Sheila, Elli

Summary:

Yesterday morning durng maintenance we went out quickly to measure the beam parameter ar AS-Air.   IThe beam waist is 3.7e-4 (1e-4) m at the AS_Air camera.  The fit of the beam divergence is pretty bad, we were hurrying to get done quickly and I didn't take enough data points to get a good fit for the beam divergence.  Using measured waist size and the (very poor) fit of beam divergence, the beam parameter at the AS_AIR camera is calculated to be 0.14+0.22i in the vertical axis and 0.16+0.29i in the horizontal axis.  UPDATE:  Today during the Earthquake at lunchtime I went out and took some more measurement points.  I am fitting these new points at the moment, and hopefully they will improve this measurement.

Details:

We used a straight shot through corner station with PRM, ITMX, SRM misaligned.  We placed a flipper mirror 0.434m upstream of the AS_Air camera, which sends the AS-Air beam across ISCT6 where we used a Thorlabs BP-104 slit scanning beam profiler to measure the beam profile at 5 locations (see optical layout diagram).  This beam scanner is mounted on its side, so that Y is the horizontal axis and X is the vertical axis.  The laser power was turned up to 24W to get enough light on Thorlabs beamscan to take a measurment (and the backgorund noise was still high at 24W).

The background noise was to high for the beam profiler to automatically fit beam waist, so we saved the beam shape in the x and y axes and fitted a waist size to these beam profiles after the measurement was over.  Puzzlingly, the maximum intensaity in the X and the Y directions is not the same.The fit of the gaussian beam in x and y is good.  Then plotted beam radius vsd distance to fit the waist location and size with a hyperbolic function.  There aren't enough points/too much error in each point to get a satisfying fit.  I have attached a fit which looks kind of reasonable, but really there is too much variation in the points to call this a good fit .(See picture of alternative fit which is almost as good, and very different.)  So the Beam waist location/size can't be pinned down using this meaurement.  This measurement could be inproved by taking more data points.