Patent Abstract:
An optoelectronic device and a method of making same. The optoelectronic device comprises a substrate, at least one dielectric waveguide in the substrate, and at least one active semiconductor layer physically bonded to the substrate and optically coupled to the at least one dielectric waveguide in the substrate, the at least one active semiconductor layer being able to generate light, detect light, amplify light or otherwise modulate amplitude or phase of light.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional application No. 60/332,370 filed Nov. 15, 2001 for “Waveguide-Bonded Optoelectronic Devices” by Daniel Yap and Keyvan Sayyah, the disclosure of which is hereby incorporated herein by reference.  
         [0002]    This application is related to a provisional patent application entitled “Agile RF-Lightwave Waveform Synthesis and an Optical Multi-Tone Amplitude Modulator” (Attorney Docket 618837-7) bearing serial No. 60/332,367 and filed Nov. 15, 2001, and its corresponding non-provisional application bearing Ser. No.______and filed on the same date as the present application (Attorney Docket 619578-0), the disclosures of which are hereby incorporated herein by this reference. These related applications are owned by the assignee of this present application. 
     
    
     
       TECHNICAL FIELD  
         [0003]    This invention relates a new class of guided-wave optoelectronic devices that provide low waveguide loss and efficient coupling to optical fibers while also being able to conduct electrical current as well as having electrically controlled parameters that can be changed rapidly. These devices are comprised of low-loss dielectric waveguides that are optically coupled to electronically active semiconductor elements that may provide functions such as in/out coupling and filtering of the guided light, absorption or generation of the light, and modulation of the amplitude and/or phase of the light. The devices, and photonic circuits containing those devices, are generally formed on a substrate that contains a network of dielectric waveguides, with the semiconductor elements being thin islands or appliques that are bonded onto the waveguide-containing substrate. Multiple active elements of different types may be coupled to the same waveguide network.  
         BACKGROUND OF THE INVENTION  
         [0004]    Electronically active guided-wave optoelectronic devices, such as lasers, photo-detectors and electroabsorption modulators fabricated from semiconductor materials, are able to conduct electrical current and transduce signals between the electrical and optical domains. These devices typically have high optical-propagation loss and couple poorly to optical fibers. Lenses are typically used to reduce the fiber-coupling loss. However, the need for lenses complicates the assembly/packaging procedure and greatly increases component costs. Electronically passive waveguides are preferably fabricated in dielectric materials, such as silica or lithium niobate, since they enjoy an order-of-magnitude lower propagation loss and because they couple efficiently with optical fibers. However, it generally is not possible to fabricate electronically active optoelectronic devices in dielectric materials, since they are insulators. Only optical filters with fixed response and modulators/switches based on temperature change or the electro-optic effect (which depends on an applied voltage rather than current) have been demonstrated in dielectric waveguides. The present invention can lead to the realization of electronically active guided-wave devices, such as those listed above, with dielectric waveguides that are low loss and have efficient coupling to optical fibers.  
           [0005]    Many advanced optoelectronic devices and integrated photonic circuits, such as mode-locked lasers or optoelectronic transceivers, consist of various electronically active devices that are interconnected by passive waveguide networks. In general, the waveguides in these advanced devices are fabricated from a semiconductor material and thus tend to have a substantially higher loss and poorer coupling to optical fiber than do dielectric waveguides. In the prior art, in which the active devices are interconnected by dielectric waveguides, separate substrate chips of semiconductor or waveguide material must be optically aligned and properly butted against each other or coupled through optical lenses in order to function. The packaging effort associated with this method of combination is costly. The present invention achieves the integration of semiconductor active devices and dielectric waveguide networks on a single substrate. Also, multiple devices, each of which could contain multiple active elements, can be fabricated on a given substrate.  
           [0006]    An example of a prior-art electronically active device that monolithically integrates a semiconductor waveguide with a photodetector is described in  IEEE Photonics Technology Letters,  v. 5, pp. 514-517 (1993). The photodetector of this device is located above a portion of the semiconductor waveguide. The interface region between the photodetector and the waveguide layers is controlled because that interface region is produced as part of the single epitaxial growth for both the photodetector and waveguide materials. This approach has the disadvantage previously discussed above because it uses a semiconductor waveguide.  
           [0007]    Examples of prior-art approaches for butt coupling of dielectric waveguides with lasers or photodetectors that are comprised of separate, complete chips are described in  IEEE J. Selected Topics in Quantum Electronics,  v. 6, pp. 4-13 (2000). A large variety of components have been realized with this approach. The difficulties of this approach are discussed above.  
           [0008]    A prior-art approach that could be used to fabricate an electronically active waveguide device involves directly bonding two pieces of semiconductor materials. One piece contains fabricated semiconductor waveguides and the other piece could contain the active element. This approach is described in  IEEE Photonics Technology Letters,  v. 11, pp. 1003-1005 (1999). In this example, two semiconductor waveguides are bonded to a semiconductor microdisk resonator. The two semiconductor pieces are directly bonded together at high temperature (750° C. for GaAs materials and 400° C. for InP based materials). This approach also has the disadvantage previously discussed above because it uses a semiconductor waveguide.  
           [0009]    A prior-art approach that could be used to bond thin electronically active device elements onto dielectric substrates is described in  IEEE Photonics Technology Letters,  v. 11, pp. 1244-1246 (1999). According to this approach, partially processed active device elements are bonded onto a carrier or transfer substrate by using a layer of organic polymer. Although GaAs was used as the transfer substrate in this prior-art example, other substrates such as quartz also could be used. In this example, the active device element is a guided-wave modulator that is based on semiconductor waveguides. The active device element is flipped top-side down above the transfer substrate. The top-side metal electrodes are fabricated on the active-device piece before it is bonded onto the transfer substrate and thus are located between the active-device epilayers and the transfer substrate. The semiconductor growth substrate for the active device element is then etched away and back-side metal electrodes are fabricated. In contrast to this prior-art approach, the present invention forms both the top-side and back-side metal electrodes after the active device pieces are bonded onto the dielectric waveguide material. This is because the interface between the active device piece and the dielectric-waveguide substrate of the present invention is used to couple light between the active device element and the dielectric waveguide and thus should be controlled carefully. The prior art approach does not involve coupling of light between the active device element and the transfer substrate (the substrate to which the active epilayer is attached temporarily).  
           [0010]    Some of the examples of new devices that can be realized by the approach of the present invention involve microresonators whose properties are adjusted electrically. The closest prior art makes use of modifying a polymer overlay that is deposited above the microresonator to irreversibly trim the resonant wavelength of the microresonator. This prior-art approach is described in  IEEE Photonics Technology Letters,  v. 11, pp. 688-690 (1999). An advantage of the present approach over the prior art is that the electrical adjustment is reversible.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0011]    In one aspect, the present invention provides an optoelectronic device having a substrate; at least one dielectric waveguide in the substrate; and at least one active semiconductor layer physically bonded to the substrate and optically coupled to the at least one dielectric waveguide in the substrate, the at least one active semiconductor layer being able to generate light, detect light, amplify light or otherwise modulate amplitude or phase of light.  
           [0012]    In another aspect, the present invention provides a method of making an optoelectronic device comprising: providing a first substrate; forming at least one dielectric waveguide in the first substrate; providing a second substrate having layers or islands of semiconductor material grown thereon; bonding an upper most layer of the second substrate onto an exposed surface of the first substrate; and etching at least portions of the layers or islands of semiconductor material initially grown on the second substrate to define at least one active device, the at least one active device being physically bonded to the first substrate and optically coupled to the at least one dielectric waveguide in the first substrate. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 provides a view of a basic embodiment of the invention;  
         [0014]    [0014]FIG. 1 a  is a detailed view of three interface layers between an active device and a dielectric waveguide for mounting the active device to the dielectric waveguide;  
         [0015]    [0015]FIG. 2 a  is block diagram of a wavelength selective amplitude encoder that may be used to form an optical-frequency filter with shaped frequency response;  
         [0016]    [0016]FIG. 2 b  is a perspective view of a single microresonator device used in the encoder of FIG. 2 a;    
         [0017]    [0017]FIG. 2 c  is a section view though a split-ring version of a single microresonator;  
         [0018]    [0018]FIG. 2 d  is plan view of the split-ring version of the single microresonator of FIG. 2 c;    
         [0019]    [0019]FIG. 3 a  is a schematic diagram of an all-optical discriminator for an optical frequency-modulation receiver;  
         [0020]    [0020]FIG. 3 b  is a perspective view of an optical amplifier; and  
         [0021]    [0021]FIG. 3 c  is a perspective view of microresonator filters and photodetectors used in the discriminator of FIG. 3 a.   
     
    
     DETAILED DESCRIPTION  
       [0022]    [0022]FIG. 1 is a side elevation view through two devices  10 L and  10 R and their associated waveguides  121 ,  122  and  123 . Two different embodiments of a waveguide-bonded optoelectronic device  10  are depicted. The device on the left hand side ( 10 L) is a detector while the device on the right hand side ( 10 R) is a disk-shaped mircoresonator  102  (see also FIG. 2 b ). Each of the devices  10 R and  10 L of the waveguide bonded optoelectronic devices are associated with at least one dielectric optical waveguide  121 ,  122 ,  123  in a waveguide layer  12  which is optically coupled with one or more electrically active semiconductor elements  14  defining, for example, devices  10 L and  10 R. The waveguide layer  12  is formed in a substrate  5 . Elements  14  can be of different types, and perform different functions, such as generation of light (e.g. lasers), absorption and detection of light (e.g. photodetectors), and modulation of light (e.g. electroabsorption modulators). One or more optional bonding interface layers  16  can be located between the waveguide layer  12  and the semiconductor elements  14 . The functions of these one or more interface layers  16  can include (i) promoting the bonding between the semiconductor elements  14  and the surface  12 - 1  of layer  12  containing the dielectric optical waveguides  121 ,  122 ,  123  and (ii) assisting to optically couple light between the waveguides  121 ,  122 ,  123  and their associated active elements  14 . The layer  12 , containing the dielectric optical waveguides, can be located above a separate backing substrate  18 , which can be comprised of materials such as silicon, alumina or quartz. Electrical contacts  20  to the active semiconductor elements  14  are preferably located on the sides of the elements  14  that face away from the dielectric waveguides  121 ,  122 ,  123 .  
         [0023]    In FIG. 1 only two devices  10  and only three waveguides  121 ,  122 ,  123  are depicted for ease of illustration. In practice, many more devices and waveguides will likely be utilized at a given time in a given embodiment.  
         [0024]    In FIG. 1 device  10 L is preferably centered on its associated waveguide  121  whereas device  10 R straddles two waveguides  122  and  123 . Preferably, the centers C of the waveguides  122  and  123  are equally spaced from a centerline CL of device  10 R if equal coupling between the device  10 R and its two associated waveguides  122  and  123  is desired. In some embodiments, non-uniform coupling may be desirable, in which case the center C of one optical waveguide may approach the centerline CL of device  10 R more closely than does the center of the other optical waveguide.  
         [0025]    The substrate  5  carrying dielectric optical waveguides  121 ,  122 ,  123  and the epitaxial materials for the active semiconductor elements  14  are fabricated separately. Initially, semiconductor epitaxial materials  14 - 1 ,  14 - 2  and  14 - 3  are grown on another semiconductor substrate  15 , comprised of compounds such as GaAs or InP, in order to fabricate the active semiconductor elements  14 . The resulting piece(s) of active semiconductor material is (are) then mounted top-side down onto the top surface  12 - 1  of the dielectric waveguide layer  12  or top-side down onto the top surface  16 - 1  of the optional interface layers  16  (if utilized) and are used to form semiconductor elements  14 . Thus, the upper-most (as grown) active semiconductor layer  14 - 1  is placed in close contact either with the dielectric waveguide layer  12  directly or its associated interface layer  16 . This close contact is desirable because, in use, light will be coupled between the dielectric optical waveguide in layer  12  and the semiconductor layers  14 - 1 ,  14 - 2  and  14 - 3  of an associated semiconductor element  14 .  
         [0026]    Various bonding approaches, to be described later, can be used to bond the semiconductor pieces onto the surface  12 - 1  of waveguide layer  12 , or the optical interface layer surface  16 - 1 . The bonded pieces are shown by the solid and dashed lines defining semiconductor layers  14 - 1 ,  14 - 2  and  14 - 3  and defining substrate  15  in FIG. 1. As will be soon described, after one or more semiconductor pieces are bonded in place, the semiconductor substrate material  15  is removed from those piece(s), leaving the epitaxial material needed for the active semiconductor elements  14 , which are defined or further defined using conventional semiconductor processing techniques to remove the dashed line portions shown in FIG. 1. Thus, in FIG. 1, the bonded pieces are shown by solid and dashed lines, while the semiconductor elements formed therefrom are shown by solid lines. The numeral  14  is used to refer to the semiconductor elements generally and also to the layered semiconductor piece(s) from which the elements are formed.  
         [0027]    An example of epitaxial semiconductor materials which might be used is the epitaxial semiconductor layers  14 - 1 ,  14 - 2 ,  14 - 3  of a PIN diode structure of a laser, photodetector or modulator. The components of epitaxial semiconductor layers  14 - 1 ,  14 - 2 ,  14 - 3  may be identical to those of conventional semiconductor guided-wave devices. The thicknesses of semiconductor layers  14 - 1 ,  14 - 2 ,  14 - 3 , however, generally are different since those layers are optically coupled to a dielectric optical waveguide in layer  12  rather than to a semiconductor waveguide. The design and growth of layers  14 - 1 ,  14 - 2 ,  14 - 3  involve known principles and techniques and therefore those matters are not discussed further here. However, it should be noted that layer  14 - 3  is the P layer, while layer  14 - 2  is the I layer and layer  14 - 1  forms the N layer of the PIN diode structure.  
         [0028]    The dielectric optical waveguide layer  12  is formed in substrate  5 . In addition to the waveguide layer  12 , substrate  5  can include multiple layers  18 ,  19  (the top most layer  19 , which is in contact with waveguide layer  12 , must be a dielectric material) or it can be monolithic (that is, comprise a single layer of dielectric material). In one embodiment, one layer  18  is silicon while layer  19  is preferably SiO 2  having a thickness, for example, of about 30 μm. Waveguides in layer  12  are doped SiO 2 . Layer  12  may have a thin optional layer of undoped SiO 2  immediately adjacent optional interface layer  16  (if utilized). In another embodiment, the optical waveguides in layer  12  are Ti-doped lithium niobate while layers  18  and  19  are preferably formed by a single layer of undoped lithium niobate.  
         [0029]    The one or more semiconductor pieces  14  bonded to the surface  12 - 1  (if no layer  16  is utilized) or to the interface surface  16 - 1  (if one or more layers  16  are utilized) of the dielectric waveguide substrate  5  are then processed to fabricate active device elements. This processing typically involves steps such as photolithographic definition, etching of the semiconductor materials, deposition and patterning of additional dielectric films, deposition of metal films  14 - 4  (for the electrical contacts) and annealing or sintering. Such steps are known in the art and are used commonly in the fabrication of the standard semiconductor guided-wave devices. Some of these processing steps could involve exposure to vacuum pressures and high temperatures (exceeding 400° C.). The bonding technique used to attach the semiconductor pieces  14  to the dielectric waveguide surface must produce a bond that can survive this additional processing.  
         [0030]    Various known bonding methods may be used to bond the semiconductor pieces  14  onto the dielectric waveguide surface  12 - 1  or interface surface  16 - 1  (if optional layer  16  is utilized). As previously mentioned, the dielectric waveguide layer  12  is typically fabricated from a material such as silicon dioxide or lithium niobate (another oxide). Before bonding, the exposed waveguide layer surface  12 - 1  or  16 - 1  may be polished to ensure that it is flat and cleaned to remove any particles or residue. Similarly, the upper layers  14 - 1  of the semiconductor pieces are preferably cleaned. The surfaces of those pieces  14  usually will be flat already. One bonding method is described in  IEEE Photonics Technology Letters,  v. 11, pp. 958-960 (1999). When this method is used to fabricate the devices of this invention, thin films of silicon nitride  22  followed by silicon dioxide  24  are deposited on the upper layer  14 - 1  of the semiconductor pieces (which, in this embodiment, are preferably formed by GaAs or GaAs compatible materials) before bonding. These layers of silicon nitride  22  and silicon dioxide  24  are not shown in FIG. 1, but can be seen in the more exploded view of FIG. 1 a.  A thin film  16  of borophosphosilicate glass (BPSG) is preferably deposited on the exposed optical waveguide surface  12 - 1  of the dielectric waveguide layer  12 . The surfaces  16 - 1  and  24 - 1  of the BPSG  16  and the silicon dioxide  24 , respectively, are cleaned and then brought into contact under pressure at room temperature so that a hydrophilic bond is formed between them. FIG. 1 a  is a detailed view showing the two films  22  and  24  formed on layer  14 - 1  and also showing the mating of surface  24 - 1  with surface  16 - 1 . The substrate, including the mounted pieces  14 , is then annealed to a sufficiently high temperature that the bond is strengthened. Temperatures of 150 to 250° C. may used for this anneal. The films  22  and  24  of, for example, deposited silicon nitride and silicon dioxide preferably have thicknesses on the order of tens of nm, and hundreds of nm, respectively. The film  16  of BPSG preferably has a thickness on the order of hundreds of nm. The precise thicknesses chosen would depend on the specific fabrication steps used for the active semiconductor elements  14  and the specific design of the optical waveguides in layer  12  and the active semiconductor elements  14 , since these interface layers  16 ,  22  and  24  affect the coupling of light between the optical waveguides in layer  12  and their associated active semiconductor elements  14 . The thicknesses of the layers of BPSG, SiO 2  and SiN, in this particular embodiment, are adjusted so that their light refracting indexes are used to bend light from the optical waveguides  121 ,  122 ,  123  into the semiconductor devices  14 .  
         [0031]    Another bonding technique makes use of a film of borosilicate glass (BSG) to achieve the bonding. This approach is described in  J. Crystal Growth,  v. 195, pp. 144-150 (1998). When this approach is used, the BSG film is preferably deposited on both the separately grown semiconductor pieces  14  and the dielectric optical waveguide layer  12 . The bonding temperature is about 550° C., with mechanical pressure being applied to hold the semiconductor piece(s)  14  against the BSG layer  16  deposited on the dielectric optical waveguide layer  12 . Despite its higher bonding temperature, this method still can be used for the present invention since that temperature still is substantially below the growth temperature for the epitaxial layers of the semiconductor piece(s)  14 . Therefore, those epitaxial materials will not be degraded by the bonding process.  
         [0032]    Still another bonding technique appropriate for the present invention makes use of spin-on glass (SOG) as the interface material. This approach is described in  Electronics Letters,  v. 36, pp. 677-678 (2000). The SOG film may be deposited on either surface, but it may be more convenient to deposit that film on the optical waveguide layer  12 . Typical SOG film thicknesses are in the hundreds of nm. The bonding is done at room temperature and the substrate supporting the piece(s)  16  is annealed at temperatures ranging from 200-225° C.  
         [0033]    Two examples of waveguide-bonded optoelectronic devices utilizing the present invention will now be discussed. The first device is a wavelength selective amplitude encoder. Such an encoder may be used to form an optical-frequency filter with a shaped frequency response. A block diagram of this shaped filter is shown in FIG. 2 a.  This device is also described, for use in a specific application, in U.S. Provisional Patent Application Serial No. 60/332,370 filed Nov. 15, 2001 and its and its corresponding non-provisional application bearing Ser. No.______filed on the same date as the present application. The shaped filter consists of an optical waveguide trunk  100  that is coupled to a sequence of microresonator elements  102   1 ,  102   2  . . . (the subscripts are used to identify particular ones of the elements  102  in the sequence). Each microresonator element  102   1 ,  102   2 , . . . is coupled to an associated outlet waveguide segment  106   1 ,  106   2 , . . . . Each microresonator element  102   1 ,  102   2 , . . . couples light of a selected range of frequencies from the waveguide trunk  100  into its associated outlet waveguide  106   1 ,  106   2 , . . . . The frequency band and the amount of light coupled out can be adjusted electrically. The electrical adjustment is accomplished by applying control voltages to the microresonator elements  102 , which functions as a waveguide-coupled electroabsorption modulator, via its control line  104   1 ,  104   2 , . . . . The frequency band is controlled by controlling the length of time it takes light to travel the circumference of the microresonator, which is done by applying a voltage across the contacts  20  depicted in FIG. 1, so each control line  104  could be implemented by a pair of wires coupled to the contacts  20  of each microresonator  102 .  
         [0034]    A single microresonator element  102  is illustrated in FIG. 2 b  and it can be considered a resonator-enhanced optical modulator that has complementary outputs. The size of the microresonator  102  and the refractive index of its resonant optical mode determine the resonant frequency of the microresonator element  102 , which is the frequency of the light that is coupled out by that element  102 . Typically a number of microresonator elements  102  are used cooperatively (as shown by FIG. 2 a ) and would be disposed on a single substrate  5  having optical waveguides therein (as such optical waveguides  100  and  106  shown in FIG. 2 b  or optical waveguides  121 ,  122  or  123  shown in FIG. 1).  
         [0035]    The encoder&#39;s optical waveguide trunk  100  is coupled optically to multiple circular microresonators  102  (only one circular micro-resonator  102  is shown in FIG. 2 b,  it being understood that typically a number of circular microresonators  102  would preferably be formed on substrate  5  and coupled as shown by FIG. 2 a ). Each micro-resonator  102  has a slightly different diameter. One or more electrical control lines  104  are supplied to each of the microresonators  102 . The control signals on these lines  104  adjust the optical refractive index and/or the optical absorption of the associated micro-resonator  102 . Optional outlet waveguide segments  106  can be optically coupled to each of the microresonators  102 . Light incident of the optical waveguide trunk  100  is in the form of multiple RF tones (f 1 , f 2 , f 3 , f 4 , . . . ) that are amplitude modulated onto a single-wavelength lightwave carrier. Each tone has a specific lightwave frequency and generally both upper and lower amplitude-modulation sidebands would be represented in the comb. Light exiting the optical waveguide trunk is comprised of the same tones but the amplitudes of those tones have been adjusted by different weights (a, b, c, d, . . . ). These weighing factors a, b, c, d, . . . are all less than or equal to unity unless the resonator has gain, in which case the weighing factors a, b, c, d, . . . are less than, equal to or greater than unity. Gain is obtained by making layer  14 - 2  (see FIG. 1) generate light. For example, GaInAs/InAlAs multiple quantum wells or GaInAsP can be used for generating light at wavelengths near 1550 nm.  
         [0036]    The microresonator element  102  preferably has a PIN structure. In one example, layer  14 - 3  is the P layer, while layers  14 - 2  and  14 - 1  form the I and N layers, respectively, of the PIN device. Layer  14 - 3  is doped p-type and a p contact is made above it by depositing an appropriate metal contact metalization layer  14 - 4 . Layer  14 - 2  is comprised of lightly doped, undoped or intrinsic material. Layer  14 - 1  of the semiconductor material at the bottom of disk  102  is doped n-type and the N-contact is made to an exposed portion of layer  14 - 1  by depositing an appropriate metalization layer  14 - 4 . The microresonator  102  formed by layers  14 - 1  through  14 - 4  corresponds to device  10 R of FIGS. 1, 2 c  and  2   d.  The refractive index of the optical mode can be changed by applying a voltage to the PIN structure of the electroabsorption modulator element as a result of the electro-refraction properties of the semiconductor material. The amount of light coupled out from the waveguide trunk  100  can be changed by adjusting the optical absorption of the microresonator element  102 . The use of absorption in a resonator to control the optical coupling between the resonator and a waveguide is discussed in  IEEE Photonics Technology Letters,  v. 10, pp. 816-818 (1998). As the absorption is increased, more of the light bypasses the microresonators  102  and remains in the waveguide trunk  100 .  
         [0037]    The disk-shaped microresonator device  10 R of FIGS. 1, 2 c  and  2   d  is preferably annular shaped in that at least the P layer ( 14 - 3 ) and its contact  20  has an annular opening  14 - 5  therein. The annular opening  14 - 5  preferably stops at layer  14 - 1 , but may optionally penetrate to layer  16  or stop at intervening layer  14 - 2 . FIGS. 2 c  and  2   d  show a modified structure where the ring PIN structure is split into two associated devices by notches  14 - 6 . As such, layer  14 - 3  is divided into two portions which are aligned below their associated metal contact layer portions  20 - 2  and  20 - 3 . The notches  14 - 6  penetrate the P layer  14 - 3 . This notched embodiment can be conveniently used with the embodiment of FIG. 2 b  with the bias applied to contact layer portions  20 - 2  and  20 - 1  controlling the operating frequency and the bias applied to contact layer portions  20 - 3  and  20 - 1  by control lines  104  (see FIG. 2 a ) controlling the amplitude of the optical signal in waveguide  100  which is dumped into outlet waveguide segment  106 . In FIG. 2 d  the relatively smaller semiconductor device (as noted by its relatively smaller metal contact  20 - 3 ) is arranged toward outlet waveguide segment  106 .  
         [0038]    In FIGS. 2 a  and  2   b  the optical waveguide truck is identified by numeral  100  while the segments into which light is dumped are identified by the numerals  106 . In FIGS. 1 and 2 d  the waveguides are identified by the numerals  122  and  123 . When the embodiments of the microresonators  10 R of FIGS. 1, 2 c  and  2   d  are used in the application of FIGS. 2 a  and  2   b,  then waveguide  122  is equivalent to waveguide  100  while waveguide  123  is equivalent to waveguide  106 .  
         [0039]    Another example of a waveguide bonded optoelectronic device is an all-optical discriminator  200  for an optical frequency-modulation receiver. This discriminator  200  is illustrated in FIG. 3 a.  The basic idea for this discriminator is described in PCT Application Number PCT/U.S.00/23935 published as WO/______on______. The input frequency-modulated (FM) light is first amplified by a semiconductor optical amplifier (SOA)  202  and then is divided into two paths by an optical-waveguide splitter  204 . Light in each path is passed through a separate frequency (or wavelength) filter  206 - 1 ,  206 - 2  whose center frequency is shifted by a pre-set amount to a higher (see the small bandpass graph for filter  206 - 1 ) or lower optical frequency from the center frequency of the input FM signal. The filtered light is then sensed by two photodetectors  208 - 1  and  208 - 2  that are connected electrically in a differential configuration. The SOA  202 , the filters  206  and the photodetectors  208  are active semiconductor elements. The waveguide network consists of an input section  201 , the splitter  204 , and the interconnect segments  205 - 1  and  205 - 2  between the splitter  204  and the filters  206 - 1  and  206 - 2  as well as the interconnect segments  207 - 1  and  207 - 2  between the respective filters  206 - 1  and  206 - 2  and the respective photodetectors  208 - 1  and  208 - 2 . The metalization contacts  20  as shown in FIGS. 1, 2 c  and  2   d  would be utilized, but are not shown in this view for ease of illustration. The filters  206  can be implemented by the microdisk resonators previously discussed with reference to FIGS.  1  (device  10 R),  2   a,    2   b,    2   c  and  2   d  and made using the manufacturing techniques discussed herein. If a resonator of type shown in FIGS. 2 c  and  2   d  is used for each filter  206 - 1  and  206 - 2 , then, as shown by FIG. 3 c,  filter input waveguides  205 - 1  and  205 - 2  would be formed by the waveguide  122  of each filter and filter output waveguides  207 - 1  and  207 - 2  would be formed by the waveguide  123  of each filter. The SOAs  202  can also be implemented and made using the devices and manufacturing techniques discussed herein (see device  10 L in FIG. 1, for example, and the device discussed with reference to FIG. 3 b  below).  
         [0040]    [0040]FIG. 3 b  depicts a semiconductor optical amplifier (SOA) which can be easily made in accordance with the techniques described above. It is yet another example of a waveguide bonded optoelectronic device. The amplifier sits atop, and is preferably longitudinally centered on, waveguide  121 . The main body portion  220  has P, I and N layers  14 - 3 ,  14 - 2  and  14 - 1 , respectively, as also has the devices  10 L and  10 R of FIG. 1. The N layer  14 - 1  is preferably formed as a wide slab  220  with layers  14 - 2  and  14 - 3  forming mesas thereon. These layers may be conveniently made using the techniques discussed above with reference to FIGS. 1 and 1 a.  The SOA of this embodiment also preferably has tapered portions  222  which comprise a portion of the N layer (and may also include a portion of the I layer as well—as is depicted in FIG. 3 b —and may further include a portion of the P layer—which is not shown in the figure). The tapered portions  222  have a longitudinal axis  224  which preferably aligns with the longitudinal centerline of the main body portion  220  and with the centerline C of waveguide  121  (see also FIG. 1). The N layer of the slab and the N layer of the tapered portions  222  are preferably defined in a single etching process.  
         [0041]    An implementation of the filters  206  and photodetectors  208  of FIG. 3 a  is shown in FIG. 3 c.  The filters  206  are implemented as microresonator-coupled waveguides  205 - 1  and  205 - 2  that can be tuned electrically. This permits the discriminator to be adjusted actively to track any changes to the center frequency of the FM signal. Also, the adjustment of the microresonators permits corrections to be made to any fabrication-related offsets in the filter performance. Again, each microresonator element  206  is an optical modulator having a structure like the structures described with reference to FIGS.  1  (device  10 R) and  2   b - 2   d.  However, in the present case, layer  14 - 2  is not made of a material that is electro-absorptive at the optical wavelength, but rather is electro-refractive or electro-optic. Thus, the refractive index of layer  14 - 2  changes as the voltage applied to that layer changes. The photodetectors  208  consist of semiconductor PIN diode structures  14  that are disposed above a waveguide in optical waveguide layer  12 . Light is coupled out of the resonators  206  through waveguides  207 - 1  and  207 - 2  and absorbed in the photodetectors  208 - 1  and  208 - 2  (generally in the I layer of the diode). Known techniques can be used to design such a waveguide-coupled photodetector. Each photodetector element  208  looks like a mesa on a wide slab  210  of N type material. The P and I layers as well as the top contact comprise the mesa. The N layer and the bottom contact comprise the slab. There may be an additional “index-matching” layer located underneath the N layer of the diode to facilitate coupling of light from the dielectric optical waveguide into the photodetector layers.  
         [0042]    A detailed drawing of the SOA  202  element is given in FIG. 3 b.  The SOA element  202  looks very much like the PIN photodetector elements  208  and has a PIN diode configuration. However, the SOA  202  element is comprised of different epitaxial layers of different layer thicknesses than those of the photodetector element  208 . These layers are common to the ones found in conventional SOA devices. Since light is coupled both from the dielectric waveguide into the SOA and out of the SOA into the dielectric waveguide, it is beneficial to form waveguide transition regions as an etched taper  222  in the SOA layers. Such tapered structures are used for conventional SOAs that contain semiconductor waveguides. The design of the SOA element is based on known techniques. This SOA and indeed all of the waveguide bonded optoelectronic devices disclosed herein are different from the prior art in that an active device preferably based on a PIN structure cooperates with and receives light from, sends light to, or exchanges light with a dielectric optical waveguide.  
         [0043]    The examples described above should provide an indication of the utility and versatility of the present invention. Obviously, many more devices can be developed that involve combinations of waveguides, waveguide networks, waveguide splitters, couplers and interferometers with active device elements such as modulators, photodetectors, optical amplifiers and lasers, all of which all can benefit from the ability to use semiconductor active regions which cooperate with one or more dielectric waveguides. As such, the invention as set forth in the appended claims is not to be limited to the particular embodiments disclosed, except as specifically required by the appended claims.

Technology Classification (CPC): 6