Abstract:
Vertical cavity optical devices, and a method of manufacturing therefor, are provided where the method includes partially forming a first vertical cavity optical device on a wafer, adjusting the lasing wavelength of the first vertical cavity optical device, and fixing the lasing wavelength of the first vertical cavity optical device to complete the forming thereof.

Description:
BACKGROUND  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to vertical cavity optical devices, and more specifically to fixed wavelength vertical cavity optical devices or arrays of vertical cavity optical devices.  
           [0003]    2. Background Art  
           [0004]    Vertical cavity surface emitting lasers (VCSELs) are revolutionizing the field of telecommunications. They generally consist of a pair of semiconductor mirrors defining a resonant cavity containing a gain medium of semiconductor materials for amplifying light.  
           [0005]    VCSELs have relatively high efficiency, small size, low weight, low power consumption, and the capability to be driven by low-voltage power. They can operate in a single longitudinal mode, or frequency, and produce a circular beam that can easily be coupled into fibers. The surface emission feature allows devices to be packed densely on a wafer, so that two-dimensional arrays are fabricated relatively easily.  
           [0006]    VCSELS use semiconductor materials comprised of elements such as aluminum, indium, gallium, arsenic, nitrogen, and phosphorous as the gain medium, and alternating high and low index of refraction materials such as silicon and silicon dioxide for the semiconductor mirrors or distributed Bragg reflectors (DBRs).  
           [0007]    The lasing wavelength of a VCSEL is determined by the optical height of its resonant, Fabry-Perot cavity. Most commonly the optical cavity height, and thus the wavelength, is determined by the thicknesses of the semiconductor layers in the devices. These thicknesses are set during the growth of the semiconductor layers and are nominally the same for all the lasers on given wafer.  
           [0008]    The resonant cavity of some VCSELs also includes an air gap, where the size of the air gap partly determines the output wavelength of the laser.  
           [0009]    Alignment of the wavelength of maximum gain for the laser gain media and the cavity modes of the laser, set by the optical cavity height of the structure significantly increases the difficulty of producing VCSELs. This is a big yield problem in VCSEL manufacturing because the optical cavity height is permanently set during the material fabrication process making it impossible to adjust later. Also, this limits the temperature range of useful performance for VCSELs since the optical cavity height changes with temperature due to the thermal expansion of the laser material.  
           [0010]    An array of monolithic multiple-wavelength VCSELs requires side-by-side fabrication of VCSELs on a wafer where the VCSELs need to be exactly the same except with controlled, different lasing wavelengths. This presents a problem because the processing used on the wafer must assure that the threshold gain at which lasing begins, the current usage, the efficiency, the losses of light in the resonant cavity, the amplification of the gain material, and the light transmission of the DBR all remain the same. At the same time, the same processing must produce different lasing wavelengths, which is most commonly realized by changing the optical height of the resonant cavity.  
           [0011]    Solutions to these problems have been long sought but have long eluded those skilled in the art.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides vertical cavity optical devices, and a method of manufacturing therefor, where the method includes partially forming a first vertical cavity optical device on a wafer, adjusting the lasing wavelength of the first vertical cavity optical device, and fixing the lasing wavelength of the first vertical cavity optical device to complete the forming thereof. In this way, the Fabry-Perot resonance and the semiconductor gain-peak can be aligned subsequent to the fabrication of the semiconductor material.  
           [0013]    The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a cross-sectional view of a vertical cavity surface emitting laser (VCSEL) in accordance with the present invention;  
         [0015]    [0015]FIG. 2 is a cross-sectional view of a monolithic array of VCSELs of the present invention in one stage of manufacturing;  
         [0016]    [0016]FIG. 3 is a cross-sectional view of the monolithic array of VCSELs in accordance with the present invention;  
         [0017]    [0017]FIG. 4 is a schematic illustration of how an optical communication system is assembled in accordance with the present invention; and  
         [0018]    [0018]FIG. 5 is a simplified flow chart of the method of manufacturing the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    For purposes of the present invention, the vertical resonant cavity device is referred to as a vertical cavity optical device (VCOD). This is because in addition to VCSELs the invention can also be used to make detectors, Fabry-Perot filters, or other optical devices that require a range of resonant cavity wavelengths.  
         [0020]    In addition, for purposes of convenience, the term “horizontal” as used in herein is defined as a plane parallel to the conventional plane or surface of a wafer upon which the VCODs are formed regardless of the orientation of the wafer. The term “vertical” or “thickness” refers to a direction perpendicular to the horizontal as just defined. Terms, such as “on”, “above”, “below”, “upper”, “lower”, “over”, and “under”, are defined with respect to the horizontal plane. The descriptions of the positions of the various elements in the various embodiments are not intended to be limiting and many other positions would be evident to those skilled in the art.  
         [0021]    Referring now to FIG. 1, therein is shown a cross-sectional view of a vertical cavity surface emitting laser or VCSEL  20  in accordance with the present invention. As explained above, the VCSEL is only one example of a VCOD. Above a substrate  22  is a lower mirror or distributed Bragg reflector (DBR)  24 , which consists of a plurality of alternating high and low index of refraction materials. The DBR  24  can be made from a number of different combinations of materials including semiconductor layers, dielectric materials such as TiO 2  and SiO 2 , or hybrid combinations of semiconductor, dielectric and metal layers. Each of the layers has an optical thickness of ˜¼ of a wavelength.  
         [0022]    Above the DBR  24  is a vertical resonant cavity  26 , which consists of a photoactive semiconductor structure  28  and a fixative material  30 . The photoactive semiconductor structure contains a quantum well for providing laser light. The fixative material  30  fills the space above the photoactive semiconductor structure  28  and below a reflector support  32 , which is above the photoactive semiconductor structure  28  and held by an anchor  34 . Mounted on and above the reflector support  32  is an upper mirror or DBR  36 , which has exactly the opposite arrangement of alternating layers of λ/4 thick low and high index of refraction materials from the DBR  24  so as to cause reflection of light from the DBR  36  back to the DBR  24 . Depending upon the relative reflectivity of the DBRs  24  and  36 , light will be emitted upward through the DBR  36  and the fixative material  30  or downward through the DBR  24  and the substrate  22 .  
         [0023]    The VCSEL  20  also includes an electrode  38  connected to the photoactive semiconductor substrate  28  and an electrode  40  connected to a capacitive conductor  42 , which are used during the manufacturing process.  
         [0024]    Referring now to FIG. 2, therein is shown a cross-sectional view of a partly formed monolithic array of VCSELs  60  of the present invention in one stage of manufacturing. By way of example, first, second, and third VCSELs  52 ,  54 , and  56  have been partially formed on a monolithic substrate. A common substrate  62  carries a lower DBR  64 , and a photoactive semiconductor structure  66 . A reflector support  68  has been formed to define a plurality (three in the present example) of air gaps  70 - 72  and is held in place by an anchor  76 . Mounted on and above the reflector support  68  are a plurality of upper DBRs  80 - 82 , which are respectively over the air gaps  70 - 72 . Adjacent to each of the upper DBRs  80 - 82  is a capacitive conductor  84 - 86 , respectively, having an electrode  88 - 90 , respectively.  
         [0025]    The lasing wavelength of the first, second, and third VCSELs  52 ,  54 , and  56  associated with the upper DBRs  80 - 82  is capable of being tuned by adjusting the heights of the respective air gaps  70 - 72  and this can be accomplished by imposing a voltage across the photoactive semiconductor structure  66  and the respective arms  88 - 90 .  
         [0026]    During the manufacturing process in accordance with the present invention, a voltage generator  92  is connected to a common electrode  94 , which is connected to the photoactive semiconductor structure  66 , and to the electrodes  88 - 90 . Different voltages are applied to provide different height air gaps  70 - 72  and a structure is used to hold the different height air gaps  70 - 72 . For example, a fixative material is introduced into the air gaps  70 - 72  to fix the air gap heights and therefore the lasing wavelengths.  
         [0027]    Referring now to FIG. 3, therein is shown a cross-sectional view of a completed monolithic array of VCSELs  95  in accordance with the present invention. The same numbers represent the same elements as in FIG. 2.  
         [0028]    The first, second, and third VCSELs  52 ,  54 , and  56  have been set to different lasing wavelengths by adjusting the air gaps  70 - 72  of FIG. 2 to different heights H1, H2, and H3, and filling them with fixative material  96 ,  97 , and  98 . As shown, the fixative material  96  and  97  are transparent, such as a transparent epoxy, and the fixative material  98  is shown as an annular ring of fixative material, which could be an opaque epoxy with an open center or an opaque epoxy with a transparent epoxy center so the center lasing area is kept clear.  
         [0029]    By selection of the fixative material with the proper thermal expansion coefficients, it is possible to eliminate or at least minimize the effects of thermal expansion, which cause changes in the lasing wavelength of typical VCSELs.  
         [0030]    Referring now to FIG. 4, therein is shown a schematic illustration of how an optical communication system  100  is assembled in accordance with the present invention. A wafer  102  will have a plurality of VCSELs formed on it in accordance with the present invention. A portion having a monolithic array  104  of VCSELs from the wafer  102  will be packaged and used with a fiber optic communication system represented by an optical fiber  106 .  
         [0031]    Referring now to FIG. 5, therein is shown a flow chart  120  of a method for manufacturing the optical communication system  100  according to the present invention. A first step  122  is forming the VCSELs  52 ,  54 , and  56  on a wafer  62 . A second step  124  is adjusting the lasing wavelengths of the VCSELs  52 ,  54 , and  56  by changing the air gaps. A third step  126  is fixing the lasing wavelengths by filling the air gaps with the fixative material  96 - 98 . A fourth step  128  involves forming the monolithic array  104 . And a fifth step  130  is placing the monolithic arrays  104  in the optical communication system  100 .  
         [0032]    It will be understood that the flow chart  120  is merely exemplary and many other steps may be added and some removed as would be evident to those having ordinary skill in the art from a reading of the above disclosure.  
         [0033]    While the invention has been described in conjunction with a specific best mode, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit of the included claims. All matters hither-to-fore set forth herein or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.