Badr-Eddine Benkelfat

An electro-optic modulation technique for direct and accurate measurement of birefringence

We report a convenient and accurate technique for measuring the optical path difference induced by a birefringent crystal. The method is based on the transmission of a low-frequency signal by the electro-optic amplitude modulation of a monochromatic light source. The linear superposition principle of the induced birefringence and the electro-optic effect are employed in the analysis of the optical parameters. Birefringence measurements at the wavelength 0.6328 lm for some common nematic liquid crystal cells are presented. The measurement accuracy is close to 10 � 3 . In addition, the phase shift of a twisted-nematic liquid crystal spatial light modulator, which is now widely used in optical processing systems, is determined as a function of the gray-scale level. 2003 Elsevier Science B.V. All rights reserved.

Technological implementation of Bragg grating reflectors in Ti:LiNbO 3 waveguides by proton exchange

The authors propose and demonstrate a simple and novel method for fabrication of efficient Bragg grating reflectors at telecommunication wavelengths in titanium-indiffused lithium niobate single-mode channel waveguides. This technique is based on the patterned proton-exchanged process. The validity of the method is verified by the good agreement of the spectral responses and the Bragg phase-matching condition. Reflectivity as high as 94% at 1546 nm was measured with 4-mm distributed parameter waveguides in z-cut lithium niobate.

Wavelength-Selective Ti:LiNbO

This paper described a wavelength-selective couplers dedicated to drop functionality (spectral-selective couplers dedicated to wavelength demultiplexing). It is consist of two Y junctions: an input two drop channels and an output. The Y-branches distributors of optical power were carried out by a specific geometry of Ti:LiNbO3 waveguides. Each Y junction is composed by two identical bends jointed at one with a weak inclination angle to decrease losses. On each arm of the Y-branches, a Bragg grating (BG) is etched by Focused Ion Beam (FIB) in order to extract a specific wavelength band. A two-channel configuration is adopted which demonstrate a drop functionality of two spectral bands centered at 1123 nm and 1527 nm. The complementary spectrum is avalaible at the coupler output.

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In this paper we propose an efficient technique for micromachining lithium niobate that is used in Ti-diffused waveguides. The use of Focused Ion Beam (FIB) etching allows obtaining homogeneous periodic microstructures. Bragg gratings with a period of 1.05 μm and an aspect ratio of 6:1 (depth-to-half period ratio) have been achieved. A reflectivity greater than 95 % associated with a bandwidth at half maximum of about 100 nm within a window centered at 1550 nm, is demonstrated for a Bragg grating of period Λ = 1.8 μm and a length of 144 μm, in good agreement with theoretical predictions.

Multiple Y-Branch Coupler Based on Focused Ion Beam Milled Bragg Reflectors

We report on InGaAs/InGaAlAs electro-absorption modulators for ultrafast all-optical signal processing. The structure design is based on the use of small conduction and valence band offsets. Switching windows as short as 5 ps for a 2 V applied reverse bias have been obtained by cross-absorption modulation.

Optical Performance of Bragg Gratings Fabricated in Ti:LiNbO

We describe the performance characteristics of a continuously tunable hybrid liquid-crystal filter over extended spectral ranges for implementation of multiwavelengths systems. This device is based on the linear electrooptic (Pockels) effect where tuning is achieved via externally applied voltage. A folded ten-stage S/spl caron/olc configuration is adopted which demonstrates a tuning range of 34 nm around 1550 nm. This opens the way to the achievement of various wavelength-division-multiplexing functions based on filtering [(de)multiplexers, add and drop devices, tunable emitters, etc.].

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We propose and demonstrate a new modified Lyot filter to independently ensure the channel amplitude equalization and the bandwidth tuning functions. The device consists of hybrid liquid crystal interferometers. We show numerically and experimentally that for a modified four-stage Lyot filter, the two first interferometers ensure equalization function with a maximum extinction ratio, while the last interferometer allows full-width at half-maximum tuning function by using, in particular, the optical canceling approach. Such a device is highly desirable in optical telecommunications, especially where in the core network the optical filters must perform several other functions in addition to their basic amplitude filtering action.

Waveguides by Focused Ion Beam Milling

The realization of a Fabry-Pérot (F-P) cavity based on a Ti:LiNbO3 optical waveguide obtained by using a concept which consists of milling the Bragg gratings (BGs) in the cavity by the focused ion beam technique is shown in this letter. This F-P cavity is formed by two BGs which represent the two reflecting mirrors of the interferometer. The length of the cavity is defined by the distance between the two BGs. Thirty-one peaks and a free spectral range (FSR)=1.75 nm have been experimentally obtained in the range of wavelengths [1505-1548] nm. The experimental results are in accordance with those of theory. The fabrication process, the experimental results, as well as improvements to increase finesse F and FSR are discussed.

Ultrafast InGaAs/InGaAlAs multiple-quantum-well electro-absorption modulator for wavelength conversion at high bit rates

We propose and demonstrate a novel optical implementation to simultaneously improve the contrast and increase the finesse of a classical N-stage Lyot filter. The new device based on the multiple-passage concept uses two mirrors placed at the extremities of the filter. The light beam passes through the structure back and forth and each passage has a significant influence in reducing the sidelobes and the bandwidth of the filter transmittance. We show that for a two-stage Lyot filter, the contrast is increased by 100% after each passage and the finesse is improved by over 50% after four passages and more. Such a device is highly desirable in a wide field where the filtering function requires high performances in term of contrast and finesse.

Low-voltage continuous tunable hybrid filter for tailored optical-bandwidth operation

We have optimized the structure of InGaAlAs multiple quantum-well electroabsorption modulators for all-optical signal processing. As wavelength conversion is a key function for all-optical telecommunication network, we have based our optimization upon the main parameters of this function, namely, polarization insensitivity, exciton saturation, and photogenerated carrier sweep-out time. Four samples have been studied with different well tensile strain and barrier height. We find low polarization dependence over a range of applied bias. Less than 10-ps absorption-saturation recovery time at lower biases and smaller saturation power than in the literature are reported.