Philip St.J. Russell

Solitary thermal shock waves and optical damage in optical fibers: the fiber fuse

Fresh experimental and theoretical results on thermally induced catastrophic breakdown (the fiber fuse) in optical fibers are presented, including the observation that the damage is not always irreversible and an analysis of the complex unsteady absorption-heat-conduction process that controls the effect. Good agreement with experiment is obtained with just two independent parameters. The analysis shows that the fiber fuse is a new kind of solitary thermal shock wave in whose leading edge the temperature gradients can reach several thousand kelvins per micrometer.

Acousto-optic superlattice modulator using fiber Bragg grating

Summary form only given. Fiber Bragg gratings (FBGs) written with ultraviolet light are becoming of increasing importance in fiber systems because of their high performance, versatility, and excellent physical characteristics. In this paper, we report a technique that transforms a FBG into a Bragg cell or acousto-optic modulator operating in reflection mode.

Photonic Bloch waves and photonic band gaps

Photonic band gap materials are dielectrics with a synthetic, three dimensional, multiply periodic microstructure (lattice constant of order the optical wavelength) whose distinguishing feature is a very large modulation depth of refractive index. When appropriately designed, these “photonic crystals” exhibit ranges of optical frequency where light cannot exist — the photonic band gaps 7. The current interest in these materials 1–22 has led us to re-appraise propagation in structures that, while not exhibiting a complete photonic band gap (PBG), nevertheless display anomalous and intriguing propagation effects in the vicinity of their Bragg conditions54–58. In most cases, around each Bragg condition appear incomplete momentum and energy gaps (i.e., ranges of, respectively, wavevector and frequency where propagation is forbidden) with widths that are given approximately by the product of the index difference with, respectively, the vacuum wavevector and h times the optical frequency. With the exception of the multi-layer dielectric stack, most conventional electromagnetic gratings, such as those encountered in holography 27, waveguides 45, distributed feedback lasers 35,37,38, acousto-optic 47 and x-ray 61 diffraction, consist of weak periodic perturbations about a mean refractive index. In these gratings, while strong spatial and temporal dispersion are present around each Bragg condition, the ranges of angles and frequencies over which this occurs are very narrow; and although PBG’s do appear, they are incomplete and mostly very weak.

Effect of poling conditions on second-harmonic generation in fused silica.

A systematic study of the effects of poling time and applied voltage on second-harmonic (SH) generation in thermally poled silica glass reveals that the SH signal is proportional to the square of the applied voltage and that the speed of the poling process increases with the applied voltage. Prior treatment of the samples is found to affect the poling process, and the optimum poling conditions are observed to depend on the poling atmosphere. The mechanism of thermal poling is discussed in the light of these new results.

All-fiber sliding-frequency Er{sup 3+}/{ital Yb}{sup 3+} soliton laser

We report a simple all-fiber sliding-frequency soliton laser incorporating a recently developed low-insertion-loss acousto-optic fiber frequency shifter. The frequency shifter simultaneously provides the spectral filtering and polarizing properties required for pulsed operation to be obtained. {copyright} {ital 1995 Optical Society of America.}

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

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.

Electron implantation: a new technique for creation and modification of second-order susceptibility in glasses

Recent work on glass poling and depoling using electron implantation is reviewed, including preliminary results on the level of second order nonlinearity induced in glasses of different composition, experiments to clarify the form of the induced second order tensor in thermally- poled silica, and proposals on the underlying mechanisms both of electron implantation and thermal poling.

Optical fibre electrets and electro-acousto-optic transduction

Optical fibres have been used to sense many different physical quantities (magnetic fields, vibrations, temperature, rotation etc.). There is however, one important exception; optical fibres are insensitive to electrical fields due to the inversion symmetry of the glass matrix, which ensures that the Pockels coefficients are zero. Most of the electric field sensors reported so far are hybrid devices, employing an extrinsic active component constructed from a piezoelectric or electrooptic crystal. Only one report exists of the observation of a Pockels effect in optical fibre poled with high electric fields, but the value of the induced electrooptic coefficient was very small (of the order of 10 pm/V) and unstable. A recent break-through is the recent observation of high second-order nonlinearities of the order of 1 pm/V in glasses and 0.2 pm/V in optical fibres using a variety of different techniques: thermal poling, corona poling, and electron implantation. Based on these results it is possible to expect a value of electrooptic coefficient in poled fibre of the same order as in crystalline quartz (1 pm/V). This value is high enough to construct an all fibre-optic electrooptic field sensor. In this paper we report the observation of a strong response of thermally poled fibre to an electric field. Phase shifts as high as 1 rad have been obtained at applied voltages of 50 V