An optical transmission circuit device generates first and second optical signals from differential set and reset signals each time digital electric input signals of N bits are received. The differential set signal indicates an increase in signal strength relative to immediately preceding N bits of the digital electric input signals and the differential reset signal indicates a decrease in signal strength relative to the immediately preceding N bits. An optical reception circuit device generates first and second digital signals from the first and second optical signals and a third digital signal from the first and second digital signals, a magnitude of the third digital signal being increased by the first digital signal and decreased by the second digital signal. The optical reception circuit further generates a digital output signal of N bits from the third digital signal and an immediately preceding N bits of the digital output signal.
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