Abstract:
Long semiconductor laser cavities are placed in relative short length chips through the use of total internal reflection (TIR) surfaces formed through etched facets. In one embodiment, a laser cavity is formed along the perimeter edges of a rectangular semiconductor chip by using three 45° angled TIR facets to connect four legs of a ridge or buried heterostructure (BH) waveguide that defines the laser cavity. In other embodiments, even more TIR facets and waveguide legs or sections are employed to make even longer laser cavities in the shape of rectangular or quadrilateral spirals. These structures are limited in the spacing of adjacent waveguide sections, which if too small, can cause undesirable coupling between the sections. However, use of notches etched between the adjacent sections have been shown to decrease this coupling effect.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit, under 35 USC 119(e), of US Provisional Application No. 61/406,529, filed Oct. 25, 2010, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to semiconductor diode lasers and, more specifically, to fitting a long cavity laser in a compact chip using total internal reflection surfaces formed through etched facets. 
         [0004]    2. Description of the Related Art 
         [0005]    Semiconductor lasers typically are fabricated on a wafer by growing an appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an epitaxial structure having an active layer parallel to the substrate surface. The wafer is then processed with a variety of semiconductor processing tools to produce a laser optical cavity incorporating the active layer and incorporating metallic contacts attached to the semiconductor material. Laser facets are typically formed at the ends of the laser cavity by cleaving the semiconductor material along its crystalline structure to define edges, or ends, of the laser optical cavity so that when a bias voltage is applied across the contacts, the resulting current flow through the active layer causes photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow. Since the semiconductor material is cleaved to form the laser facets, the locations and orientations of the facets are limited; furthermore, once the wafer has been cleaved, typically it is in small pieces so that conventional lithographical techniques cannot readily be used to further process the lasers. 
         [0006]    The foregoing and other difficulties resulting from the use of cleaved facets led to the development of a process for forming the facets of semiconductor lasers through etching. This process, as described in U.S. Pat. No. 4,851,368, also allows lasers to be monolithically integrated with other photonic devices on the same substrate, the disclosure of which is hereby incorporated herein by reference. This work was further extended and a ridge laser process based on etched facets was disclosed in the IEEE Journal of Quantum Electronics, volume 28, No. 5, pages 1227-1231, May 1992, the disclosure of which is hereby incorporated herein by reference. 
         [0007]    The formation of total-internal-reflection (TIR) facets within an optical cavity through the use of etched facets at angles greater than the critical angle for light propagating within the cavity was also disclosed in U.S. Pat. No. 4,851,368 for broad area lasers and U.S. Pat. No. 5,031,190 for ridge lasers, the disclosures of which are hereby incorporated herein by reference. 
         [0008]    High power semiconductor lasers are of significant interest for many applications, such as optical storage applications. As the power requirements for semiconductor lasers has increased, manufacturers have simply increased the cavity length of the laser chip, as shown in the graph of  FIG. 1  from U.S. Patent Application 2006/0274802, where chip lengths are over 2000 μm long for 400 mW power required for double layer DVD applications. US Patent Application 2006/0274802, the disclosure of which is hereby incorporated herein by reference, states: “high power of a laser is demanded to improve a writing speed of an optical recording disk and therefore, an increase in laser resonator length is absolutely necessary to attain the high power. In this case, there is a problem in that the increase in chip size incurs a rise in chip cost.” This patent application also introduces a tapered multimode interference (MMI) waveguide in the middle of a ridge laser diode allowing the chip length to be reduced to 1300 μm for 300 mW output, resulting in a chip length that is about two-thirds of the chip length for a standard ridge laser (without the tapered MMI). 
         [0009]    Early efforts using total internal reflection (TIR) facets included the following. “Rectangular and L-shaped GaAs-AlGaAs lasers with very high quality etched facets,” Applied Physics letters, Vol. 54, page 493, used a 45° TIR etched facet to demonstrate broad area L-shaped laser. “Continuous-wave operation and mirror loss of a U-shaped GaAs/AlGaAs laser diode with two totally reflecting mirrors,” Applied Physics letters, Vol. 56, page 1617, described a broad-area laser with two TIR facets, using cleaved front and back facets. “CW operation of folded-cavity semiconductor lasers with etched turning mirrors,” Electronic Letters, Vol. 28, page 21 described a folded cavity using ridge lasers of 80 and 5 μm ridge width with a cleaved back facet. “Precise determination of turning mirror loss using GaAs/AlGaAs lasers with up to ten 90° intracavity turning mirrors,” IEEE Photonics Technology Letters, Vol. 4, page 24, used wide ridge lasers with 10 μm ridge width with cleaved front and back facets. 
         [0010]    The market continues to demand increased numbers of chips from a given wafer for variety of high power applications, such as DVD and Blu-ray read/write systems. 
       SUMMARY OF THE INVENTION 
       [0011]    According to the present invention, a large semiconductor laser cavity is placed in a relative short length chip through the use of total internal reflection (TIR) surfaces formed through etched facets. 
         [0012]    In one embodiment, a laser cavity is formed along the perimeter edges of a rectangular semiconductor chip by using three 45° angled TIR facets to connect four legs of a ridge or buried heterostructure (BH) waveguide that defines the laser cavity. In other embodiments, even more TIR facets and waveguide legs or sections are employed to make even longer laser cavities in the shape of rectangular or quadrilateral spirals. These structures are limited first in the spacing of adjacent waveguide sections, which if too small, can cause undesirable coupling between the sections. However, use of notches etched between the adjacent sections has been shown to decrease this coupling effect. A second limitation in the spiral structured cavities is the size of the open area in the center of the spirals which must allow for a minimum bond pad diameter or width (e.g. 50 μm). However, the need for an open area can be eliminated if the bond pad is placed above at least a portion of the laser cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The foregoing and additional objects, features and advantages of the invention will become apparent to those of skill in the art from the following detailed description of the invention taken with the accompanying drawings, which are briefly described as follows. 
           [0014]      FIG. 1  is a graph taken from US Patent Application 2006/0274802 that illustrates the cavity length required for a ridge-type semiconductor laser for use in CD and DVD applications. 
           [0015]      FIGS. 2A-2D  show a 2000 μm long etched facet ridge waveguide laser cavity fitted in chips all of 250 μm width, using: (a) no total internal reflection (TIR) facets resulting in a chip length of 2010 μm; (b) four TIR facets resulting in a chip length of 585 μm; (c) eight TIR facets resulting in a chip length of 325 μm; and (d) twelve TIR facets resulting in a chip length of 240 μm. 
           [0016]      FIG. 3  shows a chip having a laser cavity using 45° incidence angle TIR facets around the perimeter of the chip so that light experiences a 90° angle change after reflection at each TIR facet. 
           [0017]      FIGS. 4A-4C  show examples of the impact of ridge-to-ridge (RR) spacing, cavity length, and bond pad width or diameter, while the chip length and width is kept constant at 300 μm and 250 μm, respectively, the number of TIR facets is kept constant at eight, and there is a constant 50 μm gap at the back-facet.  FIG. 4D  is combination table and graph that illustrates the relationships among RR spacing, cavity length, and bond pad width. 
           [0018]      FIGS. 5A-5C  show examples of the impact on chip length while the RR spacing is kept constant at 16 μm, the bond pad diameter is kept constant at 100 μm, the chip width is kept constant at 250 μm, the number of TIR facets is kept constant at eight, and there is a constant 50 μm gap at the back-facet.  FIG. 5D  is combination table and graph that illustrates the relationships between cavity length and chip length. 
           [0019]      FIGS. 6A and 6B  are graphical depictions of the E x  mode profiles for ridge waveguides with 0.40 μm and 0.50 μm residual upper cladding thickness, respectively.  FIGS. 6C and 6D  graphically depict the power exchange between two ridge waveguides spaced 5 μm apart with  FIG. 6C  showing the results for 0.40 μm residual upper cladding thickness and  FIG. 6D  showing the results for 0.50 μm residual upper cladding thickness. 
           [0020]      FIG. 7A  shows an embodiment of the invention in which a notch is introduced between two adjacent ridge waveguides to reduce light coupling between the adjacent ridge waveguides.  FIG. 7B  graphically depicts the reduced coupling between the adjacent ridge waveguides when the notch is employed. 
           [0021]      FIG. 8  shows an embodiment of the invention in which a first laser cavity using 45° incidence angle TIR facets around the perimeter of the chip is employed together a second laser cavity without any TIR facets. 
           [0022]      FIG. 9  shows a first epitaxial laser structure on a substrate, and a second epitaxial laser structure on the first epitaxial laser structure which can be employed with the embodiment of  FIG. 8 , for example. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Turning now to a more detailed description of the invention,  FIGS. 2A-2D  illustrate several examples of a 2000 μm long etched facet ridge laser cavity fitted in laser chips, all of 250 μm width.  FIGS. 2B through 2D  show examples where the cavity lengths are significantly longer than the chip length. Without any total internal reflection (TIR) facets, as is illustrated in  FIG. 2A , the chip length is slightly longer than the cavity length since etched facets are used and the chip cannot be singulated exactly at the position of the etched facet without destroying the etched facet. The example in  FIG. 2B  contains four TIR facets and results in a chip length of 585 μm.  FIG. 2C  shows an example having eight TIR facets resulting in a chip length of 325 μm, while the example in  FIG. 2D  has twelve TIR facets resulting in a chip length of 240 μm. By using more TIR facets, significant reduction is obtained in the chip size, allowing more chips to be produced from a given wafer. By using a rectangular spiral-shaped cavity that starts about the perimeter of the chip, the maximum cavity length can be obtained with the minimum number of TIR facets. It is desirable to reduce the TIR facets in a cavity, since there is some loss associated with each TIR facet. 
         [0024]      FIG. 3  illustrates a laser  10  formed on a chip  12  where four legs  14  of a ridge or buried heterostructure (BH) waveguide define a laser cavity  16  and the legs  14  are placed close to the perimeter of the laser chip  12  using three TIR facets  18 . The cavity  16  also includes an etched front facet  20  and an etched back facet  22 . Preferably, the TIR facets  18  are positioned at 45° to the legs  14  so that the incidence angle to each TIR facet  18  is 45° and the light experiences a 90° change in direction after the reflection at the TIR facet  18 . The lateral location of the laser cavity  16  is defined preferably by a distance between the ridge or the buried heterostructure (BH) waveguide and the side of the chip  12  as indicated by a in  FIG. 3 . This distance a is critical in insuring that the singulation process that turns a wafer into chips does not damage the waveguide. The width of the ridge waveguide or the BH waveguide is shown as b. The resulting cavity  16  is a quadrilateral structure that preferably is rectangular in shape, though minor variances in the TIR facet angles could result in the cavity structure not having a true rectangular shape. In addition, a gap c is deliberately left between the back facet  22  that is the termination of the fourth leg and the ridge or the BH in the first of the legs  14 . The reason for this gap is to avoid unwanted reflections from reaching the back facet  22 . Examples of the values of a, b, and c are 20 μm, 2 μm, and 50 μm, respectively. 
         [0025]    In the present invention, it is preferable that the ridge waveguide width be of dimensions smaller than 3 μm to ensure single lateral mode operation. The BH waveguide is one that only supports a single lateral mode. 
         [0026]      FIGS. 4A-4C  show examples of the impact of the distance between adjacent ridge waveguides (although adjacent ridge waveguides will be discussed from here only, it will be understood that adjacent BH waveguides could also be a used), cavity length, and bond pad width, while the chip length and width is kept constant at 300 μm and 250 μm, respectively, the number of TIR facets is kept constant at eight, and there is a constant 50 μm gap (parameter c). The example shown in  FIG. 4A  has a ridge-to-ridge (RR) spacing of 10 μm which results in a cavity length of 1920 μm and a bond pad width of 130 μm. The example shown in  FIG. 4B  has an RR spacing of 20 μm which results in a cavity length of 1720 μm and a bond pad width of 80 μm. Finally, the example shown in  FIG. 4C  has an RR spacing of 30 μm which results in a cavity length of 1520 μm and a bond pad spacing of 30 μm. The foregoing relationships among RR spacing, cavity length and bond pad width are illustrated in the table and graph of  FIG. 4D . 
         [0027]      FIGS. 5A-5C  show three examples of the impact on chip length for different length cavities while the distance between adjacent ridge waveguides is kept constant at 16 μm, the bond pad diameter is kept constant at 100 μm, the chip width is kept constant at 250 μm, the number of TIR facets is kept constant at eight, and there is a constant 50 μm gap (parameter c). In  FIG. 5A , the chip length is 200 μm and the cavity length is 1300 μm; in FIG. 5B the chip length is 300 μm and the cavity length is 1800 μm; and in  FIG. 5C , the chip length is 400 μm and the cavity length is 2300 μm. The foregoing relationships between cavity length and chip length are illustrated in the table and graph of  FIG. 5D . 
         [0028]    In practice, bonding to a bond pad of a diameter below 50 μm becomes very difficult with present-day wire-bonders. The bond pad can be placed above at least a portion of the laser cavity and the space allocated for the bond pad eliminated. 
         [0029]    Compared to prior art work in U.S. Patent Application 2006/0274802 where the chip length was reduced by only 33%, we see that using TIR in the manner described according to the present invention, significant reduction in chip length is possible, for example 83% reduction. This allows significantly more chips to be fabricated from a given wafer, thereby reducing cost drastically. 
         [0030]      FIG. 3  shows the case where no adjacent ridge waveguides or adjacent BH waveguides are present, however,  FIG. 2B  illustrates the situation where adjacent ridge waveguides are present in the rectangular spiral shaped cavity and the distance between them becomes a consideration. 
         [0031]      FIGS. 6A-6D  illustrate the importance of lateral beam profile for the case of an 808-nm laser in causing coupling between two adjacent ridges.  FIGS. 6A and 6B  are depictions of the mode profiles for 0.40 μm and 0.50 μm residual upper cladding thickness, respectively. The thickness of the remaining upper cladding outside the ridge is called residual upper cladding thickness. In these figures, both contours of modes are 4 contours at 0.1, 0.2, 0.3 and 0.4 of the electric field amplitude, E x . The peak value of E x  is normalized to 1. The inner most contour is at 0.4 and the outermost contour is 0.1. When the ridges are spaced apart by 20 μm or more, there is essentially no coupling between ridges with 0.40 μm residual upper cladding thickness and very little coupling between ridges with 0.50 μm residual upper cladding thickness. However, as illustrated in the graphs of  FIGS. 6C and 6D , which show the power exchange between two close ridges when the RR spacing is 5 μm, substantial coupling occurs between the ridges, especially in the case of ridges with 0.50 μm residual upper cladding thickness ( FIG. 6D ). 
         [0032]    To address the foregoing coupling problem between closely spaced adjacent ridges, the technique illustrated in  FIG. 7A  can be employed.  FIG. 7A  shows a partial cross sectional view of a laser structure  50  that is configured in accordance with the embodiments of the invention illustrated in  FIGS. 4A-4C  and  5 A- 5 C, for example. The laser structure  50  includes at least first and second adjacent ridge waveguides  52  and  54  that are spaced from one another by a distance RR. To reduce the coupling between the two ridge waveguides  52  and  54 , a notch  56  is formed in the laser substrate  58  between the two ridge waveguides  52  and  54 . The notch  56  is of a predetermined etch depth and width. The impact of the coupling between the two adjacent ridges is significantly reduced thereby as illustrated in the graphs of  FIG. 7B  as compared to the graphs of  FIG. 6D , which shows the results of the same structure but without the notch. 
         [0033]    It should be understood that the invention is applicable to laser structures in different material systems. Three examples of such structures are described below, all of which have been demonstrated to allow etched facets. 
         [0034]    A wafer is fabricated with an n-InP cladding layer, an n-AlGaInAs SCH (separate confinement heterostructure) layer, AlGaInAs strained multi-quantum well active layers, a p-AlGaInAs SCH layer, a p-InP cladding layer and a highly doped p-InGaAs contact layer, formed successively on an n-type ( 100 ) InP substrate. The active region consisted of five 6 nm compressively strained AlGalnAs quantum wells separated by 10 nm tensile-strained AlGalnAs barrier layers; the band gap of the quantum well was about 1300 nm. 
         [0035]    A wafer is fabricated with 2.0 μm n-Al 0.4 Ga 0.6 As lower cladding; 0.1 μm Al 0.25 Ga 0.75 As to Al 04 Ga 0.6 As lower graded region; an active region containing a single 7.5 nm thick GaAs quantum well; 0.1 μm Al 0.4 Ga 0.6 As to Al 0.25 Ga 0.75 As upper graded region; 2.0 μm thick p-Al 0.4 Ga 0.6 As upper cladding; and highly p-doped GaAs cap layer, formed successively on a GaAs substrate. The lasing wavelength is around 830 nm. 
         [0036]    A wafer is fabricated with a lower cladding region of n-doped AlGaN, an active region with quantum wells and barriers of InAlGaN (Al and/or In can be zero in this composition), an upper cladding layer of p-doped AlGaN, and a highly p-doped cap layer of GaN and/or GaInN. The upper and lower cladding regions of the photonic structure had a lower index than the index of the active region, while the GaN and/or GaInN cap layer was provided to allow ohmic contacts. Although this example is based on providing a blue laser device on a GaN substrate, it will be understood that these devices can be formed to emit other wavelengths such as green and placed on other substrates such as Sapphire, SiC or AlN. As was described in U.S. Pat. No. 7,408,183, the disclosure of which is hereby incorporated herein by reference, structures of InAlGaN may have regions of low defect density and regions of high defect density. The cavity of the present invention would be positioned only in the low defect density region. 
         [0037]    The invention is also applicable to lasers that have a front output facet and/or back facet that are etched at 90±1° to the substrate so that the laser is an edge-emitter or 45±1° to the substrate to allow the laser light to be emitted perpendicular to the surface of the substrate. Description of such structures is provided in U.S. Pat. No. 7,245,645, and U.S. Patent Application No. 12/725,894 the disclosures of which are hereby incorporated herein by reference. The front and back facets are both etched facets and using dielectric layers can have modifications in reflectivity, as described in U.S. Pat. No. 7,245,645. 
         [0038]      FIG. 8  illustrates a laser structure  100  that is the combination of a first laser  102  like the laser from  FIG. 3  including a first laser cavity  104  together with a second laser  106  including a second, straight laser cavity  108 , both on the same chip  110 . For example, the first laser  102  is formed in a first epitaxial deposited structure, forming a laser with a very long cavity  104 , while the second laser  106  is formed in a second epitaxially deposited structure, forming a laser with a cavity length about the same as the length of the chip  110 . Front and back facets  112  and  114 , respectively, of the first laser cavity  104  are both etched, as are front and back facets  116  and  118 , respectively, of the second laser cavity  108 . If longer length is necessary, the second laser  106  may include TIR facets. An example of the use of such a chip is in dual wavelength operation where the first laser is a DVD laser and the second laser is a CD laser. 
         [0039]      FIG. 9  illustrates the case when the first epitaxial laser structure  200  is deposited on a substrate  202  and the second epitaxial laser structure  204  is deposited on the first laser epitaxial structure  200 . Epitaxial deposition in this manner is less expensive than etching away a certain sections on the wafer to allow for a second epitaxial structure, since both epitaxial laser structures are deposited one after the other. 
         [0040]    The epitaxial structure of  FIG. 9  is used with 45±1° etched facets for the front facets  112  and  116  of the first and second lasers  102  and  104 , respectively, of  FIG. 8 . The second epitaxial structure is removed from the first laser  102  to allow contacts to be applied to its structure. For example, this chip contains inexpensive dual cavity surface-emitting lasers for dual wavelength operation, such as DVD at 650 nm and CD at 780 nm, or blue and green emitting lasers. 
         [0041]    Although the present invention has been illustrated in terms of preferred embodiments, it will be understood that variations and modifications may be made without departing from the true spirit and scope thereof as set out in the following claims.