Electrically tunable Bragg gratings in single-mode polymer optical fiber

Temperature and Humidity Sensitivity of Polymer Optical Fibre Sensors Tuned by Pre-Strain

Polymer optical fibre Bragg grating (POFBG) sensors are of high interest due to their enhanced fracture toughness, flexibility in bending, and sensitivity in stress and pressure monitoring applications compared to silica-based sensors. The POFBG sensors can also detect humidity due to the hydrophilic nature of some polymers. However, multi-parameter sensing can cause cross-sensitivity issues in certain applications if the temperature and humidity measurements are not adequately compensated. In this work, we demonstrate the possibility of selectively tuning sensors' temperature and humidity sensitivities to the desired level by applying a certain amount of fibre pre-strain. The temperature sensitivity of POFBG sensors fabricated in perfluoropolymers (CYTOP) can be selectively tuned from positive to negative values, having the option for insensitivity in specific temperature ranges depending on the amount of the applied pre-strain. The humidity sensitivity of sensors can also be changed from positive values to insensitivity. The importance of thermal annealing treatment of POFBG sensors for improved repeatability in temperature measurements is also reported. An array of 4 multiplexed POFBGs was fabricated, and each sensor was pre-strained accordingly to demonstrate the possibility of having targeted temperature and humidity sensitivities along the same fibre.

Spectral shaping of fiber Bragg gratings based on non-rigid origami

We present a spectrum-tunable fiber Bragg grating (FBG) based on a unique compress-twist deformation mode of non-rigid origami. By applying axial displacement on the FBG-bonded Kresling-ori, a non-uniform strain field emerges. The mechanics-induced non-uniform strain can shift the wavelength of an apodized unchirped FBG and/or transform an apodized unchirped FBG to a chirped one. For the spectrum-shaping mode, the bandwidth of the FBG was tuned from 0.32 nm up to 2.9 nm, measured at the -6dB level. For the wavelength-shift mode, a maximum wavelength shift of 0.6 nm can be achieved.

Optically tunable fiber Bragg grating based on a photo-mechanical tuning mechanism

We present an optically tunable fiber Bragg grating (FBG) based on a photo-mechanical tuning mechanism. Azobenzene containing polymer was utilized as a photo-mechanical tuning agent coated over a bare section of FBG. Controlled by ultraviolet (UV) irradiation, the polymer coating acts as a micro-actuator that deforms the fiber structure. Reversible tunability was obtained by the coating locally stretching the embedded FBG fiber by the intensity and the spatial distribution of the UV light. We found that our tunable FBG could linearly tune its center wavelength as large as 2.2 nm in a symmetric UV irradiation. We also found that a chirp of grating periods could be produced by an asymmetric irradiation, where the center of the UV irradiation profile is offset to that of the FBG device. The spectral width measured at 10 dB could be broadened from 0.4 to 1.2 nm by simply adjusting the relative position of the UV light source. The direction of the chirp, in either normal or anomalous dispersion, could be easily switched as well. Our observations demonstrated that our device provides a versatile means of tunable filters or dispersion generators, which is useful in various applications.

Microfluidic Flows and Heat Transfer and Their Influence on Optical Modes in Microstructure Fibers

A finite element analysis (FEA) model has been constructed to predict the thermo-fluidic and optical properties of a microstructure optical fiber (MOF) accounting for changes in external temperature, input water velocity and optical fiber geometry. Modeling a water laminar flow within a water channel has shown that the steady-state temperature is dependent on the water channel radius while independent of the input velocity. There is a critical channel radius below which the steady-state temperature of the water channel is constant, while above, the temperature decreases. However, the distance required to reach steady state within the water channel is dependent on both the input velocity and the channel radius. The MOF has been found capable of supporting multiple modes. Despite the large thermo-optic coefficient of water, the bound modes’ response to temperature was dominated by the thermo-optic coefficient of glass. This is attributed to the majority of the light being confined within the glass, which increased with increasing external temperature due to a larger difference in the refractive index between the glass core and the water channel.