Fig. 1. Experimental setup

Fig. 2. Measurement system for DFB frequency chirp

Fig. 3. Beat signals under different reference wavelengths. (a) Beat signal when reference wavelength λ=1548.973 nm; (b) beat signal when reference wavelength λ=1549.088 nm

Fig. 4. Measurement results of internal modulation chirp. (a) Spectrogram of beat signal when continuous wavelength λ=1548.973 nm; (b) spectrogram of beat signal when continuous wavelength λ=1549.088 nm; (c) pulse frequency change after stitching

Fig. 5. Detection signal patterns. (a) Detected signal of internally modulated direct detection type φ-OTDR system; (b) detected signal of traditional direct detection type φ-OTDR system

Fig. 6. Comparison of detection signals of two systems. (a) Original time domain signal of internally modulated direct detection type φ-OTDR system; (b) original time domain signal of traditional direct detection type φ-OTDR system; (c) first-order differential of disturbance signal of internally modulated direct detection type φ-OTDR system; (d) first-order differential of disturbance signal of traditional direct detection type φ-OTDR system

Fig. 7. Statistical distribution characteristics of disturbance signals of internally modulated direct detection type φ-OTDR system and traditional direct detection type φ-OTDR system and noise. Insert is fitting curve of SNR statistical distribution with time at disturbance position z0, and fitting curve is calculated according to equation (4) for every 10 minutes

Fig. 8. System with two channels of φ-OTDR and OTDR