Abstract:
A surface emitting diode, such as a laser, including an active region positioned between first and second semiconductor layers and extending longitudinally. The active region and at least portions of the first and second semiconductor layers defining first and second facets positioned at opposite ends of the length with the first facet defining a light output for the active region. The active region is adjusted to emit a single mode of light. A reflective element is positioned adjacent to the first facet and at an angle with the first facet for receiving light output from the active region and directing the light perpendicular to the active region.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to lasers and more particularly to lasers that generate relatively long wavelengths.  
         BACKGROUND OF THE INVENTION  
         [0002]    Vertical cavity surface emitting lasers (VCSELs) include first and second mirror stacks formed on opposite sides of an active area. The active area generally includes one or more quantum wells capable of generating light as electrical carriers are supplied. Each mirror stack includes a plurality of pairs of mirrors designed to reflect a portion of light generated in the active area back into the active area for regeneration. The pairs of mirrors in the mirror stacks are formed of a material system generally consisting of two materials having different indices of refraction to provide the reflectivity.  
           [0003]    VCSELs have a number of advantages over edge emitting and other types of lasers. One advantage is that a VCSEL emits a circular beam of light which is easier to use or direct through normal optics. Further, VCSELs have a low operating threshold and can be tested on-wafer to reduce costs expended on failed packaged units. However, manufacturing VCSELs that operate in a range including, for example, 1300 nm and 1550 nm is very difficult because of poor monolithic reflectivity at these wavelengths.  
           [0004]    In conventional VCSELs, which operate in the 760 nm to 1050 nm range, conventional material systems such as AlGaAs perform adequately. However, for VCSELs outside of this range, other material systems, whose overall performance is poorer, must be used. For example, longer-wavelength light can be generated by using a VCSEL having an InP-based active region. When an InP-based active region is used, however, the epitaxial DBRs lattice matched to the supporting substrate and the active region do not provide enough reflectivity for the VCSELs to operate because of the insignificant difference in the refractive indices between the two DBR constituents. Dielectric mirror stacks can be used instead, but they suffer from poor thermal conductivity. Since the performance of these long-wavelength materials is very sensitive to temperature, the thermal conductivity of the mirror stacks is very important.  
           [0005]    Accordingly it is highly desirable to provide a surface emitting long wavelength laser which is easy and inexpensive to manufacture.  
           [0006]    It is an object of the present invention to provide new and novel surface emitting long wavelength lasers.  
           [0007]    It is another object of the present invention to provide surface emitting long wavelength lasers with short cavities to produce single longitudinal mode operation.  
           [0008]    It is another object of the present invention to provide surface emitting long wavelength lasers that are easy to manufacture.  
           [0009]    It is another object of the present invention to provide surface emitting long wavelength lasers that do not require expensive and complicated epitaxially grown mirrors.  
           [0010]    It is still another object of the present invention to provide surface emitting long wavelength lasers that are tailored to enhance a desired or single longitudinal mode of operation.  
           [0011]    It is a further object of the present invention to provide surface emitting long wavelength lasers with surface emission designed to emit a circular beam.  
         SUMMARY OF THE INVENTION  
         [0012]    To achieve the objects and advantages specified above and others, a single mode surface emitting laser is disclosed which includes an active region positioned between first and second semiconductor layers so as to extend longitudinally parallel to the surface of a substrate. The active region and at least portions of the first and second semiconductor layers define first and second facets positioned at opposite ends of the length with the first facet defining a light output for the active region. The active region is adjusted to emit a single mode of light. A reflective element is positioned adjacent to the first facet and at an angle with the first facet for receiving light output from the active region and directing the light at an angle to the active region.  
           [0013]    In a preferred embodiment, the active region is adjusted to produce a single mode of light by forming the active region with a predetermined length. For example, a single mode surface emitting laser is constructed with an active region length such that the wavelength difference between adjacent modes of operation is sufficient to provide single mode operation. The wavelength difference, Δλ, is defined by the equation 
           Δλ=(λ 2 /2 nL ) Δ m,   
           [0014]    wherein L is the length of the active region cavity, n is the effective index of refraction of the active region cavity, λ is the operating wavelength of the active region cavity, and Δm is the difference between the different order of modes of operation. When Δm=1, Δλ represents the wavelength difference between the adjacent modes.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Referring to the drawings:  
         [0016]    [0016]FIG. 1 is a simplified sectional view of a short cavity surface emitting laser in accordance with the present invention;  
         [0017]    [0017]FIGS. 2, 3, and  4  are simplified sectional views of other embodiments of short cavity surface emitting lasers with modified reflecting surfaces, in accordance with the present invention;  
         [0018]    [0018]FIGS. 5, 6,  7 , and  8  are sectional views illustrating sequential steps in a method of fabricating a short cavity surface emitting laser in accordance with the present invention;  
         [0019]    [0019]FIG. 9 is a simplified sectional view of another embodiment of a short cavity surface emitting laser in accordance with the present invention;  
         [0020]    [0020]FIG. 10 is a graphical representation of the band pass of a portion of a filter used in the short cavity surface emitting laser of FIG. 9; and  
         [0021]    [0021]FIG. 11 is a simplified sectional view of another embodiment of a short cavity surface emitting laser in accordance with the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    Turning now to FIG. 1, a simplified sectional view is illustrated of a single mode surface emitting laser  10  in accordance with the present invention. Here it should be understood that, while the preferred embodiment of the present invention is a laser (sometimes referred to as a laser diode), the structure is basically a diode and light emitting diodes, other than lasing diodes, are intended to be included in the disclosure. Laser  10  includes a substrate  12  depicted by a broken line. An area  14  can include a buffer layer for lattice matching the substrate to the subsequent structure. A first cladding layer  15  is epitaxially grown on area  14 , an active area  16  is grown on first cladding  15 , and a second cladding layer  17  is grown on active area  16 . In this embodiment, cladding layers  15  and  17  and active area  16  are referred to generally as an active region. However, it will be understood that many different configurations can be used to produce an active region including quantum wells, quantum dots, etc. and a variety of layers may be used to provide the waveguiding function of cladding layers  15  and  17 . Also, while a single layer  16  is illustrated for the active area, it will be understood that the active area can consist of multiple layers. A cap layer  18  is grown on  17  to complete and protect the structure.  
         [0023]    Two facets  20  and  21  are formed on either end of the active region to define a length L. Facets  20  and  21  and cladding layers  15  and  17  cooperate to produce a lasing action within active region  16  such that a beam of light is emitted through facet  21 . The beam of light emitted through facet  21  generally has an elliptical cross section.  
         [0024]    Laser  10  usually operates at many different modes if the mode spacing is small and many cavity modes can be supported by the gain profile of the active region  16 , but the length L of active area  16  can be adjusted to allow single mode operation. A single mode surface emitting laser can be constructed by reducing the cavity length such that the wavelength difference between adjacent modes of operation is large enough, compared to the width of the gain profile, such that only one cavity mode is supported by the gain of the active region  16 . In this way, a laser with a single longitudinal mode of operation, is produced. Here it should be understood that the term “single longitudinal mode operation” describes the fact that the wavelength spacing of the adjacent modes is increased sufficiently so that amplification occurs only for one selected single mode. In this specific embodiment, the mode selection (i.e. adjacent mode spacing Δλ) is accomplished by adjusting the length L of the active region according to the equation 
         Δλ=(λ 2 /2 nL ) Δ m,   
         [0025]    where L is the length of the active region cavity, n is the index of refraction of the active region cavity, λ is the operating wavelength of the active region cavity at a given mode, and Δm is the difference between the different order of modes of operation. For the adjacent modes, Δm=1. Therefore, reducing the length L of active area  16  increases the mode wavelength spacing, Δλ, between adjacent modes. Generally, the length L of active area  16  can be reduced to less than 50 μm to provide a laser to operate in a single longitudinal mode, at a wavelength of around 1300 nm or 1550 nm.  
         [0026]    As a specific example, if Δm=1, λ=1300 nm, n=3.5, and L=50 μm, using the above equation Δλ=4.8 nm. By reducing the length L to 25 μm, the mode wavelength spacing (Δλ) increases to 9.6 nm. The width of the gain profile of the active region  16  can be around 10-20 nm. Therefore, a carefully designed laser with the right cavity length will have only one cavity mode located near the center of the gain profile, while the adjacent mode will be at the edge or outside of the gain profile. Only the cavity mode near the center of the gain profile will be amplified and lase.  
         [0027]    A reflective element  25 , which in this embodiment is illustrated as a flat mirror surface, is positioned adjacent to first facet  21  and at an angle with first facet  21 . In this embodiment, element  25  is formed as an integral part of the structure by etching, selective deposition, or photolithography method, etc. Also, element  25  is oriented at a 45 degree angle such that light emitted by active area  16  is directed normal to the length of the active region. Also, it will be understood that because a flat mirror is used, the reflected beam will have an elliptical cross section (assuming the laser generates a beam with an elliptical cross-section).  
         [0028]    Referring additionally to FIG. 2, a single mode surface emitting laser similar to that described in FIG. 1 is illustrated. However, in this embodiment, a reflective element  27  having a curved surface is included. The surface of reflected element  27  is curved such that the elliptical beam emitted from active area  16  is converted to a beam of a circular cross section. The curve of reflective element  27  is easily obtained and may, for example, be similar to the curve in a standard converging lens.  
         [0029]    Referring additionally to FIG. 3, another embodiment of a single mode surface emitting laser similar to that described in FIG. 1 is illustrated. Opposed facets  20  and  21  are formed in the epitaxially grown layers, generally as described above. In a preferred embodiment, a microcleaving process is used to form facets  20  and  21 , but other forms of cleaving and/or etching and polishing can be used if desired. In this embodiment, the area for the reflecting element is cleaved or etched to provide a flat surface  30 . Flat surface  30  is designed to receive an easily etchable or formable material  32 . For example, a polymer material (e.g. photoresist SU-8) may be used and exposed at a 45 angle to define the reflective surface. The photoresist can then be cured and a reflective material can be deposited thereon. A reflective surface  34  is then formed in material  32  by any convenient method including developing, etching, chemical mechanical polishing, etc. In some instances is may be convenient to deposit a thin reflective layer (e.g. such as metal) on surface  34  to enhance the reflective qualities. In a further example, illustrated in FIG. 4, a reflective element  40  is formed on flat or curved surface  34  using E-beam evaporation technology.  
         [0030]    Referring to FIGS. 5 through 8, a specific example of one method of fabricating a single mode surface emitting laser is illustrated. Referring specifically to FIG. 5, a first substrate  50  is depicted by a broken line. An area  54  can include a buffer layer for lattice matching the substrate to the subsequent structure. A first cladding layer  55  is epitaxially grown on area  54 , an active area  56  is grown on first cladding  55 , and a second cladding layer  57  is grown on active area  56 . In this embodiment, cladding layers  55  and  57  and active area  56  are referred to generally as an active region. However, it will be understood that many different configurations can be used to produce an active region including quantum wells, quantum dots, etc. and a variety of layers may be used to provide the waveguiding function of cladding layers  55  and  57 . Also, while a single layer  56  is illustrated for the active area, it will be understood that the active area can consist of multiple layers. A cap layer  58  is grown on  57  to complete and protect the structure. Opposed facets  51  and  52  can be formed by cleaving, microcleaving, etching, etc.  
         [0031]    Referring additionally to FIG. 6, a second substrate  60  is illustrated. Substrate  60  is etched or otherwise formed to define a reflective surface  62  and a flat mounting surface  64 . In a preferred embodiment, reflective surface  62  is curved so as to convert an elliptical beam to a circular beam as described above. It will be understood that reflective surface  62  can be flat or any desired shape to provide the desired beam shape and direction. It should also be understood that the reflective surface  62  can be formed separately on a different structure and positioned on flat mounting surface  64 . For example, reflective surface  62  can be formed with photoresist, exposed at a 45 angle, developed, cured, and coated with a reflective material. A layer of electrically conductive material  65  is deposited on mounting surface  64  and, as will be described below, is used as an external electrical contact for the laser.  
         [0032]    Referring additionally to FIG. 7, the surface of cap layer  58  in the structure of FIG. 5 is bonded to the surface of the contact material  65  so that light emitted from active area  56  is directed onto reflective surface  62 . In addition, a portion of contact material  65  remains exposed to provide an external electrical contact to the capped layer of the laser  58 . Substrate  50  is then removed, as illustrated in FIG. 8, by etching, chemical mechanical polishing, etc., to allow the light beam redirected by surface  62  to be emitted at an angle relative to active region  56 . A second contact metal  66  is deposited on the surface of the laser exposed by removal of substrate  50 . It should be understood that reflective element  62  can be formed directly on substrate  50  of FIG. 5 to form the structure in FIG. 8. Wafer bonding and substrate removal process is not necessary.  
         [0033]    Referring now to FIG. 9, a single mode surface emitting laser  70  is illustrated, generally as described above. Laser  70  has opposed facets  71  and  72  defining the length L. To reduce the laser threshold current, facets  71  and  72  are coated with dielectric materials  75  and  76 , respectively, which increase the reflectivity of facets  71  and  72 . As the length L is reduced, the gain of laser  70  is reduced, but by adding the coating more light is reflected and amplified because the facet losses decrease. At least one of the coatings  75  or  76  can be narrow band, as illustrated in FIG. 10. The coating bandwidth Δλ coating  is best designed to be smaller than the mode wavelength spacing Δλ, which substantially eliminates adjacent modes. While in the embodiment illustrated in FIG. 9, coating  76  could be narrow band, either or both of the coatings  75  and  76  could be narrow band in other embodiments.  
         [0034]    Referring now to FIG. 11, a coupled cavity single mode surface emitting laser  80  is illustrated. Laser  80  includes a substrate  82  having a mounting surface  83  on which is positioned a first cavity  84  and a second cavity  85 . First cavity  84  and second cavity  85  are each similar to the active region described in conjunction with FIG. 1. Also, cavities  84  and  85  are spaced apart to provide a gap  86  there between. By correctly spacing cavities  84  and  85 , gap  86  forms a Fabry-Perot resonator with only one wavelength transparent due to resonance. This gap provide a filter effect equivalent to FIG. 10. Thus, only one longitudinal mode of operation will be supported that is resonant to this coupled cavity. Here it will be understood that laser  80  can be fabricated in an embodiment similar to laser  10  of FIG. 1 by forming the active area with two facets  87  and  88  and forming gap  86  between the facets.  
         [0035]    In a different embodiment, an array of coupled cavity lasers (e.g. each similar to laser  80 ) can be made on one wafer. One coupled cavity laser is resonant to λ 1  and emits light at the wavelength λ 1  by setting the gap  86  to one appropriate spacing, while the adjacent coupled cavity laser is resonant to λ 2  and emits light at λ 2  by setting the gap  86  to a different spacing. Thus, an array of lasers with different gap spacings can be provided to obtain lasers of varying wavelength for multi-signal operation, such as wave division multiplexing.  
         [0036]    Thus, single mode surface emitting lasers have been disclosed which generate longer-wavelength light and which are easy to manufacture because there are only a few layers to grow (no DBR stacks). An In-based active region can be used to emit long wavelength light because high reflectivity stacks of mirrors are not required. Also, because high reflectivity stacks are not required, electrical contacts and heatsinking are relatively easy to make. Thus, a novel surface emitting long wavelength laser with short cavities to produce single longitudinal mode operation is disclosed. Also, the present surface emitting long wavelength laser is easy to manufacture. Several of the lasers can be combined into an array to provide multi-signal operation, such as WDM.  
         [0037]    While we have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. We desire it to be understood, therefore, that this invention is not limited to the particular forms shown and we intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.