Faramarz Farahi

Simultaneous measurement of temperature and strain: cross-sensitivity considerations

Interferometric sensors using optical fibers as a transduction medium have been shown to be sensitive to a variety of physical measurands. A result of this is that the resolution of a system designed to sense strain, for example, may be compromised by fluctuations in the temperature of the environment. The possibility of simultaneously determining the strain and temperature applied to the same piece of highly birefringent fiber is discussed. Second-order effects are shown to be important for long sensing lengths or in the presence of high strains or temperature changes. The results of experiments carried out to verify the theoretical predictions are also described. >

Simultaneous measurement of strain and temperature using Bragg gratings written in germanosilicate and boron-codoped germanosilicate fibers

A new fiber Bragg grating sensor configuration is presented for simultaneous measurement of strain and temperature. The sensor utilizes the effect of boron codoping on the temperature dependence of the refractive index in germanosilicate fibers. By writing gratings with dose wavelengths in undoped acid boron doped fibers, different temperature sensitivities are obtained while strain sensitivities remain the same. These gratings are then spliced to obtain a simple sensor head suitable for applications in smart structures and composite materials.

All-fiber Mach-Zehnder curvature sensor based on multimode interference combined with a long-period grating

A novel Mach-Zehnder interferometer based on a fiber multimode interference structure combined with a long-period fiber grating (LPG) is proposed. The multimode interference is achieved through the use of a MMF section spliced between two single-mode fibers, with a length adjusted to couple a fraction of light into the cladding modes. A LPG placed after the MMF couples light back into the fiber core, completing the Mach-Zehnder interferometer. This novel configuration was demonstrated as a bending sensor.

Optical Fiber Sensing Using Quantum Dots.

Recent advances in the application of semiconductor nanocrystals, or quantum dots, as biochemical sensors are reviewed. Quantum dots have unique optical properties that make them promising alternatives to traditional dyes in many luminescence based bioanalytical techniques. An overview of the more relevant progresses in the application of quantum dots as biochemical probes is addressed. Special focus will be given to configurations where the sensing dots are incorporated in solid membranes and immobilized in optical fibers or planar waveguide platforms.

Coherence multiplexing of remote fibre optic fabry-perot sensing system

Abstract A monomode fibre optic coherence multiplexing sensor system incorporating passive miniature fibre optic probes is demonstrated using a coherence tuned local interferometer to sequentially interrogate the individual sensors. An example of its use as a high resolution relative point temperature measurement system is described with a range to resolution in excess of 10 4 .

Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement

In this work, sensitivity to strain and temperature of a sensor relying on modal interferometry in hollow-core photonic crystal fibers is studied. The sensing structure is simply a piece of hollow-core fiber connected in both ends to standard single mode fiber. An interference pattern that is associated to the interference of light that propagates in the hollow core fundamental mode with light that propagates in other modes is observed. The phase of this interference pattern changes with the measurand interaction, which is the basis for considering this structure for sensing. The phase recovery is performed using a white light interferometric technique. Resolutions of +/- 1.4 microepsilon and +/- 0.2 degrees C were achieved for strain and temperature, respectively. It was also found that the fiber structure is not sensitive to curvature.

Simultaneous measurement of strain and temperature using interferometrically interrogated fiber Bragg grating sensors

The potential of different fiber Bragg grating pairs for simultaneous sensing of strain and temperature is analyzed. We demonstrate that interferometric interrogation of a fiber grating written in bow-tie fiber enables strain and temperature to be simultaneously determined. This is achieved by independent measurement of the shift in the wavelengths of the reflected light from the grating components along the fast and the slow axes of the hi-bi fiber. A detailed theoretical analysis is presented that includes the basic sensing principle, sensor design and demodulation scheme. The performance of the proposed technique in simultaneous measurement of temperature and strain is experimentally demonstrated and resolutions of ± 2.5° c/?Hz and ± 26 ??/?Hz are obtained for a fiber with birefringence of B = 5.5x 10-4.

Temperature and strain insensitive bending measurements with D-type fibre Bragg gratings

We present a new sensor configuration based on the intrinsic bend sensitivity of Bragg gratings written in D-type fibres. This sensor can be embedded in any layer of a composite material to evaluate curvature in a way independent of axial strain and temperature. It can also be used to measure other parameters such as acceleration, angle and acoustic pressure.

Fabry–PÉrot Cavity Based on a Suspended-Core Fiber for Strain and Temperature Measurement

A fiber-optic Fabry-Perot sensing structure based on the utilization of a suspended-core fiber is presented. The interferometric structure is formed when a small length of the suspended-core fiber is spliced to the end of a standard single-mode fiber. The interfering waves are generated by the refractive-index mismatches between the two fibers in the splice region and at the end of the suspended-core fiber. Thermal and strain responses of two different sensing heads associated with suspended-core fibers with three and four holes are characterized.

Quantum dots as self-referenced optical fibre temperature probes for luminescent chemical sensors

The use of semiconductor nano-particles as temperature probes in luminescence chemical sensing applications is addressed. Temperature changes the intensity, the peak wavelength and the spectral width of the quantum dots luminescent emission in a linear and reversible way. Results are presented that show the feasibility of implementing a self-referenced intensity-based sensor to perform temperature measurements independent of the optical power level in the sensing system. A resolution of 0.3 °C was achieved. In addition, it is demonstrated that self-referenced temperature measurements at multiple points could be performed using reflection or transmission based optical fibre configurations.