UV-induced Refractive Index Change Research Articles

UV-induced refractive index changes due to silver molecular clusters in photo-thermo-refractive glass

It is shown experimentally that in photo-thermo-refractive glasses the transformation of charged silver subnanosized molecular clusters to neutral state by UV irradiation results in the increase of glass refractive index. Quantum chemistry calculations suggest that such refractive index change is mainly dominated by silver neutral cluster polarizability. In addition to discussion of proposed mechanism, the examples of possible applications are also presented.

Kinetics of Thermally Activated Physical Processes in Disordered Media

We describe a framework for modeling the writing and erasure of thermally-distributed activated processes that we can specifically apply to UV-induced refractive index change, particularly in fibers. From experimental measurements (isochrons and/or isotherms), this framework allows to find the distribution function of the activation energy by providing only a constant, which can be determined by a simple variable change when a few assumptions are fulfilled. From this modeling, it is possible to know the complete evolution in time of the system. It is also possible to determine the annealing conditions for extending a lifetime. This approach can also be used for other physical quantities, such as photodarkening, stress relaxation, and luminescence decay, provided that it can be described by a distribution function.

Ultraviolet laser-induced degradation of CsLiB_6O_10 and β-BaB_2O_4

We investigated ultraviolet (UV) laser-induced degradation of nonlinear optical crystal CsLiB6O10 (CLBO) and β-BaB2O4 (BBO) using a high-repetition-rate pulsed laser. In this research, we found that the degradation of CLBO was caused by a UV-induced refractive index change at a peak power density higher than a few tens of MW/cm2. On the other hand, BBO exhibited lower UV transmittance at a higher intensity of 10 MW/cm2 and the UV absorption increased with time due to the formation of an absorption center. Moreover, we confirmed that the degradation resistance of CLBO with fewer light scattering defects was improved.

Fabrication and ultraviolet photosensitivity of Ge-doped silica films using FHD for optical waveguide

UV photosensitivity of Ge-doped silica films deposited on Si (1 0 0) substrates using flame hydrolysis deposition (FHD) has been investigated. The ratio of Ge and Si of the sample was estimated by XPS as 10:90, and it was shown by AFM as smooth and homogeneous. It seems to have not germanium oxygen deficiency centers which play a role in the change in refractive index after UV irradiation. The irradiation of a H 2-unloaded and a loaded film with fluency of 190 mJ/cm 2/pulse at 10 Hz to KrF excimer laser induced a relative value of the refractive index change of 5.46×10 −4 and 2.94×10 −3 at 1550 nm, respectively. Optical absorption and PL spectra of our FHD H 2-loaded sample demonstrate that Ge 2+ center (constituting GeO defect) was produced by a reaction of the germanosilicate glasses with the H 2 molecule, which could lead the higher UV-induced refractive index change after irradiation. Therefore, H 2 loading is sufficient to significantly increase the photosensitivity of Ge-doped SiO 2 film to 248 nm light.

Enhanced photosensitivity in sol–gel derived 20GeO2:80SiO2 thin films

We investigate the photosensitivity of binary 20GeO2:80SiO2 (germanosilicate) inorganic films. The samples have been fabricated by the sol–gel spin-coating method and the densification has been performed by rapid thermal annealing at various temperatures ranging from 500 °C to 1000 °C. The –OH absorption bands in the Fourier-transform infrared (FTIR) spectra and the refractive-index data show that the films annealed below 900 °C are porous and the films annealed at 900 °C and above are dense. An ultraviolet (UV) KrF laser at 248 nm has been used to induce the change in the refractive index of the samples. We have achieved a large refractive-index change (Δn) of 0.0098 after UV illumination in excess of 1 min for our dense germanosilicate films. This UV-induced refractive-index change is attributed to the formation of GeE’/SiE’ centers from Ge–Ge/Si–Si (neutral oxygen monovacancy) and Ge2+ centers and to the creation of oxygen deficiency related defects. From our experiments, the oxygen deficiency related defects correspond to the absorption band at 620–740 cm-1 in the FTIR spectra and these are the defects which make a large contribution to Δn. The attenuation coefficient of the as-deposited and UV-illuminated dense samples is about 0.42 dB/cm at 1550 nm. For porous samples, UV exposure has densified the samples to some extent.

Birefringence suppression of UV-induced refractive index with grooves in silica-based planar lightwave circuits

An effective method for suppressing the birefringence of a UV-induced refractive index change is demonstrated for phase trimming of waveguides in planar lightwave circuits. By employing grooves on both sides of the waveguide to control the UV-induced strain, a significant reduction is achieved from 1.46×10−4 to 0.11×10−4 in the UV-induced birefringence for a π phase shift averaged between the TE and TM modes in a 2 mm-long waveguide.

Improved supercontinuum generation through UV processing of highly nonlinear fibers

We demonstrate that UV exposure of highly nonlinear, germano-silicate fibers can significantly broaden the infrared supercontinuum generated by femtosecond pulses in these fibers. Both simulations and measurements of the fiber chromatic dispersion show that UV-induced refractive index changes increase the waveguide dispersion by up to 5 ps/(nm-km) at 1570 nm and shift the dispersion zero by over 100 nm. We examine fibers with a range of UV exposure levels and show that the short wavelength edge of the supercontinuum can be continuously changed by more than 100 nm. We also show that the long wavelength edge is extended beyond that of the unexposed fiber. The resulting continuum spans from 0.85 to 2.6 /spl mu/m. Cutback measurements show that the supercontinuum in the exposed fiber is generated in as little as 1 cm of fiber. A nonlinear Schro/spl uml/dinger equation (NLSE) model of the supercontinuum generation in the nonlinear fiber shows that the short wavelength behavior of the continuum is primarily controlled by changes in the fiber dispersion caused by the UV-induced change in refractive index of the fiber core.

UV-Induced Refractive Index Change of SiN Film and Its Application to Center Wavelength Trimming of Vertically Coupled Microring Resonator Filter

In this study, to develop an accurate center wavelength trimming, we investigated the UV-induced refractive index change of the SiN film formed by plasma-enhanced chemical vapor deposition (PECVD) method using SiH4 gas and found that the refractive index of the SiN film was largely changed by -1.3×10-2 by the UV irradiation. Utilizing this phenomenon, we developed a novel center wavelength trimming using SiN as a ring core and irradiating UV laser beam. A wide range (-12.1 nm) of center wavelength was demonstrated and the shifted center wavelength was almost stable for more than 1200 h. A narrow channel spacing of filter array was realized by this technique.

UV Photosensitivity in a<tex>$hboxTa_2 hboxO_5$</tex>Rib Waveguide Mach–Zehnder Interferometer

The ultraviolet (UV) photosensitivity of Ta/sub 2/O/sub 5/ measured in a rib waveguide Mach-Zehnder interferometer (MZI) is reported. With a cumulative fluence of 72 J/cm/sup 2/ at a wavelength of 248 nm incident upon one arm through a 3-mm-long window, the MZI exhibits a phase shift of 8/spl pi/ radians at wavelength /spl lambda/=1.55 /spl mu/m, corresponding to a saturated refractive index change of 2.1/spl times/10/sup -3/. Real time measurements of the MZI output during exposure are given and the UV-induced refractive index change is found to be negative.

Wide Range Center Wavelength Trimming of Vertically Coupled Microring Resonator Filter by Direct UV Irradiation to SiN Ring Core

We developed a precise and wide range ultraviolet (UV) trimming of the center wavelength of vertically coupled microring resonator, by using the direct UV irradiation to the SiN ring core. We discovered that the UV-induced refractive index change of SiN was -1.3/spl times/10/sup -2/ and there was no aging of refractive index after the UV irradiation. Utilizing this phenomenon, -12.1 nm of center wavelength shift was demonstrated without the aging effect for 1200 h.

UV written waveguides using crosslinkable PMMA-based copolymers

Crosslinkable copolymers poly(methylmethacrylate/2-methacryloylethylmethacrylate) (P(MMA/MAOEMA)) were developed for waveguide applications. P(MMA/MAOEMA) can crosslink under either UV exposure or heating. The UV-induced refractive index change in unreacted P(MMA/MAOEMA) is found to depend on the fluence. UV exposure of thermally crosslinked P(MMA/MAOEMA) can induce further structure change and thus index change, and therefore, was found to be useful for creating the core layers in optical waveguides. The photosensitivity of the thermally crosslinked polymers is sufficient for the fabrication of low loss (<1 dB/cm) channel waveguides in the thermally crosslinked copolymer system.

UV written waveguide devices using crosslinkable PMMA-based copolymers

Integrated optical devices, S-bends and Y-splitters, have been fabricated in the poly(methylmethacrylate/2-methacryloylethylmethacrylate) copolymer system (P(MMA/MAOEMA)) by direct UV writing. This fabrication technique uses a UV-induced refractive index change without additional etching. Y-splitter junction losses of 0.4 dB and a splitting ratio of 50±1% were obtained.

Direct Evidence of Two Types of UV-Induced Glass Changes in Silicate-Based Optical Fibers

Two spatially distinct types of UV-induced refractive index changes in phosphosilicate glass fibers have been differentiated by the authors. The index change close to the core-cladding interface is less stable than the index change in the bulk fiber core. It is shown that by using presensitization techniques, in which the use of hydrogen is minimized so as to optimize the index changes possible within the core center, gratings of better stability than those prepared in fully hydrogen-loaded optical fiber are obtained.

Analysis of multiple wavelength DFB fiber lasers

The behavior of multiwavelength distributed feedback fiber lasers is simulated in a comprehensive numerical model. Multiple fundamental modes can coexist, even though the coupling coefficients of their respective refractive index gratings are not equal, due to induced dynamic gain gratings. We have investigated the effect on the lasing state of variations in coupling strengths, gain parameters, grating Bragg frequency separation, relative phase between the gratings, length of the structure relative to the beat lengths between the gratings, grating center phase-shift errors, and saturation level of the UV-induced refractive index change.

Photochemical changes in hydrogen-loaded optical fibres with application to Bragg grating formation

The sensitization to UV-induced refractive-index changes, by hydrogen, of germanosilicate glass is important in the production of in-fibre Bragg gratings. During grating formation fluorescence from germanium lone-pair centres is excited in the core of the fibre by the UV exposure. This fluorescence has been used to monitor photochemical reduction of substituted tetravalent germanium atoms by hydrogen. The photoreduced atoms occupy sites and absorb fluorescent radiation. Together with ions they are responsible for the refractive-index changes exploited in the holographic formation of Bragg gratings in hydrogen-loaded, germanium-doped fibres.

Optical power transversely scattered from fiber Bragg gratings: far-field measurements and modeling

Far-field properties of the optical power scattered from type IIA Bragg gratings photowritten within high-N.A. germanosilicate fibers have been studied under various experimental conditions. Experiments were carried out in order to measure the angular distribution of the diffracted beam intensity. The optical power detected in the far field was shown to be scattered around a cone centered on the fiber axis. The azimuthal distribution of scattered optical power was nonuniform. A theoretical model, which takes into account the LP01-mode coupling to radiation modes for wavelengths well below the Bragg wavelength, properly fits the features of the observed pattern. This theoretical model assumes the existence of a nonuniform UV-induced refractive index change across a plane perpendicular to the fiber axis.

Photosensitivity in Ce^3+-doped optical fibers

A UV-induced absorption change has been studied in Ce3+-doped preforms and fibers made by a solution-doping modified-chemical-vapor-deposition technique. A reduction in the Ce3+ concentration and the creation of a broad absorption band extending from the UV to the IR were seen on UV exposure. In addition, a peak in the absorption band is created at 245 nm, attributed to Ce4+. The reduction in Ce3+ concentration was observed to remain for at least 3 months, whereas the created broad absorption bands at 632.8 nm recovered by 90% in 2 h. Large UV-induced refractive-index changes (10−4) in Ce3+-doped fibers are also demonstrated. A linear dependence on the average UV intensity is found below 4 kW/m2, above which saturation occurs. The index change also increases with increasing Ce3+ concentration, again saturating at high levels. A decrease in magnitude with increasing wavelength is observed, but the index change at 1.55 μm is still more than 50% of that at 0.5 μm, demonstrating the potential of the technique for producing gratings with optical communication applications.