Thermal diagnostic of the Optical Window on board LISA Pathfinder

TL;DR

This paper characterizes the thermal response of the Optical Window in LISA Pathfinder, using laboratory tests and models to ensure precise thermal control for interferometric measurements in space.

Contribution

It introduces a transfer function model for the Optical Window's thermal response, aiding in noise reduction and thermal diagnostics for space-based interferometry.

Findings

Single pole ARMA transfer function effectively models OW thermal response

Derived relationship between transfer function and physical processes

System noise can be reduced using the identified transfer function

Abstract

Vacuum conditions inside the LTP Gravitational Reference Sensor must comply with rather demanding requirements. The Optical Window (OW) is an interface which seals the vacuum enclosure and, at the same time, lets the laser beam go through for interferometric Metrology with the test masses. The OW is a plane-parallel plate clamped in a Titanium flange, and is considerably sensitive to thermal and stress fluctuations. It is critical for the required precision measurements, hence its temperature will be carefully monitored in flight. This paper reports on the results of a series of OW characterisation laboratory runs, intended to study its response to selected thermal signals, as well as their fit to numerical models, and the meaning of the latter. We find that a single pole ARMA transfer function provides a consistent approximation to the OW response to thermal excitations, and derive a relationship with the physical processes taking place in the OW. We also show how system noise reduction can be accomplished by means of that transfer function.