Recent advances in devices and applications of high-birefringence fiber loopmirror sensors are addressed. In optical sensing, these devices may be used as strain andtemperature sensors, in a separate or in a simultaneous measurement. Other describedapplications include: refractive index measurement, optical filters for interrogate gratingsstructures and chemical etching control. The paper analyses and compares different types ofhigh-birefringence fiber loop mirror sensors using conventional and microstructured opticalfibers. Some configurations are presented for simultaneous measurement of physicalparameters when combined with others optical devices, for example with a long periodgrating.

https://www.mdpi.com/1424-8220/7/11/2970/pdf

Recent Advances in High-Birefringence Fiber Loop Mirror Sensors

This work presents an optical sensor based on a highly birefringent photonic crystal fiber (Hi-Bi PCF) loop mirror. The length of the sensing head is 380 mm and its corresponding wavelength spacing between two interferometer minima is 8 nm. The optical sensor was characterized in strain and in temperature with an uncoated Hi-Bi PCF and with an acrylate coated Hi-Bi PCF. Different results for strain and temperature sensitivity were obtained. Relatively to the strain measurement, the sensor with the uncoated Hi-Bi PCF presents slightly less sensitivity (1.11 ) when compared with coated Hi-Bi PCF (1.21 ). For the temperature measurement and with the uncoated Hi-Bi PCF, the optical sensor is insensitive to temperature (0.29 pm/K).

Temperature-Independent Strain Sensor Based on a Hi-Bi Photonic Crystal Fiber Loop Mirror

A Sagnac interferometer with a section of a polarization maintaining side-hole fiber for multiparameter measurement is proposed The sensor was experimentally demonstrated to be sensitive to torsion, temperature, and longitudinal strain, simultaneously The birefringence in the investigated side-hole fiber is induced simultaneously by the elliptical shape of a germanium-doped core and by field overlap with the air holes surrounding the core The latter effect is purely geometrical and causes high chromatic dispersion of the group birefringence in the long wavelength range, which results in a different period of spectral interference fringes A different wavelength response is obtained for each interference fringe peak when the fiber is subjected to torsion, temperature, or longitudinal strain A matrix equation for simultaneous measurement of the three parameters--torsion, temperature, and longitudinal strain--is also proposed

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Simultaneous measurement of multiparameters using a Sagnac interferometer with polarization maintaining side-hole fiber.

A novel temperature insensitive all-fiber compact polarization-maintaining photonic crystal fiber (PMPCF) based interferometer was presented. The coupling between the PMPCF and single mode fiber (SMF) was investigated experimentally and analytically. With an all-fiber compact SMF-PMPCF-SMF structure, the cladding modes propagating in the PMPCF can be effectively excited by finely core-offsetting one splice between the PMPCF and SMF. With the increase of the core-offset, the interference depth of the interference pattern enlarges accordingly. Under a suitable core-offset, the PMPCF based interferometer with high interference depth can be obtained. Moreover, it is temperature insensitive due to its ultra-low thermal characteristics. We also demonstrated its applications in strain and refractive index measurement. The sensors have the advantages of small size, simple and compact all fiber structure, high sensitivity, and temperature insensitiveness.

Temperature Insensitive All-Fiber Compact Polarization-Maintaining Photonic Crystal Fiber Based Interferometer and Its Applications in Fiber Sensors

In this Letter, we present a fiber loop mirror configuration based on a suspended twin-core fiber for sensing applications. Using the suspended twin-core fiber, the fringe pattern is due to the differential optical patch of the light in the two cores associated with a refractive index difference of ~10−3, which indicates an advantage of this approach compared with those based on high-birefringent fibers, namely, the possibility of using a small length of fiber. The sensing configuration was characterized for torsion, temperature, and strain. Using the fast Fourier transform technique, it is possible to obtain measurand-induced amplitude variations of the fringe pattern. The results obtained indicate the viability of a temperature- and strain-independent torsion sensor.

Temperature- and strain-independent torsion sensor using a fiber loop mirror based on suspended twin-core fiber.

A novel liquid-level sensor with a high-birefringence-fiber loop mirror (HBFLM) based on a uniform-strength cantilever beam (UCB) is proposed and demonstrated. Part of the high-birefringence fiber is pasted onto the central surface of the UCB. A hollow suspending pole is utilized to apply force at the end of the beam. The applied force varies with the change of the liquid level, leading to a change of transmission intensity. Thus the variation of liquid level can be determined via the laser wavelength within the quasi-linear transmission range of the HBFLM filter. Its sensitivity, resolution, and linear measurement range reach 0.047/cm, 10 mm, and 140 mm, respectively. The advantages of the sensor include simple structure, high sensitivity, low cost, and good repeatability, etc. The sensing signal can be directly detected by a photodetector and does not require complicated demodulation devices.

Dong Bo

Papers

Liquid-level sensor with a high-birefringence-fiber loop mirror