The ultra-stable optical bench (designed and built by University Glasgow) is at the core of the optical metrology system on the LTP. Two laser beams with slightly differing frequencies are injected onto the bench through custom made fibre injectors. The beams are split multiple times and follow various paths before being recombined at four interference points. The four main outputs come from:

	The Reference Interferometer, formed by optical paths that are fully contained on the optical bench and providing a stable reference measurement for the main readouts.
	The Frequency Interferometer, again with optical paths fully contained on the bench, but with unequal arm lengths (arising from different input fibre lengths) to give a measure of the frequency stability of the laser; this is then used to monitor or stabilise the laser frequency.
	The X1 Interferometer which has an optical path that includes one of the free-falling test-masses, thus making a precise measurement of the motion of the bench (and hence the spacecraft) relative to the test-mass.
	The X12 Inteferometer which has an optical path that includes both free-falling test-masses, therefore making a precise measurement of the differential motion of the two free-falling test-masses.

The optical inteference of each of the four interferometers is read-out by quadrant photodiodes whose signals are then measured by the phasemeter and further processed in the payload computer (the DMU) to produce longitudinal and attitude measurements of two free-falling test masses.