An etalon has a first surface which is partially reflective. The reflectivity of the first surface varies with position. Thus, this reflectivity can in effect be adjusted by varying the point at which an optical beam is incident upon the etalon. At one point of incidence, the optical beam experiences one reflectivity. At a different point of incidence, the optical beam experiences a different reflectivity.

A system and method for measuring the group delay of an optical device under test (DUT) utilizes an optical frequency counter in conjunction with a test interferometer to compensate for the non-uniform frequency changes of an input optical signal used by the test interferometer to measure the group delay. The group delay of the optical DUT is measured using the zero-crossings of an AC coupled heterodyne beat signal produced by the test interferometer from the input optical signal. In the measurement of the group delay, phase changes in the heterodyne beat signal caused by the non-uniform frequency changes of the input optical signal are compensated by using the measured optical frequency of the input optical signal. The optical frequency is detected by the optical frequency counter. The detected optical frequency is indicative of the non-uniform frequency changes of the input optical signal.

A system for matching a optical filter characteristic of a first filter tunable in wavelength with an optical first signal comprises a modulator for modulating at least a part of the first signal with a modulation signal before being applied to the first filter. An analyzing unit derives a control signal for tuning the first filter by analyzing the modulated first signal after passing the first filter in conjunction with the modulation signal.

A dispersion compensation system includes one or more etalons optically coupled in series. The optical path length is used to achieves the desired dispersion compensation. In one example, at least some of the etalons are tunable in OPL and have a non-tunable front reflective interface. The dispersion compensation system can be tuned, for example to compensate for different amounts of dispersion, different combinations of dispersion offset and dispersion slope and/or manufacturing variations. In another example, the etalons all have a non-tunable front reflective interface. The optical path length is selected to compensate for chromatic dispersion that varies from one wavelength channel to the next.

A method and system for providing wavelength-locked multi-channel signals with respect to a wavelength reference. Wavelengths of a plurality of radiation sources are introduced to at least one etalon having a gas medium. The etalon produces a plurality of equally spaced spectral lines and is tuned by varying its optical index of refraction, by varying the characteristics of its gas medium. The optical index of refraction is then fixed, by fixing the characteristics of its gas medium, and thus locking the plurality of spectral lines with respect to the wavelength reference. In particular, the etalons may be tuned by varying the pressure or composition of their gas-tunable mediums until the spectral lines of the etalon align with the ITU grid.