Wavelength division multiplexing (WDM) is a technique of multiplexing optical signals of wavelengths (optical carrier frequencies) that differ from one another. According to WDM, baseband modulation is executed for each carrier wave among carrier waves of differing wavelengths and an optical coupling and decoupling device executes multiplexing and de-multiplexing of the carrier waves. To multiplex at narrow wavelength (frequency) intervals, highly precise wavelength control is required of the transmitters and wavelength de-multiplexer. Consequently, multiplexing at narrow wavelength (frequency) intervals is difficult.
Optical frequency division multiplexing (FDM) may be performed. In a system employing optical FDM, along an optical transmission path such as an optical fiber, plural nodes are disposed and carrier light is transmitted from a light source of a transmitting apparatus disposed at the starting point of the optical transmission path. Data signals at different sub-carrier frequencies are multiplexed and transmitted with the carrier wave from the plural nodes. The receiving side (a receiving apparatus) disposed at the end point of the optical transmission path receives information transmitted from the nodes (see, e.g., Japanese Laid-Open Patent Publication No. 2011-215603).
In the system employing optical FDM, the data signals are multiplexed with carrier light in a single optical transmission path and therefore, the receiving side merely has to receive the carrier light and therefore, can receive the information transmitted by the plural nodes by a simple configuration compared to that of the WDM.
However, the efficiency of frequency use cannot be improved for a system employing the conventional optical FDM. The sequence in which signals are transmitted by each node in the optical FDM system varies. Consequently, unused frequency bands occur for the frequencies (the sub-carriers) of the unused nodes in the entire frequency band used by the optical FDM.
In a case where the unused frequency bands are discrete, if frequency multiplexing is to be executed using a modulation signal corresponding to a single carrier, the configuration becomes complicated. Plural mixers and frequency oscillating devices are necessary to determine the frequency position (frequency difference) of the coupled signal against that of the carrier wave (at the frequency ν0). Therefore, the configuration becomes complicated and the cost increases.
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