Abstract:
An integrated optical circuit is described. The optical circuit includes a silicon substrate and waveguides disposed thereon, at least one photonic crystal is provided as a waveguide.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an integrated optical circuit having a silicon substrate and waveguides disposed thereon. 
     BACKGROUND INFORMATION 
     Integrated optical circuits are needed in communications engineering for various purposes, such as for the distribution, combining, spectral partitioning or switching of information-modulated light fluxes. In addition, it is also possible to implement other circuits with the aid of optical structures, such as computer circuits. 
     At present, integrated optical circuits are constructed using waveguides made of polymers or III-V compound semiconductors which are structured by lithographic processes. 
     Suitable as the optically active elements of such circuits are, inter alia, photonic crystals which, because of their small geometrical dimensions, require a waveguide pattern into which they are inserted in order to develop their full effect. Such waveguide patterns are usually strip waveguides made of polymer or semiconductor material. 
     These waveguide patterns can be produced in a complementary structure which, through its form, prevents the propagation of the photon pulses in the matter and, through selective built-in defects, allows propagation into otherwise completely reflecting matter. In this context, there is not a step change (sudden change) in refractive index as in the guiding of waves in optical waveguides formed by doping or in the form of strip waveguides, instead—theoretically stated—forbidden bands limit the state solution of the eigensolutions desired for specific wavelengths for propagating these waves. These waveguides are described, for example, in a reference by A. Mekis et AL in Physical Review Letters, Volume 77, No. 18, p. 3787. 
     SUMMARY 
     An object of the present invention is to provide an integrated optical circuit in which such waveguides are used for various functions and which can be manufactured with the requisite precision. 
     This objective is achieved according to the present invention, in that at least one photonic crystal is provided as a waveguide. Further waveguides may be provided in the form of strip waveguides, an insulating layer being disposed between the strip waveguides and the silicon substrate, and the photonic crystal extending from a plane below the lower boundary surface of the waveguides to beyond the upper boundary surface of the waveguides. 
     The commercially available material “silicon on insulator”, for example from the manufacturer SOITEC SA., Grenoble, France, can advantageously be used to manufacture the circuit according to the present invention. This material has good transmission properties for wavelengths of 1.55 μm. Silicon has a very high dielectric constant of 12 for such waveguides, which can also be used in the case of photonic crystals. Special photonic crystals, inserted with very low insertion loss at defined locations of the circuit, ensure the functioning of the circuit, for example as a computing circuit, it being possible for the entire circuit to be made very small. Thus, for example, 6 periods of the lattice of the photonic crystals with a lattice spacing of ⅓ of the wavelength are sufficient to achieve an attenuation of 35 dB. 
     One advantageous embodiment of the circuit according to the present invention is that the at least one photonic crystal is formed by needles having a high dielectric constant in the form of a two-dimensional periodic lattice with imperfections. However, it is also perfectly possible for the at least one photonic crystal to be formed by a body having a high dielectric constant with holes of low dielectric constant in the form of a two-dimensional periodic lattice with imperfections photonic crystals formed in this manner are described, for example, in German Patent No. 195 33 148. 
     Depending on the specific requirements, the needles may stand on the insulating layer, which is less thick in the region of the photonic crystal than under the waveguides, or the needles may stand on the silicon substrate. 
     An advantageous further development of the circuit according to the present invention is that the spaces between the needles are filled with non-linearly optical material, and that the refractive index of the non-linearly optical material is adjustable by a voltage applied to field electrodes. It is thus possible to control, for example, the behavior of filters designed as integrated optical circuits; See also German Patent No. 195 42 058. 
     In a further advantageous embodiment the needles or holes are at an angle with respect to the optical axis. This allows the branching of light in one part of the wavelength range into a further plane of the integrated optical circuit. An alternative thereto is provided by another embodiment of the present invention, in which the at least one photonic crystal, due to the arrangement of the imperfections, represents a branch filter in which branched-off light of a selected wavelength range escapes laterally. The laterally emerging light can be guided further in various manners. 
     In another example embodiment of the present invention, laterally emerging light of different wavelength ranges is capable of being focused on different locations of a parallel-extending photonic crystal. Thus, a plurality of computing planes can be connected in a simple manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross section of a segment from a circuit according to the present invention. 
     FIG. 2 shows a top view of the segment shown in FIG.  1 . 
     FIG. 3 shows a top view of a part of a further exemplary embodiment according to the present invention. 
     FIG. 4 shows an example of an optical connection of two planes of the integrated optical circuit according to the present invention. 
     FIG. 5 shows a schematic representation of an example of an optical connection of a plurality of computing planes in a circuit according to the present invention. 
     FIG. 6 shows a Mach-Zehnder interferometer implemented with an example circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the exemplary embodiment shown in FIG. 1, located on a silicon substrate  1  is an insulating layer  2  of silicon oxide, on which are applied optical strip waveguides  3 ,  4  made of silicon. Situated between waveguides  3 ,  4  is a photonic crystal  5  formed by a lattice of needles  6 . 
     In the exemplary embodiment, needles  6  stand on insulating layer  2 , which has a cavity in the region of photonic crystal  5 . This, combined with the fact that the needles jut out beyond the upper boundary plane of waveguides  3 ,  4 , means that the field conducted in edge areas outside of the waveguide is also covered by the photonic crystal. 
     Needles  6  may be produced in conventional manner by corpuscular-beam deposition. A process for this purpose is described, for example, in German Patent No. 195 33 148. 
     As was demonstrated in S. Y. Lin, G. Arjavalingam: Optics Letters, Vol. 18, No. 19, 1666 (1990) with reference to experiments with millimeter waves, just six periods of the lattice with a lattice constant of one-third of the wavelength are sufficient to achieve an attenuation of 35 dB. Within the wavelength range thus attenuated, using selective imperfections, i.e., by the omission of needles, it is possible to create wavelength ranges of reduced attenuation. In the exemplary embodiment shown in FIGS. 1 and 2, light of a plurality of wavelengths is guided from waveguide  3  to waveguide  4 , while light of a selected wavelength escapes at a branch  7 . The selected spacings of the needles in the central region of the photonic crystal represent merely an example of a precise configuration for obtaining the respective filter characteristics to be achieved. 
     In the exemplary embodiment shown in FIG. 3, photonic crystals are provided not only for a filter, but also for the inlet and outlets, inlet  11  and outlets  12 ,  13  in each case being in the form of all-pass filters, in that no needles are provided in the central region. 
     FIG. 4 shows an exemplary embodiment in which needles  14  forming the photonic crystal are inclined. A covering layer  15  is provided in selected regions, so that light escapes there and is focused through mounted lenses  16 , made for example of polymer material, into entrance windows (not shown) of an over-lying plane. This allows three-dimensional structures, such as in a computer circuit. The lenses may be produced in conventional manner by electron-beam lithography or using optical processes. 
     FIG. 5 shows a segment from a circuit according to the present invention, in which a plurality of branches  21  to  25  are formed by a photonic crystal  26 , a lens  27  to  31  focusing the light emerging from the branch onto entrance surfaces  32  to  36  disposed on further optical elements  37 ,  38  extending next to photonic crystal  26 . 
     FIG. 6 shows an exemplary embodiment in the form of a Mach-Zehnder interferometer. In this case, all the elements, particularly waveguide, filter, mirror and beam splitter, are formed by photonic crystals. The interferometer is to be used to measure the transit time in a reflecting object to be measured  41  that is schematically shown in FIG.  6 . For this purpose, the light supplied at  42  is first guided through an adjustable filter  43 , with whose aid the wavelength to be used for measuring is selected. Using a beam splitter  44 , the light emerging from filter  43  is guided in equal parts straight ahead to an adjustable phase shifter  45  and, in reflected form, to the object to be measured  41 . 
     Adjustable filter  43  and adjustable phase shifter  45  are each made of a photonic crystal, the interspaces being filled with non-linearly optical material whose dielectric constant, and thus the optically active spacings of the needles, is controllable by voltages applied to electrodes  46 ,  47  and  48 ,  49 . 
     Phase shifter  45  is adjoined by a completely reflecting mirror  50  which supplies the light emerging from phase shifter  45  to a further beam splitter  51 . 
     Disposed in front of the object to be measured  41  is a photonic crystal in the form of a directional filter  42 , with the effect that the light arriving from beam splitter  44  is guided into the object to be measured  41 , and the light reflected in the object passes via a waveguide  43  to the further beam splitter  51 . Both luminous fluxes overlap at output  54 . Using a suitable measuring transducer, the intensity emerging from output  54  can be measured, and the phase shift in the object to be measured  41  can be determined by adjusting the phase at  45  to a minimum of the intensity at output  54 . For the reasons described above, it is also possible to design the circuit shown in FIG. 6 to be extremely small, for example with an overall length of approximately 20 μm.