A temperature/static electric field poling process for inducing a second-order nonlinearity in fused silica is discussed. Time-resolved measurements support a model of sodium migration leading to a large internal electric field in the bulk fused silica. In addition, UV- photoluminescence studies suggest dipole orientation as another possible mechanism.

Evidence for different microscopic mechanisms of coherent photovoltaic effect in lead glass and semiconductor microcrystallite-doped glass is obtained by temperature studies of photoinduced second harmonic generation.

We have investigated the influence of Nd:YAG laser light at 1064 nm on the grating growth and erasure processes for 790/395-nm light in Tm-doped fibers. The addition of 1064-nm light to the writing pair of 790/395 nm was observed to cause at most a slight increase in writing speed in a low-Tm fiber; in a high-Tm fiber, however, it caused a rapid drop in second- harmonic generation (SHG). This drop is determined to result from Nd:YAG-induced photodarkening, which can be partially reversed by prolonged exposure to 790-nm light. These results imply either that intense Nd:YAG irradiation does not produce mobile electrons or that the mechanism of second-harmonic generation in these fibers does not involve static macroscopic electric fields.

We report experimental studies of photoinduced second-harmonic generation (SHG) in lead glasses with different PbO concentrations. The dependencies of photoinduced second-harmonic saturation efficiency on PbO concentration and calculated third-order nonlinearity are observed. The optical concentration of PbO for photoinduced SHG is found to be approximately 50 wt%. Experimental results are interpreted on the base of photovoltaic model.

The nonlinear optical process which permits the optical preparation of centrosymmetric materials for frequency doubling is described in terms of an orientational hole-burning effect induced through non-zero E³ resonant six-wave mixing. The transient all-optical poling of solutions of organic molecules is demonstrated in a configuration of phase-conjugation pumped at half frequency. A permanent all-optical poling is achieved by seeding preparation under backward and forward geometries in an azoaromatic acrylic copolymer. Figures of merit of the optical nonlinearities relevant to the frequency-doubling process are established. This offers a new molecular engineering route to the control and optimization of phase- matched oriented microstructures for frequency conversion.

Ultraviolet light irradiation of optical fibers or silica-on-silica channel optical waveguides photoinduces a permanent refractive index change in the core of the optical waveguide. The effect called `photosensitivity' provides a versatile photolithographic means for processing glass in the form of optical fiber or planar optical waveguides in order to fabricate optical waveguide devices that have applications in optical fiber communications and optical sensor systems. This paper reports on some recent experimental results on photosensitivity in optical fibers and planar optical waveguides and its use in the fabrication of optical waveguide devices.

Recent developments in the photosensitivity of germania doped silica glass are reviewed. The cause and nature of the absorption band centered near 240 nm are discussed and possible structures for this defect are reported together with a summary of the proposed mechanisms for the UV index change. Experimental techniques for assessing the refractive index change are listed and this leads to a consideration of the effects of non-ideal grating formation. Mechanisms for enhancing the photosensitivity of fibers and germania doped silica films are assessed using a systematic study of the photosensitivity of fibers fabricated under the different techniques. Finally some of the possible applications for photosensitivity are briefly discussed.

Using a KrF excimer laser operating at (lambda) equals 248 nm, we have demonstrated that fiber gratings having essentially 100% reflectivity can be manufactured using a single 20 ns pulse. In the course of studying the dynamics of the grating formation process, it was found that transient optical gains of as much as 8 dB/cm could be obtained in germanosilicate fiber. The first distributed feedback fiber laser has been made using this mechanism.

Second harmonic generation and the Bragg reflection gratings building up have attracted interest as novel nonlinear optical properties of SiO₂:GeO₂ fibers. The presence of optical absorption band centered at 5 eV is indispensable for emergence of these functions. In this article, we review a series of our recent works on optical properties of the 5 eV band and structural models for the responsible defects. A way to enhance concentrations of the defect is also shown.

Photoinscriptions of Bragg gratings have been carried out within several germanosilicate fibers via UV side writing near 243 nm. Real-time measurements of the spectral transmission, of the FWHM bandwidth and of the resonance wavelengths of the Bragg gratings lead to noteworthy observations when long irradiation times are used. Thus, second order diffraction efficiency has been observed from Bragg gratings written within some fibers. Second order diffraction can be detected once the first order grating reflectivity has begun to saturate. Whereas the dynamics of the UV induced excess losses may vary from one sample to another, no obvious direct correlation can be established between the dynamical evolutions of the loss spectra and the features of the grating growth.

The mechanisms of photorefractivity in germanosilicate fibers are considered. Evidence is presented that high intensity irradiation significantly changes the state of the glass network in the irradiated core. The large changes of refractive index (>>10^-4) are attributed to these structural changes. The possible roles of stress relief, nanocavitation and spinodal decomposition are discussed.

Thermal stabilities of photoinduced paramagnetic defects in second-harmonic (SHG) generation Ge-P-doped silica fibers and in Ge- and Ge-P-doped silica preforms are studied by electron-spin-resonance and compared with that of the reported self-organized gratings. Our data suggests that the charge trapping sites for the electric-field-induced SHG are Ge(1), Ge(2) and Ge E'_d1 centers. The defects responsible for the Bragg gratings are Ge E'-type centers. Thermal darkening in Ge-doped silica core fibers reported is likely due to the thermal induced Ge E'_d1 center.

A kind of fluorescence around 800 - 950 nm, 620 - 650 nm, 440 - 450 nm and 390 - 420 nm has been achieved in silica-based optical fibers by high power pumping at 1.06 or 0.532 micrometers . We show that it is non-dependent on the index profile but on the doping. The first and the fourth emissions arise from a reduced germanium defect. The second and the third ones are from the silica network. Life-time measurements performed with a streak-camera show that all emissions have the same time behavior as the pump. Adding the observation that these lines are often accompanied by conjugated lines through Four Photon Mixing, we suggest for blue and visible lines, a combined nonlinear process: the Four Photon Mixing Stimulated Fluorescence. We point out that there can be other fluorescence induced nonlinear processes like with stimulated Raman scattering or second harmonic generation.

The growth of Second Harmonic Generation (SHG) in fibers has been observed to saturate with time as well as with length and can actually decrease after prolonged exposure to fundamental light. We have measured the length dependence of the SHG as a function of time during preparation, saturation and subsequent exposure to IR light. We have also measured the time dependence of erasure of the (chi) ⁽²) grating with exposure to various amounts of green light. Based on these measurements we have been able to determine the parameters of the asymmetric photoionization model, specifically the current and two photon absorption rate. We have performed preliminary experiments using planar waveguides to measure the asymmetric photocurrent.

Phenomenological theory of photoinduced second-harmonic generation is considered under the assumption that third-order photogalvanic effect is responsible for spatial separation of charges and formation of electrostatic field grating in glass. Third-order photogalvanic effect of three types is taken into account and the main features of kinetics of formation of the grating are discussed.

In this work we use a simple and powerful technique (Heat scan) to characterize frequency doubling gratings in fibers. The technique is based on scanning a small heating element along the fiber, thereby affecting the phase relationship of the involved waves in a controlled way. We present recent measurements of the time evolution of the (chi) ⁽²) grating during the preparation process, and discuss aspects of the grating phase.

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.

It is demonstrated that simultaneous exposure of optical fibers to continuous wave ultraviolet, 1.064 micrometers and 0.532 micrometers radiation prepares it for efficient second-harmonic generation. A theory of second harmonic generation is proposed on the basis of the observations.

Photogalvanic effect induced by joint action of optical wave and `external' coherent waves of arbitrary origin arising in the second-order approximation in powers of both optical electric field and `external' field and microscopic models of the effect caused by point defects in silica glass are considered. Spatially-periodical gratings of electrostatic field with arbitrary spatial period may be formed in the glass owing to the forth-order photogalvanic effect caused by optical and acoustical waves acting together.

The side-writing of Bragg gratings in fibers by exposure to a periodic UV intensity pattern is discussed. The spectral characteristics of gratings in depressed index fiber are described including the formation of higher-order reflection bands and coupling to cladding modes. Exposure requirements are compared with photosensitivity measurements of the refractive index change induced by individual KrF laser pulses. Commercial germaosilicate thin film waveguides are also found to be photosensitive. Results are presented on UV-induced lateral wave confinement in H₂-loaded film and grating formation in standard buried channel guides.

The spectra of Nd³⁺ and Pr³⁺ doped fiber lasers utilizing two intracore Bragg grating reflectors written at 1084 nm Bragg wavelength have been investigated under 30 MHz resolution. A spectral vernier effect was induced by the subcavities created by reflections from the fiber end faces and from Bragg gratings. We made use of this effect to enhance the sensitivity of the measurement of the fiber temperature rise which arises from absorption of the pump power. Fresnel reflection was also used to reduce the linewidth of the laser emission.

Photogenerated mode coupling filters for temperature and strain sensing are described. Mach- Zehnder fiber interferometers have been fabricated in elliptical-core, high-birefringence fibers where the directional couplers of a conventional fiber interferometer are replaced by two polarization rocking filters written on the same fiber. One interferometer having an operational bandwidth of 18 nm was used as a temperature sensor with a measured sensitivity of 9.1 mrad/($DEGC cm). Photoinduced polarization couplers written with 266 nm light can survive temperatures of 200$DEGC. This allows development of a temperature sensor based on the shift of the resonant wavelength of the coupler with temperature. A scheme for the simultaneous measurement of strain and temperature based on the use of a Bragg grating and a mode coupling filter is proposed.

The cathodoluminescence technique has been used to analyze germanosilicate optical fiber preforms fabricated by the MCVD process. The distributions E' and drawing-induced defects have been measured and the effects of some co-dopants and fabrication conditions on the distributions have been investigated. The results have been explained in terms of a simple reaction scheme involving the electron donor and acceptor properties of co-dopants.

The luminescence of a Ge-doped fiber preform excited by 240 nm light was studied. Luminescence at 290 nm, 400 nm, and 650 nm was observed, and lifetimes were measured, as well as the spatial profiles. For the first two components, the spatial profile is different from the index profile and the absorption profile, while the 650 nm luminescence follows the index profile. We also measured different lifetimes at different positions across the core.

An overview is given of permanent changes in the refractive index and optical transmission which occur in some rare earth-doped inorganic glasses and optical fibers when resonantly excited into specific 4f and 5d manifolds. These phenomena are believed to be electronic in nature involving color centers. They are important for both refractive index grating devices as well as for the optical reliability of Er³⁺-doped optical fiber amplifiers.

Rare earth-doped glasses exhibit high initial photosensitivity but their response saturates at relatively modest values of (Delta) n (approximately 5 X 10^-7), which greatly limits their usefulness for device applications. In the context of our model, saturation results from either exhaustion of photosensitive rare earth sites, trap sites, or through competition between two photon creation and one photon bleaching processes. In this paper we report the results of new experiments designed to further elucidate the photosensitivity process with specific emphasis on the saturation mechanisms(s). Based on these new experimental results we present a refinement of our earlier model.

The index change induced by two-photon absorption of green light in Ge-doped optical fiber is partly anisotropic. We review the experimental facts that form the basis for a physical model of bleachable oriented defects. We also describe how a density matrix formalism, based on a simili three-level system, can be used to determine the tensorial properties of the photoinduced index change.

Photorefractive index gratings written by holographic method in commercial single mode silica fiber have been considerably studied. In this paper, we experimentally demonstrate the formation of an index grating in a multimode LiNbO₃ monocrystal fiber written by ordinary holographic method. Experimental techniques and results as well as theoretical analyses are presented.

Optically induced second harmonic generation in glasses with large PbO content was studied. Tensor and phase properties of encoded (chi) ⁽²) susceptibility indicate the presence of electrostatic field, which is proportional to E(EE) where E is a sum of real recording waves E_(omega ) and E_2(omega ). The model describing charge separation as a result of (chi) ⁽³) optical rectification is discussed.

Single-photon photodarkening of a germanosilicate optical fiber preform is reported and discussed in the context of oxygen deficiency defects. Qualitative and semi-quantitative aspects of a multiphoton interference model of photorefractive change in germanosilicate fibers are explored. The conditioning of production of yellow Cerenkov radiation by a fiber is reported and attributed to homogeneously induced (chi) ⁽²) susceptibility. Two nonlinear processes are again thought to be involved. The need for mode detuning to take full advantage of waveguide nonlinear effects is highlighted.

Optical low coherence reflectometry (OLCR) is applied to the precise characterization of Bragg gratings in optical fibers. Grating with lengths below two millimeters have been fabricated in standard telecommunication fibers. Their OLCR spectra can be used to determine the induced index changes, which range from 0.64*10^-3 to 1.16*10^-3, with an accuracy of a few percent. These changes are obtained by adjusting the position of the oscillation minima in the OLCR spectra.

Focusing intense laser light along with some of its second harmonic into a glass sample transforms the glass into a frequency doubler. We present a new method to measure the optical phase shift between the second-harmonic beam used to seed the glass and the second- harmonic beam subsequently produced by the glass sample. Determination of this phase shift is essential for understanding the growth dynamics of the effect, and its value can discriminate between proposed theoretical models.