Fiber-optic delay-line stabilization of heterodyne optical signal generator and method using same

A heterodyne generator circuit, comprising:

• a dual optical signal source having a first signal and a second signal and means for combining said first and second signals for producing a combination of a first optical output signal and a second optical output signal respectively, wherein said first optical output signal is offset in frequency from said second optical output signal;
• delay line means comprising a long optical fiber receiving at one end thereof said combination of said first and second optical output signals and transmitting at an opposite end thereof a delayed version thereof;
• phase detector means for detecting a change in a phase relationship between said combination of said first and second optical output signals and said delayed version thereof;
• and
• means responsive to said phase detector means for changing the frequency of one of said first and second optical output signals relative to the other so as to counteract said change in said phase relationship;
• wherein said heterodyne generator device comprises first and second continuous wave lasers each with co-linear polarization states for producing said first optical output signal and said second optical output signal, respectively, and a coupler receiving said first optical output signal and said second optical output signal.

A method for stabilizing an optical heterodyne generator having a first signal and a second signal and means for combining said first and second signals for producing a combination of a first optical output signal and a second optical output signal, wherein said first optical output signal is offset in frequency from said second optical output signal by a difference frequency, said method comprising:

• providing a fiber optic delay line;
• transmitting through said fiber optic delay line said combination of said first and second optical output signals to produce a delayed version of said first and second optical output signals;
• generating an error signal by sensing a phase difference between said combination of said first and second optical output signals and the delayed version thereof;
• and
• feeding back said error signal derived from said phase difference between said combination of said first and second optical output signals and the delayed version thereof to said heterodyne generator so that a frequency of said second output signal is changed relative to the other in such a way to decrease said error signal.

A method for stabilizing an optical heterodyne generator having a first signal and a second signal and means for combining said first and second signals for producing a combination of a first optical output signal and a second optical output signal, wherein said first optical output signal is offset in frequency from said second optical output signal by a difference frequency, said method comprising:

• providing a fiber optic delay line;
• transmitting through said fiber optic delay line said combination of said first and second optical output signals to produce a delayed version of said first and second optical output signals;
• sensing a phase difference between said combination of said first and second optical output signals and the delayed version thereof, whereby to generate an error signal;
• feeding back said error signal to said heterodyne generator so that a frequency of said second output signal is changed relative to the other in such a way to decrease said error signal;
• further comprising the following preliminary steps for setting said difference frequency:
• changing the frequency of one of said first and second optical output signals;
• counting zero-crossings sensed by the step of sensing a phase difference;
• and
• fixing said frequency of said one output signal upon a predetermined number of zero-crossings being counted in said counting step.

A circuit for stabilizing a frequency of a heterodyne generator having a first signal and a second signal and means for combining said first and second signals for producing a combination of a first optical output signal and a second optical output signal respectively, wherein said first optical output signal is offset in frequency from said second optical output signal by a difference frequency, said method comprising:

• a optical fiber delay line for producing a delayed version of said combination of said first and second optical output signals;
• a phase detector for generating an error signal by sensing a phase difference between said combination of said first and second optical output signals and the delayed version thereof;
• and
• means for feeding back said error signal derived from said phase difference between said combination of said first and second optical output signals and the delayed version thereof to said generator to effect a change in frequency of one of said first and second optical output signals relative to the other to reduce said error signal.