Coupling of optical fibers
In order to obtain good coupling efficiency, the characteristics of the focused beam (usually the laser beam) must be matched to the fiber parameters. The general guidelines are that (1) the focused spot should be comparable to the core size, (2) the focused beam should be at the center of the core, and (3) the incident cone angle should not exceed the numerical aperture NA of the fiber. conditions (1) and (2) are shown on the left side of Fig. 1, and condition (3) is shown on the right side of Fig. 1. For multimode fibers, the first two conditions are easily adapted due to the large core diameter. Therefore, matching the coupling lens to the numerical aperture NA of the fiber achieves good coupling efficiency in multimode fibers.
From this point of view, coupling to single-mode fibers is a more difficult problem. The smaller core diameter of a single-mode fiber requires more opto-mechanical elements to enable the focused beam to be coupled with sub-micron positioning accuracy. In addition, the mode of the incident laser must match the mode of the fiber. In other words, the coupling efficiency depends on the overlap integral of the Gaussian mode of the input laser beam and the approximate Gaussian fundamental mode of the fiber.
Fig. 1 Schematic diagram of optical coupling into a multimode or single-mode fiber (left). Incidence conditions in a multimode fiber such that light overflows (top right) and underfills (bottom right) the fiber.
Types of optical fibers
Many different types of optical fibers can exhibit very different geometries, as shown in Figure 2. Standard single-mode fibers used in communications have small core diameters (less than 10 microns), while multimode fibers have core diameters ranging from sixty to several hundred microns. Multimode fibers may have gradual or step refractive index profiles. Specialty fibers are also common, including bias-preserving fibers, high-power transmission fiber optic cables, bend-insensitive fibers, and fibers for extreme temperatures.
Figure 2 Different fiber types
Due to their prevalence and usefulness, two specific types of specialty fibers are described in detail below: rare earth element doped fibers and photonic crystal fibers (PCF).
Rare earth element doped fibers: Rare earth element doped fibers are particularly important for fiber lasers because these dopants (e.g. Nd, Yb and Er-Yb co-doped) can be used as laser gain media. The use of double-clad rare-earth-element doped fibers allows efficient matching of the pump beam, either by free-space focusing or by transmission through another fiber, and these doped fibers can also be used to make Bragg grating (FBG) light-sensitive fibers (Bragg grating is a periodic modulation of a material's refractive index that reflects light at wavelengths that are twice the period of the grating).
High-quality FBGs can be constructed from the periodic pattern of UV light to which the photosensitive fiber is exposed (rare earth element dopants strongly absorb UV). Gratings are formed when the fiber is exposed to periodic patterns of UV light, which are typically generated by a phase mask. From a production standpoint, this method of fabrication is fast, reliable and clearly attractive. Bragg gratings (FBGs) are capable of achieving high reflectivity (up to 99%) over narrow wavelength bands (see Fig. 3), which is beneficial for generating cavity mirrors in fiber lasers or for use as spectral filters in fiber-optic communication systems.
Photonic Crystal Fiber (PCF): Photonic crystals are microstructured materials in which the refractive index varies periodically with position. In a PCF, this periodic variation is achieved by regular vacancies or air holes parallel to the axis (see Figure 2). Unlike conventional optical fibers, both the core and the cladding are made of the same material. Therefore, all waveguiding properties in PCFs arise from the presence of vacancies.PCFs are usually divided into two main categories: refractive index guided fibers with solid cores and photonic bandgap fibers with periodic microstructural elements and cores made of low refractive index materials (e.g., hollow cores).PCFs have properties not found in ordinary optical fibers, such as the ability to operate in a single mode with large mode field diameters from UV to IR, very high nonlinearity, and the ability to have values for NA PCFs are widely used in spectroscopy, metrology, biomedicine, imaging, telecommunication, industrial processing and defense applications.
Figure 3 FBG schematic and representative transmission and reflectance spectra.
