Linearizing optical frequency-sweep of a laser diode for FMCW reflectometry

• Published in: Journal of lightwave technology Vol. 14; no. 2; pp. 173 - 178
• Main Authors: Iiyama, K., Lu-Tang Wang, Ken-Ichi Hayashi
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.02.1996; Institute of Electrical and Electronics Engineers
• Subjects: Applied sciences; Diode lasers; Electronics; Exact sciences and technology; Frequency; Interference; Linearity; Optical amplifiers; Optical interferometry; Optical modulation; Optical waveguides; Reflectometry; Stimulated emission; Testing, measurement, noise and reliability; Frequency modulation; Continuous wave; Test; Frequency sweeping; Reflectometry; Optical waveguide; Injection laser; Experimental study; Optical frequency; Linearization
• ISSN: 0733-8724; 1558-2213
• DOI: 10.1109/50.482260

We propose and demonstrate a novel linearizing method of optical frequency-sweep of a laser diode for frequency-modulated continuous-wave (FMCW) 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 amplifier. The error signal from the lock-in amplifier is fed back to the modulating signal of the injection current of the laser. Thus, a phase-locked loop composed of optical and electric circuits can be established, and the beat signal frequency is locked to the frequency of the reference signal. The optical frequency of the laser diode is, therefore, excellently linearly swept in time. In order to experimentally confirm the linearity of the proposed method, we apply this frequency-swept laser diode to the FMCW reflectometry. Resultingly, the improvement of the linearity is estimated to be about 10 dB. And the theoretically limited spatial resolution of the FMCW reflectometry is achieved. The backscattered light in optical waveguide devices is measured by the FMCW reflectometry using the proposed light source, and the propagation loss of a single-mode glass waveguide is successfully evaluated.