Optical transmitter and frequency control method in optical transmission

A frequency control method in optical transmission comprising:

• generating an optical signal in an uplink direction by modulating output light of a laser that outputs the light of a first frequency with a subcarrier on which transmission data is superimposed;
• receiving an optical signal in a downlink direction by heterodyne detection, wherein the optical signal in a downlink direction being an optical signal of a same frequency transmitted from an opposing apparatus to a plurality of optical transmission apparatuses;
• and
• calculating a first intermediate frequency based on a frequency of a downlink signal received by the heterodyne detection and the first frequency of the laser and controlling the first frequency of the laser when there is a difference between the calculated first intermediate frequency and a reference second intermediate frequency by a threshold or more, wherein
• receiving, by the heterodyne detection, an optical signal in the downlink direction transmitted from an optical transmitter that generates the downlink optical signal by performing single side band modulation or vestigial sideband modulation of output light of a laser that outputs light of a second frequency different from the first frequency with the subcarrier on which transmission data is superimposed,
• wherein the optical signal in an uplink direction is a double sideband modulated signal,
• wherein the first intermediate frequency is a value satisfying a condition indicated in the following Equation (1), where B0 being a bandwidth of one sideband of the optical signal in the uplink direction, B1 being a bandwidth of one sideband of the optical signal in the downlink direction, and fIF being the first intermediate frequency
• B0+B1>fIF>B1 Equation (1).

An optical transmission system comprise a plurality of first optical transmission apparatuses and a second optical transmission apparatus that communicates with each of the plurality of first optical transmission apparatuses,

• wherein the second optical transmission apparatus include a transmitter configured to transmit an optical signal in a downlink direction which is an optical signal of a same frequency to the plurality of first optical transmission apparatuses,
• wherein the plurality of first optical transmission apparatuses include an optical transmitter configured to generate an optical signal in an uplink direction by modulating output light of a laser that outputs light of a first frequency with a subcarrier on which transmission data is superimposed;
• a heterodyne detector configured to receive the optical signal in the downlink direction by heterodyne detection;
• and
• a controller configured to calculate a first intermediate frequency based on a frequency of a downlink signal received by the heterodyne detection and the first frequency of the laser, and controls the first frequency of the laser when there is a difference between the calculated first intermediate frequency and a reference second intermediate frequency by a threshold or more, wherein
• the heterodyne detector receives, by the heterodyne detection, an optical signal in the downlink direction transmitted from an optical transmitter that generates the downlink optical signal by performing single side band modulation or vestigial sideband modulation of output light of the laser that outputs light of a second frequency different from the first frequency with the subcarrier on which transmission data is superimposed, wherein the optical signal in an uplink direction is a double sideband modulated signal,
• wherein the first intermediate frequency is a value satisfying a condition indicated in the following Equation (1), where B0 being a bandwidth of one sideband of the optical signal in the uplink direction, B1 being a bandwidth of one sideband of the optical signal in the downlink direction, and fIF being the first intermediate frequency
• B0+B1>fIF>B1 Equation (1).