Abstract:
The invention is a semiconductor laser, laser module, and method of manufacture. The laser includes an active region having a first refractive index, and at least one confinement layer with a second refractive index, which is lower than the first refractive index. An anti-guiding layer having a third refractive index which is lower than the second refractive index is positioned so that the confinement layer is between the active region and the anti-guiding layer. A cladding layer having a fourth refractive index which is greater than the third refractive index is positioned so that the anti-guiding layer is between the cladding layer and the confinement layer.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to semiconductor lasers, and in particular to a structure and method of manufacture which improves coupling of light from such lasers into an optical fiber or other optical waveguide.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical systems have become the backbone of modern telecommunications systems primarily due to their tremendous information-handling capacity. Such systems typically include one or more semiconductor lasers as an optical source, and optical fiber as the transmission medium. Another key component is the optical amplifier, which includes a rare earth -doped fiber whose impurities are excited by another laser (pump laser) in order to provide amplification of the optical signal from the source. Another type of optical amplifier is the Raman amplifier, which also relies on pump light for amplification. While the remainder of this application will focus primarily on these pump lasers, it should be realized that the invention is applicable to any semiconductor laser.  
           [0003]    Pump laser light is typically transmitted to the optical amplifier through a single mode fiber. In order to provide optimum coupling efficiency of the light into the fiber, it is generally desirable to narrow the vertical far field angle of the light as much as possible. Unfortunately, reducing the far field angle usually results in adverse effects on other parameters of the laser, such as threshold current and slope efficiency. For example, in a standard double heterostructure AlGaAs/GaAs pump laser operating at 980 nm, the far field angle can be reduced by lowering the Al concentration of the cladding layers which are adjacent to the quantum well active layer. However, this modification also tends to degrade the threshold current and the slope efficiency, and can also lead to increased leakage currents.  
         SUMMARY OF THE INVENTION  
         [0004]    The invention is a semiconductor laser and laser module which includes an active region having a first refractive index, and at least one confinement layer with a second refractive index which is lower than the first refractive index. An anti-guiding layer having a third refractive index which is lower than the second refractive index is positioned so that the confinement layer is between the active region and the anti-guiding layer. A cladding layer having a fourth refractive index, which is greater than the third refractive index is positioned so that the anti-guiding layer, is between the cladding layer and the confinement layer.  
           [0005]    In accordance with another aspect, the invention is a method of forming a semiconductor laser which includes forming an active region having a first refractive index over a semiconductor substrate, and forming a confinement layer having a second refractive index over the active region. An anti-guiding layer having a third refractive index which is less than the second refractive index is formed over the confinement layer, and a cladding layer having a fourth refractive index which is greater than the third refractive index is formed over the anti-guiding layer. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0006]    These and other features of the invention are delineated in detail in the following description. In the drawing;  
         [0007]    [0007]FIG. 1 is a schematic diagram of a typical optical system, which may incorporate the invention;  
         [0008]    [0008]FIG. 2 is a schematic cross sectional view of a typical laser package, which may be incorporated into the system of FIG. 1;  
         [0009]    [0009]FIG. 3 is an enlarged view of a portion of the laser package of FIG. 2;  
         [0010]    [0010]FIG. 4 is a cross sectional view of a semiconductor laser incorporating features of the invention in accordance with one embodiment;  
         [0011]    [0011]FIG. 5 is a diagram of the refractive indices of the structure of FIG. 4;  
         [0012]    FIGS.  6 - 8  are graphs of confinement factor (Gamma) and far field angle for various embodiments of the invention. 
     
    
       [0013]    It will be appreciated that, for purposes of illustration, these Figures are not necessarily drawn to scale.  
       DETAILED DESCRIPTION  
       [0014]    [0014]FIG. 1 illustrates a typical optical system,  10 , which may include the inventive features. An optical source,  11 , provides a signal which is typically several wavelengths in a range about 1550 nm. The source is usually one or more semiconductor lasers such as standard Distributed Feedback (DFB) or Distributed Bragg Reflector (DBR) lasers. The signal is transmitted by an optical fiber,  12 , to a multiplexer,  13 . Light from a pump laser,  14 , which typically has a wavelength of 980 nm is transmitted to the multiplexer,  13 , by means of a single mode optical fiber,  16 . The combined signal light and pump light are transmitted to an optical amplifier,  17 , which is typically an Erbium Doped Fiber Amplifier (EDFA), but could also be a Raman amplifier. The signal light is amplified and transmitted to another multiplexer,  18 . The multiplexer,  18 , also receives light from a second pump laser,  19 , which typically has a wavelength of 1480 nm. The light from the second pump is transmitted to the multiplexer  18 , by means of optical fiber,  20 , and then on to the EDFA,  17 . The signal light is transmitted to a receiver,  22 , typically by means of an optical fiber,  21 .  
         [0015]    It will be appreciated that the diagram of FIG. 1 omits several components, such as optical isolators, which are usually included in an optical system. Also, it may not be necessary to include two pump lasers as shown operating at different wavelengths. Rather, a single pump laser may be sufficient.  
         [0016]    [0016]FIG. 2 illustrates a simple package design for a laser pump module,  25 . The laser,  14 , is typically mounted on a platform,  26 , which is on the bottom surface of a hermetic enclosure,  27 . The optical fiber,  16 , with or without a lens, extends through the enclosure and is aligned with the laser,  14 , so as to receive the light emanating therefrom. Other elements usually included in the package, such as backface monitoring components, drivers and temperature control devices, are omitted for the sake of clarity.  
         [0017]    It will be appreciated that an important design consideration is the efficient coupling of the light from the laser to the fiber. This efficiency can be improved by narrowing the far field angle of the laser light, i.e. the angle θ of divergence of the laser light as illustrated in the enlarged view of FIG. 3.  
         [0018]    [0018]FIG. 4 illustrates one embodiment of a laser structure,  14 , with a narrowed far field angle, and, consequently, improved coupling efficiency. The structure is built on a substrate,  40 , which in this example, comprises GaAs. A series of epitaxial layers, to be described, are formed on a major surface of the substrate. In this example, the layers are formed by molecular beam epitaxy (MBE), but other techniques could be employed. The first layer,  41 , is a cladding layer which comprises AlGaAs and typically has a thickness of 1-3 microns. In this example, the layer,  41 , has a refractive index of approximately 3.45. As known in the art, a cladding layer functions to confine the optical mode to the active region (to be described).  
         [0019]    Formed on the cladding layer,  41 , is an anti-guiding layer,  42 , which in this example comprises AlGaAs and has a thickness of approximately 10-200 nm. As understood in this application, an “anti-guiding” layer is one which has a refractive index less than that of the layers adjacent to it in the structure. Thus, as illustrated in FIG. 5, the refractive index of layer,  42 , is less than the refractive index of layer  41 , and less than the refractive index of layer,  43 , to be described. (For examples of lasers employing anti-guiding layers, see U.S. Pat. No. Re 36,431 issued to Muro et al, and U.S. Pat. No. 5,438,585 issued to Armour, et al.) More details regarding this layer are given below.  
         [0020]    Formed on the anti-guiding layer,  42 , is an optional spacer layer,  43 , which in this example comprises AlGaAs. Formed on the spacer layer is a standard graded Separate Confinement Layer (SCL),  44 , which in this example comprises AlGaAs with a varying amount of aluminum to produce the graded refractive index profile illustrated in FIG. 5 (i.e., where the refractive index of layer  44  increases in a direction away from the spacer layer,  43 ). As known in the art an SCL layer functions primarily to confine charge carriers to the active region.  
         [0021]    An active region,  45 , is formed on the SCL layer,  45 . As known in the art, the laser light is produced in this region as the result of recombination of charge carriers. In this example, the region comprises a series of alternating quantum well and barrier layers comprising InGaAs/AlGaAs. As illustrated in FIG. 5, this region has the highest refractive index in the structure, which is typically greater than 3.6.  
         [0022]    Formed successively over the active region are a further SCL layer,  46 , spacer layer,  47 , anti-guiding layer,  48 , and cladding layer,  49 . These layers can be (but need not be) identical to their corresponding layers  41 - 44  in thickness and composition, except for having an opposite conductivity type. Finally, a cap layer,  50 , in this example comprising GaAs with a thickness of 0.1 micron is formed on the cladding layer,  50 . Appropriate electrodes (not shown) are also formed on the top and bottom of the structure.  
         [0023]    While not being bound by any theory, it is believed that placement of anti-guiding layers,  42  and  48 , between the SCL layers,  44  and  46 , respectively, and the cladding layers,  41  and  49 , respectively, and in close proximity to their respective SCL layers,  44  and  46 , respectively, pulls a portion of the optical beam outside of the SCL layers. This widens the beam in the near field, resulting in a narrowing of the far field angle as the light enters the optical fiber. While the embodiment described above uses two anti-guiding layers, it will be appreciated that a single such layer could also have beneficial effects.  
         [0024]    In one example, applicant employed as the anti-guiding layers,  42  and  48 , AlGaAs with an aluminum concentration which was higher than that of the cladding layers,  41  and  49 . In particular, the cladding layers had a 28 percent aluminum concentration, while the anti-guiding layers had a 40 percent aluminum concentration. No spacer layers were employed. As illustrated in FIG. 6, the far field angle, curve  60 , is lowered from approximately 27 degrees with no anti-guiding layers to approximately 17 degrees with anti-guiding layer thicknesses of 0.2 microns. However, there is also some loss of optical confinement (Gamma) as indicated by curve  61 . While adequate for some applications, it is usually preferred to avoid such loss of confinement.  
         [0025]    Consequently, in a presently preferred embodiment, spacer layers,  43  and  47  of FIGS. 4 and 5, were introduced. These spacer layers can be, but need not be, identical to the cladding layers,  41  and  49 . As illustrated in FIG. 7, for a spacer layer thickness of approximately 500 angstroms, (50 nm) the far field angle, curve  70 , is again reduced to below 20 degrees with anti-guiding layer thicknesses of 0.2 microns, but the optical confinement (Gamma) changes very little. In fact, as illustrated in FIG. 8, with a spacer layer thickness of approximately 1000 angstroms (100 nm), the optical confinement increases (curve  81 ), although far field angle (curve  80 ) is not reduced as much as the previous example.  
         [0026]    Increasing the aluminum concentration in the anti-guiding layers to approximately 40 percent is expected to give an even further reduction in far field angle. For example, with no spacer layers, anti-guiding layer thicknesses of approximately 2000 angstroms (200 nm) is expected to give the lowest far field angle. Spacer layer thicknesses of 500 angstroms (50 nm) and anti-guiding layer thicknesses of approximately 3000 angstroms (300 nm) are expected to give moderate far field angle improvement and Gamma. Increasing the spacer layer thicknesses to 1000 angstroms (100 nm) with anti-guiding layer thicknesses again at 3000 angstroms (300 nm) is expected to give the best Gamma.  
         [0027]    In view of these considerations, it is generally preferred to have the aluminum concentration of the anti-guiding layers in the range 20 to 40 percent. Concentrations above 40 percent may introduce problems with doping and non-radiative recombination centers, although such higher concentrations may be usable for certain applications. The thicknesses of the anti-guiding layers are preferably in the range 10 to 200 nm. Spacer layer thicknesses are preferably in the range 0 to 100 nm.  
         [0028]    Various modifications of the invention as described are possible. For example, the SCL layers,  44  and  46 , need not have a graded index of retraction, but could have a constant or stepped index. Further, the invention is applicable to other types of lasers, such as InP-based lasers useful for pumping Raman amplifiers. In that case, InP could be used as an anti-ginding layer, while InGaAsP could be used for the remaining layers.