Danny, Georgia
This morning we worked on the ETMY HWS table. All mirror labels are referenced from this diagram.
- We installed a new 50 um core fiber on the output of the SLED, in place of the old 200 um core fiber. There is still some clipping on the table but we believe the clipped beam is only the cladding modes of the fiber. The first two attached pictures show the beam with a bright center (core) and dimmer halo (cladding): the first is a picture of the beam ~50 cm after the SLED (before any mode matching optics), the second is at HWS-M4.
- We replaced the mirror mount of HWS-M2 with a picometer controlled mount (though have not installed the picometer controllers).
- We tweaked up the alignment on the table, getting the SLED beam and ALS beam reflected off the ETM co-aligned, and the alignment onto the Hartmann camera.
- The SLED beam reflected off the test mass is visible on the HWS (third attachment). It looks like there is some clipping in yaw.
This looks very similar to what we got with the 50 micron fiber in May: aLOG 42204. Where is the beam from the fiber launcher converging in this system?
I had expected the 50-micron fiber to improve things but it, counter-intuitively, made the beam much bigger. With nominally the same NA on the fibers, the extra modes in the 200 micron fiber should produce a larger beam size. This suggests that the mode-matching is not exactly what I think it is.
One possible test is to scan the range of the translation stage (in steps of 2mm) and get images of the beam profile on the HWS for each step with the fiber launcher iris fully open, partly open (say 6mm diameter) and almost completely closed (say 2mm diameter aperture).