Patent Publication Number: US-2005117622-A1

Title: Distributed feedback laser with differential grating

Description:
RELATED APPLICATIONS  
      This application is a divisional, and claims the benefit, of U.S. patent application Ser. No. 10/284,128, entitled DISTRIBUTED FEEDBACK LASER HAVING A DIFFERENTIAL GRATING, filed Oct. 30, 2002, incorporated herein in its entirety by this reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. The Field of the Invention  
      The present invention generally relates to semiconductor laser devices. More particularly, the present invention relates to a distributed feedback laser device having a structure that improves both manufacturing yield and operating performance of the laser device.  
      2. The Related Technology  
      Semiconductor lasers are currently used in a variety of technologies and applications, including optical communications networks. One type of semiconductor laser is the distributed feedback (DFB) laser. The DFB laser produces a stream of coherent, monochromatic light by stimulating photon emission from a solid state material. DFB lasers are commonly used in optical transmitters, which are responsible for modulating electrical signals into optical signals for transmission via an optical communication network.  
      Generally, a DFB laser includes a positively or negatively doped bottom layer or substrate, and a top layer that is oppositely doped with respect to the bottom layer. An active region, bounded by confinement regions, is included at the junction of the two layers. These structures together form the laser body. A coherent stream of light that is produced in the active region of the DFB laser can be emitted through either longitudinal end, or facet, of the laser body. One facet is typically coated with a high reflective material that redirects photons produced in the active region toward the other facet in order to maximize the emission of coherent light from that facet end.  
      A grating is included in either the top or bottom layer to assist in producing a coherent photon beam. For example, the grating is typically produced in the top layer of the DFB laser body by depositing a first p-doped top layer having a first index of refraction atop the active region, then etching evenly spaced grooves into the first top layer to form a tooth and gap cross sectional grating structure along the length of the grating. A second p-doped top layer having a second index of refraction is deposited atop the first top layer such that it covers and fills in the grating structure. During operation of the DFB laser, the tooth and gap structure of the grating, which is overlapped by optical field patterns created in the active region, provides reflective surfaces that couple both forward and backward propagating coherent light waves that are produced in the active region of the laser. Thus, the grating provides feedback, thereby allowing the active region to support coherent light wave oscillation. This feedback occurs along the length of the grating, hence the name of distributed feedback laser. After reflection is complete, the amplified light waves are then output via the output end facet as a coherent light signal. DFB lasers are typically known as single mode devices as they produce light signals at one of several distinct wavelengths, such as 1,310 nm or 1,550 nm. Such light signals are appropriate for use in transmitting information over great distances via an optical communications network.  
      DFB lasers as described above are typically mass produced on semiconductor wafers. Many DFB laser devices can be formed on a single wafer. After fabrication, the DFB lasers are separated from one another by a cleaving process, which cuts each device from the wafer. This cleaving process creates each end facet of the DFB device body. Unfortunately, limitations inherent in the cleaving process do not allow the laser device to be cut such that a precisely desired distance is established between the end facet and the nearest adjacent grating tooth.  
      The inherent variability of the distance between the end facet and the adjacent grating tooth created as a result of cleaving can cause several problems. First, the end facet, especially an end facet that is coated with a high reflective coating, may be disposed adjacent the nearest grating tooth such that the laser during operation will exhibit poor sidemode suppression, which in turn results in undesired optical frequencies being amplified within the laser device. These undesired optical frequencies can spoil the monochromatic nature of the DFB laser output and result in reduced performance for the apparatus in which the laser device is disposed.  
      Other problems that can arise from the arbitrary cleaving process include an increased incidence of chirp and low power output from the DFB laser device. Chirp, or the drifting of the optical output wavelength over time, is magnified by improper distances between the grating and the high-reflective end facet caused by the cleaving process. Similarly, low power output is evidence of less-than-ideal cleaving of the DFB laser device.  
      If one or more of the above-described problems is detected in a particular DFB laser device after manufacture and testing, it often must be discarded, thereby lowering the yield of acceptable DFB laser devices that are produced from a wafer. In some cases, the percentage of rejected devices suffering from any of the above problems can exceed 50% per wafer.  
      Attempts to mitigate the effects of low precision cleaving have involved the addition of one or more quarter phase shifts in the grating. However, the typical DFB grating has a continuous pattern over the entire wafer. This continuous pattern allows for the lithography to be simple. Yet, the installation of one or more quarter phase shifts requires the use of a special lithography apparatus. Additionally, special techniques are required in order to add such phase shifts. These special requirements necessarily increase the cost of production of each DFB device.  
      In light of the above, it would be desirable to enable the production of DFB laser devices where the yield per wafer is substantially increased. Further, a need exists for the DFB laser to exhibit good sidemode suppression while limiting chirp and output power loss. Moreover, such a solution should be simply implemented, thereby limiting production cost increases.  
     BRIEF SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION  
      Briefly summarized, embodiments of the present invention are directed to a DFB laser device that overcomes the problems created by imprecise cleaving operations performed on DFB devices during their manufacture. Specifically, exemplary embodiments of the invention are concerned with a DFB laser having a differential grating configuration suitable for high yield manufacture and desirable operating characteristics, such as good sidemode suppression, low chirp, and controlled reflectivity and optical emission.  
      One exemplary DFB laser includes a body that has first and second end facets. The DFB laser is implemented in a stack configuration that includes an active region interposed between a first top layer and a substrate. A second top layer is disposed on the first top layer and has an index of refraction different from that of the first top layer. Additionally, a grating is defined in one of the top layers and extends from the first end facet to the second end facet. The grating includes a tooth/gap structure whose configuration varies between the first end facet and the second end facet. Finally, an antireflective (AR) coating is disposed on the first end facet and on the second end facet.  
      These and other aspects of exemplary embodiments of the invention will become more fully apparent from the following description and appended claims.  
    
    
     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 that 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  is a perspective cutaway view of a distributed feedback laser device manufactured in accordance with one embodiment of the present invention;  
       FIG. 2  is a cross sectional view of the laser of  FIG. 1 ;  
       FIG. 3A  is a close-up cross sectional view of a portion of the grating structure shown in  FIG. 2  according to one embodiment thereof;  
       FIG. 3B  is a close-up cross sectional view of a portion of the grating structure shown in  FIG. 2  according to another embodiment thereof;  
       FIG. 3C  is a close-up cross sectional view of a portion of the grating structure shown in  FIG. 2  according to yet another embodiment thereof;  
       FIG. 4  is a cross sectional view of a distributed feedback laser made in accordance with one embodiment of the present invention;  
       FIG. 5A  is a close-up cross sectional view of one portion of the grating structure shown in  FIG. 4 ;  
       FIG. 5B  is a close-up cross sectional view of another portion of the grating structure shown in  FIG. 4 ;  
       FIG. 5C  is a close-up cross sectional view of yet another portion of the grating structure shown in  FIG. 4 ;  
       FIG. 6A  is a close-up cross sectional view of one portion of the grating structure shown in  FIG. 4  according to an alternative embodiment;  
       FIG. 6B  is a close-up cross sectional view of another portion of the grating structure shown in  FIG. 4  according to an alternative embodiment; and  
       FIG. 6C  is a close-up cross sectional view of yet another portion of the grating structure shown in  FIG. 4  according to an alternative embodiment.  
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION  
      Reference will now be made to figures wherein like structures will be provided with like reference designations. It is understood that the drawings are diagrammatic and schematic representations of presently preferred embodiments of the invention, and are not limiting of the present invention nor are they necessarily drawn to scale.  
       FIGS. 1-6C  depict various features of embodiments of the present invention, which is generally directed to a distributed feedback (“DFB”) laser that is configured so as to exhibit improved operating characteristics. The present DFB laser further comprises a design that enables it to be manufactured such that laser device yield per wafer is substantially improved.  
      Reference is first made to  FIG. 1 , which shows a cutaway view of a DFB laser device made in accordance with one embodiment of the present invention, and which is generally designated at  10 . The DFB laser  10  includes an n-doped bottom layer or substrate  12  on which a bottom confinement layer  14  is disposed. An active layer  16 , comprising a plurality of quantum wells or other similar structure, is disposed atop the confinement layer  14  and is covered by a top confinement layer  18 . A p-doped first top layer  20  overlies the confinement layer  18 . A grating  22  is defined in the first top layer  20 . A p-doped second top layer  24  is disposed atop the grating  22 . Alternatively, the first and second top layers  20  and  24  can be n-doped, while the substrate  12  is p-doped. A contact layer  26  for providing an electrical signal to the DFB laser  10  is disposed atop the second top layer  24 . The various layers described above extend between a first end facet  28  and a second end facet  30 , partially shown in  FIG. 1 .  FIG. 1  illustrates several basic components that generally comprise the DFB laser device  10 . It is appreciated that additional or alternative layers or structures can be incorporated into the present laser device as will be understood by those of skill in the art.  
      Reference is now made to  FIGS. 2 and 3 A, which show the DFB laser device  10  of  FIG. 1  in cross section. In the illustrated embodiment, the first and second end facets  28  and  30  are shown having an anti-reflective (“AR”) coating  32  disposed thereon. The AR coating  32 , which is typically applied after cleaving, reduces reflection of internal light waves off of the end facets  28  and  30  during laser operation and instead allows the light to exit the laser device  10  through the end facets. Though it allows light waves to exit out of both end facets  28  and  30 , the present DFB laser device  10  is configured to direct a majority of the coherent light produced by the laser through only one end facet, as will be described.  
      As mentioned, the grating  22  is disposed on a top surface of the first top layer  20 . In greater detail, the grating  22  comprises a periodic series of closely-spaced grooves that are etched or otherwise defined in the first top layer  20 . As will be seen, the grooves, when seen in cross section, define a series of teeth  34  protruding from the first top layer  20 , and gaps  36  between adjacent teeth. The gaps  36  are filled with the second top layer  24  such that continuous contact is established between the first top layer  20  and the second top layer. Definition of the grating  22  on the first top layer  20  can be accomplished using known grating techniques, including electron beam lithography.  
      Though both the first top layer  20  and second top layer  24  are similarly doped, each has a distinct index of refraction with respect to one another. As seen in  FIG. 3A , which is magnified to show grating detail, the first top layer  20  has an index of refraction n 1 , while the second top layer has an index of refraction n 2 . This relative refractive index disparity is required to enable each tooth  34  to act as a feedback surface for reflecting light waves and enabling their coherent propagation within the DFB laser  10 , as is known in the art. Thus, the grating serves as a boundary between two similarly doped materials having distinct indices of refraction. Further details concerning the grating  22  are discussed further below.  
      As already described, both the first and second end facets  28  and  30  are coated with AR coating  32 . By making each end facet light transmissive via the AR coating  32 , problems that otherwise arise with respect to the imprecise distance between a respective end facet and the last tooth  34  adjacent thereto is eliminated. In other words, lightwaves that would otherwise reflect off the high reflective end facet (as in the prior art) that is potentially not properly positioned with respect to adjacent teeth  34  of the grating  22  are not, in fact, reflected to undesirably interact with the coherent light waves within the laser device, but are instead allowed to pass through the anti-reflective facet and exit the device. In this way, any problems normally created as a result of the inherent randomness in the cleaving process that defines the end facets  28  and  30  of the DFB device  10  are eliminated by making each end facet anti-reflective via the AR coating  32 . This in turn improves the yield of DFB laser devices from a given wafer while improving the sidemode suppression characteristics of each such device.  
      Notwithstanding the improvements in light emission integrity made possible by the above AR-coated end facets  28  and  30 , this by itself is insufficient to optimize coherent light emission from the DFB laser  10 . Without further modification, a laser device having AR-coated end facets will emit approximately one-half of its coherent light through either facet. This results in a significant waste of light emission.  
      To alleviate the above situation, the grating  22  is modified according to principles taught according to the present invention so as to direct the majority of coherent light emission through only one of the end facets. This is accomplished by anisotropically altering the physical configuration of the grating  22  as a function of position along the grating length. For example,  FIG. 3A , which is a close-up view of the circled portion in  FIG. 2 , shows a portion of the grating  22  near a longitudinal mid-point  38  of the grating length. The grating  22  is bifurcated by an imaginary bifurcation line at the mid-point  38  wherein each half of the grating length defines a distinct tooth and gap structure. On the left side of the mid-point  38  as seen in  FIG. 3 , a first half  22 A of the grating structure, comprising periodic teeth  34 A and gaps  36 A, is substantially uniform. In contrast to this, a second half  22 B of the grating  22 , beginning at and continuing to the right of the mid-point  38 , is characterized by a second order tooth structure, wherein every second tooth  34 B that would otherwise be present (shown in dashes) is missing, and in its place a gap  36 B having twice the normal length is disposed. This second order structure shown in  FIG. 3A  continues from the mid-point  38  along the entire length of the second grating half  22 B to the second end facet  30  shown in  FIG. 2 . Thus, a non-uniform tooth and gap structure is established along the length of the grating  22 .  
      Because of the non-uniform grating structure along the length of the grating  22 , the reflectivity per unit length of the grating, or kappa (“κ”), which is related to the particular configuration of the grating, is also non-uniform along the grating length. In the present embodiment, κ is high on the uniform first grating half  22 A, and relatively lower on the second grating half  22 B. Because κ is directly related to the number of times a light wave will be reflected from the surfaces of the grating teeth, a lower κ number associated with the second grating half  22 B indicates that light waves will be reflected by the grating less than those waves traveling through the first grating half  22 A, which possesses a higher κ value. This is so because of the particular tooth and gap structure of each grating half  22 A and  22 B. In the first grating half  22 A, for instance, a propagating light wave created in the active region  16  will encounter a tooth/gap interface, and thus a reflective opportunity, at every interval “a,” corresponding to the repetitive period of the teeth  34  and gaps  36 . On the other hand, a light wave propagating through the second order structure of the second grating half  22 B encounters a tooth/gap interface only half as many times as in first half  22 A. Thus the light wave is reflected fewer times, which in turn increases the number of light signals that are able to progress to and pass through the second end facet  30 . Correspondingly, because a light signal passing though the higher κ value first grating half  22 A encounters more reflective opportunities, relatively fewer signals are able to reach and pass through the first end facet  28 . Consequently, a substantial majority of light waves pass through the second end facet  30  when the DFB laser  10  is configured with a grating as shown in  FIGS. 2 and 3 A.  
      In addition to the second order tooth and gap configuration shown in the second grating half  22 B, the grating could be modified to alternatively include a third, fourth, or higher order tooth and gap configuration, if desired. For instance, in a third order configuration, every third tooth is missing from the grating structure. Such grating configurations can be designed so as to achieve the desired reduction or increase in the κ value for the particular grating portion involved.  
      It is noted that the bifurcated grating structure in  FIG. 3A  is separated by the imaginary bifurcation line located at the mid-point  38 . However, it is not necessary that the bifurcation occur at the mid-point  38 . Indeed, and in agreement with the teachings herein, the division of grating structure topology can be defined at any appropriate point along the length of the grating  22 . Thus, in the present example the second order grating structure can alternatively occupy one-third of the length of the grating  22  nearest the second end facet  30 , while the uniform grating structure portion is defined along the remainder of the grating length. Moreover, the transition from uniform grating structure to second order structure is seen in  FIG. 3A  occurring abruptly at the mid-point  38 . However, the present invention is not restricted to such a configuration. Indeed, the transition from one grating structure to another can occur abruptly or gradually, as may be desired for a particular application. These principles explained here also apply to the following additional embodiments as well.  
       FIG. 3B  illustrates how the circled portion of the grating  22  in  FIG. 2  would look if modified in accordance with another embodiment of the present invention. As in the previous embodiment, the grating length here is virtually bifurcated about the mid-point  38  to define first and second grating halves  22 A and  22 B. In this embodiment, as in the previous embodiment, the first half  22 A of the grating  22  has a uniformly periodic length and tooth and gap configuration, wherein each tooth  34 A′ has a substantially similar width w 1 . The second half  22 B of the grating, however, is modified in its per-tooth duty cycle such that each substantially similar tooth  34 B′ has a width w 2  that is less than the width w 1 . This correspondingly increases the length of each gap  36 B′ disposed between the teeth  34 B′.  
      In a similar manner to the previous embodiment, the grating configuration shown in  FIG. 3B  features a reduced κ value on the second grating half  22 B in comparison with the κ value of the first grating half  22 A. Specifically, the relatively skinnier teeth  34 B′ of the second grating half  22 B having width w 2  are less effective at creating reflections of light waves, and therefore allow a relatively greater number of coherent light waves to proceed without significant reflection to exit through the second end facet  30 . Correspondingly, the relatively wider teeth  34 A′ of the first grating half  22 A having width w 1  cause substantially more light wave reflection, thereby reducing the overall light emission from the first end facet  28 , as desired.  
      The differences in width between the teeth  34 A′ and  34 B′ in  FIG. 3B  are relative. Accordingly, the width w 1  can vary relative to the width w 2  in a variety of possible configurations, in addition to those described here.  
       FIG. 3C  depicts another embodiment of the present invention, wherein the amplitude or height of the grating teeth is modified in order to alter the κ value on the grating  22 . Here, the first grating half  22 A features teeth  34 A″ having a height h 1  and periodic length a. The second grating half  22 B, disposed to the right of mid-point  38 , possesses teeth  34 B″ having the same periodic length a, but with a relatively lower height h 2 .  
      During operation of the DFB laser  10 , the grating  22  shown in  FIG. 3C  operates in a similar manner to previous embodiments in biasing coherent light emission toward the second end facet  30 . In particular, the relatively tall teeth  34 A″ of the first grating half  22 A possess a relatively high K value in comparison with the relatively lower-height teeth  34 B″ of the second grating half  22 B. This differential in κ values biases light emission toward the second end facet  30  nearest the second grating half  22 B, as in previous embodiments.  
      It should be noted that in each of the embodiments depicted in  FIGS. 3A-3C , the periodic length of adjacent tooth/gap pairs remains substantially constant within the respective grating halves. For example, in  FIG. 3B , each tooth/gap pair has the same period both on the first grating half  22 A and the second grating half  22 B. In  FIG. 3A , the tooth/gap pairs that are present on the second grating half  22 B have the same period as those teeth on the first grating half  22 A if the missing teeth of the second grating half are considered. The same principle applies to the tooth/gap pairs in  FIG. 3C .  
      It should also be noted that the designation of a particular end facet for transmission of the majority of coherent light waves is not limited to that described in the accompanying figures. The DFB laser  10  could be alternatively configured such that the majority of coherent light waves exit to the left through the first end facet  28 .  
      It is appreciated that the shape of the teeth  34  in the various embodiments discussed herein can also vary from that depicted. For instance, instead of a square shape, the teeth could have rounded tops or comprise triangular shapes. Notwithstanding the shape of the grating teeth, the present invention can be practiced as described herein.  
      Reference is now made to  FIGS. 4-6C , which depict additional alternative embodiments of the present DFB laser. As cross sectionally seen in  FIG. 4 , a DFB laser device is generally depicted at  110  and comprises a similar structure to the DFB laser  10  shown in  FIGS. 1 and 2 . Specifically, the DFB laser  110  comprises an n-doped substrate  112 , a bottom confinement layer  114 , an active layer  116 , and a top confinement layer  118  disposed atop one another in a sandwich fashion. Overlying the top confinement layer  118  is a p-doped first top layer  120  having a grating  122  comprised of closely-spaced grooves defined thereon, and a p-doped second top layer  124  overlying the grating  122 . The first and second top layers  120  and  124  each possess differing indices of refraction. A contact layer  126  is disposed atop the second top layer  124 . First and second end facets  128  and  130 , respectively, are shown having AR coatings  132  applied thereon. Though the details of the grating  122  are not apparent in  FIG. 4 , they will be further described below in connection with  FIGS. 5-6C .  
      Reference is now made to  FIGS. 5A, 5B , and  5 C, together with  FIG. 4 , in describing details of the grating  122 .  FIGS. 5A, 5B , and  5 C are close-up cross sectional views of the designated circled portions A, B, and C, respectively, of the grating  122  shown in  FIG. 4 . The grating  122 , as before, comprises a tooth and gap configuration defined in the first top layer  120  having a plurality of teeth  134  and gaps  136  disposed between adjacent teeth. In contrast to the previous embodiments depicted in  FIGS. 2-3C , the tooth period, or distance between the beginning of one tooth/gap pair and the beginning of a succeeding tooth/gap pair, designated as “a,” is varied along the grating length. For example, it can be seen in  FIGS. 5A-5C  that the period a 1  of the grating teeth  134 A disposed near the first end facet  128  is substantially less than the period a 2  of the teeth  134 B disposed near the mid-point of the grating  122 , indicated at  138 . Similarly, the period a 3  of the grating teeth  134 C near the second end facet  128  is substantially less than those teeth  134 B disposed near the mid-point  138 . Thus, the period of the teeth disposed along the length of the grating  122  in the present embodiment continuously increases toward the mid point  138  of the grating, and continuously decreases toward the end facets  128  and  130 . The total magnitude of period change shown in these and in the foregoing and following figures is merely exemplary; indeed, the magnitude of change can be varied as desired for a particular application. The continuously shaped tooth period of the grating  122  shown in  FIGS. 5A-5C  serves to improve the power, frequency response, and chirp characteristics of the DFB laser  110  by reducing spatial hole burning (i.e., non-uniform light intensity within the laser). In particular, reduced hole burning can enhance the power output, as well as frequency response, in a single-wavelength laser device. Additionally, because spatial hole burning can influence the wavelength of the coherent light waves emitted by the laser, reduction of such hole burning via the alteration of the grating period as described above can reduce the amount of chirp produced by the laser during modulation. Because the effects due to spatial hole burning are most evident in lasers having a large product of K and L (the length of the laser), it is in such lasers where the most significant reductions in chirp are anticipated. These principles can therefore be used to improve the operating characteristics of the laser device.  
       FIGS. 6A, 6B , and  6 C depict yet another alternative embodiment of the present invention.  FIGS. 6A-6C  depict the three regions A, B, and C, of the grating  122  shown in  FIG. 4  according to the present embodiment. As illustrated, the period of the grating teeth  134  is continuously varied along the length of the grating  122 : the grating teeth  134 A′ disposed near the first end facet  128  have a relatively large period, as indicated by a 1  in  FIG. 6A , while the teeth  134 B′ disposed near the mid-point  138  are intermediately sized, as indicated by a 2 . The teeth  134 C′, disposed near the second end facet  130 , possess a relatively small period, as indicated by a 3 . This configuration of the grating  122  enables not only the chirp and frequency response of the DFB laser  110  to be improved, but also biases the coherent light signal toward the first end facet  128 , given the higher κ value possessed by the grating portion having a period equal to, or nearly equal to, a 1 . The embodiment illustrated in  FIGS. 6A-6C  therefore combines principles taught in connection with the variation of the grating teeth period with those relating to the variation of the K value along the grating length. This produces a DFB laser device having desirable operating characteristics that minimize problems, such as yield or sidemode suppression, that are associated with imprecise cleaving of the laser device end facets during fabrication.  
      In light of the present embodiment, it is generally appreciated that the teachings of the various embodiments as disclosed herein can be combined to produce grating configurations not explicitly illustrated here. For example,  FIGS. 5A-6C  illustrate a grating having teeth that continuously vary in period along the length of the grating. However, it is also possible to configure the grating such that the tooth period abruptly changes at a specified point along the grating length, as seen in  FIGS. 3A-3C . Thus, these and other modifications are contemplated as falling under the present invention.  
      If desired, it is also possible for a phase shift, such as a quarter wavelength phase shift, to be added to the grating structure to further enhance coherent light output from the DFB laser. Such a phase shift can be added at any appropriate location along the grating length, such as near either end facet, or at the mid-point.  
      Finally, it is noted that, though the gratings discussed herein have been shown as primarily disposed above the active region, it is also possible to dispose a grating made in accordance with the principles taught herein under the active region, such as in the laser substrate.  
      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, not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.