Optical frequency division multiplexer and polarization control method

An optical frequency division multiplexer comprising:

• a nonlinear medium that multiplexes, on a carrier light beam, a signal light beam of a modulation signal that a carrier signal at a characteristic frequency is modulated with an information signal, cross-phase-modulates the carrier light beam with the signal light beam, frequency-multiplexes the information signal in the signal light beam on the carrier light beam, generates an optical frequency division multiplexed signal, and outputs the optical frequency division multiplexed signal;
• and
• a control unit that controls a polarization state of the signal light beam in a direction in which an intensity of a modulation component takes a maximum value, based on the intensity of the modulation component involved in a modulation signal at a desired characteristic frequency of the optical frequency division multiplexed signal when the intensity of the modulation component involved in the modulation signal at the desired characteristic frequency of the optical frequency division multiplexed signal is reduced beyond a predetermined threshold from a maximum value, wherein the control unit includes:
• an intensity converting unit that converts a part of the optical frequency division multiplexed signal into an intensity modulated signal;
• an optoelectric conversion unit that optoelectrically converts the converted intensity modulated signal;
• an extracting unit that extracts the modulation component involved in the modulation signal at the desired characteristic frequency of the optoelectrically converted intensity modulated signal;
• and
• an intensity detecting unit that detects the intensity of the extracted modulation component.