An approach to achieve simultaneous measurement of refractive index and temperature is proposed by using a Mach-Zehnder interferometer realized on tapered single-mode optical fiber. The attenuation peak wavelength of the interference with specific order in the transmission spectrum shifts with changes in the environmental refractive index and temperature. By utilizing S-band and C / L-band light sources, simultaneous discrimination of refractive index and temperature with the tapered fiber Mach-Zehnder interferometer is demonstrated with the corresponding sensitivities of Ϫ23.188 nm/RIU ͑refractive index unit͒ and 0.071 nm/°C, and Ϫ26.087 nm/RIU ͑blueshift͒ and 0.077 nm/°C ͑redshift͒ for the interference orders of 169 and 144, respectively. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3115029͔In situ monitoring of physical, chemical, and biological parameters is of great importance for process control in manufacturing industries, protection of ecosystems, and prevention of global warming. Refractive index ͑RI͒ and temperature are the most important parameters in these applications, especially in chemical or food industries for quality control and in biosensing for monitoring molecular bindings or biochemical reactions. Traditionally, the standard technique to measure refractive index is a refractometer, for which many well-known apparatus such as Pulfrich and Abbe refractometers have been used for many decades. 1 Surface plasmon resonance ͑SPR͒ has also been adopted for refractive index measurement through evanescent waves in waveguide configurations. 2 However, all these apparatuses are essentially bulky prism systems.In recent years, fiber-optic sensors have received significant attention for their unique advantages such as immunity to electromagnetic interference, compact size, potential low cost, and the possibility of distributed measurement over a long distance. 3 Earlier work on fiber-optic sensors for refractive index measurement reported metal-coated side-polished fibers, tapered fibers, or multimode fibers with relatively thin cladding layers to excite SPRs. 2,4 Besides the approach with evanescent waves from nanometer fiber tips coated with gold particles, 5 a majority of fiber sensors for refractive index measurement utilized fiber gratings, i.e., long-period gratings ͑LPGs͒ and fiber Bragg gratings ͑FBGs͒. Recent work that reported refractive index measurement with LPGs are either gold coated, 6 arc-induced phase shifted, 7 asymmetric, 8 or inscribed in air-and water-filled photonic crystal fibers. 9 Reported refractive index measurement with FBGs includes a metal-coated grating in a special single-mode fiber of larger core ͑26 m͒ and thinner cladding ͑30 m͒, 10 a tilted FBG with gold coating in single-mode fiber, 11 and cladding mode resonances of etched-eroded FBG. 12 A few papers reported simultaneous sensing of refractive index and temperature using LPGs, modified FBGs, and hybrid LPG-FBG structures, 13-15 with complicated design, instable system, or high cost, which restricts their pr...

Graphene is a very attractive material for broadband photodetection in hyperspectral imaging and sensing systems. However, its potential use has been hindered by tradeoffs between the responsivity, bandwidth, and operation speed of existing graphene photodetectors. Here, we present engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which enable simultaneous broadband and ultrafast photodetection with high responsivity. These nanostructures merge the advantages of broadband optical absorption, ultrafast photocarrier transport, and carrier multiplication within graphene nano-stripes with the ultrafast transport of photocarriers to gold patches before recombination. Through this approach, high-responsivity operation is realized without the use of bandwidth-limiting and speed-limiting quantum dots, defect states, or tunneling barriers. We demonstrate high-responsivity photodetection from the visible to infrared regime (0.6 A/W at 0.8 μm and 11.5 A/W at 20 μm), with operation speeds exceeding 50 GHz. Our results demonstrate improvement of the response times by more than seven orders of magnitude and an increase in bandwidths of one order of magnitude compared to those of higher-responsivity graphene photodetectors based on quantum dots and tunneling barriers.

The femtosecond laser-induced fiber Bragg grating is an effective sensor technology that can be deployed in harsh environments. Depending on the optical fiber chosen and the inscription parameters that are used, devices suitable for high temperature, pressure, ionizing radiation and strain sensor applications are possible. Such devices are appropriate for aerospace or energy production applications where there is a need for components, instrumentation and controls that can function in harsh environments. This paper will present a review of some of the more recent developments in this field.

A multiplexed fiber Bragg grating sensor for simultaneous salinity and temperature measurement

Recent developments in fiber-optic sensing have involved booming research in the design and manufacturing of novel micro-structured optical fiber devices. From the conventional tapered fiber architectures to the novel micro-machined devices by advanced laser systems, thousands of micro-structured fiber-optic sensors have been proposed and fabricated for applications in measuring temperature, strain, refractive index (RI), electric current, displacement, bending, acceleration, force, rotation, acoustic, and magnetic field. The renowned and unparalleled merits of sensors-based micro-machined optical fibers including small footprint, light weight, immunity to electromagnetic interferences, durability to harsh environment, capability of remote control, and flexibility of directly embedding into the structured system have placed them in highly demand for practical use in diverse industries. With the rapid advancement in micro-technology, micro-structured fiber sensors have benefitted from the trends of possessing high performance, versatilities and spatial miniaturization. Here, we comprehensively review the recent progress in the micro-structured fiber-optic sensors with a variety of architectures regarding their fabrications, waveguide properties and sensing applications.

In this study, a novel fiber-optic sensor consisting of a tapered bend-insensitive fiber based Mach-Zehnder interferometer is presented to realize damped and continuous vibration measurement. The double cladding structure and the central coating region of the in-fiber interferometer ensure an enhanced mechanical strength, reduced external disturbance, and a more uniform spectrum. A damped vibration frequency range of 29-60 Hz as well as continuous vibration disturbances ranging from 1 Hz up to 500 kHz are successfully demonstrated.

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A new fiber-optic sensor system consisting of a fiber Bragg grating cantilever as a transducer is proposed and demonstrated to realize simultaneous measurement of fluid flow rate and direction. For the fiber Bragg grating mounted on either a stainless steel or a spring steel substrate, a change in the water flow rate gives rise to a monotonic shift in the Bragg resonance wavelength of the grating while the flow direction results in either a redshift or a blueshift in the Bragg wavelength due to a stretched or shrunk state of the grating. Shifts in the Bragg resonance wavelength of 0.077 and 0.826 nm at a water flow rate of 90 cm3 s−1 were achieved with the fiber Bragg grating stainless steel and spring steel cantilever sensors, respectively. The experimental results are in good agreement with the theoretical analysis.