Patent Publication Number: US-7215853-B2

Title: Optical module formed on common substrate for branching an optical signal

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical module to be utilized in an optical branching and inserting apparatus for branching a signal light from a trunk line toward a relay station and inserting the signal light transmitted from the relay station to the trunk line in an optical communication field, for example. 
     2. Description of the Related Art 
     In an optical communication using wavelength division multiplexing, an optical branching and inserting apparatus disclosed in JP-A-2000-183816 has been known as an apparatus to be used for branching a signal having a specific wavelength into a relay station and inserting the signal having a specific wavelength from the relay station. 
     As shown in  FIG. 3 , the optical branching and inserting apparatus has an optical branching device  3  for branching a wavelength multiplexing light input from a light transmission path  1  for input, and an optical coupling device  4  for coupling lights having respective wavelengths which are once branched and transmitting the lights to an output transmission path  2 . The optical branching and inserting apparatus comprises a plurality of optical switches  5  corresponding to optical paths having respective wavelengths which serves to select to branch a light having each wavelength branched by the optical branching device  3  into a receiver  7  of a relay station  8  and to newly insert a signal transmitted by a transmitter  6  of the relay station  8  or to exactly transmit the light having each wavelength branched by the optical branching device  3  to the optical coupling device  4 . 
     In such a branching and inserting apparatus, a filter module having the function of fixing a wavelength selecting filter or a lens onto an emitting optical path from an optical fiber and separating a single wavelength component from a multiple wavelength signal or the function of inserting the single wavelength component into the multiple wavelength signal is often used in the optical branching device  3  or the optical coupling device  4 . 
     Such a filter module has a structure in which collimators including a lens and an optical fiber are provided opposite to each other with a wavelength selecting filter interposed therebetween as described in JP-T-10-511476 and JP-A-10-311905, for example. 
     In such a filter module, generally, a wavelength selecting filter, a lens and an optical fiber are inserted and fixed into a common cylindrical housing with an optical axis adjusted. Such a module is generally referred to as an Add/Drop Multiplexer (ADM). 
     Since the optical branching device  3  and the optical coupling device  4  in the optical branching and inserting apparatus of  FIG. 3  are to carry out the same coupling or branching for a plurality of wavelengths, they have such a structure that a plurality of filter module units having different wavelength separating characteristics is used and the optical fibers on signal input/output ends thereof are sequentially connected by a method such as fusion. Such a module is generally referred to as “Mux/DeMux”. A light to be input to the optical branching device  3  or the optical coupling device  4  sequentially passes through a plurality of filter modules to be branched to have each wavelength or a light having each wavelength is sequentially coupled (for example, see JP-A-11-337765). In general, the single modules connected sequentially are attached to a single case. 
     In the optical branching and inserting apparatus using the filter module, if the number of channels to be used for an optical communication is increased, it is necessary to correspondingly increase the number of single filter modules to be used. For this reason, the price of a raw material component is equal to or more than a multiple of the price of the single filter module. Moreover, there is provided the step of fusing the optical fiber on the input/output end of the filter module. Therefore, the step is complicated and a cost is increased, and furthermore, a connecting loss is caused by a transverse offset during fusing connection. Furthermore, the single filter module has such a structure as to be fixed into the housing. Consequently, there is a problem in that an unnecessary volume other than functional parts is required and the volumes of necessary components are also increased with an increase in the number of the channels. 
     In order to eliminate these drawbacks, the inventors tried to reduce the price, size and loss of an optical module in a minimum volume without using unnecessary components by a structure in which an exterior member to be the housing of the filter module is eliminated and the components are fixed onto a single substrate, and a light is spatially propagated between the components. 
     However, it was found that the shift of an optical axis is generated on a light emitted from each component so that optical coupling cannot be easily carried out and an expected performance cannot be obtained in the case in which the element components in the module are to be actually separated and provided on the substrate. 
     The factor for the shift of the optical axis can include the following: 
     the end faces of an optical fiber and a refractive index profile type lens are set to be oblique end faces in order to reduce a reflection loss; 
     the optical axis is shifted when a light is transmitted through the substrate of a dielectric multilayer film filter to be a wavelength selecting filter; 
     fabrication can be carried out with precision in the external shape of each component which is equal to or less than precision in a processing required for optically coupling single mode fibers; and 
     fabrication can be carried out with precision in a processing of a substrate to be provided with these components which is equal to or less than precision required for optically coupling the single mode fibers. 
     The contents will be specifically described. For the optical coupling of the optical fibers, particularly, the single mode fibers, precision in alignment on a submicron level is required because a core diameter is 10 μm or less. In passive optical components such as a fiber pigtail and a lens, a component tolerance and a manufacturing tolerance exceed the same precision. Actually, the fabrication cannot be carried out with the same precision. Even if the fabrication can be individually carried out, moreover, there is a problem in that an emitted light is shifted from the optical axis in a collimator fabricated by a manufacturing method which is a current mainstream. 
       FIG. 4  shows a collimator fabricated by the manufacturing method which is the current mainstream, that is, in combination of a fiber pigtail  11  and a refractive index profile lens  12 . In order to reduce a reflection loss, an angle of approximately 8 degrees is formed on each of the end faces of the pigtail  11  and the lens  12 . Consequently, a position shift δ and an angle shift θ are generated on an emitted light as compared with the position of an incident light. In particular, the amount of the shift of the optical axis caused by the angle shift θ is increased if a coupling distance L is increased as shown in  FIG. 5 . In a collimator pair provided in a V groove on the same straight line, accordingly, the optical coupling is almost zero when a space is several mm or more. 
     In the case in which the V groove for fixing the collimator onto a substrate is fabricated by grinding, moreover, it is desirable that two V grooves provided with the collimator pair should be formed in parallel with each other at a request of a work. For the above reason, the collimator pair for implementing effective optical coupling cannot be fabricated on the V groove. 
     Moreover, an interference filter such as a wavelength selecting filter is usually fabricated by forming a film on a glass substrate  15  having a finite thickness as shown in  FIG. 6  and has a thickness of approximately 1 mm to avoid a breakdown against a generated film pressure. The parallel positional shift amount δ of a light incident at an angle of incidence θ on a medium  2  having a thickness h and a refractive index n 2  from a medium  1  having a refractive index n 1  (=a difference between an optical path to be passed when the medium  2  is not present and an actual optical path) can be expressed in the following equation. 
     
       
         
           
             
               
                 
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       FIG. 7  shows a relationship between the shift amount δ (μm) of the optical axis and the angle of incidence θ (Degree) when a light passes through a substrate having various thicknesses (0.5 to 1.5 mm) as shown in  FIG. 6 . As shown in  FIG. 7 , the shift of the optical axis is generated depending on the thickness of the substrate and the angle of incidence. Even if the optical coupling of the collimator pair is previously carried out before the interference filter is inserted, therefore, the optical path is shifted by the simple insertion of the filter so that a loss can be greatly increased or the coupling cannot be carried out. 
     Even if all the shifts are estimated to carry out a design, furthermore, a processing error and an assembly error of a component and a substrate are generated on each component. In addition, these errors have a level which clearly departs from necessary precision for the optical coupling, which is insignificant. 
     As described above, there is a problem in that the shift of the optical axis is actually great and sufficient optical coupling cannot be obtained if each component is simply arranged in parallel in each V groove for component fixation which is formed on the same substrate as in a conventional trial. 
     SUMMARY OF THE INVENTION 
     The invention has been made in order to solve the problems and has an object to provide an optical module having components such as a collimator and an interference filter arranged on the same substrate in which the shift of an optical axis can easily be corrected and excellent optical coupling can be obtained. Moreover, it is another object of the invention to provide an optical module having a low loss, a small size and a low price which can be utilized as an optical branching device or an optical coupling device to be used in an optical communication field. 
     A first aspect of the invention is directed to an optical module comprising, on a common substrate, a collimator for an input light which collimates a light input from an outside, a collimator for an output light which collects an incident light to be output to the outside and transmits the light to the outside, an interference filter provided on an optical path from the collimator for an input light to the collimator for an output light, and a mirror for correcting an optical path which is provided on an optical path between the collimator and the interference filter or a prism capable of producing the same advantages as those of the mirror. The mirror and the prism will be hereinafter referred to as components for correcting an optical path. 
     According to the invention, the component for correcting an optical path is provided on the optical path between each collimator and the interference filter. By adjusting each component for correcting an optical path, therefore, it is possible to easily correct the shift of an optical axis between the collimators. Consequently, excellent optical coupling can be implemented. Moreover, each component is fixed onto the common substrate and a light is spatially propagated between the components. Therefore, unnecessary components are not used so that the price and size of the optical module can be reduced in a minimum volume. 
     As in a second aspect of the invention, it is possible to use, as the interference filter, at least any of: 
     (a) a wavelength selecting filter for transmitting only a light in a specific wavelength band in incident lights and reflecting lights having other wavelengths; 
     (b) a gain equivalent filter for correcting a light intensity to flatten an intensity of an incident light if the intensity is not uniform for a wavelength; and 
     (c) a filter for taking out only a part of an amount of the incident light. 
     A third aspect of the invention is directed to an optical module comprising a collimator for an input light which collimates a wavelength multiplexing light transmitted from a light transmission path for input into a parallel light, a wavelength selecting filter for transmitting only a light in a specific wavelength band in the wavelength multiplexing lights incident through the collimator for an input light and reflecting lights in other wavelength bands, a collimator for a branched light which collects a light transmitted through the wavelength selecting filter and transmits the light to an external light transmission path for branching, a collimator for an inserted light which collimates a light in a specific wavelength band transmitted from an external light transmission path for insertion into a parallel light and causes the light to be incident on the wavelength selecting filter, a collimator for an output light which collects a synthesized light of a light incident on and transmitted through the wavelength selecting filter by the collimator for an inserted light and any of the wavelength multiplexing lights reflected by the wavelength selecting filter and transmits the collected light to an external light transmission path for output, and a mirror for correcting an optical path which is provided on an optical path between the collimator and the wavelength selecting filter or a prism capable of producing the same advantages as those of the mirror, wherein each of the collimators, the wavelength selecting filter and the component for correcting an optical path are provided on a common substrate. 
     According to the invention, the mirror or prism for correcting an optical path is provided on the optical path between each collimator and the wavelength selecting filter. By adjusting each mirror or prism, therefore, it is possible to easily correct the shift of an optical axis between the collimators. Consequently, excellent optical coupling can be carried out. Accordingly, it is possible to implement an optical branching and inserting apparatus having a low loss. Moreover, each component is fixed onto a single substrate and a light is spatially propagated between the components. Therefore, unnecessary components are not used so that the price and size of the optical module can be reduced in a minimum volume. 
     A fourth aspect of the invention is directed to an optical module wherein a plurality of wavelength selecting filters having a branching function of transmitting only a light having a specific wavelength in incident lights and reflecting lights having other wavelengths and a coupling function of coupling a light having a specific wavelength which is incident from one side and is transmitted and a light having another wavelength which is incident from the other side and is reflected is provided with the specific wavelength varied, the wavelength selecting filters are provided in such a manner that a light reflected by the filter is incident in order from an upstream side toward a downstream side in a direction of advance of the light, a collimator is provided on an optical path for a light incident on the wavelength selecting filter at the most upstream, an optical path for a light transmitted through each of the wavelength selecting filters and an optical path for a light reflected by the wavelength selecting filter at the most downstream respectively, a mirror for correcting an optical path is provided on an optical path between each collimator and the wavelength selecting filter, and the collimator, the wavelength selecting filter and the mirror or prism for correcting an optical path are provided on a common substrate. 
     In the invention, it is possible to sequentially branch and take out lights having different wavelengths from a wavelength multiplexing light or to sequentially couple the lights having different wavelengths, thereby carrying out wavelength multiplexing. In that case, the collimator and the wavelength selecting filter are fixed onto a single substrate and a light is spatially propagated between components. Consequently, unnecessary components are not used so that it is possible to reduce the price and size of the optical module in a minimum volume. Moreover, the mirror for correcting an optical path or the prism capable of producing the same advantages as those of the mirror is provided on the optical path between each collimator and the wavelength selecting filter. By adjusting each component for correcting an optical path, therefore, it is possible to easily correct the shift of an optical axis between the collimators, thereby carrying out excellent optical coupling. Accordingly, it is possible to fabricate a plural wavelength optical branching device and a plural wavelength optical coupling device which have a low loss. 
     A fifth aspect of the invention is directed to the optical module according to the fourth aspect of the invention, wherein the collimator provided on the most upstream is set to be a collimator for an input light which receives a wavelength multiplexing light from an external light transmission path for input, the other collimators are set to be collimators for a branched light which take out a light transmitted through or reflected by the wavelength selecting filter to an outside, and the wavelength selecting filter is utilized as an optical unit for branching, thereby constituting a plural wavelength optical branching device for sequentially branching the wavelength multiplexing light. 
     A sixth aspect of the invention is directed to the optical module according to the fourth aspect of the invention, wherein the collimator provided on the most downstream is set to be a collimator for an output light which transmits an output light to an external light transmission path for output, the other collimators are set to be collimators for an inserted light which cause lights having different wavelengths to be incident on the wavelength selecting filter from an outside, and the wavelength selecting filter is utilized as an optical unit for coupling, thereby constituting a plural wavelength optical coupling device for sequentially coupling lights having different wavelengths. 
     According to the fifth aspect of the invention, the wavelength multiplexing light can be sequentially branched into lights having different wavelengths at a low loss. According to the sixth aspect of the invention, the lights having different wavelengths can be coupled to carry out wavelength multiplexing at a low loss. 
     A seventh aspect of the invention is directed to the optical module according to any of the first to sixth aspects of the invention, wherein each of the collimators is provided on a V groove formed on a common substrate. 
     In the invention, each collimator is fixed to the V groove formed on the common substrate. Therefore, assembly can easily be carried out. 
     An eighth aspect of the invention is directed to the optical module according to the seventh aspect of the invention, wherein each of the collimators is provided on a V groove formed on the common substrate in parallel. 
     In the invention, each collimator is fixed to the V groove formed on the common substrate in parallel. By adjusting the optical path through the mirror or prism for correcting an optical path, therefore, it is possible to easily correct the shift of the optical axis between the collimators while using the parallel V groove which can easily be processed. Thus, excellent optical coupling can be carried out. 
     As in a ninth aspect of the invention, it is desirable that the collimator should be constituted by an optical fiber and a collimate lens provided on an emitting or incident end of the optical fiber. As in a tenth aspect of the invention, moreover, it is desirable that a mirror of a Gimbal type should be used as the mirror for correcting an optical path. 
     In the case in which a prism is to be used as the component for correcting an optical path, it is desirable that a prism of a total reflection type or a vertical angle portion of a wedge-shaped prism having an optional angle should be used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a ) and  1 ( b ) show the views of the structure of an optical module according to a first embodiment of the invention, (a) being a plan view and (b) being a side view, 
         FIGS. 2(   a ) and  2 ( b ) show the plan views showing an optical module according to a second embodiment of the invention: (a) being an explanatory view showing the case in which the optical module is used as an optical branching device; (b) being an explanatory view showing the case in which the optical module is used as an optical coupling device, 
         FIG. 3  is a view showing the schematic structure of a conventional light branching and inserting apparatus, 
         FIG. 4  is an explanatory view showing the shift of an optical axis of a collimator, 
         FIG. 5  is a chart showing a characteristic of the shift of an optical axis in the collimator, 
         FIG. 6  is an explanatory view showing the shift of an optical axis of a wavelength selecting filter, 
         FIG. 7  is a chart showing a characteristic of the shift of the optical axis in the wavelength selecting filter, 
         FIGS. 8(   a ) and  8 ( b ) show the plan views showing an optical module structure according to a third embodiment of the invention, 
         FIGS. 9(   a ) and  9 ( b ) show the plan views showing an optical module having the total prism according to a third embodiment of the invention: (a) being an explanatory view showing the case in which the optical module is used as an optical branching device; (b) being an explanatory view showing the case in which the optical module is used as an optical coupling device, and 
         FIGS. 10(   a ) and  10 ( b ) show the plan views showing an optical module having the wedge-shaped prism according to a third embodiment of the invention: (a) being an explanatory view showing the case in which the optical module is used as an optical branching device; (b) being an explanatory view showing the case in which the optical module is used as an optical coupling device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Mode for Carrying Out the Invention 
     Embodiments of the invention will be described below with reference to the drawings. 
       FIG. 1  is a view showing the structure of an optical module  100  according to a first embodiment of the invention. (a) is a plan view and (b) is a side view. The optical module  100  has the function of an optical branching and inserting apparatus (that is, the function of branching a light having a specific wavelength to the outside for a wavelength multiplexing light to be input and of coupling a light having a specific wavelength to be input from the outside to a light which is not branched and outputting the coupled light), and has such a structure that four collimators  111 ,  112 ,  113  and  114  and one wavelength selecting filter  115  are provided on a substrate  130  and mirrors  121 ,  122 ,  123  and  124  for correcting an optical path are provided between the wavelength selecting filter  115  and the collimators  111 ,  112 ,  113  and  114  respectively, thereby spatially propagating a light between components. 
     The wavelength selecting filter  115  has a branching function of transmitting only a light having a specific wavelength in incident lights and reflecting lights having other wavelengths and a coupling function of coupling a light having a specific wavelength which is incident from one side and is transmitted to a light having another wavelength which is incident from the other side and is reflected. In the wavelength selecting filter  115 , an optical multilayer film (for example, a dielectric multilayer film) is formed on a transmitting substrate such as glass or resin and a filter characteristic can be exhibited depending on the material of the optical multilayer film and a layer structure. In general, the optical multilayer film has such a structure that a material having a small refractive index and a material having a great refractive index are alternately provided. 
     In the optical module  100 , the collimator  111  for an input light, the collimator  112  for an output light, the collimator  113  for a branched light and the collimator  114  for an inserted light are provided as the four collimators. 
     The collimator  111  for an input light serves to collimate a wavelength multiplexing light (an input light) input from a light transmission path for input (a trunk optical fiber) to be spatially transmitted and to cause the same light to be incident on the surface of the wavelength selecting filter  115 , the collimator  113  for a branched light serves to collect a light having a specific wavelength which is incident on the surface of the wavelength selecting filter  115  and is transmitted and to cause the same light to be incident on a light transmission path for branching, the collimator  134  for an inserted light serves to collimate a light incident through an external light transmission path for insertion to be spatially transmitted and to cause the same light to be incident on the back face of the wavelength selecting filter  115 , and the collimator  112  for an output light serves to collect a light obtained by coupling a light having a specific wavelength input from the outside to the wavelength selecting filter  115  through the collimator  114  for an inserted light to lights having other wavelengths which are reflected by the wavelength selecting filter  115  and to output the same light to a light transmission path for output. 
     Each of the collimator  111  for an input light, the collimator  112  for an output light, the collimator  113  for a branched light and the collimator  114  for an inserted light which are used in the embodiment is constituted by a collimate lens for optical coupling to an optical fiber and fulfils the function of converting (collimating) the signal light of the optical fiber into a parallel light and emitting the parallel light toward a space, and collecting the parallel light transmitted spatially onto the end face of the optical fiber and causing the same light to be incident. 
     It is preferable that a rod-shaped lens should be used for the collimate lens in respect of easy handling. Examples of the rod-shaped lens include a refractive index profile type rod-shaped lens and a lens having a spherical or aspheric surface formed on one end of a uniform rod. While the collimator is constituted by an optical fiber and a lens provided on a substrate respectively, the optical fiber and the lens may be previously created in combination and may be provided on the substrate. The latter collimator can be formed by fixing an optical fiber to a holding tool having an equal diameter to the diameter of the collimate lens and inserting and fixing the collimate lens and the holding tool having the optical fiber into a common cylindrical member made of metal such as glass or stainless, for example. 
     The collimators  111  to  114  are positioned and fixed onto the V grooves  131  to  134  formed in parallel with each other on the same substrate  130 , respectively. The first V groove  131  fixing the collimator  111  for an input light and the fourth V groove  134  fixing the collimator  114  for an inserted light are formed through cutting, and the second V groove  132  fixing the collimator  112  for an output light and the third V groove  133  fixing the collimator  113  for a branched light are formed through cutting. 
     Accordingly, the collimator  111  for an input light and the collimator  112  for an output light are provided adjacently to each other in a close position to one of the sides of the rectangular substrate  130 , and the collimator  113  for a branched light and the collimator  114  for an inserted light are provided adjacently to each other in a close position to a side opposite thereto. Moreover, the first V groove  131  and the fourth V groove  134  are formed through cutting. Consequently, the emitting ends of the collimator  111  for an input light and the collimator  114  for an inserted light are opposed to each other, and the incident ends of the collimator  112  for an output light and the collimator  113  for a branched light are opposed to each other. The wavelength selecting filter  115  is provided on almost the center of the substrate  130  so as to be interposed between two pairs of collimators  111 ,  114 ,  112  and  113  opposed to each other. 
     The mirrors  121  and  123  for correcting an optical path are provided on an optical path between the collimator  111  for an input light and the wavelength selecting filter  115  and an optical path between the collimator  113  for a branched light and the wavelength selecting filter  115  in such a manner that a light emitted from the collimator  111  for an input light is incident on the surface (one side) of the wavelength selecting filter  115  at a predetermined angle and a light transmitted through the wavelength selecting filter  115  is incident on the collimator  113  for a branched light, respectively. Moreover, the mirrors  124  and  122  for correcting an optical path are provided on an optical path between the collimator  114  for an inserted light and the wavelength selecting filter  115  and an optical path between the collimator  112  for an output light and the wavelength selecting filter  115  in such a manner that a light emitted from the collimator  114  for an inserted light is incident on the back face (the other side) of the wavelength selecting filter  115  at a predetermined angle and a light obtained by coupling a light emitted from the collimator  114  for an inserted light and transmitted through the wavelength selecting filter  115  to a light emitted from the collimator  111  for an input light and reflected by the surface of the wavelength selecting filter  115  is incident on the collimator  112  for an output light, respectively. 
     For the substrate  130  to be used for fixing each of the collimators  111  to  114  and the wavelength selecting filter  115 , a silicon substrate, a glass substrate, a metal substrate such as aluminum and a plastic substrate which have small coefficients of thermal expansion can be used in order to prevent a positional shift between components after the assembly. Moreover, it is preferable that the substrate  130  should have such a thickness as to obtain a sufficient rigidity. Furthermore, the V grooves  131  to  134  for collimator arrangement to be formed on the substrate  130  can be formed by grinding. In the case in which the glass or the plastic substrate is used, it is also possible to form the V groove by transferring the shape of a mold through press molding. Moreover, a groove such as a slit for fixing the wavelength selecting filter or the mirror may be provided on the substrate. 
     The mirrors  121  to  124  are used for changing an optical path and correcting the shift of an optical axis which is generated depending on precision in the external shape of a component and the shift of an optical axis which is generated during passage through the components. Accordingly, it is preferable to use a mirror having a Gimbal mechanism or a mirror having an adjusting mechanism corresponding thereto. The mirror having the Gimbal mechanism can adjust an inclination by setting one point (usually a center) of the mirror to be a rotation center. It is suitable that a metal mirror such as aluminum or gold should be used for these mirrors  121  to  124  because of an excellence in a reflectance and a durability. 
     The optical module  100  can be manufactured in the following manner. 
     First of all, the substrate  130  having the V grooves  131  to  134  formed thereon is prepared. Next, the collimators  111  to  114  are provided and fixed onto the V grooves  131  to  134  provided on the substrate  130 . At this time, the collimators  111  to  114  may be fixed temporarily or permanently. It is preferable that they should be bonded to the substrate  130  collectively and permanently later in order to shorten a time required for a curing process such as heat curing or UV curing. 
     When the collimators  111  to  114  are provided, two mirrors  121  and  123  are then provided between the collimators  111  and  113  to adjust positions and inclinations thereof in such a manner that a light is experimentally emitted from the collimator  111  for an input light and is coupled to the collimator  113  for a branched light. These two mirrors  121  and  123  have the function of adjusting the directions and inclinations thereof to convert an optical path three-dimensionally. Irrespective of the positional relationship of the collimator  113  for a branched light, therefore, a light emitted from the collimator  111  for an input light can be incident thereon and their optical coupling can be carried out at a low loss. In order to confirm the optical coupling of the collimator pair  111  and  113 , a light source for generating a light having an optional wavelength is connected to the collimator  111  on the emitting side, and the amount of a light of the light source which is collimated and the amount of a light incident on the collimator  113  on the other side are monitored by an optical multimeter. Similarly, the mirrors  124  and  122  are provided between the two collimators  114  and  112  to adjust positions and inclinations thereof in such a manner that a light is emitted from the collimator  114  for an inserted light and proper optical coupling is carried out together with the collimator  112  for an output light. 
     Next, the wavelength selecting filter  115  is provided on almost the center of the substrate  130 . The wavelength selecting filter  115  can change its own direction and inclination in the same manner as the mirrors  121  to  124 . First of all, a light having a wavelength reflected by the wavelength selecting filter  115  is emitted from the collimator  111  for an input light. Then, the position and inclination of the wavelength selecting filter  115  is adjusted in such a manner that a light emitted from the collimator  111  for an input light is reflected by the mirror  121  and is incident on the wavelength selecting filter  115  and a light reflected by the wavelength selecting filter  115  is further reflected by the mirror  122  and is incident on the collimator  112  for an output light. 
     Subsequently, a light having a transmission wavelength of the wavelength selecting filter  115  is emitted from the collimator  111  for an input light. Then, the shift of an optical axis generated by a transmission through the wavelength selecting filter  115  is adjusted by the mirror  123  provided between the wavelength selecting filter  115  and the collimator  113  for a branched light. Moreover, the light having a transmission wavelength of the wavelength selecting filter  115  is emitted from the collimator  114  for an inserted light and a light transmitted through the wavelength selecting filter  115  is adjusted by the mirror  124  provided between the collimator  114  for an inserted light and the wavelength selecting filter  115  to be optically coupled to the collimator  112  for an output light. 
     By adjusting the mirrors  121  to  124  as described above, it is possible to cause the optical paths between all the components to be coincident with each other. Thus, it is possible to fabricate the optical module  100  having the function of branching and coupling a light corresponding to the wavelength selecting characteristic of the wavelength selecting filter  115  (=the light branching and inserting function). According to the optical module  100  fabricated actually, optical coupling can be carried out at a coupling loss which is less than 0.2 dB between the collimators. It is preferable that each component provided on the substrate  130  should be fixed onto the substrate  130  after the adjustment of the optical axis. If the mirrors  121  to  124  are not moved in usual use, the inclinations thereof may be finely adjusted later without fixation. 
     Next, the action of the optical module  100  having the structure will be described. 
     First of all, a wavelength multiplexing light (including lights having wavelengths λ 1  to λn) supplied from a light transmission path for input is emitted as a parallel light from the collimator  111  for an input light. The wavelength multiplexing light thus emitted is incident on the surface side of the wavelength selecting filter  115  through the mirror  121 , and only a light having a specific wavelength (assumed to be the wavelength λ 1 ) is transmitted through the wavelength selecting filter  115  and lights having other wavelengths (the wavelengths λ 2  to λn) are reflected by the wavelength selecting filter  115  depending on the wavelength selectivity of the wavelength selecting filter  115 . 
     The light having the wavelength λ 1  which is transmitted through the wavelength selecting filter  115  is incident on the collimator  113  for a branched light through the mirror  123  and is transmitted to the outside. On the other hand, the light having the wavelength λ 1  which is input from the outside is emitted as a parallel light by the collimator  114  for an inserted light. The light emitted from the collimator  114  for an inserted light is incident on the back side of the wavelength selecting filter  115  through the mirror  124  and is transmitted therethrough, and furthermore, is coupled to the lights having the wavelengths λ 2  to λn reflected by the wavelength selecting filter  115  to produce a wavelength multiplexing light including the wavelengths λ 1  to λn, and the wavelength multiplexing light is incident on the collimator  112  for an output light through the mirror  122  and is transmitted to a light transmission path for output. For the wavelength multiplexing light thus input, a signal light having a specific wavelength is branched and inserted into the outside. 
     According to the optical module  100 , the collimators  111  to  114  are used in the input/output portion of a light to spatially propagate the light between the components. Therefore, it is not necessary to connect the components through an optical fiber. Consequently, manufacture can easily be carried out and the size of the apparatus can be reduced, and furthermore, the components can be replaced readily when they are defective. Moreover, only one wavelength selecting filter  115  is used in a branching and inserting process for one wavelength. Therefore, the number of expensive filters can be decreased so that a manufacturing cost can be reduced. 
     Furthermore, the shifts of the optical axis between the collimators  111  and  113  and the collimators  114  and  112  are corrected by means of the mirrors  121  to  124  provided between the wavelength selecting filter  115  and the collimators  111  to  114 . By adjusting the mirrors  121  to  124 , consequently, it is possible to carry out a light branching and inserting process in which sufficient optical coupling can be obtained and a loss can be reduced. Moreover, the collimators  111  to  114 , the wavelength selecting filter  115  and the mirrors  121  to  124  are provided on the same substrate  130  and a light is spatially propagated between the components. Consequently, unnecessary components are not used and the price and size of the optical module  100  can be reduced in a minimum volume. Since the collimators  111  to  114  are fixed onto the V grooves  131  to  134  formed on the substrate  130  in parallel with each other, particularly, assembly can easily be carried out. 
     While the case in which the wavelength selecting filter  115  is used as an interference filter has been described in the embodiment, it is also possible to use an interference filter having other filter characteristics, for example, a gain equivalent filter for flattening and correcting the light intensity of an original signal when the light intensity is not uniform for a wavelength or a filter for taking out only one part of the amount of an incident light. 
     Next, a second embodiment of the invention will be described. 
       FIG. 2  is a plan view showing an optical module  200  according to the second embodiment of the invention. (a) shows, in an arrow, a direction of incidence/emission of a light when the optical module  200  is used as an optical branching device and (b) shows, in an arrow, a direction of incidence/emission of a light when the optical module  200  is used as an optical coupling device. 
     The optical module  200  comprises a plurality of collimators  210  to  215 , a plurality of wavelength selecting filters  115   a  to  115   d  and a plurality of mirrors  220  to  225  provided and fixed onto a common substrate  230 . Each of these elements has the same function as that described in the first embodiment. For example, the wavelength selecting filters  115   a  to  115   d  have a branching function of transmitting only a light having a specific wavelength in incident lights and reflecting lights having other wavelengths and a coupling function of coupling a light having a specific wavelength which is incident from one side and is transmitted and a light having another wavelength which is incident from the other side and is reflected. 
     There are provided the wavelength selecting filters  115   a  to  115   d  having specific wavelengths varied. The first wavelength selecting filter  115   a  has such a characteristic as to transmit a light having a specific wavelength λ 1  , the second wavelength selecting filter  115   b  has such a characteristic as to transmit a light having a specific wavelength λ 2  , the third wavelength selecting filter  115   c  has such a characteristic as to transmit a light having a specific wavelength λ 3 , and the fourth wavelength selecting filter  115   d  has such a characteristic as to transmit a light having a specific wavelength λ 4 . 
     These wavelength selecting filters  115   a  to  115   d  are provided in such a manner that the lights reflected by them are incident in order from the upstream side toward the downstream side in the direction of the advance of the light. Based on the case of utilization as the optical branching device in (a), the wavelength selecting filters  115   a  to  115   d  are provided in this order in such a manner that the lights reflected by them are incident in order from the upstream side toward the downstream side in the direction of the advance of the light. More specifically, each of the filters is provided to have such a positional relationship that the first and third wavelength selecting filters  115   a  and  115   c  and the second and fourth wavelength selecting filters  115   b  and  115   d  are obliquely opposed to each other and such a positional relationship that the first and third wavelength selecting filters  115   a  and  115   c  and the second and fourth wavelength selecting filters  115   b  and  115   d  are adjacent to each other. 
     A collimator  210  for input and output is provided on an optical path for an incident light on the wavelength selecting filter  115   a  at the most upstream, first to fifth collimators  211  to  215  for branching and insertion are provided on an optical path for the light transmitted through each of the wavelength selecting filters  115   a  to  115   d  and an optical path for the reflected light of the wavelength selecting filter  115   d  on the most downstream, and furthermore, mirrors  220  to  225  for correcting an optical path are provided on optical paths between the collimators  210  to  215  and the wavelength selecting filters  115   a  to  115   d  respectively. The collimator  210  for input and output and the second and fourth collimators  212  and  214  are provided in this order along one of the short sides of the substrate  230  having a rectangular shape, and the first, third and fifth collimators  211 ,  213  and  215  are provided in this order along the other short side of the substrate  230 . 
     The mirrors  221  to  225  adjust an optical path in such a manner that a light advances in the following manner. First of all, a light emitted from the collimator  211  for input and output is incident on the first wavelength selecting filter  115   a  and the light transmitted therethrough is incident on the first collimator  210  for branching and insertion. Moreover, a light reflected by the first wavelength selecting filter  115   a  is incident on the second wavelength selecting filter  115   b  and the light transmitted therethrough is incident on the second collimator  212  for branching and insertion. Furthermore, a light reflected by the second wavelength selecting filter  115   b  is incident on the third wavelength selecting filter  115   c  and the light transmitted therethrough is incident on the third collimator  213  for branching and insertion. In addition, a light reflected by the third wavelength selecting filter  115   c  is incident on the fourth wavelength selecting filter  115   d  and the light transmitted therethrough is incident on the fourth collimator  214  for branching and insertion. A light reflected by the fourth wavelength selecting filter  115   d  is incident on the fifth collimator  215  for branching and insertion. 
     The optical module  200  can be manufactured in the following manner. 
     First of all, the substrate  230  having a V groove (not shown) formed thereon is prepared. Next, each of the collimators  210  to  215  is provided and fixed onto the V groove formed on the substrate  230 . A work to be carried out for the fixation and the confirmation of optical coupling are the same as those in the first embodiment. 
     Subsequently, two mirrors  220  and  221  are provided between the collimators  210  and  211  to adjust positions, directions and inclinations thereof in such a manner that a light having an optional wavelength is emitted from the collimator  210  for input and output and is coupled to the first collimator  211  for branching and insertion. The two mirrors  220  and  221  provided between the collimators  210  and  211  have the function of adjusting the directions and inclinations thereof, thereby converting an optical path three-dimensionally. Irrespective of the positional relationship between a pair of collimators  210  and  211 , their optical coupling can be carried out at a low loss. 
     Then, the first wavelength selecting filter  115   a  is provided in a position between the two mirrors  220  and  221  in which a light emitted from the collimator  210  for input and output strikes. The first wavelength selecting filter  115   a  has a function capable of varying its own direction and inclination in the same manner as the mirrors  220  and  221 . Moreover, the mirror  222  is provided in such a manner that a light emitted from the collimator  210  for input and output and reflected by the first wavelength selecting filter  115   a  is input to the second collimator  212  for branching and insertion. More specifically, the positions, directions and inclinations of the first wavelength selecting filter  115   a  and the mirror  222  are adjusted in such a manner that a light having a wavelength (a wavelength other than λ 1 ) to be reflected by the first wavelength selecting filter  115   a  is emitted from the collimator  210  for input and output and is properly incident on the second collimator  212  for branching and insertion. 
     Although the first wavelength selecting filter  115   a  is inserted so that the optical axis of incidence on the first collimator  211  for branching and insertion is shifted, correction can easily be carried out later by finely adjusting the direction and inclination of the mirror  221  in such a manner that a light having a transmission wavelength (λ 1 ) of the first wavelength selecting filter  115   a  is emitted from the collimator  210  for input and output and the light transmitted through the filter  115   a  is properly incident on the first collimator  211  for branching and insertion. Accordingly, it is possible to obtain optical coupling which is equivalent to that acquired before the insertion of the filter. 
     Next, the second wavelength selecting filter  115   b  is provided in a position between the first wavelength selecting filter  115   a  and the mirror  222  in which a light emitted from the collimator  210  for input and output (a light reflected by the wavelength selecting filter  115   a ) strikes. In the same manner as the mirror, the second wavelength selecting filter  115   b  also has a function capable of varying its own direction and inclination. Moreover, the mirror  223  is provided in such a manner that a light emitted from the collimator  210  for input and output and reflected sequentially by the first and second wavelength selecting filters  115   a  and  115   b  is input to the third collimator  213  for branching and insertion. More specifically, the positions, directions and inclinations of the second wavelength selecting filter  115   b  and the mirror  223  are adjusted in such a manner that a light having a wavelength (a wavelength other than λ 1  and λ 2 ) to be reflected by the first and second wavelength selecting filters  115   a  and  115   b  is emitted from the collimator  210  for input and output and is properly incident on the third collimator  213  for branching and insertion. 
     Also in this case, although the second wavelength selecting filter  115   b  is inserted so that the optical axis of incidence on the second collimator  212  for branching and insertion is shifted, correction can easily be carried out later by finely adjusting the direction and inclination of the mirror  222  in such a manner that a light having a transmission wavelength (λ 2 ) of the second wavelength selecting filter  115   b  is emitted from the collimator  210  for input and output and the light transmitted through the filter  115   b  is properly incident on the second collimator  212  for branching and insertion. Accordingly, it is possible to obtain optical coupling which is equivalent to that acquired before the insertion of the filter. 
     Subsequently, the third wavelength selecting filter  115   c  is provided in a position between the second wavelength selecting filter  115   b  and the mirror  223  in which a light emitted from the collimator  210  for input and output (a light reflected by the wavelength selecting filters  115   a  and  115   b ) strikes. In the same manner as the mirror, the third wavelength selecting filter  115   c  also has a function capable of varying its own direction and inclination. Moreover, the mirror  224  is provided in such a manner that a light emitted from the collimator  210  for input and output and reflected sequentially by the first, second and third wavelength selecting filters  115   a ,  115   b  and  115   c  is input to the fourth collimator  214  for branching and insertion. More specifically, the positions, directions and inclinations of the third wavelength selecting filter  115   c  and the mirror  224  are adjusted in such a manner that a light having a wavelength (a wavelength other than λ 1 , λ 2  and λ 3 ) to be reflected by the first, second and third wavelength selecting filters  115   a ,  115   b  and  115   c  is emitted from the collimator  210  for input and output and is properly incident on the fourth collimator  214  for branching and insertion. 
     Also in this case, although the third wavelength selecting filter  115   c  is inserted so that the optical axis of incidence on the third collimator  213  for branching and insertion is shifted, correction can easily be carried out later by finely adjusting the direction and inclination of the mirror  223  in such a manner that a light having a transmission wavelength (λ 3 ) of the third wavelength selecting filter  115   c  is emitted from the collimator  210  for input and output and the light transmitted through the filter  115   c  is properly incident on the third collimator  213  for branching and insertion. Accordingly, it is possible to obtain optical coupling which is equivalent to that acquired before the insertion of the filter. 
     Then, the fourth wavelength selecting filter  115   d  is provided in a position between the third wavelength selecting filter  115   c  and the mirror  224  in which a light emitted from the collimator  210  for input and output (a light reflected by the wavelength selecting filters  115   a ,  115   b  and  115   c ) strikes. In the same manner as the mirror, the fourth wavelength selecting filter  115   d  also has a function capable of varying its own direction and inclination. Moreover, the mirror  225  is provided in such a manner that a light emitted from the collimator  210  for input and output and reflected sequentially by the first, second, third and fourth wavelength selecting filters  115   a ,  115   b ,  115   c  and  115   d  is input to the fifth collimator  215  for branching and insertion. More specifically, the positions, directions and inclinations of the fourth wavelength selecting filter  115   d  and the mirror  225  are adjusted in such a manner that a light having a wavelength (a wavelength other than λ 1 , λ 2 , λ 3  and λ 4 ) to be reflected by the first, second, third and fourth wavelength selecting filters  115   a ,  115   b ,  115   c  and  115   d  is emitted from the collimator  210  for input and output and is properly incident on the fifth collimator  215  for branching and insertion. 
     Also in this case, although the fourth wavelength selecting filter  115   d  is inserted so that the optical axis of incidence on the fourth collimator  214  for branching and insertion is shifted, correction can easily be carried out later by finely adjusting the direction and inclination of the mirror  224  in such a manner that a light having a transmission wavelength (λ 4 ) of the fourth wavelength selecting filter  115   d  is emitted from the collimator  210  for input and output and the light transmitted through the filter  115   d  is properly incident on the fourth collimator  214  for branching and insertion. Accordingly, it is possible to obtain optical coupling which is equivalent to that acquired before the insertion of the filter. 
     Next, description will be given to the case in which the optical module  200  is used as a plural wavelength optical branching device. In the case in which the optical module  200  is used as the optical branching device, as shown in  FIG. 2(   a ), the collimator  210  for input and output is set to be a collimator for input which receives a wavelength multiplexing light emitted from an external light transmission path for input and the other collimators  211  to  215  are set to be collimators for a branched light which take a light transmitted or reflected by the wavelength selecting filters  115   a  to  115   d  to the outside, and the wavelength selecting filters  115   a  to  115   d  are utilized as optical units for branching. Thus, the function of sequentially branching the waveform multiplexing light can be exhibited. 
     In the case in which the wavelength multiplexing light having the wavelengths λ 1  to λ 5  is input to the collimator  210  for input and output, only the light having the wavelength λ 1  is transmitted through the first wavelength selecting filter  115   a  and is incident on the first collimator  211  for branching and insertion, and the other lights having the wavelengths λ 2  to λ 5  are reflected toward the second wavelength selecting filter  115   b . In the second wavelength selecting filter  115   b , similarly, only the light having the wavelength λ 2  is transmitted and incident on the second collimator  212  for branching and insertion, and the other lights having the wavelengths λ 3  to λ 5  are reflected toward the third wavelength selecting filter  115   c . In the third wavelength selecting filter  115   c , moreover, only the light having the wavelength λ 3  is transmitted and incident on the third collimator  213  for branching and insertion, and the other lights having the wavelengths λ 4  and λ 5  are reflected toward the fourth wavelength selecting filter  115   d . In the fourth wavelength selecting filter  115   d , only the light having the wavelength λ 4  is transmitted and incident on the fourth collimator  214  for branching and insertion, and the other light having the wavelength λ 5  is reflected toward the fifth collimator  215  for branching and insertion. Consequently, a light having each wavelength is branched sequentially. 
     Subsequently, description will be given to the case in which the optical module  200  is used as a plural wavelength optical coupling device. In the case in which the optical module  200  is used as an optical coupling device, as shown in  FIG. 2(   b ), the collimator  210  for input and output is set to be a collimator for an output light which serves to transmit an output light to an external light transmission path for output and the other collimators  211  to  215  are set to be collimators for an inserted light which cause lights having different wavelengths to be incident on the wavelength selecting filters  115   a  to  115   d  from the outside, and the wavelength selecting filters  115   a  to  115   d  are utilized as optical units for coupling. Thus, the function of sequentially coupling the lights having different wavelengths is exhibited. 
     In the case in which the lights having the wavelengths λ 1  to λ 5  are sequentially input to the collimators  211  to  215  for branching and insertion, the lights having the wavelengths λ 5  and λ 4  are coupled in the fourth wavelength selecting filter  115   d , the lights having the wavelengths λ 5  to λ 3  are coupled in the third wavelength selecting filter  115   c , the lights having the wavelengths λ 5  to λ 2  are coupled in the second wavelength selecting filter  115   b , and the lights having the wavelengths λ 5  to λ 1  are coupled in the first wavelength selecting filter  115   a . Then, the light emitted from the first wavelength selecting filter  115   a  is incident on the collimator  210  for input and output and is transmitted to the external light transmission path. 
     As described above, the optical module  200  according to the embodiment can also be used as both the optical branching device and the optical coupling device. In that case, in addition, the shift of the optical axis which is generated in passage through each component is corrected by the mirrors  220  to  225  provided between the wavelength selecting filters  115   a  to  115   d  and the collimators  210  to  215 . Therefore, sufficient optical coupling can be obtained and a light branching process or a light coupling process can be carried out at a low loss. In the optical module  200 , moreover, each of the components is provided on the substrate  230  and a light is spatially propagated between the components. As compared with an optical branching device or an optical coupling device of such a type as to connect a plurality of filter modules through an optical fiber by using the filter modules according to the conventional art, therefore, it is possible to obtain an optical branching device or an optical coupling device which has a lower loss, a smaller size and a lower price. In particular, the optical module according to the embodiment can be more advantageous if the number of channels is increased. 
     [0077-2] 
     A third embodiment. Also in the case in which the mirror used as the component for correcting an optical path is replaced with a prism in the first and second embodiments of the invention, the same advantages can be produced as shown in optical modules  300 ,  400 ,  500  and  600 . In the optical modules  300  and  400 , a total reflection prism is used as the prism so that an optical path for a signal light can be corrected in the same manner as the mirror. In the optical modules  500  and  600 , moreover, a wedge-shaped prism having an optional angle is used as the prism to utilize an angle of refraction of the signal light so that an optical path can be corrected. In both the total reflection type prism and the wedge-shaped prism, if an antireflection coating is provided on a transmitting surface, the same performance as that of a reflecting mirror can be obtained. In case of the wedge-shaped prism, particularly, a beam generated by low angle reflection is not enlarged. Therefore, the size of the prism itself can be reduced. Consequently, it is also possible to produce such an advantage that a whole size can be reduced. 
     The arrangement of the components such as each collimator, a mirror, a prism and a wavelength selecting filter is not restricted to that in each of the embodiments but they may be provided in other ways if a necessary optical path can be formed. While the wavelength selecting filter is used as the interference filter in the embodiments, moreover, an interference filter having another function may be used. If necessary, furthermore, other optical components, for example, a polarizing unit and a lens may be provided on the same substrate in addition to the collimator, the mirror, the prism and the filter. 
     ADVANTAGES OF THE INVENTION  
     As described above, according to the invention, a mirror for correcting an optical path is provided on the optical path between a collimator and an interference filter and the shift of an optical axis between the collimators is corrected by means of the mirror or prism. Therefore, it is possible to implement excellent optical coupling. Moreover, each component is fixed onto a common substrate and a light is spatially propagated between the components. Consequently, unnecessary components are not used and the price and size of an optical module can be reduced in a minimum volume. According to the third aspect of the invention, moreover, it is possible to implement a light branching and inserting apparatus having a low loss by carrying out excellent optical coupling. According to the fourth aspect of the invention, furthermore, it is possible to fabricate a plural wavelength optical branching device (the fifth aspect of the invention) and a plural wavelength optical coupling device (the sixth aspect of the invention) at a low loss.