Guided-wave Optic Direct Interconnection For Two-dimensional Optical Patterns

Many optical interconnection technologies have been proposed, most of which can be broadly classified as either guided-wave or free-space type optics. Guided-wave optic interconnections using optical fibers or waveguides can be practically implemented because they match with conventional fiber-optic transmission technology. By contrast, free-space optic interconnections can carry large amounts of data in the form of two-dimensional optical patterns, though they are more difficult to integrate. In this paper, we propose a new class of optical interconnections, in which a two-dimensional optical pattern is directly transmitted through a multimode fiber or waveguide. It could offer both the advantages of the flexiblity of guided-wave optics and the massive parallelism of free-space optics. Although direct image transmission utilizing phase conjugation has been proposed', a viable implementation has yet to be realized because of the practical difficulty of preparing two identical fibers for transmission. However, use of a phase conjugation mirror (PCM) with an image-input function enables guided-wave optic interconnection for optical patterns, as shown in Fig. 1. With this approach, reference and object light from the receiver site are conveyed to the transmitter through a single-mode and a multimode waveguide, respectively. When the reference beam is perpendicularly incident on the PCM, the conjugated wave is reflected and transmitted back through the multimode waveguide. When an interference pattern on the PCM is modulated by the optical data pattern, the optical pattern input on the PCM can be reconstructed at the receiver site through an imaging lens because the phase distortion of returned beam through the multimode waveguide is compensated. . The interconnection proposed above has been experimentally demonstrated using a multimode fiber (1 mm in diameter by 200 mm in length) and an optically-addressed spatial light modulator (SLM). The SLM2, consisting of an a-Si:H photoconductor and a ferroelectric liquid crystal (FLC), acts as a PCM with an image-input function. Two coherent beams from a He-Ne laser were incident on the photoconductor via the FLC to record the interference pattern3. The optical pattern to be transmitted was focused on the photoconductor with another incoherent light source. The pattern illumination modulated the interference pattern on the SLM since the interference pattern were erased in the illuminated regions. One result of the experiment is shown in Fig. 2. The intensities of the coherent beams and data illumination light are 0.5 mW/cm*. This interconnection technology could also be applied for reconfigurable interconnection among processors and memories, where parallel access to optical storage would support a huge throughput capacity4. Switchable parallel access from two receiver sites to the same optical data has been demonstrated, as shown in Fig. 3. A He-Ne laser beam is split into three beams. One is a reference beam, and the others are object beams which are separately sent to the SLM through a fiber and a shutter. Two shutters (Sl,S2) are placed in the path of each object beam for access switching. Experimental results are shown in Fig. 4., where the optical data were readout by CCD sensor configured for each fiber. And the same data pattern was also readout by both sensors simultaneously. The significant implication is that multiple processors can access the same memory in parallel at the same time. In conclusion, we have proposed a guided-wave optic interconnection for optical patterns that uses a spatial light modulator which acts as a phase conjugation mirror with an image-input function. This technology could have a wide range of applications, including parallel access to optical memories from multiple processors.