Abstract:
A folded cavity laser for generating a laser beam, includes a substrate provided with a distributed Bragg reflector (DBR); an active medium formed above the DBR for amplifying the laser beam; a first and a second mirrors formed on sides of the active medium, respectively, for making a horizontal cavity and for reflecting the amplified laser beam to the DBR; and a microlens, formed on the substrate opposite the DBR, for making the amplified laser beam astigmatic after passing therethrough.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a folded cavity laser; and, more particularly, to a folded cavity surface emitting laser by employing an astigmatic microlens which can correct the asymmetric beam shape due to the asymmetric of active medium.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    With popularization of Internet, a speedy optical communication network is actively under-construction and with increasing demands, the optical communication network spreads from a backbone network to an individual user. Now, cost competitiveness of a photo device is being a big barrier in populating the optical communication.  
           [0003]    A conventional edge-emitting laser can be easily manufactured, however, since a cavity of the conventional edge emitting laser is formed by cleaving its function must be measured after a packaging. Moreover, the conventional edge emitting laser has an astigmatism, meaning, a shape of an emitting beam has distorted by an asymmetry of a laser active medium, along to a horizontal and a vertical direction in an epitaxial layer. The distorted beam shape makes a mode coupling efficiency deteriorate and increases a packaging cost.  
           [0004]    A surface-emitting laser is a kind of laser devices, which complements the above problems, and has two types, i.e., a folded cavity type and a vertical cavity type.  
           [0005]    The folded cavity type makes 45° reflecting mirrors by using an ion etching and a chemical wet etching, instead of a cleaved facet, wherein the 45° reflecting mirrors emit a light beam to the epitaxial layer, vertically, thereby measuring an element characteristics in a state of wafer. However, the asymmetrical radiation angle problem is not solved still.  
           [0006]    In the vertical cavity type, a distributed Bragg reflector (hereinafter, referred to as a DBR) is grown top and bottom of the active medium, through epitaxial growth, so that an oscillation direction is being vertical on the epitaxial surface. Therefore, the vertical cavity type controls its beam shape to have a circular symmetry.  
           [0007]    Recently, an 850 nm vertical cavity surface emitting laser device has been developed.  
           [0008]    Actually, the progress of vertical-cavity surface-emitting laser development in 1.3 μm to 1.5 μm, which can be used over 300˜500 m, is slow. It is because, materials which are used in a long-wavelength laser have inferior thermal characteristics and the vertical cavity surface emitting laser requires a strict optical gain condition.  
           [0009]    Especially, in a 1.3 μm wave length, InGaAsN and InGaAsP materials are realized as an edge emitting laser device in GaAs and InP substrates, however, the vertical cavity surface emitting laser device using this material has not been reported yet and it shows manufacturing difficulties of the above-mentioned device.  
         SUMMARY OF THE INVENTION  
         [0010]    It is, therefore, an object of the present invention to provide a folded cavity laser which complements a radiation difference that is caused by an asymmetry of a laser mode, by integrating an astigmatic microlens with the laser device.  
           [0011]    Another object of the present invention is to present a folded cavity laser appropriate for mass production through a semiconductor manufacturing process without extra beam alignments, and provide a folded cavity laser which has improved characteristics comparable to those of the vertical-cavity surface-emitting laser.  
           [0012]    In accordance with an aspect of the present invention, there is provided a folded cavity laser for generating a laser beam, comprising: a substrate provided with a distributed Bragg reflector (DBR); an active medium formed above the DBR for amplifying the laser beam; a first and a second mirrors formed on sides of the active medium, respectively, for making a horizontal cavity and for reflecting the amplified laser beam to the DBR; and a microlens, formed on the substrate opposite the DBR, for making the amplified laser beam astigmatic after passing therethrough.  
           [0013]    In accordance with another aspect of the present invention, there is provided a folded cavity laser incorporating therein a microlens which is in the form of cylinder.  
           [0014]    In accordance with another aspect of the present invention, there is provided a folded cavity laser incorporating therein a microlens which is a polymer pattern, formed by a photolithography process.  
           [0015]    In accordance with another aspect of the present invention, there is provided a folded cavity laser incorporating therein mirrors which are inclined approximately 45° with respect to an optical axis of a horizontal cavity.  
           [0016]    In accordance with another aspect of the present invention, there is provided a folded cavity laser incorporating therein a laser active medium which is InGaAsN material and a substrate which is GaAs material.  
           [0017]    In accordance with another aspect of the present invention, there is provided a folded cavity laser incorporating therein an active medium which is any one selected from a group consisting of InGaAsP, AlInGaAs or InAs quantum dot, and a substrate which is InP material.  
           [0018]    In accordance with another aspect of the present invention, there is provided a folded cavity laser incorporating therein a microlens which is a GaAs pattern formed by patterning and etching a substrate.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1 is a cross-sectional view illustrating a light beam path in a laser device where an astigmatic microlens and an active medium are formed in accordance with the present invention;  
         [0021]    [0021]FIGS. 2A and 2B are perspective views showing an astigmatic microlens forming structure in accordance with a first preferred embodiment of the present invention;  
         [0022]    [0022]FIGS. 3A to  3 C are diagrams showing an astigmatic microlens forming process in accordance with a second preferred embodiment of the present invention; and  
         [0023]    [0023]FIGS. 4A and 4B are diagrams showing a light beam passing through a substrate in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Hereinafter, a folded cavity laser according to the present invention will be described in detail referring to the accompanying drawings.  
         [0025]    Referring to FIG. 1, a DBR layer  13  and an active medium  12 , made of InGaAsN which is formed by an epitaxial growing method are formed on a substrate  10 B. Preferably, the substrate  10 B is made of a material such as GaAs. An stigmatic microlens  14 , e.g., made of a polymer, is formed on a backside of the substrate  10 B. Meanwhile, a pair of reflecting mirrors are formed on sides of the active medium  12  in such a way that they are inclined at an angle, e.g., approximately 45°, with respect to an axis of the active medium  12 .  
         [0026]    As shown in FIG. 1, a light beam in the laser device is amplified due to an optical gain obtained by traveling the active medium  12  back and forth. Although a material such as InGaAsN can be grown on a GaAs substrate, it is difficult to form a vertical surface emitting laser because it has a 1.3 μm central frequency and a low optical gain. However, the material can meet a laser oscillation condition by extending the length of cavity to hundreds of μm as an edge emitting laser. By using the above-mentioned structure, InGaAsN can be grown on the GaAs substrate and meet a laser oscillation condition.  
         [0027]    The reflecting mirror  11  is formed on the active medium  12  by using an ion etching and a chemical wet etching in such a way that its side surfaces  11 A,  11 B are inclined at an angle, e.g., approximately 45°, with respect to the axis of the active medium  12 , whereby the light beam changes its path approximately 90° at the side surfaces  11 A,  11 B. The deflected light beam impinges onto a DBR layer  13 , which is formed under the active medium  12 , then a portion of the light beam is transmitted thereto in a direction normal to an interface between the DBR layer  13  and the substrate  10 B and the remaining portion of the light beam is reflected back to the side surfaces  11 A,  11 B.  
         [0028]    It is possible that the left side surface  11 A can be formed in such a way that it is inclined at an angle, e.g., 90°, with respect to the axis of the active medium  12 .  
         [0029]    In this case, the left side surface  11 A plays role as a cleaved facet. And also, in place of the DBR layer  13 , an air layer formed by an undercut etching can be used as a reflecting layer. As described above, a folded cavity is formed, whereby the light beam is emitted to a direction vertical to the substrate  10 B. At this time, since the DBR layer  13  has an asymmetric geometry, the light beam has a beam spot asymmetric after passing through the DBR layer  13 . To compensate for the above problem, an oval-shaped microlens  21  in FIG. 2A or a cylindrical microlens  22  in FIG. 2B are formed on a backside of the substrate  10 B.  
         [0030]    As described above, by integrating the astigmatic microlens into the substrate  10 , the asymmetric beam shape becomes a circular beam shape after passing through the astigmatic microlens, thereby increasing an optical fiber coupling efficiency. In accordance with the preferred embodiment, the astigmatic microlens can modulate a focal length of the light beam and may introduce directly the light beam into the optical fiber, so it reduces a packaging cost by replacing optical components, e.g., lenses for coupling the light beam into the optical fiber, arranged between the optical fiber and the folded cavity laser with the astigmatic microlens.  
         [0031]    Referring to FIG. 3A, a method of manufacturing the astigmatic microlens is described hereinafter in accordance with a preferred embodiment. First, a rectangular shaped polymethylgluterimide (hereinafter, referred to as a PMGI) pattern  32  is formed on a backside of the substrate by using a UV lithography. Particularly, the PMGI pattern  32  method can be implemented by following the manual of Microchem Corporation. After patterning the PMGI pattern  32 , a heat treatment is carried out at an oven at a temperature of approximately 290° C. during about 30 minutes for reflowing the PMGI pattern  32 . After the reflow, the rectangular shaped PMGI pattern  32  is changed to a curved surface pattern by a surface tension, as shown in FIG. 3B.  
         [0032]    At this time, a portion of the PMGI pattern  32  contacting with the substrate  10 B is not changed usually, however, a pedestal is formed by slightly etching the substrate  10  in the substrate  10 B to prevent the reflowed polymer from spreading to cope with a deformation problem of PMGI pattern  32 .  
         [0033]    The curved surface made by the above method has a cylindrical shape which has a large and a small curvatures to a longitudinal direction and a lateral direction, respectively. A polymer resist itself may be used as an astigmatic microlens, but the polymer resist may be transferred to the substrate  10  to increase mechanical intensity. That is, the polymer, such as PMGI, can be etched by an ion etching process, such as Cl, and the GaAs which is used in the substrate  10 , also can be etched by the Cl ion too, so the cylindrical shape can be transferred to the substrate  10 . The cylindrical shape of lens, which is transferred to the substrate  10  after carrying out an ion etching, is described in FIG. 3C.  
         [0034]    Likewise, before the reflow process, if the polymer pattern is not a rectangle shape but an oval shape, then the oval shaped astigmatic microlens  21  is formed after the reflow process, as shown in FIG. 2A.  
         [0035]    Also, the etch rate of the substrate  10  and the polymer can be controlled based on an asymmetrical shape of the polymer pattern and a condition of the ion etching so that differences of a focal length of the astigmatic microlens  21  are modulated. The cylindrical lens  22  is classified into any one of astigmatic lenses, which has an infinite focal length in sagital plane and a finite focal length in tangential (meridional) plane. The sagital plane corresponds to the junction plane before being deflected by 45° mirror. Thus, the beam has larger divergence angle in tangential plane than in sagital plane. Hereafter, we refer to the bundle of rays lying on the tangential plane as a tangential ray, and to the bundle of rays lying on the sagital plane as a sagital ray.  
         [0036]    Two examples will be presented for helping to understand the concrete implementation of this device. The first one deals with focusing laser emission into a fiber without external optics. The second one deals with the combination of the invention and an additional microlens to correct the beam shape.  
         [0037]    For the first example, if a tangential ray, vertical to a junction plane of the laser device, has an optical mode of 0.8 μm and a radiation angle of 30°, and a sagital ray, parallel to the junction plane of the laser device, has an optical mode of 1.6 μm and a radiation angle of 15°.  
         [0038]    If a thickness of the substrate  10  is 100 μm and the light beam is diverged in the substrate  10  with a half angle approximately 30°, a diameter of the light beam becomes approximately 110 μm at the astigmatic microlens  21 . When a diameter of the astigmatic microlens  21  is approximately 100 μm, the astigmatic microlens  21  can converge most of the light beam. Also, if a curvature of the astigmatic microlens  21  is approximately 52 μm, an image is focused on an image plane which is at a distance of 100 μm from the astigmatic microlens  21 . The height of the astigmatic microlens  21  corresponding to the curvature is approximately 50 μm, and this can be realized by using microlens manufacturing method.  
         [0039]    A size of a geometrical image, formed on the tangential plane is approximately 2.4 μm to an axis vertical thereto and approximately 4.8 μm to an axis parallel thereto. If a difference between two focal lengths of the astigmatic microlens  21  is approximately 2.0 μm at the junction plane, a symmetrical mode of 4.8 μm can be formed by designing the astigmatic microlens  21  to have a difference between focal lengths approximately 2.0 μm.  
         [0040]    After passing through the astigmatic microlens  21 , the light beam become increased in its diameter by approximately 3.0 μm due to diffraction. Therefore, the beam has a shape of a symmetrical mode within a range of 9.0 μm, which is an optical fiber diameter of a single mode, thereby increasing an optical coupling efficiency with the optical fiber.  
         [0041]    For the second example, we suggest a case of a cylindrical lens integrated on the back side of substrate, a curvature and a height thereof are 67 μm and 22 μm, respectively, a light beam may be collimated in tangential plane. In this case, the sagital ray has 50 μm of diameter and 15° of divergence angle when the light beam reaches the cylindrical lens. If the radiation angle increases about three times, e.g., 45°, due to a refractive index difference between the substrate  10  and an air, on the surface, the width of the sagital ray is became same as that of tangential ray after traveling 25 μm away from microlens normal to the substrate.  
         [0042]    In the above-mentioned location, by placing the cylindrical microlens  42  with its cylindrical axis perpendicular to that of microlens  41 , the sagital ray is collimated too. As a result, a symmetrical type of light beam is formed and the symmetrical type of light beam is focused by GRIN lens  43  into the optical fiber. Referring to FIG. 4B, a cylindrical microlens is fabricated on one side of the GRIN (graded refractive index) lens  45 , to simplify an optical system and an arrangement.  
         [0043]    In the second optical system, by using the oval microlens instead of the cylindrical microlens  42 , the light beam on the axis with small radiation angle decreases the radiation angle to provide mobility in the position of the lens, and increases allowed tolerance in the axis direction. Eventually, those lenses can be replaced with one microlens. Finally, the laser integrated with the cylindrical microlens may compensate for an astigmatism, which is generated from the laser, with usage of another astigmatic microlens, thereby increasing a coupling efficiency to the optical fiber and decreasing the number of optical components used in packaging.  
         [0044]    The present invention compensates for the difference of a radiation angle, which is caused by an asymmetry of the laser mode, by monolithically integrating an astigmatic microlens. The above-mentioned devices can be manufactured by semiconductor manufacturing process, and without separate optical alignments to each of the devices, thereby providing a laser device that the characteristics of the conventional folded cavity surface emitting laser improves as much as that of the vertical cavity surface emitting laser. Additionally, the symmetrical mode increases a coupling efficiency of a light beam of the optical fiber and reduces packaging cost by decreasing optical devices used in packaging and by increasing alignment tolerance. Also, the device can be characterized characteristic in wafer condition, which is an advantage of the surface emitting laser, and thereby increasing production yield.  
         [0045]    Especially, in case of the InGaAsN material, which can manufacture an edge-emitting laser but has difficulties in making the vertical cavity surface emitting laser due to low optical gain, by using the symmetrical modes, the InGaAsN material can use the edge-emitting laser manufacturing process by maintaining an advantage of the mode characteristic of the vertical cavity surface emitting laser.  
         [0046]    Moreover, the InGaAsN material takes advantage of GaAs substrate, such as processing on a large size wafer and good thermal conductivity of AlGaAs material.  
         [0047]    While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.