Optical frequency-division multiplexer and demultiplexer

A device, comprising:

• a coarse WDM optical filter with a first resolvable minimum channel spacing, having a plurality of input terminals and a single output terminal;
• a plurality of fine WDM optical filters with a second resolvable minimum channel spacing less than said first resolvable minimum channel spacing, each fine WDM optical filter having a plurality of input terminals to receive WDM channels at said second resolvable minimum channel spacing and an output terminal to output a multiplexed signal with received WDM channels and connected to a respective input terminal of said coarse WDM optical filter, wherein different fine WDM optical filters configured to receive WDM channels at different frequencies,
• each fine WDM optical filter comprising a plurality of cascaded optical units, each optical unit having:
• a polarizing beam splitter (PBS) to reflect light in a first polarization in an input light beam into a first optical beam and to transmit light in a second, orthogonal polarization as a second optical beam;
• a first filter positioned to receive said first optical beam from said PBS and operable to transmit light at a first wavelength and reflect light at other wavelengths;
• a first polarization rotator positioned in an optical path between said PBS and said first filter to produce a polarization rotation of about 90 degrees on each beam that passes through said first polarization rotator twice;
• a second filter positioned to receive light in said second polarization that is reflected from said first filter and transmits through said PBS, said second filter operable to transmit light at a second wavelength different from said first wavelength and reflect light at other wavelengths;
• and
• a second polarization rotator positioned in an optical path between said PBS and said second filter to produce a polarization rotation of about 90 degrees on each beam that passes through said first polarization rotator twice.

A method, comprising:

• directing an input light beam, of a first polarization, along an input signal direction;
• using a polarizing beam splitter to reflect the input light beam to a first signal direction perpendicular to the input signal direction;
• filtering the input light to transmit light of a first wavelength along the first signal direction and to reflect light of other wavelengths along a second signal direction opposing the first signal direction;
• rotating the polarization of the reflected light by about 90 degrees to a second polarization to transmit through the polarizing beam splitter;
• filtering the reflected light to transmit light of a second, different wavelength along the second signal direction and to reflect light of other wavelengths along the first signal direction;
• rotating the polarization of the reflected light along the first signal direction to be in the first polarization;
• and
• reflecting the reflected light in the first polarization by using the polarizing beam splitter back to the input signal direction.

An optical device, comprising:

• an input terminal having a polarizing beam splitter which splits an input light beam into a first beam in a first polarization and a second beam in a second, orthogonal polarization, a reflector to direct said second beam to propagate in parallel to said first beam in a path shifted from said first beam, and a polarization rotator to rotate the polarization of said second beam into said first polarization;
• an optical filtering stage to receive said first and said second beams from said input terminal that are parallel and are in said first polarization and operable to change a spectral composition of said first and said second beams to produce modified first and second beams;
• and
• an output terminal, similarly constructed as said input terminal, positioned to combine said modified first and second beams to form a modified output beam.