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We present a distributed fiber sensor capable of discriminating between temperature and strain while performing low-noise, dynamic measurements. This was achieved by leveraging recent advances in Brillouin and Rayleigh based fiber sensors. In particular, we designed a hybrid sensor that combines a slope-assisted Brillouin optical time domain analysis system with a Rayleigh-scattering-based frequency scanning optical time domain reflectometry system. These sub-systems combine state-of-the-art sensitivity with the ability to perform both dynamic and quasi-static measurements. This enabled a hybrid system capable of temperature/strain discrimination with a quasi-static temperature resolution of 16 m°C and a strain resolution of 140 nɛ along 500 m of single mode fiber with 5 m spatial resolution. In contrast to previously reported techniques, this approach also enabled dynamic measurements with a bandwidth of 1.7 kHz and temperature (strain) noise spectral density of 0.54 m°C/√Hz (4.5 nɛ/√Hz) while temperature/strain cross-sensitivity was suppressed by at least 25 dB. This represents a dramatic improvement in measurement speed and sensitivity compared with existing techniques capable of temperature/strain discrimination in standard single mode fiber.

We demonstrate a distributed multimode fiber Φ-OTDR sensor using holographic detection to record the backscattered speckle field. The sensor detects frequencies up to 400 Hz over 2 km with a sensing aperture of 20 m.

We present a -OTDR sensor that recovers quantitative strain information from the Rayleigh backscattering ϕ amplitude of a sequence of frequency-shifted pulses. This approach allows for higher launch power, enabling low-noise ( 1.3 p ε / Hz ) dynamic strain sensing.

We present a distributed acoustic sensor using micro-machined fiber with localized reflectors. The enhanced reflectors enable a phase noise of −91.5 dB (re rad2/Hz), 20 dB below the noise achieved using the Rayleigh backscattered light.

While the vast majority of Φ-OTDR sensors use single mode fiber, multimode fiber is also widely deployed by the telecom industry. From a sensor design perspective, multimode fiber also offers advantages compared with single mode fiber, such as higher nonlinear thresholds and immunity to interference fading. However, most attempts to perform distributed strain sensing in a multimode fiber rely on interrogation systems designed for single mode fiber. As a result, these systems discard most of the backscattered light by coupling the multimode fiber under test to a single mode fiber based receiver. In this work, we present a technique that combines a high-speed camera with a time-gated local oscillator to construct a distributed multimode fiber sensor capable of using the entire backscattered speckle field. We demonstrate quantitative, fully distributed strain sensing across a 2 km multimode fiber with a spatial resolution of 20 m, a bandwidth of 400 Hz, and a noise floor of −61 dB re rad2/Hz (4.9 pε/√Hz). The same system can be electronically reconfigured to probe any single sensor position with a bandwidth of up to 20 kHz and a noise floor of −86 dB re rad2/Hz (0.27 pε/√Hz).

We report an amplitude-measuring Rayleigh-based sensor that uses a series of frequency-shifted pulses to extract quantitative distributed strain measurements. By using frequency multiplexing, we are able to inject a train of 10 pulses into the fiber at once. This allows us to use a higher average input power than standard phase-sensitive optical time domain reflectometry systems, improving the sensitivity. The sensor recovers the strain by tracking the time-dependent amplitude of the Rayleigh backscattered light from all 10 pulses. This approach enables a sensor with a noise floor of 1.5 p ε / √ H z over 10 km of fiber with 12 m spatial resolution, a 5 kHz bandwidth, and a dynamic range of 80 dB at 1 kHz. The sensor exhibits a high degree of linearity and is immune to interference fading.

We present a low-noise distributed acoustic sensor using enhanced backscattering fiber with a series of localized reflectors. The point reflectors were inscribed in a standard telecom fiber in a fully automated system by focusing an ultra-fast laser through the fiber cladding. The inscribed reflectors provided a reflectance of −53 dB, significantly higher than the Rayleigh backscattering level of −70 dB/m, despite adding only 0.01 dB of loss per 100 reflection points. We constructed a coherent φ-OTDR system using a double-pulse architecture to probe the enhanced backscattering fiber. Using this system, we found that the point reflectors enabled an average phase noise of −91 dB (re rad2/Hz), 20 dB lower than sensors formed using Rayleigh backscattering in the same fiber. The sensors are immune to interference fading, exhibit a high degree of linearity, and demonstrate excellent non-local signal suppression (>50 dB). This work illustrates the potential for low-cost enhanced backscattering fiber to enable low-noise, long-range distributed acoustic sensing.

We report an amplitude-measuring multimode fiber sensor capable of making quantitative strain measurements and extracting the algebraic sign of the strain. The Rayleigh-based sensor probes the fiber with pulses of alternating optical frequency and records the backscattered speckle patterns on a high-speed camera. We show that measuring the change in the speckle pattern induced by a change in optical frequency provides a form of in situ calibration, enabling the sensor to recover the magnitude and algebraic sign of the strain. The sensor, which can be positioned anywhere along 2 km of fiber, has a linear strain response, a 10 kHz bandwidth, and a strain noise of 10.2 p ε / √ H z .