# A Fading Suppression Method for \(\Phi\)-OTDR Systems Based on Multi-Domain Multiplexing
## 3 Schematic of the Proposed Multi-Domain Multiplexing System  
In the previous section, the MDM principle was introduced. In this section, we describe a test experiment using an MDM \(\Phi\)-OTDR system, and the schematic diagram of it is shown in Figure 1.  
The principle of this system shown in Figure 1 is as follows. A narrow linewidth laser (NLL) emits polarization-maintaining continuous-wave (CW) laser light with a wavelength of 1550.12 nm, a linewidth of 3 kHz, and a power of 13.1 dBm. After passing through a 1 \(\times\) 2 polarization-maintaining optical coupler (OC1) with a splitting ratio of 90:10, 90% of the optical power enters a 1 \(\times\) 4 polarization-maintaining fiber coupler with the same splitting ratio and is injected into acousto-optic modulator (AOM) crystals with frequency shifts of 80 MHz, 100 MHz, 120 MHz, and 150 MHz as the probe light, respectively. The remaining 10% of the optical power enters a polarization diversity receiver (PDR) as the local light. The four AOM crystals are then combined using a 4 \(\times\) 1 polarization-maintaining fiber coupler with the same splitting ratio to form multifrequency probe pulse lights, which is manufactured by Nanjing Fiber Technology Co., Ltd. in Nanjing, China. These pulse lights are amplified by an erbium-doped fiber amplifier (EDFA) to compensate for insertion losses and enhance pulse energy. These amplified probe pulse lights enter the circulator (CIR) through port A, and then enter the fiber under test (FUT) through port B. The RBS signal from the FUT enters the polarization diversity receiver (PDR) with a bandwidth of 300 MHz through port C of the CIR. The PDR acts as an optical receiver, receiving the RBS and local light and outputting dual IF electrical signals with polarization states P (P state) and S (S state). The IF signals of the P state and the S state are filtered through Filter 1 and Filter 2. Filter 1 and Filter 2 are exactly the same. The center frequency  
Figure 1: Schematic of the proposed MDM system. of the filter is 115 MHz. The in-band flatness within the passband ranging from 67 MHz to 163 MHz is less than 0.8 dB, and the out-of-band rejection ratio at the frequencies of 50 MHz and 184 MHz exceeds 50 dB. Hence, the signals with frequencies of 80 MHz, 100 MHz, 120 MHz, and 150 MHz are within the passband, and the wide-spectrum out-of-band noise is suppressed to improve the signal-to-noise ratio of the intermediate-frequency signal, which is conducive to improving the accuracy of the demodulated phase. In Figure 2, the generation process of the multifrequency probe pulse light is illustrated further. A 10 MHz clock signal and pulse are generated by the DAQ and synchronized with the multifrequency modulation control module. As shown in Figure 2, a highly stable high-precision 10 MHz reference clock is obtained after the crystal oscillator clock passes through the phase-locked loop (PLL), and a high-precision timing pulse is obtained through frequency division. The high-precision clock signal output by the PLL is input into the carrier generation circuit, and the pulse signal is input into the pulse generation circuit. The pulse generation circuit counts according to the output clock of the phase-locked loop to generate a trigger signal and a delayed pulse. The carrier generation circuit generates continuous sinusoidal carrier signals with frequencies of 80 MHz, 100 MHz, 120 MHz, and 150 MHz according to this trigger signal. The carrier signal and the delayed pulse are mixed using a mixer to generate an electrical pulse signal in the radio frequency band, which is then amplified by a power amplifier to finally obtain an amplitude-modulated radio frequency (AM-RF) signal. The AM-RF signal is used to drive the acousto-optic modulators (AOMs) to modulate the continuous light wave into a probe pulse light with multiple frequency shifts. Through this modulation method, the data sampling clock of the DAQ is highly synchronized with the generation of the multifrequency modulation probe pulse light.