Patent Publication Number: US-7711016-B2

Title: Semiconductor laser with side mode suppression

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/880,655, filed Jun. 30, 2004, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to the field of semiconductor lasers. More particularly, the present invention relates to systems and methods for suppressing second order modes in semiconductor lasers. 
     2. The Relevant Technology 
     Lasers are some of the primary components of optical networks. They are often used, for example, in optical transceivers to generate the optical signals that are transmitted over an optical network. Lasers are also used to pump various types of optical amplifiers, such as Raman amplifiers and erbium-doped amplifiers. 
     Edge-emitting lasers such as Fabry-Perot lasers, Distributed Feedback lasers (DFB lasers), and distributed Bragg reflector lasers (DBR lasers), etc., are examples of semiconductor lasers used in optical environments. Ridge waveguide lasers are a form of edge-emitting lasers that have an etched ridge. 
     The output spectrum of semiconductor lasers such as edge-emitting lasers is often related to the length of the laser&#39;s cavity. Because edge-emitting lasers tend to have relatively long cavities, there are several wavelengths that may lase within the cavity. As a result, many edge-emitting lasers are often referred to as multiple longitudinal mode (MLM) lasers. 
     The output spectrum of MLM lasers may have a spectral width of around 10 nanometers, but the spectral width can vary from one laser to the next. Although MLM lasers can be useful in various applications, MLM lasers become less useful as the speed of an optical network increases. In other words, using MLM lasers in high speed optical networks typically leads to chromatic dispersion. In addition, wavelength division multiplexing (WDM) systems experience substantial crosstalk when MLM lasers are used. 
     One of the ways that the spectral width of a semiconductor laser is reduced is to used distributed reflectors such as in a distributed feedback laser (DFB laser) or a DBR laser. A DFB laser or a DBR laser both typically have a spectral width that is more narrow than a simple Fabry-Perot laser. In some lasers, a mesa or ridge may also be formed by etching away part of the semiconductor laser. The ridge also helps to make the semiconductor laser emit a single mode. 
     In fact, the modes emitted by the laser can be affected by the width of the mesa or ridge. If the ridge is too wide, the laser may support a 2 nd  order transverse mode, so that two modes exist for each longitudinal mode. Unfortunately, a narrow width, which may result in a more narrow spectral width, has performance disadvantages such as reduced optical confinement that leads to higher threshold currents and higher voltages which lead to poor thermal performance. The ability to fabricate a single mode laser is also complicated by the material gain of the laser. At lower temperatures, the material gain of a semiconductor laser tends to blueshift (move to shorter wavelengths). Because the second order transverse mode of a laser is located on the blue side of the main mode of the semiconductor laser, the second order mode may affect the performance of the semiconductor laser. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other limitations are overcome by embodiments of the present invention, which relate to systems and methods for reducing or removing optical modes from a semiconductor laser. Semiconductor lasers including distributed feedback (DFB) lasers and DBR lasers may exhibit a poor side mode suppression ratio and embodiments of the invention change the side mode suppression ratio by removing or reducing the second order mode. The poor side mode suppression ratio of a semiconductor laser becomes more prominent at lower temperatures because the gain of the material tends to move to shorter wavelengths (blueshift) at lower temperatures. The second order mode is located on the short wavelength side of the primary mode. As a result, a shift to shorter wavelengths can lead to poor side mode suppression ratios. 
     In one embodiment, a waveguide layer such as a planar slab waveguide layer is included in a semiconductor laser. The waveguide layer may be located beneath the active region of the laser structure such that it may be unbounded laterally. The waveguide layer is typically lattice matched to the structure of the laser and is also configured such that the phase velocity of a mode it supports corresponds to the phase velocity of the active region&#39;s second order mode. At the same time, the phase velocity associated with the waveguide layer is further away from the phase velocity of the primary mode. As a result, the strength of optical coupling between the waveguide layer is stronger for the second order mode than the primary or main mode. 
     Because the coupling between the waveguide layer and the secondary mode is strong, the confinement of the secondary mode to the active region of the laser is reduced and the secondary mode is effectively stripped from the laser into the waveguide layer. At the same time, the optical confinement of the primary mode is not significantly reduced because the coupling between the waveguide layer and the active region for the primary mode is much weaker. As a result, the side mode suppression ratio improves. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  illustrates an example of a primary mode and a secondary mode in a semiconductor laser; 
         FIG. 2A  illustrates an example of a semiconductor laser with a waveguide layer that optically couples with a secondary mode; 
         FIG. 2B  illustrates one embodiment of an active region of a semiconductor laser that includes a waveguide layer to reduce a side-mode suppression ratio; 
         FIG. 3  illustrates another embodiment of the waveguide layer to reduce side-mode suppression ratio; 
         FIG. 4  illustrates yet another embodiment of the waveguide layer used to reduce side-mode suppression ratio; 
         FIG. 5A  illustrates a plot of the modal index of the waveguide layer for a primary mode and a secondary optical mode with respect to the modal index of the active region as a function of wavelength; and 
         FIG. 5B  illustrates a plot of the confinement of a primary optical mode and a secondary optical mode in an active region of a semiconductor laser. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Semiconductor lasers including edge-emitting lasers often emit more than one longitudinal mode. To reduce problems associated with, for example, chromatic dispersion and crosstalk, it is desirable to produce lasers that have a narrow wavelength spectrum. In other words, it is often desirable to produce lasers that emit a single mode. As previously stated, however, edge-emitting lasers typically have relatively long cavities that can support many longitudinal modes. 
     The ability of a semiconductor laser to generate a particular mode while suppressing or filtering other modes may be quantified using a side-mode suppression ratio (SMSR). For example, the SMSR may determine how well a second order mode (or other mode) is suppressed relative to the main or primary mode of the laser. 
     Embodiments of the present invention relate to systems and methods for improving the SMSR of semiconductor lasers including edge-emitting lasers and/or ridge waveguide lasers. In one embodiment, the present invention improves the SMSR of a semiconductor at low temperatures when the material gain blueshifts such that the primary mode has reduced gain relative to the secondary or second order mode. Examples of edge-emitting lasers and/or ridge waveguide lasers may further include, but are not limited to, Fabry Perot lasers, DFB lasers, DBR lasers, external cavity lasers, and the like. 
       FIG. 1  illustrates a plot of a primary mode and a secondary mode of a semiconductor laser. The plot  100  illustrates a primary mode  102 . The primary mode  102  corresponds to a longitudinal mode and is emitted, in this example, at a wavelength of about 1310 nanometers. This example illustrates the onset of a secondary mode  104  that is to the shorter wavelength side or blue side of the primary mode  102 . As previously described, a secondary mode in systems such as WDM systems can induce crosstalk as well as chromatic dispersion. 
     At low temperatures, the material gain of semiconductor lasers blueshifts or moves to shorter wavelengths. As a result, the power in the secondary mode  104  increases at lower temperatures. The onset of the secondary mode  104  can decrease the SMSR of the semiconductor laser as previously described. 
       FIG. 2A  illustrates one embodiment of the present invention to reduce the second order mode. The laser  200  includes a substrate  212  with various layers arranged over the substrate  212 . In this example, an n-type semiconductor layer  210  is arranged over the substrate  212  and the active region  204  is arranged over the semiconductor layer  210 . A p type semiconductor layer  208  is arranged or formed over the active region  204 . 
     The laser  200  may be a DFB or a DBR laser, for example, and the laser  200  may also include a grating  206 . After the grating  206  is etched and defined, InP or other suitable material may be regrown and the laser is then fabricated using a standard ridge waveguide process. The grating  206  serves as the etch stop layer during the ridge waveguide process. 
     The active region  204  is effectively located at a pn junction. In one embodiment, the active region  204  includes a multi-quantum well structure. The material system of the quantum wells is InGaAlAs in this example. The active region  204  may include 6 quantum wells interleaved with 7 barrier layers, although one of skill in the art can appreciate that the invention may be implemented using more or fewer quantum wells and that other materials may be used in the quantum wells. 
     Each quantum well may have a thickness on the order of 5 nm while the barrier layers may each have a thickness on the order of 8.5 nm. In some embodiments, the barrier layers and/or the quantum wells are tensile strained or compressive strained. In one embodiment, the quantum wells are compressive strained and the barrier layers and tensile strained. The photoluminescence of the active region  204  in this example is on the order of 1300 nm. One of skill in the art can appreciate an active region having a photoluminescence that is greater or lower. 
       FIG. 2B  illustrates one embodiment of an active region  204 . More particularly in  FIG. 2B , the active region  204  may also include graded index separate confinement heterostructure (GRINSCH) layers  222 ,  226 . The GRINSCH layers  222 ,  26  sandwich the multi-quantum well structure  224  and are each on the order of 120 nanometers thick. For each GRINSCH layer  222 ,  226 , the bandgap ramps down from 960 nanometers to 850 nanometers moving away from the multi-quantum well structure  116 . Each GRINSCH layer  222 ,  226  is followed by a layer of InAlAs  220 ,  228  that is 50 to 100 nanometers thick. 
     Thus the InAlAs layer  228  would be formed on the layer  210 . The p-type semiconductor layer  208  is formed on the other InAlAs layer  222  in this example. The p-type semiconductor layer  208  is typically formed from InP. The grating layer  206  is formed or grown on the layer  208  and is followed by a cap layer in which a ridge waveguide process is performed to form a ridge in the laser  100 . The grating layer  206  acts as an etch stop. The metal contact may then be deposited or formed after the ridge is formed in one embodiment. 
     The laser  200  also includes a waveguide layer  202  as illustrated in  FIG. 2 . The waveguide layer is typically formed beneath the active region  204 . In an InP based laser, the waveguide may be InGaAsP lattice matched to InP. The specific composition or other parameters of the waveguide layer  202  can be altered as described below to improve the SMSR. 
       FIGS. 3 and 4  illustrate alternative embodiments of the invention. In  FIG. 3 , the waveguide layer  302  of the laser  300  includes multiple layers. In one embodiment, the multiple layers in the waveguide layer  302  are distributed Bragg layers.  FIG. 4  illustrates a laser array  400 . In  FIG. 4 , a ridge waveguide structure  401  is formed laterally adjacent to a ridge waveguide structure  403 . The structure  402  includes a waveguide layer  402  that can optically couple with the active region  404  of the structure  403 . The waveguide layer  402  is located sufficiently near the active region  403  for optical coupling to occur. 
     Returning to  FIG. 2 , the waveguide layer  218  has a thickness  218  (t). When t=0 or when the waveguide layer  202  is not present, and for typical width values of the mesa  214 , the laser  200  may support a second order mode that can lead to the SMSR failure previously described. As the thickness  218  increases, the speed of light (or the phase velocity) in the waveguide layer approaches the phase velocity of the second order mode. As a result, the second order mode couples with the waveguide layer  202 . In this example, the waveguide layer may have a thickness of about 115 nanometers and have a photoluminescence peak of about 1200 nanometers. 
     The waveguide layer  202  is a planar waveguide in this example and the second order mode is effectively coupled into the waveguide layer  202 . In other words, the optical confinement of the second order mode is reduced from its value when the thickness  218  equals zero. The confinement of the primary mode is not affected because the phase velocity of the waveguide layer is further away from the phase velocity of the primary mode. Thus, the optical confinement of the primary mode is not affected. 
     The effect of the waveguide layer on the confinement of the optical modes of the active region is further illustrated in  FIG. 5A .  FIG. 5A  plots the modal index (phase velocity) of a mode of the waveguide structure as well as the modal index of the modes of the active region as a function of wavelength. The strength of coupling between the waveguide layer and the active region modes can be either wavelength dependent or wavelength independent. The waveguide layer is typically configured such that the modal index curve illustrated in  FIG. 5A  is approximately parallel to the curve of the active region. When the curve of the waveguide layer is substantially parallel to the curve of the active region, the optical coupling is wavelength independent. 
     The curves  504  and  502  correspond to the primary and secondary modes of the laser active region, respectively. The curve  512  corresponds to a mode of the waveguide layer. Because the modal index curve of the secondary mode  502  and the modal index curve of the waveguide layer  512  are close together, the secondary mode of the active region strongly couples with the waveguide layer. At the point  508 , the strength of coupling is the strongest and the loss of confinement of the secondary mode is maximized at this point. The primary mode does not strongly couple with the waveguide layer because its modal index, as represented by the curve  504 , is not well matched to the modal index of the waveguide layer  512 . This suggests that the optical confinement of the primary mode is not substantially reduced while the optical confinement of the second order mode is reduced. The primary mode and secondary mode lase at different wavelengths, but the above conditions are satisfied over a larger wavelength range than the separation of the primary and secondary lasing wavelengths, which is typically 5 to 10 nm. 
       FIG. 5B  plots the confinement of the primary and secondary mode to the active region.  FIG. 5B  plots the confinement of the primary and secondary modes to the active region, normalized to their values when the thickness  218 =0. In this example, the distance  216  (D) in  FIG. 2A  is 1.2 micrometers and the waveguide layer has a bandgap of 1200 nanometers. The curve  550  corresponds to the optical confinement, the curve  552  corresponds to the optical confinement of the secondary mode and the curve  554  corresponds to the primary mode. As the phase mismatch goes through zero and becomes positive when the thickness of the waveguide layer is on the order of 100 nanometers, the mode confinement of the secondary mode is essentially zero. As the thickness increases, the waveguide layer begins to influence the primary mode. One result is that the waveguide layer permits the active region to be effectively single mode well above the width limit of a conventional waveguide laser. 
     Various parameters of the feedback layer may also be adjusted to impact the mode confinement. Examples of parameters include, but are not limited to, thickness of the waveguide layer, location of the waveguide layer with respect to the active region (thickness of the layer  106 , for example), material composition or formulation of the waveguide layer, refractive index of the waveguide layer, modal index of the waveguide layer, and any combination thereof. Another parameter may be the number and type of layers in the waveguide layer. One of skill in the art can also appreciate that the formulation of the active region can also be adjusted to impact the mode confinement and/or the material gain of the active region. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.