In 57169  I described coupling the Prometheus 1064 and 532 beams to fibers to be used in the OPO + VIP rebuild in the optics lab, using Thorlabs fibers.  Peter sent us an in air Diamond fiber (E2000536) from MIT, which will be the type of fiber used for 532 in O4.  These fibers have a special cap for coupling to free space which should be less easily damaged by high power.

I've measured the mode out of the diamond fiber and a Thorlabs PAF-X-5-A.  I attempted to pre align and colimate the fiber port using the instructions on the thorlabs website, however, with the lens as far from the fiber as possible, the beam  is still diverging.  It seems that if we want to get a well collimated beam with this fiber we would need to use a shorter effective focal length port or collimator. This results in a horizontal waist of 256um 0.40m behind the fiber port, and a vertical waist of 218um 0.27m behind the fiber port. 

The prometheus is matched to this using a PLCX-25.4-206.0-UV lens placed 0.53 meters from the prometheus and the fiber port 1.73meters from the Prometheus.  I normally measure the coupling efficiency into a fiber using a power meter to check the input power and holding the end of the fiber up to the power meter, but I found that with the APC end of this fiber the reading I got from the power meter was inconsistent and I could sometimes get much higher powers by adjusting the angle and distance from the power meter.  Instead I used a thorlabs PDA36A, responsivity at 532 ~0.2A/W on the 0dB setting with 1.51kOhm transimpedance.  I saw 3.03V with 10.6mW in, which would mean nearly 95% coupling efficency.

Beam width measurements are in the attached a la mode script.