Impressing technology of optical Bragg’s gratings on planar optical sol-gel waveguides

The aim of the presented investigations was to develop a technique of producing Bragg’s grating couplers on planar waveguides. Waveguides are obtained by means of the sol-gel technology. The introduction of a light beam into the structure of the waveguide is in the case of planar or strip optical systems always an essential technical problem, requiring simple and reproducible solutions without extending excessively the waveguide structure. The paper presents a technology of producing grating couplers by impressing the pattern of the network while forming the planar waveguide structure applying the sol-gel method. Some remarks concerning the sol-gel technology are also presented. The results of investigations on grating couplers obtained in such a way have been discussed, too. Attention has been drawn to the possibility of using such structures in optoelectronic sensors, particularly gas sensors, including sensors of water vapour as well as toxic gases.     

Flexible hollow waveguides for delivery of excimer-laser light

A hollow waveguide consisting of a glass tube with a thin aluminum film on the inner surface is proposed for delivery of excimer-laser light. The waveguide, fabricated by a chemical-vapor-deposition method, shows low losses for KrF excimer laser light: the straight loss of a 1-mm-bore waveguide was 0.4 dB:m.     

Propagation characteristics of Ag+ ion-exchanged stripe waveguides

A detailed experimental study of stripe waveguides formed by Ag⁺/Na ion-exchange has been carried out. Propagation constants were precisely measured by excitation of single output pq-lines. The resulting data allowed us to approximate the transverse silver distribution by a rectangular profile. The effective index method is used for calculating theoretical propagation constants, assuming no sideways diffusion. A comparison between experiment and theory is made over the range of mask widths studied.     

Flat-top profile of an excimer-laser beam generated using beam-splitter gratings

We present a simple solution to the beam-shaping problem in which a wide, typically rectangular spatial distribution of an excimer-laser beam should be transformed into a flat-top distribution, a few millimeters wide in both directions. The set-up can be made practically lossless and it allows the energy density of the pulse to be increased by a factor of 50. The method finds use, e.g., in pulsed laser deposition, UV lithography and micro machining.     

Silver nanoclusters formation in ion-exchanged waveguides by annealing in hydrogen atmosphere

Light waveguides containing silver, introduced by ion-exchange process in soda-lime glass, has been annealed in hydrogen atmosphere at temperatures in the 120–250°C range. Annealings cause a near-surface precipitation of metallic silver to form nanometer-size clusters with good uniformity in size and spatial distribution. Hydrogen permeation and ion-exchange between hydrogen and sodium (which remains in the glass matrix after silver-for-sodium exchange) are steps of the annealing process. A further step is the diffusion of silver ions towards the surface, and its eventual precipitation, with an activation energy close to that measured for silver-sodium interdiffusion in glasses of comparable composition.     

Bragg diffraction of light by rectangular phase gratings

Bragg diffraction of light by rectangular phase gratings is considered, utilizing two-dimensional coupled-wave theory. Diffraction characteristics for two important cases of incident plane waves and Gaussian-profile beams are presented. It is shown that the diffraction properties depend on the following normalized parameters: the grating strength and a geometry parameter, the beam width and a displacement. Values of these parameters for the realization of both nearly transmitting and nearly reflecting regimes of diffraction are determined. The results obtained are used for the choice of the design parameters of integrated optic Bragg-type devices.     

光致折变布拉格光栅实验研究

本文介绍了 2 48nm、Kr F准分子激光经过石英相位掩膜版的空间调制 ,在载氢增敏处理过的通信光纤与 B/Ge光纤上分别写入光致折变布拉格光栅的实验研究。经实验测定 ,在光栅布拉格波长上 (λH2 =15 46 .45 5 4nm ,λB/Ge=15 45 .15 5 nm) ,两种光栅反射率均大于 99% ,3d B带宽分别为 0 .12 9nm和 0 .182 nm。经比较 ,B/ Ge光纤比氢载光纤具有更好的光敏性和热稳定性     

Criteria for Bragg regime diffraction by phase gratings

Four alternative criteria for the occurence of Bragg regime diffraction by phase gratings are presented and evaluated.     

Experimental investigation of intracavity Bragg gratings

We report the results of an experimental investigation of the diffractive properties of a thick index grating embedded in a Fabry-Perot resonator, the so-called intracavity Bragg grating. We compare the performance of this device with that of a bare Bragg grating with same thickness and the same index modulation and establish the improvement in performance with the resonator. The experimental data also fit predictions calculated from a theoretical model.     

Characteristics of hydrogenated fiber Bragg gratings

By utilizing both hydrogen loading for photosensitivity enhancement and a phase mask for holographic exposure, we have fabricated fiber Bragg grating with controllable reflectance and bandwidth. The evolution of hydrogen diffusion into a single-mode optical fiber before exposure, and out of the fiber after exposure was characterized, and the results were consistent with the theoretical modeling. The shifting of Bragg wavelength and thermal reliability were found related to the hydrogen behavior inside the fiber. One solution to prevent Bragg wavelength from drifting after the grating was formed and hence to increase the grating's reliability was to anneal it so that the residual hydrogen was forced out of the optical fiber in a short time. Received: 12 June 1996/Accepted: 17 June 1996     

High performance diffraction gratings made by e-beam lithography

Gratings are essential components in different high performance optical set-ups such as spectrometers in space missions or ultrashort-pulse laser compression arrangements. Often such kinds of applications require gratings operating close to the technological accessible limits of today??s fabrication technology. Typical critical parameters are the diffraction efficiency and its polarization dependency, the wave-front error introduced by the grating, and the stray-light performance. Additionally, space applications have specific environmental requirements and laser application typically demand a high damage threshold. All these properties need to be controlled precisely on rather large grating areas. Grating sizes of 200?mm or even above are not unusual anymore. The paper provides a review on how such high performance gratings can be realized by electron-beam lithography and accompanying technologies. The approaches are demonstrated by different examples. The first example is the design and fabrication of the grating for the Radial-Velocity-Spectrometer of the GAIA-mission of the ESA. The second grating is a reflective pulse compression element with no wavelength resonances due to an optimized design. The last example shows a three level blazed grating in resonance domain with a diffraction efficiency of approximately 86?%.     

Dispersion properties of planar Bragg waveguides

We reformulate the leaky mode condition of planar Bragg waveguides as two simple, separate conditions on the propagation constant; a phase match condition and a loss formula. These relations enable efficient numerical calculations of the mode properties. Furthermore, we use the phase match condition as a starting point to discuss the dispersion of waveguides with advanced cladding structures. In particular, we point out that chirped claddings, where the effective reflection point is dependent on frequency, do not give dispersion characteristics significantly different from metallic waveguides or waveguides with periodic claddings.     

Synthesis of 1D Bragg gratings by a layer-aggregation method

We present what we believe to be a novel method for the synthesis of complex 1D (fiber and waveguide) Bragg gratings, which is based on an impedance reconstruction layer aggregation technique. The main advantage brought by the method is the possibility of synthesizing structures containing defects or discontinuities of the size of the local period, a feature that is not possible with prior reported methods. In addition, this enhanced spatial resolution allows the synthesis of very strong fiber Bragg grating devices providing convergent solutions. The method directly renders the refractive index profile n(z) as it does not rely on the coupled-mode theory.     

Temporal characterization of short pulses stretched by fiber Bragg gratings

Short femtosecond pulses at 1054 nm are stretched up to 400 ps by long broadband chirped-fiber Bragg gratings. The temporal profile of the reflected pulses is measured with a cross-correlation scheme. The advantage of this technique over the usual spectral measurements is its high sensitivity, since the temporal profile of reflected pulses is revealed to be strongly affected by the presence of defects in the grating. We show that this method is a good test of the quality of fiber Bragg gratings.     

Transverse load sensing by use of pi-phase-shifted fiber Bragg gratings

The birefringence of a pi -phase-shifted fiber Bragg grating can be determined with high accuracy by measurement of the polarization-induced spectral splitting of its narrow central transmission window. The use of this feature for sensing of a load applied in the direction transverse to the optical fiber is demonstrated. A distributed force resolution of 1.4x10(-3) N/mm was obtained, which corresponds to a difference in the principal strains of the fiber core of 0.5mu? . We also show that the transverse load response of the sensor is insensitive to temperature.     

Planar surface-wave waveguides of finite width

Dispersion equations for scalar (acoustic) surface waves propagating along an impedance band on an absolutely rigid plane and along an impedance tape are derived and analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 43–48, January, 1971.In conclusion the authors thank V. G. Myshkin for critical comments offered in a discussion of these results.     

High-reflectivity Bragg gratings fabricated by 248-nm excimer laser holographic ablation in thin Ta2O5 films overlaid on glass waveguides

We demonstrate strong Bragg grating reflection in Ta₂O₅ (tantalum pentoxide) thin films overlaid on potassium ion-exchanged channel waveguides in BK-7 glass, inscribed using 248-nm excimer laser holographic ablation. The experimental data presented are divided into two sections: the first section refers to the study of the grating ablation process of thin Ta₂O₅ films with respect to the exposure conditions, while the second focuses on the implementation of these relief gratings in functional waveguide devices. Firstly, experimental data on grating morphology versus exposure conditions, accomplished with scanning electron microscopy microscans, are presented. In the second section diffraction spectra for waveguide gratings are presented and analysed. Spectral notches in transmission of depth -18 dB for the TM polarisation were obtained from 16-mm-long gratings patterned in waveguides overlaid with a 105-nm-thick Ta₂O₅ film, using 50 pulses of 60-mJ/cm² energy density. PACS 42.82.Cr; 42.79.Dj; 81.65.Cf     

Motion characteristics of Bragg grating solitons in rectangle-apodized fiber Bragg gratings

We introduce both concave and convex rectangular apodizations in the middle of fiber Bragg gratings to achieve slow light. Based on the nonlinear coupled mode equations (NLCMEs), the transmission characteristics of grating solitons in rectangle-apodized gratings are numerically simulated and analyzed. The rectangular apodization can change the grating coupling coefficient to give rise to slow and capture the solitons in gratings. The effects of the soliton energy parameters, the width of rectangular apodization and the variation of the coupling coefficient on the soliton transmission are presented. The results show that, the velocity of solitons can be slowed down, and the capability to capture a soliton depends on the energy of input solitons, coupling coefficient, and the rectangular width. Two kinds of soliton capture methods are proposed and compared with each other.