Patent Publication Number: US-2004047556-A1

Title: Optical module

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to an optical module that has two optical parts, such as a fiber array and a lens array, which mutually transmit optical signals.  
       [0002] In the prior art, an optical module as shown in FIGS. 7 and 8 has been proposed. The optical module is formed as a collimator array and includes an optical fiber array  21 , which retains optical fibers  20  arranged in a line, and a lens array  23 , which includes microlenses  22  arranged in a line. (For example, Japanese Laid-Open Patent Publication 2001-305376) Such an optical module is manufactured by securing opposing surfaces of the optical fiber array  21  and the lens array  23  after aligning the optical fiber array  21  with the lens array  23 . The opposing surfaces of the optical fiber array  21  and the lens array  23  are secured by adhesive as shown in FIG. 9 or by holders as shown in FIG. 10.  
       [0003] In the securing method shown in FIG. 9, the opposing surfaces of the optical fiber array  21  and the lens array  23  are directly adhered with adhesive  24  to secure the optical fiber array  21  and the lens array  23 . In the securing method shown in FIG. 10, the optical fiber array  21  is secured to an optical fiber holder  25 , and the lens array  23  is secured to a lens holder  26 . The opposing surfaces of the holders  25 ,  26  are then secured by adhesive or YAG laser welding.  
       [0004] However, when the optical module is manufactured using the securing method shown in FIG. 9, the adhesive is located in an optical path. Therefore, the adhesive is damaged when high power optical signals are used, which decreases the performance of the optical module. Therefore, the high power output signals cannot be used and the application of the optical module is restricted. When the optical module is manufactured using the securing method shown in FIG. 10, the fiber holder  25  and the lens holder  26  increase the outer dimensions of the entire optical module, and increase the number of parts. This increases the manufacturing cost of the optical module.  
       SUMMARY OF THE INVENTION  
       [0005] Accordingly, it is an objective of the present invention to provide an inexpensive optical module that has no resin material in an optical path and is adoptable with high power optical signals.  
       [0006] To achieve the above objective, the present invention provides an optical module, which includes a first optical part, a second optical part, and a hollow spacer. An optical signal is mutually transmitted between the first optical part and the second optical part. The first and second optical parts are secured to each other in a state in which the first and second optical parts are aligned with each other. An optical path is formed between the first and second optical parts to permit the optical signal through. The hollow spacer is located between opposing surfaces of the first and second optical parts. A hollow portion is formed in the hollow spacer. The hollow portion has a size that is large enough so that the hollow spacer does not interrupt the optical path of the optical signal. The first and second optical parts are secured to each other by adhering the hollow spacer to each of the first and second optical parts.  
       [0007] Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008] The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
     [0009]FIG. 1( a ) is a side view illustrating an optical module according to a first embodiment of the present invention;  
     [0010]FIG. 1( b ) is a front view illustrating a hollow spacer;  
     [0011]FIG. 2 is a plan view illustrating the optical module shown in FIG. 1( a );  
     [0012]FIG. 3 is a perspective view illustrating the optical module shown in FIG. 1( a );  
     [0013]FIG. 4 is a cross-sectional view illustrating an optical module according to a second embodiment;  
     [0014]FIG. 5 is a cross-sectional view illustrating an optical module according to a third embodiment;  
     [0015]FIG. 6 is a side view illustrating an optical module according to a working example;  
     [0016]FIG. 7 is a plan view illustrating a prior art optical module;  
     [0017]FIG. 8 is a side view illustrating the optical module shown in FIG. 7;  
     [0018]FIG. 9 is a plan view illustrating the optical module shown in FIG. 8 that is secured by adhesive; and  
     [0019]FIG. 10 is a cross-sectional view illustrating the optical module shown in FIG. 8 that is secured by holders.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0020] An optical module  30  according to a first embodiment of the present invention will now be described with reference to drawings.  
     [0021] FIGS.  1 ( a ),  1 ( b ),  2 , and  3  show an optical module  30  according to a first embodiment. The optical module  30  includes a first optical part, which is an optical fiber array  31  in the first embodiment, and a second optical part, which is a lens array  32  in the first embodiment.  
     [0022] The optical fiber array  31  includes optical fibers (single mode optical fibers)  33  and a capillary  34 , which retains the optical fibers  33  arranged in a line. The lens array  32  is constituted by a flat microlens array in which microlenses  36  are arranged in a line on a right end  35   a  of a transparent lens substrate  35 .  
     [0023] The optical module  30  is constituted as a collimator array in which optical signals are mutually transmitted between the optical fiber array  31  and the lens array  32 . That is, outgoing light from each of the optical fibers  33  is converted to a parallel beam by the corresponding microlens  36 . In contrast, a parallel beam entered from each of the microlens  36  is converged by the microlens  36  and is connected to the corresponding optical fiber  33 .  
     [0024] A hollow spacer  37  is arranged between opposing surfaces of the optical fiber array  31  and the lens array  32 , that is, between a right end  34   a  of the capillary  34  and a left end  35   b  of the lens substrate  35 . The hollow spacer  37  has a hollow portion  37   a  the size of which is large enough so that the hollow spacer  37  does not interrupt a light path L (see FIG. 6) through which the optical signals pass. The optical fiber array  31  and the lens array  32  are secured by adhering the opposing surfaces of the optical fiber array  31  and the lens array  32  with each other via the hollow spacer  37  using adhesive. The hollow spacer  37  has a high machining performance and a low thermal expansion, and is made of inexpensive metal material, such as stainless steel (SUS) and covar. A first adhesive layer (not shown) is formed between the optical fiber array, or the first optical part  31 , and the hollow spacer  37 . A second adhesive layer (not shown) is formed between the lens array, or the second optical part  32 , and the hollow spacer  37 .  
     [0025] A method for securing the optical fiber array  31  and the lens array  32  using the hollow spacer  37  will now be described.  
     [0026] The securing method includes the following steps.  
     [0027] In a first step of the securing method, an adhesive, such as an UV cure adhesive or a thermosetting adhesive, is applied to the left side surface of the hollow spacer  37  that faces the capillary  34 . The hollow spacer  37  is then adhered to the right end  34   a  of the capillary  34 . At this time, the first adhesive layer is formed.  
     [0028] In a second step, an adhesive, such as an UV cure adhesive or a thermosetting adhesive, is applied to the left end  35   b  of the lens substrate  35  in advance. After aligning the optical fiber array  31  and the lens array  32  with each other, the left end  35   b  is adhered to the hollow spacer  37 . At this time, the second adhesive layer is formed.  
     [0029] As described above, the optical fiber array  31  and the lens array  32  are adhered to each other via the hollow spacer  37 .  
     [0030] The first embodiment formed as above provides the following advantages.  
     [0031] (1) The optical fiber array  31  and the lens array  32  are secured to each other by adhering the opposing surfaces of the optical fiber array  31  and the lens array  32  via the hollow spacer  37 , which has the hollow portion  37   a  the size of which is large enough so that the hollow spacer  37  does not interrupt the optical path. Therefore, a structure in which an adhesive does not exist in the optical path is obtained with a simple structure. Thus, the structure in which an adhesive does not exist in the optical path is achieved at a low cost, and the optical module that is adoptable with high power optical signals is manufactured.  
     [0032] (2) Since the opposing surfaces of the optical fiber array  31  and the lens array  32  are adhered to each other via the hollow spacer  37 , it is not necessary to be careful that an adhesive does not enter the optical path during adhering process as in a case where the opposing surfaces of the optical fiber array  31  and the lens array  32  are directly adhered to each other. This facilitates the adhering process, which facilitates the process for securing the optical fiber array  31  and the lens array  32  with each other.  
     [0033]FIG. 4 shows an optical module  30 A according to a second embodiment. The optical module  30 A has the same structure as the optical module  30  of the first embodiment shown in FIGS.  1 ( a ),  1 ( b ),  2 , and  3  except that in the optical module  30 A, an adhesive  40 , such as epoxy resin, is applied about a securing portion where the opposing surfaces of the optical fiber array  31  and the lens array  32  are secured to each other via the hollow spacer  37 . That is, the adhesive  40 , such as epoxy resin, is applied about the first and second adhesive layers. The adhesive  40  reinforces the outer perimeter of the optical fiber array  31 , the lens array  32 , and the hollow spacer  37 .  
     [0034] Therefore, in the second embodiment, the securing portion between the optical fiber array  31  and the lens array  32  is reinforced by the adhesive  40 , and the moisture resistance of the securing portion is improved by the adhesive  40 . Accordingly, an optical module  30 A is obtained having a high reliability.  
     [0035] The first adhesive layer between the optical fiber array  31  and the hollow spacer  37  and the second adhesive layer between the lens array  32  and the hollow spacer  37  are joined with the layer of the adhesive  40 , or a third adhesive layer, which covers the entire outer perimeter of the hollow spacer  37 .  
     [0036]FIG. 5 shows an optical module  30 B according to a third embodiment. The structure of the optical module  30 B of the third embodiment is substantially the same as the optical module  30 A of the second embodiment shown in FIG. 4. In the optical module  30 B, the outer dimensions of the hollow spacer  37  are smaller than the outer dimensions of the optical fiber array  31  and the lens array  32 . That is, the outer dimensions of the hollow spacer  37  are smaller than the outer dimensions of the right end  34   a  of the capillary  34  and the outer dimensions of the left end  35   b  of the lens substrate  35 .  
     [0037] In the third embodiment that is formed as mentioned above, an annular recess is formed between the outer surface of the hollow spacer  37 , the right end  34   a  of the capillary  34 , and the left end  35   b  of the lens substrate  35 . The adhesive  40  is filled in the recess to reinforce the securing portion between the optical fiber array  31  and the lens array  32  with the adhesive  40 . Since the annular recess is formed, the adhesive  40  is prevented from protruding from the outer perimeter of the optical fiber array  31  and the lens array  32 . Accordingly, the outer dimensions of the entire optical module  30 B are substantially uniform.  
     [0038] A working example of the optical module corresponding to the first embodiment shown in FIGS.  1  to  3  is described with reference to FIG. 6.  
     [0039] The solid state properties of the optical module  30  according to the working example are as described bellow. The lens array  32  of the optical module  30  uses a flat microlens array in which the microlenses  36  are arranged in a line at 0.25 mm pitch.  
     [0040] The outer diameter φ of each microlens  36  is 0.25 mm. The focal distance f of each microlens  36  is 0.750 mm (Wave length: 1550 nm). The refractive index n of the lens substrate  35  is 1.523. The thickness t1 of the lens substrate  35  is 0.8 mm. The refractive index n of the core of each optical fiber  33  is 1.467. The optical fibers  33  are single mode optical fibers. The thickness t2 of the hollow spacer  37  is 0.3 mm.  
     [0041] The optical module  30  (collimator array) having the working distance WD of 10 mm and the insertion loss that is less than or equal to 1.0 dB is manufactured by the members having the above mentioned solid state properties. The working distance refers to the maximum collimator length. The distance between the right end  35   a  of the lens substrate  35  and the beam waist of the parallel beam corresponds to half the working distance WD.  
     [0042] It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.  
     [0043] In the above embodiments, the optical module  30  having the optical fiber array  31  and the lens array  32  are described. However, the present invention is not limited to have such structure. The present invention may be widely applied to optical modules having first and second optical parts, which mutually transmit optical signals and are secured to each other by adhering the opposing surfaces after alignment. In this case, the same advantages as the first embodiment are provided by arranging, between the first and second optical parts, a hollow spacer having a hollow the size of which is large enough so that the hollow spacer does not interrupt an optical path through which optical signals pass, and securing the opposing surfaces of the first and second optical parts via the hollow spacer.  
     [0044] The hollow spacer  37  used in the above embodiments may have any shape as long as the hollow spacer  37  has the hollow portion  37   a  the size of which is large enough so that the hollow spacer  37  does not interrupt the optical path L (see FIG. 6) through which optical signals pass.  
     [0045] When the hollow spacer  37  is thin, the hollow spacer  37  is not easily affected by thermal expansion. The hollow spacer may be formed by resin material instead of the metal material.  
     [0046] In the above embodiments, the thickness of the hollow spacer  37  is preferably set taking into consideration of the thicknesses of the adhesive layers formed between the hollow spacer  37  and the right end  34   a  of the capillary  34  and between the hollow spacer  37  and the left end  35   b  of the lens substrate  35 . For example, the hollow spacer  37  is formed to have the thickness obtained by subtracting the thicknesses of the adhesive layers formed on both sides of the hollow spacer  37  from the predetermined distance between the optical fiber array  31  and the lens array  32 .  
     [0047] In the first embodiment, the lens array  32  is constituted by the flat microlens array in which the microlenses  36  are formed on the transparent lens substrate  35  by an ion-exchange method. However, the present invention is not limited to have such structure, but several types of microlenses may be used. For example, after forming a lenticular resin layer on a glass, a lens array may be manufactured by reactive ion etching (RIE) method using anisotropic etching, or a resin lens array may be manufactured by molding. The lens array  32  may be formed by arranging microlenses, which are gradient index rod lenses.  
     [0048] In the above embodiments, the optical modules  30 ,  30 A, and  30 B are constituted by the optical fiber array  31 , which has the optical fibers  33 , and the lens array  32 , which has the microlenses  36 . However, the present invention is not limited to have such structure. That is, the present invention may be applied to a collimator that includes a single core capillary, which has an optical fiber, and a flat microlens, which has a microlens, or a rod lens.  
     [0049] The present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.