## 3 Experimental Setup and Results & Discussion  
The proof-of-concept experimental system for the proposed DLFM-OTDR is shown in Figure 5. The system is implemented on an existing conventional OTDR development board without any hardware change. A 1625-nm multiple quantum well (MQW) Fabry-Perot (FP) laser diode was used as the optical source, which can only be turned on and off, and it was controlled by a Cyclone V series (Cyclone V 5 CGXFC6U19I7N; Altera) field-programmable gate array (FPGA) through a constant current source circuit. The waveform of the digital LFM signal is shown in Figure 6. In order to be able to generate D-LPM pulse sequences on conventional OTDR development board, the conventional OTDR development boards we use are developed by ourselves, and their signal transmission and signal receiving parts are not as good as commercial OTDRs. The bandwidth of the laser driver circuit on the conventional OTDR development board was only 25 MHz. To generate a relatively complete digital LFM signal, its sweep frequency range was only from 4 MHz to 8 MHz (\(B=4\) MHz), and the pulse width and period were 4 us and 1.2 ms, respectively. Direct modulation of laser current introduces some spurious phase modulation, in addition to intensity modulation. However, firstly, the pulse signal modulation frequency is from 4 M to 8 M, the modulation frequency is very low, and the generated spurious phase modulation is negligible. Secondly, the receiving end uses intensity detection, which is insensitive to phase modulation. Therefore, the spurious phase modulation has little effect on the system. The backscattered signal was directly detected and converted into a current signal through the avalanche photodiode (APD) (NR8360 JP-BC) with a 1 GHz bandwidth. After passing the transimpedance and the signal amplification circuits, the signal was sampled by an analog-to-digital converter (ADC) (AD9230) with a sampling rate of 100 MHz. The STFrFT for the acquired signal was implemented on the computer. For the experiment, two 50-km fibers made up the FUT with a length of 100 km. Figure 4: STFrFT with the Hamming window (time normalizes with respect to T). (**a**) Compressed narrow pulse by STFrFT and (**b**) zoom-in of the main lobe in (a) with a 3-dB width, which is equal to \(1.47/KT\), and a peak sidelobe ratio of approximately 41 dB. The average results of 16384 experiments are shown in Figure 7. By default, the STFrFT window is rectangular. Figure 7a illustrates the measurement result considering only the primary LFM signal in the backscattered signal, showing that the Fresnel reflection event in the FUT can be successfully measured only in this case. Three reflection peaks can be observed. The first and second peaks, respectively at 0 km and 50 km, are caused by the connector. The third peak, at 100 km, is the reflection peak at the fiber end. The spatial resolution, which is the FWHM of the mainlobe of the reflection peak, is 30 m, validating the theoretical spatial resolution of 26 m calculated by substituting \(n=1.446\) and \(B=4\) MHz into Equation (21). If a wide-bandwidth laser driver circuit generating a digital LFM signal with a broader sweep frequency range is adopted in DLFM-OTDR, the spatial resolution is expected to be further improved. Note, the fiber attenuation due to the Rayleigh scattering in the fiber cannot be measured when considering only the primary LFM signal. Since the Rayleigh scattering is a random process, the backward Rayleigh scattering signal at the receiver is a superposition of Rayleigh backscattered light at each point in the fiber. Therefore, the power of the backward Rayleigh scattering signal is an average value, which is mainly reflected by the direct current (DC) component of the backscattered signal. The blue curve in Figure 7b represents the measurement result taking into account both the primary LFM signal and the DC component, this DC component is obtained by a digital low-pass filter with a bandwidth of 1 MHz.