Low noise and narrow linewidth external cavity semiconductor laser for coherent frequency and time domain reflectometry

A method for coherent frequency and time domain reflectometry using an external cavity laser, comprising the steps of:

• generating a coherent light beam at an origination point;
• phase modulating said coherent light beam and linearly varying an optical frequency of the laser;
• selecting and stabilizing one longitudinal mode of the laser cavity for lasing;
• varying the length of the laser cavity and thereby tuning the optical frequency of said selected longitudinal mode to match the transmission frequency of an etalon by maintaining maximum power intensity;
• narrowing the linewidth and reducing the noise of the laser;
• and
• communicating and directing a portion of said coherent light beam back to said origination point, whereby a reference portion of said coherent light beam is coherently mixed with a back scattered signal portion of said light beam to yield coherent frequency and time domain reflectometry measurement or detection.

A semiconductor laser having an external cavity used for coherent frequency and time domain reflectometry comprising:

• a light source generating and emitting a coherent light beam;
• a phase modulator receiving said coherent light beam and linearly varying an optical frequency of the laser;
• an etalon receiving and selectively transmitting said coherent light beam thereby selecting and stabilizing one longitudinal mode of the laser external cavity for lasing and narrowing the linewidth and reducing the noise thereof;
• means for lengthening and shortening a total path length of said coherent light beam and thereby tuning the optical frequency of said selected longitudinal mode for maintaining maximum power intensity and stabilizing the selected longitudinal mode;
• means for communicating and directing a portion of said coherent light beam back to said coherent light source;
• and
• said coherent light source amplifying said returned coherent light beam and further emitting said amplified coherent light beam in the same direction, whereby a reference portion of said coherent light beam is coherently mixed with a back scattered signal portion of said light beam to yield coherent frequency and time domain reflectometry measurement or detection.