Digitally controlled chirped pulse laser for sub-terahertz range fiber structure interrogation
View Patent ↗The present disclosure provides a sweep velocity-locked laser pulse generator (SV-LLPG) controlled using a digital phase locked loop (DPLL) circuit. The SV-LLPG is utilized for the interrogation of sub-terahertz-range fiber structures for sensing applications that require real-time data collection with mm-level spatial resolution. A laser generates chirped laser pulses via injection current modulation and a DPLL circuit locks the optical frequency sweep velocity. A high-quality linearly chirped laser pulse with a frequency excursion of 117.69 GHz at optical communication bands using a distributed feedback laser is provided.
1. A pulsed laser source for use in an interrogation system for an optical fiber including at least one sensing structure, the pulsed laser source comprising:
a laser;
a laser driver circuit operating the laser at a constant sweep velocity;
a coupler splitting output of the laser into a sensing portion and a control portion;
a Mach-Zender Interferometer (MZI) receiving said control portion of the coupler output, said MZI having a fixed delay and outputting an analog beat signal;
a PhotoDiode (PD) receiving output from the MZI and generating an AC-coupled signal output;
an Automatic Gain Control (AGC) amplifier operative to adjust an amplitude of the AC-coupled signal output of the PD;
a voltage comparator receiving output from said AGC and converting said output into digital signals;
a Digital Reference Clock;
a Digital Phase Comparator receiving said digital signals from said AGC and from said Digital Reference Clock, and generating a digital phase error signal; and
a loop filter receiving said digital phase error signal and generating a time varying laser control signal provided to said laser driver circuit,
wherein the laser is modulated by the time varying signal through the laser driver to produce a sweep velocity-locked laser pulse.
2. The pulsed laser source of claim 1 wherein the tuning bandwidth is about 117 GHz.
3. The pulsed laser source of claim 1 wherein the sweep velocity is about 14 GHz/ms.
4. The pulsed laser source of claim 1 further comprising an isolator at an output of said laser.
5. The pulsed laser source of claim 1 wherein said coupler is a 90/10 coupler providing 10% of the laser output for the control loop portion.
6. An interrogation system for distributed measurements of an optical fiber comprising:
a sensing module including an optical fiber having at least one sensing structure; and
a sweep velocity-locked laser pulse generator (SV-LLPG) controlled with a digital phase-locked loop (DPLL) circuit having an output coupled said sensing module,
said (SV-LLPG) interrogating said optical fiber,
said sensing module further including a sensing path for detecting changes in reflections and interference patterns caused by physical changes in said optical fiber.
7. The interrogation system of claim 6 wherein said sensing structure comprises a continuous grating written along an entire length of said optical fiber.
8. The interrogation system of claim 6 wherein said sensing structure comprises at least one sub-terahertz-range fiber structure.
9. The interrogation system of claim 6 wherein said sensing structure comprises an intrinsic or modified Rayleigh backscatter profile.
10. The interrogation system of claim 6 wherein said sensing module comprises a circulator receiving an output from said laser pulse generator and having an interrogation path including said optical fiber, and further having parallel reference and sensing paths which are recombined in a downstream coupler.
11. The interrogation system of claim 10 wherein said sensing module further comprises a PhotoDiode (PD) receiving output from said downstream coupler, an analog to digital converter (ADC) receiving analog output from said PD and a Digital Signal Processor (DSP) receiving digital output from said ADC.
12. The interrogation system of claim 6 wherein said interrogator system comprises
a laser;
a laser driver circuit operating the laser at a constant sweep velocity;
a coupler splitting output of the laser into a sensing portion and a control portion;
a Mach-Zender Interferometer (MZI) receiving said control portion of the coupler output, said MZI having a fixed delay and outputting an analog beat signal;
a PhotoDiode (PD) receiving output from the MZI and generating an AC-coupled signal output;
an Automatic Gain Control (AGC) amplifier operative to adjust an amplitude of the AC-coupled signal output of the PD;
a voltage comparator receiving output from said AGC and converting said output into digital signals;
a Digital Reference Clock;
a Digital Phase Comparator receiving said digital signals from said AGC and from said Digital Reference Clock, and generating a digital phase error signal; and
a loop filter receiving said digital phase error signal and generating a time varying laser control signal provided to said laser driver circuit,
wherein the laser is modulated by the time varying signal through the laser driver to produce a sweep velocity-locked laser pulse.
13. The interrogation system device of claim 10 wherein said sensing module comprises a circulator receiving an output from said laser pulse generator and having an interrogation path including said optical fiber, and further having parallel reference and sensing paths which are recombined in a downstream coupler.
14. The interrogation system device of claim 13 wherein said sensing module further comprises a PhotoDiode (PD) receiving output from said downstream coupler, an analog to digital coverter (ADC) receiving analog output from said PD and a Digital Signal Processor (DSP) receiving digital output from said ADC.