D.L. Williams

Photosensitive index changes in germania-doped silica glass fibers and waveguides

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.

Photoconduction in germanium and phosphorus doped silica waveguides

Photoconduction measurements on Ge‐P doped silica planar ridge waveguides exposed to several miliwatts of ultraviolet radiation are reported. It is shown that there is a strong reduction in the photocurrent with time of exposure pointing to a depletion in the available photoactivated carriers. The density of defects is directly inferred from the measurements.

Second-harmonic Generation in Glass Fibres

Abstract Experiments on frequency doubling of 1064 nm and 1053 nm radiation with non-phase-matched and (close to) phase-matched germania-doped silica fibres are reported. The greatest conversion efficiencies are obtained with non-phase-matched fibres that are single-mode at the second-harmonic wavelength, in which phase-matching is thought to arise from a self-written grating. Competition between grating self-writing and self-erasure in these fibres in the absence of an external seed is also reported. Phase-matched fibres allow the formation of the nonlinearity to be studied independently of the phase matching. The nonlinearity is optically induced by exposing the fibre to short-wavelength visible radiation. An enhancement of 104 to 105 in conversion efficiency between the fundamental and the phase-matched LP31 second-harmonic mode over the intrinsic background second harmonic has been observed. We also report on the occurrence of phase-matched second-harmonic generation (SHG) and non-phase-matched SHG in...

Ultraviolet absorption studies on photosensitive germanosilicate preforms and fibers

Detailed measurements of the ultraviolet absorption spectra of germanosilicate preforms, multimode and single‐mode fibers are presented. Significant differences between the spectra of preforms and fibers are revealed and these differences are thought to be caused by the stresses induced during fiber pulling. Measurements are also presented of the changes in the absorption spectra in single‐mode fibers into which reflection gratings have been written using an ultraviolet laser. These results suggest that changes in the absorption between 200 and 600 nm are not responsible for the inferred changes in the refractive index.

UV spectroscopy of optical fibers and preforms

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 fibers 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.

Spectral, temporal, and spatial study of UV-induced luminescence in Ge-doped fiber preform

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.

Photosensitive germanosilicate preforms and fibers

A detailed study of the ultraviolet loss spectra of germanosilicate preforms and fibers is reported. The fiber samples had peak losses of the order of 10 dB/cm, which was several orders of magnitude lower than in preform samples. All spectra could be described by a number of previously observed absorption bands, many of which had been associated with particular defects in germanosilicate glasses. The incorporation of phosphorous as a core codopant eliminated a loss band centered at 325 nm in as drawn fibers. The effect on the ultraviolet absorption spectra of increasing doses of irradiation from an ultraviolet fiber- coating curing-lamp source and also the change in fiber absorption before and after writing a 35% reflection grating are presented. The results are not consistent with the previously proposed model for photosensitivity based on the Kramers-Kronig mechanism.

Spatial distribution of UV-excited luminescence in Ge-doped fiber preforms

Abstract The spatial distribution of the 240 nm absorption and its associated luminescence measured in a Ge-doped silica optical fibre preform are presented. The 400 nm luminescence was found to be maximum at the edges and near the centre of the core, while the absorption follows the Ge concentration in the core, implying that the luminescence is a nonlinear function of the Ge concentration. Quenching of the luminescence by either the effect or Ge concentration is proposed to explain this observation.

Photosensitive glass integrated optical devices

Photosensitive glasses for applications in integrated optical devices have recently become a popular research topic, due primarily to the success of the germania doped silica fibre counterpart. Whole ranges of devices could in principle be fabricated utilising the photosensitivity phenomenon, from directly written waveguides to more complex grating structures. A key parameter in all applications is the change in material refractive index after light exposure. Considerable effort has been devoted to understanding mechanisms responsible for the phenonema with attempts to maximise the effect. To date index changes achieved are ~ 10-3 which is sufficient for the production of highly efficient holographic gratings within a waveguide, but not quite large enough for direct waveguide formation. Various writing schemes have been developed, but most are based on UV laser sources operating in the 240 - 270nm wavelength range to coincide with a germanium related defect absorption. High quality gratings, with reflection efficiencies of 14dB have been written. These gratings have been used as feed back elements in external cavity lasers and when incorporated in a Mach-Zender interferometer, a channel dropping filter has been demonstrated. With continued effort, more compact optical devices utilising gratings and the photosensitivity effect can be expected in parallel with the fibre development work.

Phase-matched second-harmonic generation of infrared wavelengths in optical fibers

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.