Minutes of meeting on design of the 40m DC readout experiment Thursday, July 14 2005, 10am. Attending: Mike, Rana, Jay, Seiji, Bob, Ryan, Ben, Marcus, Steve, Osamu, David, JuLi, Rob, Virginio, Alan. Minutes by Alan. ___ The Review committee will be chaired by Ken Strain. He has asked what we think are the main points that the review committee should focus on. Please suggest some. ___ Layout and optical design: We'll pick off the AS beam and send some light to an in-air table: the ASRF beamline. Most of the beam will go to the ASDC beamline: the in-vac output mode-matching telescope (OMMT), in-vac output mode cleaner (OMC), in-vac DCPDs, and in-air OMCR and OMCT monitor photodetectors. A beamsplitter will send ~1/3 of the AS light to the ASRF beamline, and 2/3 to ASDC, split into 2 DCPDs. ___ ISS: Do we need 1 or 2 ISS PDs in vacuum or can we do it in air? either way, we may need to pick off more light for the ISS PDs. Rob says that for 19 pm offset, DC shot noise is not dominated by intensity noise if we can get the intensity noise < 3e-8 @ 300 Hz, which can get in air. So, tentatively, keep the two ISS PDs in air on the PSL table (but get more light onto them), and maybe move them to IMCT. ___ Output mode-matching-telescope (OMMT): Mike's current drawing shows the OMMT cantilevering, weighing down the small output optic chamber (OOC) stack. Is this a worry? Steve doesn't think so. Mike will try to shorten the MMT, just in case it's too long to fit in the OOC. ___ Beam path from SRM to OMMT: The beam barely fits through the input MMT (IMMT) on its way to the OOC. There is very little leeway for steering the beam without hitting the IMMT. We must align carefully! ___ In-air Alignment: In the past, we found it most useful to align the IFO using the main PSL beam, either flashing through the input mode cleaner (IMC) or just passing through MC1,MC3. After going through PRM, BS, SRM, there will be very little light available to align the DC readout path. So, we need another plan: Use the SRM as a reference. Send an autocollimator beam in through the ASRF pickoff and the in-vac PZT steering mirror, retro-reflect it off of the SRM, and thence back to the ASDC beamline, to align everything. Any error in the SRM alignment can be corrected for after pumpdown, using the in-vac PZT steering mirror. Should work! ____ OMC: Rana believes he can obtain a LIGO PSL PMC as a backup OMC or diagnostic device. The 40m PMC has a relatively high finesse (T: 0.4%, 500ppm on back; finesse = 730); the ones at the sites are lower finesse (true?); we have the choice to use either for an OMC. The PMC would need to be vacuum-prepped; presumably, the only nasty stuff is the glue used to stick the curved mirror on the PZT. __ Meanwhile, we want to design our own OMC. Base it on Keita's 4-mirror design, but no fancy cylindrical lenses to cancel astigmatism or equalize Guoy phase in H&V. __ 3-mirror vs 4-mirror - - 4-mirror has degenerate HOMs, halving the chance that a HOM will be resonant. - 4-mirror passes both the correctly-polarized AS beam and any wrong polarization. Because the IFO is polarization-insensitive to lowest order, any wrong polarization AS beam carries GW information. But, because of the BS polarization dependence, the wrong polarization carries arm imbalance signal which is noise, not signal. Better off rejecting it! - designing, building, aligning and commissioning a 3-mirror or 4-mirror OMC are probably about the same. - 4 mirrors costs 4/3 as much as 3 mirrors. We decided to go for a 4-mirror OMC, laid out as suggested by Keita. The mirrors could be mounted to a metal fixed spacer using mechanical mounts, rather than glue. Mike knows how to design and build this. The only glue would be to mount the mirror on the PZT length actuator. The input and output mirrors would be wedged. The folding mirrors do not need to be wedged. Remember to include the losses in the folding mirrors when calculating the finesse and HOM/RF suppression! __ Fixed spacer material: - Fused silica is expensive and long lead-time, CTE is 0.6 ppm/deg. - Al is cheap and easy to machine, but CTE is 23ppm/deg. For a half-length of 0.25 m, and a temperature excursion of 0.1degree, that's 0.6 um, not negligible compared to range of PZT (which is?). - SS is only a little more expensive to buy and machine than Al, and CTE is 11 ppm/degree. - Invar is much more expensive to buy and machine; CTE < 1 ppm/deg. Mike will price both Al and SS; he believes the price difference is small and his suggestion is to go with SS because of the lower CTE. __ Aspect ratio: Keita's design has the beam reflecting off of the folding mirrors at an angle of ~6 degrees. Rob's cartoon has phi = atan(2/20) = 5.7 degrees. I *think* the PMC is 2*atan(2.5/18.5) = 15 degrees... If some of the reflected light is scattered through that angle, it will counter-propogate, resonate, reflect back into the IFO, then back out, carrying a noisy signal to the dark port. The amount of reflected light scattered back as a function of angle is called the Bi-directional Reflection Distribution Function (BRDF); for the CVI mirrors, the BRDF is unknown. Maybe we should consider tweaking Rob's cartoon to increase the reflection angle. __ g-factor: Marcus showed plots suggesting that g = (1-L/R) = 0.44 (with L = OMC half-length; R = radius of curvature of mirror) had the least amount of HOM transmission, going up to m,n=3. However, the 166 MHz sideband TEM01 mode is ~40% transmitted. Meanwhile, Rob suggested L = 0.2446, R = 1 meter, g = 0.7554 which has < 1e-3 transmission for all HOMs of carrier and RF sidebands, until n+m=6, which has ~2% transmission for carrier. Rob also determined that the suppression is robust for errors on R of ~ 2-3%. CVI laser mirrors come with radii of 0.25, 0.3, 0.5, 0.75, 1.0. 1.5, 2.0 meters. http://www.cvilaser.com/Catalog/Pages/Template6.aspx?pcid=19&filter=0 __ Finesse: input and output couplers with 0.5% transmission -> Finesse=600, f_pole = 0.5 MHz 1.0% transmission -> Finesse=300, f_pole = 1.0 MHz 2.0% transmission -> Finesse=150, f_pole = 2.0 MHz Higher finesse means better filtering of RF sidebands and HOMs. It also makes the OMC more sensitive to length noise, makes it harder to lock (higher BW required on servo), and has higher stored power, making it easier for contaminants to burn on to the mirrors. And, the exact finesse obtained depends more strongly on the losses in the OMC. The lower finesse should be sufficient for filtering, but we run the risk of accidental HOM resonances due to some imperfection. Let's let the review committee decide this for us! ___ DCPDs: We did not have time to discuss this! Ben will draw up a DCPD design for the Tuesday meeting. ___ OMC controls, monitoring and DAQ electronics We did not have time to discuss this! Jay will draw up a block diagram, with parts list and channel list, for the Tuesday meeting.