A fiber optic sensor system comprises at least one measuring sensor 1 providing an optical output dependent upon one or more parameters to be measured, e.g. temperature, and at least one reference sensor 2 providing a reference output for comparison with the measuring sensor output. The reference sensor is provided in a birefringent fiber. The system includes a detecting means 13,14 whereby a reference beat signal f.sub.2 is derived by measuring the optical frequency splitting between frequency ...

An apparatus for controlling the temperature of a part of a human or animal body 2 during a cryosurgical procedure is disclosed. The apparatus comprises a radiative energy transmitter 26 for radiating energy to a zone of the body 2 to be protected from excessive cooling. There is also provided a means 12 for controlling the energy radiated by said radiative energy transmitter 26. Preferably the radiative energy transmitter comprises an infra-red laser diode coupled to a diffuser 34 by an optical...