UV-induced Refractive Index 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.

UV-Writing of a Superstructure Waveguide Bragg Grating in a Planar Polymer Substrate.

We report on the fabrication of a superstructure Bragg grating in a planar polymer substrate. Based on a twofold illumination process an integrated waveguide and a superstructure Bragg grating are subsequently written into bulk polymethylmethacrylate by UV-induced refractive index modification. The measured reflected spectrum of the superstructure Bragg grating exhibits multiple reflection peaks and is in good agreement with performed standard simulations based on the beam propagation method and coupled mode theory algorithms. By applying a varying tensile load we determine the strain sensitivity to be about 1.10 pm/µε and demonstrate the applicability of the superstructure Bragg grating for strain measurements with redundant sensing signals.

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.

Role of point defects in the photosensitivity of hydrogen-loaded phosphosilicate glass

It is shown that point defect modifications in hydrogen-loaded phosphosilicate glass (PSG) do not play a central role in determining its photosensitivity. Photochemical reactions that involve a two-step point defect modification and pre-exposure effect are incapable of accounting for photoinduced refractive index changes. It seems likely that a key role in UV-induced refractive index modifications is played by structural changes in the PSG network. Experimental data are presented that demonstrate intricate network rearrangement dynamics during UV exposure of PSG.

UV-induced refractive index modulation of photoreactive polymers bearing N -acylcarbazole groups

In this study, we report on the synthesis and characterization of photoreactive polymers bearing N-acetylcarbazole and N-formylcarbazole groups, respectively. These polymers were easily accessible by polymer analogous acylation of commercially available poly-(2-vinylcarbazole). While poly(1-(2-vinyl-9H-carbazol-9-yl)ethanone) (poly-1) undergoes a partial photochemical Fries rearrangement, poly(2-vinyl-9H-carbazole-9-carbaldehyde) (poly-2) decarbonylates smoothly when exposed to UV irradiation. The difference in reactivity between the two acylated polymers is because of the lower stability of the formyl radical, which is formed in the first stage of this photoreaction. Ellipsometric measurements of thin films showed that the photo-Fries rearrangement in poly-1 causes a change in refractive index by Δn = +0.01 at 650 nm. UV illumination of poly-2 results in a change of the refractive index by Δn = +0.03 at 650 nm, which can be explained by the high yield of the photodecarbonylation of the N-formylcarbazole groups. Refractive index patterns can be easily realized using lithographic techniques as demonstrated by optical microscopy using a phase contrast set-up for visualization. Patterned films of poly-1 and poly-2 with feature sizes of about 5 μm were obtained with a mask aligner. Photoreactive polymers bearing N-acylcarbazole groups are of potential interest for optical applications such as waveguides, optical switches, and data storage devices. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Dependence of the Refractive Index of a Coating on a Long-Period Fiber Grating on the Initial Coupling Strength

The dependence of the refractive index of a coating on the transmission characteristics of a longperiod fiber grating (LPFG) on the initial coupling strength between core and cladding modes is discussed. The coupling strength is modified by changing the UV-induced refractive index, and the transmission of a LPFG varies like Cos(κL), where κ and L are a coupling coefficient and a grating length, respectively. Since the cladding modes coupled from the guided core mode are interfaced with the external environment directly, the coupling strength and the transmission characteristics of a LPFG can be changed readily by the refractive index of a coating material, which induces a variation of both the resonant wavelength and the peak depth. The dependence of the LPFG on the variation of the ambient index of a coating material is changed by using the initial coupling strength of the grating. We fabricate two types of LPFGs with different coupling strengths and measure their transmission characteristics with changing ambient index. The transmission peak depth of the overcoupled LPFG increases as the ambient refractive index is changed from 1.5 to 1.7. Compared with the sensitivity of the 1st saturated LPFG to the ambient index change, the total of the overcoupled LPFG is improved by changing the coupling strength.

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.

Effective-index increase, form birefringence and transition losses in UV-side-illuminated photosensitive fibers

We present a simple variational method for the analysis of UV-side-illuminated single-mode fibers. We use a shiftable, two-dimensional elliptical Gaussian function as a field trial for the fundamental-mode. By this method, the actual UV-induced peak refractive-index increase can be quantitatively related to the measured effective-index increase. The asymmetry of the UV-induced refractive-index profile due to the absorption of the writing UV light causes both form birefringence and transition losses between UV-illuminated and non-illuminated fiber sections. These characteristics are easily calculated from the results of the variational method. We show that UV-illumination of the fiber from two opposite sides reduces both the form birefringence and the transition losses.

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.

Modal field deformation and transition losses in UV side-written optical fibers

The asymmetry of the UV-induced refractive-index profile in photosensitive optical fibers causes a deformation of the modal fields, resulting in transition losses between UV-exposed and unexposed fiber sections up to 0.1 dB for UV-induced index changes of the order of <or=10(-2). A numerical analysis of the transition losses is developed. It is verified experimentally that the loss is caused by coupling of power of the fundamental mode to cladding modes at such a transition.