Method and apparatus for measuring a frequency of an optical signal

A method of detecting a frequency of an optical signal, such method comprising the steps of:

• defining a carrier frequency grid whereby the frequency space between optical carriers on said grid is determined;
• providing a means by which an optical carrier frequency is maintained on a carrier grid frequency;
• defining a reference frequency grid whereby the frequency space between optical references on said reference grid is determined and different from that of the carrier frequency grid;
• providing a means by which an optical reference signal is maintained on a reference grid frequency within a predetermined bandwidth from that of the optical carrier frequency;
• mixing the optical carrier frequency with the optical reference signal frequency thus generating a difference signal as a beat frequency;
• and
• measuring a frequency of a difference signal.

An apparatus for detecting a frequency of an optical signal within a communications band having a plurality of optical carriers separated by a predetermined fundamental frequency, such apparatus comprising:

• providing a reference optical signal spectrally offset from an optical carrier of the plurality of optical carriers by a spectral quantity within a predetermined bandwidth;
• mixing the reference optical signal with the plurality of optical carriers;
• detecting the difference frequency between the reference optical signal and a closest optical carrier frequency as a heterodyne beat frequency;
• and
• measuring a frequency of a heterodyne beat signal.

An apparatus for detecting a frequency of an optical signal within a communications channel having a plurality of optical carriers separated by a predetermined frequency spacing, such apparatus comprising:

• a stabilized laser adapted to provide a reference optical signal spectrally offset from an optical carrier of the plurality of optical carriers by a spectral quantity within a predetermined bandwidth;
• a photodetector adapted to mix the reference optical signal with the plurality of optical carrier frequencies to create a heterodyne beat frequency with a closest optical carrier frequency;
• and
• an analyzer adapted to measure a frequency of a heterodyne beat signal.