A light modulation/demodulation system includes a transmission unit (1) and a reception unit (2). The system includes: a multiplication unit (14) which prepares a modulation signal as a complex product of a transmission symbol m and a normalized differential symbol obtained by normalizing a differentiated transmission symbol before the symbol time; a delay adjustment unit (16) which obtains a delay differential symbol by delaying the differentiated symbol by the time of m symbols; and a normalization unit (15) which normalizes the delay differential symbol in the intensity direction. The multiplication unit (14) makes the delay differential symbol normalized by the normalization unit (15) to be the normalized differential symbol. The reception unit (2) has: a front end unit (21) which executes a delay detection of the m-symbol time on the received optical signal and convert the optical signal into an electric signal; a division unit (23) which divides the electric signal by the absolute value of the symbol complex electric field; and an identification unit (24) which identifies the code of the transmission data in accordance with the division result obtained by the division unit (23).
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