From: Chris Shelton Date: May 9, 2006 11:14:35 AM PDT To: Mitchell Troy , "'Hal Petrie'" Cc: Antonin Bouchez , Ed Kibblewhite , Viswa Velur , Jenny Roberts , "'John Angione'" Subject: BTO laser combining Reply-To: Jean.C.Shelton@jpl.nasa.gov Mitch et al., Antonin and I have come up with what we think is a good layout for red/yellow combining and polarization control. We think this will go a long way towards having the BTO stay locked on the red pilot beam with or without the yellow high power beam. It takes advantage of the high degree of linear polarization of the yellow and red lasers, and has the following features -- o Yellow (589nm) and red (660nm) are combined in a polarizing beam splitter with yellow in the 99.5% path. o Mirror angles are simplified and angles squared up so that S and P are not mixed, linear polarization is preserved, and laser table P maps to polar axis S. This is the desired polarization at zenith. A motorized half-wave plate rotates this polarization to match the hour angle of the telescope. o This linear polarization minimizes 589nm losses in the beam transfer to the launch telescope. A quarter-wave plate in the vertical leg of the LLT converts linear to circular for launch, which maximizes return from the sodium layer. o The polarization of the red beam maximizes the signal from quad cells Q1 and Q2, at the expense of the red signal at Q3 and red power to the sky. o It is desirable to bring the 589 nm and 660 nm beams both into the dynamic range of the quad cells at the same time. With this polarization scheme, this can be done with a sheet of plastic polarizer per quad cell, which can be rotated to attenuate yellow as desired without affecting red, and/or an amateur astronomy sodium light filter can be used. o The signal at Q2, which sees the leak-through of M3, has traditionally been quite weak with pure S-polarization. It can be brought up to a usable level by offsetting the linear polarization angle by 2 or 3 degrees. The red laser is P-pol and 660 nm, which together gave a usable signal in the last run. o The quarter-wave plate at coude is no longer needed and is left out. o The replacement red laser, 20 mW at 660 nm, will have polarization-preserving fiber. The fiber end will be in a rotating holder. o It will be possible to register the yellow beam onto the polar axis using only local adjustments, without putting a hand into the laser works. o The layout is compact enough that one can conceive of a cover for it. The red laser and its optics and the polarizing beam combiner will be mounted on a new horizontal small breadboard mounted inside the existing steel bracket. The existing angled table will hold the FSM and the half-wave plate. Antonin will be making drawings and working out details of the coude assembly. I will talk with Hal about how to get a quarter-wave plate into the LLT. Chris