Tunable optical device and optical system using the tunable optical device as coding filter

Abstract: According to the present invention the tunable optical device comprises a first optical wave guide and a second optical wave guide defining a space between the two wave guides with space comprises the first cell with liquid crystal material and a second cell with liquid crystal material spaced by a third wave guide and the first and the second cell for liquid crystal material having orientation of the liquid crystal material perpendicular to each other.

Description: Field of the invention This invention relates to a tunable optical device that is used in a telecom system especially in a wavelength division multiplexing communication system or an optical code division multiplex system. Background Differentiation is made between access networks and transport networks for the transmission of data. The access networks, which ensure the access to the consumer, are characterised by complex network structures. The system diversity of these primarily service-specific access systems is very great. Within a communications network, the access network has the function of enabling the subscribers to access switching devices, for example servers. From his location, each subscriber must firstly be connected to a supply node in order to avail himself of the offered services. All information technology means are basically suitable for the connection of the subscribers. In the long-distance network, the routing of the telecommunications traffic is concentrated in point-to-point connections between the different network- and switching nodes. On the other hand, in the access network very different requirements must be fulfilled. Standardised functions must be implemented for the technical cooperation between the subscriber exchange and the terminals. The subscribers themselves are not homogeneously distributed over the entire access area; rather, there are local areas with high subscriber densities as well as areas in which the subscribers occur only sporadically. On the basis of the relevant communications requirements, it is possible to distinguish between highly differing subscriber groups, such as private customers and smaller or larger business customers to whom appropriate transmission-technology solutions must be offered. It is in the case of the access networks that willingness is required on the part of the network operator to invest a large capital outlay. The components are exposed to environmental influences and must each be made available individually to the access network subscribers. A method for constructing access network is known form the prior art German application 199 04 940.8. A transmission system of this kind for coded optical signals consists of optical transmission lines, optical splitters and optionally optical amplifiers insert to transmit code multiplexed optical signals. Each transmitter contains a coder in which the signal to be transmitted are coded prior to their transmission into the optical transmission network. The coding takes place optically by frequency coding using an optical filter. Each receiver, which wishes to receive the data per specific transmitter, must contain a decoder, which is tuned to the coder of this specific transmitter. In the simplest case the frequency ranges which are conductive for an optical signal and the frequency ranges which are blocked for optical signals on the same in the coder and in the decoder. The code division multiplex methods are particularly as suitable for the transmission of not too high bit rates. The code division multiplex message is suitable for use as multipoint- to- multipoint network such as for example a LAN. An example for a broadcasting network the unpublished German application 100 51 633.5. All this examples for access network require optical coding means with high performance. To achieve the coding effect especially optical cavities of Fabry-Pérot types are used. To achieve the tunability phase shifters in the cavity of the optical coding means are needed. All effects that are utilized in refractive optical switches are commonly suitable for generating the additional phase shift in a cavity. In particular these are as follows: a) The electrical optical effect that for example occurs in lithium niobate, plastic or III/V semiconductors for example InGaAsP/InP or GaAlAs/GaAs and whereby an electrical field produces a change in refractive index. b) Charge carrier injections into III/V semiconductors for example InGaAsP/InP or GaAlAs/GaAs wherein the change in refractive index is defined by the strength of the injection and c) The thermo-optical effect that for example occurs in material systems of glass and silicon and where the temperature T determines the change in refractive index. Electro-optical phase shifters are polarization dependent and induce high losses. The thermo-optical effect in fibers or wave-guide optics is not very reliable or too expensive due to the fact that the devices consume a high electrical power. Also known are mechanical influences on fibers or wave-guide by stretching the wave-guides. Which is also known from prior art are liquid crystal phase shifters that are easy to build and consume not too much electrical power. It is known from the US 6 154 591 to use a tunable optical device comprising a substrate and a superstrate first in the second optical wave guide sandwiched between that substrate and that superstrate so as to define the base between that wave guides an optical resonant cavity extending across that space, that space containing a liquid crystal material to commit tuning of the cavity and alignment means to orientate the liquid crystal material so that it responds to an applied electrical field wherein the alignment means is exposed on at least one of the substrate and the superstrate. Normally the polarization dependent due to the anisotropy structure of the liquid crystal molecules induces the strong liquid crystal material to commit tuning of the cavity and alignment means to orientate the liquid crystal material so that it responds to an applied electrical field wherein the alignment means is exposed on at least one of the substrate and the superstrate. Normally the polarization dependent due to the anisotropy structure of the liquid crystal molecules induces the strong birefringence. To avoid this polarization dependence it is also known to use a symmetric arrangement of liquid crystal structures to make a tunable devices polarization insensitive. The publication "Electrically driven polarization insensitive liquid crystal narrow bandpass intensity modulator", E. Nowinowski-Kruszelnicki et al. Sensors and Actuators A, Elsevier Sequoia S.A. Lausanne, Vol. 68 describes an intensity modultor in a form of a tandem Fabry Perot filter. This filter comprises liquid crystal layers. The embodiment described can not be integrated in an integrated optical device. Summary of the invention According to the present invention the tunable optical device according to claim 1 comprises a first optical waveguide and a second optical wave guide defining a space between the two wave guides which space comprises the first cell with liquid crystal material and a second cell with liquid crystal material spaced by a third waveguide and the first and the second cell for liquid crystal material having orientation of the liquid crystal material perpendicular to each other. Especially the solution allows avoiding effects of polarization dependency. Also according to the present invention the tunable device form an optical cavity by high reflection means in that space where the first and the second cell of liquid crystal is in between the optical cavity, using a Bragg grating as reflection means. This allows integrating the device easily in an integrated circuit. The invention includes a method of fabricating the device according to claim 5 and a method for tuning the device according to claim 7. Description of the invention An exemplary embodiment of the invention is illustrated in Figure 3 and explained in detail in the following description. In the drawing: Figure 1 shows a symmetrical arrangement of liquid crystal cells Figure 2 an example of a tunable optical device Figure 3 an embodiment of the tunable optical device. Figure 1 show as an example two liquid crystal cells 1, which are aligned from the inputted light along the optical axes 2. The cells contain liquid crystal material. Liquid crystal molecules are typical rod shaped organic molecules with a function to order in temperature. The nematic phase is characterized by the orientation order of the constituent molecules. The molecule orientation can be controlled with applied electrical field. A well known nemactic cell is made of two bounding plates using glass slides each with a transparent conductive coding such as indium tin oxide that act as an electrode and spacers to control the cell gap precisely. The surface of the transparent electrodes in contact with the liquid crystal is coated with a thin layer of polymer, which has been rubbed or brushed in one direction. Nematic liquid crystal molecules tend to orient with their long axe parallel to this direction. For the phase shifter the glass plates are arranged so that the molecules adjacent to the top electrode are orientated parallel to those of the bottom. The state of polarization of the input light with an electrical field vector within the plane of the drawing is affected by the electrical voltage across the first liquid crystal cell 1 via the tilt angel of the liquid crystal molecules in the cell. The higher the tilt angle is the higher the retardation effect on the inputted light vector. In the case the molecules are arranged perfectly parallel to the glass plates and to the inputted light vector the retardation is zero. The voltage does not affect the polarization state with the electrical field vector perpendicular to the plane of the drawing. It is neither attenuated. By crossing the orientation of the molecules in two cascaded liquid crystal cells by 90 degrees relative to each other, both polarization axes of the input light will be retarded to the same amount or if required to different amounts according to the applied voltages and to the geometrical dimensions of the single cells. An example of a polarization independent phase shifter is shown in figure 2. This solution is a micro-optical implementation with separated wave-guides 3 built as a separate component for individual use in telecommunication systems. The optical device is constructed by fused silica substrate material. To avoid optical losses the device is coated with an anti-reflection layer at the input and output side of the device. Indium tin oxide layers 5 are evaporated for applying electrical voltage across the liquid crystal cells 1. Orientation layer 9 can be integrated separately or in the Indium tin oxide layers 5. The filter cavity is defined by the position of the two high reflective coatings 6. Tuning of the optical component is accomplished by applying a single electrical voltage across the filter using the two Indium tin oxide layers 5. In another embodiment when the wave-guide 3 in the middle of the device is too thick additional indium tin oxide layers 5 are deposited between the orientation layers 9 and the wave-guides 3. Than the two cascaded liquid crystal cells 1 are tuned separately. The two liquid crystal cells are tilted by 90 degrees relative to each other. Figure 3 shows an integrated planar wave-guide solution for the invention. A substrate 8 is structured to form optical wave-guides 3. The wave-guides 3 are cut at two positions to fill them with the liquid crystal material. Before the liquid crystals are filled in the defined grooves the electrical layers and reflective layers must be evaporated. The cells are closed by another silica layer which is fixed on the silica substrate. Also other materials and different fixing methods, as gluing melting, are used to close the liquid crystal cells In figure 3 the reflective mean for the optical cavity is realized in a Bragg grating 7, which is written into the optical wave-guide 3. Also in the scope of the invention is a method for tuning an optical filter structure with the help of the defined phase shift in the optical cavity. For this issue the two liquid crystal cells 1 are connected to an electrical source. The voltage applied to the indium tin oxide layers 5 influences the index of refraction in the cavity and allows a very high performing tuning of the optical cavity. Due to the symmetrical structure of the optical device polarization dependence disappears. In another embodiment the two liquid crystal cells 1 are applied separately with separate voltage values. The tunable filter device is preferably used in an optical code division multiplex system. The filter codes the broadband signal of the different light sources or decodes the received signals at the receiver side. The filter can be used for a translation device to translate one coded signal to another. The use in an optical code division multiplex system does not limit the scope of the use of the filter. One process for create the optical filter is a LIGA procedure. This method produces moulds with a high precision in different polymer materials. Therefore the grooved for the optical filter can be structured in a mould of ceramic, a polymer or silica, which can than be processed to comprise the liquid crystal cells. Also any other technology, which is known by persons skilled in the art, is possible for shaping the filter structure. Planar waveguide technology with several steps of masking, aligning and etching in a preferred embodiment used to structure a silica filter. For the ITO layers the well-known techniques of vaporization are used.

Claims: A tunable optical device (8) with a first cell (1) with liquid crystal material and a second cell (1) with liquid crystal material and the first and the second cell for liquid crystal have an orientation of the liquid crystal material perpendicular to each other, the device forming an optical cavity by two high reflection means (7) where the first and the second cell of liquid crystal is within the optical cavity, characterized in the that device comprises a first optical wave guide (3) and a second optical wave guide (3) defining a space between the two wave guides , whereby said space comprises the optical cavity including the first and the second cell for liquid crystals spaced by a third wave guide (3); and in that the reflection means (7) are Bragg gratings. A tunable optical device according claim 1 where the electrodes for orientation of the liquid crystal material are evaporated on the surface of the liquid crystal cells perpendicular to the optical axis in the optical path. A tunable optical device according to claim 1 with a compact structure comprising along the optical axis in a symmetrical arrangement: a first part of silica as the first optical wave guide, a first electrical layer, a first high reflection mean, the first liquid crystal cell with an orientation layer, a second part of silica as the third optical wave guide the second liquid crystal cell comprising an orientation layer, a second high reflective mean, a second electrical layer and a third part of silica as the second optical wave guide. A tunable optical device according to claim 3 integrating parts, layers and cells in a planar substrate structure. A method of fabricating a tunable optical device according to claim 3 by integrating parts, layers and cells in a planar substrate structure using planar wave guide or LIGA technology for forming the integrated structure. Optical transmission system with broadband light sources, coding means, transmission lines and receivers, using a optical code division multiplex transmission scheme, where the coding means are tunable optical devices according to claim 1. Method for tuning an optical device according to claim 1 by influencing the index of refraction of a material in the cavity, where index of refraction is adapted by applying electrical field across two liquid crystal cells.

Inventors: PFEIFFER THOMAS DR, PFEIFFER, THOMAS, DR.
Applicants: CIT ALCATEL, ALCATEL
Application year: 2001
Main classification: G02F1
IPC codes: G02B6, G02F1