Electrooptic Glasses for Information Transmitting and Processing Systems
Tech Area / Field
- PHY-OPL/Optics and Lasers/Physics
8 Project completed
Senior Project Manager
Vavilov State Optical Institute (GOI), Russia, St Petersburg
- University of Southampton / Optoelectronics Research Center, UK, Southampton\nIPHT-Jena / Institut für Physicalishe Hochtechnologie e.v., Germany, Jena\nInstituto di Ricerca sulle Onde Elettromagnetiche Nello Carrara, Italy, Florence\nFraunhofer Institute Angewandte Optik und Feinmechanik, Germany, Jena
Project summaryElectrooptic (EO) effect, that is, the change of refractive index of transparent media under applied electric field, is widely used nowadays for modulation of light. The phase, or polarization, or amplitude modulation can be gained by using a variety of schemes of the EO structure.
Usage of optical waveguide structures operating on the same principle as the micro-wave waveguides, allows one to confine the light propagation within the region with a characteristic cross-section of several wavelengths, that is, of several micrometers. As a result, in those structures the driving voltage for light modulation is lower than in bulk structures because the necessary electric field (up to 10 V/mm) can be provided using the voltage of several volts. Since value of EO effect increases with optical path, the driving fields and voltages can be decreased with increasing the length of EO structures.
Conventional (bulk) and optical waveguide (integrated) EO structures are widely used nowadays in information processing and transmission. The bulk structures are mainly used in the systems of image processing. Integrated schemes of EO systems are used as light modulators and as modulating and switching elements of fiber optical lines in the multipurpose information systems, specifically in telecommunication systems.
The basic features required for EO structures are the following:
- low value of optical losses inserted by the structures;
- the lowest possible driving voltages;
- fast response (time of response to an operating signal is about 10-10 s);
- possibility for efficient coupling with fibers in structures used in fiber optical systems, which assumes a coincidence of the characteristic sizes and refractive index profiles of the elements to be connected;
- technological effectiveness and low cost of mass production.
It is customary to use the anisotropic crystals for producing EO structures with linear EO effect (linear dependence between the change of refractive index and the magnitude of the driving electric field, that is Pockels effect). Those materials demonstrate fast response, the driving field being ~1 V/mm. Up to now, the technology of producing slab and strip EO waveguides on the substrates made of LiNbO3 and KTP is developed.
Crystals have the following disadvantages:
- restricted dimensions;
- complexity of the synthesis technology;
- it is impossible to draw optical fibers from crystals;
- when the light modulator is placed in the fiber optical channel, a high level of light losses appears due to the significant difference between refractive indices of the fiber and the crystal used;
- low flexibility of the technology of optical waveguide structures on the base of crystals.
Dependence between the change of refractive index and the electric field in isotropic materials (specifically in glass) is of quadratic character (Kerr effect). As a rule, the electric field, which is necessary for reaching the same change of refractive index as in the case of linear EO effect, is significantly higher (~ 30 V/mm). Those values of the electric field, as a rule, cannot be put into practice, because the required voltage is too high for being applied to a modulator. Therefore, in spite of the fact that EO response of glasses is in picosecond range, glasses are not used as working media for EO modulators, although their advantages over the anisotropic media used from the viewpoint of their cost, production technology, possible size as well as an opportunity to form integrated and fiber optically-waveguiding structures (by means of the effective implementation of ion-exchange technology and glass drawing) are obvious.
It is important that the usage of optical waveguide structures allows reaching the necessary driving electric field at lower voltage as compared with the voltage for bulk devices. Increase in the length of such structures will make it possible to decrease the driving electric field.
Therefore, elaboration of EO glasses suitable to produce optical fibers and graded-index optical microstructures if the changes of index are of the same order as in the crystalline materials that are presently in use and the refractive index of the glasses can be varied with driving electric fields down to 10 V/mm will allow:
- to elaborate optical fibers of novel type — EO fibers that can provide modulation of propagating optical radiation. That will result in joining functions of transmission and processing of a signal in a single element. Weak EO phenomenon can be used in this case since rather long optical fibers can be made;
- to design fast EO modulators, switches and sensors of new generation combining the low cost and low level of inserting losses with an opportunity of effective coupling with optical fibres;
The main obstacle to solve these problems is low EO efficiency of glasses.
It is well known that EO efficiency of glasses can be increased if formation of highly dispersed crystalline phase in a glass can be provided, the crystallites having good EO sensitivity. There is a hope that a small fraction of the dispersed in glass matrix crystalline phase leads to increasing EO sensitivity and the specific features of glassy media, like low light scattering losses and possibility to produce optical fibers, will be preserved.
For the first time EO effect in glass was demonstrated in 60s by Corning Glass Company. That was due to the precipitation of sodium niobate in the niobium-containing glass. However, no commercial technology was developed since EO parameters of the glass were worse as compared with ones of well-known crystalline materials.
Nevertheless, the possibility found last years to make glasses doped with increased concentration of micro- and nanocrystalline phase, development of technology of optical fibers drawing, elaboration of glasses suitable to form graded-index optical microstructures (concentrating light within micron-size regions) by means of ion-exchange techniques have jointly opened a way to solve the above-mentioned problems.
Previous research in the State Optical Institute for the first time in Soviet Union resulted:
- in finding compositions of niobium containing glasses with EO properties;
- in demonstrating low light scattering losses in designed glass with embedded alkali niobate nanocrystals;
- in demonstrating a possibility to form in the glass different graded-index regions with index variation in the range 0.001-0.08 by means of heat or ion exchange treatment;
- in developing methods to calculate conditions of ion-exchange treatment ensuring the preassigned distribution of refractive index in a glass.
All that ensure a possibility to develop the graded-index integrated optical structures with electrically controllable optical parameters on the basis of those glasses;
Purpose of the Project:
Elaboration of novel EO materials and elements of optical devices on the basis of glass through the usage of the experience accumulated in All-Russia Scientific Center "S.I.Vavilov State Optical Institute".
Tasks of the Project:
- elaboration of novel highly EO sensitive glasses which will be suitable to draw optical fibers and will demonstrate high parameters of ion exchange diffusion;
- development of techniques of drawing and thermal processing the optical fibers to induce EO sensitivity;
- working out the techniques and technological conditions of glass treatment in graded thermal field and melted salt to form the necessary for EO microstructures index distributions;
- preparation of trial samples of EO fiber and EO graded-index optical-waveguiding microstructures.
Technical Approach and Methodology
For the investigation of the most promising glasses to be done, the joint application of a set of the physical and physical-chemical structure-sensitive techniques of investigation of glasses, of ion-exchange diffusion and of phase decomposition will be employed. They are: electron probe microanalysis, X-ray diffractometry, small angle X-ray scattering, the Rayleigh and Mandel'shtam-Brillouin scattering spectroscopy, differential thermal analysis, microinterferometer technique of the measurement of refractive index distribution and EO effect in the glasses, etc. This approach will allow one to develop the most promising compositions of EO glasses, their melting conditions, schedules of drawing, heat and ion-exchange treatment, ensuring the combination of high values of Kerr constant and ion exchange parameters as well as low light scattering and absorption losses.
It is supposed that one can expect the usage of the glass sample shaped into fiber instead of the bulk sample to lead to essential increasing the effectively of the usage of EO effect due to increasing the optical path where the effect works.
- elucidation of the main regularities of Kerr effect in glasses containing micro- and nanocrystals;
- design of novel glasses with increased Kerr constant about of 10-12 m/V2 and high parameters of ion-exchange diffusion ensuring a possibility of their usage in fiber and x integrated optics;
- generation of the scientific groundwork for fabrication of graded-index EO microstructures driven by the voltage about of 30 V;
- development of techniques for EO fiber drawing;
- manufacturing the pilot samples of EO fibers and graded-index EO optical waveguide structures.
Scientific, technical and commercial consequences of the project execution:
- new results of high scientific importance for physics and chemistry of glass, modern optics, and optical technology will be obtained;
- the mass transfer and ion exchange phenomena in micro- and nanosize inhomogeneous glassy matrices will be studied. It will allow one to prepare graded-index microstructures with preassigned refractive index distribution;
- the theoretical-experimental and technological fundamentals for the shortrun production of novel type fibers, that is, fibers with EO effect, will be worked out. These fibers can be used for direct electric field modulation of the propagating optical radiation;
- basics for the manufacturing process of strip and slab EO waveguide made of EO glass will be worked out;
- as a result, the possibilities will appear to modify the element set of lightguide (fiber and integrated optics) devices used in the systems of information transmission and processing and of optical sensing;
- possible commercial result of the project execution is a prospect for a new technology and a novel product to appear in the high technology market.
Potential role of the foreign collaborators:
The project participants suppose that cooperation with foreign collaborators will be most essential in testing EO fibers, analysis of Western market and in consulting the participants in the fields of applying results of the project. That cooperation will be arranged through the arranging of regular seminars, current consultations and through e-mail and fax communication as well as working visits to the collaborators.
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