BOSEM fieldbox


This is a specialised circuit to drive "BOSEMs" used in interferometer test mass suspensions. These contain an LED shining light onto a "flag" - a rectangular card that blocks incident light - attached to a suspended test mass used in laser interferometers. The light transmitted past the flag is sensed by a single element photodiode. A nearby voice coil is used to feed back to the flag position to keep it static in the presence of external disturbances, reducing the coupling of ground motion to the test mass.

The BOSEM LED needs a stable current so the light power is constant, otherwise fluctuations in power could be mistaken for fluctuations in flag position. The circuit therefore provides a stable current source, photodiode readout and coil driver together in one package. Extra features include various diagnostic readouts and error detection mechanisms.

The current driver is based on the Libbrecht-Hall design. The coil driver is adapted from an electron microscope driver from the Art of Electronics, 3rd Edition. The error checking circuits are my own design. The inputs and outputs interface with the control system we use in the lab.

Since BOSEMs are used only in LIGO and the AEI 10 m prototype, this circuit is likely not directly useful to anyone else; however, the current and coil drivers and error checking circuits may be useful references for others.

I'm working on an updated version of this circuit to fix a few minor issues.