The characteristic curve and site-selective laser excitation of local relaxation in glass

The so-called characteristic curve describing photosensitivity change is elaborated and shown to be a powerful tool for understanding and characterizing photosensitive growth both at a fundamental and practical level. It has been used successfully to diagnose when optimal hypersensitization has been achieved and the physical basis for this is explained. By way of example, previous results using 355 nm hypersensitization are re-examined. Evidence of single site-selective glass relaxation through direct laser excitation offers a new approach to accessing and studying induced relaxations.

Hydrogen loading of optical waveguides by use of host diluent gases

Recent dramatic improvements in photosensitivity gas mixtures when very low hydrogen concentrations (as low as 0.1%) were used are shown to involve condensation of the host gas. The vapor pressure of the host determines the effective partial pressure of the hydrogen, and this accounts for the bulk of the observed enhancements.

All-fibre photonic crystal distributed Bragg reflector (PC-DBR) fibre laser

We describe an Er3+-doped aluminosilicate core photonic crystal fibre laser incorporating distributed Bragg reflectors written by two-photon 193nm irradiation through an optical phase mask as the feedback elements. The laser is diode pumped at 980nm and evidence of dual linewidth laser operation close to threshold is observed. However, at higher pumping levels gain competition preferentially selects one laser line.

Analysis and removal of fracture damage during and subsequent to holey fiber cleaving

The damage caused by cleaving holey fibers is investigated as a function of cleaving force. Comparisons are made with standard optical fibers and holey fibers. Optimum cleaving forces are determined for a number of holey fiber air fractions and fiber diameters. A simple technique for removing cleave damage is also presented.

Propagation in air by field superposition of scattered light within a Fresnel fiber

Propagation of light at 1.5 microm with peak intensity in an air hole is achieved within an air-silica-structured Fresnel waveguide. Thus a simpler fiber design alternative to photonic crystal fibers is possible for high-peak-power propagation with reduced nonlinear interactions. Multiple foci are observed in the far field.

Retaining and characterising nano-structure within tapered air-silica structured optical fibers

Air-silica fiber 125m in diameter has been tapered down to ~15m. At this diameter, it is commonly assumed that the nanostructured fiber holes have collapsed. Using an Atomic Force Microscope, we show this assumption to be in error, and demonstrate for the first time that structures several hundred nanometers in diameter are present, and that hole array structures are maintained. The use of Atomic Force Microscopy is shown to be an efficient way of characterising these structures.

Hypersensitization of rare-earth-doped waveguides for distributed-feedback amplifier and laser applications

Deterioration of the gain coefficient as a result of UV processing of hydrogen-loaded Er(3+) - and Er(3+)/Yb (3+) -doped silicate optical waveguides can be reduced by initial hypersensitization of the waveguide prior to bulk irradiation. This allows improved performance in active-grating devices such as distributed-feedback fiber amplifiers and lasers.

Stability of thermally hypersensitised phosphosilicate waveguides and the characteristic growth curve

Low temperature (80 0 C) hypersensitised hydrogen-loaded phosphosilicate optical fibre is found to be unstable, decaying progressively at room temperature. However, the hypersensitisation process linearises the grating growth characteristic curve. Further, a negative index contribution is inferred at low fluence in the presence of hydrogen.

Low-temperature hypersensitization of phosphosilicate waveguides in hydrogen

Conventional hydrogen loading of phosphosilicate optical fibers at relatively low temperatures (80 degrees C) is sufficient to enhance the fiber's photosensitivity after hydrogen outdiffusion, allowing permanent Bragg grating structures to be produced. Thermal sensitization is proposed to be a major contributor to stable index change.

Fabrication of long-period fiber gratings by use of focused ion-beam irradiation

Long-period gratings have been made in nonphotosensitive optical fibers by irradiation of the core of a fiber with a focused beam of high-energy protons. The irradiated fibers exhibit relatively low loss, even before thermal annealing, and possess strongly wavelength-dependent transmission. The absence of a mask provides the opportunity to tailor the grating to a desired profile, and a variety of grating profiles were explored. The profile most resembling a sinusoid was found to produce the cleanest transmission spectra.

Ultraviolet-induced absorption losses in hydrogen-loaded optical fibers and in presensitized optical fibers

Reduced hydroxyl formation in presensitized fibers exposed to cw 244-nm light after hydrogen outdiffusion is reported. The OH band in the presensitized fiber shifts toward 1390 nm. In the fully hydrogen-loaded fiber the OH band is centered at 1397 nm and does not shift with fluence.

Birefringence control in plasma-enhanced chemical vapor deposition planar waveguides by ultraviolet irradiation

Complete birefringence compensation is demonstrated in plasma-enhanced chemical vapor deposition waveguides by 193-nm postexposure. A single relaxation process dominates the decay in stress anisotropy, indicating that compressive stress from the substrate leads to an elastic stress anisotropy at the core.

Bending-induced colouring in a photonic crystal fibre

A photonic crystal fibre has been fabricated with a photonic crystal that is surrounded by a number of silica cores. Bending of the fibre induces an interaction between the core and photonic crystal areas, resulting in a highly wavelength-dependent loss of the core modes. White-light transmission experiments are presented which show that the colour of the transmitted light changes as a function of the fibre-bending radius. We compare the results to a simple model and find agreement.

Annealing properties of gratings written into UV-presensitized hydrogen-outdiffused optical fiber

Accelerated aging of gratings written in UV presensitized hydrogen outdiffused optical fibers show that these gratings are more stable than standard gratings written in hydrogen-loaded fibers. They are observed to grow initially by as much as 2%. The predicted decay after 25 years at 80 degrees C is ~0.1% . The results suggest that the index modulation decay is so slow that postfabrication annealing can be avoided.

Carbon dioxide laser-assisted poling of silicate-based optical fibers

A novel poling method, carbon dioxide laser-assisted poling (CLAP), is demonstrated. Localized heating during CLAP is achieved through silicate absorption of the CO>(2) laser radiation. Electro-optic coefficients of 0.2 pm/V are achieved within a 55-s scan of a twin-hole fiber. It is shown that there is a range of CO>(2) laser powers for which the residual electro-optic coefficient is maximized.

Engineering large anisotropy in amorphous glass

By exploiting the strain-related rollover thresholds that occur between UV-induced positive and negative index regimes in germanosilica glass it is possible to engineer large anisotropy into the structure. This is qualitatively analyzed for a planar waveguide and has been confirmed experimentally. However, the technique described here is not confined to waveguide devices and can in principle be applied to any glass structure that is capable of undergoing strain-sensitive transitions.

Locking in photosensitivity within optical fiber and planar waveguides by ultraviolet preexposure

Strong photosensitivity is locked in permanently after hydrogen outdiffusion of hydrogen-loaded waveguides presensitized with UV light by either pulsed 193-nm or cw 244-nm laser output.

Correlation of ultraviolet-induced stress changes and negative index growth in type IIa germanosilicate waveguide gratings

The stress anisotropy in planar germanosilicate waveguides is used to follow the changes in stress, measured as a birefringence, with UV irradiation. Compressive stress increases with the growth of the negative index grating, saturating when the grating modulation saturates.

High-temperature stable gratings in germanosilicate planar waveguides

Negative index gratings in planar germanosilicate waveguides were found to be stable up to 500 degrees C. The annealing properties are similar to those of negative index fiber gratings.

Grating-based transmission bandpass filters using dispersion-matched mode conversion

A narrow-bandpass filter with a 15-dB signal-to-noise level is demonstrated using Bragg-grating assisted mode conversion within a tapered waveguide. The potential effects of such filters in wavelength-division multiplexing systems are discussed.

Complex photosensitivity observed in germanosilica planar waveguides

Photosensitive effects distinguished as type I and type IIA photosensitivity within optical fibers were observed in a much more pronounced form within germanosilica waveguides deposited by hollow-cathode plasma-enhanced chemical-vapor deposition. With increasing exposure to 193-nm UV light, positive index changes greater than 2 x 10(-3) were observed, followed by negative index changes greater than -5 x 10(-3) . These behaviors are attributed to an increase in macroscopic polarizability and a reduction in material density, respectively. The negative index change is more temperature resistant and is fully annealed only at 900 degrees C, whereas the positive one is annealed at 500 degrees C.

Characterization of apodized fiber Bragg gratings for rejection filter applications

Apodized fiber Bragg gratings with > or = 30-dB sidelobe suppression +/-1 nm from the Bragg center wavelength are spatially characterized with side-scatter and side-diffraction techniques. Resonant side scatter is suppressed, indicating that little dispersion arises from reflections off the grating boundaries.

Modal interferometer for in situ measurements of induced core index change in optical fibers

A simple interferometer based on modal interference is described. Without compromising interferometer stability it is possible to measure index changes of less than 1 x 10(-6).

Incoherent scattering losses in optical fiber Bragg gratings

Scattered-light losses from UV-processed optical fibers and optical fiber Bragg gratings are investigated for wavelengths in the vicinity of the Bragg wavelength. Broadband incoherent scattering loss arising from nonuniform grating elements was measured ranging from 5 x 10(-5) to 0.2 dB/cm, depending on fiber type and (or) processing conditions. Asymmetry of the radially scattered-light profile is consistent with the grating elements' being localized to within a few micrometers of the core -cladding interface.

Rayleigh longitudinal profiling of optical resonances within waveguide grating structures using sidescattered light

The resonant characteristics of Bragg gratings in fibers are characterized by measurement of the wavelength dependence of the Rayleigh scattering of light propagating in the core along the grating length. This technique is found to give information on the standing-wave intensity profile in the grating that cannot be deduced from the standard transmission or reflection spectra.