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

Abstract

Abstract-We propose and demonstrate a novel linearizing method of optical frequency-sweep of a laser diode for frequencymodulated 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 lockin 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.

Precise control of broadband frequency chirps using optoelectronic feedback

“…However, the bandwidth and the speed of demonstrated linear frequency sweeps have been limited by the inherent non-linearity of the frequency modulation response of the SCL vs. the injection current, especially at high speeds. A feedback system to overcome this nonlinearity using a fiber interferometer and a lock-in technique has been reported [8]; however the rate of the frequency sweep was limited to about 100 GHz in 10 ms.…”

Linearizing optical frequency sweep of a laser diode by injection current modulation with a rectangular-signal-superimposed triangular signal for FMCW reflectometry

“…The first method entails feeding back the error signal generated by comparing the phase of the interference signal of the auxiliary interferometer with that of the external electrical signal to the injection current of the laser diode [79]. The second method consists of modifying the modulation waveform of the laser diode by using a digital filter whose frequency response is given by the inverse of the frequency modulation response of the laser diode [10]. In this paper, we propose a simple method for linearizing the optical frequency sweep of a laser diode by modulating the injection current with a triangular signal on which a rectangular signal, synchronized with the modulating triangular signal, is superimposed.…”

Coherence enhancement of a chirped DFB laser for frequency-modulated continuous-wave reflectometry using a composite feedback loop

“…In Ref. 13, an experiment based on the servo-loop control method using self-heterodyning delay interferometers [13][14][15][16][17][18][19] is carried out for nonlinearity frequency error correction and frequency stabilization in low Fourier frequencies. However, the loop bandwidth is limited by the long fiber delay line in the interferometer and slow response time of the acousto-optic modulator, in which case the loop is not able to suppress broadband frequency noise.…”