Abstract:
When an optical system is housed in a plastic case, an optical connection in the system might be changed by a deformation of the case due to heat and thermal resistance might be increased. It is important that the case has not only a function of housing the elements of the optical system, but also a function of ensuring that light is stably produced in the optical system, which, is the most important function of the case. 
     Therefore, the deformation of the case due to a change in temperature is prevented by a frame inserted into the opening portion of the case and further by mounting the case on an external aluminum substrate. That is, stability in the optical connection is improved by improving the structure of the case and by mounting an case on the external substrate. Further, an increase in the thermal resistance is prevented in the same way by provision of the frame and the external aluminum substrate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor laser coupling device for applying light to and receiving light from an optical fiber; and, more particularly, the invention relates to a semiconductor laser coupling device in which a light-receiving element, a laser element, and an optical fiber are aligned with each other on a substrate and are coupled to each other and are assembled in a case, to product an assembly which is suitable for outputting light to the outside, and to a method of mounting the assembly on a circuit board. 
     A conventional case of assembling a semiconductor element in an optical module is disclosed in Japanese Patent Laid-Open No. 152527/1997. The optical module described in this publication comprises an optical fiber, a substrate on which an optical element is mounted, a cap for fixing the optical fiber in position on the substrate, and a micro-capillary into which the optical fiber is inserted, the optical fiber being inserted into and fixed to the micro-capillary. A portion of the optical fiber having a length nearly equal to the micro-capillary is inserted into the micro-capillary to cause the tip end thereof to protrude from the tip end of the micro-capillary, and the protruding end of the optical fiber is arranged on a V-shaped groove on the substrate and is fixed thereto by the cap, whereby it is optically coupled to the optical element. The optical element and the optical fiber are then packaged by resin molding. 
     In the above-mentioned conventional optical module, the constituent parts for stable optical coupling of the optical module, that is, the substrate, the optical fiber, the micro-capillary, and the resin, which are constituent members of the optical module, are made of different kinds of materials and are coupled to each other. Therefore, when each member is subjected to a change in temperature, a thermal stress is produced in the members. That is, when the members made of different materials are expanded or constricted by a change in temperature, they are each subjected to a different amount of expansion or constriction, whereby they are bent. In the conventional optical module, no consideration is given to the fact that this bending deformation produces an optical axis deviation in the optical coupling of the optical element to the optical fiber. 
     Further, when an electric current is switched on the optical element to laser light, heat is produced at the same time. In some instances, depending on the structure or the material of the case, in particular, when using a resin case, the following problems are produced: heat dissipation takes a long time because the thermal conductivity of the resin is low, which increases the temperature of the semiconductor and reduces the optical output and produces a change in optical output and a change in lasing wavelength with a lapse of time. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a semiconductor laser coupling device which solves the above described problems and has a high accuracy and a long life. 
     In order to achieve the object described above, the present invention provides a semiconductor laser element, which is optically coupled to an optical fiber, and a light-receiving element, which is provided for monitoring the light of the semiconductor laser element provided, and these elements are assembled in a case whose main component is resin. A reinforcing member having a strength higher than the resin and having a constraining force is provided at the bottom portion of the case and a further reinforcing member is provided at the opening portion of the case. The elements described above are assembled in the case and embedded by pouring resin in the case. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is a longitudinal sectional view of a semiconductor laser coupling device representing one preferred embodiment in accordance with the present invention. 
     FIG.  1 ( b ) is a plan view of FIG.  1 . 
     FIG. 2 is a perspective view showing the inside of a case configured in accordance with the present invention. 
     FIGS.  3 ( a ) to  3 ( d ) are diagrams which illustrate the parts of a case configured in accordance with the present invention. 
     FIGS.  4 ( a ) to  4 ( d ) are diagrams showing in cross-section different examples of a frame  19  in accordance with the present invention. 
     FIG. 5 is a graph showing the results of measurements of deformation caused by a change in temperature of the surface of a Si substrate. 
     FIG. 6 is a side section view of one embodiment in which a semiconductor laser connecting device in accordance with the present invention is mounted on an external circuit substrate. 
     FIGS.  7 ( a ) and  7 ( b ) are top plan and side sectional views, respectively, of another embodiment in which a semiconductor laser connecting device in accordance with the present invention is mounted on an external circuit substrate. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One preferred embodiment in accordance with the present invention will be described with reference to FIG.  1 ( a ) to FIG.  3 ( d ). 
     FIG.  1 ( a ) shows a semiconductor light emitting device in which a semiconductor laser element  10  is made to emit light in response to an electrical signal from an external electrode, and the emitted light is directed into an optical fiber  12 , thereby to deliver the emitted light to another location via the optical fiber. A semiconductor light receiving device in which a semiconductor light receiving element  11  is mounted at the position of the semiconductor laser element  10  such that it receives light from the optical fiber  12  has the same construction as the semiconductor light emitting device. The semiconductor light emitting device and the semiconductor light receiving device will collectively be called a semiconductor laser coupling device in the following description. 
     The semiconductor laser element  10  and the semiconductor light receiving element  11  are bonded to a substrate comprising an Si single crystal (crystal face: (100)) by solder, such as Au—Sn or the like. 
     The semiconductor laser element  10  is an end surface light-emitting element which emits light from the right and left end surfaces of the element (z direction in FIG.  1 ( a )). Although a conventional semiconductor laser element  10  emits light at a radiation angle of 30 to 40 degrees, in order to increase the optical coupling rate to the optical fiber  12 , it is desirable when the radiation angle is close to an angle of incidence of light entering the optical fiber  12 . To be more specific, a mode-enlarging structure is provided in the light-emitting portion of the semiconductor laser element so that the radiation angle of the semiconductor laser element  10  is 10 to 15 degrees. 
     A single mode fiber (outer diameter: 125 μm) is used as the optical fiber  12  and, in order that the light is effectively directed into a core (outer diameter: 7 μm) portion which transmits the light in the optical fiber, it is important that the light-emitting end of the semiconductor laser element is aligned in a proper position relative to the end of the optical fiber (in the X, Y directions in FIG.  1 ( a )). An amount of deviation in position of about 2 μm in a direction perpendicular to an optical axis (in the X, Y direction) between the light emitting end of the semiconductor laser element  10  and the end portion of the optical fiber  12  will cause a relative deviation of 1 dB in the light being transmitted. Therefore, in order to make a highly reliable semiconductor laser coupling device, it is necessary to correctly fix the optical fiber  12  and the semiconductor laser element  10  at predetermined positions. Accordingly, in order to fix the semiconductor laser element  10  with good accuracy, a metallized film and a marker are formed near the center of the Si substrate, and the semiconductor laser element  10  is positioned on the Si substrate by a passive alignment method so that the marker on the Si substrate is aligned with the marker made on the semiconductor laser element  10 . Then, the semiconductor laser element  10  is soldered to the Si substrate. The semiconductor light receiving element  11  is fixed to the Si substrate in the same way. 
     On the other hand, in order to make it possible to stably fix the optical fiber  12 , the optical fiber  12  is fixed in a V-shaped groove formed on the Si substrate  13  in the direction of the emitting light (in the Z direction) from the position of a light-emitting point of the semiconductor laser element  10 , which is connected by the above-described method. The V-shaped groove can be formed with a high accuracy of ±0.5 μm or more by an anisotropic etching method with an angle of 70.6 degrees to the top surface of the Si substrate  13 . Since the optical fiber  12  is fixed along the V-shaped groove, the optical fiber  12  can be prevented from deviating on the Si substrate  13  and can be positioned at the correct position. 
     The optical fiber  12  is bonded and held to the Si substrate  13  not only by the V-shaped groove, but also by a press plate  14 . The optical fiber  12  can be stably fixed to the Si substrate  13  by suitably shaping the part of the press plate  14  which is in contact with the optical fiber  12 . Preferably, the press plate  14  has a V-shaped groove at a portion in contact with the optical fiber  12 , but in place of the V-shaped groove, a U-shaped groove or a square groove may be used. A gap of 5 to 8 μm is suitable for a bonding gap between the press plate  14  and the Si substrate  13 . The gap is not limited to this value, but it may be 5 μm or less. The same Si single crystal material as the Si substrate  13  is suitable for the material of the press plate  14 . 
     The order of assembly is as follows: the metallized film and the marker are formed at predetermined positions on the Si substrate  13  and then the V-shaped groove is formed by anisotropic etching; the semiconductor laser element  10  and the semiconductor light receiving element  11  are bonded to the surface of the Si substrate  13 ; then, the optical fiber is placed along the groove of the Si substrate  13  and its position is adjusted such that it is in parallel to the optical axis and such that the distance between it and the semiconductor laser element  10  is the most suitable; then, the press plate  14  is placed on the optical fiber  12  and bonded thereto using an adhesive; then, the Si substrate  13  with the optical fiber  12  is assembled in a case  15  and is bonded to the center of the bottom surface of the case  15 ; then, the optical fiber  12  is guided out through a hole made through the side wall of the case  15  to which a U-shaped fiber guide  20  is fixed for protecting the optical fiber  12 . 
     The optical fiber  12  is basically covered with a nylon coating  21  protecting itself against an external force, and when the optical fiber  12  is mounted on the Si substrate  13 , the coating  21  is removed to fix the optical fiber  12  with high positioning accuracy and with reliability. A plate  16  with good thermal conductivity is mounted on the bottom surface of the case  15 . 
     After having been assembled in the case  15 , an external electrode terminal  17  and a respective electrode  18  on the Si substrate  13  are electrically connected to each other. The external electrode terminals  17  are mounted on the side wall of the case, as shown in FIG.  1 ( a ), such that each external electrode terminal  17  is at the same level from the bottom surface of the case as the top surface of the Si substrate  13 . Finally, a resin is potted on the semiconductor laser element  10  and the semiconductor light receiving element  11  to protect them against dust, moisture, and the like. 
     Next, one preferred embodiment of the inside structure of the case  15  will described with reference to the perspective view in FIG.  2  and the illustration of the parts in FIGS.  3 ( a ) to  3 ( d ). 
     In the fundamental structure of the case  15  of the present preferred embodiment, a plate  16  having a size of approximately 80% of the bottom surface of the case is inserted into the bottom of the case as a reinforcing member having a good heat dissipativity, and a square frame  19  is inserted into a portion near to the top surface of the case as a reinforcing member. Then, resin is poured. In addition, the external electrode terminals  17  for electrical connection to an external part are mounted at a middle portion between the top and bottom surfaces of the case. It is suitable that the relation between the height of the external terminals  17  and that of the square frame  19  is such that the square frame  19  is mounted, as shown in FIG.  1 ( a ), at least above the external electrode terminals  17  and near to the top surface of the case. 
     The main constitutional material of the case  15  is a resin, and the plate  16  is fixed by the resin at the periphery thereof and the square frame  19  is embedded in the resin. 
     Epoxy, PBS (polybutyl styrene), liquid crystal polymer and the like are suitable for use as the resin, or a mixture of these resins with fillers (glass, AlN, Al 2 O 3 , quartz, carbon powder, aluminum powder, or silver powder) or glass fiber is also suitable, and, in addition to a resin, a mixture of ceramic with copper powder, aluminum powder, silver powder produces the same effects. 
     FIG.  3 ( a ) shows the frame  19 , FIG.  3 ( b ) shows the external electrode terminals  17 , FIG.  3 ( c ) shows the plate  16  which is fixed to the bottom surface of the case, and FIG.  3 ( d ) shows the appearance of the resin case  15  made by pouring a resin on these parts. 
     The parts shown in FIG.  3 ( a ), FIG.  3 ( b ), and FIG.  3 ( c ) may be separate parts or they may be integral with each other. A metal frame made of Sn-containing Cu(Cu-1.0Sn), Fe-containing Cu-(Cu-1.5Fe), or CuW(Cu-30W), or a ceramic frame made of SiC, SiN, or AlN is suitable for use as the plate  16  inserted into the bottom of the case. The frame  19  may be made of the same material as the plate  16 . Further, one plate may be three-dimensionally etched into or plastically deformed into one integral structure to form the frame  19  and the plate  16 . The frame  19  is not necessarily square in the strictest sense, and may be shaped in the form of a polygon having a shape close to a continuous square. Further, small dips or bumps on the surface of the frame  19  are preferable because they put the frame  19  into further tight mechanical contact with the resin. 
     Examples of the cross sectional shape of the frame  19  are shown in FIG.  4 ( a ) to FIG.  4 ( d ). As shown in FIG.  4 ( a ) to FIG.  4 ( d ), the cross section of the frame  19  may be (1) square, (2) circular, (3) trapezoidal, or (4) T-shaped and protruding from the top surface of the case. 
     The deformation of the case on the Si substrate inserted into and fixed to the case produced at the time when the temperature of the whole case is increased from 25° C. to 85° C. was determined by experiments for the two structural examples in which ( 1 ) a metal plate (Cu-1.0Sn) is inserted only into the bottom of the case and ( 2 ) a metal plate (Cu-1.0Sn) is inserted into the bottom of the case and a square made from (Cu-1.0Sn) inserted into the top portion of the case. The resin used was PBS base resin and the size of the Si substrate was 2 mm wide and 4 mm long, with the deformation of the Si substrate being measured in the direction of the length of 4 mm. 
     The results of measurement of the deformation of the Si substrate are shown in FIG.  5 . The measurements were performed at the center of the width (2 mm) of the Si substrate. While there was a difference of 5 μm in deformation between the center of the Si substrate and the end thereof in the case ( 1 ), there was a difference of only 0.2 μm or less in deformation in the case ( 2 ); that is, the Si substrate with no frame  19  was deformed convexly. 
     The reason why there was a difference in deformation between the case ( 1 ) and the case ( 2 ) is that the thermal expansion coefficient of the resin is larger than that of the metal plate and that the center of the bottom of the case protruded convexly upward as the temperature of the case was increased. In particular, in the case ( 1 ), the top portion of the case was expanded largely to produce a large deformation. On the other hand, in the case ( 2 ), the square frame inserted into the top portion of the case constrained the thermal expansion of the resin as compared with the thermal expansion of the resin itself to make the amount of expansion of the case bottom nearly equal to that of the case top, which prevented the case from bending. 
     If the case bends, the Si substrate bonded and fixed to the bottom of the case is also deformed. The optical fiber  12  and the semiconductor laser element  10  which are bonded to the surface of the Si substrate  13  are arranged in the direction of the length of the Si substrate  13 . Therefore, if the Si substrate  13  is deformed, the angle of the end surface of the optical fiber  12  and that of the semiconductor laser element  10  are changed so as to produce an adverse effect on the optical coupling, which results in a change in the optical coupling with a change in temperature. Therefore, in the case ( 2 ), that is, in the resin case in which the metal frames are inserted into the bottom of the case and in the portion near the top of the case, even if the temperature is changed, the optical coupling is not changed. Thus, the case ( 2 ) represents a semiconductor laser coupling device which produces a stable optical output over a wide temperature range. 
     There are some other methods for controlling the deformation of the case when the temperature of the case is changed. In one method, a cover is bonded to the opening of the top surface of the case; in another method, the ends of the cover are folded inside to cover the opening; and, in a third method, a cover provided with a spring force is fitted in the opening of the case. However, the cover bonded to the opening of the case produces a problem in that the bonded portion may be easily separated or deformed and can not be used for a long period of time. The cover fitted in the opening of the case also produces a problem in that, if the case is used for a long period of time, the resin of the case yields to reduce the spring force. 
     According to the present invention, the frame  19  embedded in the resin prevents the case from being deformed without the need for bonding the cover to the case or making the resin yield, thereby producing an effect of preventing a change in optical coupling. 
     FIG. 6 is a cross sectional view of an assembly in which the semiconductor laser coupling device  100  is mounted on a circuit board  200 . 
     The circuit board  200  has a hybrid circuit constituted by an IC  210  for controlling the operation of the semiconductor laser coupling device  100 , a resistance constituting a circuit, and the like. The circuit board  200  is an alumina ceramic board provided with multilayer wiring and has a PGA terminal  230  easily mounted on a circuit board (not shown). The semiconductor laser coupling device  100  uses a SOP signal terminal as an external output terminal and the SOP signal terminal is soldered to the bottom surface of the case after applying thermally conductive grease to the bottom surface of the case. The external terminal of the case is a SOP terminal having an international pin arrangement. This produces an effect that the semiconductor laser coupling device  100  can be easily mounted oh the external circuit. 
     The terminal of the IC  210  is a QFP (Quad Flat Package) and is similarly mounted on the flat plane. The circuit board  200  is surface-mounted with a hybrid circuit, the semiconductor laser coupling device  100  or the like and then is provided with a metal cover  240  to cover the whole unit, the circuit board  200  and the metal cover  240  being fixed to each other with solder  250 . It is not necessary to fix the whole periphery of the circuit board  200 , but only half or more of the whole periphery need be bonded. 
     In the semiconductor laser coupling device  100 , as described above, the metal frame  16  made of Cu-1.0Sn, Cu-1.5Fe, or Cu-30W, or a ceramic frame made of SiC, SiN, or AlN is inserted into the bottom of the case. If thermally conductive grease is applied to the bottom of the case and the circuit board  200 , the heat of the semiconductor laser coupling device  100  can be quickly dissipated. 
     Further, since the metal cover  240  is bonded to the circuit board  200 , the metal cover  240  can also dissipate the heat. Brass (Cu-3OZn), copper (Cu), or SPCC is suitable for the material of the metal cover  240 . If fins shaped like bumps and dips are formed on the surface of the metal cover  240 , they can improve the heat dissipation effects. Further, the metal cover  240  has an effect of electrically shielding the hybrid circuit and, hence, can produce a stable lasing wavelength and an optical output without increasing the temperature of the semiconductor laser element. Since the circuit board  200  is provided with the PGA terminal  230  having an international pin spacing, it can be mounted on an external board substrate of any type. 
     According to the preferred embodiment, the frame  19  is inserted into the portion near to the opening of the case receiving the Si substrate, on which the optical fiber, the semiconductor laser element, and the semiconductor light receiving element are mounted, and the thermally conductive plate  16  is inserted into the bottom of the case. This constitution can prevent the Si substrate from being deformed when the whole module is subjected to a change in temperature. Therefore, this constitution does not produce a change in optical coupling between the semiconductor laser element and the optical fiber or between the semiconductor light receiving element and the optical fiber and can produce a stable optical input/output and a stable lasing wavelength. Further, since the thermally conductive plate  16  is inserted into the bottom of the case and the case is mounted on the external circuit substrate and the whole external circuit substrate is covered with a cover, a stable optical input/output and a stable lasing wavelength can be produced even if the outside temperature is changed. 
     Next, another arrangement of the semiconductor laser coupling device mounted on a circuit board in accordance with the present invention will be described with reference to FIG.  7 ( a ), which is a plan view of the arrangement of the semiconductor laser coupling device, and FIG.  7 ( b ), which is a cross sectional view taken on a line A-A′ in FIG.  7 ( a ). 
     In this preferred embodiment, the bottom cover  240 B is divided into a portion to accommodate the circuit board  200  and a heat-dissipating block  240 C to be provided with the semiconductor laser coupling device. 
     The bottom cover  240 B is formed of plastic, plastic plated with metal, or a thin iron or aluminum plate. 
     Aluminum, stainless steel (SUS430), or copper is suitable for the material of the heat-dissipating block  240 C and the heat-dissipating block  240 C has fins on the surface thereof to increase the heat-dissipating area. 
     In the plan view in FIG.  7 ( a ), two semiconductor laser coupling devices are shown, one being a receiving device and the other being a lasing device. 
     The semiconductor laser coupling devices are mounted on the circuit board  200  as follows: electrical parts, such as a resistor  220 , a control IC  210 , a condenser, and the like, are bonded and fixed to the circuit board  200  in advance and then the semiconductor laser coupling device is mounted at a predetermined position on the circuit board  200 . The circuit board  200  assembled in this manner is placed on and fixed to the bottom cover  240 B coupled to the heat-dissipating block  240 C. In order to sufficiently dissipate the heat of the semiconductor laser coupling device, it is important to reliably fix the semiconductor laser coupling device to the heat-dissipating block  240 C. For example, the heat dissipating block  240 C is secured to the semiconductor laser coupling device by fastening screws into threaded holes made in the semiconductor laser coupling device (the threaded holes are not shown in the drawing). A top cover  240 A is placed on the top surface of the circuit board  200  (not shown in FIG.  7 ( a ) and shown by broken lines in FIG.  7 ( b )), so that the parts are assembled into one device. 
     According to the present invention, the optical coupling of the optical fiber to the semiconductor laser element or the semiconductor laser light receiving element is not changed by an environmental change, and so a stable optical output can be produced. Further, since the case can be made of plastic, which is inexpensive and is manufactured by mass production, the semiconductor laser coupling device can be produced at a low cost. 
     Further, since the semiconductor laser element or the semiconductor light receiving element is received in the case and then is potted with resin or provided with a metal cover, it can be operated without being subjected to an external force or the moisture or the dust in the atmosphere. Further, since the semiconductor laser coupling device uses an external output terminal corresponding to the international standard, the semiconductor laser coupling device is easily mounted on an external board.