Patent Document

RELATED APPLICATIONS 
     This Application claims priority of U.S. patent application Ser. No. 60/526,172, filed Dec. 2, 2003, entitled VERTICALLY COUPLED LARGE AREA AMPLIFIER (VECLAA), the entire disclosure of which is hereby incorporated by reference as if being set forth in its entirety herein. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     The invention was made with U.S. government support, and the U.S. Government has certain rights in the invention, as provided for by the terms of Contract number MDA972-03-C-0043 (DARPA) awarded by the U.S. Army Research Laboratory. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to optical systems and amplifiers. 
     BACKGROUND OF THE INVENTION 
     The amplification of optical signals is known to be important. High power optical amplification is useful in telecommunications and ablation applications, both by way of non-limiting example only. It is believed to be desirable to achieve faithful amplification powers of optical signals of about 500 watts or more, also by way of non-limiting example. 
     The principles of the transmission of optical signals via optical fibers are well understood. In deed, optical fibers are used to transmit light in many conventional applications. It is also believed to be desirable to provide a high power optical amplifier that is well-suited for use with optical fibers—in addition to other types of optical waveguides as well. 
     SUMMARY OF INVENTION 
     A monolithic light amplification system including: an un-doped waveguide; a ridge waveguide positioned over the un-doped waveguide; and, at least a doped layer between the un-doped waveguide and ridge waveguide; wherein, the un-doped waveguide and ridge waveguide cooperate to amplify light input to the un-doped waveguide. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings, wherein like numerals refer to like parts and: 
         FIG. 1  illustrates a diagrammatic view of a system according to an aspect of the present invention; 
         FIG. 2  illustrates a mode profile corresponding to section  2 - 2  of the system of  FIG. 1 ; 
         FIG. 3  illustrates exemplary performance characteristics according to an aspect of the present invention; 
         FIGS. 4A and 4B  illustrate near- and far-field performance characteristics according to an aspect of the present invention; and, 
         FIG. 5  illustrates a diagrammatic view of a system according to an aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements found in typical optical systems and methods of making and using the same. Those of ordinary skill in the art will recognize that other elements are desirable and/or required in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. 
     Referring now to  FIG. 1 , there is shown a diagrammatic representation of an amplifier system  10  according to an aspect of the present invention. Amplifier  10  generally includes a substrate  20 , amplification/guiding region  30  and waveguide region  40 . Amplifier system  10  may be monolithic in nature. Amplifier system  10  may be used with other photonic components as part of a photonic integrated circuit and/or in combination with non-monolithic components, such as optical fiber. For example, amplifier system  10  may be provided alone, in combination with other amplifiers, or in combination with other photonic components, such as sources like lasers, or photonic switches, all by way of non-limiting example only. 
     Substrate  20  may take the form of any suitable optical system substrate suitable for use with other materials found in the system. Substrate  20  provides a structural base upon which the regions  30 ,  40  may be supported, for example. Substrate  20  may be composed of GaAs, by way of non-limiting example only. 
     Region  30  serves as a primary propagation and amplification region during operation. Region  30  may be composed of one or more waveguiding materials suitable for use with other materials found in the system. For example, region  30  may be composed of AlGaAs. According to an aspect of the present invention, region  30  may he undoped. According to an aspect of the present invention, region  30  may be composed of undoped ALGaAs. According to an aspect of the present invention, the cross-sectional area of region  30  may be large compared to the active layer of waveguiding region  40 , and devoid of quantum well structure. Region  30  is vertically, optically coupled to region  40 , such that activation of region  40  serves to amplify optical signals in region  30 . 
     Region  40  may take the form of a ridge waveguide structure including an active layer sandwiched between cladding layers. For example, region  40  may include quantum well(s) in a layer  44  sandwiched between an upper cladding  42  and a lower cladding  46 . By way of further example, quantum well(s) layer  44  may take the form of a single quantum well (SQW) structure or multi-quantum well (MQW) structure. Quantum well(s) layer  44  may be composed of any suitable material system, such as one including InGaAs/GaAs, by way of non-limiting example only. Upper cladding  42  may take the form of a suitable cladding material for use with layer  44 , such as p-AlGaAs. Lower cladding  46  may also take the form of a suitable cladding material for use with layer  44 , such as n-AlGaAs. Contact  43  may be provided for upper cladding layer  42 . Contacts  45  and  47  may be provided for lower cladding layer  46 . In one configuration, contact  43  may provide a p-contact for region  40 , while contacts  45 ,  47  provide n-contacts for region  40 , as will be understood by those possessing an ordinary skill in the pertinent arts. 
     The ridge waveguide structure may optionally be provided with passivation and/or cap layers. Further, lower cladding layer  46  of the ridge waveguiding structure may be coupled to an upper surface  30 ′ of region  30 —thereby vertically, optically coupling region  30  to region  40 . 
     Referring now also to  FIG. 2 , there is shown an operating mode profile  35  of the amplifier system of  FIG. 1  according to an aspect of the present invention. As may be seen therein, region  30  provides large-mode waveguide functionality for ridge waveguide structure  40 . As may be seen, according to an aspect of the present invention a low modal overlap to doped layers ( 42 ,  46 ) may be provided. For example, the overlap may be less than about 0.02%. Accordingly, system  10  exhibits low optical internal loss characteristics, such as below about 0.8 cm −1 . According to an aspect of the present invention, the cladding layers ( 42 ,  46 ) may be highly doped, such as on the order of about 10 18  cm −3 . Further, according to an aspect of the present invention, top cladding layer(s) ( 42 ) may be relatively thin, such as on the order of about 0.5 μm. As will be understood by those possessing an ordinary skill in the pertinent arts, these characteristics lead to a device exhibiting low electrical resistance, on the order of about 10 −5  Ω·cm 2  for example, and a low temperature resistance, on the order of about 2° C./Watt·mm for example. Accordingly, a device of the present invention is suitable for use with high current injection. Thus, device  10  exhibits a high added power. 
     Further, as may be seen in  FIGS. 1 and 2 , according to an aspect of the present invention a large optical mode  35  may be provided. Thus, device  10  exhibits good coupling characteristics for optical fiber, providing for high coupling efficiencies that may be greater than about 80%, for example. 
     Further yet, as the majority of the optical mode of a device of the present invention propagates in an undoped material, device  10  exhibits low optical loss and low heat generation. 
     Referring now also to  FIG. 3 , there is shown an exemplary optical intensity for an about 5 mm long structure in accordance with  FIGS. 1 and 2 . As is demonstrated thereby, significant amplification may be achieved through utilization of the amplifier of  FIG. 1 , where a first end facet  30   i  of layer  30  is used as an input light facet and an opposite end facet  30   o  is used as an output light facet. Optical signals input into amplifier  30  via facet  30   i  may be amplified as they propagate longitudinally along layer  30  towards output facet  30   o  responsively to excitation of region  40 . 
     Referring now also to  FIGS. 4A and 4B , an amplifier system according to the present invention may be well suited for use with optical fiber. Optical fiber may be used to deliver optical signals to amplifier  10  and receive amplified signals from amplifier  10 , such as via input and output facets  30   i ,  30   o  ( FIG. 3 ). As is shown in  FIGS. 4A and 4B , good spot size geometry is maintained by layer  30  both in the near field (i.e., nearer facet  30   i ) and far field (nearer facet  30   o ) and is well suited for use with optical fibers. 
     According to an aspect of the present invention, a multi-ridge system may be used to provide for enhanced amplification powers. In such a case, individual gain regions, such as in the form of system  10  of  FIG. 1 , may be connected in series to form a multi-ridge system, with each of the devices electrically isolated. This may advantageously provide for more even current injection in each device and reduced power fluctuation in operation. The devices may be formed monolithically, and be provided on a common substrate or separately. The devices may be coupled together using a planar waveguide or optical fiber, all by way of non-limiting example only. 
     For non-limiting purposes of further explanation only, the following example may also be considered. Referring now to  FIG. 5 , there is shown an amplifier system  500  according to an aspect of the present invention. System  500  may include a semi-insulating substrate  100 , such as a GaAs substrate. A lower cladding layer  110  may be provided upon substrate  100 . Lower cladding  110  may be about 1.6 μm thick and be undoped. Lower cladding  110  may be composed of AlGaAs, and have an Al content of about 0.25%. A large-mode waveguide layer  120  may be formed over lower cladding  110 . Large-mode waveguide layer  120  may be composed of AlGaAs, and have an Al content of about 0.21%. Large-mode waveguide  120  may be about 3 μm thick and also be undoped. A graded waveguide layer  130  may be provided over large-mode waveguide  120 . Graded waveguide layer  130  may have an aluminum content that is graded from about 0.3 to about 0.25%. The graded layer  130  may be graded substantially linearly and transversely with respect to the layers structure, with the lower percentage nearer the quantum well(s) layer. Graded waveguide layer  130  may be about 0.3 μm thick. Graded layer  130  may be n-doped to less than about 3×10 18  cm −3 . For example, layer  130  may be doped to about 1×10 18  cm −3 . Graded waveguide  130  may be provided with an n-contact layer, composed of AlGaAs, for example. The n-contact AlGaAs may have an Al content of about 20% to about 30%, and a doping level on the order of about 10 18  cm −3 . The n-contact may have an electron mobility of at least about 1000 V/cm sec. 
     A graded waveguide layer  140  may be provided over layer  130 . Layer  140  may be about 0.2 μm thick. Graded waveguide layer  140  may have an aluminum content that is graded from about 0.25% to about 0.2%. The graded layer  140  may be graded substantially linearly and transversely with respect to the layers structure, with the lower percentage nearer the quantum well(s) layer. Graded waveguide layer  140  may be undoped. A GaAs barrier layer  150  may be provided over graded layer  140 . Barrier layer  150  may be about 10 nm thick, for example. Barrier layer  150  may be undoped. An InGaAs quantum well layer  160  may be provided over barrier layer  150 . Layer  160  may have an In content of about 0.2%, and be about 7 nm thick, for example. Layer  160  may also be undoped. A GaAs barrier layer  170  may be provided over quantum well layer  160 . Barrier layer  170  may be about 10 nm thick and be undoped as well. A graded waveguide layer  180  may be provided over barrier layer  170 . Layer  180  may be about 0.6 μm thick. Layer  180  may be composed of AlGaAs, and have an Al content that is graded from about 0.2% to about 0.4%. The graded layer  180  may be graded substantially linearly and transversely with respect to the layers structure, with the lower percentage nearer the quantum well(s) layer. Layer  180  may be undoped. A p-cladding layer  190  may be provided over layer  180 . Layer  190  may be about 1 μm thick. Layer  190  may be composed of AlGaAs, and have an Al content of about 0.4%. Layer  190  may be p-doped to about 2×10 18  cm −3 . An intermediate graded layer  200  may be provided over layer  190 . Layer  200  may be about 200 nm thick. It may be composed of AlGaAs, and have an Al content that is graded from about 0.4% to about 0%. Layer  200  may be graded substantially linearly and transversely with respect to the layers structure with the higher Al content nearer the quantum wells layer. Layer  200  may be p-doped to about 2×10 18  cm −3 . Finally, a contact layer  210  may be provided over layer  200 . Layer  210  may be about 50 nm thick. Layer  210  may be composed of GaAs and be p-doped to about 10 19  cm −3 . 
     The layers  100 - 210  may be provided and shaped using conventional processing methodologies suitable for use with the materials thereof, such as deposition and etching for example. According to an aspect of the present invention, one may precisely control the Al content of layer  120  to provide for good waveguiding and amplification properties. For example, it may prove important to control the Al content to within about 1% of the target content. 
     According to an aspect of the present invention, layers  100 ,  110  may be suitable for use as substrate  20  of  FIG. 1 . Layer  120  may be suitable for use as region  30  of  FIG. 1 . Finally, layers  130 - 210  may be suitable for use as region  40  of  FIG. 1 . 
     It will be apparent to those skilled in the art that various modifications and variations may be made in the apparatus and process of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modification and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Technology Category: 5