Abstract
First demonstrated over four decades ago, fiber gratings, and in particular fiber Bragg gratings (FBGs), have become indispensable and ubiquitous components within fiber laser cavities as well as optical communication networks and sensor systems. Because of their overwhelming use, the bulk of published work to date has concentrated on fibers that are composed of silica-based glasses, yet those become virtually opaque at wavelengths above 2 μm, i.e., in the technically increasingly important mid-infrared part of the electromagnetic spectrum. Soft glass fluoride fibers on the other hand, in particular those composed of ZBLAN (abbreviation for ZrF4-BaF2-LaF3-AlF3-NaF), offer low-loss transmission out to wavelengths of up to almost 4 μm. However, while fiber manufacturing itself has now reached a high level of maturity, with passive fiber attenuation levels as low as 1 dB/km becoming commercially available, grating fabrication in these fibers remains challenging, and research into suitable inscription techniques is still a very active and ongoing scientific field. This review aims to provide an overview of work that has been done in this area to date with an emphasis on femtosecond-laser based fabrication methods.
Highlights
Whenever the effective refractive index of the core of an optical fiber is modulated periodically along a section which is typically a few millimeters to several centimeters in length, a fiber grating is formed
Diffraction grating, such gratings are known as a fiber Bragg gratings (FBGs) that find a vast number of applications, for example as wavelength filters [2], fiber laser mirrors [3] and optical sensors [4]
In a landmark paper that was published in 1996, Davis et al demonstrated the feasibility of using laser pulses with a duration of 120 fs and a center wavelength of 810 nm to induce a permanent and highly localized refractive index change in silica, borate, soda lime silicate, and fluorozirconate glasses [20]
Summary
Whenever the effective refractive index of the core of an optical fiber is modulated periodically along a section which is typically a few millimeters to several centimeters in length, a fiber grating is formed. In a landmark paper that was published in 1996, Davis et al demonstrated the feasibility of using laser pulses with a duration of 120 fs and a center wavelength of 810 nm to induce a permanent and highly localized refractive index change in silica, borate, soda lime silicate, and fluorozirconate glasses [20] This paved the way for the afterwards rapidly expanding field of femtosecond laser direct-writing [21] and provided the basis for the fabrication of fiber gratings into virtually any type of fibers, independent of their geometry, material or photosensitivity.
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