Coherent Noise Reduction in High Visibility Phase-Sensitive Optical Time Domain Reflectometer for Distributed Sensing of Ultrasonic Waves

Abstract

Abstract-Phase-sensitive optical time domain reflectometry ( OTDR ) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. Up to now, OTDRs have been used mostly for the measurement of sub-kHz vibrations, normally in the context of intrusion sensing. In this work, the authors present an experimental and theoretical description of a high-visibility OTDR and its performance when used for ultrasonic vibration measurements. The use of a semiconductor optical amplifier (SOA) in the setup allows to suppress coherent noise and also to improve the spectral response of the pump pulses. These two advantages greatly decrease the detected intra-band noise thus allowing frequency measurements in the limits set by the time of flight of the light pulses while maintaining the simplicity of the scheme, as no post-processing, extremely high coherence lasers or coherent detection methods are required. The sensor was able to measure vibrations of up to 39.5 kHz with a resolution of 5 m over a range which could go up to 1.25 km. This is the first time to our knowledge that a fully distributed measurement of ultrasonic waves was achieved. The statistical behavior of the system was also described theoretically and characterized experimentally.

Chirped-Pulse Phase-Sensitive Reflectometer Assisted by First-Order Raman Amplification

“…In order to eliminate the amplified spontaneous emission (ASE) added by the EDFA, a dense wavelength division multiplexer (DWDM) with a spectral width of 100 GHz is used. The remained ASE centered at the pulse frequency (distributed temporally outside the pulse), induces intra-band noise, which generally is an important limitation in phase-sensitive reflectometers [22]. To avoid this, in addition to the optical filter, the pulse is temporally gated using an optical switch with rise/fall times of 100 ns and a typical ER of 25 dB.…”

“…The PZT was fed with a sinusoidal signal whose frequency was linearly modulated from 70 Hz to 150 Hz with a period of 10 s, thus applying a controlled vibration in the perturbed fiber section. The optical pulses were injected into the fiber with a repetition rate of 1 kHz, allowing us to measure up to 500 Hz vibrations, which is very close to the limit imposed by the pulse time of flight (700 Hz) [22]. The optical pulse length is 100 ns, its spectral content is 630 MHz and the traces were digitized with 40 GSps sampling rate.…”

Harmful intrusion detection algorithm of optical fiber pre-warning system based on correlation of orthogonal polarization signals

“…It can be used to detect concurrent intrusions with every small resolution cell. Fixed threshold detection is adopted in Φ-OTDR initially, Fukun BI et al: Harmful Intrusion Detection Algorithm of Optical Fiber Pre-Warning System Based on Correlation of Orthogonal Polarization Signals 227 but its performance degrades dramatically in complicated application environment [7][8][9][10][11]. Furthermore, the constant false alarm rate (CFAR) methods are introduced to detect the intrusion signal of optic fiber [12][13][14][15][16].…”

Chirped-pulse phase-sensitive optical time-domain reflectometry

“…Distributed optical fiber sensing using phase sensitive optical time domain reflectometry (ϕOTDR) [1] is gaining increasing attention due to its potential application in pipeline surveillance and perimeter protection [2]. In ϕOTDR a pulse of highly coherent light is injected into a conventional single-mode fiber and the light reflected from different scattering centers interferes coherently to produce the detected optical power trace [1].…”