Yes, I checked this voltage. With current power, it is 2.8V. This is already quite close to 4V, so we don't have much space for improvement.

Anyway, I tried to remove one ND filter to have 4V, but the spectrum is quite similar with the 2.8V one.

PR and BS can accpet more power, but most of the light was lost since the suspended mirror surface is not HR for 635nm. So we couldn't have more power.