# US 2008/0187324 A1 · Optical transmitter and control method therefor > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2008/0187324 A1 - Document ID: US20080187324A1 - Status: Published application - Kind: A1 - Application number: 12/013,857 - Filed: 2008-01-14 - Published: 2008-08-07 - Earliest claimed: 2007-01-15 - Assignees: Fujitsu Limited - Inventors: Yuichi AKIYAMA; Hideyuki Miyata; Toshiki Tanaka; Masato Nishihara - Web page: https://patentgrep.com/patent/US20080187324A1 ## CPC - H04B 10/5053: {using a parallel, i.e. shunt, combination of modulators} - H04B 10/50: Transmitters - H04B 10/50575: {to control the modulator DC bias} - H04B: TRANSMISSION ## Abstract The disclosed optical transmitter and control method include performing phase modulation of a light propagating through a corresponding optical path in accordance with a data signal, supplying bias voltages for regulating an operating point of each of phase modulation performed, imparting a predetermined phase difference, supplying bias voltages for phase difference regulation and coupling lights output from the corresponding optical paths. The optical transmitter includes superimposing a pilot signal on either one of bias voltages where the pilot signal has a frequency lower than a frequency of a bit rate of the data signal, and performing a feedback control in accordance with a result of monitoring. ## Claims ### Claim 1 An optical transmitter for generating and transmitting a multiphase modulated signal, comprising: a light source generating an optical light; a splitter splitting the light received from the light source into a plurality of lights; a plurality of optical paths each inputting a respective light split by the splitter; a plurality of phase modulator portions each performing phase modulation of a respective light propagating through a corresponding optical path in accordance with a data signal; a first bias supply portion supplying bias voltages for regulating an operating point of each of the plurality of phase modulator portions; phase shifter portions each imparting a predetermined phase difference into the respective light propagating through the corresponding optical paths; a second bias supply portion supplying bias voltages for phase difference regulation to a side of the phase shifter portions; a coupler coupling lights output from the corresponding optical paths; a pilot signal superimposition portion superimposing a pilot signal on either one of bias voltages being supplied to a side of at least one of the plurality of phase modulator portions and to the side the phase shifter portions, the pilot signal having a frequency lower than a frequency of a bit rate of the data signal; a monitor portion monitoring an output of the coupler; and a bias control portion performing a feedback control of the first bias supply portion and the second bias supply portion in accordance with a result of monitoring performed by the monitor portion. ### Claim 2 (depends on 1) An optical transmitter according to claim 1, wherein the monitor portion performs a synchronous detection using the pilot signal in accordance with the output of the coupler, and the bias control portion performs the feedback control of the first bias supply portion and the second bias supply portion so that an output signal level of the synchronous detection in the monitor portion becomes near zero. ### Claim 3 (depends on 1) An optical transmitter according to claim 1, wherein the monitor portion monitors a power of an AC (alternating current) component of the output of the coupler, and the bias control portion performs the feedback control of the first bias supply portion and the second bias supply portions so that a power to be monitored in the monitor portion is minimized. ### Claim 4 (depends on 1) An optical transmitter according to claim 1, wherein the coupler includes an optical coupler that includes a plurality of input ports corresponding to the plurality of optical paths and two output ports and that utilizes a multi-mode interference, and the monitor portion monitors an output of one of the two output ports of the optical coupler utilizing the multi-mode interference. ### Claim 5 (depends on 1) An optical transmitter according to claim 1, wherein the monitor portion includes an optical branch coupler provided in an optical path connecting with an output end of the coupler, and monitors a signal split by the optical branch coupler. ### Claim 6 (depends on 1) An optical transmitter according to claim 1, wherein the phase shifter portions vary phases of the corresponding lights propagating through the plurality of optical paths to thereby impart a predetermined phase difference into the lights, the second bias supply portion supplies a plurality of bias voltages respectively corresponding to the plurality of optical paths to the side of the phase shifter portions, and when superimposing the pilot signal on the plurality of bias voltages on the side of the phase shifter portions, the pilot signal superimposition portion superimposes the pilot signal on at least one of the plurality of bias voltages being supplied to the phase shifter portions. ### Claim 7 (depends on 1) An optical transmitter according to claim 1, wherein the monitor portion has a narrower band range in comparison to a symbol frequency of the signal. ### Claim 8 (depends on 1) An optical transmitter according to claim 1, further comprising: an intensity modulator portion performing intensity modulation of the signal output from the coupler, in accordance with a clock signal of a frequency corresponding to the bit rate of the data signal. ### Claim 9 (depends on 8) An optical transmitter according to claim 8, wherein the monitor portion monitors an output of the intensity modulator portion. ### Claim 10 (depends on 1) An optical transmitter according to claim 1, further comprising: an intensity modulator portion performing intensity modulation of the light from the light source, in accordance with a clock signal of a frequency corresponding to the bit rate of the data signal. ### Claim 11 (depends on 1) An optical transmitter according to claim 1, wherein the pilot signal superimposition portion superimposes the pilot signal on driving signals corresponding to respective data signals being supplied to the plurality of phase modulator portions, and the optical transmitter further comprises an amplitude control portion performing a feedback control of an amplitude of the driving signal in accordance with a result of monitoring performed by the monitor portion. ### Claim 12 (depends on 11) An optical transmitter according to claim 11, wherein the pilot signal superimposition portion superimposes a pilot signal on a driving signal, the pilot signal being different in frequency from the pilot signal that is superimposed on the one of the bias voltages. ### Claim 13 An optical transmitter for generating and transmitting a multiphase modulated signal, comprising: a light source generating an optical light; a splitter splitting the light received from the light source into a plurality of lights; a plurality of optical paths each for inputting a respective light split by the splitter; a plurality of phase modulator portions each performing phase modulation of a respective light propagating through a corresponding optical path in accordance with a data signal; a first bias supply portion supplying bias voltages for regulating an operating point of each of the plurality of phase modulator portions; phase shifter portions each imparting a predetermined phase difference into the respective light propagating through corresponding optical paths; a second bias supply portion supplying bias voltages for phase variation to a side of the phase shifter portions; a coupler coupling the lights output from the corresponding optical paths, the coupler including an optical coupler that includes a plurality of input ports corresponding to the plurality of optical paths and two output ports and utilizes a multi-mode interference; a pilot signal superimposition portion superimposing a pilot signal on bias voltages being supplied to at least the side of the phase shifter portions, the pilot signal having a frequency lower than a frequency of a bit rate of the data signal; a monitor portion monitoring an output of one of the two output ports of the optical coupler utilizing the multi-mode interference; a bias control portion performing a feedback control of at least the second bias supply portion in accordance with a result of monitoring performed by the monitor portion. ### Claim 14 A control method for controlling an optical transmitter that generates and transmits a multiphase modulated signal, comprising: generating an optical light using a light source; splitting the light received from the light source into a plurality of lights; providing a plurality of optical paths each for inputting a respective light split by said splitting; performing phase modulation of a respective light propagating through a corresponding optical path in accordance with a data signal; supplying bias voltages for regulating an operating point of each phase modulation; providing phase shifter portions each imparting a predetermined phase difference into the light propagating through the corresponding optical paths; supplying bias voltages for phase difference regulation to a side of the phase shifter portions; and coupling the light output from the corresponding optical paths, superimposing a pilot signal on only one of bias voltages being supplied to a side of at least one of a plurality of phase modulator portions and to the side the phase shifter portions, the pilot signal having a frequency lower than a frequency of a bit rate of a respective data signal; monitoring an output of said coupling; and performing a feedback control of each supplying of said bias voltages in accordance with a result of said monitoring. ### Claim 15 A method of controlling an optical transmitter, comprising: supplying bias voltages for regulating an operating point of each phase modulation performed on light portions propagating through optical paths based on a data signal; imparting a predetermined phase difference into a light among said light portions; supplying bias voltages for phase difference regulation of said light portions and coupling the light portions; selectively superimposing a pilot signal on only one of said bias voltages, the pilot signal having a frequency lower than a frequency of a bit rate of the data signal; and performing a feedback control of each supplying of bias voltages in accordance with a result of a monitoring of an output of the coupling.