Optical multiplexer/demultiplexer

Abstract

A monolithic version of optical multiplexer/demultiplexer with an improved spectral characteristics is provided by two diffraction gratings arranged optically in tandem and with a field stop in the coupling between them, the gratings also being arranged to provide free spectral ranges differing by a factor of at least two, and having a coupling between them that carries over into the second grating information concerning the dispersion afforded by the first grating. The field stop is constituted by a pair of etched troughs arranged in the pattern of an open chevron.

I claim:  
     
       1. An optical multiplexer/demultiplexer for the multiplexing/demultiplexing of optical signal channels at a substantially uniform optical frequency spacing, which multiplexer/demultiplexer includes, in an integrated waveguide optics structure, a set of input/output ports optically coupled with an output/input port via a tandem arrangement of first and second optical waveguide diffraction gratings that provide multiple optical waveguide paths from each member of the set of input-output ports to the output/input port via different grating elements of the gratings, 
       wherein the difference in optical path length occasioned by paths via adjacent optical waveguide elements of the first grating is greater than that occasioned by paths via adjacent optical waveguide elements of the second grating,  
       wherein said difference in optical path length defines for its associated grating a frequency range, the Free Spectral Range, being the frequency range over which said optical path length difference produces a phase difference whose value ranges over 2π,  
       wherein the Free Spectral Range of the first diffraction grating is matched with the optical frequency spacing of the optical signal channels,  
       wherein the Free Spectral Range of the second diffraction grating is at least as great as the sum formed by the addition of the difference in frequency between adjacent frequency channels of the multiplexer/demultiplexer the difference in frequency between the highest and lowest frequency channels of the multiplexer/demultiplexer,  
       wherein the portion of the optical coupling between the set of input/output ports and the output/input port that extends between the first and second diffraction gratings couples spatial information between the two gratings in addition to intensity information, and includes a field stop constituted by a pair of reflecting facets, inclined at an acute angle to each other, created by the provision of wells in the integrated waveguide optics structure, and defining between them an aperture through which all light coupled between the first and second diffraction gratings is coupled, and  
       wherein the length of the facets and the angle between them are related such that light emitted from either diffraction grating is not able, by multiple reflection in the facets, to couple into the other diffraction grating, or back into the same diffraction grating from which that light was emitted.  
     
     
       2. An optical multiplexer/demultiplexer as claimed in  claim 1 , wherein the dispersion of the first grating is substantially matched with that of the second. 
     
     
       3. A wavelength division multiplexed optical transmission system including at least one multiplexer as claimed in  claim 2 . 
     
     
       4. A wavelength division multiplexed optical transmission system including at least one demultiplexer as claimed in  claim 1 .