Diffraction grating with reduced polarization sensitivity

#The invention relates to the field of optical communications and the routing of different wavelength signals. Improvements to reflective diffraction gratings, which are used in communication systems for dispersing light by wavelength, are featured.

#Optical communication systems include devices, referred to as multiplexers, that route different wavelength signals from individual pathways into a common pathway and other devices, referred to as demultiplexers, that route the different wavelength signals from the common pathway back into the individual pathways. Often, the only difference between these devices is the direction of light travel through them.

#The multiplexer/demultiplexer designs gaining widest acceptance are based on planar spectrographs containing phased arrays or reflective diffraction gratings. Within these devices, two optical mechanisms are used for routing the optical signals between the common and individual pathways--dispersion and focusing. Dispersion angularly distinguishes the different wavelength signals, and focusing converts the angularly distinguished signals into spatially distinguished signals.

#FIG. 1 is a diagram of a multiplexer/demultiplexer based on a Rowland circle design.

#FIG. 2 is a diagram of an enlarged portion of my reflective diffraction grating relating various parameters of the design.

#FIG. 3 is a graph of diffraction efficiency varying as a function of wavelength over a transmission bandwidth for two directions of polarization.

#A multiplexer/demultiplexer 10 shown in FIG. 1, is implemented as an integrated planar optic interconnecting a common waveguide (pathway) 12 for conveying a plurality of different wavelength signals ".lambda..sub.l -.lambda..sub.n " with an array of individual waveguides pathways) 14a-14b for separately conveying the different wavelength signals ".lambda..sub.l " and ".lambda..sub.n ". For simplicity of illustration, only two of the individual waveguides 14a and 14b are shown, but many more are ordinarily used. A reflective diffraction grating 16 couples the common and individual waveguides 12 and 14a-b through a central slab waveguide (central pathway) 18.

#While it is possible to use a flat diffraction grating for wavelength dispersion and to use a separate optical element, such as a lens or mirror for focusing, the reflective diffraction grating 16 is curved for performing both the dispersing and the focusing functions. A stepwise succession of facets 20 having centers 22 are positioned along an arc 24 defined by a radius "R" and a center of curvature "C". The facets 20 are oriented at blaze angles ".theta..sub.b " (see also FIG. 2) so that lines 26 extending perpendicular to the facets 20 from their respective centers 22 converge toward a single blaze point "B". Both the blaze point "B" and the center of curvature "C" are located along a so-called Rowland circle 28 that is typically tangent to the reflective grating 16 with a diameter equal to the grating's radius of curvature "R". A focal point "F", which has infinity as its conjugate, coincides with the center of the Rowland circle 28.

#Also located along the Rowland circle 28 are inner end 32 of the common waveguide 12 and inner ends 34a and 34b of the individual waveguides 14a and 14b. The Rowland circle 28 defines a focal line along which image and object conjugates of the inner ends 32, 34a, and 34b are located. In a demultiplexing mode, each of the different wavelength signals ".lambda..sub.l -.lambda..sub.n " launched by the common waveguide 12 is projected as an image of its inner end 32 onto a different one of the individual waveguide inner ends 34a and 34b. In a multiplexing mode, images of the inner ends 34a-b of the individual waveguides 14 are collectively projected onto the inner end 32 of the common waveguide 12.