UV Sensor Based on Fiber Bragg Grating Covered with Graphene Oxide Embedded in Composite Materials.

Polymer-matrix composites degrade under the influence of UV radiation in the range of the 290-400 nm band. The degradation of polymer-matrix composites exposed to UV radiation is characterized by extensive aging of the epoxy matrix, resulting in deterioration of their mechanical properties. Glass fibers/epoxy resin composites were made by an out-of-autoclave method whereas a fiber optic sensor was placed between different layers of laminates. In our work, we used a fiber Bragg grating sensor covered with graphene oxide and embedded in a polymer matrix composite to monitor UV radiation intensity. Measurements of UV radiation may allow monitoring the aging process of individual components of the polymer composite. In order to estimate the number of microcracks of epoxy resin, microstructure observations were carried out using a scanning electron microscope.

Ultra-narrow linewidth DFB-laser with optical feedback from a monolithic confocal Fabry-Perot cavity.

We present a compact, ultra-narrow-linewidth semiconductor laser based on a 780 nm distributed feedback diode laser optically self-locked to a mode of an external monolithic confocal Fabry-Perot resonator. We characterize spectral properties of the laser by measuring its frequency noise power spectral density. The white frequency noise levels at 5 Hz(2)/Hz above a Fourier frequency as small as 20 kHz. This noise level is more than five orders of magnitude smaller than the noise level of the same solitary diode laser without resonant optical feedback, and it is three orders of magnitude smaller than the noise level of a narrow linewidth, grating-based, extended-cavity diode laser. The corresponding Lorentzian linewidth of the laser with resonant optical feedback is 15.7 Hz at an output power exceeding 50 mW.

Thermoacoustic optical path length stabilization in a single-mode optical fiber.

We present a simple technique to actively stabilize the optical path length in an optical fiber. A part of the fiber is coated with a thin, electrically conductive layer, which acts as a heater. The optical path length is thus modified by temperature-dependent changes in the refractive index and the mechanical length of the fiber. For the first time, we measure the dynamic response of the optical path length to the periodic changes of temperature and find it to be in agreement with our former theoretical prediction. The fiber's response to the temperature changes is determined by the speed of sound in quartz rather than by slow thermal diffusion. Making use of this fact, we succeeded in actively stabilizing the optical path length with a closed-loop bandwidth of 3.8 kHz.