[Ed, Evan]
We are preparing to make a measurement of the length of the PRC using the phase-locked auxiliary laser technique described by Chris Mueller (T1400047). Previously, this has been used to measure the Livingston IMC length (LLO alog 9599).
We set down a 520 mW Lightwave NPRO on the IOT2R table, along with a Faraday isolator and steering mirrors. We will inject this beam into the PRM_refl side of the IOT2R periscope. The beam will hit the back of IM4, and a small fraction (2400 ppm) will be transmitted toward the PRM. This gives 1.2 mW of auxiliary power on the PRM, compared to 9 mW of 45MHz PSL single-sideband power on the PRM.
Most of the auxiliary power should reflect from the back of IM4 and return to the IOT2R table via the IO_forward side of the periscope. For mode-matching, we hope that we can simply send part of the IO_forward beam onto a New Focus 1611 and maximize the observed beat. Currently, there is 3 mW of power in the IO_forward beam.
Using this beat, or otherwise, we will phase-lock the auxiliary laser to the PSL carrier beam. Then with PRMI locked on the PSL sideband, we will sweep the offset to the auxiliary PLL and monitor the RF coming out of POPAIR_B. We should see the strength of the RF reach a maximum whenever the auxiliary beam is coresonant with the the PSL sideband. By tracing out the Lorentzian profile of the RF amplitude across successive resonances of the PRC, we can extract the FSR of the PRC. Given a design length of 57.6557 m, we expect an FSR of 2 599 850 Hz. If we can measure the FSR to within 100 Hz, we can get the PRC length to within 2 mm.
Yesterday we got the NanoScan back from EX and Ed used it to measure the beam parameter coming out of the Faraday isolator. The waist is about 100 µm and located more or less in the middle of the isolator. The size is maybe a bit smaller than we want, but we appear to be able to get more than 90% of the power through, with a reasonably Gaussian mode.
After the FI, we placed a HWP to set the beam to be s-polarized. After this, we placed a New Focus 5104 as a first steering mirror. As a second steering mirror, we use IO_PRMR_BS1.
We removed a lens from between IO_PRMR_M3 and IO_PRMR_BS1. It was unlabeled, and anyway there is nothing after that lens except beamsplitters and dumps.
We did an ALM optimization to mode match to the PRM. Joe Gleason's IOT2R layout (D0902284) gives the distance from the bottom of the IOT2R periscope to the PRM as 3.6 m. The spot size is 2.24 mm, with a ROC of 11 m (T0900407, p 5). ALM told us to put an f = 500 mm lens about 3 inches before IO_PRMR_M3 ("before" meaning "closer to the FI").
We put down two irises in order to constrain the pointing of the PRM_Refl beam. We then blocked this beam and steered the auxiliary beam through the irises. With a little tweaking, we were able to see our beam coming out on the IO_Forward part of the periscope. We measured the power of this beam and found that it was only about 5% of what we were putting in. This initially confused us, until we realized that our path in HAM2 has to go through a 90% reflector which is intended for the ISS. Given that IO_PRMR_BS1 is a 90% reflector and ROM LH1 (in HAM2) is also a 90% reflector, we in fact only expect 90% × 90% × 10% = 8% of the power to come back onto the IOT2R table.
Yesterday, we put down the New Focus 1811, aligned the PSL and auxiliary beams from IO_forward onto the PD. We found a beat with the auxiliary laser temperature around 37.7 °C. By tweaking the auxiliary input pointing, we were able to get -4 dBm of RF beat out of the 1811 with about 1 mW of DC power from each beam in front of the PD (so 2 mW total).
We were then able to implement a PLL using an HP function generator and the LB1005 servo box. We set the function generator to ~30 MHz and +7 dBm, and used it to drive the LO of a mixer. We took the beat and put it into the mixer RF. The IF was terminated, filtered at 1.9 MHz, and then fed into the LB1005. The output of the LB1005 was then fed into the fast input of the laser. We were able to catch lock by turning the laser's temperature control knob to push the beat toward 30 MHz. The lock would hold for about 1 minute before the controller saturated. To maintain sanity, I suspect it will be necessary to implement a slow temperature loop to relieve the fast controller.