Abstract:
A vertical external cavity surface emitting laser (VECSEL) using end pumping in which a pumping beam is recycled using a pumping beam reflection layer to increase pumping beam absorption is provided. The VECSEL includes: an active layer for generating and emitting signal light; an external mirror that is separated from and faces a top surface of the active layer and transmits a first portion of the signal light and reflects a second portion of the signal light to the active layer; a first reflection layer contacting a lower surface of the active layer and reflecting the signal light to the external mirror; a pump laser for emitting the pumping beam toward the lower surface of the active layer to excite the active layer; and a second reflection layer contacting the top surface of the active layer and reflecting a portion of the pumping beam back to the active layer.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2005-0107031, filed on Nov. 9, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The present disclosure relates to an external cavity surface emitting laser, and more particularly, to an end pumping external cavity surface emitting laser in which pumping beam is recycled using a pumping beam reflection layer to increase absorption of the pumping beam by an active layer. 
     2. Description of the Related Art 
     A vertical cavity surface emitting laser (VCSEL) oscillates in a single longitudinal mode of a very narrow spectrum and emits a beam having a small radiation angle. VCSELs can be integrated easily with other devices, but the output of the VCSELs is low. 
     A vertical external cavity surface emitting laser (VECSEL) is a high output laser with the above-described advantages of the VCSEL. The VECSEL has an external mirror instead of an upper mirror, resulting in an increased gain region, and can thus output several to dozens of watts of light. 
       FIG. 1  is a schematic view of a VECSEL  10 . The VECSEL  10  is a front pumping laser in which light is pumped by a pump laser  15  which is disposed obliquely in the front of the VECSEL  10 . As illustrated in  FIG. 1 , the VECSEL  10  includes a heat sink  11 , a Distributed Bragg Reflector (DBR) layer  13  and an active layer  14  sequentially stacked on the heat sink  11 , an external mirror  17  that faces the active layer  14  and is separated a predetermined distance from the active layer  14 , and a pump laser  15  disposed obliquely toward the top surface of the active layer  14 . A heat spreader  12  may be further formed on the top surface of the active layer  14  to spread the heat generated by the active layer  14 , and a second harmonic generation (SHG) crystal  18  which doubles the frequency of the light output may be placed between the active layer  14  and the external mirror  17 . Also, the VECSEL  10  includes a collimating lens  16  that collimates the pumping beam emitted from the pump laser  15 . For instance, the active layer  14  may have a multiple quantum well structure having a resonant periodic gain (RPG) structure and is excited by the pumping beam to emit light with a predetermined wavelength λ 2 . The pump laser  15  inputs a pumping beam having a shorter wavelength λ 1  than the wavelength λ 2  of the light emitted from the active layer  14  to excite the active layer  14 . 
     In the above described configuration, a pumping beam with a relatively short wavelength λ 1  emitted from the pump laser  15  is incident on the active layer  14 , and the active layer  14  is excited to emit light with a predetermined wavelength of λ 2 . The emitted light is reflected repetitively between the DBR layer  13  and the external layer  17  through the active layer  14 . Thus, a portion of the light amplified in the active layer  14  is output to the outside via the external mirror  17 . When the SHG crystal  18  is interposed between the active layer  14  and the external mirror  17 , for example, light in the infrared region emitted from the active layer  14  is converted into visible light and then output. 
       FIG. 2  is a schematic view of a conventional VECSEL  20  using end pumping. In the VECSEL  10  using front pumping illustrated in  FIG. 1 , the incident surface of the pumping beam in the active layer  14  and the emission surface of the output light are the same. That is, a pumping beam is incident through the top surface of the active layer and the output light is emitted through the top surface of the active layer  14 . On the other hand, as illustrated in  FIG. 2 , in the VECSEL  20  using end pumping, a pumping beam is incident through the lower surface of the active layer  23  and the output light is emitted through the top surface of the active layer  23  to an external mirror  25  facing the active layer  23 . For example, a DBR layer  22  and an active layer  23  are stacked sequentially on a light transmissive heat spreader  21  which is formed of diamond or silicon carbide (SiC), and a pump laser  24  faces the active layer  23  with the light transmissive heat spreader  21  interposed therebetween. Accordingly, a pumping beam emitted from the pump laser  24  passes through the light transmissive heat spreader  21  and is incident on the lower surface of the active layer  23 . Such end pumping enables easy assembly of the VECSEL and reduces light loss since a pumping beam is perpendicularly incident on the active layer. 
     However, in the conventional VECSEL, a pumping beam emitted from the pump laser may not be completely absorbed in the active layer, and a portion of the pumping beam is dispersed by the heat sink or passes through the active layer and then emitted. In VECSELs using front pumping, a portion of the pumping beam which is not completely absorbed in the active layer passes through the DBR layer and is wasted. Also, in VECSELs using end pumping, a portion of the pumping beam which is not absorbed by the active layer is emitted through the top surface of the active layer. Accordingly, conventional VECSELs cannot efficiently use the energy of the pumping beam, and thus have low efficiency. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure may provide a vertical external cavity surface emitting laser (VECSEL) using end pumping in which a pumping beam emitted from a pump laser is recycled to increase the pumping beam absorption in an active layer. 
     According to an aspect of the present invention, there may be provided a VECSEL comprising: an active layer for generating and emitting signal light; an external mirror that is separated from and faces a top surface of the active layer, and is adapted to transmit a first portion of the signal light generated by the active layer and to reflect a second portion of the signal light to the active layer; a first reflection layer positioned beneath a lower surface of the active layer and adapted to reflect the signal light generated by the active layer to the external mirror; a pump laser for emitting a pumping beam toward the lower surface of the active layer through the first reflection layer, the pumping beam being adapted to excite the active layer; and a second reflection layer positioned on the top surface of the active layer and adapted to reflect a portion of the pumping beam transmitted through the active layer back to the active layer. 
     The second reflection layer may be adapted to transmit the signal light emitted from the active layer. 
     The second reflection layer may be a Distributed Bragg Reflector comprising two semiconductor layers with different refractive indexes alternately stacked. 
     The two semiconductor layers with different refractive indexes may be an Al x Ga 1-x As layer and an Al y Ga 1-y As layer, where 0≦x,y≦1, x≠y. The thickness of each of the semiconductor layers may be approximately one fourth of the wavelength of the pumping beam. When H refers to the semiconductor layer with a first refractive index and L refers to the semiconductor layer with second refractive index and the first refractive index is higher than the second refractive index, the second reflection layer may have a multi-layer structure of ((L/2)H(L/2)) N , where N is a positive integer. 
     The second reflection layer may be a dielectric stack reflection layer with a multi-layer structure in which two dielectric layers with different refractive indexes are alternately stacked. The thickness of each of the dielectric layers forming the second reflection layer may be approximately one fourth of the wavelength of the pumping beam. 
     The second reflection layer may be a metal layer. The thickness of the metal layer may be 50 nm or less. 
     The reflectance with respect to the pumping beam of the second reflection layer may be more than 50%. 
     An anti-reflection layer positioned on the top surface of the second reflection layer and adapted to enhance the transmission of the signal light through the second reflection layer may be further included. 
     The first reflection layer may be adapted to transmit the pumping beam emitted from the pump laser and reflect the signal light emitted from the active layer. 
     The first reflection layer may be a DBR layer with a multi-layer structure comprising two semiconductor layers with different refractive indexes alternately stacked. The thickness of each of the semiconductor layers forming the first semiconductor layer may be approximately one fourth of the wavelength of the signal light. 
     A light transmissive heat spreader positioned beneath the lower surface of the first reflection layer and adapted to cool down the active layer may be further included. The light transmissive heat spreader may be formed of a material selected from the group consisting of diamond, silicon carbide (SiC), aluminum nitride (AlN), and gallium nitride (GaN). 
     A second harmonic generation (SHG) crystal that doubles the frequency of the signal light emitted from the active layer and is interposed between the active layer and the external mirror may be further included. 
     The active layer may comprise a plurality of quantum well layers, and each of the quantum well layers may be disposed in an anti-node of a standing wave generated by the signal light resonating between the external mirror and the first reflection layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will be described in detailed exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a schematic view of a conventional vertical external cavity surface emitting laser (VECSEL) using front pumping; 
         FIG. 2  is a schematic view of a conventional VECSEL using end pumping; and 
         FIG. 3  is a schematic view of a VECSEL with a pumping beam reflection layer according to an embodiment of the present invention; 
         FIG. 4  illustrates the recycling process of a pumping beam by the pumping beam reflection layer shown in  FIG. 3 ; 
         FIGS. 5 and 6  are graphs illustrating reflectivity according to wavelength of a pumping beam in the pumping beam reflection layer shown in  FIG. 3 ; 
         FIG. 7  is a graph illustrating an increase in the absorption of the pumping beam in an active layer due to a pumping beam reflection layer according to an embodiment of the present invention; and 
         FIG. 8  is a graph illustrating an increase in the output of a VECSEL according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. 
       FIG. 3  is a schematic view of a VECSEL  30  with a pumping beam reflection layer according to an embodiment of the present invention. Referring to  FIG. 3 , the VECSEL  30  uses end pumping and includes a pumping beam reflection layer to reflect a pumping beam. That is, the VECSEL  30  according to an embodiment of the present invention includes an active layer  33  emitting signal light having a predetermined wavelength λ 2 , an external mirror  36  separated from and facing the top surface of the active layer  33 , a signal light reflection mirror (layer)  32  contacting the lower surface of the active layer  33  to reflect the signal light generated by the active layer  33  to the external mirror  36 , a pump laser  35  emitting a pumping beam with a wavelength λ 1  toward the lower surface of the active layer  33  to excite the active layer  33 , and a pumping beam reflection layer  34  contacting the top surface of the active layer  33  to reflect a portion of the pumping beam transmitted through the active layer  33 . 
     A heat spreader  31  may be further included on the lower surface of the signal light reflection mirror  32  to emit the heat generated by the active layer  33  to the outside. The heat spreader  31  may be light transmissive so that the pumping beam can pass through the heat spreader  31 . The light transmissive heat spreader  31  may be composed of diamond, silicon carbide (SiC), aluminum nitride (AlN), or gallium nitride (GaN). 
     A second harmonic generation (SHG) crystal  37  may be further included between the active layer  33  and the external mirror  36  to double the frequency of the signal light emitted from the active layer  33 . When the SHG crystal  37  is interposed between the active layer  33  and the external mirror  36 , the light in the infrared region emitted from the active layer  33  may be converted into visible light and then output. 
     As is well known in the art, the active layer  33  generating the signal light having a predetermined wavelength λ 2  may have a Resonant Periodic Gain (RPG) structure including a plurality of quantum wells and barriers interposed between the quantum wells. In order to obtain a gain, each quantum well is disposed in an anti-node of a standing wave which is generated by the signal light resonating between the external mirror  36  and the signal light reflection layer  32 . The active layer  33  is excited by absorbing the pumping beam emitted from the pump laser  35  and emits signal light. To excite the active layer  33 , the wavelength λ 1  of the pumping beam may be shorter than the wavelength λ 2  of the signal light generated by the active layer  33 . For example, when the active layer  33  emits signal light in the infrared region between 920 nm and 1060 nm, the wavelength λ 1  of the pumping beam may be approximately 808 nm. 
     The external mirror  36  is separated a predetermined distance from the top surface of the active layer  33 , reflects most of the incident signal light which is generated by the active layer  33  so that the signal light can resonate, and transmits a portion of the signal light to the outside. As illustrated in  FIG. 3 , the reflection surface of the external mirror  36  is concave to converge the reflected signal light on the active layer  33 . 
     The signal light reflection layer  32  contacting the lower surface of the active layer  33  reflects the signal light generated by the active layer  33  and helps to resonate the signal light between the external mirror  36  and the signal light reflection layer  32 . The signal light reflection layer  32  should be transmissive at the wavelength λ 1  of the pumping beam so that the pumping beam emitted from the pump laser  35  can pass through the signal light reflection layer  32  to be absorbed into the active layer  33 . For this, as is well known in the art, the signal light reflection layer  32  may be a Distributed Bragg Reflector (DBR) layer which is designed to have maximum reflectance at the wavelength λ 2  of the signal light. That is, the signal light reflection layer  32  may be formed of two alternately stacked semiconductor layers having different refractive indexes and each having a thickness of approximately one fourth of the wavelength of the signal light, that is λ 2 /4. For example, the DBR layer which reflects the signal light and transmits the pumping beam can be obtained by alternately repeating an Al x Ga 1-x As layer and an Al y Ga 1-y As layer (0≦x,y≦1, x≠y). 
     Also, a pumping beam reflection layer  34  formed on the top surface of the active layer  33  reflects a portion of the pumping beam which is not absorbed by the active layer  33  back to the active layer  33 . The pumping beam reflection layer  34  may be transmissive at the wavelength of the signal light so that the signal light generated by the active layer  33  can resonate between the external mirror  36  and the signal light reflection layer  32 . Accordingly, the pumping beam reflection layer  34  reflects the pumping beam emitted from the pump laser  35  and transmits the signal light emitted from the active layer  33 . For example, the pumping beam reflection layer  34  may be a DBR which is designed to have maximum reflectance at the wavelength λ 1  of the pumping beam and to have minimum reflectance at the wavelength λ 2  of the signal light. That is, the pumping beam reflection layer  34  includes two semiconductor layers and each of the semiconductor layers has a thickness of approximately one fourth of the pumping beam wavelength λ 1 , that is λ 1 /4. The thickness of each layer forming the pumping beam reflection layer  34  can be designed optimally according to simulation. The two semiconductor layers may be respectively an Al x Ga 1-x As layer and an Al y Ga 1-y As layer (0≦x,y≦1, x≠y), similar to the signal light reflection layer  32 . The pumping beam reflection layer  34  satisfying the above conditions may have various structures. 
     For example,  FIG. 5  is a graph illustrating reflectivity according to wavelength in the pumping beam reflection layer  34  having a multiple layer structure of ((L/2)H(L/2)) N , where L refers to a semiconductor layer having a relatively low refractive index, and H refers to a semiconductor layer having a relatively high refractive index among the two semiconductor layers, and N is a positive integer. For example, when N is 5, the pumping beam reflection layer  34  is stacked in the sequence of (L/2)H(L/2)(L/2)H(L/2)(L/2)H(L/2)(L/2)H(L/2)(L/2)H(L/2), that is, (L/2)HLHLHLHLH(L/2). As illustrated in  FIG. 5 , when the wavelength of the pumping beam is 808 nm, the pumping beam reflection layer  34  reflects most of the pumping beam. Also, when the signal light is 920 nm or 1060 nm, the pumping beam reflection layer  34  transmits most of the signal light. In  FIG. 5 , three graphs are shown for three different values of N. As the value of N, that is, as the number of the stacked semiconductor layers increases, the reflectivity of the pumping beam reflection layer  34  with respect to the pumping beam increases. 
     Table 1 shows data for another structure of the pumping beam reflection layer  34 . 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Layer 
                 Thickness (nm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Al 0.2 Ga 0.8 As 
                 356.81 
               
               
                   
                 AlAs 
                 61.28 
               
               
                   
                 Al 0.2 Ga 0.8 As 
                 47.8 
               
               
                   
                 AlAs 
                 58.36 
               
               
                   
                 Al 0.2 Ga 0.8 As 
                 54.87 
               
               
                   
                 AlAs 
                 68.69 
               
               
                   
                 Al 0.2 Ga 0.8 As 
                 58.73 
               
               
                   
                 AlAs 
                 71.95 
               
               
                   
                 GaAs 
                 30.93 
               
               
                   
                 Al 0.2 Ga 0.8 As 
                 27.22 
               
               
                   
                 AlAs 
                 72.05 
               
               
                   
                 Al 0.2 Ga 0.8 As 
                 59.86 
               
               
                   
                 AlAs 
                 72.47 
               
               
                   
                 Al 0.2 Ga 0.8 As 
                 60.33 
               
               
                   
                 AlAs 
                 72.9 
               
               
                   
                 Al 0.2 Ga 0.8 As 
                 60.57 
               
               
                   
                 AlAs 
                 71.67 
               
               
                   
                 Al 0.2 Ga 0.8 As 
                 24.22 
               
               
                   
                 GaAs 
                 5 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, a GaAs layer is formed immediately above the top surface of the active layer  33  with a thickness of 5 nm, Al 0.2 Ga 0.8 As layers and AlAs layers with the thicknesses shown in the right column of Table 1 are alternately stacked on the GaAs layer.  FIG. 6  is a graph illustrating reflectivity according the wavelength in the pumping beam reflection layer  34  according to Table 1. As illustrated in  FIG. 6 , the pumping beam reflection layer  34  has maximum reflectance with respect to a pumping beam with a wavelength of 808 nm and the minimum reflectance with respect to the signal light with a wavelength of 920 nm. 
     The pumping beam reflection layer  34  may have other structures than the DBR layer. For example, the pumping beam reflection layer  34  may be a dielectric stack reflection layer with a multiple layer structure in which two dielectric layers having different refractive indexes are alternately stacked. Like in the semiconductor DBR layer, each of the dielectric layers that form the dielectric stack reflection layer may have a thickness of one fourth of the pumping beam wavelength λ 1 . The dielectric stack reflection layer may be formed of a dielectric material such as SiO 2 , Al 2 O 3 . 
     Also, the pumping beam reflection layer  34  may be a very thin metal layer. When a thin metal layer is used, the thickness of the pumping beam layer  34  may be 50 nm or less in order to reduce the reflection of the signal light. With the thickness of 50 nm or less, the reflectance with respect to the pumping beam of the pumping beam reflection layer  34  may be decreased. However, the reflectance with respect to the pumping beam of the pumping beam reflection layer  34  according to certain embodiments of the present invention does not have to be almost 100% and approximately 50% is sufficient. 
     Although not shown in the drawings, an anti-reflection layer may be further coated on the top surface of the pumping beam reflection layer  34  so that the pumping beam reflection layer  34  can transmit the signal light generated by the active layer  33  more efficiently. 
     In the above described configuration, the pumping beam passing through the signal light reflection layer  32  and incident on the lower surface of the active layer  33  enters the active layer  33  and is absorbed to excite the active layer  33  as illustrated in  FIG. 4 . Then, the portion of the pumping beam which is not absorbed by the active layer  33  is emitted through the top surface of the active layer  33 . Conventionally, such a pumping beam passing through the active layer  33  is not recycled and is lost. However, according to certain embodiments of the present invention, a pumping beam reflection layer  34  is formed on the top surface of the active layer  33  to reflect the portion of the pumping beam passing through the active layer  33  back to the active layer  33 . Accordingly, the portion of the pumping beam which is not absorbed in the active layer  33  can be recycled to excite the active layer  33 . 
       FIG. 7  is a graph illustrating an increase in the portion of the pumping beam absorbed in the active layer  33  due to the pumping beam reflection layer  34  according to an embodiment of the present invention. As illustrated in graph A of  FIG. 7 , the pumping beam which is directly emitted from the pump laser  35  enters through the surface of the active layer  33  and is attenuated as it proceeds through the active layer  33 . Accordingly, the amount of the pumping beam absorbed decreases as the pumping beam passes through the active layer  33 . Consequently, the power absorbed from the pumping beam at a depth of 1.5 μm from the surface of the active layer  33  is less than a threshold power, and thus the active layer  33  cannot emit signal light from a depth greater than 1.5 μm. Accordingly, in this instance, the optimum thickness of the active layer  33  may be approximately 1.5 μm. The portion of the pumping beam which is not absorbed by the active layer  33  is emitted through the top surface of the active layer  33 . When the above described pumping beam reflection layer  34  is formed on the top surface of the active layer  33 , the portion of the pumping beam emitted through the top surface of the active layer  33  is reflected by the pumping beam reflection layer  34  and is incident again on the top surface of the active layer  33 . As illustrated in graph B of  FIG. 7 , the reflected pumping beam is absorbed by the active layer  33 . As a result, the overall absorption of the pumping beam in the active layer  33  is increased as illustrated in graph C of  FIG. 7 , and the variation of the pumping beam absorption according to the depth in the active layer  33  is decreased as well. Accordingly, the overall density of carriers in the active layer  33  is increased to increase the output of the laser device, and the output according to the depth is relatively uniform thus improving the characteristics of the laser device. 
       FIG. 8  is a graph illustrating the increase in light output of the VECSEL  30  according to an embodiment of the present invention. The lower graph of  FIG. 8  is from a VECSEL without a pumping beam reflection layer formed on the top surface of the active layer, and the upper graph of  FIG. 8  is from a VECSEL with a pumping beam reflection layer on the top surface of the active layer. With other conditions being equal, as illustrated in  FIG. 8 , when a pumping beam reflection layer is used to recycle the pumping beam, the output power is increased by more than 30% for a high input current over conventional structures. Conventionally, when the input current is more than 30 A, the output saturates. However, according to certain embodiments of the present invention, the input and the output of the VECSEL can have a linear relationship. 
     As described above, the VECSEL according to exemplary embodiments of the present invention includes a pumping beam reflection layer on the top surface of the active layer to recycle the portion of the pumping beam which is not absorbed by the active layer and wasted otherwise. As a result, the efficiency of the pumping beam usage is increased so that a laser device with large output can be provided. Also, the laser device can have a thinner active layer and less power consumption. Furthermore, the output variation with respect to the input variation has a large slope, increasing the linearity of variation, and the laser devices according to exemplary embodiments of the present invention can be used as modulators. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.