Special Fiber Bragg Gratings Research Articles

A Fiber Bragg Grating Anchor Rod Force Sensor for Accurate Anchoring Force Measuring

This paper presents a novel anchor rod force sensor based on fiber Bragg grating (FBG) for accurate anchoring force measurement. A special FBG strain sensor with enhanced sensitivity has been designed and embedded in the centre line of an elastic structural body, which forms the main body of the anchor rod force sensor. The mechanical structure design and measuring principle of the proposed anchor rod force sensor have been presented detailedly. Then a sensor prototype has been manufactured and fully tested. The experimental results show that the sensor provides a sensitivity of 11.58 pm/kN with a fitting linear correlation coefficient of 0.9996 in a measurement range of 50 kN, a repeatability error of 3.26%, and a hysteresis error of 3.87%. Furthermore, good resistance to bending effects of anchor rod, good creep resistance (fluctuant value is within 2 pm), and good temperature compensation capacity (maximum value is 11 pm) have also been verified. These results promise the sensor better applicability in the health monitoring of large civil engineering structures.

Fiber-Optic Vector Magnetometer Based on Magnetic Fluid and Fiber Bragg Grating Written on a Multi-Clad Fiber

The magnetic fluid is used to integrate with a special fiber Bragg grating (FBG) inscribed over the core and innermost cladding of a multi-clad fiber, which acts as a bridge between magnetic field measurement and FBG. The FBG is etched by hydrofluoric acid and packaged with a magnetic fluid inside a capillary. The reflection of the sensor provides two resonance modes of which the cladding mode intensity shows a great response to both the magnetic field intensity and the direction. Meanwhile, the fundamental core mode intensity is quiet stable, and thus the proposed sensor clearly demonstrates the potential to simultaneous measurement of magnetic field and temperature.

High-efficiency LD-pumped all-fiber Raman laser based on a 100 µm core graded-index fiber

We demonstrate a high-power, high-efficiency Raman laser based on a 100 µm core graded-index (GRIN) fiber directly pumped by 915 nm multimode laser diode modules in all-fiber configuration. Optimization of GRIN fiber length and pumping scheme was performed. As a result, 62 W of CW power has been obtained at a wavelength of 954 nm with a slope efficiency of 85%. The joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the GRIN fiber core results in significant beam quality enhancement for the generated Stokes beam ( < 3) in comparison with that of the pump radiation ( > 30). In addition, the exact value of the operating wavelength near the Raman gain maximum was varied. To the best of our knowledge, this is the first study of the impact of a Raman gain spectral profile on the power, spectrum and beam quality parameter M2 of multimode Raman laser. It appears that they very much dependent on the spectral region of Raman gain at which we work, when the wavelength is selected by a fiber Bragg grating inscribed in a multimode GRIN fiber.

Multimode Fiber Raman Lasers Directly Pumped by Laser Diodes

Raman fiber lasers (RFLs) are usually based on single-mode fibers core-pumped by high-power rare-earth-doped fiber lasers with single transverse mode output that leads to a rather complicated design of RFLs. One of interesting possibilities to simplify the RFL design is its direct pumping by cheap and reliable high-power multimode laser diodes (LDs). It is attractive to use standard graded-index multimode passive fibers characterized by very high quality and low cost due to their wide use in telecom. In this case, one can directly couple the multimode radiation of high-power LDs with moderate brightness into the core of multimode graded-index fiber with much higher efficiency than into the core of single-mode fiber. Using commercially available multimode LDs with operating wavelengths of 915–940 nm, it is possible to obtain high-power high-beam-quality Raman lasing in wavelength range of 950–1000 nm, which is problematic for rare-earth-doped fiber lasers. Here, we review the results obtained in this direction and report on the demonstration of all-fiber LD-pumped CW Raman laser based on the graded-index fiber. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of fiber core, results in high-efficiency conversion of multimode (M2∼26) pump at 915 nm into a high-quality output beam at 954 nm. Fibers with core diameter of 62.5, 85, and 100 μ m are compared. With core enlargement, the conversion efficiency increases sufficiently (from 47% to 84%) at the expense of slight beam-quality parameter reduction (M2 = 1.3–3). The generated spectrum remains to be rather narrow ( 60 W.

Fiber optic accelerometer based on grating inscribed over innermost-clad of multi-clad fiber

A special fiber Bragg grating (FBG) that is inscribed over the core and innermost depressed-index cladding of a short multi-clad fiber (MCLF) by femtosecond laser side-illumination technique is proposed and demonstrated for orientation-dependent measurement, experimentally. Splicing the MCLF with standard single-mode fiber (SMF) provides a cladding-mode generation mechanism via core mismatch, and then “cladding FBG” in MCLF can simultaneously generate two resonances in reflection. The cladding mode shows a significant response to fiber bending due to novel refractive-index profile of MCLF and coupling in the splicing junction. And the asymmetry “cladding FBG” configuration relatively to fiber core center enables it to perform strong orientation-dependent response to the fiber bending. These two properties are well applied to measure acceleration with high resolution of 0.02 m/s2 and orientation-dependence. The cladding and core mode also present linear wavelength-shifting with temperature variation but no intensity fluctuations. Furthermore, the accidental power perturbation or cross-sensitivity can be calibrated out by monitoring the fundamental core mode resonance.

Generating high-quality beam in a multimode LD-pumped all-fiber Raman laser.

We report on the first demonstration of an all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode fiber-coupled laser diodes. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the graded-index fiber core, results in high-efficiency conversion of a multimode (M2~26) pump at 915 nm into a high-quality (M2~2.6) output beam at 954 nm. About 50 W output power has been obtained with slope efficiency of 67%. The proposed development and integration of key multimode fiber technologies opens the door to new type of LD-pumped high-power high-beam-quality fiber lasers that may operate at almost any wavelength defined by available LDs.

Instantaneous Microwave Frequency Measurement Using a Special Fiber Bragg Grating

A photonic approach to realizing instantaneous microwave frequency measurement based on frequency to optical power ratio mapping using a special fiber Bragg grating (FBG) is proposed and demonstrated. The special FBG is designed to have a spectral response with two slopes that are inversely proportional to optical frequency, which enables the realization of a linear power ratio function, leading to an increased measurement range and improved measurement accuracy. The FBG is fabricated and the incorporation of the FBG in a system for frequency measurement is experimented. Instantaneous microwave frequency measurement with a measurement range of 1-10 GHz and a measurement accuracy of ±0.2 GHz under different input microwave power levels is realized.

Arbitrary-order all-fiber temporal differentiator based on a fiber Bragg grating: design and experimental demonstration

A new technique to design an all-fiber temporal differentiator that has a large bandwidth and an arbitrary differentiation order is proposed and investigated. The proposed temporal differentiator is a special fiber Bragg grating (FBG) that is designed by controlling its magnitude and phase responses with the discrete layer peeling (DLP) method. There are three important features of this technique: 1) the temporal differentiator has an arbitrary magnitude response and a controllable bandwidth; 2) the temporal differentiator can be designed and fabricated with an arbitrary differentiation order that is realized in a single FBG; 3) the required maximum index modulation of the FBG-based differentiator is largely decreased by using a Gaussian windowing function. The use of the proposed technique to design temporal differentiators with a differentiation order up to the fourth and with a bandwidth up to 500 GHz is studied. A proof-of-concept experiment is then carried out. A first- and a second-order temporal differentiator with a bandwidth of 25 GHz are experimentally demonstrated.