Optical EngineeringPower, efficiency, and polarization analysis of a beamsplitter for the equipartition of infrared input power in the presence of angular, film-thickness, and spectral deviations
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A high-efficiency beamsplitter for the equipartition of infrared input power using combined reflection and transmission is described. The new design does not use a back metal reflector coating, and hence is more efficient than those previously described. The beamsplitter uses a parallel slab of fused silica that is strip coated with a germanium film on the front and back sides to generate four beams of equal powers. A specific design for operation at the 1.55-μm fiber communication wavelength is presented. Power, efficiency, and polarization analysis of fractional beams in the presence of angular, film-thickness, and spectral deviations near the equipartition condition are discussed. The following results are obtained. An angular deviation of ±0.5 deg has no significant effect on power (<0.1%), efficiency (<0.02%), or polarization (ellipsometric) parameters (< 1 deg) of the fractional beams. A film-thickness deviation of ±10 nm results in a small change in power (<2%), efficiency (<0.1%), and polarization (<0.5 deg) of the fractional beams. A spectral variation of ±25 nm also has a small effect on power (<1.5%), efficiency (<0.01%), and polarization (<1 deg).