TonyS and RickS

Today, we measured the transmissivity of one of the SPI HR mirrors and the AR surface reflectivity of one of the SPI beamsplitters, as requested by Jeff Kissel.  The reflectivities of the beamsplitters were measured earlier, see this aLog entry.

HR mirror reflectivity:  We used a set of Gentec power meters procured for the Pcal effort.  An SPI HR mirror (see attached photo for details).  Aligned at 45 deg. AOI with p-polarized light, the power transmissivity (P_trans/P_inc) was measured to be 23 e-6 W/ 567 e-3 W or 4.1e-5 (0.004%). 

BS AR surface reflectivity: The reflectivity of the AR surface of a 50/50 beamsplitter was measured in two ways, both after mounting the BS with the AR surface closest to the input beam.

1. Using the PS5 Pcal power sensor and blocking the beam reflected from the 50 % reflectivity surface near the beamsplitter, the output voltage (after zeroing the no-laser-light level) was - 0.387 mV.  With both the main reflected beam (~ 50 % of the incident light) and the beam reflected from the AR surface, the voltage was - 1.3699 V.  Doubling the voltage for what would have been expected with the full incident power, the estimated AR reflectivity is  0.387 e-3 / (2 x 1.3699) = 1.4 e-4 or  0.014 %.

2. Using power meters to measure the power in the beam reflected from the AR surface and the power incident on the BS, we measure 52.6 e-6 W / 564 mW = 9 e-5 or about 0.01 %.