Fiber Bragg Gratings

If the strength of the index modulation in a grating is constant over some length, and suddenly drops to zero outside that range, the reflection spectrum exhibits side lobes, in particular if the peak reflectance is high (see Figure 2). These side lobes are sometimes disturbing, e.g. in some applications of fiber Bragg gratings as optical filters. They can be largely removed with the technique of apodization: the strength of the index modulation is smoothly ramped up and down along the grating. Of course, one then needs an increased overall length of the grating to achieve a certain peak reflectance. For the exact profile of the index modulation in an apodized fiber Bragg grating, there is a trade-off between optimum side-lobe suppression and maximum reflectance for some restricted grating length and a given maximum strength of index modulation.

Fiber gratings with aperiodic index modulation can have interesting properties, such as reflectance curves without side lobes, multiple tailored reflection bands, or special chromatic dispersion profiles. Particularly for dispersion compensation, so-called chirped fiber gratings are used [28], where the Bragg wavelength monotonously varies with position. It is possible e.g. to achieve very large group delay dispersion in a moderate length of fiber, sufficient for compensating the dispersion of a long span of transmission fiber in an optical fiber communications system. Another application is pulse compression, e.g. within a chirped-pulse amplifier system.

Chirped fiber gratings are also interesting for application as distributed fiber-optic sensors with intragrating sensing, i.e., monitoring e.g. the temperature along the length of the device.

Chirped gratings can be obtained in different ways, e.g. with point-to-point laser inscribing, with chirped phase masks or by tapering the fiber after writing the grating.

Some fiber Bragg gratings are fabricated such that the planes of constant refractive index are not normal to the fiber axis, as usual, but are tilted against the axis by some angle (often a few degrees). If that tilt is strong enough, the coupling to backward core modes may become quite weak; instead, one has a coupling of core modes to cladding modes. Effectively, one obtains a transmission loss without the corresponding reflection into the core. That can be used for a notch filter, attenuating light in a certain spectral region, which may be widened if the grating is chirped at the same time. An application of this can be the reduction of stimulated Raman scattering in high-power fiber lasers and amplifiers [33, 34], for example.

Typical FBGs have grating periods of a few hundred nanometers, coupling counterpropagating waves in the core. A second possibility is to use long-period gratings (LPG) [19] with periods of the order of hundreds of microns or more (often with tilted grating planes) and a length of e.g. a few centimeters. Note that these are generally not regarded to be Bragg gratings, as that term is reserved for reflecting gratings.

Such gratings can only couple modes with the same propagation direction because the difference of wavevectors for counterpropagating waves would be too large to be matched by the grating. For example, the fundamental mode of a multimode fiber can be coupled to a certain higher-order mode, or a core mode can be coupled to cladding modes propagating in a similar direction. In the latter case, the coupling effectively introduces propagation losses because light in cladding modes normally experiences strong losses in the fiber coating.

Long-period gratings can even be made by pressing a short length of fiber against a plate with periodic grooves [18]. This kind of grating is reversible and potentially tunable.

Long-period gratings are used for introducing carefully controlled wavelength-dependent losses, e.g. for gain equalization in erbium-doped fiber amplifiers or for suppressing effects of stimulated Raman scattering, but are also used for fiber-optic sensors.

It is also possible to write FBGs in polymer optical fibers. As with silica fibers, one usually uses ultraviolet light, but the physical mechanisms are somewhat different. An advantage of Bragg gratings in polymer fibers is the larger wavelength tunability: polymer fibers can be stretched more strongly, and they react more strongly to temperature changes.

It is possible to fabricate Bragg gratings in multimode fibers. However, a substantial problem for various applications is that the Bragg condition depends on the involved fiber modes, which generally have different propagation constants. Therefore, the wavelength of maximum reflectance is generally different between different modes. That is a substantial impediment for the use of FBGs in multimode fibers. For obtaining high reflectance for all guided modes at a certain wavelength, one can use a chirped (and correspondingly longer) Bragg grating, but generally the multimode nature of a fiber imposes substantial restrictions.

Some applications (like filtering of star light in astronomical telescopes) require multimode fibers but cannot tolerate mode-dependent reflections. A solution which has been developed for such applications are photonic lanterns, allowing one to send the light into a number of single-mode fibers, where Bragg gratings perform well, and later back to a multimode fiber. The lantern principle also works with other kinds of waveguides, in particular with multi-core fibers. Fortunately, it is possible with carefully optimized methods to write rather uniform Bragg gratings even into multi-core fibers with a large core count [31].

The fabrication of fiber Bragg gratings often involves the illumination of the fiber core material with ultraviolet laser light (e.g. from a KrF or ArF excimer laser or other type of ultraviolet laser). Ths UV light induces some structural changes and thus a permanent modification of the refractive index. Such Type-I gratings are written with moderate intensities and exhibit an index grating right across the core.

The photosensitivity of the core glass is strongly dependent on the chemical composition and the UV wavelength: silica glass (as is often used for the cladding) has a very weak photosensitivity, whereas germanosilicate glass exhibits a much stronger effect, making possible a refractive index contrast up to ≈ 10⁻³. A significant further increase in photosensitivity is possible by loading the fiber with hydrogen (hydrogenated fibers). For that purpose, the fiber is kept in a high-pressure hydrogen atmosphere for some time. Another possibility is to use boron co-doped fibers.

Phosphate glasses are normally regarded as unsuitable for FBG fabrication, but special methods make this possible [26].

The first fiber Bragg gratings [1] were fabricated with a visible laser beam propagating along the fiber core, but in 1989 a more versatile technique was demonstrated by G. Meltz et al. [3], using the interferometric superposition of ultraviolet beams which come from the side of the fiber (transverse holographic technique). The angle between the ultraviolet beams determines the period of the light pattern in the fiber core and thus the Bragg wavelength. The two ultraviolet beams are often generated by exposing a periodic phase mask (photomask) with a single UV beam [4] (phase mask technique), using the two first-order diffracted beams. Non-periodic phase masks can be used to obtain more complicated patterns. Another technique is the point-by-point technique [22, 30], where the regions with increased refractive index are written point by point with a small focused laser beam. This is an appropriate (and very flexible) technique particularly for long-period Bragg gratings (see above), although extra propagation loss may occur at shorter wavelengths.

Instead of ultraviolet light, infrared light in the form of intense femtosecond pulses can also be used for writing Bragg gratings [5, 20] in various kinds of glasses. In that case, two-photon absorption essentially occurs only near the focus of the laser beam. It is even possible to write such gratings through the polymer coating of a fiber [24], since the intensity in the coating is much lower when the beam is focused on the fiber core. A totally different method also using infrared light is the fabrication of long-period FBGs in photonic crystal fibers by irradiation with a CO₂ laser beam.

Type-II gratings are often written by applying a substantial number of femtosecond pulses to each spot. However, it also possibel to use a single nanosecond pulse from an excimer laser (single-shot damage gratings). In that way, they can be written on the drawing tower [6] just before the fiber is coated. With such draw tower gratings, one avoids the process of removing an already fabricated coating, and obtains a grating with the full mechanical strength of ordinary fiber.

For applications at extremely high temperatures, where even Type II gratings might degrade or standard silica fibers drift, regenerated fiber Bragg gratings (RFBGs) can be the solution. These are fabricated by writing a strong “seed” grating (typically Type I) into a hydrogen-loaded fiber and then annealing it at high temperatures (e.g., 800 to 1000 °C). During this process, the original seed grating is erased, but a new, highly stable index modulation structure forms — likely due to crystallization or chemical rearrangement in the glass matrix. Regenerated gratings typically exhibit lower reflectivity than the seed grating but offer stability up to 1000 °C or even 1295 °C (in sapphire fibers), making them ideal for sensing in turbines, engines, and nuclear reactors.