## 2 Integration of Fiber Communication System and BOTDR Sensor System  
The experimental configuration for the integrating BOTDR sensor system and fiber communication system is shown in Figure 1. The BOTDR sensor system is illustrated in a red dashed box of Figure 1. A distributed feedback (DFB) laser at a 1546.12 nm wavelength and 100 kHz linewidth is used. Thereafter using a 50/50 coupler, the signal is split into two pathways where the upper path signal is used for the optical pulses and the lower path is uses for the local oscillator (LO). The electrical pulses (generated from the pulse generator) are modulated into optical pulses using a dual-drive Mach-Zehnder modulator (DD-MZM). Using the Erbium-doped fiber amplifier (EDFA), the signal is amplified to an optimized power level. The amplified spontaneous emission (ASE) filter (pass-band: 1550 \(\pm\) 5 nm) is employed to exclude the ASE noise generated from the EDFA. Using MZM 1, the LO signal is modulated by a 10 GHz frequency, which is upshifted and downshifted by 10 GHz from the pump signal. The polarization noise is the dominant noise source in Brillouin based distributed fiber sensor systems, therefore we used a simple and low-cost passive depolarizer to reduce the polarization noise [16]. The backscattered signal from the coupler is connected to the balanced photodetector(PDB470C, 400 MHz bandwidth) and analyzed using an electrical spectrum analyzer (ESA) in zero span mode. The experimental setup of the fiber communication system transmitter and receiver is illustrated in Figure 1 (showed in blue dashed box). A DFB laser diode provides a continuous wave (CW) at 1550 nm wavelength with a temperature controller, to maintain a constant laser temperature at 25 \({}^{\circ}\)C. The laser emission is then externally modulated using an MZM (Photoline, MXAN-LN-20). The polarization controller (PC) is used before the MZM to attain maximum power at the MZM output. In this configuration, the data signal is generated by a vector signal generator (Agilent-E4438C) with three modulation formats of QPSK, 16-QAM and 64-QAM with OFDM signal independently. The vector signal generator produces a real-time OFDM signal with a carrier frequency of 2.6 GHz and the channel bandwidth of 10 MHz via Agilent Signal Studio\({}^{\text{TM}}\) software [17]. After the MZM, the signal is then amplified to the required launch power using an EDFA and then transmitted over a 25 km single-mode silica fiber link. At the receiver end, the RF signal is detected by a PD (Thorlabs, PDA8 GS) and amplified using a low noise RF amplifier (LNA, PE15A1008). Thereafter, the detected signal is analyzed using an electrical spectrum analyzer (Agilent-9020A-MXA), which is automated using Agilent 89601B VSA software. Using the VSA software, the EVM of detected symbols is estimated [18]. The system parameters used in the experiments are defined in Table 1.  
Since the BOTDR system requires only single end access of the fiber, the complete BOTDR sensor system is located at one end, as the input pump signal and backscattered signal operate at the same end of the fiber. The pulse width was set to 50 ns, corresponding to 5 m spatial resolution and the pulse period was 255 \(\upmu\)s. The communication system transmitter was composed of a QPSK, 16-QAM and 64-QAM independently with bit rates of 16, 33 and 50 Mbps, respectively. It is significant to employ an isolator after the communication system transmitter; therefore, the signal reflections do not influence the data transmitter operation. The data signal and the sensor signal are simultaneously sent through a multiplexer and then to a 25 km single-mode silica fiber. At the receiver, a band-pass filter (BPF) is used to eliminate the sensor signal and extract the communication data signal. In the experiment, we employ a 1550 \(\pm\) 2 nm polarization maintaining BPF (AFR's-1550 nm bandpass filter) with a low insertion loss of 1 dB and maximum optical power handling of 300 mW. Therefore, data detection at the receiver will not be influenced by the sensor system signal.