Abstract:
A vertical laser cavity includes a non-planar top mirror in order to improve the optical performance of the laser cavity. In one approach, the top mirror is curved to form a plano-concave geometry with the bottom mirror, as opposed to the typical plano—plano geometry. This can reduce diffraction losses and otherwise improve optical performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 60/365,466, “Vertical laser cavity with a non-planar top mirror,” by Daniel A. Francis and Chris Decker, filed Mar. 18, 2002. The subject matter of the foregoing is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to semiconductor-based vertical laser cavities (e.g., as used in VCSELs). More specifically, the top mirror of the laser cavity is non-planar, thus enhancing the performance of the laser cavity. 
     2. Description of the Related Art 
     As the result of continuous advances in technology, optical devices are becoming more important and more prevalent. For example, the increasing demand for communications bandwidth has resulted in an increased interest in optical communications systems, including those that transmit data over optical fibers. This, in turn, has resulted in increased demand for optical devices for use in these systems. 
     One general class of optical devices is those that are based on vertical laser cavities. In these devices, a laser cavity is formed by a bottom mirror and a top mirror and is oriented vertically with respect to a supporting substrate. In one common approach, different layers of material are epitaxially grown on the substrate to build up the vertical laser cavity one layer at a time. Pumping the active region within the laser cavity above its lasing threshold results in laser action. The laser action can be used for different purposes, depending on the design of the rest of the device. For example, in vertical cavity surface emitting lasers (VCSELs), the device is used as a source of laser light. The laser radiation generated by the laser cavity is output through one of the mirrors. In vertical lasing semiconductor optical amplifiers (VLSOAs), the device is used as an amplifier. The laser action within the vertical laser cavity gain clamps the active region. A second optical signal passing through the active region experiences amplification without significant gain saturation. For example, see U.S. patent application Ser. No. 10/014,679, “Integrated Optical Device including a Vertical Lasing Semiconductor Optical Amplifier,” by Jeffrey D. Walker and Sol P. Dijaili, filed Dec. 11, 2001, which is incorporated herein by reference. Other devices and uses exist for vertical laser cavities. 
     One drawback of vertical laser cavities is they can be difficult to fabricate and operate. Because of the geometry, the round trip laser path typically is not very long within the active region. This results in relatively low gain for each round trip. As a result, care typically must be taken to ensure that round trip losses are not too large. Otherwise, lasing cannot be achieved. The top and bottom mirrors are one significant component in determining the overall loss and performance of a vertical laser cavity. However, their design is often driven by fabrication limitations, resulting in less than optimal mirror designs. 
     Thus, there is a need for vertical laser cavities with improved mirror designs. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the limitations of the prior art by providing a vertical laser cavity in which the top mirror is non-planar, thus improving the optical performance of the laser cavity. In one approach, the top mirror is curved to form a plano-concave geometry with the bottom mirror, as opposed to the typical plano-plano geometry. This can reduce diffraction losses and otherwise improve optical performance. 
     In one embodiment, an optical device includes a planar bottom mirror, a buried active region (e.g., a buried heterojunction) and a non-planar top mirror all integrated on a substrate. The two mirrors form a vertical laser cavity. A current path traverses the buried active region. Electrically pumping the buried active region provides gain for the vertical laser cavity. 
     In one approach, the non-planar top mirror is formed by underfilling. The vertical laser cavity is fabricated by forming a first set of layers on a substrate. This set includes the bottom mirror. Some of these layers are removed in lateral areas away from the vertical laser cavity. They are replaced by the lateral areas away from the vertical cavity are underfilled. In one approach, the underfill creates a current blocking layer. A second set of layers is formed on top of this. This set includes the top mirror. The underfill causes the non-planar shape of the top mirror. 
     In a different approach, the non-planar top mirror is formed by introducing a bump in the structure. The vertical laser cavity is fabricated by forming a first set of layers (including the bottom mirror) on a substrate. A seed bump is formed over the first set of layers. A second set of layers (including the top mirror) is then formed over that. The seed bump causes the non-planar shape of the top mirror. 
     In one particular application, the optical device is designed for use at either the 1.3 micron or the 1.55 micron window. The underlying substrate is n-doped InP, the planar bottom mirror is a Bragg reflector with alternating layers of InP and InGaAsP, the buried active region is either InGaAsP or InAlGaAs, and the non-planar top mirror is a Bragg reflector with alternating layers of InP and InGaAsP (either n-doped or p-doped, depending on the rest of the structure). The vertical laser cavity is electrically pumped. Current blocking layers may be used to confine the pumping current. 
     Other aspects of the invention include devices (e.g., VCSELs and VLSOAs) that utilize these vertical laser cavities and methods for fabricating these vertical laser cavities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  is a cross-section of a vertical laser cavity according to the invention. 
         FIGS. 2A–2B  are cross-sections illustrating fabrication of one implementation of the vertical laser cavity of  FIG. 1 . 
         FIGS. 3A–3B  are cross-sections illustrating fabrication of another implementation of the vertical laser cavity of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a cross-section of a vertical laser cavity  100  according to the invention.  FIG. 1  is simplified to show only the relevant portions of the laser cavity  100 . Other portions of the cavity (e.g., to achieve control over the laser mode, the pumping mechanism, etc.) have been omitted for clarity and can be designed using conventional techniques. The laser cavity  100  includes a bottom mirror  120  and a top mirror  130 , and an active region  150  located in an optical path between the two mirrors  120 ,  130 . The entire structure is fabricated on a substrate  110 . It is vertical in the sense that the laser cavity  100  is oriented vertically with respect to the substrate surface. The laser cavity  100  is primarily a planar structure, but the top mirror  130  is non-planar. More specifically, the center portion  132  of the top mirror  130  is raised relative to the outer portions  134 . In effect, there is a “bump”  135  in the materials underneath the top mirror  130 . Then, when the top mirror  130  is formed on top of the bump  135 , the mirror is non-planar and has a somewhat curved shape. 
     The vertical cavity  100  is a layered structure, allowing the structure  100  to be fabricated using standard semiconductor fabrication techniques, preferably including organo-metallic vapor phase epitaxy (OMVPE) or organometallic chemical vapor deposition (OMCVD). Other common fabrication techniques include molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), photolithography, e-beam evaporation, sputter deposition, wet and dry etching, wafer bonding, ion implantation, wet oxidation, and rapid thermal annealing, among others. In one approach, the different features shown in  FIG. 1  are planar and built upon a substrate  110 ; the active region  150  is implemented as a buried feature. 
     The choice of materials system for the vertical cavity  100  will depend in part on the wavelength of the optical signal, which in turn will depend on the application. Wavelengths in the approximately 1.3–1.6 micron region are currently preferred for telecommunications applications, due to the spectral properties of optical fibers. In particular, currently there are two common transmission windows for optical fiber: one at approximately 1.3 micron and another at approximately 1.55 micron. The 1.55 micron window currently can include shorter wavelengths (e.g., the S-band, 1450–1530 nm), centered wavelengths (e.g., C-band 1530–1560 nm), or longer wavelengths (e.g., L-band 1570–1610 nm). In addition, the approximately 1.28–1.35 micron region is currently also preferred for data communications over single mode fiber, with the approximately 0.8–1.1 micron region being an alternate wavelength region. The term “optical” is meant to include all of these wavelength regions. 
     The active region  150  can be implemented in a number of ways. In one embodiment, the active region  150  includes a multiple quantum well (MQW) active region. MQW structures include several quantum wells and quantum wells have the advantage of enabling the formation of lasers with relatively low threshold currents. In alternate embodiments, the active region  150  may instead be based on a single quantum well or a double-heterostructure active region. The active region  150  may be based on various materials systems, including for example InAlGaAs on InP substrates, InAlGaAs on GaAs, InGaAsP on InP, GaInNAs on GaAs, InGaAs on ternary substrates, and GaAsSb on GaAs. Nitride material systems are also suitable. The surrounding materials will depend in part on the composition of active region  150 . The active region  150  can be pumped by many different mechanisms, including electrical and optical pumping. In the figures, the active region  150  is shown as having a finite lateral extent but this is not required. 
     Examples of top and bottom mirrors  130  and  120  include Bragg reflectors, non-Bragg reflectors such as metallic mirrors, and hybrid mirrors consisting of a Bragg reflector in combination with a metallic mirror. Bragg reflectors may be fabricated using various materials systems, including for example, alternating layers of GaAs and AlAs, SiO 2  and TiO 2 , InAlGaAs and InAlAs, InGaAsP and InP, AlGaAsSb and AlAsSb or GaAs and AlGaAs. Gold is one material suitable for metallic mirrors. 
     Laser cavity  100  operates as follows. The active region  150  provides gain. When the active region  150  is pumped above the lasing threshold, the cavity  100  lases. This produces a lasing field  170  within the cavity and optionally also a laser output from the mirror(s). The longitudinal mode of the laser field  170  lies in the x direction; the transverse modes lie in the y and z directions. 
     The non-planar top mirror  130  is beneficial because it reduces optical losses in the cavity. Vertical cavities typically have low roundtrip gain and therefore require low losses in order to achieve threshold. Optical losses typically must be held to about 1% or less. The non-planar top mirror  130  can help reduce optical losses by a number of mechanisms. One source of loss is diffraction loss at the mirrors. This loss is particularly important for vertical cavities that have a small cross sectional area, for example cavities designed for single mode operation. Threshold currents typically increase dramatically as the lateral dimension of cavities is decreased. At communications wavelengths, single mode cavities typically have lateral dimensions of about 5 microns or less. By introducing some curvature to one of the mirrors, the diffraction loss can be reduced relative to a plano—plano cavity configuration. Another possible source of loss is scattering from rough surfaces. As will be described below, in some cases, the process for creating the non-planar top mirror  130  also tends to improve the surface quality of the mirror  130 , thus reducing this type of scattering. 
     Examples of devices that utilize a vertical laser cavity include vertical cavity surface emitting lasers (VCSELs) and vertical lasing semiconductor optical amplifiers (VLSOAs). In a VCSEL, the laser cavity generates a laser that is the output of the device. In a VLSOA, the laser field acts as a ballast to gain clamp the overall gain of the amplifier. 
       FIGS. 2–3  are cross-sections of different embodiments of the vertical laser cavity  100 . In  FIG. 2 , the bump  135  is formed by underfilling. In  FIG. 3 , the bump  135  is formed by depositing material. 
     The laser cavity  200  in  FIG. 2  is fabricated as follows. The process begins with an n-doped substrate  210 , which is produced using conventional crystal growing and doping techniques. In a first epitaxy stage, OMCVD is used to grow the following layers: the alternating layers of Bragg reflector  120 , n layer  220 , active region  150 , and p layer  230 . The wafer is then removed and a mask, typically either an oxide or nitride mask, is placed over selected areas of the wafer in order to define the lateral extent of the active region  150 . Unmasked areas are removed (e.g. using a bromine-based wet etch process), leaving the portions of layers  220 ,  150  and  230  shown. 
     In a second epitaxy stage, a current blocking layer  170  is grown (e.g. p-doped InP and n-doped InP pn blocking layers, or semi-insulating InP layers). This example is electrically pumped and the current blocking layer  170  confines the current to the active region  150 . No material grows on top of n layer  130  since it is still masked. The second epitaxy is underfilled so that the surface is not planarized, as shown in  FIG. 2A . This is the genesis of the bump  135 . The wafer is then removed in order to remove the mask. 
     In a third epitaxy stage, p layer  240  and Bragg layers for top mirror  130  are grown, resulting in the structure of  FIG. 2B . Due to the underfill of the second expitaxy stage, the Bragg layers are non-planar. The curvature of the top mirror  130  is smoother than the abrupt transition introduced by the underfill because the intervening layers smooth out the transition. For single mode cavities at communications wavelengths, the lateral mode typically is about 5 microns wide or less. Corresponding bumps  135  typically are wider (but not necessarily so) with a height of 0.1–1.0 microns, although actual optimum shapes will depend on specifics of the cavity. For example, in one embodiment for single mode applications at communications wavelengths, the genesis of the bump has a width of about 3 microns and the bump  135  itself has a width of about 5 microns. 
     In one particular design, the vertical laser cavity can be designed for use in either the 1.3 micron or the 1.55 micron window. The substrate  210  is n-doped InP. The Bragg reflector  120  is alternating layers of InP and InGaAsP. The n layer  220  is n-doped InP, the active region  150  is either InGaAsP or InAlGaAs, and the p layer  230  is p-doped InP. In one variation, layer  230  also includes a tunnel junction which is a backward diode that allows the change of carriers from n type to p type. For example, see U.S. Patent Application Serial No. 60/365,464, “Electrically Pumped Semiconductor Active Region with a Backward Diode, for Enhancing Optical Signals” filed by Jeffrey D. Walker et al. on the same date as this application, and which is incorporated by reference herein. In the embodiment where layer  230  is p doped only, the p layer  240  is p-doped InP and the top mirror  130  preferably is a p doped mirror of InP/InGaAsP. In the embodiment where  230  includes a tunnel junction, the layer  240  is n-doped InP and top mirror  130  preferably is an n doped mirror of InP/InGaAsP. The layers  220 ,  150 , and  230  are etched using a bromine-based wet etch process and then replaced (but underfilled) by current blocking layer  170 , which is a semi-insulating InP layer. In a different design, the current blocking layer  170  is implemented by a pn structure using p-doped InP and n-doped InP. In these examples, the genesis of the bump has a width of about 3–5 microns and a height of 0.2 microns, and the resulting bump  135  has a width of about 50 microns and a height of 0.2 microns. The distance between the top and bottom mirrors  130  and  120  is approximately 3–4 microns. The active region  150  is electrically pumped. 
     The curved top mirror  130  typically reduces diffraction losses due to a lens effect. In addition, the fabrication process described above can also result in a cleaner and smoother top mirror  130 , thus reducing scattering loss. Because of the curvature of the bump, irregularities and impurities tend to migrate towards the lower regions  134 , which are the regions away from the laser mode. 
     In  FIG. 3 , a primarily planar process is used to fabricate the vertical laser cavity. However, at some point in the processing, a “seed bump”  310  is introduced as shown in  FIG. 3A . As further layers are built up, the seed bump  310  results in the final bump  135  and curved top mirror  130 , as shown in  FIG. 3B . The dimensions given previously are also applicable to these types of bumps. 
     The seed bump  310  can be created in different ways. For example, standard photolithography and etching can be used to remove material on the sides, leaving the seed bump  310 . Alternately, material can be deposited in selected areas using conventional techniques, thus forming the seed bump  310 . The seed bump  310  can be used directly as the final bump  135 . The seed bump  310  may be the same or different material as the surrounding materials. 
     Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments will be apparent. For example, the invention is not restricted to the specific laser cavity designs shown here. Nor is it restricted to the particular fabrication techniques described. These are merely examples.