This invention relates to optical waveguide transmission devices and more particularly to optical waveguide devices suitable, for example, for employment in optical multiplexer and demultiplexer, and router applications employing Wavelength Division Multiplexing (WDM) which may find application in optical fiber based communication networks.
Increasingly, state-of-the-art optical transmission devices for wavelength division multiplexing utilize waveguide gratings. In a typical device structure a waveguide grating may be placed between input and output slab waveguides. Multi-wavelength light from a point source, the end of the input waveguide, is transmitted through the grating and different individual wavelengths are focused on different points of the output waveguide surface. The grating itself may include a number of different physical length waveguides with a constant length increment. This results in a wavefront rotating with the wavelength change. The grating can be also used to focus an output waveform and to collimate an input waveform. For this purpose the ends of the grating may be located on a circular surface provided by each slab waveguide, at which the waveguides are located close to each other to assure strong coupling needed for high efficiency. However, in the grating itself, coupling between individual waveguides is not desirable, so that the waveguides are spaced relatively far apart from each other. This implies a relatively large grating structure. In an optical multiplexer, multi-wavelength light from a point source, the end of the input waveguide, is transmitted through the grating and different individual wavelengths are focused on different points of the output waveguide surface.
Close coupling between waveguides, at the interface between the slab waveguide and the grating, has typically been considered essential to the efficiency of the device. To alleviate concerns about resulting aberrations, it has been proposed to position the foci of interface arcs between the input waveguide and the slab region, and the slab region and the waveguide grating, some distances from the arcs themselves. This approach of positioning the focal points of the arc boundary (center of the interface arc) away from the opposite arc, originating from the desire to maintain strong coupling, has continued. The emphasis on strong coupling was consistent with grating based multiplexer designs for handling a small number of channels (for example, 1xc3x974 to 1xc3x978) and high efficiency, i.e. low transmission loss.
Another desideratum in multiplexer design is a compact device, and in particular a compact waveguide grating section. This is driven by cost reduction associated with reduced size, as well as lower sensitivity to material non-uniformities. One approach addressing the compactness issue has been to introduce a reflector in the grating section. Possible reflecting arrangements include waveguide Bragg reflectors and mirrors, including mirrors with stepped surfaces. However, such approaches have only partially addressed the overall issue of compactness and transmission efficiency.
The present invention addresses these concerns by providing an optical waveguide device comprising a free space region having a plurality of optical signal ports for coupling to input and output waveguide sections and an optical waveguide grating including an array of laterally spaced grating waveguides coupling the free space region to a reflector surface to provide a folded structure. The optical waveguide grating includes tapered optical waveguide sections laterally spaced apart from each other and extending from the free space region, each of the tapered optical waveguide sections having a wider end adjacent to the free space region and an opposite, narrower end extending to a respective one of the array of optical grating waveguides extending between the narrower ends of the tapered optical waveguide sections and the reflector surface. Each of the grating waveguides differs in length from a neighboring grating waveguide by a constant increment, preferably an optical path length increment. The grating waveguides also include intermediate curved portions having respective curvatures which increase progressively according to the sequential location of the grating waveguides from a reference grating waveguide in said array.
An advantageous feature of the invention is that the tapered waveguide sections and the grating waveguides are laterally spaced apart sufficiently to provide optically isolated transmission paths for light waves between said free space region and said reflector surface. The tapered waveguide sections can be suitably configured to enhance collection of light waves transmitted across the free space region and to separate the collected light waves into optically isolated paths, even at the interface between the wider ends of the tapered waveguide sections and the free space region.
The free space region preferably has arcuate first and second end surfaces. The optical signal extend radially from the arcuate first end surface and the tapered waveguide sections extend radially from the arcuate second end surface. Advantageously, the arcuate first end surface has a radius of curvature originating on the arcuate second end surfaces, and said arcuate second end surface has a radius of curvature originating on the first arcuate end surface.
In the waveguide grating section, the curved portions of the grating waveguides preferably have radii of curvature that decrease in an approximately parabolic manner as the numerical sequential location of a said curved portion increases from an intermediate grading waveguide location, suitably at or close to the central region of the grating waveguide array, to respective locations at opposite sides of the grating waveguide array. The rate of sequential decrease in the radii of curvature of said curved portions may be modified to result in preferential attenuation of optical signal reflection at said curved portions located near the edges of the grating waveguide array to implement apodization.
By implementation of features of the invention, an optical waveguide device can be designed which is particularly compact. For example the overall length of the optical waveguide grating readily can be designed to be less than the overall length of the free space region, both measured along the general direction of transmission of light wave signals.
Structures embodying the invention may be used to implement a WDM device capable of handling a larger number of channels (e.g. 40 channels with a channel separation as close as 50-100 GHz) while giving rise to low cross talk between adjacent channels (e.g. less than 50 db) and significantly reduced variation of transmission efficiency across the response spectrum of the device. An important contribution to the improved functionality device is the decoupling of the waveguide sections along the length of the waveguide grating, and in particular at the ends of the waveguide sections at the waveguide slab end. The particular mode of deployment of the tapered waveguide sections described above significantly contributes to this desired separation without adversely affecting overall transmission efficiency.
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