This paper reviews the recent progress in long-wavelength photodetectors and integrated photoreceiversdeveloped for long-distance and large-capacity optical transmission systems. The InGaAsavalanche photodiodes (APDs) successfully operating in the 1.6 Gb/s systems are now being developedfor the 10 Gb/s range, where the reduction of avalanche buildup time is crucial. In InGaAsPIN photodiodes, flip-chip bonding structure shows ultra-low diode and stray capacitance suitablefor PIN/FET integrations. Monolithically integrated Twin-PIN photodiodes of dual balanced opticalreceivers (DBORs) for coherent optical transmission systems exhibit the high-sensitivity much betterthan direct detection. For the future devices, possibility of the very high-speed and low-noise operationof the staircase APDs is discussed as well as a new PIN photodiode in the wavelength range largerthan 2μm, where the theoretical loss of a fluoride fiber is minimized.

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This paper reports a two-dimensional vector velocimeter on the basis a the time-varing spatialfiltering method using a rotating disk with two transmission gratings. The filtering characteristics ofthe spatial filter employed in the practical experiment were studied theoretically. The velocimeter isused to measure the flow velocity of water in a small glass pipe. The flow velocity was measured inreal time by using a new signal analizing system. The experimental resurts show the usefulness ofthe proposed velocimeter for the measurements of flow velocities.

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We have developd a new optical unit for YAG laser soldering of high-density, super-multiple-pin LST 's. The unit enablesus to irradiate facing lines of LST 's lead arrays simultaneously with two rec -tangular cross-section laser beams. We evaluated the unit by using an experimental assembled systemcomposed of two rectangular robots, a velt conveyor, and a mounting system with pattern recognitioncapability. The following results obtained with 100 pin flat package IC 's are summerized asfollows (1) The most suitable solder amount was 2.5mg/cm-7mg/cm when solder paste was continuouslydispensed across lead pad arrays and subsequently fused by rectangular laser beams. (2) Four lines of flat package IC lead arrays were soldered in 4 seconds, which is less than half of thatfor a previously developed conventional laser soldering method. (3) The photographs taken duringlaser soldering revealed that the flux in the solder paste evaporated in the first quarter of a unitsoldering time; the dispensed solder then fused and flowed to leads and pads during the next quar-ter, and subsequently they got wet completely with the fused solder in the following half of the unitsoldering time. (4) Suitable laser irradiation time for the FPIC 's having different pitch and volume ofleads was found to be approximately proportional, at a constant laser power to their lead volume perlead pitch.

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