Semiconductor laser module

Abstract: A semiconductor laser module is described in which the vicinity of one end of optical fiber (22) is fixed to one side wall of the semiconductor laser module (20), an electronic cooling element (23) is secured to the interior bottom surface of a module package (21), and a substrate (24) composed of a metal plate having a two-level construction in which the upper-level portion (24U) and the lower-level portion (24D) have a level difference h, the upper surface of the electronic cooling element (23) being secured to the lower surface of upper-level portion (24U), a semiconductor laser (25) being secured to the end portion of the upper surface of upper-level portion (24U), and a lens (26) being mounted on the upper surface of lower-level portion (24D) of the substrate; whereby the overall height occupied by all components is reduced by the level difference has compared with a semiconductor laser module in which the lower surface of the substrate is a single plane.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a semiconductor laser module used in, for example, optical communication or optical information processing, and in particular, to a semiconductor laser module in which an optical beam emitted from a semiconductor laser is coupled to an optical fiber and the temperature of the semiconductor laser is regulated by a cooling element. 2. Description of the Related Art: Semiconductor laser modules are widely used as signal light sources used in optical fiber transmission devices. Advances in the miniaturization of transmission devices employing optical fiber necessitate circuit boards of higher integration. Moreover, each type of component incorporated into a circuit board is miniaturized and has a lower-profile structure, thereby creating a demand for semiconductor laser modules which are also of thinner construction. Fig. 1 shows the schematic structure of a semiconductor laser module of the prior art. One end of an optical fiber 12 for transmitting optical signals is accommodated on one side wall of module package 11 of semiconductor laser module 10. Electronic cooling element 13 is disposed on the interior bottom surface of module package 11, and on top of this element, interposed by substrate 14, are arranged semiconductor laser 15 and lens 16 for optically coupling a light beam emitted by semiconductor laser 15 with optical fiber 12. In this semiconductor laser module 10 of the prior art, the following measures are taken to reduce the height of module package 11 to a minimum. The height of electronic cooling element 13 has been reduced to the minimum capable of providing a prescribed cooling power. The portion of substrate 14 arranged on cooling element 13 for mounting lens 16 is processed to a minimum thickness. The portion in which semiconductor laser 15 is mounted is set to a height such that the laser beam coincides with the optical axis of lens 16. The height of module package 11 can be reduced to the extent that the diameter of lens 16 is decreased, but because a prescribed numerical aperture (NA) must be maintained, the diameter is therefore set to the smallest aperture that meets this requirement. In the construction of such a semiconductor laser module of the prior art, the portion of substrate 14 for accommodating lens 16 is made as thin as possible in order to reduce the overall height of interior components to the minimum. However, the overall height occupied by the components (chip-on-carrier) such as electronic cooling element 13 and substrate 14 and semiconductor laser 15 disposed upon electronic cooling element 13 the theoretically cannnot be made less than sum of height of electronic cooling element 13 and the diameter of lens 16 even if, for example, the thickness of substrate 14 is set to processing limits. A specific example of the above-described construction is described in Japanese Patent Laid-open No. 356989/92 or in DE 3 902 579 A (Fig. 4). SUMMARY OF THE INVENTION The object of the present invention is to provide a semiconductor laser module in which the overall height occupied by components within a package can be reduced through the disposition of these components. Another object of the present invention is to provide a semiconductor laser module in which the overall height of components can be made less than the sum of the height of the cooling element and the diameter of the lens. To this end, the present invention concerns a semiconductor laser module as defined in claim 1. According to the invention as defined in claim 1, the semiconductor laser module is thus provided with: (1) a module package of a prescribed shape; (2) a substrate of good heat conductivity composed of a step portion, an upper-level portion of which is a flat plate arranged perpendicular to the upper portion of one side wall surface of the step portion, and a lower-level portion of which is a flat plate arranged perpendicular to the lower portion of the opposite side wall surface of the step portion; (3) a temperature regulation element for regulating temperature having its lower surface secured to the interior bottom surface of the module package and having its upper surface secured to the lower surface of the upper-level portion of the substrate; (4) a semiconductor laser that emits a laser light and that is disposed in the vicinity of one end of the upper-level portion of the substrate; (5) an optical fiber having one end secured at the level of the side-wall portion of the module package which is irradiated by the laser light; and (6) a lens arranged on the upper surface of the lower-level portion of the substrate which optically couples the semiconductor laser and optical fiber. In other words, according to the invention as defined in claim 1, a substrate, which is disposed on a temperature regulation element such as a Peltier element for regulating temperature, is constructed from an upper-level plate portion, a lower-level plate portion, and a step portion that connects these two plate portions, the lower face of the upper-level plate portion being secured to the upper face of the temperature regulation element such that the lower-level plate portion aligns with the temperature regulation element. The lower-level plate portion is thus arranged in a lower position than the upper-level portion by means of the step portion, and by arranging the semiconductor laser on the upper-level portion and the lens on the lower-level portion, the height from the bottom surface of the module package to the top surface of the lens can be reduced from that of the prior art. The invention as defined in claim 1 is characterized in that the level difference between the upper-level portion and the lower-level portion is set such that the upper surface of the lower-level portion of the substrate is lower than the upper surface of the temperature regulation element. If the level difference is made relatively large, the upper surface of the lower-level portion of the substrate can be made lower than the upper surface of the temperature regulation element. The height of such a semiconductor laser module can thus definitely be reduced from that of a module of the prior art. Of course, the upper surface of the lower-level portion of the substrate need not be lower than the upper surface of the temperature regulation element. The invention as defined in claim 2 is further characterized in that the lower surface of the lower-level portion of the substrate is arranged in a non-contacting state at a prescribed spacing from the interior bottom surface of the module package. Heat is therefore not directly conducted between the substrate and the module package, thereby allowing effect utilization of heat energy as well as miniaturization of the temperature regulation element. The invention as defined in claim 3 is further characterized in that the temperature regulation element is an electronic cooling element for electronically cooling the semiconductor laser. Of course, the invention is not limited to a cooling element, and according to necessity, allows combination with a heating element or the use of a heating element alone. The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate an example of a preferred embodiment of the present invention. BREIF DESCRIPTION OF DRAWINGS Fig. 1 is a schematic structural view showing the construction of a semiconductor laser module of the prior art. Fig. 2 is a schematic structural view showing the construction of a semiconductor laser module according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: The present invention will next be described in detail with respect to an embodiment. Fig. 2 shows the construction of a semiconductor laser module of one embodiment of the present invention. The semiconductor laser module 20 of this embodiment employs a somewhat long and narrow module package 21, the vicinity of one end of optical fiber 22 for transmitting optical signals being secured to one side wall of module package 21 by YAG (yttrium-aluminum-garnet) laser welding. Electronic cooling element 23 composed of a Peltier element is secured by solder to the interior bottom surface of module package 21. The subtrate 24 consisting of the upper-level portion 24U and the lower-level portion 24D is constituted by a metallic plate and is structered into levels 24U and 24D with a prescribed level difference h; the plate or substrate 24 is arranged in a manner that said its lower-level portion 24D extends parallel to the interior buttom surface of the module package between said electronic cooling element and said side-wall portion of the module package supporting the optical fiber. The upper-level portion 24U is secured to the upper surface of the electronic cooling element 23. The upper surface of said lower-level portion 24D is lower than the upper surface of the electronic cooling element 23. A semiconductor laser is secured by solder to the end of the upper-level portion 24U which is closer to the optical fiber 22. Optical fiber 22 is secured at a pre-set position such that its optical axis coincides with the height of the light-emitting portion of semiconductor laser 25. Lens 26 is secured by YAG laser welding to the upper surface of lower-level plate portion 24D of substrate 24. The level difference h between the two level portions 24U and 24 D of substrate 24 is set to a value such that the optical axes of this lens 26 and optical fiber 22 coincide. The bottom surface of the lower-level portion 24D of substrate 24 is in a non-contacting state with the interior bottom surface of module package 21. In a semiconductor laser module 20 constructed in this manner, semiconductor laser 25 and optical fiber 22 are optically coupled by means of lens 26. In addition, semiconductor laser 25 discharges heat through electronic cooling element 23 by way of substrate 24 so as to maintain semiconductor laser 25 within a prescribed temperature range. Moreover, according to the semiconductor laser module 20 of this embodiment, a level difference h is provided in substrate 24, thereby allowing the height of the upper surface of lower-level portion 24D to be lower than the height of the upper surface of electronic cooling element 23. Accordingly, not only can the overall height of all components within module package 21 be made smaller than a value produced by merely adding the height of lens 26, the height of electronic cooling element 23 and the thickness of substrate 24, but by adjusting the height of electronic cooling element 23 and the set value of level difference h, the overall height can be further greatly reduced to a value less than a value produced by merely adding the height of electronic cooling element 23 and the diameter of lens 26. As a result, the invention allows a greater degree of freedom in selecting each type of component, such as electronic cooling element 23, while remaining within the range of permissible height for the overall height of the components, and furthermore, affords greater margins in performance and reliability of the components. As a matter of course, the height of the semiconductor laser module can be greatly improved by setting the height of each type of component at its limit. In addition, although one semiconductor laser was arranged within the semiconductor laser module of the embodiment, the present invention may also be applied in a semiconductor laser module in which two or more semiconductor lasers are arranged as described in, for example Japanese Patent Laid-open No. 281442/93. Furthermore, while one example of the shape of the lower-level portion of the substrate has been presented in the present embodiment, the shape and position of arrangement, and height of each type of component (chip-on-carrier) disposed on the temperature regulation element may be modified in various ways depending on the size or shape of the components. According to the invention as described above in claim 1, the substrate arranged on the temperature regulation element is a construction made up of an upper-level portion, a lower-level portion, and a step portion connecting these two portions, and the lower surface of the upper-level portion is secured to the upper surface of the temperature regulation element such that the lower-level portion is at the same level as the temperature regulation element. The lower-level portion is thus arranged at a position lower than the upper-level portion by the amount of the level difference, and arranging the semiconductor laser on the upper-level portion and the lens on the lower-level portion allows a reduction of the height from the bottom surface of the module package to the top of the lens. This construction therefore affords an extra margin in the performance and size of each of the constituent components while reducing the height of the semiconductor laser module and contributing to the economy and compactness of the transmitter. In addition, by miniaturizing each individual constituent component, the semiconductor laser module can be made even more compact. Furthermore, according to the invention as described in claim 2, the upper surface of the lower-level portion of the substrate can be made lower than the upper surface of the temperature regulation element by employing a relatively large level difference, and accordingly, the thickness of the substrate need not differ between the portion at which the lens is secured and the portion at which the semiconductor laser is secured. As a result, the invention has the advantage of allowing easy fabrication of the substrate by, for example, simply bending a flat plate. Moreover, according to the invention as described in claim 3, the lower surface of the lower-level portion of the substrate is at a prescribed distance from, and in a non-contacting state with, the interior bottom surface of the module package. For this reason, heat is not directly conducted between the substrate and the module package, thereby allowing both efficient utilization of heat energy and miniaturization of the temperature regulation element.

Claims: A semiconductor laser module (20), comprising a module package (21) of a prescribed shape containing: a semiconductor laser (15) for emitting a laser light, an end portion of an optical fiber (12) secured at the level of the side-wall portion of said module package irradiated by said laser light, a lens (26) optically coupling said semiconductor laser and said optical fiber, a substrate (24) of good heat conductivity on which are mounted said semiconductor laser and said lens, and a temperature regulation element (23) for regulating the temperature, having its lower surface secured to the interior bottom surface of said module package and its upper surface secured to at least part of the lower surface of said substrate; characterized in that said substrate (24) is constituted by a plate shaped in a manner such as to have: a step portion extending parallel to the part of the upper portion of the lateral surface of the temperature regulation element which is the nearest to the optical fiber; an upper-level portion (24U) arranged perpendicular to the upper portion of the side surface of said step portion which faces the temperature regulation element, the lower surface of said upper-level portion being secured to the upper surface of said temperature regulation element, and said semiconductor laser being mounted on a portion of the upper surface of said upper-level portion which is in the vicinity of said step portion; and a lower-level portion (24D) arranged perpendicular to the lower portion of the opposite side surface of said step portion which faces the optical fiber so as to extend parallel to the interior bottom surface of said module package, between said temperature regulation element and said side-wall portion of said module package supporting said optical fiber, with the upper surface of said lower-level portion being lower than the upper surface of said temperature regulation element, said lens being mounted on the upper surface of said lower-level portion. A semiconductor laser module according to claim 1 characterized in that the lower surface of said lower-level portion (24D) of said substrate (24) is arranged in a non-contacting state at a prescribed spacing from the interior bottom surface of said module package. A semiconductor laser module according to claim 1 characterized in that said temperature regulation element (23) is an electronic cooling element for electronically cooling said semiconductor laser.

Inventors: KOSUGI TOMONARI, KOSUGI, TOMONARI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: G02B6
IPC codes: H01S5, G02B6