Abstract:
An integrated-optical device including a substrate having waveguide and a plurality of electrodes to receive electrical signals for controlling the light transmission through the pathways. The waveguide pathways are in an interaction zone of an electro-optically active waveguide material whose refractive index changes in response to electrical signals applied to the electrodes, and also in an access zone providing optical access to the interaction zone. The active waveguide material in the interaction zone is preferably a different material from the waveguide material in the access zone, enabling improved performance and/or simpler fabrication in a number of described respects.

Description:
This application claims benefit of provisional application Ser. No. 60/186,359, filed May 16, 2000. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to integrated optical devices, and also to methods of making such devices. 
     Integrated optical devices generally include a substrate formed with waveguide pathways each having a higher refractive index than the substrate for guiding the transmission of light therethrough, and a plurality of electrodes to receive electrical signals for controlling the light transmission through the pathways. The waveguide pathways in an interaction zone are of an electro-optically active waveguide material whose refractive index changes in response to electrical signals applied to the electrodes. Waveguide pathways in an access zone provide optical access to the interaction zone. 
     The invention is particularly useful in cavity-assisted directional-coupler devices in which the interaction zone includes an optical cavity having front and back ends defined by reflector facets perpendicular to the longitudinal axis of the optical cavity. The invention is therefore described below particularly with respect to this type of device, but it will be appreciated that the invention, or various aspects thereof, may also be used in other types of integrated optical devices. 
     Integrated optical devices are characterized by extremely short response times, in the sub-nano-second order, which makes them ideally suited in optical communications systems. Such devices generally, and cavity assisted directional-coupler devices in particular, are described in a large number of publications, including the Ph.D. thesis by the inventor in the present application: D. Nir, “Novel Integrated Optic devices Based On Irregular Waveguide Features”, Ph.D. thesis, Tel Aviv University, 1996. 
     The extension of such devices to ever-increasing applications depends to a large degree on the operational efficiency attainable by such devices, and also on the complexity in fabricating such devices. Efforts are continuously being made to increase the operational efficiency of such devices, and to simplify their fabrication, in order to extend their use to many additional applications. 
     For example, a fundamental feature of cavity-assisted directional-couplers is a very short optical cavity, typically 25-250 μm in length. The cavity is created when two reflectors confine a waveguide section. 
     The reflector structures, in particular at the input side, are generally trench structures created by etching out material, as by reactive-ion-beam etching (RIBE). The reflector facets must be perfectly flat, smooth and perpendicular to the optical cavity in order to minimize cavity losses because of scattering by imperfections. The back facet of the front trench is coated with a semi-reflecting film to input the light, whereas the front facet in the back trench is coated with a fully reflecting film to produce total reflection through the optical cavity between the latter two films. 
     Because of the trench structure produced by etching, the front facet of the front trench (facing the input waveguide) is coated with an anti-reflecting film to improve the light transmission. However, providing such a film adds to the complexity of fabrication; it also contributes to the optical losses in such devices. 
     OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide integrated optical device of the foregoing type, and methods for making them, to improve the operational efficiency of the devices, and/or to reduce the complexity in their fabrication. 
     According to one aspect of the present invention, there is provided an integrated optical device, comprising: a substrate including waveguide pathways each having a higher refractive index than the substrate for guiding the transmission of light therethrough, and a plurality of electrodes to receive electrical signals for controlling the light transmission through said pathways; 
     the waveguide pathways being included in an interaction zone and being of an electro-optically active waveguide material whose refractive index changes in response to electrical signals applied to the electrodes; the waveguide pathways also being included in an access zone providing optical access to the interaction zone; characterized in that the active waveguide material in the interaction zone is a different material from the waveguide material in the access zone. 
     As will be described more particularly below, this broad aspect of the invention enables a number of techniques to be used for improving the operating efficiency of such devices, as well as for reducing the complexity in their fabrication. 
     According to another aspect of the present invention attainable by the above feature, there is provided an integrated optical device of the optical cavity type characterized in that the reflector facets for the optical cavity (or cavities) are defined by trenchless formations in the substrate and consist only of a semi-reflecting facet at the front end of the optical cavity and a fully-reflecting facet at the back end of the optical cavity. Such a construction, obviating the need for trenches and an anti-reflecting facet at the inlet end of the optical cavity, not only enables the operation efficiency of the device to be improved by eliminating optical losses in the anti-reflecting coating on the front facet of the front trench, but also enables the fabrication of such devices to be simplified. 
     According to another aspect of the present invention also attainable by the foregoing feature the invention provides cavity-assisted directional couplers including a single optical cavity on the interaction zone, characterized in that both the input waveguide pathway and the output waveguide pathway are coupled to the optical cavity on the same side of the substrate, as distinguished from the prior art constructions, as described below (and illustrated in FIG. 1 a ) wherein they are on opposite sides of the substrate. Such a feature may be highly desirable in many designs to increase the flexibility and/or compactness of the design. 
     According to another aspect of the present invention, there is provided a method of making a cavity-assisted directional-coupler in which the interaction zone includes an optical cavity having front and back ends defined by reflector facets perpendicular to the longitudinal axis of the optical cavity produced by dicing and polishing, rather than by precise etching. As will be described more particularly below, such a method enables attaining both an increase in the operating efficiency of the device, as well as a reduction in the complexity of its fabrication. 
     According to another aspect of the present invention, there is provided an integrated optical device characterized in that a second substrate is bonded to the substrate formed with the interaction zone waveguide pathways and is of a material having a higher heat capacity than the material of the latter substrate so as to serve as a heat sink for that substrate. Such a construction permits the substrate including the waveguide pathways to be made of a first material, such as LiNbO 3 , having a relatively low heat capacity and a relatively high thermal sensitivity, and the second substrate to be made of a material, such as silicon, having a high heat capacity so as to serve as a heat sink for the first substrate and thereby to minimize its temperature change during the operation of the device. 
     According to a still further aspect of the present invention, there is provided a method of producing an integrated optical device including waveguide pathways defining an optical cavity of an interaction zone, and waveguide pathways in an access zone; the method comprising: forming the waveguide pathways of one zone in a first substrate; bonding the first substrate to a second substrate to embed the waveguide pathways; etching one of the substrates to produce perpendicular facets at the front and back ends of the optical cavity of the interaction zone; and applying reflector coatings to the perpendicular facets. 
     In the preferred embodiment of the invention described below, the second substrate is silicon and is etched to form a mask for etching the first substrate to produce the waveguide pathways of the interaction zone, and particularly the perpendicular facets at the opposite ends of the optical cavity. Since silicon is easily etchible by conventional wet etching techniques, as distinguished from LiNbO 3  which generally requires reactive ion beam etching (RIBE), this aspect of the invention enables relatively perfect reflector facets to be produced at the opposite ends of the optical cavities by wet etching rather than by RIBE. 
     Further features and advantages of the invention will be apparent from the description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, somewhat schematically and by way of example only, with reference to the accompanying drawings, wherein: 
     FIGS. 1 a  and  1   b  are top views of a single cavity device SCD and double cavity device (DCD), respectively, of the prior art; 
     FIG. 1 c  is a side view illustrating the construction of such prior art devices; 
     FIGS. 2 a  and  2   b  are top and side views, respectively, of the prior art devices more particularly illustrating the electrodes thereof; 
     FIG. 3 is a top view illustrating an integrated optical device constructed in accordance with one aspect of the present invention; 
     FIGS. 4 a - 4   i   2  illustrate various stages in one method for manufacturing an integrated optical device in accordance with the present invention; 
     FIGS. 5 a   1 - 5   e   3  illustrate various stages in a second method of manufacturing such devices in accordance with the present invention; 
     FIGS. 6 a   1 - 6   h   2  illustrate various stages in a third method of manufacturing such devices in accordance with the present invention; 
     FIGS. 7 a   1 - 7   i  illustrate various stages in a fourth method of manufacturing such devices in accordance with the present invention; 
     FIGS. 8 a - 8   g   2  illustrate various stages in a fifth method of manufacturing such devices in accordance with the present invention; 
     FIGS. 9 a - 9   e   4  illustrate various stages in a sixth method of manufacturing such devices in accordance with the present invention; 
     FIGS. 10 a - 10   i   2  illustrate various stages in a seventh method of manufacturing such devices in accordance with the present invention; 
     FIGS. 11 a - 11   h  illustrate various stages in an eighth method of manufacturing such devices in accordance with the present invention; and 
     FIGS. 12 a  and  12   b  illustrate a further advantageous feature of the present invention. 
    
    
     BRIEF DESCRIPTION OF PRIOR ART DEVICES 
     FIGS. 1 a  and  1   b  are top views schematically illustrating the two main type of known cavity devices, namely: a single cavity device (SCD) illustrated in FIG. 1 a , and a double-cavity device (DCD) illustrated in FIG. 1 b . FIG. 1 c  is an enlarged view schematically illustrating the construction of the optical cavity in either of these devices. 
     Thus, as shown in FIG. 1 a , the SCD includes a substrate  10  provided with a plurality of waveguide pathways each having a higher refractive index than the substrate for guiding the transmission of light therethrough. The SCD in FIG. 1 a  includes an input waveguide pathway  11  on one side of the substrate, a single optical cavity  12 , and an output waveguide pathway  13  on the opposite side of the substrate; whereas the DCD of FIG. 1 b  includes an input waveguide pathway  11 , two optical cavities  12   a ,  12   b , and an output waveguide pathway  13  on the same side as input waveguide pathway  11 . 
     FIG. 1 c  schematically illustrates the construction of the optical cavity  12  in the SCD of FIG. 1 a , or in each of the two optical cavities  12   a ,  12   b  in the DCD of FIG. 1 b . In the conventional cavity devices, each optical cavity  12  is defined by a front trench  14  and a back trench  15  at the opposite ends of the waveguide pathway defining the optical cavity  12 . The front facet of the front trench  14  is provided with an anti-reflecting coating  14   a ; the back facet of the front trench  14  is provided with a semi-reflecting coating  14   b ; and the front facet of the back trench  15  is provided with a fully-reflecting coating  15   a.    
     Thus, the light transmitted through the inlet waveguide pathway  11  passes through coatings  14   a  and  14   b  to enter the optical cavity  12  and is reflected back through the optical cavity by the reflecting coating  15   a . When the optical length of the cavity is properly phase tuned, the introduced light is amplified to a level depending on the structure parameters. The light in the optical cavity  12  is coupled directly to the output waveguide  13  in the SCD of FIG. 1 a , and via the second optical cavity ( 12   b ) to the output waveguide  13  in the DCD of FIG. 1 b.    
     The transmission state of the device is controlled by electrical signals applied to electrodes carried by the device. The electrical field produced by these electrical signals applied to the electrodes changes the refractive index of the waveguide material at the interaction zone, i.e., the optical cavity  12  and the outlet waveguide  13  coupled to it in the SCD of FIG. 1 a , or the two optical cavities  12   a ,  12   b  in the DCD of FIG. 1 b . This change in the refractive index is produced by the electro-optic effect and enhances or reduces power transfer-rate between the input and output waveguide pathways  11 ,  13 . 
     FIGS. 2 a  and  2   b  are top and side views, respectively, illustrating a DCD including such electrodes in the interaction zone of the two optical cavities  12   a ,  12   b . The illustrated construction includes two outer electrodes  21 ,  22  overlying the outer region on opposite sides of the two optical cavities  12   a ,  12   b , and an inner electrode  23  overlying the inner region between the two cavities. All three electrodes  21 ,  22 ,  23  are coplanar and are insulated from the waveguide layer by a dielectric buffer layer  24 . 
     Since such integrated optical devices are well known and extensively described in the literature, further details of the construction or operation of these devices are not set forth herein. The literature describes many electro-active materials which may be used, including LiNbO 3  (lithium-niobate), GaAs (gallium arsinide), InP (indium phosphide), silicon and electro-optic (EO) polymers. 
     DESCRIPTION OF THE BROAD CONCEPTS OF THE INVENTION 
     FIG. 3 illustrates one broad concept of the present invention, namely of using one material for the waveguide pathways in the interaction zone and a different material for the waveguide pathways in the access zone. 
     For purposes of example, FIG. 3 illustrates a double cavity device (DCD) corresponding to the prior art FIG. 1 b , including an input waveguide pathway  11 ; a pair of optical cavities, generally designated  12 , confined between a front reflector facet  14  and a back reflector facet  15 ; and an output waveguide pathway  13 . It will be seen that the optical cavities  12   a ,  12   b  occupy an interaction zone, generally designated  31 , which controls the transmission state of the device in accordance with electrical signals applied to their electrodes (corresponding to electrodes  21 - 23 , FIG. 2 a ); while the two waveguide pathways  11  and  13  occupy an access zone, generally designated  32 , providing optical access to the interaction zone  31 . 
     Whereas in the prior art, the waveguide material in the access zone  32  was generally the same as in the interaction zone  31 , according to one important aspect of the present invention the materials are different in the two zones. Thus, the waveguide material in the interaction zone  31  must be electro-optically active, i.e., one whose refractive index changes in response to electrical signals applied to the electrodes; while the material in the access zone  32  need not be electro-optically active but rather can be electro-optically passive, since the function of these waveguides is merely to provide optical access to the interaction zone. 
     As will be described more particularly below with respect to specific embodiments of the invention selected for purposes of example, the foregoing concept enables many important advantages to be attained, including the following: 
     (a) improved facet quality in the trench reflectors, thereby adding to the transmission quality of the device; 
     (b) improved electro-optical efficiency, thereby reducing the electrical drive power required; and/or: 
     (c) improved thermal stability, thereby enabling one waveguide material having high heat capacity to be used as a heat sink for the other waveguide material having good electro-optical properties but also a high thermal sensitivity to temperature changes. 
     While the foregoing is broadly an important concept of the present invention, the invention also involves several other concepts all stemming from this broad concept, as will be brought out in the description below of several methods of making integrated optical devices in accordance with the present invention. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     For purposes of illustrating the various aspects and the scope of the invention, the invention is described below with respect to a number of examples constituting preferred embodiments of the invention at the present time. 
     EXAMPLE 1 
     (FIGS.  4   a - 4   i   2 ) 
     This example utilizes a silicon wafer  40 , as illustrated in FIG. 4 a , with &lt;110&gt; orientation as purchased from the vendor. Such a wafer is typically 400 μm thick and is provided with a top layer  41  doped in a manner to form an index step for light waves at wavelength over ˜1100 nm; therefore, light injected into layer  41  propagates in it and does not escape into the bulk of the substrate. 
     The waveguide layer  41  is first patterned by photolithography and etched selectively, e.g., by RIE (reactive ion etching) or by RIBE (reactive ion beam etching) to define the waveguide channels in the access zone ( 32 , FIG.  3 ), typically supporting propagation of the fundamental mode. The cavity in the interaction zone ( 31 , FIG. 3) is then patterned and chemically “wet” etched (typically a KOH bath) to form the front and back facets of the cavity with a high degree of flatness, smoothness and perpendicularity. FIGS. 4 b   1  and  4   b   2  are side and top views, respectively, illustrating the results at this stage of the method producing the input waveguide pathway  42 , the output waveguide pathway  43 , and the cavity  44  between the flat, smooth, and perpendicular facets  44   a ,  44   b , at the front and back ends, respectively, of the cavity. 
     In the next processing step, the two cavity facets  44   a ,  44   b  are coated with their respective reflecting coatings  45   a ,  45   b , as shown in FIG. 4 c , (side view) and FIG. 4 c   2  (top view). These coatings may be either metallic (single film), or dielectric multi-layer pair stacks (alternating paired index sequence: n 1 , n 2 : n 1 , n 2 ; n 1 , n 2  - - - ) as known in the art. A dielectric coating has lower losses and can also be designed to admit a specified spectral window. As described above, the front facet coating  45   a  is semi-reflecting, whereas the back facet coating  45   b  is fully reflecting. 
     The next processing step involves the formation of the various elements in the interaction zone within cavity  44 , as illustrated in FIGS. 4 d   1 - 4   d   3 . Thus, FIG. 4 d   1 , which is an enlarged fragmentary view, illustrates the isolating dielectric buffer layer  46  which is first applied, and then the metal electrode  47  deposited thereover. Electrode  47  serves as a common bottom electrode for the three coplanar electrodes  21 - 23  of FIG. 2 a . FIGS. 4 d   2  and  4   d   3  are side and top views, respectively, schematically illustrating the device at this stage. 
     The next step involves the application of the electro-optically active waveguide material, as shown FIGS. 4 e   1 - 4   e   3 , over the electrodes  47 . As seen in the enlarged fragmentary view of FIG. 4 e , and the side view of FIG. 4 e   2 , the electro-optically active waveguide material is in a middle core covered by a bottom cladding  48   a  and a top cladding  48   b ; and as seen in the top view of FIG. 4 e   3 , it defines a pair of optical cavities between the reflector facets  45   a ,  45   b . Waveguide material  48  is preferably an electro-optically active polymer (EO polymer). 
     FIG. 4 f  is an enlarged sectional view more particularly illustrating the various elements within the cavity  44 , namely the dielectric buffer  46 , the bottom electrode  47 , and the EO polymer  48  in the form of two parallel optical cavities, and the bottom and top claddings  48   a ,  48   b  of the EO polymer  48 . 
     As an example, the bottom cladding  48   a  may be a passive material spun, cured and trimmed to proper thickness by RIE or by RIBE; the EO polymer  48  may be spun, cured, trimmed and than patterned to define the two optical cavities; and the top cladding  48   b  may be a passive material applied to fill the space between the EO polymer  48 , as well as topping it typically by 2 μm, cured and trimmed. 
     The next processing step, as illustrated in the side and top views of FIGS. 4 g   1 ,  4   g   2 , respectively, produce the top electrodes  49 , e.g., patterned to correspond to the three coplanar electrodes  21 - 23  in FIG. 2 a . These top electrodes may be applied by a conventional photolithographic technique, e.g., by photoresist patterning, vacuum deposition (of metals), electroplating, etching, etc. 
     The top electrodes  49  have two functions: (1) initially, to pole the EO polymer  48  defining the two optical cavities in the interaction cavity zone  44 ; and (2) operationally, to control the transmission state of the optical device. 
     Poling is required for creating un-isotropic polarization in the polymer in order to make it electro-optically active. Poling is typically carried out at a field of 3.5 V/μm and at 300° C. Cooling the material with the field still applied fixes the polarization; and the direction of the field lines determines the polarization orientation. 
     The top electrodes  49  may also be used for controlling the transmission state of the device. As known in the art, other electrode arrangements may be provided for this purpose. 
     The integrated optical device may then be completed and packaged to provide a packaged device as illustrated in the side view of FIG. 4h 1  and the top view of FIG. 4 h   2 , to include a platform  49   a  to which the substrate  40  is bonded; a metallic casing  49   b  housing the device; input and output lead-through waveguides  49   c ,  49   d ; r.f. feedthroughs  49   e  bonded to the electrodes; and an electrical feedthrough  49   f . These operations can be performed by conventional techniques. 
     FIGS. 4 i   1  and  4   i   2  are side and top views. respectively, illustrating a possible modification to this method. In this modification, instead of forming the access waveguides (e.g.,  42 ,  43 ) from the waveguide layer  41  of the silicon wafer  40 , these access waveguides may be formed of the same polymer as the waveguides in the interaction zone, except they would not be poled to make them electro-optically active. 
     Thus, as shown in FIGS. 4 i   1  and  4   i   2 , the initial silicon wafer  40  need not be provided with a waveguide layer  41 , but rather can be formed with a narrow upwardly-projecting strip  41   a  and a larger upwardly-projecting strip  41   b  to define between them the facets at the opposite ends of the cavity zone  44 . 
     Strips  41   a  should be typically 3-10 μm wide so that the propagating wave will sense only a minor disturbance between the interfaces; with wider dimensions, the beam will spread and scatter away from the path. 
     The semi-reflecting coating  45   a  at the front end of the cavity zone  44  would be applied to the back face of strip  41   a , and the fully-reflecting coating  45   b  at the back end of the cavity zone would be applied to the front face of the upwardly-projecting strip  41   b . However, in this case an anti-reflecting coating  45   c  would be applied to the front face of strip  41   a . After the above reflecting coatings have been applied to the opposite ends of the cavity zone  44 , an electro-optically active waveguide material, such as an EO polymer, may be applied in the interaction cavity zone  44  to define the pair of optical cavities ( 48 , FIG. 4 f ); and also in the access zone to define the two waveguide pathways ( 42 ,  43 FIG. 4 b   2 ). 
     It will thus be seen that this modification eliminates a patterning step, but requires an additional coating step for coating the anti-reflection layer  45   c  on the access zone facet (front facet of strip  41   a ). Also, since there are no silicon waveguides, the original silicon substrate is not required to have a waveguide layer. All the other steps described above with respect to this method would also apply to this modification. 
     It will be appreciated that the foregoing method provides a number of important advantages over the prior art, including the following: 
     The method, except for the modification of FIGS. 4 i   1 ,  4   i   2 , produces trenchless formations at the front and back ends of the optical cavity, and therefore obviates the need for the anti-reflection coating at the front end of the optical cavity, thereby simplifying the fabrication process as well as reducing optical losses. With respect to the modification of FIGS. 4 i   1 ,  4   i   2 , while the anti-reflecting coating is required, the fabrication process is simplified by eliminating one of the patterning steps. 
     In addition, employing different waveguide materials for the interaction zone and each zone, the access zone may be made of a material which has superior characteristics for its respective function. For example, by making the waveguide material in the access zone of silicon (&lt;110&gt;), wet etching of the ( 111 ) facets produces perfectly perpendicular, flat, and smooth facet surfaces. In addition, by applying the electro-optically active waveguide material in the interaction zone, this material may be applied to tightly fill the space between the reflector facets, thereby to produce trenchless formations, and also to become automatically aligned wit the facets. In addition, improved thermal stability may be obtained by using a relatively thick substrate having high heat capacity, such as silicon, for the access zone, and a relatively thin layer, such as an EO polymer, for the interaction zone. 
     While the method described above utilizes silicon as the substrate and also as the waveguide material in the access zone, with an EO polymer as the waveguide material in the interaction zone, it will be appreciated that other materials and combinations can be used according to particular applications. For example, the substrate may also be LiNbO 3 , GaAs, or InP; the active waveguide material in the interaction zone may be the same as that of the substrate; and the waveguide material in the access zone may be a polymer, silicon or SiO 2 . 
     EXAMPLE 2 
     (FIGS.  5   a   1 - 5   e   3 ) 
     In this example, the formation of the waveguide pathways in the interaction zone and access zone is reversed as compared to the above-described Example 1 method. That is, in this Example 2 method, the waveguide pathways in the interaction zone are first formed with the reflector facets of the optical cavity, and then the waveguide pathways in the access zone are formed. 
     FIGS. 5 a   1  and  5   a   2  are side and top views, respectively, illustrating the starting substrate  50 , in this case lithium niobate. The first step, shown in FIGS. 5 a   1 ,  5   a   2 , is to form the waveguide pathways in the interaction zone, namely the two optical waveguides  51   a ,  51   b.    
     These waveguide pathways may be formed by conventional techniques, e.g., by Ti indiffusion. Waveguide pathways  51   a ,  51   b  are thus made of electro-optically active waveguide material and constitute the interaction zone of the device. 
     The next step is to form the electrodes. FIGS. 5 b   1  and  5   b   2  are side and top views, respectively, illustrating this step in the process, wherein the waveguide pathways  51   a ,  51   b  are first covered by a thin dielectric buffer film  52  (typically SiO 2 ), and then by a metal film  53   a ,  53   b  (typically gold). 
     FIGS. 5 c   1 ,  5   c   2  are side and top views, respectively, illustrating the next step, which involves the formation of the facets  54   a ,  54   b , at the opposite ends of the waveguide pathways  51   a ,  51   b  for producing the mirror facets confining the optical cavities defined by pathways  51   a ,  51   b . Preferably, the facets  54   a ,  54   b  are produced by RIBE. 
     FIGS. 5 d   1 ,  5   d   2 , are side and top views, respectively, illustrating the next step in this method, namely the application of the reflector coatings  55   a ,  55   b  to the facets  54   a ,  54   b . These coatings may be metal films, but preferably are dielectric multi layer pair-stacks vacuum deposited on the facets  54   a ,  54   b.    
     FIGS. 5 e   1 - 5   e   3  illustrate the next stage in this method, namely the application of the waveguide material in the access zone of the device. As shown particularly in the enlarged fragmentary illustration of FIG. 5 f   1 , this waveguide material includes a core  56  with bottom and top claddings  56   a ,  56   b . The bottom cladding  56   a  is first deposited, followed by the deposition of the core layer  56  patterned according to conventional techniques to define the input and output waveguide pathways  57 ,  58 , followed by the deposition of the top cladding layer  56   b.    
     The fabrication of the optical device may then be completed in any conventional manner, or as described above with respect to Example 1. 
     It will be seen that this Example 2 also provides a number of important advantages over the conventional techniques, including elimination of the trench formation for the input mirror, thereby obviating the need for the anti-reflecting coating in the front cavity facet, among the many other advantages described above with respect to the first method. 
     EXAMPLE 3 
     (FIGS.  6   a   1 - 6   h   2 ) 
     In this example, a substrate  60 , such as lithium niobate, is processed in the same manner as in the first two steps of Example 2 described above to produce the two waveguide pathways  61 , the dielectric film  62 , and the two electrodes  63 , as illustrated in the side views of FIGS. 6 a ,  6   b   1 , and their corresponding top views FIGS. 6 a   2 ,  6   b   2 . 
     The face of the lithium niobate substrate  60  carrying the waveguide pathways  61  and electrodes  63  is then bonded and fused to a silicon substrate  64  to embed the foregoing elements in the bonded faces of the two substrate, as shown in FIG. 6 c.    
     The lithium niobate substrate  60  is then thinned to a thickness of about 5-7 μm, e.g., by mechanical or chemical means (FIG. 6 d ). A thin buffer film  65   a  is then applied, and additional electrodes  65  are patterned thereon (FIG. 6 e ). 
     The foregoing layers are then subjected to an etching process, e.g., RIBE, to define the cavity facets  66   a ,  66   b . At the same time, the etching process removes the thinned lithium niobate of the original substrate  60  except for the film between the two electrodes  63 ,  65 , thereby exposing the silicon substrate  64  (FIG. 6 f ). 
     The reflector coatings  67   a ,  67   b , are then applied to the two facets  66   a ,  66   b  to define the opposite ends of the two optical cavities produced by the waveguide pathways  61  in the interaction zone of the device (FIG. 6 g ). 
     The waveguide pathways of the access zone are then provided by bonding a substrate  68  of silicon, polymer or the like and patterning the input and output waveguide pathways  68   a ,  68   b  on the substrate in accordance with conventional techniques. 
     The result, as shown in the top and side views of FIGS. 6 h   1  and  6   h   2 , is an optical device in which the two interaction zone waveguides  61  are of thin lithium niobate; the access waveguide pathways  68   a ,  68   b  are of silicon, polymer, or the like; and both waveguide pathways are bonded to a relatively thick base of silicon, which has high heat capacity. The silicon base therefore serves as a heat sink particularly for the waveguide pathways in the interaction zone, thereby substantially increasing the thermal stability of the optical device. 
     It will be appreciated that many of the other advantages described above are also attainable by this method. 
     EXAMPLE 4 
     (FIGS.  7   a   1 - 7   i ) 
     The process illustrated in FIGS. 7 a   1 - 7   d   2  of this example is generally similar to that described above with respect to FIGS. 6 a   1-6   e  of Example 3 to produce a silicon substrate  74 , a bottom metal electrode  73 , an active waveguide pathway  71 , a thinned layer of lithium niobate  70 , and a top metal electrode  75  generally, corresponding to elements  60 - 65  in Example 3, except both metal electrodes  73  and  75  are isolated on both faces by buffer layers  73   a ,  73   b  and  75   a ,  75   b . FIG. 7 d   2  illustrates the conductive deposit  76  serving as the connection to the electrode  75 . 
     In this Example 4, however, the silicon substrate  74  is used as a mask for etching the facets at the opposite ends of the optical cavities defined by the waveguide pathways  71  in the interaction zone. 
     Thus, as shown in the side view of FIG. 7 e   1  and in the top view of FIG. 7 e   2 , a photo resist mask  77  is applied to the outer face of the silicon substrate  74 . This mask is used for etching the silicon substrate  74 , as well as the opposite edges of the metal layers  73  and  75 , to produce trenches  77   a  having a very high aspect ratio (width to depth). This etching may be effected either dry (e.g., by the RIE “Bosch Process”), or wet of ( 111 ) planes of &lt;110&gt; silicon to the thinned lithium niobate layer  70 . 
     The thinned lithium niobate layer  70  may then be etched through the trenches  77   a  of the silicon substrate. The etching of the lithium niobate layer is greatly facilitated because of the high aspect ratio, smoothness and perpendicularity of the walls of the trenches  77   a  produced by etching the silicon substrate, such that smooth, flat and perfectly perpendicularity facets can be produced by using a suitable etchant, such as SF 6 . This is shown in the side and bottom views of FIGS. 7 f   1 ,  7   f   2 , respectively. 
     With the RIE process, the mask thickness is limited to 100-200 μm. However, a thicker mask could be used with the Ion-Beam-Milling process (Bombardment of the material with high energy ions, a hot chemical process like RIE/RIBE). 
     The reflecting coatings, in this case three coatings  78   a ,  78   b  and  78   c  may then be applied to define the reflector facets of the optical cavities in the waveguide pathways  71 , as shown in FIGS. 7 g   1 ,  7   g   2 . The reflecting coatings  78   a - 78   c  may be applied via photo resist masks from the lithium niobate layer  70  side of the silicon substrate  74 . 
     The optical device may then be completed and packaged in the manner described above with respect to Example 1 (FIGS. 4 i   1 ,  4   i   2 ) to include a mounting base (not shown, corresponding to base  49   a  of FIG. 4 h   1 ), a housing  79   b  corresponding to housing  49   b , the input and output waveguide pathways  79   c ,  79   d , the r.f. feedthrough  79   e , and the electrical feedthrough  79   f , as shown in FIGS. 7 h   1 ,  7   h   2  and  7   i.    
     EXAMPLE 5 
     (FIGS.  8   a - 8   g   2 ) 
     The first steps of the method in this example, as illustrated in FIGS. 8 a - 8   e , are similar to the first steps in the method of Example 4, to produce the silicon substrate  84 , the metal electrode  83 , its buffer layers  82   a ,  82   b , the waveguide pathway  81  and the lithium niobate initial substrate  80 , with the trenches  87   a  being formed through the silicon substrate  84 . In this case, however, the lithium niobate substrate  80  is not thinned; only its upper surface is etched; and the silicon substrate  84  is subsequently removed, as shown in FIG. 8 f.    
     The reflector coatings  88   a ,  88   b  and  88   c  (FIG. 8 g ) are then applied to the facets defined by the trenches  87   a  to produce the reflector facets at the opposite ends of the interaction zone defined by the active waveguide pathways  81  on the lithium niobate substrate  80 . 
     If the waveguide pathways  81  in the interaction zone define a single optical cavity to produce an SCC, the access zone would include an input waveguide pathway  89   a  on one side of the substrate, and an output waveguide pathway  89   b  on the opposite side, as shown in FIG. 8 g , and as described above with respect to FIG. 1 a . On the other hand, if the waveguide pathway  81  in the interaction zone defines two parallel optical cavities, as shown in FIG. 1 b , the input waveguide pathway and the output waveguide pathway in the access zone would be on the same side of the substrate, as shown at  89   a ′ and  89   b ′ in FIG. 8 g   2 . 
     EXAMPLE 6 
     (FIGS.  9   a - 9   e   4 ) 
     This method involves the same sequence of steps as shown in FIGS. 8 a - 8   f  of Example 7 to produce the lithium niobate substrate  90  carrying the waveguide pathway  91  in the interaction zone, the metal electrode  93 , and its buffer layers  92   a ,  92   b , as shown in FIG. 9 a.    
     In this method, however, a photoresist mask  98  is applied over the above-described elements in the interaction zone (FIG. 9 b ). The masking by photoresist of the interaction zone permits: (1) applying the reflective coatings; and (2) processing the access waveguides. The flat facets (of the mirrors) were produced at an earlier stage. 
     The access waveguides may then be applied by applying another substrate  99  (e.g., of silica or a polymer to one side of substrate  80 , to produce a double cavity device (DCD) in which the input and output waveguide pathways  89   a ,  89   b  are located on the same side of substrate  80 , as shown in FIGS. 9 e   1  and  9   e   2 ; or a single cavity device (SCD), as shown in FIGS. 9 e   3 ,  9   e   4 . FIGS. 9 e   3 ,  9   e   4  illustrate the input and output waveguide pathways  99   a ′,  99   b ′ on opposite sides of substrate  90  (output waveguide pathway  99   b ′ being formed in a separate substrate  99   c  applied to the opposite side of substrate  90 ), but it will be appreciated that it could also be on the same side as described in Example 5. 
     All the foregoing Examples 1-6 involve the need for precise etching in order to produce the required smooth, flat and perpendicular reflector facets at the opposite ends of the cavity, or pair of cavities, in the interaction zone. Such precise etching is critical to avoid loss of optical power, and is particularly difficult with respect to certain materials that may otherwise have optimum properties for integrated optical devices. Moreover, in many of these methods, the reflector facets required trench formations at the opposite sides of the optical cavity or cavities, and therefore need, besides the normal front semi-reflecting coating and the back fully-reflecting coating, also an anti-reflecting coating at the inlet side of the optical cavity, which not only increases the complexity of the fabrication process, but also contributes to optical power losses. 
     FIGS. 10 a - 11   i   2  describe two examples of methods for making integrated optical devices in accordance with the present invention, which obviate the need for precise etching to produce the required reflector facets at the opposite ends of the interaction zone. In the following two methods, the high-quality reflector facets are produced, not by precise etching, but rather by cutting or dicing a substrate formed with waveguide pathways of one of the zones (e.g., the interaction zone), polishing the cut edges, applying the reflector coatings to the polished edges, and then bonding the substrate to another substrate to provide waveguide pathways of the other zone (e.g., the access zone). 
     EXAMPLE 7 
     (FIGS.  10   a - 10   i   3 ) 
     In this method, a lithium niobate substrate  100  is patterned to define waveguides  101 ; and at the interaction zone, the electrode layer is Applied by first applying a buffer coating  102 , the patterned electrode layer  103 , and an overlying buffer layer  104  (FIG. 10 b ). 
     As shown in FIG. 10 c , the back end of the interaction zone is then cut or diced along a line  105  perpendicular to the longitudinal axis of the optical cavity to be produced in that zone. The diced edge is then polished, as shown in FIG. 10 d , and the back reflector coating  106   a  is applied as shown in FIG. 10 e.    
     Another substrate  107   a  (e.g., silicon) is then bonded to that face of substrate  100 , and a further substrate  107   b  (e.g., also of silicon) is bonded to the underface of both substrates  100  and  107   a  (FIG. 10 f ). The resulting block is then cut along another perpendicular line  105   b  (FIG. 10 g ) to define the front facet of the interaction zone. The front facet is then polished and coated with the front reflector coating  106   b  (FIG. 10 h ). 
     A further substrate  108  (e.g., also of silicon or of silica, a polymer, etc.) is then bonded to this face, as shown at FIG. 10 i , to define the access waveguide pathways for the optical device. 
     FIG. 10 i  illustrates a double cavity device (DCD) wherein the input waveguide pathway  108   a  and the output waveguide pathway  108   b  are on the same side of the substrate, as in the conventional construction shown in FIG. 1 b.    
     FIG. 10 i   2  illustrates a single cavity device (SCD), wherein the input waveguide pathway  108   a ′ and the output waveguide pathway  108   b ′ are also on the same side of the substrate, as distinguished from the conventional construction illustrated in FIG. 1 a . Providing the SCD with the input and output waveguide pathways on the same side of the substrate permits greater flexibility in design, which may be advantageous in many applications of integrated optical devices. 
     EXAMPLE 8 
     (FIGS.  11   a - 11   i   2 ) 
     Steps  11   a - 11   e  of this example are similar to the steps illustrated in FIGS. 7 a   1 - 7   d   2  in the above-described Example 4, to produce a thinned lithium niobate substrate  110  formed on its upper surface  110   a  with a waveguide pathway  111 , and bonded to a thick silicon substrate  114  embedding, at the interaction zone, a patterned metal electrode layer  113 , and its buffer layers  112   a ,  112   b , over the waveguide pathway  111  at the interaction zone. The opposite surface of the thinned substrate  110  carries another patterned metal electrode layer  116 , between buffer layers  116   a ,  116   b , also in the interaction zone. 
     As shown in FIG. 11 e , the device is cut along a perpendicular line  117   a  to define the back facet of the interaction zone. This edge is polished, and the reflector coating  118   a  is applied to serve as the back reflector facet of the optical cavity to be produced. This back reflector coating  118   a  is shown in FIG. 11 f  after another silicon substrate  119   a  has been bonded to that face of substrate  114 , and a further supporting substrate  119   b  has been bonded to the faces of both substrates  114  and  119   b  opposite to the waveguide layer  111 . 
     The two substrates  119   b  and  114  are then cut along a perpendicular line  117   b  to define the front facet of the optical cavity to be produced in the interaction zone; and after this cut edge is polished, the front reflector coating  118   b  is applied. This is followed by the bonding of a third substrate  119   c  to define the access zone or a part thereof, including the input and output waveguide pathways  119   a ,  119   b , as described above. It will be appreciated that, this method may also be used to produce optical devices in which the input and output waveguide pathways in the access zone are on the same side of the interaction zone, not only in double cavity devices (DCD) as in the prior art, but also in single cavity devices (SCD) as distinguished from the prior art. 
     A Further Advantageous Feature (FIGS.  12   a - 12   b ) 
     FIGS. 12 a  and  12   b  illustrate a further advantageous attainable by the novel methods of the present invention, particularly where the integrated optical device is constructed of a thinned lithium niobate substrate carrying the waveguide pathways, and a thick silicon substrate acting as a heat sink as well as a mechanical supporting structure. 
     FIG. 12 a  is a cross-sectional view of the interaction zone of the DCD or SCD, wherein it will be seen that the thinned lithium niobate substrate  120  is bonded to the silicon substrate  121  and carries a bottom electrode  122 , the optical cavity or cavities ( 123   a ,  123   b ) in the interaction zone, and the top electrodes  124   a ,  124   b . Such as construction may be produced by any one of the Example 3-8 methods described above. 
     If a notch is cut between the two top electrodes  124   a ,  124   b  on the thinned lithium niobate face  120 , as shown at  125  in FIG. 12 b , the electrical field is confined to the paths between the opposite electrodes since the field trajectory through the lithium niobate layer  120  is made significantly longer than the path between the bottom electrode  122  and the top electrodes  123   a ,  124   a . This improves the overlap in the electrical-optical fields, and therefore improves the efficiency and reduces the power requirements of the device. 
     While this improvement, as well as other features of the invention described above are particularly applicable to cavity-assisted directional couplers, it will be appreciated that such features could be applied as well to other optical devices particularly those based on thinned substrates, and made of other electro-optic materials. 
     Also, while the invention has been described with respect to several preferred embodiments, it will be appreciated that these are set forth merely for purposes of example, and that many other variations, modifications and applications of the invention may be made.