of 4-5 mm should be ok for us.
==for mirror with 1m ROC==
If the ROC is 1.0m, the coating diameter can be 8mm. For the cavity with 1.45" long, the spot radius on the mirror will be 215um (182um with 0.5m mirror). This changes the noise budget of the setup a little bit. The total noise level is lower by a factor of ~ 1.2. (see below figure) at 100 Hz.
fig2: Noise budget comparison between setup with 0.5 m and 1.0m RoC mirrors, plotted on top of each other. Noises that change with spotsize are coating brownian, substrate brownian, thermoelastic in substrate, and thermo-optic.
==What do we choose? 0.5m or 1.0m==
For both 0.5 and 1m, the cavity will be stable (see T1200057-v11, fig11). So either choice is fine
if we use 1.0 m,
So at this point, I'm thinking about going with 1.0 m mirror.
We should be able to mode match into a cavity with 1.0 m ROC mirrors using only the optics we already have on the table.
Current mirrors: 0.5 m ROC (has -1114 mm FL)
Proposed mirrors: 1 m ROC (has -2227 mm FL)
The various waists for the proposed mode matching are equal to or larger than the waists for the current mode matching, so I don't think we should be any more worried about sensitivity than we already are.
Today I installed the Faraday isolator after the PMC. Tara and I then spent some time trying to figure out why the PDH error signal suddenly had a huge DC offset (it was because I accidentally knocked the angle control on one of the HWP mounts while installing the FI beam dump). Before installing the FI, we had observed that the loop oscillates noticeably at about 100 kHz and had hoped it was caused by back-reflection into the laser (which the FI would fix). Installing the FI seems to have no effect on the oscillations. After installing the FI I adjusted the HWP immediately following and retuned the phasing of the PDH loop by adding some extra cable to the PD SMA input. I've attached a picture showing the sweeps of the cavity refl response and PDH error signal, and a picture showing the oscillations when the loop is engaged.
I tried minimizing the rejected light out of the FI to optimize the angle of the QWP directly in front of the cavity, but this light appears to be dominated by reflections other than those off of the cavity. The rejected light consists of two distinct spots which can be seen with an IR card. I think one of them is a reflection from the lens immediately following the FI, and the other is a reflection from the 14.75 MHz EOM.
Tara and I repositioned the QWP and PBS immediately preceding the periscope so that we could move the 64.4-mm ROC lens closer to the cavity. For space reasons, this lens is now forked directly to the table rather than mounted on a translation stage. I tried for a while to adjust this and the 38.6-mm ROC lens to improve the mode matching, but I can't seem to do much better than 80% visibility. We may have to adjust the 103-mm ROC lens directly after the PMC in order to go further.
In better news, we were able to couple some power into the fiber that runs into the ATF. The beam is picked off with the PBS immediately following the EAOM and then sent through two mode-matching lenses and a HWP before hitting the fiber. We're sending 10 mW in and currently getting 0.85 mW out. More work is needed to get the polarization correct and to improve the coupling efficiency. This setup will probably have to be redone at some point, since the current pickoff beam is downstream of the cavity EOM and therefore has sidebands on it. Also, we will have to redo the coupling if we touch the 103-mm ROC lens to improve the cavity mode matching.
I redid the mode matching for both refcav, the visibilities are up to ~ 93% and 95% for RCAV and ACAV.
I'll add the new layout for the current situation soon.
I installed the beat board back behind the cavities. I still have not finished aligning both beams to the 1811.
Both cavities are locked (not optimized yet). Since it has been awhile that both are locked, here is a picture.
Rcav is locked by Fast feedback only. I still have to check the polarity for PC feedback. I adjusted the phase between the LO and PD for RCAV loop to get a nice error signal. I noticed that there is an offset in the error signal, I will try to adjust the polarization of the beam in front of the EOM to see if I can reduce this offset from RFAM.
Since we are trying to optimize a layer structure for AlGaAs coatings. It is a good idea to summarize some notes about all the coatings details. Thanks Koji for the discussion about the coaitngs.
==some background about SiO2/Ta2O5 QWL with 1/2 wave cap coatings==
For quarter wave layer stack (QWL) SiO2/Ta2O5 coatings, SiO2 and Ta2O5 are the material with low (nl) and high refractive indices (nh), respectively. Due to the stronger structure of SiO2, we usually have a cap of SiO2 as a protective layer on top. This cap has thickness of 1/2 wave length. The reason is that the reflected beam from the interface between the cap and the next layer will be in phase with the first reflected beam at the air-coating surface, see the figure below (top).
If the SiO2 cap is 1/4 thick, the reflected beam from the interface between the cap and the next layer will destructively interfere, causing the reflectivity to go down (see the picture below, middle).
However, if the cap is Ta2O5 (nH) material, it can be QWL thickness, and the phase from every reflected beams still interferes constructively (picture below, bottom).
Note: As we can see, the incoming beam and the reflected beam are 180 degree out of phase. It means that the E field at the coatings surface will always be zero. This will prevent the burning on the surface of the coating. With this, the standing wave in the cavity will always have zero E field at the coating surface, see below picture.
This is not AR coat, since all the reflected beams interfere constructively. The reflected beams from AR coating will destructively interfere among each layer.
To sum up for the SiO2/Ta2O5 coatings:
For GaAs/Al0.92Ga0.08As (AlGaAs) coatings, the situation is a bit different from SiO2/Ta2O5. The cap has to be GaAs (nH) because Al0.92Ga0.08As will oxidize and change its material properties. Now that the cap will be nH, the thickness has to be 1/4 wavelength. The last layer next to the substrate has to be GaAs (nH) too (I think because of both the better reflectivity and the fabrication process).
There is an assumption about the layer structure used in the optimization code that the cap is nL(SiO2), 1/2 layer. The coatings layers are even number ( doublets of SiO2/Ta2O5). I'm making sure all the assumptions in the code are fixed. Here is a preliminary result.
above: Layer structure, the first layer (cap) is GaAs (nH). In the optimization, I keep the cap thickness to be 1/4, and vary the rest.
above: Noise budget of the optimized layer. TO noise is below BR noise from DC up to 1kHz.
The reflectivity of the coatings is -0.9997 + 0.0209i (reflection phase = 180 - 1.2 degree). I'm not sure if this is good enough, maybe better optimization can be done.
Note: My layer structure is really different from what rana did in T1200003. For my structure, the layers near the cap vary a lot before getting close to 0.25 when the layers are close to the substrate. The result from 1200003 is the opposite. The layers near the cap are about 0.25, and start to diverge when the layers are close to the substrate.
above: Optimized coatings result from T1200003. The optimization probably assume the cap of low index material, but the following layers evolution are opposite of what I got. That's why I'm not sure about my optimization.
I'll upload my codes soon so that people can check my optimization.
The codes for optimizing Thermo-optic noise in coatings are up on svn.
I adopt some codes that have been on svn for awhile and modified them for AlGaAs coatings. There are two main codes
This file is modified from DoETM.m found in .../iscmodeling/coating/AlGaAs/doETM.m . The optimization method is using Matlab's fmincon function to search for coatings structure that minmize TO noise. Some modifications include:
This code calls on optAlGaAs.m when running fmincon.
This file is the modification of optETM.m found in ../iscmodeling/coating/AlGaAs/optETM.m .It calculates the reflectivity and the TO coefficients from the given layer structure. The modifications are:
This code is used in optAlGaAs.m it calculates the reflectivity and impedance of the given coatinns structure. There is no modification to it. The code can be found in .../coating/coating_optimization_new/.
To run the codes
check out .../iscmodeling/ folder from the svn. The optimization is in .../iscmodeling/coating/AlGaAs_TO_opt_CTN/ folder, but you need other functions in other folders.
Once you run DoAlGaAs.m, the optimized layer will be in matlab workspace called xout. This is the layer structure withtout 1/4 cap. Check if there is a layer with thickness of 0.002 or not. I ran the code several times, sometime it shows up. Just rerun the code and get the layer that is around 0.1 or thicker. The 0.002 is just the lower bound used in fmincon search in doAlGaAs.m.
Plotting noise budget
The noise budget of the optimized layer can be plotted with /coating/AlGaAs_Refcav/nb_algaas.m . Currently, at line 38-39, the code will take xout and create a layer structure with 1/4 cap on top of it. The reflectivity of the coatings is in rCoat workspace item after running the noise budget code. It should be close to -1 + 0i