Abstract
Fiber optic modal interferometry has been around as a sensing concept since the outcome of fiber optic sensing. Initially supported by the utilization of standard Hi-Bi fibres associated to polarimetric modal interference, later this sensing approach evolved to modal interference based on spatial modes propagating in the core, on spatial modes propagating in the core and in the cladding with coupling performed by fibre devices such as long period gratings and tapers, and more recently on several types of modes propagating in photonic crystal fibers. This paper will address fiber optic sensing based on modal interferometry, and configurations of different type researched in last years will be presented and their performance compared.

Abstract
A multimode interferometer based-fiber optic sensor with a silica tube section aimed to measure refractive index (RI) variations of surrounding liquids is presented. The sensing head is a silica tube section fusion spliced to single mode fibers operating in transmission. In the splice regions tapers were made to allow the light to be guided in the silica tube while the core is formed by air. This configuration permits measurements of refractive index variations with sensitivities of 101.1, 106.29, and 107.97 nm/RIU considering resonances with different wavelengths. The same resonances were tested with temperature variations with sensitivities achieved of 7.8, 8.7, and 9.3 pm/ degrees C, respectively. The spectral variation associated with one degree temperature change corresponds to a refractive index change of similar to 8 x 10(-5), proving the low temperature dependence compared with sensitivity to RI variations. (C) 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3646393]

Abstract
We describe an all-fiber Mach-Zehnder interferometric configuration based on a suspended twin-core fiber. Because of the birefringence of the fiber cores, two interferometers are obtained by illuminating the fiber with polarized light. Applying strain, curvature, and temperature to the sensing head, different sensitivities are observed that permit the use of the matrix method to discriminate these three measurands. (C) 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3553482]

Abstract
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 degrees C/root Hz and +/-26 mu epsilon/root Hz are obtained for a fiber with birefringence of B = 5.5 x 10(-4). (C) 2000 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(00)00808-4].

Abstract
An interferometric Fabry-Perot cavity based on hollow-core ring photonic crystal fiber (HCR-PCF) for pressure sensing is proposed. The sensing head is formed by splicing a small section of HCR-PCF to standard single mode fiber. The spectral response depends on the cavity length due to the geometry of the HCR-PCF. The sensing head is subjected to methane pressure variations, where it exhibits a sensitivity of 0.82 nm/MPa. Its response to nitrogen pressure variation is also studied. The sensing head's intrinsic sensitivity to the nitrogen refractive index variations inside the hollow-core is also estimated. Finally, temperature measurement is performed and a sensitivity of 3.77 pm/degrees C is obtained for temperatures below 200 degrees C.

Abstract
The spectral behavior in the C-band of fiber Bragg gratings (FBGs) was analyzed as a function of temperature and strain. The FBGs were fabricated in pure silica four-leaf-clover- shaped suspended-core fibers by (DUV) femtosecond laser exposure (3.6 W at 800 nm, 130 fs, 1 kHz frequency tripled to 350 fs, 650 mW at 267 nm). A defect fiber (with a hollow hole in the core) and nondefect fiber were compared both yielding approximate to 1 pm/mu epsilon sensitivity to strain but different sensitivity to temperature (from 3.0 pm/degrees C to 8.4 pm/degrees C for the defect fiber and 10 pm/degrees C for the nondefect fiber). The 16% to 70% relative difference between the thermal coefficients of the two fibers, together with their similar strain sensitivity enables the simultaneous measurement of strain and temperature.