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:

•  a lens with f = 343.6 m (plcx R =154.5mm)
•  one more curve mirror with R = 0.3m for the second AOM.
• aom adaptor plate (need to submit this to the work shop to have it done
• periscope sets for both ACAV and RCAV (we need 4 in total, but we have only 2 sets)
• second VCO

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.

As we removed some optics on the table, we use pressurized air to blow away dust/dirt on the mechanical parts (mount/ post/ lens holder) Optics have not been cleaned yet. We will clean it before we put everything back on the table. The cleaned parts are kept in a plastic box.

Today we removed the optics behind the PMC, ACAV, and cleaned the table.

•   Optic mounts and posts are cleaned by pressurized air, and kept in a plastic box.
•   Lens and mirrors are kept in optic cases.
•   ACAV is moved to the end of the table, ion pump is unplugged.
• Table is cleaned with methanol, but some grease( under acav) is still on it.

NOTE: I just realized that the HEPA filter above the table close to the entrance ( the one that has the laser) is unplugged.

I could not find any available outlet to plug it back yet. We should turn it on soon.

  Today we started working on the layout. There is one mistake in the layout, the mirror behind AOM for REFCAV is too close to the insulation box, so we have to fix the layout.

 The oil on the table actually comes from holes on the table. About 5-6 screw holes had lot of oil, so I flushed them with methanol a few times.

The HEPA filter is plugged in and turned on. I unplugged one of the monitors and used the outlet for the filter.

 The window is measured to be ~ 6 inches in diameter. Thus, the assumption in the design that the centers between two cavities are 3 inches is ok. If necessary, it can go up to 4 or 5 inches.

The layout is updated. The spot size in both AOMs are adjusted to 220 um.

 We preparing optics for the new layout. To reduce scattering noise, most of the Y1-1064 mirrors we have been using will be replaced by super polished mirrors. 

We think Y1-1064 mirrors can cause scattering noise in the setup because the coating surfaces look very milky.

fig1: Y1-1064 mirror.

     We have ~ 10-20 super polished mirrors. Some of them are good, some of them are rejected from the site. The good one will be used for periscope/ beat setup.  I tested a couples of the rejected mirrors, but they can reflect both p and s beams with high efficiency. We will ask Peter to find out what is wrong with them.

fig2: Left and right, super polished mirror.

fig3: left, mirrors' case, right, certificate.

    I have cleaned about half of the required optics, I think we should be able to lock the first cavity before next Wednesday.

  RCAV is locked, I have not optimized the mode matching yet, the coupling efficiency is ~ 67%.

This new setup has a double passed AOM. The frequency is shifted by 160 MHz.

I will try to optimize the mode matching tomorrow, then I can check the loop performance that it works as before.

 I optimized mode matching for RCAV. The coupling is ~75%.  I also minimized RFAM from the 35.5MHz EOM.

          For RCAV mode matching, I moved only two lenses in front of RCAV (R=51.5 and 20.6 mm) to optimize the mode matching.

I have not tried moving other lenses or the mirror behind AOM yet, because I think 75% is enough for now.

 The reflected beam will cause the shot noise level to be higher, but it should not be critical for our current situation.

 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 !

ASAP!

 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:

EOM

4-axis stage

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.

==Motivation==

   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.

Next step:

• We will use a Marconi with some amplifiers for LO to drive the EOM and demodulate the signal later. The current function generator is noisy, but it's a good start to see how much better we can gain from resonant EOM.
• Use Jenne laser to characterize the 14.75 MHz RFPD. Then we can calculate how much gain we get, and how much improvement we need.

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:

•  Beam splitter that divides the beam to ACAV and RCAV path. Now the new BS is 1" cube for large beam
• Fixed the orientation of mirrors on the periscope and the turning mirror for ACAV RFPD. A few of them were flipped back causing stray light in the beam path.
• Two of the mirrors on periscope had transmission of 2% or so, we replaced with a high reflective ones (0.1% transmission).

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 need to damp most of our optics. The current plan is to use a rubber cone and just place on top of the mirror mounts. We are also thinking about better damping schemes. 
• There are not much different between a stainless steel mount [add model#], and an aluminium mount[add model#]. It is probably unnecessary to change mirror mounts.
• We will order more of the rubber cones for damping.

 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)

•  A&N: ($775), no info on optical quality. This is probably just a regular viewport similar to the one we use.
• Pfeiffer also offers viewports for visually monitoring, so I think they are not good enough.
•  MDC fused silica window, for 10" flange ($ 3,297), no optics properties. Only lens grade for ultraviolet are specified, but they claim that for IF also available (viewport)
• Nor-Cal also has flange for 10" and 2.75" with glass,fused quartz, fused silica material. no optics properties shown.

 Plan2: 10" diameter blank with 2 smaller windows (1.5"/2" diameter)

•  Thorlabs: 2.75" OD window, 1.18" window (windows are replacable, I think we can switch to CVI windows ) ($244 x2) + machining .
•   MDC offer 2.73" with 1.5" window, no info on optics properties 

 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

• I think the window size of 2.75" diameter is the largest size for us ( with 3" clearance between the 2 beams). Thorlabs has a 2.75" window with 1.5" optic,with 1.18" opening, so it might not be compatible with its wedged window:http://www.thorlabs.us/NewGroupPage9.cfm?ObjectGroup_ID=5546 which is only 1" diamter.
• Or we can order 2.75" flange with 1" bored, from N-C. to use with Thorlabs' window. Though I'm not sure how to assemble the two together.

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

double viewports

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:==

       Pros

• easy to replace for damaged optics,
• ready in short time (parts are in stock)
• minimum time on machine shop
• The thickness of the window is only 0.63", It should be able to fit in the set up which has ~ 1" clearance.

      cons:

• Have more rubber seals in the system due to the design, I'll check Cryo:194 to see what will be the minimum pressure we need.

==Making custom parts (like LIGO, see quote window)==

     Pros:

• There are only one o-ring used for each window (better vacuum pressure)

     cons:

• Spend more time on designing/machining, probably more money for making the parts as well.
• Take longer time if we need more spare pieces.
• The available space for the window is quite limited (1"). If we follow what Zach did for Gyro, it is already to thick for our setup

I asked Steve about the choices, he thought the Thorlabs window should be ok for us.

      What Steve suggested are:

• The seal between the 10" blank and the windows should be copper seal, (the window already comes with knife edge),so
• the blank will have knife edge seals for two small windows as well.
• Thorlabs window does not have an o-ring between the window frame and the optic, we should add the o-ring between them to avoid a direct metal-glass contact.

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.

------------------------

Hi Tara,

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
to help.
-------------

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:

•  The design is intend for 2" OD, 0.25" thickness window. The blank has 2 degree wedge surface for the window.
•   The grooves for the o rings are based on the instruction on previous entry.
•  The material for the small window, and the 10" window will be stainless steel.
•   I feel that the drawing is a bit unclear, I'll try to draw it properly.

 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 on concave side of the mirror for 0.5m x6 (I'm not sure if they can do the transfer on 0.5m mirror now) for 1.0m x6 for flat mirror x3 -
• for circularly polarized light, normal incidence
• Transmission @1064 = 100ppm +/- 10ppm. 10% error is still within the acceptable value for 10ppm loss (T ~ 67-73%), see T1200057v11 -
• Absorption + scatter loss < 10ppm, this is what Garrett told us. -
• coatings diameter = 8mm (The number is from Garrett), the loss around the edge for our beam with diameter=364 um is less than 10^-10 ppm. -
• Max scratch surface and point defects are not determined yet. I can look up the specs from our current SiO2/Ta2O5 mirror since they are ok for us. -
• I think we are aiming for the thermo-optic optimized coatings. The layer structure can be found in T1200003-v1.

==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,

• we loss the signal level a bit,
• but we are more certain that the coating will work. 
• The procurement should be faster (as promised by Garrett)
• have large area coating up to 8mm diamter
• need to check if we can mode match or not (I'm positive that we can, but I'll check or let Evan check)

So at this point, I'm thinking about going with 1.0 m mirror.