Since the discovery of photosensitivity in optical fibers there has been great interest in the fabrication of Bragg gratings within the core of a fiber. The ability to inscribe intracore Bragg gratings in these photosensitive fibers has revolutionized the field of telecommunications and optical fiber based sensor technology. Over the last few years, the number of researchers investigating fundamental, as well as application aspects of these gratings has increased dramatically. This article reviews the technology of Bragg gratings in optical fibers. It introduces the phenomenon of photosensitivity in optical fibers, examines the properties of Bragg gratings, and presents some of the important developments in devices and applications. The most common fabrication techniques (interferometric, phase mask, and point by point) are examined in detail with reference to the advantages and the disadvantages in utilizing them for inscribing Bragg gratings. Reflectivity, bandwidth, temperature, and strain sensitivity of the Bragg reflectors are examined and novel and special Bragg grating structures such as chirped gratings, blazed gratings, phase-shifted gratings, and superimposed multiple gratings are discussed. A formalism for calculating the spectral response of Bragg grating structures is described. Finally, devices and applications for telecommunication and fiber-optic sensors are described, and the impact of this technology on the future of the above areas is discussed.

Fiber Bragg gratings

A large second-order nonlinearity [ χ(2)~1 pm/V~0.2 χ22(2) for LiNbO3] is induced in the near-surface (~4 μm) region of commercial fused-silica optical flats by a temperature (250–325°C) and electric-field (E ~ 5 × 104 V/cm) poling process. Once formed, the nonlinearity, which is roughly 103–104 times larger than that found in fiber second-harmonic experiments, is extremely stable at room temperature and laboratory ambient. The nonlinearity can be cycled by repeated depoling (temperature only) and repoling (temperature and electric field) processes without history effects. Possible mechanisms, including nonlinear moieties and electric-field-induced second-order nonlinearities, are discussed.

Large second-order nonlinearity in poled fused silica.

Photoinduced guided index changes approaching 10−4 in the range 488–784 nm, measured using a simple interferometric technique, are reported in germanosilicate single-mode optical fibers exposed to the 488-nm line of an Ar+ laser running multifrequency. The wavelength dependence and dynamics of the writing process are characterized, and the material dispersion of the induced Δn(λ) is shown to be weak. The effect is placed in the context of related research on color centers in these fibers, and two different mechanisms are proposed that lead to quantitative estimates in rough agreement with the measured Δn values.

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Photoinduced refractive-index changes in germanosilicate fibers.

Fibres are typically used as passive devices, whether in fibre-optical cables used in telecommunciations or as yarns for clothing. The demonstration of polymer-based piezoelectric fibres that can be drawn to tens of metres in length, and whose acoustic response can be actively controlled, suggests possible applications in, for example, medical imaging or acoustic sensing.

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Multimaterial piezoelectric fibres

This review presents the state of the art of the study of germanium-related colour centres in silica glass and silica-based fibres. Most attention is concentrated on the dominant colour centres such as the Ge-related oxygen-deficient centres and paramagnetic Ge(n) centres, all of them having ultraviolet absorption bands. The hypothetical models and formation mechanisms proposed so far for these colour centres are discussed in detail. The origins of the models and formation mechanisms and their weak and strong points are analysed. The origin of the less well studied Ge-related colour centres in the visible spectral range (GeH, GeX, drawing-dependent defects) is also discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/6/35/003/pdf

Colour centres in germanosilicate glass and optical fibres

It is shown that large permanent enhancements in second-order optical nonlinearity can be induced in germanosilicate fibers (both pure and codoped with phosphorus) by application of a transverse dc electric poling field in the presence of high-intensity light. The macroscopic inversion symmetry of the core material is broken by the excitation and alignment of defect centers. Significant frequency doubling results despite the absence of phase matching. The saturation (with both increasing dc field and increasing intensity) of this effect is investigated.

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Generation of permanent optically induced second-order nonlinearities in optical fibers by poling.

We report evidence in support of the view that induced loss and nonlinear transmission in pure germanosilicate fibers at blue-green wavelengths are governed by the formation (through two-photon absorption), spontaneous and stimulated transformation, and bleaching (through single-photon events) of Ge(1), Ge(2), and Ge(3) color centers. Using a tunable pulsed-dye laser, the excitation spectrum of the induced absorption, its spectral attenuation, and the effects of Ge concentration and thermal annealing are investigated.

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Nonlinear transmission and color-center dynamics in germanosilicate fibers at 420-540 nm.

The present understanding of colour centres in germanosilicate glass fibres and the diverse effects attributed to colour centre activity are reviewed. Drawing on a wide range of up-to-date research results, an attempt is made to piece together as far as possible a unified picture of the defect processes behind second harmonic generation, nonlinear transmission and photorefractive grating formation in optical fibres.

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Frequency doubling, absorption, and grating formation in glass fibers: effective defects or defective effects?

Over the past five years, a color-center model for the dynamics of the absorption induced in germanosilicate fibers upon exposure to blue/green light has been under development at Southampton. This model is introduced and its predictions used for the first time to test our proposed Kramers-Kronig mechanism for the concurrent refractive index changes induced in the visible and the infrared. It is found that the predicted color-center population changes in the UV are insufficient to explain these refractive index changes. A possible alternative model, based on density changes in the glass triggered by color-center formation, is assessed experimentally and analytically. The implications of this result to photonically driven self-organization in fibers is briefly assessed, and reference made to recent experimental results.

Optically induced creation, transformation, and organization of defects and color centers in optical fibers

The formation of efficient holographic second-harmonic generation in high-birefringence phosphorus-doped germanosilicate fibers is reported. The influence of optical polarisation on the nonlinear writing and read-out processes in explored. Fiber birefringence permits phase-matched second-harmonic conversion at wavelengths within ±125 /cm (±14nm) of the writing wavelength (1.061 µm).

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L.J. Poyntz-Wright

Papers

Generation of permanent optically induced second-order nonlinearities in optical fibers by poling.

Nonlinear transmission and color-center dynamics in germanosilicate fibers at 420-540 nm.

Frequency doubling, absorption, and grating formation in glass fibers: effective defects or defective effects?

Optically induced creation, transformation, and organization of defects and color centers in optical fibers

Tunable holographic second-harmonic generators in high-birefringence optical fibers.