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In-line short cavity Fabry-Perot strain sensor for quasi distributed measurement utilizing standard OTDR

This paper presents an in-line, short cavity Fabry-Perot fiber optic strain sensor. A short air cavity inside a single-mode fiber is created by the fusion splicing of appropriately micro machined fiber tips. A precise tuning of the cavity length is introduced and used for the setting of the sensor static characteristics within the quasi-linear range around a quadrature point, which significantly simplifies signal processing. Sensor insertion losses achieved by short cavity design and optimized fusion splicing proved to be below 1 dB. Low insertion loss allows for effective cascading of the proposed strain sensors into a quasi-distributed sensor array. A practical 10-point quasi-distributed strain sensor array was demonstrated in practice, where each in-line sensor was tuned to the same operating point in the static characteristics, thus allowing for simple interrogation of the sensor array by using standard telecommunication OTDR. In addition, precise tuning of the short cavity Fabry Perot sensor was applied for an effective compensation of temperature-induced strain errors and for an increase in the unambiguous measuring range, while improving the overall linearity of the sensor system.

Multimode all-fiber quasi-distributed refractometer sensor array and cross-talk mitigation

The analysis and design of a quasi-distributed multimode fiber refractometer array is presented. The main challenge in the design of a practical quasi-distributed sensor array proved to be in mitigation of otherwise pronounced cross-talk effects among the individual sensors in the network. The cross-talk effects originate from mode filtering properties and the strong mode excitation dependence on the multimode refractometer sensors that constitute the array. The introduction of mode conditioning based on fiber mode filters and mode mixers effectively reduced the cross talk to a negligible level while providing the desired sensor response at acceptable collateral losses to the network. A comprehensive experimental analysis was carried out to provide detailed insight into the multimode sensor array behavior and to obtain data necessary for an overall and effective network design.

All-fiber quasi-distributed polarimetric temperature sensor

This paper presents an all-fiber design of a quasi-distributed polarimetric temperature sensor array that utilizes commercially available single polarization and high birefringence fibers. The modulation depth of temperature induced loss and the operational temperature range of individual sensors in the network are set by the rotational alignment of fibers before fusion splicing and through fine adjustment of the sensing fiber lengths. A practical sensor network was built with sensors that operated in the temperature range from 0 to 100 C. Individual sensors in the network generated temperature dependent loss that changed proportionally from 0.9 to 1.8 dB. With current standard telecommunication OTDRs, more than 20 prototype sensors could be interrogated.

Quasi-distributed long-gauge fiber optic sensor system

This paper presents a quasi-distributed, long-gauge, sensor system for measurement optical path length variation. This system can be directly applied to long gauge strain and/or temperature sensing. The proposed sensor system is comprised of sensing fiber, which is divided into the sensors segments separated by semi reflective mirrors made out of standard optical connectors. Short duration radio-frequency modulated optical bursts are launched into the sensing fiber and phase differences among individual reflected bursts are measured to determine the optical path-length variations among neighboring mirrors. Twenty sensing fiber segments were successfully addressed by a single-signal processor, while relying on standard telecommunication PIN diode, and a Fabry Perot laser diode. The resolution of a fiber-length variation better than 5 m was demonstrated in practice. Since the long sections of fiber can be employed for constructing individual sensors within the sensors array, a microstrain resolution can be achieved in practice. The drift of the sensors system can be predominantly attributed to the temperature sensitivity of the electronic components, which proved to be below 20 m/C. The entire system relies on simple and widely-used components that are low-cost.

All-fiber, low-cost single-point and quasi-distributed evanescent field temperature sensors with extended temperature measurement range, based on standard telecommunication graded index fibers

Fiber-optic single-point and quasi-distributed evanescent temperature sensors recoated with a blend of poly(methyl methacrylate) and poly(vinylidene fluoride) are proposed. Solid cladding enables the construction of small-size, low-cost, relatively wide-range and fast-response temperature sensors. The diameter of the sensor does not exceed the dimensions of the original optical fiber, while the response time of the sensor is 7:4ms. Different mass ratios of polymers in the blend enable fine tuning of the applied claddings refractive index. This allows the construction of sensors for different temperature ranges, while the application of all-silica graded-index multimode fibers enables the construction of quasidistributed sensor systems with considerably reduced cross talk.

Optical fiber for dispersion addressing

This letter presents a fully distributed forward propagating system, suitable for use with microbend sensors. The principle relies on selected mode launch in specially designed multimode fiber where a short pulse is launched into the fundamental mode. In the presence of microbend disturbance located down the sensing fiber, light couples from the fundamental to higher-order modes that propagate at different group velocity than the fundamental mode. The position of the disturbance is determined by the time delay between the pulse carried by the fundamental mode and by the pulse carried by higher-order modes. The group velocity difference is maximized by proper construction of the refractive index profile of the proposed fiber. Experimentally produced fibers exhibited difference of group velocities in ranges over 1%. This allows for easy reconstruction of position and amplitude of microbend deformations located down the sensing fiber.

A fiber-optic system for the quasi-distributed detection of liquids

This letter presents a multiplexed fiber-optic sensor array suitable for detection of liquid intrusions, spills, and leaks. The proposed detection systems is based on active heating and observation of temperature changes in short sections of optically absorbing (vanadium-doped) optical fibers that were interconnected into a sensing array by a standard single-mode optical fiber. The heating of the vanadium-doped fiber sections was achieved by application of a common 980-nm pump laser source. The associated signal processing and integration utilizes a simple, custom-designed optical time domain reflectometry that relies on only a few low-cost standard telecom components.