Inputs and outputs are generally coupled to ends of planar waveguides in alignment with a direction of light propagation through the waveguides. For example, optical fiber inputs and outputs are generally coupled by aligning a core layer of each fiber with a core layer of the planar waveguide. Cladding layers surrounding the fiber cores separate the cores along the ends of the waveguides. The number of fibers that can be coupled to an end of a waveguide is limited by the diameters of the fibers.
Despite the further miniaturization of features within planar waveguides, waveguide dimensions are often significantly increased to provide sufficient room for coupling the input and output fibers. Fiber diameters, which typically measure around 125 microns, can be reduced to only about 70 to 75 microns without significantly deteriorating in structure or function. Similar problems are apparent with other types of inputs and outputs such as laser light sources and photodetectors--each of which is generally larger in transverse dimension (e.g., diameter) than optical fibers.
This problem is particularly apparent in planar waveguide multiplexer/demultiplexer devices that route optical signals between individual and common inputs or outputs (e.g., optical fibers). Within many of these planar waveguides, a dispersing mechanism, such as a diffraction grating, angularly distinguishes different wavelength signals and a focusing mechanism, such as a lens, converts the angularly distinguished signals into spatially distinguished signals. An array of inputs or outputs is aligned with the spatially distinguished signals along an end of the planar waveguide.
Even a tight grouping of the inputs or outputs requires a much larger planar waveguide than would otherwise be required to accomplish its function. Either the dispersing and focusing mechanisms must be increased in size to spatially separate the signals to match the spacing between the inputs or outputs or an intermediate coupling must be added to otherwise expand the signal separation to match the input or output spacing. The intermediate coupling adds length to the planar waveguide and reduces coupling efficiencies.
Planar waveguides have also been coupled to each other and to external devices, such as lasers and photodetectors, using out-of-plane mirrors that reflect light normal to the direction of light propagation through the planar waveguides. For example, U.S. Pat. No. 4,750,799 to Kawachi et al. mounts a micro-reflecting mirror between guides of a planar waveguide for folding light through a right angle to an external device mounted on top of the planar waveguide. The micro-reflecting mirror is separately manufactured from coated glass or plastic and is mounted between the guides so that the mirror's reflective surface is oriented at 45 degrees to the direction of light propagation.
U.S. Pat. No. 5,182,787 to Blonder et al. and U.S. Pat. No. 5,263,111 to Nurse et al. teach the fabrication of similar out-of-plane mirrors as integral structures of planar waveguides. Both involve etching cavities in planar waveguides and coating an inclined side wall of the cavity with a reflective material. Blonder et al. etch the opposite side wall nearly perpendicular to minimize refraction of light emitted into the cavity. Nurse et al. coat portions of a cavity floor and shelf in addition to an inclined side wall for improving uniformity of the reflective surface.
However, none of the proposed arrangements for coupling external devices or other planar waveguides to top or bottom surfaces of planar waveguides entail any suggestions for reducing spacing requirements of multiple inputs or outputs.