Many applications require the use of narrowband optical filters to separate two signals of closely spaced frequencies. The filter's performance can be determined by its throughput and rejection. For weak signals, the filter should provide both high throughput and high rejection of the adjacent signals. The development of a double-pass configured all-fiber Mach-Zehnder interferometric optical filter satisfies these requirements. The configuration is theoretically compared with a single-pass all-fiber Mach-Zehnder and a bulk Fabry-Perot interferometer and shown to have superior performance for certain applications.
The temperature resolution of a fiber-optic distributed temperature sensor based on taking the ratio of the temperature sensitive backscattered spontaneous Brillouin signal to the corresponding Rayleigh signal depends on the optical signal-to-noise of the receiver system and the amplitude fluctuations on the Rayleigh signal. The amplitude fluctuations or coherent Rayleigh noise were investigated experimentally as a function of detection bandwidth, source bandwidth and spatial resolution and showed good agreement with theory.
We report on the use of a cost-effective erbium-doped based optical preamplifier in a single-ended distributed fiber-optic sensor to improve receiver sensitivity and hence measurement range. The improved accuracy of spontaneous Brillouin measurements was demonstrated in a 23km sensor and was limited by amplified spontaneous emission from the preamplifier, which was verified theoretically. Reduction of amplified spontaneous emission was achieved with a 0.37nm in-fiber Bragg grating. A signal-to-noise improvement of 17dB was achieved and supported by theory, which translates to approximately 40km range improvement for a single-mode sensing fiber having losses of 0.2dB/km at 1550nm.
An optical preamplifier system consisting of a 27dB-gain erbium-doped fibre amplifier, a three-port circulator and a 47GHz bandwidth in-fibre grating has improved receiver sensitivity and consequently sensor range of a spontaneous Brillouin intensity based single-ended fiber-optic distributed temperature sensor. Optical signal-to-noise has improved by 17dB. Operating at 1533nm, a sensor with a range of 23km, spatial resolution of 1.8m, measurement time of 9 minutes and temperature resolution of 6K was implemented. The temperature resolution was attributed to contamination of the backscattered spontaneous Brillouin signal by the residual Rayleigh signal after optical filtering. The optical signal-to-noise is supported theoretically by considering asscociated noise sources.
When a pulse of light is launched down a single-mode optical fiber, with a peak power below the threshold for nonlinear effects, backscattered Rayleigh and spontaneous Brillouin signals are generated. The Brillouin signal is temperature sensitive while the Rayleigh signal shows comparatively negligible sensitivity but serves as a backscattered signature for fiber attenuation and loss mechanisms. Ratioing one signal to the other extracts the temperature information. Optical separation of the Rayleigh and Brillouin signals of our sensor requires a source with bandwidth less than 90 pm. However, the narrowband Rayleigh signal generated is noisy due to coherent Rayleigh noise. A broader source generates a Rayleigh signal with reduced coherent Rayleigh noise. A Q-switched erbium-doped fiber-optic laser containing a fiber-optic switch in the laser cavity, which allows switching and selection of a narrow (<90 pm) or broad (~3 nm) bandwidth output coupler has been developed. Periodic switching should allow virtually simultaneous capturing of Brillouin and broadband Rayleigh signals thus ensuring that the spontaneous Brillouin signal is correctly referenced even if fiber attenuation, splice or bend losses should vary during extended data collection cycles.
KEYWORDS: Signal to noise ratio, Signal detection, Fiber optics sensors, Sensors, Temperature sensors, Receivers, Signal attenuation, Optical amplifiers, Temperature metrology, Electronics
A fibre-based optical preamplifier system has been used to improve the signal-to-noise capabilities of a Brillouin-based fibre-optic distributed temperature sensor. A 17dB optical signal-to-noise improvement in a 23km sensor is demonstrated.
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