Abstract
Optical coherent domain reflectometry (OCDR) can achieve a high spatial resolution that is independent of the bandwidth of the receiver, but the measurement range is usually very limited. Here we propose an external modulation OCDR system, in which a pair of linear frequency-modulated pulses generated by one modulator are employed as the probe pulse and the reference, respectively. The spatial resolution is determined by the frequency modulation range of the pulse, and the measurement speed is boosted by orders because the proposed technology can simultaneously diagnose a section of fiber with each pair of pulses, while only a single point can be accessed at a time in typical OCDR. In the demonstrational experiment, a measurement range of up to 50 km is achieved with a spatial resolution of 1.4 m and a measuring time of less than 30 s.
Highlights
The nominal frequency of the acousto-optic modulator (AOM1) is 200 MHz. It is driven by an arbitrary waveform generator (Tektronix, AFG3252C), which generates a frequency chirping radio frequency (RF) pulse with a time duration of 8 μs
The full width at half maximum (FWHM) of the frequency sweeping range is about 70 MHz, corresponding to a theoretical spatial resolution of 1.4 m in fiber according to Equation (2)
Fast Fourier transform is applied to the collected data in each measurement, and the power spectral density in the range of 77.5 MHz to 82.5 MHz is selected and normalized to compensate for the window effect, corresponding to an overlap time of no less than 7.75 μs between the reference and the Rayleigh backscattering (RBS)
Summary
Optical reflectometry technologies, including optical time domain reflectometry (OTDR) [1,2,3,4], optical frequency domain reflectometry (OFDR) [5,6,7,8,9], and optical coherent domain reflectometry (OCDR) [10,11,12,13,14,15,16,17,18,19,20], have been widely used in the monitoring of fiber link and distributed sensing applications. The OTDR measures the round-trip time of reflection and Rayleigh backscattering (RBS) of the probe pulse. It has a long measurement distance of up to hundreds of kilometers, while the spatial resolution is usually on the meter level. In the OFDR system, the round-trip time of RBS is mapped to the beat frequency because of the linearly sweeping light source. It is famous for a high spatial resolution of up to micrometer level, but the maximum measurement range is limited by the nonlinearity of frequency sweeping and phase noise
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