Search

International audience; We demonstrate that the birefringence of the liquid-crystal cell (LCC) can be varied by applying different frequency values of a single applied ac square voltage. For the experimental evaluation of the birefringence, associated with a certain wavelength λ, as a function of the frequency FLCC of the electrical signal applied to the LCC, we use, for the first time to our knowledge, what we call here a frequency-dependent transmission technique. It consists in measuring the transmission responses between crossed and parallel polarizers as a function of the frequency FLCC. Experimental tests were carried out using a 7 μm-thick E63 nematic LCC and a laser source emitting at λ=1.55 μm with a launch power of −3 dBm. The tuning voltage VLCC applied to the LCC is an alternative square wave electrical signal whose frequency ranges from 0.5 to 15 kHz. The peak to peak amplitude of the electrical signal is 5 V. The curve of the measured variations of the optical path difference of the LCC versus the frequency FLCC has a positive slope. Application to the tuning of the center wavelength of the transmission response of a one stage hybrid birefringent filter is shown as a proof-of-principle test

This paper describes a novel variable optical attenuator using a phase variation of a polymer dispersed liquid crystal in a cell with an adequate photo-resist pattern. This component is inserted between two collimated single-mode fibers and tested at telecom wavelengths. An optical attenuation range of 22 dB is obtained with a maximum polarization dependence loss (PDL) of 0.18 dB with a new phase pattern.

A polymer containing fine droplets of liquid crystal (LC) is investigated for obtaining an optical phase-shifting attenuator at 1550-nm wavelength. Two mixtures of liquid crystals and monomers are compared for a cell thickness of 15 /spl mu/m. The first method uses nanosized LC droplets to achieve 2.29-rad phase shifting for an applied voltage of 230 Vrms. The second method uses microsized LC droplets to achieve 2.35-rad phase shifting for an applied voltage of 15 Vrms. Theoretical analysis of the phase shifting is developed. This optical property is used to achieve a variable optical attenuator (VOA) between two single-mode fibers. An optical architecture using graded index rod lenses and a pattern of photoresist polymer is presented. Typical characteristics of such VOA are: 12-dB range of attenuation, a maximum polarization dependence loss of 0.5 dB, 1.1-dB insertion loss, and a saturation voltage of 20 Vrms.

Dispersions of liquid crystals in polymeric matrices (PDLC) are used as light Variable Optical Attenuators (VOAs). Electronically controllable VOAs play a crucial role for channel power equalizers in WDM systems. This article describes the theoretical model optimizing PDLC performance and the fabrication process utilizing the polymerization-induced phase separation (PIPS) method, and presents the optical attenuation (OA) and polarization dependence loss (PDL) characteristics versus fabrication conditions. Fabricated VOAs have a large attenuation range (10–30 dB) and low insertion loss (IL) at a 1.55 µm wavelength (0.5 dB). Depending on the fabrication parameters, the OA, IL, and PDL are optimized. This device needs an applied voltage V90 of 10 to 70 Vrms for 10–100 µm PDLC thickness. The flatness of the optical characteristics of this device has also been measured in the C+L band.

For the development of reflective flat displays, polymer liquid crystal composite is a good potential challenger material. In order to reduce the driving voltage and to enhance the brightness of the back-scattered off-state, it is necessary to understand and optimise the composite formation. Based on a suitable liquid crystal and polymer couple, we describe a model for the fabrication of polymer dispersed liquid crystal. The influence of the main parameters on the composite production is studied. We choose the case of polymerization-induced phase separation, by means of ultra violet light polymerizing monomer. This study shows how the polymerization parameters control the droplet size and the remaining monomer content, both of which play a role in back-scattering and saturation voltage. We bring to light, the strong temperature sensitivity of PDLC formation and the necessary precautions to reach the optimum performance of the chosen liquid crystal-polymer couple. As this step is essential to expl...

In this work, an advanced multifunctional optical switch based on multimode fibers is proposed. It can work as a 3×1 optical multiplexer/combiner, a 2×2 optical switch, a variable optical attenuator and a variable optical power splitter. All these functionalities can be developed in the same device without any hardware modification, only by using the proper ports and control electronics. The proposed switch has been developed for being used in the visible and near infrared wavelength range: 450–650 nm for optical fiber automobile applications, 650–850 nm for home broadband applications; and 850–1300 nm for multimode fiber access networks. Up to three different types of twisted nematic liquid crystal cells have been designed and fabricated for fulfilling these different wavelength ranges as part of the proposed device. The multifunctional switch has been implemented and experimentally tested. Crosstalk usually better than−15 dB at 532 nm, 660 nm and 850 nm, in any state has been measured. Switching is achieved at voltage levels of 4 Vrms. Fiber to fiber insertion losses when operating as a 2×2 optical switch, range from 10 to 15 dB within 200 nm wavelength range; with a non-optimized optics for collimation and coupling. This work was partially supported by CICYT (TEC2009-14718-C03-03-MCI), by the European Commission through the NoE BONE (Project no. 216863), and FACTOTEM-II-CM: S2009/ESP-1781 Projects. Publicado

International audience; We propose and demonstrate a novel optical implementation to simultaneously improve the contrast and increase the finesse of a classical -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 performancesin term of contrast and finesse.

Engineering educational programs increasingly rely on project-based learning. Within, it is crucial to early present to students intended learning outcomes from the outset of large projects. However, even if well formulated, many outcome-related project learning objectives are not properly understood by students out of experiences. Thus, some students struggled to identify objectives throughout activities and tended to distance themselves from acquisitions. Short concrete activities could advantageously help student self-recognize learning objectives. For such, we propose a systematic insertion of authentic starter activities in the front-end of large projects, so as to give students a better insight and deeper understanding of the knowledge, skills, and abilities they are expected to acquire. Properly designed starter activities promote enthusiasm, motivation, and increase project involvement. For program developments, this paper proposes a three-phase process, from design to improvement, for integrating starter activities and to favor a coherent treatment throughout a curriculum. This process is based on our six years of experiences with large semester projects.

Optical communication using optical fibres as the transmission medium is essential to handling the massive growth of both telecom and datacom traffic. To fully realize the potential bandwidth available on these optical fibres, other components of the optical network system have to be developed, ranging from detectors and multiplexers to buffers and switches. This book addresses the different technologies which can be applied to switching optical signals. An optical switch functions by selectively switching an optical signal delivered through an optical fibre or in an integrated optical circuit to another. Several methods are available and each relies on a different physical mechanism for its operation. The various physical mechanisms used are discussed in the main chapters in the book which cover electro-optical, thermo-optical, micro-electro-mechanical (MEMS)-based and semiconductor optical amplifier (SOA)-based optical switches. The book also covers switching based on optical nonlinear effects, liquid and photonic crystal optical switches as well as fibre, holographic, quantum optical and other types of optical switches. Each chapter discusses the choice of materials, fabrication techniques and key issues in switch design. With its distinguished editors and international team of contributors, Optical switches: materials and design is a standard reference for the telecommunications industry and those researching this important topic.

International audience; We propose the design of a multi-channel amplitude-equalizing polarization interference filter based on stacked liquid crystal birefringent blocks. Each block, which is optimized to control a predefined wavelength channel, consists of three components. The wavelength blocking/transmitting and the equalizing actions are provided by the liquid crystal component whose optical path difference varies between two particular values when its birefringence is electrically controlled. For a typical three-channel equalizing filter, the three liquid crystal cells provide all eight possible combinations to block/transmit channels and also allow us to independently equalize their corresponding levels. The benefit of such a polarization-based filter is that it operates without free-space demultiplexing optics and with no moving parts. It can be used as a key element in the present reconfigurable optical networks

In this work, a combination of a multiplexer and a variable optical attenuator in the same device (VMUX) for use in Polymer Optical Fiber is proposed. Polymer Dispersed Liquid Crystals (PDLC) are polarization independent, have high contrast and gray scale capability. For these reasons, PDLC cells with pixels can be used as switching elements in the device. Characterization of the PDLC has been carried out at two wavelengths. VMUX Complete switching is reached for driving voltages of 20 Vrms, with insertion losses less than 1.6 dB, attenuation larger than 31 dB, rise time less than 2.6 ms and decay time better than 12.4 ms have been obtained. This work was supported in part by CICYT (TIC2003-03783 and TEC2006-13273-C03-03-MIC), in part by UC3M (FAVICOBIS), in part by CAM (FACTOTEM-CM:S-0505/ESP/000417), and in part by COST 299. Publicado

A novel 1×2 wavelength routing switch is introduced and analyzed. It is based on a tunable hybrid liquid-crystal solc filter and a polarization beamsplitter. The device can be set into the bar state for an arbitrary subset of WDM channels while remaining in the cross state for the rest of channels. This is achieved via a tuning voltage applied to liquid-crystal cells. Interchannel crosstalk and its reduction are discussed. Application of the switch includes coarse and dense WDM.

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.].

Free space optical switches are very attractive because of their high-density interconnections, transparency to signal format and bit rate. Recently, multistage optical switches using liquid crystal (LC) cells have been demonstrated at the telecommunication wavelengths [1] [2] [3], By alternately cascading polarisation controller arrays (LC cells) and routing elements (bireffingent plates, polarising beam splitters, prisms, diffractive optical elements), a N input N output optical switch can be realized. To create an optical switch with low insertion loss and good crosstalk, we must resolve some technical issues such as the development of low-loss and high- contrast polarisation controllers.

Liquid crystals are frequently used as active holographic elements for beam deflection applications. Thanks to their small response time, ferroelectric liquid crystals are mostly studied for telecom applications. However, for routing applications, the reconfiguration rate is not a key parameter, on the other hand, nematic liquid crystals make it possible to produce phase ramps and therefore high efficiencies. [1 2 3 4 5 6-7]

Metals, insulators and also semi-conductors are already available from thin film technology by the way of deposition techniques. Glass substrates allow large area (100 cm 2) devices. With a rule pattern of 10 μm, the built up of 100 000 components on a single chip is quite at hand. This technology has been developed in our laboratory especially for neural networks purposes.

With a view to making highly efficient light shutters, the object of this study is to know the best structure of the liquid crystal polymer composite yielding a high extinction ratio. The main feature of this letter shows that experimental results are consistent with anomalous diffraction approach. For our demonstration, we made polymer dispersed liquid crystal (PDLC) cells having different droplet radius (a), ranging from 0.5 to 1 μm. In addition, a strong wavelength sensitivity with these radii is measured in the visible region. Beyond one micron, droplets yield higher transmission as regards both the wavelength and the optical anisotropy of the liquid crystal. These results bring forward that the most efficient PDLC films must follow the optimization relation: a(Δn)/λ=0.3, where λ is the wavelength and Δn the liquid crystal anisotropy.

A variable optical attenuator using polymer dispersed liquid crystal is presented. The fabrication process is described and the optical attenuation and polarisation dependent loss (PDL) characteristics in relation to fabrication conditions are presented. The device achieves 12.7 dB of attenuation and 0.7 dB of insertion loss at 1.55 /spl mu/m with a PDL < 1.8 dB.