A frequency shift monitoring system for monitoring a frequency shift of a beam of radiation, the system comprising: a beamsplitter for splitting the beam of radiation which is incident upon the beamsplitter, into a first and second output beam (20a, 20b) of radiation which propagate from the beamsplitter, the beamsplitter comprising: - a first beamsplitting portion for generating the first output beam and a second beamsplitting portion for generating the second output beam; - the first portion comprising a front and rear planar surface (111, 112) orientated in a substantially parallel orientation; - the second portion comprising a front and rear planar surface orientated (121, 122) in a non-parallel orientation; - the rear surface of the first portion comprising a first coating for partially reflecting and partially transmitting radiation propagating onto the rear surface of the first potion from within the beamsplitter, - the front surface of the second portion comprising a second coating for reflecting radiation propagating onto the front surface of the second portion from within the beamsplitter; - such that the first output beam propagates in a first direction and the second output beam propagates in a second direction, the first and second directions being non-parallel. an etalon comprising first and second reflecting surfaces (211, 212) for reflecting the first and second output beams (20a, 20b); a tuning arrangement for varying an optical path length of the first and second output beams (20a, 20b) through the etalon between the first and second reflecting surfaces (211, 212); and, at least one detector for detecting an intensity of the first and second output beams (20a, 20b) which is transmitted through the etalon. a beamsplitter for splitting the beam of radiation which is incident upon the beamsplitter, into a first and second output beam (20a, 20b) of radiation which propagate from the beamsplitter, the beamsplitter comprising: - a first beamsplitting portion for generating the first output beam and a second beamsplitting portion for generating the second output beam; - the first portion comprising a front and rear planar surface (111, 112) orientated in a substantially parallel orientation; - the second portion comprising a front and rear planar surface orientated (121, 122) in a non-parallel orientation; - the rear surface of the first portion comprising a first coating for partially reflecting and partially transmitting radiation propagating onto the rear surface of the first potion from within the beamsplitter, - the front surface of the second portion comprising a second coating for reflecting radiation propagating onto the front surface of the second portion from within the beamsplitter; - such that the first output beam propagates in a first direction and the second output beam propagates in a second direction, the first and second directions being non-parallel. - a first beamsplitting portion for generating the first output beam and a second beamsplitting portion for generating the second output beam; - the first portion comprising a front and rear planar surface (111, 112) orientated in a substantially parallel orientation; - the second portion comprising a front and rear planar surface orientated (121, 122) in a non-parallel orientation; - the rear surface of the first portion comprising a first coating for partially reflecting and partially transmitting radiation propagating onto the rear surface of the first potion from within the beamsplitter, - the front surface of the second portion comprising a second coating for reflecting radiation propagating onto the front surface of the second portion from within the beamsplitter; - such that the first output beam propagates in a first direction and the second output beam propagates in a second direction, the first and second directions being non-parallel. an etalon comprising first and second reflecting surfaces (211, 212) for reflecting the first and second output beams (20a, 20b); a tuning arrangement for varying an optical path length of the first and second output beams (20a, 20b) through the etalon between the first and second reflecting surfaces (211, 212); and, at least one detector for detecting an intensity of the first and second output beams (20a, 20b) which is transmitted through the etalon.