Over the past few days we've relocked the reference cavity in the optics lab, coupled the transmitted light into a fiber optic cable, and shot it down to End-Y.  This will be used to stabilize the ALS laser using a phase-locked loop, at least until the new H1 PSL has a serviceable FSS.

For the reference cavity, the setup is the same as that put in place by Matt, Bram, and Szymon last fall.  After a few days of loop tuning and optics tweaking (label your optics!  and for crying out loud, don't label something 'HWP' on one side and 'QWP' on the other!), we settled on a UGF of 120kHz with about 40 deg of phase margin.  The loop is pretty stable, but experiences an intermittent high-frequency oscillation that causes an audible buzzing in the EOM.  We're not sure what this is coming from, maybe insufficient gain in the fast path (relative to the fast-fast EOM path).  The visibility through the cavity is not great, only about 20% at the moment.

We take 95% of the transmitted light from the cavity and couple that into a fiber optic.  This coupling isn't that great either, only about 30%; the mode matching could use some work.  From there, the fiber runs to the mass storage room, and connects to a spigot of the communications cable that goes down the arm.  Four kilometers later it emerges in the VEA field rack, about ten meters from the ALS table.  We're using the first connector, in the upper left corner of the fiber patch panel.  The transmission from the optics lab down the arm is quite respectable, only about 5.7dB is lost.  Better than 1.5dB/km!

Here are some numbers:

41.6mW incident on the reference cavity
7.0 mW transmitted through the cavity
2.2 mW coupled into the fiber
0.6mW at End-Y

One potentially worrisome thing is that the fiber coupler appears to scatter light back into the reference cavity, which causes fringe wrapping or some other junk on the RFPD.  There's a bunch of noise in REFL_DC when the beam is aligned into the fiber coupler, and it goes away when you block the coupler with your hand.  We may have to install a Faraday isolator on the transmitted beam to keep this from polluting the PLL.

Another thing: does the reference cavity have some seismic isolation inside the vacuum can?  Like, is it sitting on springs?  When we hip-check the optics table there's a vertical mode at about 4Hz that gets rung up.

Currently for laser-safety reasons the fiber optic is disconnected in the optics lab, we'll plug it in once the cabling at the endstation is worked out.