Fiber optic sensor systems

A method of sensing a parameter comprising:

• providing a fiber optic measuring sensor having a measuring optical output;
• providing a birefringent fiber optic reference sensor having a reference optical output;
• and
• comparing said measuring optical output with said reference optical output,
• wherein said measuring and reference sensors have substantially the same nominal operating wavelength.

A method of measuring one or more parameters using a fiber optic sensor system, said sensor system comprising:

• at least one measuring sensor providing an optical output dependent upon said one or more parameters;
• at least one reference sensor providing a reference output, wherein the reference sensor is provided in a birefringent fiber such that said reference output has frequency components in different polarisation planes;
• and
• detecting instrumentation;
• said method comprising:
• deriving a reference beat signal by measuring an optical frequency splitting between said frequency components in different polarisation planes of the reference sensor output;
• generating a further beat signal between said measuring and reference sensor outputs;
• and
• using said reference beat signal and said further beat signal to derive a measurement of said one or more parameters.

A method of measuring one or more parameters using a fiber optic sensor system, said sensor system comprising:

• at least one measuring sensor providing an optical output dependent upon said one or more parameters;
• at least one reference sensor providing a reference sensor output wherein the reference sensor is provided in a birefringent fiber
• said method comprising:
• deriving a first beat frequency from the reference sensor output;
• deriving a second beat frequency from a comparison between the measuring sensor output and the reference sensor output;
• and
• using said first and second beat frequencies to derive an indication of said at least one parameter without the need directly to measure the absolute frequency of either sensor by optical means.

A method of measuring at least one parameter using a fiber laser sensor system having:

• at least two fiber lasers written into respective optical fibers, at least one of said fibers being birefringent and having a birefringence;
• and
• detecting instrumentation;
• said method comprising using said detecting instrumentation to generate beat signals dependent upon the birefringence of said at least one birefringent fiber.

A method of spectral analysis comprising:

• providing a tunable light source;
• passing a first part of light from said tunable light source to an optical device providing a device spectrum to be measured;
• passing a second part of the light from the tunable light source to an interferometer, thereby generating a comb spectrum;
• using said comb spectrum to provide a linearised frequency scale for measurement of the device spectrum, whereby the effect of noise in the tunable light source is reduced;
• and
• passing a third part of the light from the tunable light source to a birefringent reference grating, thereby providing an absolute wavelength reference.

A method of sensing using:

• at least two passive π-phase shifted FBG sensors written into respective fibers, at least one of which is a birefringent fiber;
• and detecting instrumentation;
• the method comprising:
• measuring a frequency splitting between said two resonances of the birefringent fiber;
• and
• said detecting instrumentation using said splitting to derive a reference signal.

A method of deriving an output signal indicative of at least one parameter comprising:

• providing at least two fiber optic sensors providing optical outputs with substantially the same nominal operating wavelength, at least one of said outputs having birefringent components;
• receiving and analysing said optical outputs;
• and
• comparing said optical outputs to derive said output signal indicative of the at least one parameter.

A fiber optic sensor system, comprising:

• at least one measuring sensor providing a measuring output dependent upon one or more parameters to be measured;
• at least one reference sensor providing a reference output for comparison with the measuring output, said reference sensor being provided in a birefringent fiber, such that said reference output has a plurality of different polarisation planes and frequency components;
• detecting instrumentation, said detecting instrumentation being adapted to derive a reference frequency difference signal by measuring an optical frequency splitting between said frequency components, and said detecting instrumentation being adapted also to generate a further frequency difference signal between the measuring and reference sensor outputs;
• wherein said system is arranged to derive a measurement of one or more parameters using said reference frequency difference signal and said further frequency difference signal.