Sensitivity of photonic crystal fiber modes to temperature, strain and external refractive index

Several strong narrowband resonances are observed in the transmission spectra of fiber Bragg gratings photo-written in photonic crystal fiber that has a refractive index-neutral germanium/fluorine co-doped core. Experimental results for the strain, temperature and refractive index sensitivities of these mode resonances are reported and compared to those of conventional single mode fiber. In particular, we identify three kinds of resonances whose relative sensitivities to strain, temperature and refractive index are markedly different and present numerical simulations to explain these properties. Potential multiparameter optical sensor applications of these mode resonances are briefly discussed

Optical fiber refractometer using narrowband cladding-mode resonance shifts

Short-period fiber Bragg gratings with weakly tilted grating planes generate multiple strong resonances in transmission. Our experimental results show that the wavelength separation between selected resonances allows the measurement of the refractive index of the medium surrounding the fiber for values between 1.25 and 1.44 with an accuracy approaching 1 × 10 -4. The sensor element is 10 mm long and made from standard single-mode telecommunication grade optical fiber by ultraviolet light irradiation through a phase mask

Highly sensitive bend sensor with hybrid long-period and tilted fiber Bragg grating

We demonstrate a new type of fiber optic bend sensor with a hybrid structure made up of a long period grating (LPG) and a tilted fiber Bragg grating (TFBG). The sensing mechanism is based on the spectrum of power transfers between the core and cladding modes from a TFBG located downstream from a LPG. We show that the curvature of a beam can be determined by the reflected power difference between the core mode and the recoupled cladding modes. We further provide design rules for the LPG and TFBG to optimize and linearize the sensor response. In addition, the temperature cross-sensitivities of this configuration are also investigated for two different types of fiber

Tilted fiber Bragg grating sensor interrogation system using a high-resolution silicon-on-insulator arrayed waveguide grating

We report a compact high-resolution arrayed waveguide grating (AWG) interrogator system designed to measure the relative wavelength spacing between two individual resonances of a tilted fiber Bragg grating (TFBG) refractometer. The TFBG refractometer benefits from an internal wavelength and power reference provided by the core mode reflection resonance that can be used to determine cladding mode perturbations with high accuracy. The AWG interrogator is a planar waveguide device fabricated on a silicon-on-insulator platform, having 50 channels with a 0.18 nm wavelength separation and a footprint of 8 mm × 8 mm. By overlaying two adjacent interference orders of the AWG we demonstrate simultaneous monitoring of two widely separated resonances in real time with high wavelength resolution. The standard deviation of the measured wavelength shifts is 1.2 pm, and it is limited by the resolution of the optical spectrum analyzer used for the interrogator calibration measurements

Fiber Bragg Grating High Impact Force Sensors with Adjustable Sensitivity and Dynamic Range

This paper presents the development of a fiber Bragg grating sensor system for loads applied perpendicularly to the fiber axis. The sensor design allows for adjustable sensitivity and high impact resistance. Two designs were tested, each containing a fiber Bragg grating embedded in a polymer material and encased in aluminum. Testing with a rate controlled load press was used to show the dependence of the response on loading conditions with loads reaching kN levels. Projectile drop tests further showed the response of both types of sensors with forces applied over millisecond durations. The magnitudes of the measured wavelength shift loading and unloading rates ranged from 0.203 to 0.507 nm/ms. Static structural simulations were completed and are in agreement with the experimental results. Loading rates of 1.667 mm/s with contact areas ranging from 38.5 to 113 mm2 resulted in respective sensitivities ranging from 0.154 to 0.064 nm/kN. Simulations and experiments further demonstrated the ability to increase the sensitivity up to 99% through modifications of lid thickness, cavity diameter, and loading area

Highly accurate micro-displacement measurement based on Gaussian-chirped tilted fiber Bragg grating

Intensity-referenced and temperature-independent high accuracy displacement measurement is demonstrated by using a Gaussian-chirped tilted fiber Bragg grating (TFBG). A specially designed bending cantilever beam is used to provide a displacement-induced Gaussian-strain-gradient and internal tilt angle modulation along the sensing TFBG. Compared with a straight FBG, the peak intensity of the Bragg resonance in the TFBG shows an improved sensitivity to the displacement but is immune from the spatially uniform temperatu