Abstract:
Waveguide designs and fabrication methods provide adiabatic waveguide eigen mode conversion and can be applied to monolithic vertical integration of active and passive elements in PICs. An advantage of the designs and methods is a simple fabrication procedure with only a single etching step in combination with subsequent well-controllable selective oxidation. As a result, improved manufacturability and reliability can be achieved.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims one or more inventions which were disclosed in Provisional Application No. 61/507,233, filed Jul. 13, 2011, entitled “Adiabatic Mode-Profile Conversion by Selective Oxidation For Photonic Integrated Circuit”. The benefit under 35 USC §119(e) of the United States provisional application is hereby claimed, and the aforementioned application is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to the field of integrated optics, and more particularly to methods of monolithic integration of active devices with passive components. 
         [0004]    2. Description of Related Art 
         [0005]    Presently many optoelectronic systems are assembled from separate components that are individually packaged into fiber modules. These components include, but are not limited to, LEDs, lasers, amplifiers, modulators, detectors, power splitters, switchers, filters, and multiplexers. However, the cost of the components is high mainly because of the package itself, where coupling optics, temperature stabilization, and precise adjustment are all required. Moreover, systems based on the discrete components are power consumable and it is difficult to make them compact in size. Joining the components into a single-package configuration, also known as a photonic integration circuit (PIC), eliminates these disadvantages. 
         [0006]    Photonic integration circuits can be based either on hybrid or on monolithic integration. Hybrid photonic integrated circuits bring together optical devices based on different material systems, for example, an III-V evanescent laser bonded on Si (A. W. Fang et al., “Electrically pumped hybrid AlGaInAs-silicon evanescent laser”, Optical Express, vol. 14, 9203-9210, 2006). An advantage of hybrid integration is that each component is optimized for one specific function, enabling deployment of state of the art components. However, there are also disadvantages including, but not limited to, an inefficient light coupling between the components, different lattice and thermal expansion constants, and diffusion of impurities between the components. 
         [0007]    On the other hand, monolithic integration joins the devices based on the same material system, avoids aligning and bonding problems, and provides exceptional thermal and mechanical characteristics (see for example U.S. Pat. No. 7,282,311 by Little, issued Oct. 16, 2006). Taking into account these benefits, monolithic integration can be preferable for certain applications with modest integration levels. 
         [0008]    Low-loss optical waveguides are normally needed in PICs for interconnection and also for some passive components, e.g. spectral and spatial filters, splitters, delay lines, and chromatic dispersion compensators. There are a few approaches for monolithic integration of the passive waveguides including different regrowth technologies, quantum well intermixing, and vertical twin-waveguide structure growth. 
         [0009]    The most straightforward passive waveguide integration technique is epitaxial growth of a second waveguide with the desired properties after the removal of the original waveguide, also known as the butt jointregrowth method (see U.S. Pat. No. 4,820,655 by Noda, issued Apr. 11, 1989). The main advantage of this integration scheme is a high degree of flexibility in the design, for example, compositions, thicknesses, and doping concentrations. However, the epitaxial crystal growth at the abutting locations creates the problem of layer misalignment and imperfect interfaces (quality and shape) between the active and passive components, which results in scattering loss, parasitic optical feedback, and low coupling efficiency. Another regrowth approach, selective area growth, uses a dielectric mask to inhibit epi-layer growth during metal organic vapor phase epitaxy (MOVPE) or metal organic chemical vapor deposition (MOCVD) and, as a result, to tailor the waveguide properties along its length (see U.S. Pat. No. 5,543,353 by Suzuki, issued Aug. 6, 1996 and U.S. Pat. No. 7,060,615 by Glew, issued Jun. 13, 2006). However, the waveguide properties cannot be strongly changed on a short distance resulting in additional absorption losses and chirp in the region of the band edge transition. Moreover, a very precise control of growth parameters is necessary. 
         [0010]    Another passive waveguide integration method is based on disordering of quantum wells, also known as quantum well intermixing (QWI), to locally change band-edges (see U.S. Pat. No. 6,989,286 by Hamilton, issued Jan. 24, 2004). Since the QWI process only slightly modifies the composition profile and does not change the average composition, there is a negligible refractive index discontinuity at the interface between adjacent sections. Different modifications of the QWI technique, such as impurity free vacancy disordering (IFVD), impurity induced disordering (IID) and laser-induced disordering (LID), suffer from their specific drawbacks, including free-carrier absorption, parasitic conductivity, residual damage from implantation, inferior quality of recrystallized material after laser melting, and degradation of the top surface caused by high-temperature annealing. Taking into account complexity, irreproducibility, and the poor area selectivity of the intermixing process, QWI technology is not a practical method for monolithic integration of multi-functional optoelectronic devices in PIC (J. H. Marsh, “Quantum well intermixing”, Semiconductor Science Technology, vol. 8, pp. 1136-1155, 1993). 
         [0011]    Vertical twin waveguide structure represents a promising integration platform technology. This integration technique can be realized by using either the waveguide modes beating concept or an adiabatic mode transformation concept. In the first case, the power transfer results from the bimodal interference between two supermodes of the vertical twin-waveguide (TG) structure (Y. Suematsu et al., “Integrated twin-guide AlGaAs laser with multiheterostructure”, IEEE Journal of Quantum Electronics, vol. 11, pp. 457-460, 1975; see also U.S. Pat. No. 5,859,866 by Forrest, issued Jan. 12, 1999). Despite the fact that active and passive functions are separated into different vertically displaced waveguides, all integrated components cannot be well optimized separately due to a requirement of resonant coupling of both waveguides. Moreover, performance characteristics of the devices based on the TG structures are not stable due to mode interaction and fluctuation in the device structure itself (layer thickness, composition, dry etching profiles). On the contrary, the adiabatic mode transformation concept, based on an asymmetric twin-waveguide (ATG) with tapered couplers, is unaffected by modal interference (see U.S. Pat. No. 6,282,345 by Schimpe, issued Aug. 28, 2001). The waveguide is designed in such a way that only one mode exists. To reduce coupling losses during the power transfer process, the lateral tapering of the active waveguide at a junction of the active-passive waveguides is used (see U.S. Pat. No. 5,078,516 by Kapon, issued Jan. 7, 1992). As the active waveguide rib is narrowed, the mode profile is smoothly transformed without any loss of power and, finally, the mode is adiabatically pushed down into the passive waveguide. This allows the independent optimization of the active/passive devices in a single epitaxial growth step. However, there are strict requirements for the etching process (at least two steps), and for the precision of sub-micron lithography with a complicated alignment procedure. In addition, ridge waveguides are rather long, and precise control of taper tips is required. 
         [0012]    Each of the above-mentioned coupling techniques suffers from one or more of the following major drawbacks: high optical/coupling losses, poor manufacturability, high cost, insufficient reproducibility, and inadequate reliability. Therefore, there is a need in the art for a novel economical and manufacturable active-passive coupling technique that permits further progress in photonic-network communication technology. 
       SUMMARY OF THE INVENTION 
       [0013]    Waveguide designs and fabrication methods for adiabatic conversion of waveguide eigen mode provide adiabatic mode-profile conversion in vertical monolithic integration of active devices with passive elements into a single photonic integrated circuit. An advantage of embodiments of the present invention is a simple fabrication procedure which includes single-step etching in combination with subsequent well-controllable selective oxidation. As a result, improved manufacturability and reliability can be achieved. 
         [0014]    A tapered single-step ridge waveguide, which includes a multilayer transverse epitaxial structure grown on a substrate, provides an adiabatic mode-profile conversion by a lateral oxidation of Al-rich layers. 
         [0015]    The transverse layered structure of the waveguide includes an active region with a plurality of quantum well, quantum dots in a well (DWELL) and/or quantum dot layers for creating an optical gain under current injection, a passive region optimized for low-loss wave propagation, and a mode-control region, including at least one Al-rich layer. A refractive index of the mode-control region can be changed by oxidation, which enables control of an overlapping of an eigen mode with the active region and the passive region, and provides anti-degradation protection of other Al-rich layers. The transverse layered structure also includes at least one cladding region having refractive indexes less than a refractive index of the active region and a refractive index of the passive region. In some preferred embodiments, the refractive index of the active region is higher than the refractive index of the passive region. A material and a thickness of the active region and the passive region are designed to provide mode localization either in the active region or in the passive region. 
         [0016]    In the longitudinal direction, the single-step ridge waveguide includes a narrow section, a wide section, and a laterally tapered section that connects the narrow section and the wide section. The narrow section has a width that is sufficiently small such that oxidation of the mode-control region results in the confinement of the eigen mode in the passive region inside the narrow section. The wide section has a width sufficiently large such that an effective refractive index of the wide section is negligibly influenced by oxidation of the mode-control region and therefore the eigen mode is confined in the active region inside the wide section. A change of a width of the lateral taper section provides gradual optical mode power transfer between the active region and the passive region, and the power losses during the mode transfer can be controlled by geometrical parameters of the lateral taper section. 
         [0017]    A method of fabricating a tapered ridge waveguide includes the step of epitaxially growing a layered structure on a substrate. The layered structure includes an active region including a plurality of quantum well, quantum dot layers, and/or quantum dots in a well (DWELL) for creating an optical gain under current injection, a passive region optimized for low-loss wave propagation, a mode-control region including at least one Al-rich layer, where an Al composition of this layer is sufficiently high to be transformed to (AlGa) x O y  by oxidation, and at least one cladding region, the cladding region having refractive indexes less than a refractive index of the active region and a refractive index of the passive region. Each region represents at least one layer with a certain thickness and composition. The composition and thickness of the layers composing the active, the passive, and the mode control regions are designed to provide mode localization either in the active region, if the mode control region is not oxidized, or in the passive region, if the mode control region is oxidized. 
         [0018]    The method also includes the step of forming a ridge waveguide in a single etching step. The ridge waveguide includes a narrow section, a wide section, and a laterally tapered section along the ridge. The laterally tapered section connects the narrow section and the wide section. 
         [0019]    The method also includes the step of selectively oxidating the mode control region, where an oxidation time and an oxidation temperature are selected such that an oxidation depth is large enough to sufficiently change an effective refractive index of the mode control region in the narrow section in order to provide mode localization in the passive region inside the narrow section. The thickness of the wide section is sufficiently high so that oxidation of the mode-control region in the wide section weakly influences effective refractive index of the wide section and the eigen mode is confined in the active region inside the wide section. A change in the width of the laterally tapered section provides gradual optical mode power transfer between the active region and the passive region, and the power losses during the mode transfer can be controlled by the geometrical parameters of the laterally tapered section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1   a  is a schematic view of a transverse structure of a waveguide of the invention, where the mode-control region is out of the active region. 
           [0021]      FIG. 1   b  illustrates the refractive indexes and mode profiles of the waveguide shown in  FIG. 1   a  before oxidation of the mode-control region. 
           [0022]      FIG. 1   c  illustrates the refractive indexes and mode profiles of the waveguide shown in  FIG. 1   a  after oxidation of the mode-control region. 
           [0023]      FIG. 2   a  is a perspective view of a first preferred embodiment of the present invention.  FIG. 2   b  is a longitudinal-section of  FIG. 2   a  along the mode-control region. 
           [0024]      FIG. 3   a  is a vertical cross-section scanning electron microscopy image along line 3 a  of  FIG. 2   a.    
           [0025]      FIG. 3   b  shows the optical field distribution of the cross-section shown in  FIG. 3   a.    
           [0026]      FIG. 3   c  is a vertical cross-section scanning electron microscopy image along line 3 c  of  FIG. 2   a.    
           [0027]      FIG. 3   d  shows the optical field distribution of the cross-section shown in  FIG. 3   c.    
           [0028]      FIG. 3   e  is a vertical cross-section scanning electron microscopy image along line  3   e  of  FIG. 2   a.    
           [0029]      FIG. 3   f  shows the optical field distribution of the cross-section shown in  FIG. 3   e.    
           [0030]      FIG. 4   a  is a plan view of the narrow section of a straight ridge waveguide. 
           [0031]      FIG. 4   b  shows the optical field distribution of the cross-section of the narrow section shown in  FIG. 4   a.    
           [0032]      FIG. 4   c  is a plan view of the narrow section of a curved ridge waveguide. 
           [0033]      FIG. 4   d  shows the optical field distribution of the cross-section of the narrow section shown in  FIG. 4   c.    
           [0034]      FIG. 5   a  shows the bending losses in dB for a 90° bend as a function of curvature radius R b  for a curved ridge waveguide according to  FIG. 4   c  for the different etching depth D e  at TE-polarization of optical mode. 
           [0035]      FIG. 5   b  shows the bending losses in dB for a 90° bend as a function of curvature radius R b  for a curved ridge waveguide according to  FIG. 4   c  for the TE and TM polarization of optical mode at fixed etching depth of D e =3.1 μm. 
           [0036]      FIG. 6  is a perspective view of a second embodiment of the present invention. 
           [0037]      FIG. 7  is a perspective view of a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    In order to overcome the drawbacks of the prior art, the waveguide design and fabrication method are based on the selective oxidation technology for adiabatic mode-profile conversion in the vertical monolithic integration of active devices with passive elements into a single photonic integrated circuit with improved manufacturability and reliability. Some optical devices for which the methods and devices of the present invention could be used include, but are not limited to, lasers, photodetectors, modulators, light emitting diodes, amplifiers, detectors, power splitters, switchers, filters, multiplexers, array waveguide gratings, and passive waveguides. 
         [0039]    “Adiabatic”, as defined herein, means gradually or smoothly, and with negligible power losses/scattering/interference. “Adiabatic mode transformation”, as defined herein, means gradual mode transformation with negligible power losses. A “single-step ridge” and “single-step ridge waveguide”, as defined herein, are ridge structures fabricated in a single etching step/process. This process forms a structure with upper surfaces at two heights (a ridge with a single “step”). “Al-rich” layers, as defined herein, are aluminum containing layers with a high aluminum composition (“rich in Al”). In preferred embodiments, “Al-rich” layers are layers with an Al composition that is sufficiently high to be transformed to (AlGa) x O y  by oxidation. In preferred embodiments, the Al composition in Al-rich layers is greater than or equal to 80%. 
         [0040]    The design and method address the problem of vertical monolithic integration of active devices with passive elements. The devices fulfill three general criteria and provide an effective active-passive coupling technique promising for use in monolithically integrated devices such as PIC:
       1) control of the refractive index profile of the waveguide and, thus, an overlapping of the eigen mode with the active and passive region by lateral oxidation of Al-rich layers;   2) mode localization either in the active region, or in the passive region depending on the status of the mode-control region (e.g. oxidized or non-oxidized mode-control region); and   3) adiabatic reversible transfer of the eigen mode between the active region and the passive region.       
 
         [0044]    A waveguide structure is depicted schematically in  FIG. 1 . The waveguide structure  10 , shown in a vertical cross-section in  FIG. 1   a,  includes an active region  5 , a passive region  3 , a mode-control region  4  and surrounding cladding regions  2  and  6 . The mode-control region  4  surrounds the active region  5 . In one preferred embodiment, one or more Al-rich layers act as the mode-control region  4 . High Al composition is preferable for oxidation (the higher the Al composition the easier (quicker) the oxidation process). In preferred embodiments, the Al composition of the Al-rich layers of the mode-control region  4  is between approximately 80% and 100%. While the mode-control region  4  in this figure surrounds the active region  5 , in other embodiments the mode-control region  4  may be inserted into the active region (see, eg.,  FIG. 7 ). In other embodiments, the mode-control region  4  is located only above or below the active region. 
         [0045]      FIG. 1   b  and  FIG. 1   c  show the refractive indexes  8  and mode profiles  9  of  FIG. 1   a  in this preferred embodiment, before and after oxidation of the Al-rich layer in the mode-control region  4 , respectively. In the case of a non-oxidized mode-control region  4 , the optical mode  9  has an effective refractive index  7  higher than the effective refractive index  8  for the passive region  3  and lower than the effective refractive index  8  for the active region  5 . The optical mode  9  is thus preferably localized in the active region  5  as illustrated in  FIG. 1   b.  However, the process of wet oxidation transforms the Al-rich layer into an aluminum oxide (e.g.—(AlGa) x O y ) and, hence, causes reduction of the effective refractive index  7  of the mode-control region  4 . As a result, the optical mode  9  has an effective refractive index  7  higher than the effective refractive index for the cladding regions  2  and  6  and lower than the effective refractive index for the passive region  3 . It is consequently mainly localized in the passive region  3  as shown in  FIG. 1   c.  To achieve such functionality, the refractive index of the active region  5  should be higher than the refractive index of the passive region  3 , and the thickness of the passive region  3  should be larger than the thickness of the active region  5 . The waveguide  10  can be designed to confine only one eigen mode (fundamental mode) in the active region  5  before lateral oxidation of the mode-control region  4 . After the oxidation of the mode-control region  4  the fundamental mode will be confined in the passive region  3 . 
         [0046]    A preferred embodiment of the present invention is illustrated in a perspective view in  FIG. 2   a.  As shown in  FIG. 2   a,  a tapered ridge waveguide  100  grown on a substrate  110  includes a bottom cladding region  120 , a passive region  130 , a mode-control region  140 , an active region  150 , a top cladding region  160  and a cap layer  170 . The active region  150 , which is grown above the passive region  130 , can include a plurality of quantum well, quantum dots in a well (DWELL), and/or quantum dot layers for creating a laser, a photodetector, a modulator, etc., while the passive region  130  is optimized for low-loss wave propagation. In accordance with  FIG. 1   a,  the cladding regions  120  and  160  have refractive indexes less than the refractive index of the active region  150  and the passive region  130 . The mode-control region  140  includes two layers, which are preferably Al-rich layers with an Al composition sufficiently high for oxidation. One of the layers  142  is placed below the active region  150  and the other layer  141  is placed above the active region  150 . The tapered ridge waveguide  100  is designed as a single-mode and single-step ridge waveguide including a wide section  101  and a narrow section  103  as well as a laterally tapered section  102  between them. The ridge is etched through the active region  150  at least down to the bottom cladding layer  120  to minimize electric device capacity and minimize optical losses at bending of the waveguide, which is critical for many devices including, but not limited to, array waveguide gratings and ring channel filters. To simplify the fabrication process for the device (one etching step and one oxidation process), the Al-rich structures of the mode-control region  140  are preferably uniformly oxidized at a certain length L ox  (oxidation depth), which results in formation of a Y-branch like oxidation profile as shown in  FIG. 2   b.  During the oxidation process, Al-rich layers  141  and  142  partly transform into aluminum oxide layers. The oxidation starts at the perimeter and then goes deeper into the structure. The longer the oxidation time, the greater the oxidation depth L ox . Section  103  is narrow and layers  141  and  142  are fully oxidized inside this section. Section  101  is wider and layers  141  and  142  are only partly oxidized inside this section (namely to the depth L ox  from both sides, marked in black). In the middle part of section  101 , layers  141  and  142  are still not oxidized (marked in white). Note that  FIG. 2   b  is a section of  FIG. 2   a  made parallel to layer  141  or layer  142 . The effective refractive index of the tapered ridge waveguide is controlled by proper selection of the waveguide widths D w  (the largest width of wide section  101 ) and D n  (the width of the narrow section  103 ) and the oxidation depth L ox  to provide localization of the eigen mode  90  (see  FIGS. 3 and 4 ) in the passive region  130  for the narrow section  103  (referred to as the oxidized mode-control region) and to provide localization of the eigen mode  90  in the active region  150  for the wide section  101  (referred to as the non-oxidized mode-control region). In other words, the width of the narrow section is preferably sufficiently small such that oxidation of the mode-control region results in the confinement of the eigen mode in the passive region inside the narrow section and a width of the wide section is sufficiently large such that an effective refractive index of the wide section is negligibly influenced by oxidation of the mode-control region and therefore the eigen mode is confined in the active region inside the wide section. The width of the wide section should not be too large in order to avoid very deep oxidation and large electrical capacitance. The width of the narrow section should not be too small in order to avoid a considerable overlap of the optical mode with the active region and bottom cladding layer, which would cause additional internal and bending loss. In preferred embodiments, the width of the wide section ranges from approximately 0.5 μm to approximately 5 μm and the width of the narrow section ranges from approximately 0.3 μm to approximately 3 μm. Mode  9  shown in  FIG. 1  is the same mode (in a one-dimensional profile) as mode  90  shown in  FIG. 3  (in a two-dimensional profile). 
         [0047]    Vertical cross-sections of the tapered ridge waveguide  100  of  FIG. 2  are shown in  FIGS. 3   a,    3   c,  and  3   e  taken along section surfaces  3   a,    3   c,  and  3   e,  respectively, as indicated in  FIG. 2   a.  Corresponding optical field distributions are shown in  FIGS. 3   b,    3   d,  and  3   f,  respectively. Referring to  FIG. 3   b,  the effective refractive index of the tapered ridge waveguide  100  in the wide section  101  is negligibly influenced by a finite oxidation depth L ox . Therefore, the eigen mode  90  has an effective refractive index higher than an effective refractive index for the passive region  130  and lower than an effective refractive index for the active region  150 , and, thus, the optical mode  90  is propagating primarily in the active region  150 . 
         [0048]    When the optical mode  90  starts propagating through the laterally tapered section  102 , the width of the tapered ridge waveguide  100  is continuously reduced from D w  to D n , hence the effective refractive index of this waveguide is monotonically decreased and the mode profile is smoothly transformed. As a result, the optical power of the eigen mode is gradually transferred from the active region  150  into the passive region  130  as shown in  FIG. 3   d.  When this reduction in the width of the waveguide of  FIG. 2  takes place over a sufficiently long distance L t , then the optical mode  90  is adiabatic with negligible power losses displaced into the passive region  130  (downward in  FIG. 3 ). The longer the distance L t , the smaller the losses. According to calculations for L t =80 μm, the losses are less than 8%, and for L t =160 μm, the losses are less than 1%. In a preferred embodiment of the tapered ridge waveguide  100 , a length of L t  for the laterally tapered section  102  is more than 350 μm to keep transformation losses below 0.01%. Finally, according to  FIG. 3   f,  the complete oxidized mode-control region noticeably reduces the effective refractive index of the waveguide of  FIG. 2  in the narrow section  103 . As a result, the effective refractive index of the optical mode  90  becomes higher than the refractive indexes of the cladding regions  120  and  160  and lower than the refractive index of the passive region  130  and, as a result, the optical mode  90  mainly propagates in the passive region  130 . 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Layer structure of a tapered ridge waveguide with refractive 
               
               
                 indices according to the first embodiment of FIG. 2. 
               
             
          
           
               
                   
                   
                 Layer 
                   
               
               
                 Layer 
                   
                 Thick- 
                 Refractive 
               
               
                 Description 
                 Composition 
                 ness (nm) 
                 index 
               
               
                   
               
               
                 Substrate 110 
                 GaAs 
                 — 
                 3.45 
               
               
                 Bottom cladding 
                 Al 0.81 Ga 0.19 As 
                 2000 
                 3.03 
               
               
                 region 120 
               
               
                 Passive region 130 
                 Al 0.72 Ga 0.28 As 
                 1600 
                 3.08 
               
               
                 Mode control region: 
                 Al 0.9 Ga 0.1 As/(AlGa) x O y   
                 60 
                 2.92/1.6 
               
               
                 bottom layer 142 
               
               
                 Active region 150 
                 GaAs 
                 150 
                 3.45 
               
               
                 Mode control region: 
                 Al 0.9 Ga 0.1 As/(AlGa) x O y   
                 60 
                 2.92/1.6 
               
               
                 top layer 141 
               
               
                 Top cladding 
                 Al 0.81 Ga 0.19 As 
                 1000 
                 3.03 
               
               
                 region 160 
               
               
                 Cap layer 170 
                 GaAs 
                 100 
                 3.45 
               
               
                   
               
             
          
         
       
     
         [0049]    In one example of a tapered waveguide  100 , the epitaxial wafer of the tapered ridge waveguide  100  is grown in a single epitaxial process on a substrate  110  of GaAs by molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD). The compositions of the layers of this waveguide  100  are summarized in Table 1. In this example of the waveguide shown in  FIG. 2 , the active region  150  is a 150 nm-thick GaAs layer, while the low-loss passive region  130  is a 1600 nm-thick AlGaAs (72%) layer. The top cladding region  160  of the tapered ridge waveguide is composed of a 1000 nm-thick AlGaAs (81%) layer overgrown with a cap layer  170  of 100 nm-thick GaAs, while the bottom cladding region  120  is a 2000 nm-thick AlGaAs (81%) layer. The mode control region  140  is composed of two 60 nm-thick AlGaAs (90%) layers. One layer  141  is placed between interfaces of the top cladding region  160  and the active region  150  (e.g. above the active region  150 ), and the other layer  142  is placed between interfaces of the active region  150  and the passive region  130  (e.g. below the active region  150 ). In this example, the tapered ridge waveguide  100  is designed for an operating wavelength of 1.3 μm. The single mode tapered ridge waveguide  100  is fabricated by optical lithography and reactive ion etching. The device has a stripe width D w  of 3 μm in the wide section  101  and a stripe width D n  of 1.6 μm in the narrow section  103 . The etching depth, D e , of the epitaxial structure is more than 3.1 Note that a stop-etching layer can be introduced into the bottom cladding layer to provide high accuracy of the deep etching process. In this example of the waveguide, the laterally tapered section  102  has a linear profile with a taper length L t  of 360 μm. A wet lateral selective oxidation technique is used to fabricate buried dielectric (AlGa) x O y  layers with a low refractive index. The oxidation depth L ox  of the Al-rich layers  141  and  142  is around 0.9 μm.  FIGS. 3   a,    3   c,  and  3   e  also depict the cross-sectional scanning electron microscopy images of the fabricated tapered ridge waveguide  100  along section surfaces  3   a,    3   c,  and  3   e,  respectively. Note that the abundant amount of oxidant into the (AlGa)O y  layer  141  with the subsequent vertical oxidation of the AlGaAs (81%) layer  160  results in the taper oxidation front of the mode control region  140 . Regarding theoretical simulation, such a complex oxidation front can result only in weak additional confinement of the optical mode and does not cause any noticeable changes in performance of the tapered ridge waveguide  100 . 
         [0050]    An important component of the modern PIC systems are curved optical channel waveguides; therefore, the issue of the excess losses due to bending is actual and important for the present invention.  FIG. 4   a  shows a straight ridge waveguide  11 , where the field of the optical mode  90  is symmetric about the field peak and occurs at the center of the passive region  130  of the waveguide  11 , as shown in  FIG. 4   b.  In contrast, in a curved waveguide  12 , the field mode profile is asymmetric, as the optical mode  90  shifts toward the outward side of the bend curvature (see  FIGS. 4   c  and  4   d ). This trend gets more pronounced with a smaller bend radius R b  and the optical mode  90  becomes leaky. Hence, the continuous radiation of mode power tangentially out of the curved waveguide  12  as light travels around the bend causes additional optical losses. Thus, the bending losses dramatically increase with a decrease in the curvature radius R b , as shown in  FIG. 5   a.  Such excess loss can be reduced by increasing the confinement of the mode field  90 . Indeed, if the mode  90  will be weakly confined in the passive region  130 , then the optical mode  90  will tend to have long exponential tails extending into the cladding region  120 , e.g. the optical mode  90  will suffer from stronger radiation. In contrast, the increased degree of modal confinement caused by the deep etching of the waveguide (etching depth at least down to the bottom cladding region  120 , D e &gt;3 μm) will result in a decrease in the bending losses (see  FIG. 5   a ). The wider the waveguide, the higher the effective refractive index, the stronger confinement of the optical mode, and, therefore, the lower the bending losses. On the contrary, the thinner the waveguide, the lower the effective refractive index, the weaker confinement of the optical mode, and the stronger power scattering on waveguide sidewalls. Moreover the higher-order modes tend to have more energy in the exponential tail outside of the passive region  130 , causing larger bending losses, which can be used for effective selection of the fundamental mode in the case of multimode waveguides. 
         [0051]    Another important aspect is the influence of polarization on bending losses. Referring to  FIG. 5   b,  proper design optimization of the waveguide  10  and increasing the confinement of the mode field enable one to keep a relatively low level of bending losses for both TM (transverse magnetic) polarization and TE (transverse electric) polarization even for small bend radiuses R b &lt;100 μm. In preferred embodiments, an effective refractive index of the waveguide for TE polarization is close to an effective refractive index for TM polarization, which can be important for polarization-independent photonic integrated circuits. In a preferred embodiment, a difference between an effective refractive index for transverse electric polarization and an effective refractive index for transverse magnetic polarization is less than 10 −3 . In summary, in preferred embodiments, the waveguide  10  should be designed as a single-mode waveguide with a deep ridge to minimize optical losses at bending, where negligible polarization sensitivity is also possible by proper selection of the waveguide width and the passive region thickness. 
         [0052]      FIG. 6  schematically illustrates a tapered ridge waveguide  200  in another embodiment of the present invention. A difference between the tapered ridge waveguide  100  and the tapered ridge waveguide  200  is that, in  FIG. 6 , the passive region  130  is grown above the active region  150 . According to the general concept, the ridge waveguide  200  is designed to confine the eigen mode in the active region  150  in the wide section  101 . The continuous reduction of stripe width of the waveguide of  FIG. 6  (laterally tapered section  102 ) results in adiabatic displacement of the optical mode  90  from the active region  150  into the passive region  130  (upward as compared to  FIG. 3 ). Finally, the optical mode is confined in the passive region  130  in the narrow section  103 . 
         [0053]    In another embodiment shown in  FIG. 7 , a design of a tapered ridge waveguide  400  is similar to that for the tapered ridge waveguide  100  except for the mode-control region  140 . The mode-control region  140  is inserted into the active region  150  and includes at least one Al-rich layer in this embodiment. The tapered ridge waveguide  400  is designed to localize the eigen mode in the active region  150  in the wide section  101 . The lateral tapering in the taper section  102  provides adiabatic transfer of optical power from the active region  150  into the passive region  130  (downward as compared to  FIG. 3 ). Finally, the optical mode is confined in the passive region  130  in the narrow section  103 . 
         [0054]    Although the laterally tapered section  102  has a linear profile for all embodiments illustrated herein, this is not intended to limit the invention to the precise embodiments disclosed herein. 
         [0055]    The linear taper should be designed to be large enough for adiabatic transfer of the eigen mode between the active region and the passive region. Note that tapers of other forms and profiles may be used within the spirit of the present invention. For example, a lateral taper with an exponential profile has smaller mode transformation losses than a linear taper and provides the adiabatic displacement of the eigen mode between the active region  150  and the passive region  130  at smaller taper lengths L. Similarly, with a non-exponential curved profile, the taper has a smaller mode transformation loss and provides the adiabatic transfer of the eigen mode between the active region and the passive region at a smaller length of the taper than with a linear profile. As another example, a two-section taper has a first section with a linear profile and a second section with an exponential profile. This two-section taper provides a trade-off between a linear profile taper and an exponential profile taper. For example, the first section of this taper results in preliminary lateral mode confinement, while the second section provides the adiabatic power transfer between the active region  150  and the passive region  130  at smaller total taper lengths L t . 
         [0056]    The devices address the issue of vertical monolithic integration of active devices with passive elements into a single photonic integrated circuit, therefore the design rules of doping in optoelectronic devices is actual. Depending on the exact application (light emitting diodes, lasers, modulators, passive waveguides, etc.), various doping profiles of a waveguide structure of the present invention to realize electrical conductivity, p-n-junction(s) or highly doped contact layers are possible. For example, the doping profile of the devices based on the first embodiment can be a p + -type doped cap layer (acting as contact layer)  170 , a p-type doped top cladding region  160 , an undoped active region  150 , a lightly n-type doped passive region  130 , and an n-type doped bottom cladding region  120  on an n + -type substrate  110 . Note that a reverse doping profile for a p + -type substrate  110  is also possible, however the passive region  130  should be preferably doped by n-type material to provide the lowest optical losses during propagation of the optical mode  90  in the passive region  130 . 
         [0057]    The so-called wet lateral selective oxidation of Al-rich layers technology has a unique feature in that it provides the opportunity to form buried insulating layers with a high structural quality and with the required electrical and optical parameters. Moreover, this technique enables smoothing of the sub-micron surface roughness of the ridge waveguide, which is especially important for adiabatic low-loss transfer of the optical power. The present invention also addresses possible solutions to some critical problems related to the oxidation technique. 
         [0058]    In fact, the accumulated stress and the amount of the intermediate products generated in the oxidation reaction result in poor mechanical stability of the oxidized structures. In addition, the residual hydro-oxides are metastable, which can result in the undesirable oxidation reaction in the future. Using in-situ high-temperature annealing allows not only the effectively removal of the intermediate products, but also the partial conversion of the amorphous oxide into the more stable polycrystalline phase. Furthermore, the use of AlGaAs layers with relatively high Ga-composition also provides improved mechanical stability compared to pure AlAs layers. 
         [0059]    Another critical issue is reproducibility and uniformity for oxidation across the epitaxial structure due to the extremely sensitive compositional, temperature, and doping (level and type) dependencies of the oxidation rates, especially in the Al-concentration range of 96-100%. However, the activation energy for the oxidation reaction of an Al-rich layer demonstrates weak composition dependency at an Al-composition less than 92% and, thus, the oxidation rate is insensitive to small deviations of Al. In combination with diffusion-limited regimes of oxidation, where the oxidation process is determined by the diffusion of water vapor through the oxide to the reaction front rather than reaction rate, the relatively high degree of oxidation selectivity between AlGaAs layers provides reproducible oxidation. An additional improvement of oxidation reproducibility can be provided by the short-time chemical etching before the oxidation process (for example NH 4 OH:H 2 O 2 :H 2 O solution) to remove the surface damage and contamination caused by the non-chemical etching, for example of reactive ion etching and dry etching. 
         [0060]    All of the references mentioned herein are hereby incorporated herein by reference. 
         [0061]    Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.