Abstract:
The present invention relates to a ferrule assembly capable of yielding a laser resonator which is less likely to be influenced by variations in manufacture and changes in its environment of use, and an optical module equipped with this ferrule assembly. The ferrule assembly comprises a fiber grating and a ferrule attached to a front end portion of the fiber grating. As one of structural features of the ferrule assembly, the portion of the fiber grating excluding a grating region is secured to a securing portion of the ferrule, whereas the grating region is accommodated in an accommodating portion of the ferrule in a state separated from the inner wall of the accommodating portion by a predetermined distance without being secured to the ferrule.

Description:
RELATED APPLICATIONS 
     This is a Continuation-In-Part application of application Ser. No. 08/633,522 filed on Apr. 17, 1996, now pending. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ferrule assembly and an optical module having the ferrule assembly, specifically serving as a light source of optical communication, optical fiber amplifiers, and the like. 
     2. Related Background Art 
     Laser modules are basically composed of a light-emitting device as a light source and an optical resonator comprising a pair of reflectors mutually reflecting the light emitted from the light-emitting device. 
     In the laser modules mentioned above, one of the reflectors constituting the resonator is one facet (back facet) of the light-emitting device itself or dielectric films having a high reflectivity formed on this back facet. A Bragg grating disposed near an end portion of the optical fiber opposing the light-emitting device is utilized as the other reflector. The Bragg grating is formed within the optical fiber by providing a periodic refractive index alteration, and reflects specific light determined by the Bragg diffraction condition. The optical fiber in which a Bragg grating is formed is described as grating faiber and is advantageous that the desirable oscillation wavelength can easily be selected by the specification of the Bragg grating. 
     In case of an optical resonator constituted by one facet of the light-emitting device and a grating fiber, a ferrule is attached to an end portion of the grating fiber, and a ferrule assembly constituted by the grating fiber and the ferrule is secured at a predetermined position. An optical coupling between the light-emitting device and the grating fiber can be realized by disposing an optical element (lens) between the facet of the light-emitting device and the end face of the grating fiber facing each other. The optical coupling can also be realized by rounding the facet of the grating fiber instead of providing the optical element. 
     In a typical configuration of the ferrule assembly, it is common that the optical axis of the light-emitting device and the grating fiber are aligned to each other. In order to prevent the light reflected by the front facet of the light-emitting device back into the waveguide of the light-emitting device again, however, the front facet of the light-emitting device is vertically inclined off the optical axis by a predetermined angle. 
     In case where the facet of the grating fiber is rounded, the ferrule assembly has such a configuration that the end portion of the grating fiber protrudes from the end surface of the ferrule. It is due to the fact that, while the facet of the light-emitting device and the end face of the grating fiber must be installed close to each other, it is structurally difficult for the ferrule to approach the light-emitting device. Hence, a part of the end of the grating fiber including the rounded facet thereof protrudes from the end surface of the ferrule. 
     As a result of detailed studies concerning the above-mentioned resonator, the inventor has found that the resonator characteristic is attributed to the structure of the ferrule assembly. 
     The first factor affecting to the resonator characteristic lies in tightly securing the whole portion of fiber grating to the inner wall of the ferrule with an adhesive. In this case, due to deformations, thermal expansions, and the like of the ferrule, a stress is directly applied to the fiber grating. The reflection characteristic of the fiber grating would change. The second factor is, when the grating fiber and the ferrule are not firmly fixed to each other, the installed position of the grating fiber may shift gradually due to the environment conditions. As a result, the coupling efficiency between the light-emitting device and the grating fiber deteriorates. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems mentioned above, it is an object of the present invention to provide a ferrule assembly which is not influenced by manufacture and environment conditions, and an optical module equipped with the ferrule assembly. 
     For achieving the object, the ferrule assembly of the present invention comprises a grating fiber and a ferrule attached to an end portion of the grating fiber. The grating fiber includes a Bragg grating disposed apart from one end face by a predetermined distance. The ferrule comprises a securing portion for securing the grating fiber and an accommodating portion for accommodating the grating region in which the Bragg grating is disposed. 
     In particular, the ferrule assembly is characterized in that, while the portion of the grating fiber excluding the grating region is secured to the securing portion, the grating region is accommodated in the accommodating portion of the ferrule in a state apart from the inner wall without being secured to the ferrule. 
     As a consequence, the grating region would not receive any stress from the ferrule even when the ferrule is mechanically or thermally deformed. Stable optical characteristics, i.e., stable resonance characteristics, can be attained. 
     Further, the ferrule assembly of the present invention may have a configuration in which an opening end of the insertion bore of the ferrule is provided with a tapered portion. The grating fiber and the ferrule are secured to each other with solder at the tapered portion. Since the solder can appropriately be lead to a desirable securing position, scattering of the securing strength between the grating fiber and ferrule among products can be suppressed. The member to be secured to the ferrule is not needed to be a grating fiber such as that mentioned above but may be a standard single-mode fiber without a Bragg grating. This configuration is widely applicable to technologies in which a ferrule and an optical fiber are secured to each other with solder. 
     The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing a cross-sectional structure of an embodiment of the optical module of the present invention; 
     FIGS. 2A to  2 D are views showing various configurations of the ferrule assembly of the present invention; 
     FIGS. 3A to  3 C are views for explaining manufacturing steps of a ferrule assembly; 
     FIGS. 4A and 4B are views for explaining steps of securing an optical fiber to a ferrule; 
     FIG. 5 is a view for explaining a problem attributable to the steps shown in FIGS. 4A and 4B; 
     FIG. 6 is a view showing a cross-sectional structure of the ferrule assembly of the present invention; 
     FIGS. 7A and 7B are views for explaining steps of manufacturing the ferrule assembly shown in FIG. 6, a process of securing the grating fiber to the ferrule in particular; 
     FIG. 8 is a view for explaining a failure case of plating between the ferrule and the optical fiber; and 
     FIG. 9 is a table showing results of evaluations of manufactured ferrule assemblies. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Detailed description of the present invention will be described referring to the accompanying drawings as follows. 
     In the method of manufacturing an optical module, a grating fiber, which is an optical component with a Bragg grating, is generally used as one of the mirrors of an optical resonator. 
     FIG. 1 is a sectional view showing a structure of the optical module of the present invention employing a ferrule assembly. 
     In the optical module, the light-emitting device  105  is mounted on the base  103  with the sub-mount  104  interposed therebetween. Also, one end  103   b  of the base  103  rises vertically and is provided with a through hole to which the ferrule  106  holding the grating fiber  107  is inserted therethrough. For monitoring the operation of the light-emitting device  105 , the light-receiving device  110  is also mounted behind the light-emitting device  105 . Assembly members such as those mentioned above and installed in the package  100  are mounted on the temperature control device  102  such as Peltier effect device, and is configured so as to maintain an operation temperature as constant as possible by feedback control through the temperature detecting device  109  mounted on the sub-mount  104 . 
     In FIG. 1, the base  103  comprises a portion  103   a  mounted with the sub-mount  104 , and portions  103   b  extending in a direction perpendicular to the portion  103   a,  whereas the portion  103   c  supports a member  133  and a member  132  for holding the light-receiving device  110 . The ferrule assembly is firmly secured by a plurality of support members  141 ,  142 ,  143 . In the drawing,  121   a,    121   b,    141   a,    142   a,  and  143   a  indicate the respective YAG-welding portions of the individual members  141  to  143 . 
     In the optical laser module shown here, the grating region separated from the end of the grating fiber  107  by a predetermined distance is provided with the Bragg grating  107   a.  The Bragg grating  107   a  constitutes an optical resonator in cooperation with the reflecting film formed in the light-emitting device  105 . In this embodiment, the Bragg grating  107   a  is formed into the end portion of the grating fiber  107 , and the rounded end face is disposed very close to the light-emitting device  105 , whereby the resonator length is made as short as possible (optical elements such as lens and the like are unnecessary as well). 
     FIGS. 2A to  2 D are sectional views showing various configurations of the ferrule assembly. In the following descriptions, all the constituents common among drawings are referred to with identical reference numerals without repeating their detailed explanations. 
     In the configuration shown in FIG. 2A, the grating fiber  107  inserted through the ferrule  106  is secured with solder  111  only at the end portion (securing portion  106   a ) of the ferrule  106 . The end portion of the grating fiber  107  protrudes from the end face of the ferrule  106  by on the order of several hundreds of micrometers to several millimeters, whereas the end face of the grating fiber  107  is rounded and provided with a anti-reflecting coating. 
     On the other hand, the end face of the ferrule  106  is tapered about the insertion bore (the securing portion of the ferrule  106  being provided with a tapered opening for soldering). Also, the portion of the tapered opening where the solder attaches is plated with gold, so that the solder favorably attaches to the ferrule  106  side. The inner diameter of the ferrule  106  on the other side (the accommodating portion  106   b  for the Bragg grating  107   a ) is made larger, i.e., on the order of 0.3 to 1 mm, so as to facilitate the insertion of the grating fiber  107  and apply no stress to the fiber grating  107   a.  The inner diameter of the securing portion where the diameter is the smallest and about 126 μm. Since the diameter of the grating fiber  107  is 125 μm, and the gap between the ferrule  106  and the grating fiber  107  is 1 μm, the axial centers of the ferrule  106  and grating fiber  107  substantially coincide with each other. 
     Though solder is used for securing the grating fiber  107  in this embodiment, various other adhesives may also be used. If the grating fiber  107  is secured as mentioned above, the ferrule can be handled integrally with the grating fiber  107 . In the configuration shown in FIG. 2A, no stress is applied to the Bragg grating  107   a.    
     In the optical module employing the ferrule assembly with the configuration shown in FIG. 2A, however, there is a possibility of the solder flowing into the gap between the ferrule  106  and the grating fiber  107 . In such a case, an unnecessary stress would be applied to the Bragg grating  107   a.  Therefore, in the configuration shown in FIG. 2B, the gap between the ferrule  106  and the grating fiber  107  is filled with a cushioning material  112 . While examples of the cushioning material  112  include epoxy resins and the like, any material can be chosen as long as it has a Young&#39;s modulus lower than that of the grating fiber  107 . Employing such a material reduces the stress applied to the accommodated grating region even when the ferrule  106  itself is deformed. 
     The configuration shown in FIG. 2C is the same as that shown in FIG. 2B except that the securing position of the ferrule  106  and grating fiber  107  is different. Such a configuration would function similarly to that shown in FIG.  2 B. 
     In the configuration shown in FIG. 2D, each of the ends of the ferrule  106  is provided with a securing portion  106   a  (at which the inner diameter of the insertion bore is smaller than that of the accommodating portion  106   b ), so that the grating fiber  107  is secured at each securing portion  106   a  with solder  111 . A grating fiber  107  longer than the accommodating portion  106   b  is accommodated within the ferrule  106 , so as to be slackened. Therefore, even when the ferrule  106  is deformed or thermally expanded, no stress would be applied to the Bragg grating  107   a.  As with the other configurations, solder or an adhesive can be used for securing. Also, in order to prevent solder from flowing into the inside of the ferrule  106 , the accommodating space of the ferrule  106  may be filled with a cushioning material. 
     As the cushioning material, any of epoxy resins is preferable. As the material of the ferrule  106 , any of metal and metal-plated ceramic is preferable. The grating fiber  107  is secured with solder to the ferrule at the plated portion of the ferrule. 
     As mentioned above, in the configuration utilizing the Bragg grating formed in the optical fiber as an external resonator, it is preferred that the end of the optical fiber and the light-emitting device are disposed as close as possible to each other. Therefore, other components such as lens and the like must avoid to insert between the optical fiber and the light-emitting device. It is essential to round the end of the optical fiber. In the state where the ferrule is attached to the grating fiber, however, it is difficult to round the end portion of the optical fiber. As a consequence, it is necessary to manufacture the ferrule assembly in which the end of the grating fiber is rounded at first and then the ferrule is attached thereto. 
     FIGS. 3A to  3 C are views for explaining steps of attaching a ferrule to a grating fiber. 
     As shown in FIG. 3A, the grating fiber  107  is basically an optical fiber comprising a cladding  172  and a core  173  in which the Bragg grating  107   a  is formed within the grating region located near an end portion thereof. In particular, in this manufacturing step, the grating fiber  107  is stripped of its coating layer  170 , whereby its end portion is exposed. Subsequently, as shown in FIG. 3B, an end portion  107   b  of such a grating fiber  107  is processed into a spherical form, and its side face is plated with a metal  171  made of Au, or laminated with one of Ni and Cr, and Au successively. The grating grating  107  with thus metal-coated side face is inserted into a ferrule  106  as shown in FIG. 3C, and is secured to one end of the ferrule  106  with solder. The gap between the end of the ferrule  106  and the grating fiber  107  is filled with a resin material  112  such as epoxy or the like. Thus integrating the grating fiber  107  with the ferrule  106  reduces the possibility of the grating fiber  107  being damaged. 
     FIGS. 4A and 4B are views for explaining the detail of the securing operation with solder in a series of steps such as those shown in FIGS. 3A to  3 C. 
     As shown in FIG. 4A, securing is carried out in a state where the grating fiber  107  is inserted into the ferrule  106 . The side face of the grating fiber  107  has already been coated with metal plating made of an Ni—Au alloy or the like. Then, as shown in FIG. 4B, several pieces of ball-shaped solder  111   a,    111   b  are disposed on the grating fiber  107  and are melted by microwave heating. After such processing, the grating fiber  107  is secured to the end of the ferrule  106  with the solder  111  as shown in FIG.  4 B. 
     However, it is difficult to place the solder balls  111   a ,  111   b,  a diameter of about 0.2 to 0.5 mm, at their appropriate positions on a flat end face of the ferrule  106  whose outer diameter is about 1 mm. 
     Also, when the grating fiber  107  which the ferrule  106  is attached thereto is used in an airtight package, it will be inevitable to use flux free solder because of an outgas with organic ingredient. However, flux free solder is less likely to wet to its priming, whereby there is a possibility of the solder  111  flowing into the gap between the grating fiber  107  and the ferrule  106  or flowing out of the ferrule  106  as shown in FIG.  5 . 
     The ferrule assembly of the present embodiment is characterized in that it comprises a structure which can overcome problems such as those mentioned above, reliably secure an optical fiber and a ferrule to each other. In the embodiment, the optical fiber to be secured to the ferrule with solder is not restricted to a grating fiber. 
     The main feature of the ferrule assembly according to the embodiment lies in that the form of the end portion of the ferrule  106  are optimized such that the solder  11  attaches to both of the optical fiber  107  and the ferrule  106  as shown in FIG.  6 . 
     In the embodiment, the end portion of the ferrule  106  is formed with a tapered opening. The inner face of the tapered opening forms an angle of at least 50° but less than 65° with respect to the side face of the optical fiber  107 . If this angle is 65° or greater, the melted solder may flow out widely. On the other hand, if the angle is less than 50°, the amount of solder need to secure the optical fiber  107  may not be accommodated in the tapered opening. Also, the area where the solder attaches to the optical fiber  107  is elongated, which is more likely to influence optical characteristics of the optical fiber  107 . 
     Further, according to a preferred mode of the embodiment, the inside diameter of the ferrule  106  is designed such that the gap between the optical fiber  107  and the inner wall face of the ferrule  106  is at least 1 μm but not exceeding 10 μm. That is, if the gap exceeds the upper limit thereof, then the melted solder may flow into the inside of the ferrule  106 , whereby the securing of the optical fiber  107  at the end portion of the ferrule  106  becomes insufficient. If the gap is narrower than the lower limit thereof, then it becomes difficult to insert the optical fiber  107 . 
     The solder used for securing the optical fiber  107  is preferably at least one of Sn, Pb, Au, Ag, and In. On the other hand, it is preferred that the region where solder is mounted on the surface of the ferrule  106  be plated with one of Ni and Cr, and Au successively, or with Au directly. 
     The configuration of the ferrule assembly according to the embodiment will now be explained with reference to drawings. The following disclosure, however, relates only to an embodiment and does not restrict the technical scope of the present invention at all. 
     FIG. 6 is a sectional view showing characteristic portions of the ferrule assembly of the present invention. This ferrule assembly is applicable to the optical modules shown in FIG.  1 . 
     As shown in FIG. 6, the end portion of the ferrule  106  is formed with a tapered opening  106   a  such that the inside diameter of the through hole increases toward the end portion. To such a ferrule  106 , the optical fiber  107  (the grating fiber provided with the Bragg grating  107   a ) inserted therein is secured with a resin and the solder  111  on the end sides of the ferrule  106 , respectively. Since the inside of the ferrule  106  would be sealed with the resin and the solder  111 , the ferrule  106  is provided with a vent hole  106   b,  to leak the air from the inside of the ferrule  106 . The vent hole  106   b  also contributes to facilitating the adhesion of solder  111 . 
     In the ferrule assembly, the angle θ between the inner face  106   a  of the tapered opening and the side face of the optical fiber  107 , and the size of the inside diameter d 1  of the ferrule  106  with respect to the outer diameter d 0  of the optical fiber  107  are defined as mentioned above. For comparison, a plurality of ferrules with the above-mentioned angle θ and inner diameter d 1  being varied were prepared, and assembled samples were evaluated with respect to each specification. 
     Each of the prepared ferrule samples is made of stainless steel, whose surface is plated with one of Ni and Cr, and Au successively, or with Au directly. A ferrule made of plastics or ceramics can also be handled in a manner similar to the embodiment when plated with one of Ni and Cr, and Au successively, or with Au directly. The ferrule  106  has an outer diameter of 1.6 mm and a length of 10 mm. The inner diameter of the tapered opening is 0.9 mm. The form of the tapered opening is designed so as to yield a depth of about 260 μm when the angle θ is 60°. Further, the length of the securing portion is set to about 0.5 mm, and the inner diameter of the remaining portion (accommodating portion) of the ferrule is set to about 0.5 mm. The step between the larger diameter portion and smaller diameter portion is left so as to keep its shape formed upon processing. 
     The prepared optical fiber  107  has an outer diameter of 125 μm in a state where the coating layer  170  is removed. The end portion is rounded, so that the end portion in the final ferrule assembly protrudes from the end face of the ferrule by about 1.0 to 1.3 mm. Further, the end portion of the optical fiber  107  is formed with the Bragg grating  107   a  over a length of about 5 mm. 
     FIGS. 7A and 7 a re views for explaining steps of manufacturing the ferrule assembly of the embodiment. 
     First, as shown in FIG. 7A, the optical fiber  107  is inserted into the ferrule  106 . The side face of the optical fiber  107  has already been provided with a metal plating  220  made of Au, or laminated with one of Ni and Cr, and Au successively. Subsequently, several solder balls are placed at the tapered opening so as to surround the optical fiber  107 , and then are melted by microwave heating. After such processing, the optical fiber  107  is secured to the ferrule  106  with the solder  111  as shown in FIG.  7 B. 
     A plurality of ferrule assembly samples were prepared by a series of steps mentioned above. Employed as the solder balls were those each having a diameter of 0.2 mm. The table of FIG. 9 summarizes evaluations of each sample. 
     In the table of FIG. 9, the cases where “Failure”, is noted in the column of “Result” refer to those in which the solder  111  flows to the outside or inside of the ferrule as shown in FIG.  8 . Also, symbol “*” in the table means ½ of the difference between the outer diameter d 0  of the optical fiber  107  and the inner diameter d 1  of the ferrule  106 . 
     As can be seen from the above-mentioned results, appropriately choosing the inside diameter of the through hole of the ferrule  106  and shaping its end portion into an appropriate form enables the optical fiber  107  to be favorably secured with the solder  111 . 
     In accordance with the present invention, as explained in detail in the foregoing, since a space for accommodating the Bragg grating is provided within the ferrule, no mechanical stress is applied to the fiber grating. Consequently, the fiber grating exhibits the initially-set characteristics, whereby the designed oscillation wave length can be obtained with a good reproducibility. Due to such effects of the invention, the yield in manufacture of the optical module and the stability of the product thereof are improved. 
     Further, since an end portion of the ferrule is provided with a tapered opening for attaching solder thereto, the ferrule assembly of the present invention can reliably secure the optical fiber and the ferrule with the solder, thus enabling the manufacturing efficiency of the finally-obtained semiconductor laser module (optical module) to improve. 
     From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims. 
     The basic Japanese Applications No. 7-090561 filed on Apr. 17, 1995, No. 10-200346 filed on Jul. 15, 1998, and No. 11-123244 filed on Apr. 28, 1999 are hereby incorporated by reference.