Abstract
In this paper, two novel hybrid multimode/single mode fiber Fabry-Perot (FP) cavities were compared. The cavities fabricated by chemical etching are presented as high temperature and strain sensors. In order to produce this FP cavity a single mode fiber was spliced to a graded index multimode fiber with 62.5 mu m core diameter. The multimode fiber was cut approximately 150 mu m away from the splice. Then the tip of the fiber containing the multimode fiber segment was dipped into a solution of 48% of HF during 8 minutes, creating a concavity due to the fact that the reaction between HF and the germanium doped fiber core is much faster than the reaction between HF and the pure silica cladding. By this method a concavity of approximately 100 mu m deep was created at the fiber tip. Two different FP cavities can be fabricated. The first cavity is obtained when a spliced with an identical tip concavity fiber (Sensor A) and the second is created when a tip concavity is spliced to a single mode fiber (Sensor B). The Fabry-Perot cavities were tested as a high temperature sensor in the range between room temperature and 800 degrees C and as strain sensors. A reversible shift of the interferometric peaks with temperature allowed to estimate a sensitivity of 0.75 +/- 0.03 pm/degrees C and 0.98 +/- 0.04 pm/degrees C for the sensor A and B respectively. For strain measurement sensor A demonstrated a sensitivity of 1.85 +/- 0.07 pm/mu epsilon and sensor B showed a sensitivity of 3.14 +/- 0.05 pm/mu epsilon. The sensors demonstrated the feasibility of low cost fiber optic sensors for high temperature and strain.

Abstract
A simple interrogation technique for refractive index measurement is proposed, using a multimode interference-based fiber tip structure. The fiber probe is a section of a multimode fiber, spliced to a single-mode fiber and interrogated in reflection. The interrogation technique uses two fiber Bragg gratings as discrete optical sources; by means of relative intensity variation of the reflected signals, those sources will provide a measurement of refractive index changes, while taking advantage of the MMI-based fiber tip. The read-out system uses a WDM and two photodetectors to separate both signals. A sensitivity of -5.87/RIU, in the refractive index range 1.30-1.38, was achieved with the proposed configuration.

Abstract
Two new configurations of high-birefringent fiber loop mirror with an output port probe are proposed. The two configurations used two couplers spliced between them with unbalanced arms and one output port is used as the probe sensor. The difference between them is that the section length of high-birefringent fiber is located between the two couplers (first configuration) or spliced in the output port probe (second configuration). The first new configuration is studied as an optical refractometer and the second configuration is analyzed when the strain and temperature are applied.

Abstract

Abstract
In this work, we analyze a remote optical sensor system composed of two Fiber Bragg Gratings (FBGs) and one Long Period Grading (LPG) capable of simultaneously sensing the temperature and the refractive index, separated by 50 km from the optical source and the interrogation unit. Since the active components of the system and the sensor head are separated over such a large distance, it is necessary to consider Raman amplification o strengthen the optical signal. We present both experimental measurements and the results of numerical simulations, which describe the signal evolution and predict the measurement results for a remote sensor based on a LPG. The simulation codes are also used to study a hybrid sensor composed of two FBGs with a LPG. We show that the power ratio between the two central wavelengths of the FBG has a linear relation with the change of refractive index of the sensored medium.

Abstract
A single-mode nonadiabatic tapered optical fiber (NATOF) sensor was inserted into a fiber loop mirror (FLM) enabling us to tune its sensitivity towards refractive index (RI). The NATOF was fabricated by the heat pulling method, utilizing a CO laser. The adjustment of the polarization controllers (PCs) inserted in loop allowed us to excite different cladding modes in the interferometric taper resulting in different optical paths for the clockwise and the counterclockwise beams. By variation of the PCs' settings, the sensitivity of the sensor for RI in the range from 1.3380 to 1.3510 could be tuned from 876.24 to 1233.07 nm/RIU. Experimental results show that the sensitivity to the external RI increased with the order of the cladding mode.