As a technique for providing a large capacity in optical fiber transmission, the wavelength division multiplexing (WDM) technique is used in which a symbol rate for a single carrier is increased to narrow the channel spacing for an increased number of multiplexes, to thereby achieve an enlarged transmission capacity.
However, an increase in symbol rate results in a more noticeable influence of a chromatic dispersion and a polarization mode dispersion (PMD) in a transmission fiber. Therefore, a sophisticated compensation technique is required. In addition, in order to obtain necessary receiver sensitivity, more power is required. This leads to a problem of suppressing a nonlinear optical effect produced in the fiber such as self phase modulation (SPM), four wave mixing (FWM), or cross phase modulation (XPM). In addition, with the increase in symbol rate, an occupied band for a single channel is expanded. This makes it difficult to increase the number of WDM channels.
Here, symbol rate is synonymous with baud rate, which is a bit rate to be transmitted divided by the number of bits of the modulation code. For example, if it is assumed that the bit rate is B (b/s) and that an M-ary modulation code is used, then the symbol rate is given by B/log 2 (M) (symbol/sec).
To solve this problem, what used to be transmitted in a single carrier manner is divided into subcarriers at a symbol rate that does not require a sophisticated compensation technique and then the subcarriers are transmitted. Thereby, it is possible to suppress chromatic dispersion and PMD, and an influence of a nonlinear optical effect.
If an orthogonal frequency division multiplexing (OFDM) modulation, which is often used in wireless transmission, is performed on data, and continuous light emitted from a continuous light source is modulated by an intensity modulator or a single sideband (SSB) modulator driven with an OFDM signal, then optical OFDM transmission can be implemented (for example, see Patent Document 1, or Non-Patent Document 1 or 2).    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2005-311722    Non-Patent Document 1: Arthur James Lowery et al., “Orthogonal Frequency Division Multiplexing for Adaptive Dispersion Compensation in Long Haul WDM Systems,” Optical Fiber Communication Conference 2006, PDP 39 (2006)    Non-Patent Document 2: Brendon J. C. Schmidt et al., “Experimental Demonstration of 20 Gbit/s Direct-Detection Optical OFDM and 12 Gbit/s with a colorless transmitter,” Optical Fiber Communication Conference 2007, PDP18 (2007).