As a method of modulating a signal into carrier light using an optical amplifier, a method of modulating the intensity of pumping light to be supplied to an amplification medium, and modulating a gain is used. An erbium-doped optical fiber amplifier (EDFA) that uses an EDF (Erbium-Doped Fiber) for an amplification medium, and a Raman amplifier that uses an optical fiber using quartz for a base material, for an amplification medium, are used as optical amplifiers. Patent Document 1 and Nonpatent Literature 1 disclose that the Raman amplifier is more suitable than the EDFA, to execute modulation in a relatively high frequency.
Patent Document 1: Japanese Patent Application Laid-Open No. H11-344732
Patent Document 2: Japanese Patent Application Laid-Open No. 2001-311973
Nonpatent Literature 1: The Institute of Electronics, Information and Communication Engineers (IEICE) Communications Society Conference B-10-107 “Study on Gain Modulation Characteristics of Distributed Raman Amplification Line”, 2002 (Imai et., al.)