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ID Date Author Type Categorydown Subject
  659   Wed Aug 17 20:41:13 2011 taraDailyProgressopticACAV path is up

I put most optics on ACAV path. I have not tried to lock the cavity yet. I'll install ACAV RFPD next.

layout_2011_08_17.jpg

 

 

  676   Tue Sep 13 01:11:51 2011 FrankNotesoptic50:50 beamsplitter data - scanned spec sheets from ATF

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.

scan0008.jpg

scan0011.jpg

scan0012.jpg

  719   Tue Nov 1 16:11:48 2011 taraNotesopticV-block for faraday isolator

I made a drawing for faraday isolator's base. I'll submit the drawing tomorrow.

Attachment 1: isolator_vblock.PDF
isolator_vblock.PDF
  726   Tue Nov 8 18:51:38 2011 taraDailyProgressopticoptics for RFAM

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.

IMG_2186_ACAV.jpg

IMG_2187_RCAV.jpg

 

  739   Tue Nov 29 01:50:40 2011 taraDailyProgressopticoptics for RFAM

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.

RFPD_setup.jpg

     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.


  746   Sat Dec 3 01:12:00 2011 taraNotesopticV-block for faraday isolator

     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.

 

  770   Tue Dec 27 19:49:27 2011 FrankNotesopticLens Kit missing!

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!

  784   Fri Jan 13 16:51:29 2012 TaraNotesopticIsolator base, EOM base's height

 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.

dual_periscope_base.PDF

Attachment 1: eom_base_2011_01_13.PDF
eom_base_2011_01_13.PDF
  785   Tue Jan 17 21:59:47 2012 taraDailyProgressoptic14.75MHz EOM

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.

  787   Thu Jan 19 01:19:31 2012 taraDailyProgressoptic14.75MHz EOM

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.

  789   Thu Jan 19 23:43:43 2012 taraDailyProgressopticRCAV is locked

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.

2012_01_19.jpg

 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.

2012_01_19b.jpg

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.

 

  790   Fri Jan 20 15:31:08 2012 FrankDailyProgressopticRCAV is locked

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!

Quote:

RCAV is locked using TTFSS.

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.

 

  797   Wed Jan 25 02:51:44 2012 frank, taraDailyProgressopticFaraday Isolator is back

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.

 

2012_01_24_1.jpg

  799   Wed Jan 25 23:41:38 2012 frank, taraDailyProgressopticnot much improvement on mode matching

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%. 

2012_01_25_3.jpg

 

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.

2012_01_25_1.jpg

  801   Thu Jan 26 12:40:21 2012 TaraNotesopticDual periscope base

 I fixed the drawing for periscope base. Will submit to the machine shop soon.

Attachment 1: dual_periscope_base.PDF
dual_periscope_base.PDF
  805   Sat Jan 28 02:41:45 2012 FrankPhotosopticphotos of cavity input optics

just for reference which part is/was where for later...

IMG009.jpg IMG010.jpg

IMG011.jpg IMG012.jpg

  844   Sun Feb 26 17:40:43 2012 taraDailyProgressopticbeam realignment after new seismic stack installation

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.

IMG_0374.jpgIMG_0373.JPG

      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.

 

  845   Mon Feb 27 10:56:43 2012 taraDailyProgressopticbeam realignment after new seismic stack installation

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.

  905   Tue Apr 10 00:50:56 2012 frank, taraDailyProgressopticbad PBS

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.

  906   Wed Apr 11 02:44:29 2012 frank, taraDailyProgressopticbad PBS

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.

beat_2012_04_10.png

  912   Fri Apr 13 03:15:12 2012 koji,frank,taraDailyProgressopticvibration damping on optics

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.

IMG_0731.jpg

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.

damped.png

 

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.
  915   Sat Apr 14 06:03:23 2012 frank,taraDailyProgressopticvibration damping on optics

 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]

IMG_0744.jpgIMG_0742.jpg

  916   Mon Apr 16 02:20:58 2012 taraDailyProgressopticvibration damping on optics

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.

beat_2012_04_15.png

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.

Attachment 1: beat_2012_04_15.png
beat_2012_04_15.png
  919   Mon Apr 16 16:21:01 2012 taraNotesopticwindows for vacuum chamber

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.

  921   Mon Apr 16 22:02:08 2012 taraDailyProgressopticvibration damping on optics

I forgot to change the code to disable the air springs, now the seismic coupling makes more sense.

beat_2012_04_15_fixed.png

Quote:

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.

beat_2012_04_15.png

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.

 

  922   Tue Apr 17 19:51:54 2012 taraNotesopticwindows for vacuum chamber

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.

 IMG_0757.jpg

 

Quote:

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.

 

  923   Wed Apr 18 00:49:07 2012 taraDailyProgressopticvibration damping on optics

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.

  926   Thu Apr 19 03:12:23 2012 taraNotesopticwindows for vacuum chamber

 

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

  931   Sun Apr 22 23:09:38 2012 taraNotesopticwindows for vacuum chamber

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

Quote:

 

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

 

  932   Mon Apr 23 16:52:16 2012 taraNotesopticwindows for vacuum chamber

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).

Quote:

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

Quote:

 

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

 

 

  938   Wed Apr 25 10:44:29 2012 FrankNotesopticwindows for vacuum chamber

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?

Quote:

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).

Quote:

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

Quote:

 

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

 

 

 

  940   Thu Apr 26 08:35:57 2012 steveNotesopticwindows for vacuum chamber

Quote:

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?

Quote:

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).

Quote:

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

Quote:

 

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

 

 

 

 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

  943   Fri Apr 27 08:46:24 2012 FrankNotesopticwindows for vacuum 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)

Quote:

Quote:

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?

Quote:

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).

Quote:

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

Quote:

 

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

 

 

 

 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

 

  949   Tue May 1 20:37:16 2012 taraNotesopticwindows for vacuum 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.
-------------
  960   Fri May 11 17:52:17 2012 taraNotesopticwindows for vacuum chamber

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.

Attachment 1: O-Rings.pdf.zip
  964   Wed May 16 22:27:13 2012 taraNotesopticwindows for vacuum chamber

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.

2_small_window.PDF

blank_10_wedge.PDF

Quote:

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.

 

  969   Mon May 21 16:53:38 2012 taraNotesopticwindows for vacuum chamber

 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.

 

window_assembly.PDF

window_assembly2.PDF

 

Attachment 1: edit_blank_10_wedge.PDF
edit_blank_10_wedge.PDF
  971   Wed May 23 15:04:53 2012 steveNotesopticwindows for vacuum chamber

 

 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

  985   Wed Jun 13 11:12:55 2012 taraNotesopticcavity mirrors

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.

IMG_1357.jpgIMG_1358.jpg

IMG_1359.jpg

 

  1119   Thu Mar 14 18:20:04 2013 taraNotesopticfused silica pmc

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.

IMG_0819.JPG

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.

IMG_0820.JPG

  1121   Fri Mar 15 11:23:45 2013 taraNotesopticAlAs/GaAs

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.

edgeloss.png

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.

noise_compare.png

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.

 

 

  1122   Sun Mar 17 16:09:10 2013 EvanNotesopticAlAs/GaAs

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)

  • 370 um PMC waist at z = 0 m = 0 in
  • 229.1 mm FL lens at z = 0.203 m = 8.0 in
  • 209 um intermediate waist
  • 85.8 mm FL lens at z = 0.923 m = 36.3 in
  • 41 um intermediate waist
  • 143.2 mm FL lens at z = 1.201 m = 47.3 in
  • 182 um cavity waist at z = 1.944 m = 76.5 in
  • Mode overlap 1.000

Proposed mirrors: 1 m ROC (has -2227 mm FL)

  • 370 um PMC waist at z = 0 m = 0 in
  • 229.1 mm FL lens at z = 0.203 m = 8.0 in
  • 209 um intermediate waist
  • 85.8 mm FL lens at z = 0.889 m = 35.0 in
  • 45 um intermediate waist
  • 143.2 mm FL lens at z = 1.166 m = 45.9 in
  • 210 um cavity waist at z = 1.952 m = 76.8 in
  • Mode overlap 0.999

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.

 

Quote:

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.

edgeloss.png

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.

noise_compare.png

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.

 

 

 

Attachment 1: 0.5m_ROC_modematch.pdf
0.5m_ROC_modematch.pdf
Attachment 2: 1m_ROC_modematch.pdf
1m_ROC_modematch.pdf
  1123   Wed Mar 20 20:44:32 2013 EvanDailyProgressopticAdded Faraday isolator

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.

I also tried improving the mode matching into the cavity by adjusting the two lenses on translation stages, but I didn't get much improvement. Right now we are at just under 50% coupling.
Attachment 1: cavity_scan.jpg
cavity_scan.jpg
Attachment 2: loop_oscillation.jpg
loop_oscillation.jpg
  1130   Tue Mar 26 22:42:23 2013 EvanDailyProgressopticMode matching extravaganza

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.

  1228   Mon Jul 8 21:52:04 2013 taraDailyProgressopticmode matching to refcavs

 I redid the mode matching for both refcav, the visibilities are up to ~ 93% and 95% for RCAV and ACAV.

  • For RCAV (refcav with PMC), the visibility was ~ 80% before, now it is ~95%. (The numbers are measured from the reflected beam on the RFPD)
  • For ACAV (refcav without PMC), the visibility is now ~ 93%. This is pretty good, compared to ~ less than 85% from previous setup when we used an AOM.

I'll add the new layout for the current situation soon.

 

==Note==

  • We care about mode matching because we already saw that any light that was not coupled into the cavity was reflected back to the laser and caused extra noise.
  • By changing the lens, the beams for fiber optic (both for Gyro and Erica's experiment) have to be re calculated. I'm sorry about that .
  1231   Tue Jul 9 19:35:20 2013 taraDailyProgressopticbeat board is back

I installed the beat board back behind the cavities. I still have not finished aligning both beams to the 1811.

  • Note about ACAV ( this path has PMC on it): After new mode matching with more visibility (from 80% to 95%), I can increase more gain and the error noise is getting lower. However, there is a problem with the beam reflected from the window of the tank. It overlaps with the main beam and cannot be blocked. I think this is the reason why we cannot suppress the error noise down to what we had before. I still need to convert the error noise back to frequency noise to see if it is below the estimated coating noise or not. If not, we have to reopen the chamber and tilt the cavity a bit. Rcav does not have this problem, the back reflection is away from the main beam and can be dump properly.
  • Note about RCAV: Erica and I plan to finish the EOM driver test tomorrow. After that I'll use it to drive the broadband EOM for locking RCAV to the cavity. The plan is to use one marconi to drive two EOM at the same frequency (14.75 MHz). We use a 4-way splitter for 2 EOM and 2 demodulations. I don't know how using same frequency for EOM will turn out (cross talk problem), but I want to see the first beat measurement within this week.
  • Note about beat setup: Evan calculated the mode matcing for beat setup, but I had to modify it. The first lenses were moved out of the board and mounted between the vacuum tank and the board due to space limitation. This might add some extra resonant peaks in the beat setup due to the long posts for lenses. The spot diameter on the PD is about 130um, which should be fine because 1811's diameter is ~300 um.
  1233   Thu Jul 11 00:19:52 2013 taraDailyProgressopticboth cavities are locked

Both cavities are locked (not optimized yet). Since it has been awhile that both are locked, here is a picture.

photo_2013_07_10.JPG

 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.

To do:

  • lock rcav with both fast and PC feedbacks
  • optimize the setup ( reducing RFAM, minimize back reflection)
  • setup the beat path (mode match + alignment)
  • setup the ISS path
  • check the beat frequency
  • re organizing the wiring on the table.
  • replace the current SMA cables with the semi-rigid ones, once all the equipments are in place.

 

 

 

  1263   Mon Jul 29 22:30:34 2013 taraNotesopticcoating optimization for AlGaAs

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).

multilayer1.JPG

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.

multilayer2.JPG

To sum up for the SiO2/Ta2O5 coatings:

  • SiO2 is stronger than Ta2O5, so we use it for the end cap.
  • Because SiO2 has lower n than that of Ta2O5, the cap thickness has to be 1/2 wave thick so that all the reflected beams interfere constructively.
  • We want the reflected phase to be 180 degree away from the incident beam so that the surface won't get burnt from the building up E field. (If the E field is non zero, it will be amplified by a factor of Finesse/pi).  My previous optimization for AlGaAs that used 1/8 cap was wrong because the reflection phase was not 180. This means that by adjusting the cap thickness to optimize the TO noise is not a good method, since the reflection phase is not close to 180 anymore. The optimization has to take the phase into account.

 

==AlGaAs 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).

==optimization code==

 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.

 

opt1_2013_07_29.png

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.

nb_opt1_2013_07_29.png

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.

T1200003_refcav.png

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.

  1269   Wed Jul 31 00:31:39 2013 taraNotesopticcoating optimization for AlGaAs

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

      1) DoAlGaAs.m

         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:

  • (Line16-18 )Number of layer. For AlGaAs, the number of layer will be odd number (start with GaAs, end with GaAs), I fixed the layer structure to be odd number.
  • (Line74) Cap. During the optimization, the first cap is kept constant. For a cap made with high refractive index material (nH), the layer thickness is 1/4 lambda, see previous entry.

This code calls on  optAlGaAs.m when running fmincon.

    2) optAlGaAs.m

        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:

  • (Line41-45) Layer structure, the cap start with nH. The material for substrate is SiO2 with nsub = 1.45.
  • (Line60) Desired transmission, as a test, I chose 200 ppm.
  • (Line88) Calculation for TO coefficients (StoZ), I switched from getCoatThermoOptics.m to getCoatThermoOPticsAGS.m. Codes with AGS suffix in /GwincDev folder are fixed for AlGaAs coatings structure. This code calls many functions in /GwincDev folder.

     2.1) multidiel1.m

       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

  1287   Wed Aug 7 20:55:47 2013 EvanDailyProgressopticPreparing the EOAM

In preparation for getting the ISS up and running, Tara and I have been fooling around with the EOAM and associated half waveplates. Additionally, Tara inserted a quarter waveplate (mounted horizontally, for space reasons) after the EOAM in order to get linear amplitude modulation. The HWP before the EOAM is at 99 degrees and the QWP after the EOAM is at 51 degrees.

There's currently 8.0 mW going into the EOAM and 4.0 mW coming out after the EOAM + QWP + PBS. When 10 V dc is applied to the EOAM, the power drops to 3.7 mW. This gives a conversion factor of 3.0×10−5 W/V. The value expected from the manual is (π/2)(8 mW / 300 V) = 4×10−5 W/V, so we're not too far off. 

For those who prefer the status quo, the original HWP angles are as follows. The HWP after the PMC was at 336 degrees, the HWP before the EOAM was originally at 150 degrees, and the HWP before the cavity (which Erica is using as a pickoff for her fiber) was at 236 degrees. Restoring these angles will not restore the previous power configuration unless the quarter waveplate is removed.
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