Storage capacity and cross talk in angularly multiplexed holograms: two case studies.

Storage capacity of a volume is derived based on the space-bandwidth products of angularly multiplexedholograms and by considering their diffraction efficiency and cross-talk limitations. An erasable medium of Fe:LiNbO(3) (10MM × 1mm × 2mm) is simulated thatyieldsacapacity of ˜ 26 Gbits for 95 holograms, each with an efficiency of ~ 10(-4), for a total of ~ -31-dB cross talk. The recording time is estimated at 0.7 s (assuming 10(3) W/m(2) and λ = 0.5 µm). A nonerasable dichromated gelatin medium 10 mm × 10 mm × 0.025 mm is also analyzed that yields a capacity of ~ 3.3 Gbits for 11 holograms with efficiency of 0.09 and ~ -25-dB cross talk.

Large interconnects in photorefractives: grating erasure problem and a proposed solution.

The known problem of grating erasure in photorefractive-based holographic optical interconnects can lead to limitations in the density of the system. This problem is analyzed for an architecture that uses spatially multiplexed beams (to eliminate cross erasure) and angularly multiplexed input beams to record multiplexed gratings in a bismuth silicon oxide (BSO) crystal to interconnect three beams to an array of 8 x 8 (64) detectors. Drawbacks of the time-exposure technique, which compensates for erasure, are analyzed. A novel photorefractive-based interconnect architecture is proposed, which uses holograms and read beams that are segregated to minimize erasure during recording and reading.

Active-fiber star coupler that uses arrays of microlenses and liquid-crystal modulators.

An active-fiber star coupler that uses arrays of N x N microlenses and liquid-crystal modulators is presented. A simplified implementation of this device uses an 8 x 8 array of Fresnel microlenses fan out an incident beam into 64 focused spots (an 18 dB fan-out loss); on-off capability (270:1 extinction ratio) at each element is provided by liquid-crystal spatial light modulators. A row of focused spots is coupled into an array of 1 x 8 multimode fibers with a measured excess loss of 12 dB along each path (with an estimated overall loss of 30 dB in an 8 x 8 device). A modification of this device that is capable of wavelength selection at individual output fibers is proposed for wavelength division multiplexing applications.

Alignment and fabrication tolerances of planar gratings for board-to-board optical interconnects.

The use of diffraction gratings to trap optical beams in substrates for optical interconnect applications has been proposed by many. We analyze the alignment and fabrication tolerances of such gratings to make them viable for such applications. The use of chirped circular gratings is proposed as a means of compensating for transverse-optical beam misalignments. Fabrication of 1-, 2-, and 4-microm-period gratings on glass substrates through ion-beam milling and their analysis are presented. High diffracted orders were confined to the substrates through total internal reflection.

Electrically controlled polarization-independent liquid-crystal Fresnel lens arrays.

We describe the properties and construction of a polarization-independent Fresnel lens array using nematic liquid crystals in which the diffraction efficiency of lenses can be electrically controlled. A novel structure is used such that the principal axis of the liquid crystals in two adjacent zones of each Fresnel lens are orthogonal. This makes the device polarization independent. We have characterized these lenses at different applied fields and input polarizations by diffracting an argon-ion beam into focus.

Integration of planar Fresnel microlenses with vertical-cavity surface-emitting laser arrays.

We report a technique of integrating planar Fresnel microlenses with InGaAs/GaAs-based vertical-cavity surfacee-mitting laser arrays by selectively ion-beam milling the substrate. Depending on the application, one can focus, collimate, and bend the individual laser beams using such microlenses. An example is presented where a 32 x 32 array of microlenses, each with an aperture of 80microm and a focal length of 108 microm is integrated with a laser array. As expected, arrays of focused beams, each with a 2-microm spot size, are generated at a distance of approximately 110 microm.

Binary phase Fresnel lenses for generation of two-dimensional beam arrays.

Two-dimensional arrays of mutually coherent optical beams are needed in holographic interconnections for both recording and reading gratings. We analyze the use and limitations of binary phase Fresnel lenses to generate beams for these applications. Two known techniques of ion beam milling and thin film deposition are compared to fabricate such lens arrays in SiO(2) and Si(3)N(4). Each lens in the 8 x 8 arrays has a 1.2-mm square aperture with a focal length of 20 mm. Diffraction of a single argon-ion beam into an 8 x 8 array of highly uniform coherent focused beams (with 12-microm spot size) was achieved by the lenses with an efficiency of approximately 30% (41% theoretical limit).