An interferometry assembly for use in an optical locker, the assembly comprising an input assembly, an interferometer, and a detector assembly, wherein: the input assembly is configured to receive a test beam, to split the test beam into a plurality of physically non-coincident input beams (903, 904), and to direct the input beams to the interferometer; the interferometer is configured to receive each input beam and to produce, for each input beam, an output beam with an intensity that depends on the wavelength of the input beam; the detector assembly is configured to produce a plurality of output signals, each output signal being dependent on the intensity of a respective output beam; the input assembly is configured to direct the input beams such that each input beam enters the interferometer at a different angle and thereby travels through the interferometer with a differing path difference, and such that the output beams arrive at the detector assembly physically separated; characterized in that: the interferometer includes a first part made of a first solid material and a second part made of a second solid material that is different than the first solid material, the interferometer includes a beam splitter that splits a respective input beam, of the input beams, into a first intermediate beam that has a path through the first solid material and a second intermediate beam that has a path through the first solid material and the second solid material, and the interferometer is configured to recombine the first intermediate beam and the second intermediate beam to produce a respective output beam. the input assembly is configured to receive a test beam, to split the test beam into a plurality of physically non-coincident input beams (903, 904), and to direct the input beams to the interferometer; the interferometer is configured to receive each input beam and to produce, for each input beam, an output beam with an intensity that depends on the wavelength of the input beam; the detector assembly is configured to produce a plurality of output signals, each output signal being dependent on the intensity of a respective output beam; the input assembly is configured to direct the input beams such that each input beam enters the interferometer at a different angle and thereby travels through the interferometer with a differing path difference, and such that the output beams arrive at the detector assembly physically separated; characterized in that: the interferometer includes a first part made of a first solid material and a second part made of a second solid material that is different than the first solid material, the interferometer includes a beam splitter that splits a respective input beam, of the input beams, into a first intermediate beam that has a path through the first solid material and a second intermediate beam that has a path through the first solid material and the second solid material, and the interferometer is configured to recombine the first intermediate beam and the second intermediate beam to produce a respective output beam. the interferometer includes a first part made of a first solid material and a second part made of a second solid material that is different than the first solid material, the interferometer includes a beam splitter that splits a respective input beam, of the input beams, into a first intermediate beam that has a path through the first solid material and a second intermediate beam that has a path through the first solid material and the second solid material, and the interferometer is configured to recombine the first intermediate beam and the second intermediate beam to produce a respective output beam.