Sheila, Nutsinee, TJ

I've updated this entry with correct numbers, I made 2 basic errors the first time.  

Last week we set up the Faraday isolator on the squeezer platform (VIP) and made transmission, isolation and backscatter measurements.  We tried to optimize the angles of the polarizers and the half wave plate for transmission to reduce losses for the squeezing.  Our results are 96% transmission, -20 dB of isolation, and less than -36dB of backscatter. As we assembled the Faraday, we tried to adjust each angle for transmission.   

Details

We roughly followed the measurement procedure from Maggie's wiki, with minor differences.  The set up is shown in the attachment (the beam path looks curved in the photo because of my poor photography). We used a chopper to measure transfer functions between two PDs, one at Pos A (monitor of laser power into set up on one side of 50/50 BS), and the other moving between Pos B (measurement of power onto VIP), Pos C (power transmitted through Faraday), and made two measurement at position D, first the return power from the Faraday with the HR retro reflecting the beam back through the Faraday to measure isolation and second with a razor blade beam dump in pos D to measure backscatter off the Faraday itself. Before the 50/50 beam splitter we added a f=1m lens to make sure the beam diameter stays well below the 5mm rotator aperture through the entire set up, and a PBS cube mounted to transmit vertically polarized light.  

We found that the efficiency of each of the PDA100A's we used depended on the angle of the PD.  We initially tried to set markers for the position of the reflection off the PDs so that we could reproduce the angle that we mounted the PDs at each time we moved them, but found that the two PDs have reflections at very different angles.  We used small C clamps on the post to mark the height, and tried to use the knobs on the C clamps to keep the PD angles consistent from position to position.  We cannot use the PDs face on because as Maggie found at LLO the scatter from position A to position D will cause measurement problems.  

Measurements: We have made these measurements twice, first with the initial assembly of the Faraday, this afternoon I went in and adjusted the thin film polarizers but ended up with slightly worse transmission and isolation.  

The ratio of a measurement of before and after the Faraday to the beamsplitter reflection (C/A)/(B/A) gives us the Faraday transmission with the PD efficiency and input power canceling out, we measured this with both PDs in each position.  With the orginial tuning of the Faraday, with PD1 before and after the Faraday, we measured 96.9% transmission, with PD2 before and after the Faraday we measured 96.5% transmission.  This afternoon with PD1 before and after the Faraday, we measured 96.4% transmission, with PD2 before and after the Faraday we measured 96.5% transmission

The ratio of the measurement with the PD after the Faraday to the return beam from the Faraday with the high reflector in place (D/A)/(C/A) is the transmission of the Faraday in reverse multiplied by the beamsplitter reflectivity.  For the initail set up we measured 22.5 dB of isolation (with both PD placements), this afternoon after "tuning" I measured -20.5 moving PD1 in transmission of the Faraday, and -19dB with PD2 in transmission of the Faraday. 

With the razor blade beam dump instead of the retro-reflector after the Faraday, we can estimate backscatter from the Faraday back towards the OPO.  If this scatter is large, it can reduce the measured isolation.  For this measurement I am using the power returning over the power incident on the PD, (D/A)/(B/A), and dividing our the beam splitter reflectivity, which gave -34 dB with both PDs for the original angle tuning.  After changing the angles, the backscatter is too small to measure, so backscatter isn't a large impact on our isolation measurements.  

Beam heights:

The thin film polarizers are 1/4 inch thick fused silica used at brewster's angle (56 deg), so they each deflect the beam down by about 2.8 mm as they are mounted.  This means that we have almost a quarter inch difference in beam height across the Faraday isolator if we start with a level beam.  Since the rotator aperture is only 5mm and it is mounted at 4 inches off the table, the beam needs to be a little high going into the first TFP and a little low coming out (top hand drawing in second attachment).  Since the VOPO beam I think will be at 4 inches, we will have to steer it up then down again, which is possible.  The bottom shows a suggestion for keeping the beam height into and out of the Faraday at 4 inches.