EXCELLENT LINEARLY FREQUENCY-SWEPT LIGHT-SOURCE FOR SENSING SYSTEM UTILIZING FMCW TECHNIQUE

We propose and demonstrate an excellent linearly frequency-swept laser diode (LD) for sensing system utilizing frequency-modulated continuou s-wave (FMCW) technique. In order to linearly sweep the optical freque ncy, we adopt a reference interferometer and an electric phase compara tor. The interference beat signal of the reference interferometer is p hase-compared with an external reference rectangular signal having a f ixed frequency near the interference beat signal frequency by a lock-i n amplifier. The error signal from the lock-in amplifier is fed back t o the modulating signal of the injection current of the LD. Thus, a ph ase-locked loop composed of optical and electric circuits can be estab lished, and the beat signal frequency is locked to the frequency of th e reference signal. The optical frequency of the LD is, therefore, exc ellently linearly swept in time. In order to experimentally confirm th e linearlity of the proposed method, we apply this light source to the FMCW reflectometry. Resultingly, the improvement of the linearity is estimated to be about 10 dB. And the theoretically limited spatial res olution of the FMCW reflectometry is achieved.