It is shown that second-harmonic generation efficiency in a photovoltaic model depends on the relationship of illuminated size to grating period. Model test experiments are proposed and realized. To explain the photo-induced second-harmonic generation (SHG) in glass optical fibers, two groups of models have been suggested, some based on separation of charges and appearance of a strong electrostatic field the others based on orientation of defects. However, a strong electrostatic field (approximately equals 10⁴ V/cm) appears also in the last models. That is why neither the experiment on the discovery of such a field nor the experiment on the measurement of the component (chi)⁽²⁾ tensor ratio gives an answer to the preference of either model. This paper shows that the efficiency of SHG by a (chi)⁽²⁾ grating, resulting from a coherent photocurrent, depends strongly (approximately equals (r₀/(Lambda) )^-4) on the ratio of the transverse size r₀ of the light beam, used for the grating preparation, and the grating period (Lambda). So we think that an experiment on SHG in a bulk sample to check this point will make it possible to clear up the mechanism of (chi)⁽²⁾ grating formation. Another possibility is an experiment with the (chi)⁽²⁾ grating writing by pump and second-harmonic waves, propagating in opposite directions in a fiber. In this case the grating period will decrease to (lambda)_p/4 approximately equals 0.25 micrometers.

We describe some of the theoretical explanations of efficient second-harmonic generation in Ge-doped glass optical fibers known in the literature, and we discuss two models that are based on the reorientation of GeE' centers by means of selective electronic transitions. The first model, which uses selective absorption of photons, stimulated experiments on fiber preparation under UV-excitation. The experimental results, in turn, stimulated modification of the model and the formulation of a selective emission model.

Phase mismatch by angular detuning of a nonlinear crystal allows continuous variation of the intensity of second-harmonic light generated in the crystal, while maintaining a controlled phase relationship between the first and second harmonics. Interference of this second- harmonic light with second-harmonic light generated in another crystal further along the beam path has been measured, and can be understood by taking beam walk-off into account. When the second crystal is replaced by an optical fiber prepared for second-harmonic generation, the observed interference allows one to determine the phase of the (chi)⁽²⁾ grating in the optical fiber. We find the phase to be 88 +/- 4 degree(s). This phase differs by 180 degree(s) from recently reported results.

The different phenomena related to photoinduced self-organization in optical fiber are reviewed, from photoinduced absorption and refraction changes, to index grating formation and harmonic generation. Some pertinent questions regarding the origin and the physics of these effects are raised.

We have observed several differing photorefractive effects on transmission in polarization maintaining optical fibers. Both the strain-induced and shape-induced birefringence fibers can exhibit strong photorefractive behavior, depending on fiber composition, structure and pre- treatment, while pre-treatment alone can influence a single fiber sample to exhibit differing behavior. Subsets of these transmission effects appear to originate from qualitatively differing photorefractive mechanisms which we have not identified. Contrasting behavior is most clearly seen in the characteristics of photo-generated components such as reflection gratings and polarization couplers. High-efficiency polarization couplers, including 100% and overcoupled versions, can be made in some fibers, but can apparently always be erased by heating to 240 degree(s)C. Reflection gratings written in a fresh fiber sample appear to be thermally stable. However, further gratings, written in the same fiber sample after optical erasure of the original grating, are thermally erasable.

Photoinduced effects in optical waveguides are compared: photoinduced conversion of radiation polarization in lithium niobate optical waveguides and photoinduced second-harmonic generation in glass optical fibers. The cause of both phenomena consists in unusual interference between radiation with orthogonal polarization in one case and between radiation with a different frequency in the other one.

Second-order susceptibility ((chi)⁽²⁾) formation is described from the phenomenological point of view as a time-dependent cumulative medium response to the third-order optical rectification. Particularly, an approximate model is outlined that describes the beginning of (chi)⁽²⁾ grating formation at the interaction of quasi-monochromatic waves with relatively strong second-harmonic seeding, and the effects of linear dispersion and Kerr nonlinearities are discussed.

Ultraviolet loss spectra of optical fibers and preforms have been measured over the wavelength range from 200 nm to 600 nm. The observed spectra consist of a number of well-known absorption bands, most of which have been associated with germanosilicate related defects. We have found that the size of the absorption peaks in fibers is typically many orders of magnitude smaller than the corresponding absorption peaks in the preforms from which the fibers were drawn. In particular, the 240 nm absorption band, so evident in preform spectra, is very much weaker in the fiber's spectrum. Exposure of the fibers to broad-band uv radiation increases the size of the absorption peak at 325 nm, and also produces new bands at 380 nm and 305 nm, not previously ascribed to any defects. Fibers exposed to high power uv-laser radiation to form Bragg reflection gratings in the infrared, show the appearance of a previously unassigned band at 300 nm. Applying the Kramers-Kronig transformation to the measured changes in the absorption spectrum of the fiber between 200 nm and 600 nm, gives a calculated index change which is not sufficient to explain the observed refractive index change of approximately equals 10^-4.

Second harmonic generation in optical fibers through the self written grating or external seeding process has been so far limited to fundamental wavelengths not much longer than 1 micron. However, it is shown in this paper that fibers prepared at the sensitive wavelengths are also phase matched at longer wavelengths but for different mode combinations. By judicious choice of the fiber parameters, phase matching at any wavelength is possible. It is also shown that a set of fundamental and second harmonic modes which are phase matched through mode dispersion at some arbitrary wavelength, (lambda) , are also phase-matched at wavelengths on either side of (lambda) . Internally written grating-phase-matching is thus possible in these fibers for the same set of modes at longer wavelengths. It is also possible to design a fiber so that the frequency doubling bandwidth characteristics at the long wavelength is the same or better than the preparation wavelength; in particular broadband frequency doubling is possible under certain conditions.

The behavior of second-harmonic generation (SHG) is germanosilicate fibers in compared to recent results on similar effects in semiconductor microcrystallite glasses. The latter offer a much more readily characterizable system where many of the bulk semiconductor properties can be invoked to explain the results. The comparison suggests that extended state enhancement of nonlinear response and electric field dependent trapping in the germanosilicate glass system would unify the encoding process in both materials.

It is our aim to examine a few aspects of SHG in fibers which have not been sufficiently discussed in the literature, and concentrate our attention on the preparation process under exposure to UV light and to the problem of the phase of the waves involved. We also describe a simple picture which helps in guiding our efforts to understand the process, discussing a few points not noticed and/or not understood earlier.

We have measured the efficiency of photoinduced second-harmonic generation (SHG) in Ge- doped silica fibers containing different concentrations of 'GeO' and 'GeH' defects. The relative concentration of these defects were varied by heat treatments in H₂ and determined using luminescence spectroscopy. The results show that the GeH defect center, which can be excited in the visible (around 500 nm) and luminescence around 700 nm, plays a role in the photoinduced SHG process. On the other hand, the results do not allow us to rule out contributions from the GeO defects.

We report on specific experiments of optical phase conjugation by non-degenerate six-wave mixing processes involving fifth-order nonlinear susceptibilities. Both processes are enhanced by simultaneous one- and two-photon resonances with the second-harmonic of Nd:YAG laser. We show that light induced second-order susceptibility is much more efficient in noncentrosymmetric (Diethylaminonitrostolbene) than in centrosymmetric (Polydiacetylene) molecules in solutions. We obtain a 2.5 fm/v 210 picosecond-lived phase-matched frequency-doubling susceptibi1ity. The effect is an orientational hole burning in the isotropic distribution of non-centrosymmetric molecules.

In this paper we report experiments on the dynamic of self-organized (chi)⁽²⁾ gratings. An important observation is that the initial growth rate of (chi)⁽²⁾ gratings is always linear in time. A fast transient and slow fluctuations are also observed in certain cases. The initial growth rate of the grating amplitude as a function of the irradiances of the seeding fundamental ((omega)) and second harmonic (2(omega)) beams was measured and was determined to have a power dependence on I(2(omega)) of exponent 1.48 +/- 0.2, and on I((omega)) of 0.9 +/- 0.2. A model based on selective ionization of dipolar defects due to an interference between two-photon and three-photon processes has been explored in the context of these results.

We study the dynamics of phase grating formation with visible light in an optical fiber. Adopting a simple two-photon local bleaching model, we show the grating approaches an ideal grating, where the writing frequency is always in the center of the local band gap, as it evolves. The evolution at each point in the fiber can be described in terms of a universal parameter.

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.

Photoinduced defect centers by harmonics of 1.06 micrometers photons in Ge-doped silica are discussed and proposed defect models reviewed in relation to the self-organization phenomena, such as Hill gratings and second harmonic generation (SHG) in Ge-doped silica core fibers. In particular, we will show that the reported preparation kinetics of SHG in both seeded and unseeded processes can be explained in terms of defect centers induced by the fourth harmonic of 1.06 micrometers photons. The SHG erasure kinetics can also be understood by the reaction of defect centers responsible for SHG with free excitons.

This panel discussion addresses the following, and other questions: Is compaction taking place in the glass? How important is the 240 nm absorption band? What is the role of phosphorous as a codopant? Can we draw a clear picture of defects in optical fibers? The panel was formed of F. Ouellette, T. E. Tsai, R. Kashyap, D. Krol, and P. St. J. Russell.

This panel discussion asks the question: is a high conversion efficiency possible with optical fibers. This is an important question to ask since it will determine if this phenomenon will have practical applications or not.

The refractive index of germanosilicate and germano-aluminosilicate fiber can be increased by a few parts in 10⁵ to upwards of five parts in 10⁴ by photobleaching the oxygen vacancy defect band of germania. Typically, the fiber core is exposed through the side of the cladding to UV radiation at a wavelength from 240 - 250 nm. Permanent phase gratings are 'written' with a specified period by using a pair of intersecting coherent beams. We have investigated the fiber photosensitivity by monitoring the grating reflectivity and wavelength spectrum during the exposure. The Bragg wavelength shifts as the grating is 'written' due to a small increase in the average refractive index of the core. From these measurements, one can determine the growth and saturation characteristics of the photoinduced changes and study the effects of composition and high temperature hydrogen annealing. We have also measured the polarization sensitivity and thermal stability of gratings and the transient and permanent changes in fiber absorption.

A self-consistent model of second harmonic generation in fibers is based upon multiphoton ionization interference effects. An atom subject to a pair of harmonically related fields can exhibit a preferred direction of photoelectron emission that is dependent upon the relative phase between the two fields. Our model focuses on defects sites that require four fundamental frequency photons for ionization. In this case, phase-matched interference effects arise between four and three photon ionization and three and two photon ionization. We also qualitatively compare the four photon case with the two photon case in which there is only one interference term. The defects are modeled using a one-dimensional picture, with plane waves for ionized electron states. Both the four and two photon cases show exponential small signal spatial gain in steady state. The saturation characteristics can be different in the two cases, however. Results for the four photon model are in reasonable agreement with experimental observations. We discuss some of the limitations of our current model and possible future enhancements.