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We will briefly review the present status of optical topography and then discuss the method of improving practicality, i.e., the signal-to-noise (S/N) ratio and the spatial resolution in observations of higher-order brain functions. The optimum wavelength pair improved the S/N ratio sixfold for deoxyhemoglobin, and new configurations of light irradiation and detection positions doubled the spatial resolution. We also report on developing application fields of optical topography. This modality will bridge the gap between natural sciences, neuroscience, and pedagogy, and show actual real-time brain activity. OCIS codes: 170.0110, 170.7050, 170.6960, 170.3880, 170.3890.

A vertical-cavity surface-emitting laser based bidirectional free-space optical interconnect has been implemented to interconnect two printed circuit boards. A total of 512 clustered channels with a density of 2844 channels/[cm.sup.2] are transmitted over a distance of 83 mm. The optical interconnect is a combination of refractive microlenses and diffractive minilens relays. OCIS codes: 200.2610, 200.4650, 220.4830, 200.4880, 250.7260, 350.3950.

We propose flee-space optical interconnections for a bookshelf-assembled terabit-per-second-class ATM switch. Thousands of arrayed optical beams, each having a rate of a few gigabits per second, propagate vertically to printed circuit boards, passing through some boards, and are connected to arbitrary transmitters and receivers on boards by polarization controllers and prism arrays. We describe a preliminary experiment using a 1-mm-pitch 2 x 2 beam-collimator array that uses vertical-cavity surface-emitting laser diodes. These optical interconnections can be made quite stable in terms of mechanical shock and temperature fluctuation by the attachment of reinforcing flames to the boards and use of an autoalignment system. OCIS codes: 200.0200, 200.4650, 160.3710, 140.2020.

Liquid-crystal microprism arrays are shown to be useful for providing electrically controlled alignment of optical beams and fixed various free-space optical interconnections. They can deflect closely spaced micro-optical beams individually to any position with high transmittance (95%), high deflection angle ([approximately]10 [degrees]), and low voltage (< 2.8 [V.sub.rms]). Various fixed optical interconnections can be made simply by changes in the voltages applied to the microprism. Key words: Optical interconnection, free space, liquid crystal, optical beam, deflector, two-dimensional optical switch.

A polymer optical fiber amplifier (POFA) of the graded-index (GI) type, with gain in the visible region, was successfully prepared for the first time, to our knowledge, with the interfacial-gel polymerization technique. An input signal of 0.85 W at 591 nm was amplified to 420 W (27-dB gain) by injection of 690 W of pump power at 532 nm into a GI POFA with a 0.5-m length. The efficiency of conversion of pump energy to signal amplification was more than 60%. Key words: Graded index, polymer optical fiber amplifier, amplification, organic dye, Rhodamine B.

We analyzed the polarization-switching characteristics of a ferroelectric liquid-crystal polarization control device using a double-layer cell structure for free-space optical processing and switching systems. We theoretically derived the optimum arrangement of layered cells for 90 [degrees] polarization switching. The theory shows that the optimum parameter range for the double-layer cell structure is wider than that for a single-layer cell. We verified our theory by measuring the polarization cross talk of experimental polarization control devices. Key words: Polarization-control device, polarization switching, ferroelectric liquid crystal, spatial light modulator, free-space optical processing.

We present a digital free-space photonic switch structure that uses exciton absorption reflection switch (EARS) arrays. The network topology of the switch is a banyan network. This switch structure uses microbeam interconnections to realize a compact switch fabric. The low-loss interconnection that is necessary for high-speed operation can be achieved by the use of an optical system based on birefringent plates. A prototype switch (based on an 8 X 8 EARS array) with fiber array pigtails is fabricated and used to demonstrate two-input, two-output switching of 8-Mbits/s signals. This digital switch may be applicable to large-scale, multistage switching networks for optical interconnections and broadband telecommunication switching systems.