Fiber optic transmission line stabilization apparatus and method

A fiber optic transmission line stabilizer for providing a phase-stabilized reference signal at a receiving end of a fiber optic transmission line, said phase-stabilized reference signal having a predetermined phase relationship with respect to a first reference signal provided at a transmitting end of said fiber optic transmission line, said stabilizer comprising:

• means for providing a first optical signal modulated by said first reference signal to said fiber optic transmission line transmitting end, said first optical signal being at a first optical wavelength;
• means for developing a second reference signal related to the modulation on said first optical signal after having passed through said fiber optic transmission line;
• means for providing a second optical signal modulated by said second reference signal to said fiber optic transmission line receiving end, said second optical signal being at a second optical wavelength;
• means for simultaneously altering the phase of the modulation on said first optical signal and the phase of the modulation on said second optical signal in response to a phase shift control signal, and
• means for generating said phase-shift control signal in response to a phase difference between said first reference signal and the modulation on said second optical signal after having been altered by said means for simultaneously altering, thereby resulting in said second reference signal having a predetermined phase relationship with respect to said first reference signal, said second reference signal then defining said phase-stabilized reference signal.

A voltage controlled phase shifter for simultaneously altering the phase of first and second optical signals comprising:

• a voltage controlled directional coupler having a reference signal input/output port, a first intermediate signal input/output port, a second intermediate signal input/output port, and a control signal port;
• means for providing said first optical signal at a first optical wavelength to said reference signal input/output port whereby said voltage controlled directional coupler provides a first intermediate signal at said first intermediate signal input/output port and a second intermediate signal at said second intermediate signal input/output port, the amplitude ratio between said first and second intermediate signals being related to a signal provided to said control signal port;
• combining means for combining said first and second intermediate signals and for dividing said second optical signal at a second optical wavelength into third and fourth intermediate signals, the combined first and second intermediate signals defining a phase altered first optical signal;
• delay means interconnecting said second intermediate signal input/output port and said combining means for simultaneously delaying said second intermediate signal as it passes from said second intermediate input/output port to said combining means and said fourth intermediate signal as it passes from said combining means to said second intermediate signal input/output port;
• and
• means for interconnecting said first intermediate signal input/output port and said combining means for simultaneously passing said first intermediate signal from said first intermediate signal input/output port to said combining means and said third intermediate signal from said combining means to said first intermediate signal input/output port whereby said third intermediate signal and said fourth intermediate signal are summed by said directional coupler as though they had an amplitude ratio the same as that between said first and second intermediate signals as they pass from said first and said second intermediate signal input/output ports, respectively, the summed third and fourth intermediate signals defining a phase altered second optical signal that exits said voltage controlled directional coupler reference signal input/output port simultaneously as said first optical signal enters said reference signal input/output port.

A method for providing a phase-stabilzied signal at a receiving end of a fiber optic transmission line, said phase stabilized signal having a predetermined phase relationship with respect to a first reference signal provided at a transmitting end of said fiber optic transmission line, said method comprising the steps of:

• providing a first optical signal modulated by said first reference signal to said fiber optic transmission line transmitting end, said first optical signal being at a first optical wavelength;
• developing a second reference signal related to the modulation on said first optical signal after having passed through said fiber optic transmission line;
• providing a second optical signal modulated by said second reference signal to said fiber optic transmission line transmitting end, said second optical signal being at a second optical wavelength;
• simultaneously altering the phase of the modulation on said first optical signal and the phase of the modulation on said second optical signal in response to a phase-shift control signal;
• and
• generating said phase-shift control signal in response to a phase difference between said first reference signal and the modulation on said second optical signal after having been altered by said simultaneously altering step, thereby resulting in said second reference signal having a predetermined phase relationship with respect to said first reference signal, said second reference signal then defining said phase stabilized signal.