Polarising beamsplitters and extinction ratio

A polarising beamsplitter separates a beam by polarisation state rather than by intensity. Inside the cube, a dielectric coating on the diagonal transmits p-polarised light and reflects s-polarised light.

Reading the extinction ratio

Extinction ratio is the intensity ratio of wanted to unwanted polarisation in a given port. The number quoted on the datasheet, commonly better than 1000:1, is nearly always the transmitted port. The reflected port is typically an order of magnitude worse, because the coating reflects some of the unwanted state as well.

The practical rule follows immediately: if you need a clean polarisation state, take it from the transmitted port and treat the reflected port as a dump or a monitor.

The variable attenuator

Put a half-wave plate in front of the cube. Rotating the plate rotates the incoming polarisation, which changes how the power divides between the two ports. You get a continuously variable, essentially lossless power split with no absorbing elements in the beam, which is why this pair appears in almost every laser laboratory.

Power handling

The cement layer is the limit. For nanosecond pulses at 1064 nm the damage threshold is a real constraint and worth checking against your pulse energy and beam diameter rather than your average power. Where the energy density is too high, a plate polariser removes the cement from the problem.