Abstract:
Bypassing the terminal current through a multiplicity of active regions contained within the same optical cavity, easily fabricated diode lasers with external differential efficiencies greater than unity are created. Such multiple-active-region lasers can enable optical links with net electrical-to-electrical signal gain as well as facilitate impedance matching at the source. These devices can also be used within tunable laser structures, vertical laser structures or other complex laser cavity structures to provide low-cost monolithic devices with unique, desirable capabilities. When the terminal current is supplied by an integrated photodetector, low-noise optical-optical signal gain can be provided in a single monolithic semiconductor component formed by a compatible materials technology.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/127,109, filed Mar. 31, 1999, entitled “HIGH-EFFICIENCY SERIES-CONNECTED MULTIPLE-ACTIVE REGION LASERS AND OPTICAL AMPLIFIERS,” by Larry A. Coldren, which application is incorporated by reference herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002] This invention was made with Government support under Grant No. ECS9634542, awarded by the National Science Foundation. The Government has certain rights in this invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    This invention relates generally to electronic devices, and more particularly to high-efficiency series connected multiple active region lasers and optical amplifiers.  
           [0005]    2. Description of Related Art  
           [0006]    Modern day usage of optical components and lasers has made communications and data transfer more efficient and more cost effective. The use of semiconductor lasers has made the fabrication and packaging of optical sources more cost effective, as well as reducing the size of the overall device.  
           [0007]    However, the requirements for communications and data transfer systems have also increased. Optical systems used as computer interconnections on the Internet, server-to-server connections, and other long distance, high data rate systems continually drive the optical sources and amplifiers to be more efficient and produce higher power outputs.  
           [0008]    Currently, many semiconductor sources, such as lasers, and optical amplifier systems are limited in their ability to transfer data and/or provide higher power outputs. Differential efficiencies on the order of unity gain for such devices are currently possible. However, a differential efficiency of greater than unity should be possible, and would increase the communications and data throughput capabilities of such devices.  
           [0009]    It can be seen, then, that there is a need in the art for optical sources and amplifiers to have increased differential efficiencies. It can also be seen that there is a need in the art for semiconductor and other lasers to have higher power outputs.  
         SUMMARY OF THE INVENTION  
         [0010]    To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and device for producing high efficiency laser outputs. The device comprises a first layer having a first dopant, a plurality of active layers, wherein at least one of the plurality of active layers is coupled to the first layer, wherein each layer in the plurality of active layers comprises a p-i-n structure, and a second layer having a second dopant, coupled to at least one of the plurality of active layers, wherein when an electrical current passes from the first layer to the second layer through at least one of the plurality of active layers, a laser light output from the plurality of active lasers is produced therein.  
           [0011]    The method comprises growing a first layer having a first dopant, growing a plurality of active layers, wherein at least one of the plurality of active layers is coupled to the first layer, wherein each layer in the plurality of active layers comprises a p-i-n structure, and growing a second layer having a second dopant, coupled to at least one of the plurality of active layers, wherein when an electrical current passes from the first layer to the second layer through at least one of the plurality of active layers, a laser light output from the plurality of active layers is produced therein.  
           [0012]    An object of the present invention is to provide optical sources and amplifiers that have increased differential efficiencies. Another object of the present invention is to provide semiconductor and other lasers that have higher power outputs. Another object of the present invention is to provide lasers and amplifiers with improved signal-to-noise ratios.  
           [0013]    Various advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there is illustrated and described specific examples in accordance with the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Referring now to the drawings in which like numbers represent similar features throughout:  
         [0015]    FIGS.  1 A- 1 D illustrate some illustrative embodiments of devices manufactured in accordance with the present invention;  
         [0016]    FIGS.  2 A- 2 C illustrate the band diagrams and optical mode overlays of stacked active region devices in accordance with the present invention;  
         [0017]    [0017]FIG. 2D illustrates a multiple-active-region vertical-cavity laser in accordance with the present invention;  
         [0018]    [0018]FIG. 3 illustrates the light output versus the current input for various active region samples in accordance with the present invention; and  
         [0019]    [0019]FIG. 4 is a flow chart illustrating the steps used in the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown byway of illustration the specific embodiment in which the invention maybe practiced. It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the scope of the present invention.  
         [0021]    Overview  
         [0022]    FIGS.  1 A- 1 D illustrate some illustrative embodiments of devices manufactured in accordance with the present invention.  
         [0023]    Semiconductor lasers with differential efficiencies greater than 1 should be possible by manufacturing lasers in accordance with FIGS.  1 A- 1 D. FIGS. 1A and 1B show series-connected axially-segmented edge-emitting laser structures. FIG. 1C illustrates a series-connected vertically-stacked active region edge-emitting laser, and FIG. 1D illustrates a series-connected vertically-stacked active region vertical-cavity laser.  
         [0024]    As shown in FIG. 1A, device  100  accepts input  102  to detector  104 . Input  102  is typically an optical input. Detector  104  converts input  102  to an electrical signal. The electrical signal output from detector  104  is passed to an edge-emitting segment  106 , which is coupled in series with edge emitting segments  108 - 116 . For an electrical input to device  100  detector  104  is omitted and the electrical input signal is directly applied to segment  106 . Each edge-emitting segment  106 - 116  is co-linear, e.g., each laser is manufactured in substantially the same mesa structure  118  such that each edge-emitting segment  106 - 116  is optically coupled to the other edge-emitting segments  106 - 116 . However, each edge-emitting segment  106 - 116  is electrically separated from adjacent edge-emitting segments  106 - 116  by ion implantation or other isolation means.  
         [0025]    For optical inputs to device  100  a bias voltage is placed between the top of detector  104  and the bottom of edge emitting segment  116 . This bias voltage, in combination with the incident light, provides photocurrent which flows through each edge-emitting laser segment  106 - 116 . The edge-emitting laser segments  106 - 116  are connected electrically in series, such that the entire terminal current passes through each edge-emitting laser segment  106 - 116 . A DC bias current may also be applied between photodetector  104  and first laser segment  106  to pre-bias the laser to or above threshold so that all of the photocurrent is used to create additional laser output. If the input to device  100  is electrical in nature, the DC bias current and the signal current are both applied to the first laser segment  106  directly, and the photodetector  104  is not necessary.  
         [0026]    The mesa structure  118 , when provided a combined bias and signal current above the lasing threshold of the device  100 , will produce a laser output  120 .  
         [0027]    In FIG. 1B, a side view of mesa structure  118  is shown. A p-doped region  122 , an active region  124 , and an n-doped region  126  are shown in their proper perspective. The p-doped region  122  and n-doped region  126  can be reversed without departing from the scope of the invention, e.g., regions  122  and  124  are typically of different dopant types, and are not restricted to having the p-dopant be proximate to detector  104 . Although shown as indium phosphide, the doped regions  122  and  126  and active region  124  can be of any laser producing material. Bias voltage  128  and ground  130  are also shown, with connections  132  shown to connect the edge-emitting laser segments  106 - 116  in series. Active region  124  now contains four segments, separated by isolators  134 . This segmentation produces multiple active regions  124  that are connected in series electrically and placed in a single optical cavity.  
         [0028]    Since detector  104  is also connected in series with device  100 , this allows for direct detection of an incoming optical signal in addition to applying an electrical bias or signal at the terminal beneath the detector  104 . Use of the optical input  102  enables optical amplification and wavelength conversion from a single integrated element if desired.  
         [0029]    [0029]FIG. 1B illustrates active region  124  segments that are arranged in series optically along the laser cavity within an edge-emitting geometry. The configuration of FIGS.  1 A- 1 B allows the entire biased signal current to flow through each segment. As a result, when N segments are created, the lasing threshold for the device  100  is achieved with only 1/N of the current required for a non-segmented device  100 .  
         [0030]    Further, an increment in current above threshold (a current modulation) also results in N times the change in output  120  optical power as compared to the normal unsegmented case. In other words, the, external, differential efficiency of device  100  is multiplied by N compared to a non-segmented device of similar geometry. The spontaneous emission power at and above threshold should be approximately equal because the threshold current density and volume of the active region  124  is approximately equal. Thus, the output will increase without any noise penalty, resulting in an increase in the signal-to-noise ratio for device  100 . To provide these advantages, a bias voltage  128  higher than that required for a non-segmented device  100  is required because of the increased resistance across device  100 . However, the dissipated power and the thermal impedance are approximately equal to a non-segmented device  100  of similar geometry.  
         [0031]    [0031]FIG. 1C illustrates a multiple-active region device  200 . For optical signal inputs, detector  202  is connected in series with active regions  204 - 208 , which are placed within a single edge-emitting cavity defined by layers  210  and  212 . A bias voltage  214  and ground  216  are also shown.  
         [0032]    Active region  204  is typical of the active regions  204 - 208 , and the description of active region  204  applies equally to all active regions  204 - 208  of device  200 . Further, a greater or lesser number of active regions  204 - 208  can be used in device  200  without departing from the scope of the present invention; three active regions  204 - 208  are shown for purposes of illustration only.  
         [0033]    Active region  204  comprises a p-i-n structure, e.g., p-doped region  218 , intrinsic region  220 , and n-doped region  222 . Active regions  204 - 208  are connected in series within a single laser cavity that emits output  226  from an edge of the laser cavity  224 . Each active region comprises multiple quantum wells (MQW) that provide the laser output  226 . The connection between the various p-i-n active regions generally requires a highly-doped n+-p+“back” or “tunnel” diode. In other words, the layer structure becomes a structure of the form p-i-n-n + -p + -p-i-n-n + -p + - . . . etc. for stacking two p-i-n active regions in series. By designing the optical mode of the device  200 , the back diodes can be placed at optical nulls of the device  200 , which reduces or eliminates the optical interference caused by the back diode highly-doped regions within the laser cavity of the edge-emitter structure.  
         [0034]    The stacking of multiple active regions  204 - 208  results in a multiplication of the differential efficiency by the number of active regions, e.g., for three active regions  204 - 208 , the differential efficiency will be multiplied by three, provided the process does not result in additional optical loss or non-radiative carrier recombination. The incorporation of highly doped regions to create the series connections may add such deleterious effects, but these effects can be reduced to substantially insignificant amounts using common fabrication practices. The threshold current to activate the multiple active regions  204 - 208  decreases depending on the bias point of the device  200 . Therefore, the spontaneous emission at and above the threshold current point would increase by less than the number of multiple active regions  204 - 208 , which increases the signal to noise ratio at higher bias voltages. Device  200  is a higher-voltage, higher impedance (due to the multiple stacked active regions  204 - 208  series resistances), with a higher heat generation density, but is easy to fabricate and the large optical cavity  224  provides better coupling to optical fibers. The integration of detector  202  provides efficient, low-noise, and inexpensive analog repeaters, wavelength converters, and optical taps.  
         [0035]    [0035]FIG. 1D illustrates a vertical cavity laser fabricated in accordance with the present invention.  
         [0036]    Device  300  comprises detector  302 , which comprises n-doped region  304  and p doped region  306 , and is coupled to a differential Bragg reflector (DBR) mirror stack  308 . Again, for an electrical input signal the detector stage is omitted. The mirror is grown or otherwise coupled to the p-doped gain material  310 . Again, active regions  312 - 316  are stacked in series in the cavity of the laser, where active region  312  comprises p-doped region  318 , intrinsic region  320 , and n-doped region  322 , active region  314  comprises p-doped region  324 , intrinsic region  326 , and n-doped region  328 , and active region  316  comprises p-doped region  330 , intrinsic region  332 , and n-doped region  334 , respectively. The active regions  312 - 316  are connected serially, and are then connected to n-doped gain material  336 . Material  336  is coupled to the other DBR mirror stack  338 . Bias voltage  340  and ground  342  are applied to device  300 . Input  344  is applied, and laser output  346  emits from the stack opposite from input  344 .  
         [0037]    Again, active regions  312 - 316  comprise p-i-n structures, e.g., p-doped region  318 , intrinsic region  320 , and n-doped region  322 . Active regions  312 - 316  are connected in series within a single laser cavity that emits output  346 . Each active region comprises multiple quantum wells (MQW) that provide the laser output  346 . As in the edge-emitter case, tunnel or back diodes are incorporated between the p-i-n multiple active regions for low voltage cascading. However, in the vertical-cavity laser case, these highly-doped regions can be placed at optical standing-wave electric-field nulls, such that little optical loss is added to the overall structure.  
         [0038]    The stacking of multiple active regions  312 - 316  results in a multiplication of the differential efficiency by the number of active regions, e.g., for three active regions  312 - 316 , the differential efficiency will be multiplied by three. The threshold current to activate the multiple active regions  312 - 316  decreases depending on the bias point of the device  300 . Therefore, the spontaneous emission at and above the threshold current point would increase by less than the number of multiple active regions  312 - 316 , which increases the signal to noise ratio at higher bias voltages.  
         [0039]    The basic characteristics of the configurations of FIGS.  1 A- 1 D can be understood by deriving the output power vs. input current starting from a set of phenomenological rate equations. Of course, the result does not illustrate the effects of correlated spontaneous emission noise, but it does contain the inherent increase in differential efficiency, reduction in threshold current, and value of spontaneous emission, which clamps at threshold due to carrier clamping.  
         [0040]    The carrier and photon density rate equations maybe written as:  
              N          t       =           η   i        I       q                   V   A         -     N   τ     -     g                   V   g          N   P                          N   P            t       =       Γ                 g                   v   g          N   p       +       Γβ     s                 p            R     s                 p         -       N   p       τ   p                               
 
         [0041]    where  
         [0042]    N=carrier density  
         [0043]    N P =photon density  
         [0044]    η i =internal efficiency  
         [0045]    I=input current that flows through each active region  
         [0046]    V=volume of each of the N A  active regions,  
         [0047]    τ=carrier lifetime in the absence of stimulated emission,  
         [0048]    v g =group velocity,  
         [0049]    g=incremental plane-wave gain of the active material,  
         [0050]    Γ=confinement factor including all active regions (Γ=Γ A N A , where Γ A =V A /V P  is the overlap of one active region with the optical mode),  
         [0051]    β SP =spontaneous emission factor, and  
         [0052]    τ P =photon lifetime.  
         [0053]    The output power above threshold is given by:  
         P   0     =         η   i            N   A          (       α   m         〈     α   i     〉     +     α   m         )              h                 v     q          (     I   -     I     t                 h         )                   fo                 r                 I     &gt;     I     t                 h                               
 
         [0054]    where  
         [0055]    &lt;αi&gt;=average internal loss  
         [0056]    αa m =mirror loss =(1/L)ln(1/R)  
         [0057]    R=mean mirror power reflectivity  
         [0058]    I=input current, and  
         [0059]    I th =threshold current  
         [0060]    This is identical to the output power of a single active region, except the differential efficiency is now given by:  
         η   d     =       N   A          (         η   i          α   m           〈     α   i     〉     +     α   m         )                             
 
         [0061]    which is N A  times larger than the single active region differential efficiency.  
         [0062]    Using an exponential gain model for the current density (J), the threshold current maybe written as:  
         I     t                 h       =       [       A                   J     t                 r           η   i       ]          exp        (       g     t                 h         g   0       )                               
 
         [0063]    where  
         [0064]    g th =[&lt;α i &gt;+α m ]/Γ 
         [0065]    J tr =transparent current density, and  
         [0066]    A=area of each active region.  
         [0067]    In the device of FIGS.  1 A- 1 B, the area A is reduced to 1/N A , so the threshold current is seen to reduce by the same amount, if the active volume, Γ, and losses remain the same. In practical situations, however, there may be some slight decreases in A and Γ, as well as some increases in average internal loss, which would increase the threshold slightly.  
         [0068]    In the devices of FIGS.  1 C- 1 D, Γ is approximately N A  times larger than a device with one active region if the optical mode remains the same, g th  is reduced by a factor of N A  and I th  is also reduced by a factor of N A . The average internal loss may also increase in the process of adding additional active regions, so the overall increase in differential efficiency may be increased slightly less than N A .  
         [0069]    Below the threshold current the output power is given by the spontaneous emission power  
           P   0          (     I   &lt;     I     t                 h         )       =       η   r          η   i            N   A          (       α   m         〈     α   i     〉     +     α   m         )              h                 v     q          β     s                 p          I                           
 
         [0070]    Thus, for a given current the spontaneous emission is N A  times larger. However, according to the exponential gain model equations above, the threshold current is reduced by some amount (up to N A  times). Therefore, at and above threshold, the spontaneous emission power may be only slightly larger than with one active region. Since the differential efficiency is N A  times larger, the ratio of stimulated-to-spontaneous emission will be increased, except very close to threshold.  
         [0071]    Using the present invention with different mirror reflectivities, cavity lengths, and/or confinement factors per active region in going from a single to a multiple-active-region design may also increase the differential efficiency of devices made in accordance with the present invention. The conclusions made above assume that the mirror loss and confinement factor for each active region are the same in both single active region and multiple-active region cases. In fact, it is possible to further improve the desired characteristics by changing these parameters in an optimized multiple-active-region design using the above-provided equations.  
         [0072]    FIGS.  2 A- 2 C illustrate a band diagram and optical mode overlays derived from experimental results for single and multiple-active-region edge-emitting devices.  
         [0073]    [0073]FIG. 2A illustrates the band diagram  400  for a two-active region stack. For this illustration, the valence band  402  and conduction band  404  are shown, with the MQW region  406  illustrated. The distance  408  from the MQW region  406  to the edge of the active region is 740 angstroms. The tunnel-diode  410  between active regions is 120 angstroms thick.  
         [0074]    [0074]FIG. 2B illustrates a single active layer  406  comprising 5 quantum wells in a 45 angstrom indium gallium arsenide (InGaAs) stack The optical mode  414  is indicated. FIG. 2C illustrates the optical mode  416  for a three active-region device, where each active region comprises a five quantum well layer  418 .  
         [0075]    [0075]FIG. 2D illustrates a multiple-active-region vertical-cavity laser in accordance with the present invention.  
         [0076]    Active regions  420 , consisting of single Quantum Wells (QWs) or MQWs are shown. Back (or tunnel) diodes  422  are shown between the active regions  420 , and connect the active regions  420  together. Schematically, the back diodes  422  are shown as diodes  424 , and active regions  420  are shown as diodes  426 . Optical path  428  is shown, and DBR mirror stacks are located at positions  430  and  432 . Note that the back diodes  422  are located at nulls of the standing wave  428 , which reduces or eliminates interference between the back diodes  422  and the standing wave  428 . Zero reference line  434  illustrates the zero of the electric field squared, and illustrates that the back diodes are located at nulls of the electric field.  
         [0077]    [0077]FIG. 3 illustrates the light output versus current input for a single active layer device and a multiple active layer device in accordance with the present invention.  
         [0078]    Graph  500  illustrates the light output for a single active layer device. The differential efficiency is 50 percent for such a device.  
         [0079]    Graph  502  illustrates the light output for a multiple, three-active layer device. Note that the differential efficiency is now 125 percent, or greater than unity (1.25).  
         [0080]    Logic  
         [0081]    [0081]FIG. 4 is a flow chart illustrating the steps used in the present invention.  
         [0082]    Block  600  represents performing the step of growing a first layer having a first dopant;  
         [0083]    Block  602  represents performing the step of growing a plurality of active layers, wherein at least one of the plurality of active layers is coupled to the first layer, wherein each layer in the plurality of active layers comprises a p-i-n structure.  
         [0084]    Block  604  represents performing the step of growing a second layer having a second dopant, coupled to at least one of the plurality of active layers, wherein when an electrical current passes from the first layer to the second layer through at least one of the plurality of active layers, a laser light output from the plurality of active layers is produced therein.  
         [0085]    Conclusion  
         [0086]    In summary, the present invention provides a method and device for producing high differential efficiency laser outputs. The device comprises a first layer having a first dopant, a plurality of active layers, wherein at least one of the plurality of active layers is coupled to the first layer, wherein each layer in the plurality of active layers comprises a p-i-n structure, and a second layer having a second dopant, coupled to at least one of the plurality of active layers, wherein when an electrical current passes from the first layer to the second layer through at least one of the plurality of active layers, a laser light output from the plurality of active lasers is produced therein.  
         [0087]    The method comprises growing a first layer having a first dopant, growing a plurality of active layers, wherein at least one of the plurality of active layers is coupled to the first layer, wherein each layer in the plurality of active layers comprises a p-i-n structure, and growing a second layer having a second dopant, coupled to at least one of the plurality of active layers, wherein when an electrical current passes from the first layer to the second layer through at least one of the plurality of active layers, a laser light output from the plurality of active layers is produced therein.  
         [0088]    Within the scope of the present invention, other embodiments or alterations of the descriptions herein are possible with the present invention. For example, the present invention is described with respect to certain materials families, e.g., indium phosphide (InP), indium gallium arsenide (InGaAs), and other materials. However, many semiconductor and other materials, such as indium aluminum gallium arsenide, aluminum gallium indium arsenide, indium arsenide, indium antimonide, gallium nitride, and other III-V, II-V, tertiary and quaternary materials may be used without departing from the scope of the present invention. Further, the detector that is described as monolithic with the laser cavity can be a separate device if desired. The device of the present invention, although typically described as a three active-layer device, can be a multiple active layer device of any number, e.g., a five active-layer device, a seven active-layer device, a ten active-layer device, etc., without departing from the scope of the present invention.  
         [0089]    The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.