Frequency separation stabilization method for optical heterodyne or optical homodyne communication

Claim 2

A frequency separation stabilization method comprising the steps of: multiplexing output beams from two light sources to form a combined beam; converting at least a portion of the combined beam to an electric signal by an optical detector; multiplying at least a portion of the electric signal, having a difference component in optical frequency between said light sources, by the output from an oscillator to form a multiplication signal; extracting a difference frequency component between said electric signal and the output of said oscillator; converting the difference frequency component to an electric signal by a frequency discrimination circuit; and changing optical frequencies of said light sources by applying said output signal from said frequency discrimination circuit to one or both of said light sources to keep the optical frequency separation between said two light sources constant.

Claim 3

The frequency separation stabilization method according to claim 1, wherein said step of multiplexing light sources includes the step of multiplexing at least one of semiconductor lasers and multi-electrode semiconductor lasers.

Claim 4

The frequency separation stabilization method according to claim 1, wherein said step of multiplexing light sources includes the step of multiplexing light sources comprised of semiconductor lasers in conjunction with of at least one of reflecting mirrors and diffraction gratings.

Claim 5

The frequency separation stabilization method of claim 3 including the step of changing at least one of an angle of said reflecting mirror and said diffraction grating, in accordance with an output signal of the frequency discrimination circuity, in order to change the optical frequency of said light sources and to keep the optical frequency separation between said two light sources constant.

Claim 6

The frequency separation stabilization method of claim 1 further comprising the step of using the output signal of said frequency discrimination circuit as a controf signal for changing temperature of at least one of said light sources in order to change an optical frequency of said light sources and to keep the optical frequency separation between said two light sources constant.

Claim 7

The method of claim 1 wherein the step of multiplexing output beams includes the step of encoding data for communication, in accordance with one of optical homodyning and optical heterodyning, by controlling a first of the two light sources which functions as a signal light source, and wherein the step of changing the optical frequencies includes the step of keeping an intermediate frequency of multiplexed output beams constant by controlling a second of the two light sources which functions as a local oscillator light source.

Claim 8

The method of claim 6, further comprising the steps of: combining, with a balanced optical receiver having optical couplers, the signal from said signal light source with the beam from said local oscillator light source; converting, with two optical detectors the two output beams to electric signals; and adding said two electric signals to each other.

Claim 9

The method of claim 6 further comprising the steps of: combining, with a polarization diversity receiver consisting of an optical coupler, the signal beam with the local oscillation beam and a polarization beam into an output beam; splitting, with a splitter, the output beam into two substantially crossing polarization components; converting, with two optical detectors, the two polarization components to two electric polarization signals; demodulating, with two demodulation circuits, said two electric polarization signals; and adding, with an adder, said two demodulation signals.

Claim 10

The method of claim 6 further comprising the steps of: separating, with a balanced type polarization diversity receiver, said signal beam from said signal light source into an x polarization component beam and a y polarization component beam crossing each other substantially at right angles; separating the beam of said local oscillator optical source into two beams substantially in conformity with the polarization directions of said x and y polarization component beams, respectively; combining, with two optical multiplexers, the two beams substantially in conformity with the polarization directions; converting, with two optical detectors, two output beams from said optical multiplexer with said x polarization component beam with the beam from said local oscillator optical source into first electric signals; adding the first electric signals to form a first added signal; demodulating, with a demodulation circuit, said first added signal to form a first demodulated signal; converting, with two optical detectors, the two output beams of said optical multiplexer for combining said y polarization component beam with the beam from said local oscillator optical source into second electric signals; adding said second electric signals to form a second added signal; demodulating, with a demodulation circuit, said second added signal to form a second demodulated signal; and adding, with an addition circuit, said first and second demodulated signals to form an added demodulated signal.

Claim 13

The stabilizing system of claim 10 wherein the frequency discrimination means includes a means for generating a first voltage level when a frequency of the electrical input signal exceeds fo, means for generating a second voltage level when a frequency of the electrical input signal equals fo, and means for generating a third electrical input signal when a frequency of the electrical input signal is less than fo.

Claim 14

The stabilizing system of claim 11 wherein the frequency discrimination means is comprised of: a signal separation circuit means for dividing the electrical input signal into first and second portions; a delay circuit means for forming a delayed second portion from the second portion of the electrical input signal; double balance mixer means for forming the control signal from the first portion of the electrical input signal and the delayed portion of the electrical input signal.

Claim 15

The stabilizing system of claim 12 wherein at least one of the first and second light sources is comprised of a combination of a semiconductor laser device and a mirror separated by a lens system, and wherein the frequency control means is comprised of means for varying a distance between a semiconductor laser device and its associated mirror in accordance with the control signal.

Claim 16

The stabilizing system of claim 12 wherein at least one of the first and second light sources is comprised of a semiconductor laser device and an optical fiber, and wherein the frequency control means is comprised of a means for varying an effective length of the optical fiber in accordance with the control signal.

Claim 17

The stabilizing system of claim 12 wherein at least one of the first and second light sources is comprised of a semiconductor laser device and a diffraction grating separated by a lens system, and wherein the frequency control means is comprised of a means for varying at least one of (a) a distance between a semiconductor laser device and its associated diffraction grating, and (b) an angle between a semiconductor laser device and its associated diffraction grating, in accordance with the control signal.

Claim 18

The stabilizing device of claim 12 wherein at least one of the first and second light sources is comprised of a semiconductor laser device, and wherein the frequency control means is comprised of a means for varying a temperature of at least one of the semiconductor laser devices in accordance with the control signal.

Claim 21

An optical communication system comprising: means for generating a first periodic electric signal having a frequency f1; means for driving a first light source with the first periodic electric signal such that the first light source has a periodic optical wave output with a frequency f1; means for generating a second periodic electric signal having a frequency f2; means for driving a local oscillation light source with the second periodic signal such that the local oscillation light source has a periodic optical wave output with a frequency f2; photo-coupler means for combining the first and second periodic optical waves into combined optical wave output having a frequency f2-f1; means for diverting at portion of the combined optical wave to an optical-to-electrical transducer; optical-to-electrical transducer means for forming an combined electrical signal from the combined optical wave such that the combined electrical signal has a frequency f2-f1; demodulation means for converting a portion of the combined electrical signal into a baseband signal; oscillator means for generating an electrical oscilla- tor signal having a selected difference frequency fs-fo, wherein fs is a desired frequency separation between f2 and f1, and wherein fo is a desired output frequency; multiplier means for multiplying the electrical oscillator signal and the combined electrical signal to form a multiplied electrical signal having a frequency (f2-f1) +/- (fs-fo); means for filtering out a portion of the multiplied signal to form an electrical input signal having a frequency (f2-f1) - (fs-fo); frequency discrimination means for generating a control signal in accordance with the electrical input signal; and frequency control means for altering a frequency of the local oscillation light source in accordance with the control signal.

Claim 22

An optical communication system comprising: means for generating a first periodic electric signal having a frequency f1; means for driving a first light source with the first periodic electric signal such that the first light source has a periodic optical wave output with a frequency f1; means for generating a second periodic electric signal having a frequency f2; means for driving a local oscillation light source with the second periodic signal such that the local oscillation light source has a periodic optical wave output with a frequency f2; photo-coupler means for combining the first and second periodic optical waves into combined optical wave output having a frequency f2-f1; means for diverting at portion of the combined optical wave to an optical-to-electrical transducer; optical-to-electrical transducer means for forming an combined electrical signal from the combined optical wave such that the combined electrical signal has a frequency f2-f1; demodulation means for converting a portion of the combined electrical signal into a baseband signal; oscillator means for generating an electrical oscilla- tor signal having a selected difference frequency fs-fo, wherein fs is a desired frequency separation between f2 and f1, and wherein fo is a desired output frequency; multiplier means for multiplying the electrical oscillator signal and the combined electrical signal to form a multiplied electrical signal having a frequency (f2-f1) +/- (fs-fo); means for filtering out a portion of the multiplied signal to form an electrical input signal having a frequency (f2-f1) - (fs-fo); frequency discrimination means for generating a control signal in accordance with the electrical input signal; and frequency control means for altering a frequency of the local oscillation light source in accordance with the control signal.