Abstract:
A monolithic laser configuration has a substrate, a laser diode disposed on the substrate for generating a light beam, a photodetector disposed on the substrate in the path of the light beam for receiving at least one part of the light beam, and a deflection device disposed on the substrate for deflecting the light beam substantially perpendicularly to the substrate surface. The monolithic laser configuration can advantageously be used in a coupling module with a plastic SMT housing of a standard configuration, it being possible to produce the optical access through an opening in the leadframe of the coupling module.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a monolithic laser configuration, a coupling module, and to a method for producing the monolithic laser configuration. 
   In particular, but not exclusively, the invention relates to a monolithic laser configuration having a substrate, a laser diode disposed on the substrate for generating a light beam, a photodetector disposed on the substrate in the path of the light beam for receiving at least part of the light beam, and a deflection device disposed on the substrate for deflecting the light beam substantially perpendicularly to the substrate surface. The monolithic laser configuration can advantageously be used in a coupling module with a plastic SMT housing of a standard configuration, it being possible to produce the optical access through an opening in a leadframe of the couple module. 
   The invention is concerned with the problems which result from the particular features of the construction of components with edge-emitting semiconductor laser diode chips for optical information transmission. 
   Laser diodes (LD) in metal housings (e.g. “butterfly” laser diodes, dual inline (DIL) laser diodes, TO laser diodes, etc.) are known in which the light-emitting window of the LD chip is coupled via air with suitable optical elements (e.g. glass or Si lenses, fiber tapers) to an optical fiber (i.e. a single mode fiber (SMF)). This requires a housing technology with correspondingly dense optical fiber feeding as far as the LD chip, or else a housing (TO) hermetically sealed with a window, and a corresponding optical configuration which images the output of the laser diode through the window onto the optical fiber. 
   Such housings are of complicated construction and expensive to produce. 
   Against this background, efforts are being made to reduce the complexity by using LD chips in the so-called leadframe technology in plastic surface mount technology (SMT) housings. In the case of such solutions, the LD chip is constructed on the leadframe and electrically connected to the associated connections by “standard wire bond” technology. 
   The optical connection of the LD chip is effected by an immersion adaptation to an optical deflection prism that optically deflects the beam and passes it through the silicon submount, which is transparent (for wavelengths &gt;1.2 μm) through the optical access in the leadframe toward the outside. The optical coupling to an SMF can be affected by an Si lens placed directly on the Si submount, a monolithically integrated lens, or the like. 
   In the solution realized hitherto, e.g. in an O-TSSOP plastic SMT housing, however, the construction of the laser chip with a monitor diode (MD) and a deflection prism requires a hybrid mounting of the individual components (mounting of the microprisms on the Si submount and subsequent mounting of the individual chips LD and MD) and also a corresponding coupling by an immersion resin prior to the molding process of the SMT housing. 
   In this construction, too, the production is therefore complicated and expensive. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a monolithic laser configuration that overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a monolithic laser configuration. The laser configuration contains a surface, a laser diode for generating a light beam, a photodetector disposed in a path of the light beam and receiving at least one part of the light beam, and a deflection device receiving the light beam and deflecting the light beam substantially perpendicularly to the surface of the monolithic laser configuration. 
   A monolithic integration of the laser diode, of the photodetector, and of a deflection device for deflecting the light beam simplifies the construction of the laser configuration to a considerable extent. The photodetector is preferably disposed between the laser diode and the deflection device. Preferably the photodetector is offset with respect to a center axis of the light beam. The photodetector may be formed as a continuation of a laser layer. 
   In particular, the laser configuration may form a monolithic Fabry-Perot (FP), an edge-emitting, or a distributed feedback (DFB) laser chip that emits its light power perpendicularly to the mounting area. 
   The photodetector forms an integrated monitor chip that generates a control signal in the form of a photocurrent proportional to the light power. 
   The photodector is preferably disposed in the forward coupled-out light path of the laser diode. As a result, it is possible to completely avoid a coupling-out via the rear-side mirror of the laser diode. The rear-side mirror of the laser diode can therefore be 100% reflective, thereby increasing the efficiency of the laser diode. 
   In one refinement, the deflection device is realized by an interface on the semiconductor substrate of the laser configuration. The interface is configured in such a way that the light beam is directed through the substrate. As a result, the monolithic laser configuration may advantageously be disposed on a leadframe, it being possible to produce the optical access to the light path via an opening in the leadframe. The interface runs at an angle of approximately 45° relative to the surface of the laser configuration and is preferably a mirrored surface of the substrate. 
   Moreover, the monolithic laser configuration may have a covering or housing on the top side of the chip. The covering completely encloses the laser diode, the photodetector and the interface. The incorporation of the chip into a standard configuration is thereby simplified. 
   The covering may contain a transparent plastic and form an optically transparent solid body. All that is essential, however, is that the mirror trenches of the laser diode are filled with a transparent filling composition. A nontransparent plastic may also be applied above the latter in order to form a solid body. This has the advantage that standard tested plastics can be used, which are generally not transparent. In particular, it is thus possible to select a plastic with regard to its mechanical properties and independently of its optical properties. Thus it is possible to produce a stable solid body with a monolithic laser configuration in a simple manner. 
   Overall, the formation of a solid body made of plastic makes it possible to create a component with an edge-emitting chip which is constructed in a simple manner as a plastic SMT housing of a standard configuration. 
   The monolithic laser configuration can advantageously be used in a coupling module for coupling a laser diode to an optical conductor such as e.g. an optical fiber. The coupling module has a leadframe for connecting the laser configuration to a current source, and also a submount made of silicon that is disposed thereon. The monolithic laser configuration is disposed on the submount. The leadframe may have a cutout through which the light beam can be passed by the deflection device. 
   Consequently, a component that radiates perpendicularly to the mounting plane with its electrical connections can be realized in a simple manner. 
   The coupling module may furthermore have a lens in the light path of the light beam in the region of the cutout in the conductor element. The lens makes it possible to improve the coupling of the laser diode to the optical waveguide. 
   The lens may also be monolithically integrated into the silicon submount, for example by etching. 
   The invention additionally provides a method for producing a monolithic laser configuration, the deflection device being produced by creating an interface by etching into the semiconductor substrate of the laser configuration. The interface forms a mirror for beam deflection. The semiconductor substrate preferably contains InP and corresponding epitaxial layers. An HCl—HBr etching solution may be used for etching into the semiconductor substrate. The mixture ratio of HCl to HBr is preferably two to one. The temperature during etching is preferably approximately 5° Celsius. 
   To summarize, the significance of the invention consists in the configuration and in the production of the optically active interfaces for the beam deflection downward at the monolithically integrated laser monitor chip, and also in the monolithic integration of the monitor chip through photolithography and epitaxy. 
   Two advantages of the invention can be seen, in particular, in the fact that:
     a) a complete monolithic integration of a laser with vertical radiation is achieved; the laser chip can then be incorporated into conventional plastic SMT housings in a simple manner; and   b) an edge-emitting LD chip is provided which can be processed completely on a wafer and which, with its integrated monitor function, can already be tested completely on the wafer at the process stage, so that use can be made of all possibilities of known wafer processes for semiconductor chips.   

   Other features which are considered as characteristic for the invention are set forth in the appended claims. 
   Although the invention is illustrated and described herein as embodied in a monolithic laser configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic, cross-sectional view through a conventional component; 
       FIG. 2  is a diagrammatic, cross-sectional view through a coupling element according to an exemplary embodiment of the invention; 
       FIG. 3  is a diagrammatic, cross-sectional view through a monolithic laser configuration according to an exemplary embodiment of the invention; 
       FIG. 4  is a diagrammatic, cross-sectional with beam guidance through the monolithic laser configuration according to an exemplary embodiment of the invention; 
       FIG. 5  is a diagrammatic, partial, cross-sectional view in a region of a deflection device in a beam path; 
       FIG. 6  is a diagrammatic, sectional view of a layer construction of the monolithic laser configuration; 
       FIG. 7  is a graph showing a dependence of an InP etching angle on a HBr/HCl ratio in a production method according to an exemplary embodiment of the invention; and 
       FIGS. 8–10  show Tables 1 to 3 illustrating expitaxy conditions during the production of a laser configuration according to an exemplary embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown diagrammatically a cross section view through a conventional component having a leadframe  1 , a submount  2  disposed thereon, and an optical configuration  4  which is disposed in a cutout  3  of the leadframe  1  and serves for focusing an output light beam. Disposed on the submount  2  is a laser diode  5  and also a monitor diode  6  with electrical connections  7  and  8 , respectively. A mirror element  9  is fitted in between the laser diode  5  and the monitor diode  6 , with a partially reflective mirror surface  10  through which one part of the light generated by the laser diode  5  is reflected in the direction of the optical configuration  4 , while another part is allowed to pass in a direction toward the monitor diode  6 , as indicated by arrows  11 . The laser diode  5 , the monitor diode  6 , and the mirror element  9  are covered by a covering or housing  12 . 
   The laser diode  5 , the monitor diode  6 , and also the mirror element  9  are separate components. 
     FIG. 2  diagrammatically shows a cross section through a coupling module  20  according to an exemplary embodiment of the invention. The coupling module  20  has a leadframe  21 , a submount  22 , and also a monolithic laser configuration  23  disposed thereon. Provided in the leadframe  21  is an opening  24  through which an optical output path of the laser configuration  23  is accessible. Provided in the opening  24  in the optical output path is a lens  25  that focuses, for example, the light beam generated by the laser configuration  23 . Furthermore, a covering or housing  13  in the form of a plastic layer is applied on a top side of the laser configuration  23 . 
   The laser configuration is produced monolithically and contains a laser diode  26  with an electrical contact  27  and an electrical lead  28 , a monitor diode  29  with an electrical lead  30 , and also a mirror surface  31  which is disposed downstream in the optical path of the light beam generated and deflects the light beam essentially perpendicularly to the surface of the laser configuration  23  through the submount  22  and the lens  25 . 
   The coupling module  20  thus forms a complete laser diode-transmitter module component with coupling optical configuration for coupling to an SMF, other waveguides or further optical configurations, or free space. 
     FIG. 3  diagrammatically shows a cross section through the laser configuration  23  according to an exemplary embodiment of the invention. The laser configuration  23  is produced monolithically and contains the laser diode  26  and the monitor diode  29  already shown in  FIG. 2 . The laser diode  26  is delimited by a front mirror pit  35  and a rear mirror pit  36 . The mirror pits  35  and  36  form a front mirror surface  37  and a rear-side mirror surface  38 , respectively, of the laser diode  26 . 
   The monitor diode  29  is disposed in the front-side light path of the laser diode  26 . As a result, the rear-side mirror surface can be made completely reflective since no light from the laser diode  26  has to be projected onto a monitor diode located in the rear-side light path. This increases the efficiency of the laser diode  26 . 
   The front-side mirror surface  37  allows a part of the generated light to pass through, which light impinges on the mirror surface  31 , as indicated by the arrows. 
   By way of example, approximately 30–40% of the generated light is reflected back into the laser diode  26  by the mirror surface, while 60–70% impinges on the mirror surface  31 . These values may vary depending on the mirroring. 
   The mirror surface  31  is at an angle of 45° with respect to the surface of the laser configuration  23 . The mirror surface  31  deflects the generated light beam downward, i.e. perpendicularly to the surface of the laser configuration  23  through the semiconductor material, as is also illustrated in  FIG. 2 . 
   The monitor diode  29  is disposed with respect to the light path between the laser diode  26  and the mirror surface  31 . The monitor diode  29  is offset relative to a center axis of the light beam (offset upward in the orientation shown in  FIG. 3 ). The configuration of the monitor diode  29  is such that a part of the light beam is coupled out laterally and is incident on the monitor diode  29  (approximately 10% in one configuration). The output signal of the monitor diode  29  can accordingly be used for regulating the output power of the laser diode  26 . 
   As an alternative to the configuration illustrated in  FIG. 3 , the monitor diode  29  may also be embodied as a continuation of the laser layer. This has the advantage that epitaxial layers and hence costs can be saved during production. 
   That part of the light beam that is not coupled out laterally is incident on the mirror surface  31 . 
     FIG. 4  diagrammatically shows a further cross section through the monolithic laser configuration  23  according to an exemplary embodiment of the invention. The laser configuration  23  is formed by an active zone  40  on a semiconductor substrate  41 . A transparent covering layer  42  (“cladding”) is applied on the laser diode  26 , the monitor diode  29 , the mirror surface  31 , and also in the trenches  35  and  36 . In particular, however, the covering layer  42  need only be transparent within the mirror trenches  35  and  36 . 
     FIG. 5  shows a detail from the cross section through the monolithic laser configuration from  FIG. 4  in the region of the monolithic beam deflection by the mirror surface  31 . As already mentioned, a part of the light beam generated impinges on the monitor diode  29  located in the forward beam path of the laser diode  26 . 
     FIG. 6  diagrammatically shows the layer construction of the monolithic laser configuration  23 , in which case cladding layers  45 , a contact  46  of the laser diode  26  (LD contact) and also a contact  47  of the monitor diode  29  (PD contact) are additionally illustrated. 
     FIG. 7  is a graph illustrating the dependence of the InP etching angle on the HBr/HCl ratio at room temperature (RT) and 5° C. (“cold”) during the production of the deflection area  31  illustrated in  FIG. 4 . 
     FIGS. 8–10  show Tables 1 to 3 illustrating expitaxy conditions during the production of the laser configuration according to an exemplary embodiment of the invention. Tables 1, 2 and 3 illustrate the method parameters during the basic epitaxy, the p-n blocking epitaxy, and the cladding epitaxy, respectively, during the production of the monolithic laser configuration. The epitaxy processes are followed by the patterning of the absorbing regions of the monitor diode (layer No. 5 and No. 6). A selective diffusion of the p-type contact layer of the laser, the layer then being situated at the top, is subsequently effected. 
   The n-type contact of the monitor diode is situated at the top in the case of this structure. 
   The invention is not restricted to the exemplary embodiments described, but rather encompasses modifications in the context of the scope of protection defined by the claims.