An optical reflectometer is a powerful optical instrument, and it is widely adopted in distributed fiber optic sensors for loss location and temperature and stress measurement. Usually, optical reflectometers include optical time domain reflectometers (OTDRs) and optical frequency domain reflectometers (OFDRs). OFDR uses a sweeping laser as the light source. Compared with OTDR, it has higher spatial resolution and larger dynamic range. However, when the detection length is longer than the coherence length of the light source, the phase noise will degrade the system performance greatly. Meanwhile, a high spatial resolution requires a wide range of sweeps of the probe light. Because of the high cost of narrow linewidth lasers and modulators, OFDR is difficult to be commercialized at present. Therefore, most OFDR studies focus on phase noise and sweep ranges. There are already many ways to suppress OFDR phase noise, such as narrow linewidth lasers and coding and phase compensation algorithms. Time-gated digital optical frequency domain reflectometry (TGD-OFDR) is also a method proposed in recent years to suppress phase noise and improve detection length. The frequency sweep method of OFDR is generally divided into external modulation and internal modulation. External modulation has better sweep performance and is easier to be controlled, but the sweep range is limited. The internal modulation has a large sweep range, but it has problems such as nonlinearity of the sweep frequency and linewidth broadening, and compensation or correction is often required through other means. Therefore, designing low-cost, long-range, and high-resolution TGD-OFDR systems is the main work in this paper.

The TGD-OFDR system has the characteristics of a simple structure, which can effectively overcome phase noise and achieve long-distance detection. By taking the advantages of low cost, easy integration, and high sweep range of distributed feedback (DFB) lasers, an internally modulated DFB laser is selected as the swept frequency light source, and a TGD-OFDR system based on the DFB laser is designed. Firstly, by analyzing the modulation method and frequency sweep characteristics of DFB lasers, a pre-distortion system based on the Mach-Zehnder interferometer (MZI) and Hilbert variation of DFB laser sweep nonlinearity is designed. The system is a closed-loop feedback system and is divided into an optical path part and a circuit part. The optical path part is an MZI with a delay path, while the circuit part includes a computer-controlled acquisition card and an arbitrary waveform generator. In addition, the calculation of the Hilbert variation and proportion integral differential (PID) algorithms realizes the pre-distortion processing of the swept frequency. Secondly, the current-modulated DFB laser is used as the swept frequency light source, and it is proposed for demodulation. A frequency-stabilized laser is added as a local detection light. At the same time, due to the uncertainty of the frequency sweep rate of DFB lasers and the system characteristics of TGD-OFDR, a photodetector is added to receive the analog reference signal, and the analog reference signal is used as a matching filter to directly perform the cross-correlation algorithm to obtain the trace curve.

In the demonstration reflectometry experiment, the duration of the pulse τ_p is set to 6 μs by an acoustic optical modulator. The pulse chirp range is about 1.1 GHz, corresponding to a theoretical spatial resolution of 9 cm. The local reference laser is a tunable single-frequency diode laser. A high speed analog to digital device (NI 5185) acquisition card with a sampling rate of 6.25 GSa/s and a resolution of 8 bit is used to sample the beat signal on the balanced photo detector. The fiber under test (FUT) is composed of three coils of single-mode fibers with a length of 24.7, 24.8, and 24.7 km, respectively. The measured trace is shown in Fig. 9(a). The dynamic range is measured to be 23.3 dB, and 2100 measurements at seven different laser wavelengths are carried out. The dynamic range can be further improved if the laser's power is large. The spatial resolution at the start of the FUT is 10 cm, as shown in Fig. 9(b), which is very close to the theoretical spatial resolution. The spatial resolution at the far end of the FUT is about 18 cm, as illustrated in Fig. 9(c). The spatial resolution degeneration is mainly caused by the phase jitter of laser2 and the accumulation of phase noise, which is much larger than the linewidth of the DFB laser. Although the spatial resolution degenerates with a longer distance, it is still the best spatial resolution ever reported for the OFDR system with a measurement range of over 60 km.

In this paper, an internally modulated DFB laser is used as the probe in the TGD-OFDR system. A frequency modulation range of up to 1.1 GHz with a fast modulation rate is achieved by using a current-modulated DFB laser, and a narrow linewidth diode laser is employed as the stable oscillator. A spatial resolution of 18 cm is realized over a fiber link of 74 km, which is believed to be the best resolution ever reported for the OFDR system of over 60 km. The system performance can be further improved by more precise pre-distortion algorithms, more stable oscillators, and stronger light source power.