LINEARIZING OPTICAL FREQUENCY-SWEEP OF A LASER-DIODE FOR FMCW REFLECTOMETRY

We propose and demonstrate a novel linearizing method of optical frequ ency sweep of a laser diode for frequency-modulated continuous-wave (F MCW) reflectometry. In order to linearly sweep the optical frequency, we adopt a reference interferometer and an electric phase comparator, The interference beat signal of the reference interferometer is phase- compared with an external reference rectangular signal having a fixed frequency near the interference beat signal frequency by a lock-in amp lifier, The error signal from the lock-in amplifier is fed back to the modulating signal of the injection current of the laser, Thus, a phas e-locked loop composed of optical and electric circuits can be establi shed, and the beat signal frequency is locked to the frequency of the reference signal, The optical frequency of the laser diode is, therefo re, excellently linearly swept in time. In order to experimentally con firm the linearity of the proposed method, we apply this frequency-swe pt laser diode to the FMCW reflectometry, Resultingly, the improvement of the linearity is estimated to be about 10 dB, And the theoreticall y limited spatial resolution of the FMCW reflectometry Is achieved, Th e backscattered light in optical waveguide devices is measured by the FMCW reflectometry using the proposed light source, and the propagatio n loss of a single-mode glass waveguide is successfully evaluated.