Surface‐normal 3×3 non‐blocking wavelength‐selective crossbar using polymer‐based volume holograms
Applied Physics Letters
We present a 3ϫ3 surface-normal wavelength-selective crossbar using polymer-based volume holograms. A prototype device is demonstrated using the center wavelength of 775 nm and ⌬ϭ10 nm. Employment of 1 4 -pitch graded-index rod lenses reduces the required nine wavelengths to three while maintaining the 3ϫ3 interconnects. The diffraction efficiencies of 75%, 83%, and 75% are experimentally confirmed for wavelength 765, 775, and 785 nm, respectively. Surface-normal configuration eliminates the
... n eliminates the conventional edge-coupling scheme which is vulnerable in a harsh environment. A 3ϫ3 crossbar is demonstrated with a two-way system insertion loss less than 3 dB and channel-to-channel cross talk less than 20 dB. © 1996 American Institute of Physics. ͓S0003-6951͑96͒04552-4͔ Crossbar-based optical interconnects represent the most desirable network due to their fast switching speed and low latency in transmitting high speed signals. In this letter, we report the formation of a surface-normal nonblocking crossbar based on a unique wavelength switching scheme in which photopolymer-based volume holograms are employed in conjugation with graded index ͑GRIN͒ rod lenses. A prototype polymer-based volume hologram for multiplewavelength 3ϫ3 crossbar is experimentally demonstrated at 765, 775, and 785 nm. The unique beam routing property of a GRIN lens reduces nine wavelengths to three wavelengths while maintaining the required nine ͑3ϫ3͒ individual interconnects. The elimination of edge-coupling significantly enhances the packaging reliability. Furthermore, such a configuration is compatible with the implementation of vertical cavity surface-emitting lasers where the characteristic of azimuthal symmetry may be maintained in the waveguide substrate. 1 The demonstrated device is shown in Fig. 1 . The volume phase gratings recorded in the photopolymer films are slanted. The central wavelength of the input surface-normal beam, i.e., 775 nm, is designed to be diffracted with a maximum efficiency at the Bragg angle 2 which is 45°in our design. The wavelengths that deviate from the center wavelength are dispersed at different substrate bouncing angles with less diffraction efficiencies where discrete substrate modes are generated and zig-zagged within the substrate. 3 The schematic of the microstructure of the designed volume holograms is shown in Fig. 2 . The central wavelength bouncing angle 0 is set at 45°. For maximum diffraction efficiency at the central wavelength, the grating spacing ⌳ must satisfy 3 where 0 is the central wavelength, ⌳ is the grating spacing and n is the polymer refractive index. Four GRIN rod lenses are employed for this demonstration. The three input GRIN lenses function as collimators for the incoming optical signals while the output GRIN rod lens functions as a focusing element on which a fiber array is integrated ͑Fig. 1͒. The GRIN lenses employed have a parabolic refractive index distribution profile of where n(0) is the refractive index of the GRIN lens axis and A is the GRIN lens property constant. The paraxial equation describing the ray position at the output GRIN lens surface from a zigzag substrate mode with a bouncing angle ⌬ away from the Bragg condition is 4 where a͒ Electronic mail: email@example.com FIG. 1. Polymer-based volume hologram crossbar for a surface-normal 3ϫ3 nonblocking wavelength selective crossbar. Note that the special characteristic of the GRIN lens reduces the nine wavelengths to three, i.e., ͚ iϭ1 3 i1 ϭ 1 ϭ765 nm, ͚ iϭ1 3 i2 ϭ 2 ϭ775 nm, and ͚ iϭ1 3 i3 ϭ 3 ϭ785 nm.