Wavelength division multiplexer and demultiplexer

DETAILED DESCRIPTION This invention is based on the observation that many filters have the characteristic property that the filter passes incident light of wavelengths within a characteristic wavelength band and reflect incident light of wavelengths outside the characteristic wavelength band, where the characteristic wavelength band varies with the angle of incidence of the incident light to the normal direction to the filter.
The variation of the characteristic wavelength band with the angle of incidence is utilized for multiplexing and demultiplexing light of a plurality of wavelengths.
For each filter, a normal direction can be defined.
In most instances, the filter comprises a multilayer coating with a substantially flat surface, so that a direction along the line normal to the flat surface of the filter pointing towards the filter can be defined as the normal direction of incidence of the filter, or simply the normal direction to the filter.
One aspect of the invention is directed towards a method for demultiplexing light of a plurality of wavelengths in an input beam by means of a filter having the characteristic property described above, said plurality of wavelengths including a first, second and a third wavelength.
The method comprises (a) directing and collimating said input beam towards a filter with said characteristic property at a first incident angle so that light of one of the at least three wavelengths is reflected and light of another of the at least three wavelengths is passed by the filter; and (b) directing and collimating said light substantially reflected or passed by the filter towards the filter at a second incident angle different from the first incident angle, the first and second incident angles being such that the selective passage and reflection of light of different wavelengths by the filter separates the input beam into three output beams, each output beam containing light of one of the at least three wavelengths.
Another aspect of the invention is directed towards an apparatus for demultiplexing light of a plurality of wavelengths in an input beam containing light of a plurality of wavelengths including at least three wavelengths