General requirement

RP requires attenuation to 14 mJ, so 1.75% at 800 mJ input.

The beam has a ~10-13 mm diameter (will depend on final collimation) and is uncompressed, so at around 200 ps pulse duration.

Pre-2022 RP attenuator

Currently installed RP attenuator.

Consists of 4 mirrors, a wave plate and thin-film polariser

Requires p-pol as input. A wave plate before the RP attenuator (not shown) changes s- to p-pol for this.

Attenuates to 0.5*0.8*0.5*0.1 = 0.02 (2%)

Maybe a bit more attenuation due to the polariser, as this technically is not enough for a 800 mJ beam.

Fig 1: Currently installed RP-attenuator. Red arrows indicate the beam path.

4 mirrors and s-pol as input (one wave plate needs to be removed).

Mirrors are 35%R from MPO, AR coating on back and 2 degree wedge.

Attenuation should be down to 0.35^4 = 0.015 (1.5%) or 800 mJ * 0.35^4 = 12 mJ

We added a wedge and AR coating to remove potential ghosts.

We determined that the thick side of the wedge needs to alternate sides when looking from the back of the mirror mounts.

That is in order to avoid that wedges compensate each other.

Quote for mirrors (arrived first week Dec):