Optical fibers are flexible waveguides of the optical range. Usually made of some kind of glass, they can potentially be very long (achieving hundreds of kilometers). Quartz glass is most often used for the manufacture of optical fibers because of its unique properties, in particular:
- extremely low energy losses of radiation propagating in it (realized only in ultra-pure material);
- surprisingly high mechanical tensile and bending strength.
Most optical fibers used in laser optics have a core with a refractive index slightly higher than that of the outer layer (the so-called shell). Light introduced into the optical fiber is distributed mainly in the region of its core, although some of the radiation can also penetrate into the shell. Light is retained in the fiber due to the effect of full internal reflection at the border of regions with different refractive index, or due to the gradient of the refractive index in the core of the optical fiber.
They less commonly use fibers that implement a different principle of light retention in them. Such optical fibers are based on the fiber Bragg grating structures and photonic crystals to form a photonic stop-band. This technique is fundamentally different from the method of retaining light in conventional fibers.
To be more precise, a fiber Bragg grating is a periodic or aperiodic perturbation of the effective refractive index in the fiber core. As a rule, the Bragg grating has a length of about a few millimeters or centimeters. The width of the reflection band of the fiber Bragg grating, which is usually much less than 1 nm, depends on both the length of the Bragg grating and the depth of the refractive index modulation. Narrow-band gratings required, for example, for the manufacture of single-frequency fiber lasers or for certain optical filters can be obtained using long gratings with weak modulation. Wide reflection bands can be obtained on short Bragg gratings with deep modulation, but also on gratings with aperiodic modulation.
Since the maximum reflection wavelength depends not only on the Bragg grating period but also on temperature and mechanical stresses, fiber Bragg gratings can be used in temperature and stress sensors. Transverse stresses compress the optical fibers of gratings between two flat plates, inducing birefringence and thus causing the Bragg wavelength to depend on polarization.
Additionally, fiber Bragg gratings are often used for the development of fiber optic sensors. Fiber optic sensors are fiber optic systems for detecting certain factors, usually temperature or mechanical stress, but sometimes also displacement, vibration, pressure, acceleration, rotation, and chemical concentrations.
Optromix is a fast-growing vendor of fiber Bragg grating (FBG) product line such as fiber Bragg grating sensors, FBG interrogators and multiplexers, Distributed Acoustic Sensing (DAS) systems, Distributed Temperature Sensing (DTS) systems. The company creates and supplies a broad variety of fiber optic solutions for monitoring worldwide. If you are interested in structural health monitoring systems and want to learn more, please contact us at info@optromix.com