Optical frequency domain reflectometry (OFDR) systems feature high precision, high resolution, large range, strong real-time performance, and high signal-to-noise ratio. Therefore, OFDR technology has broad application prospects in aviation, transportation, and communication. After decades of this technological development, the bottleneck for its further development is that it is difficult to realize the sweep light source and optimize the signal processing, and the performance, precision, and resolution of the system are directly affected by the frequency-modulated linearity of the light source. At present, the light source employed in the system is a mechanically tuned diode laser with a wide frequency-modulated range and a narrow linewidth, but it is difficult to reduce the cost and extend the service life. The direct current modulation of distributed feedback semiconductor (DFB) lasers is a potential high-quality light source for OFDR systems because of its low cost and simple frequency modulation, but the phase noise and poor linearity of frequency modulation must be solved. Thus, we discuss the frequency modulation nonlinearity of light source in the OFDR system, such as the location difference of the sensor unit, low sensor precision, narrow sensor range of the sensor, and poor system adaptability, and propose an open-loop correction method combined with an optoelectronic phase-locked loop closed-loop correction for DFB laser.

The method of open-loop correction combined with photoelectric phase-locked loop closed-loop correction for DFB lasers is adopted to control the continuous, fast, and large-range frequency scanning linearization of DFB lasers. As a result, it becomes a high-quality light source for the OFDR system and improves the OFDR resolution. First, based on the direct current modulation characteristics of DFB lasers and the idea of iterative open-loop correction, the DFB laser is initially corrected, and the correction effect is evaluated by the time-frequency curve of the beat signal and the FFT power spectrum curve. Then, the closed-loop correction method is introduced, and the closed-loop correction is realized by constructing the corresponding oscillator, loop filter, and phase discriminator structure in the photoelectric phase-locked loop. The correction results are evaluated, and the fixed fiber length is measured for the sweep frequency nonlinearity correction system and the uncorrected DFB laser to show the correction effect.

For the uncorrected DFB laser, the beat signal center frequency is 238.8 kHz, the 3 dB bandwidth is 39.5376 kHz, the power is -59.18 dBV, and the nonlinearity of the light source is 16.55% (Fig. 6). After the open-loop correction, the sweep nonlinearity of the DFB laser is reduced to 174 kHz, the bandwidth of 3 dB is reduced to 6 kHz, the power is -48.22 dBV, and the light source nonlinearity is reduced to 3.45% (Fig. 7). Meanwhile, the central frequency of the beat frequency signal is 169.4 kHz, the 3 dB bandwidth is reduced to 132.169 Hz, and the nonlinearity is reduced to 0.078% after the closed-loop correction of the photoelectric phase-locked loop, with the power of -35.09 dBV (Fig. 8). The proposed method has a good correction effect on the sweep nonlinearity of DFB lasers. Additionally, in the experiment of measuring the optical fiber length, the maximum error between the measured and true values of the system is 0.3006 m in the range of 0-15 m for the uncorrected light source (Table 1), which continues to increase the optical fiber length. The beat signal power is very close to the noise power, and the low signal-to-noise ratio makes the beat signal cannot be distinguished. The maximum error between the measured and true values is 3.79 mm (Table 2). The system shows a longer measuring range and a smaller error, and the corrected system exhibits stronger stability with a maximum standard deviation of 112.2 μm for repeatability system measurements over a 50 m probe range (Fig. 11).

We put forward the method of open-loop correction combined with photoelectric phase-locked loop correction for DFB lasers to solve the frequency modulation nonlinearity of DFB laser source in the OFDR system, thus improving the resolution of OFDR systems. The experiment shows that the nonlinearity of the corrected light source is reduced to 0.078%, and the central frequency power of the beat frequency signal is increased by 21.1 dB compared with the uncorrected one. The maximum error of 3.79 mm is achieved in the detection range of 50 m and the maximum standard deviation of repeatability measurements is 112.2 μm. The maximum detection distance of 50 m in the final experiment does not represent the detection limit of OFDR technology based on direct current modulation of DFB lasers. The important indexes closely related to the maximum detection distance are the frequency modulation nonlinearity and the light source linewidth. The experiment proves that the maximum detection distance of the system increases with the reduced frequency modulation nonlinearity of the light source, and the light source linewidth directly affects its coherence length. Therefore, the further reduction of frequency modulation nonlinearity helps further increase the maximum detection distance of the system by a more accurate open-loop correction method with narrower linewidth and higher-power DFB lasers, improving the resolution and detection distance of the OFDR system. In the future, the resolution and detection distance of the OFDR system can be further improved by a more accurate open-loop correction method or DFB lasers with narrower linewidth and higher power.