Abstract:
A method to assemble a transmitter optical module is disclosed, where the optical module installs two lenses, one of which concentrates an optical beam emitted from a laser diode, while, the other collimates the optical beam concentrated by the former lens. The method has a feature that the first lens is firstly positioned in a point to collimate the optical beam coming from the laser diode, then, moved to a point, which is apart from the former point with respect to the laser diode, to concentrate the optical beam. The process performs the steps to position the lens by a jig to extract the optical beam passing through the first lens outside of the housing.

Description:
BACKGROUND 
     1. Technical Field 
     The present application relates to a method to assemble a transmitter optical sub-assembly (hereafter denoted as TOSA), in particular, the application relates to a TOSA having a function of the wavelength division multiplexing (hereafter denoted as WDM). 
     2. Prior Arts 
     Various TOSAs implemented with the WON function have been disclosed. For instance, U.S. Pat. No. 8,036,533B, has disclosed a TOSA with optical sources, arrayed lenses, and an optical multiplexer where they are enclosed within a package. Optical beams coming from respective optical sources are collimated by arrayed lenses, enter the optical multiplexer by an inclined angle, iterate internal reflection within the optical multiplexer as being multiplexed with other optical beams, and are output as a single multiplexed beam. The inclined angle of the optical multiplexer and the thickness thereof determine the axes of the optical beams. 
     A Japanese Patent Application laid open No. H01-101511A has disclosed an optical system for multiplexing optical beams each output from a laser diode (hereafter dentoed as LD) and collimated by a collimating lens. The collimated beams enter a concentrating lens but at points different from others. The concentrating lens concentrates thus entered collimated beams on a point. 
     As a volume of information to be transmitted drastically increases, a new type of an optical transceiver widely called as Centum Form factor Pluggable (hereafter denoted as CFP) has been developed. However, there is no end to request further increase of the transmission capacity by limited power consumption. An optical transceiver with smaller sized and reduced power consumption compared with CFP is eagerly requested in the field. Such an optical transceiver generally needs an enhanced coupling efficiency between an external optical fiber and an optical signal source. The optical systems those disclosed in prior arts described above do not always satisfy the requests. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present application relates to a method to assemble a transmitter optical module that includes steps of: preparing an intermediate assembly including a semiconductor laser diode (hereafter denoted as LD), a substrate for mounting the LD, and a housing for installing the substrate with the LD; extracting an optical beam output from the LD to an outside of the housing by using a supplementary jig to offset the optical beam; aligning a first lens, as monitoring the optical beam, in a position where the optical axis of the first lens coincides with that of the LD and the optical beam passing through the first lens becomes a collimated beam; and moving the first lens, as monitoring the optical beam passing through the first lens, in another position where the LD is positioned in a focal point of the first lens. 
     The supplementary jig may include two prisms each reflecting the optical beam output from the LD to offset the optical axis of the optical beam, the optical bean output from the supplementary jig being substantially in parallel to the optical axis of the optical beam output from the LD. 
     The step to align the first lens includes a step to monitor the optical beam output from the supplementary jig two-dimensionally. Specifically, the first lens may be aligned such that a position corresponding to the maximum intensity becomes the center of the two-dimensional image, and this two-dimensional image has a predetermined size. 
     The step to move the first lens includes a step to move the first lens so as to be apart from the LD by a predetermined length as keeping the maximum intensity to be in the center of the two-dimensional image. 
     The method may include, after moving the first lens, a step to fix the first lens by steps of: lifting the first lens from the substrate slightly, applying an adhesive resin on the substrate beneath the lifted first lens; landing the first lens on the substrate; and curing the adhesive resin by irradiating the resin with ultraviolet rays and/or heating the resin. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
         FIG. 1  is a plan view of a transmitter optical module according to one embodiment; 
         FIG. 2  is a side view of the transmitter optical module shown in  FIG. 1 ; 
         FIG. 3  is a flow chart of a process to assembly the transmitter optical module shown in  FIG. 1 ; 
         FIG. 4  schematically shows a process to assemble the first lens installed in the transmitter optical module; 
         FIG. 5  is a flow chart of the process to assemble the first lens; 
         FIG. 6  schematically shows an intensity profile of an optical bema detected in the arrangement shown in  FIG. 4 ; 
         FIG. 7  schematically shows a process modified from those shown in  FIG. 4 ; 
         FIG. 8  schematically shows a process subsequent to those shown in  FIG. 4  to position the first lens; 
         FIG. 9  shows a flow chart of a process subsequent to those shown in  FIG. 5 , where the process assembles the second lens; and 
         FIG. 10  schematically shows a process to assemble the second lens. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Next, some embodiments will be described as referring to drawings. In the description of the drawings, numerals or symbols same or similar to each other will refer to elements same or similar to each other without duplicate explanations. Also, in the figures, symbols, P 1  to P 7 , denote reference planes each disposed in this order. 
     A transmitter optical module of an embodiment will be described as referring to  FIGS. 1 and 2 . The transmitter optical module  2  includes a driver  10 , sub-mounts,  11   a  to  11   d , semiconductor laser diodes (LDs),  12   a  to  12   d , first lenses,  13   a  to  13   d , a beam splitter  14 , monitor photodiodes (mPDs),  15   a  to  15   d , second lenses,  16   a  to  16   d , and an optical multiplexer  17 . The first lenses,  13   a  to  13   d , are a type of concentrating lens; while, the second lenses,  16   a  to  16   d , are a type of collimating lens. The transmitter optical module  2  further includes a substrate  1   a , a housing  1   b  and an output port  18 , where the substrate  1   a  mounts those electrical and optical elements described above; while, the housing  1   b  provides an output port  18  in one side thereof; and encloses those elements and the substrate  1   a  therein. The output port  18  provides a bore continuous from an opening provided in one side of the housing  1   b . The bore of the output port  18  has a center substantially aligned with the center of the opening of the housing  1   b.    
     The substrate  1   a  provides a surface  1   a   1  for mounting the electrical and optical components above described thereon. The LDs,  12   a  to  12   d , which have a structure same with each other in the present embodiment, are disposed along the first reference plane P 1  on the primary surface  1   a   1  of the substrate  1   a  through respective sub-mounts,  11   a  to  11   d . The LDs,  12   a  to  12   d , each emits an optical beam B 1  with a specific wavelength different from others. 
     The first lenses,  13   a  to  13   d , which have a configuration same with others, are arranged along the second reference place P 2  on the primary surface  1   a   1  of the substrate  1   a  by adhesive resin J 1 . The second reference place P 2  is in parallel with the first reference plane P 1 . Each of the first lenses,  13   a  to  13   d , concentrates the optical beam B 1  coming from respective LDs,  12   a  to  12   d , on the fourth reference plane P 4  that is also in parallel to the first and second reference planes, P 1  and P 2 . The beam splitter  14 , which is set between the second and fourth reference planes, P 2  and P 4 , divides the optical beams B 2  output from the first lenses,  13   a  to  13   d , into two parts, one of which B 2b  heads for the second lenses,  16   a  to  16   d , transmitting through the beam splitter  14 ; while, rest portions advance toward the mPDs,  15   a  to  15   d , each mounted on the beam splitter  14 . When the LDs,  12   a  to  12   d , are set on the first reference plane P 1  and the first lenses,  13   a  to  13   d , are set on the second reference plane P 2 , the optical beams B 2  output from the first lenses,  13   a  to  13   d , and partially reflected by the beam splitter  14  focus on the primary surface of respective mPDs,  15   a  to  15   d , as the optical beams B 2a . 
     The mPDs,  15   a  to  15   d , are disposed along the third reference plane P 3 , but the beam splitter  14  in a lateral center thereof is offset from the third reference plane P 3 , that is, referring to  FIG. 2 , the interface between two prisms of the beam splitter  14  has an angle smaller than 45° with respect to the primary surface  1   a   1  of the substrate  1   a . Thus, the optical beams B 2a  enter respective mPDs,  15   a  to  15   d , by an inclined angle deviating from the normal of the mPDs,  15   a  to  15   d . This optical arrangement of the mPDs,  15   a  to  15   d , and the beam splitter  14  prevents light reflected at the surface of the mPDs,  15   a  to  15   d , from returning to the LDs,  12   a  to  12   d , and entering therein. Re-entered light in an LD causes optical noises in the LD. In the present embodiment, the mPDs,  15   a  to  15   d , each has a structure same with others, and may have the type of, what is called, the top illumination or the back illumination. 
     The second lenses,  16   a  to  16   d , are the type of the collimating lens. The second lenses,  16   a  to  16   d , are positioned along the fifth reference plane P 5  on the primary surface  1   a   1  of the substrate  1   a  also by adhesive resin J 2 . The fifth reference plane P 5  is in parallel to the first to fourth reference planes, P 1  to P 4 . The second lenses,  16   a  to  16   d , each converts the optical beam B 2b  output from the beam splitter  14  and focused on the fourth reference plane P 4  into collimated beams B 3 . 
     The optical multiplexer  17  multiplexes the optical beams B 3  each output from respective second lenses,  16   a  to  16   d , depending on wavelengths thereof and outputs a single optical beam B 4  toward the output port  18 . The single optical beam B 4  is a collimated optical beam and contains four wavelengths. The third lens  19  put outside of the housing  1   b  concentrates the optical beam B 4  output from the optical multiplexer  17  and passing the output port  17  on an end of an external optical fiber  20 . 
     Next, a process to assemble the transmitter optical module  2  will be described as referring to  FIGS. 3 to 10 . The description concentrates on a process to assemble one of first lenses  13   a  and one of second lenses  16   a ; but a technique substantially same with those described will be applicable to other of the first lenses,  13   b  to  13   d , and other of the second lenses,  16   b  to  16   d.    
     Referring to  FIG. 3 , the process first prepares at step S 1  an intermediate assembly  1  that includes the substrate  1   a , the LDs,  12   a  to  12   d , the beam splitter  14 , the mPDs,  15   a  to  15   d , and the optical multiplexer  17 , where the latter four components,  12   a  to  17 , are precisely set in respective positions on the substrate  1   a , and the substrate  1   a  thus assembling the components,  12   a  to  17 , is set on the designed position within the housing  1   b  in advance. The intermediate assembly  1  thus processed is set on a laterally movable stage  26  as illustrated in  FIG. 4 . 
     The first lens  13   a  is assembled with the intermediate assembly  1  in subsequent steps, S 2  to S 10 . Specifically, at step S 2 , a supplementary jig  21  is set between the LD  12   a  and the beam splitter  14 , or between the beam splitter  14  and the optical multiplexer  17 . The supplementary jig  21  assembles two prisms,  21   a  and  21   b , with a parallelepiped body  21   c , where each of the oblique edge of the prisms,  21   a  and  21   b , faces the other as a reflecting mirror. The optical beam B 1  output from the LD  12   a  and passing through the first lens  13   a  is able to be extracted from the housing  1   b  as the optical beam B 7  by the duplicate reflection at respective interfaces,  21   a  and  21   b , of the supplementary jig  21 . 
     Step S 3  of the process temporarily adjusts the position of the first lens  13   a  set between the supplementary jig  21  and the first LD  21   a  by using the lens holder  25   a  supported by the positioner  25 . The lens holder  25   a  is a type of, for instance, the vacuum collet generally used in a semiconductor process, and/or the mechanical chuck. The positioner  25 , or the lens holder  25   a , moves the first lens  13   a  in up and down directions, and sometimes rotatively moves the first lens  13   a  around the axis of the lens holder  25   a . At step S 4 , the LD  12   a  is practically activated to emit the optical beam B 1 . 
     At step S 5 , the first lens  12   a  is practically aligned on the seventh reference plane P 7  as observing an image of the optical beam B 7  output from the supplementary jig  21  and detected by the image detector  22 . When the relative distance between the first LD  12   a , exactly, the front facet thereof, and the first lens  13   a  becomes the focal length of the first lens  13   a , the optical beam B 1  output from the LD  12   a  is converted into a collimated or a parallel beam B 6 , which is never focused on a point. The position of the first lens  13   a  where the optical beam B 6  becomes the parallel beam is assumed to be the seventh reference plane P 7 . 
     Step S 5  is further specifically described as referring to  FIGS. 4 to 6 . Step S 5  includes two sub-steps, S 5a  and S 5b , indicated in  FIG. 5 . The controller  24 , cooperating with the image analyzer  23  and the positioner  25 , adjusts the position of the first lens  13   a  through the positioner  25  such that, as detecting the intensity of the image two-dimensionally by the image detector  23 , the maximum K 1  becomes the center of the monitor of the image detector  23  as keeping the axial symmetry of the image. The peak position K 1  of the intensity of the optical beam B 7  is calculated by the image analyzer  23 . When the position of the first lens  13   a  in the optical axis thereof deviates from the axis of the LD  12   a , the image is deformed from the axial symmetry. 
     At step S 5b , the controller  24  further adjusts the position of the first lens  13   a  through the positioner  25  such that the diameter L 1  of the image of the optical beam B 7  becomes minimum, which substantially corresponds to a relative intensity of 1/e 2  with respect to the maximum intensity B. Although the intensity profile of the optical beam B 7  strongly depends on the far field pattern of the LD  12   a , the minimum diameter or the minimum profile may be estimated. Thus, the image analyzer  23 , based on the image of the optical beam B 7  detected two-dimensionally by the image detector  22 , may evaluate the maximum intensity of the image and the size thereof. The position of the first lens  13   a  thus decided is exactly aligned with the LD  12   a  and just on the seventh reference plane P 7 . 
     The controller  24 , co-operating with the image analyzer  25 , controls the positioner  25  and the movable stage  26 . For instance, the positioner  25  may move the first lens  13   a  in up and down directions, while, the movable stage  26  may move the LD  12   a  laterally. The image detector  22  is fixed in a relative position with respect to the positioner  25 . That is, the image detector  22  is movable in connection with the positioner  25 . 
     Although step S 5  above described uses the image analyzer  23  and the controller  24 ; the positioning of the first lens  13   a  on the seventh reference plane P 7  may be carried out without these devices. As illustrated in  FIG. 7 , the output of the image detector  22  is brought to the visual monitor  22   a  that indicates the profile of the optical beam B 7  two-dimensionally by light and shade patterns. Accordingly, the positioner  25 , namely the first lens  13   a , and the movable stage  26 , namely, the LD  12   a , may be manually positioned such that the peak K 1  and the size L 1  of the light and shade pattern become respective designed conditions. 
     Step S 6  further moves the first lens  13   a  from the position on the seventh reference plane P 7  to the designed position on the second reference plane P 2  apart by a distance L 2  along the optical axis. Specifically, as monitoring the image profile of the optical beam B 7  through the supplementary jig  21 , the first lens  13   a  is gradually apart from the LD  12   a . As the positioner  25  or the movable stage  26  increases a distance between the first lens  13   a  and the LD  12   a , the image monitored by the image detector  22  becomes clear and sharp. However, the optical axis of the first lens  13   a  is kept aligned with that of the LD  12   a  by keeping the center of the image and the shape of the clearer image. 
     Steps S 7  to S 10  fix the first lens on the designed position on the second reference plane P 2 . First, the positioner  25  or the movable stage  26  slightly lifts up the first lens  13   a  to form a gap against the substrate  1   a . Then, an adhesive resin is applied on the surface  1   a   1  at step S 8 . The adhesive resin is a type of ultraviolet curable resin and/or thermo-curable resin. At step S 9 , the positioner  25  or the movable stage  26  loads the first lens  13   a  down to the substrate  1   a . Finally, the adhesive resin applied at step S 8  is cured by illuminating with ultraviolet rays or heating up to cure the adhesive resin to fix the first lens  13   a  rigidly and permanently on the second reference plane P 2  on the substrate  1   a  at step S 10 . Then, the second lens  16   a  will be positioned. The supplementary jig  21  is removed after the first lens  13   a  is fixed on the substrate  1   a.    
     Because the first lens  13   a  is precisely aligned along the second reference plane P 2  as described above, the mPD  15   a  may be also precisely positioned on the third reference plane P 3 . When the first lens  13   a  is positioned on the second reference plane P 2 , the focal point of the first lens  13   a  is set on the light receiving surface  15   a   1  of the mPD  15   a  even when the mPD  15   a  deviates from the designed position on the beam splitter  14 . Specifically, even when the beam splitter  14  deviates the position thereof from the designed one, or the mPD  15   a  set on the beam splitter  14  deviates the position thereof from the designed position on the beam splitter  14 , the mPD  15   a  may effectively detect the optical beam to find the focal point of the optical beam B 2a  on the beam splitter  14 . 
     Steps S 11  to S 17  align and fix the second lens  16   a  on the designed position of the intermediate assembly  1 . Subsequent to step S 10 , the lens holder  25   a  holds the second lens  16   a  and roughly sets it on the designed position thereof on the substrate  1   a  at step S 11 . One of LDs  12   a  is practically activated at step S 12  and the positioner  25  moves the second lens  16   a  on the fifth reference plane P 5  as monitoring the optical beam B 8  by the image detector  22  such that the two-dimensional image detected by the image detector  22  has the intensity maximum in the center thereof and the preset size, which is substantially same as those for aligning the first lens  13   a  on the seventh reference plane P 7 . The position on the fifth reference plane P 5  for the second lens  16   a  is a position at which the monitored intensity of the optical beam B 8  becomes the maximum. The controller  24  manipulates the lens holder  25   a  through the positioner  25  based on the intensity profile detected by the image detector  22 . The image detector  22  in the axis thereof deviates from the axis of the second lens  16   a  to be aligned by a preset offset because the optical multiplexer  17  shifts the optical axes of the optical beams input therein. Because the second lens  16   a  is the collimating lens while the lens  27   a  is the concentrating lens, the deviation of the second lens  16   a  from the designed position is reflected in the asymmetry of the image profile. 
     Subsequently, an auxiliary component with an optical fiber  27   b  assembled with a concentrating lens  27   a  is replaced from the image detector  22 , and the optical output extracted from the optical fiber  27   b  is detected by the intensity monitor  28 , as shown in  FIG. 10 . The auxiliary component is first aligned with the optical beam B 8  output from the output port  18  of the housing  1   b  such that the intensity concentrated by the lens  27   a  becomes the maximum. Concurrently with the alignment of the auxiliary component, the second lens  16   a  is further precisely aligned along the optical axis thereof to obtain the maximum intensity by the intensity monitor  28 . The positions of the auxiliary component with respect to the optical multiplexer  17 , or the output port  18  of the housing  1   b , are memorized for the later use. 
     Then, the second lens  16   a  is fixed on the fifth reference plane P 5  on the substrate  1   a  at steps S 14  to S 17 . Step S 14  slightly lifts up the second lens  16   a  from the aligned position defined through steps S 11  to S 13 ; then, an adhesive resin is applied on the substrate  1   a ; step S 16  loads the second lens  16   a  down to the defined position; and step S 17  cures the adhesive resin. The adhesive resin used for the second lens  16   a  may be same or similar to that used in the aforementioned steps for the first lens. Specifically, the adhesive resin is the type of the ultraviolet curable resin and/or the thermo-curable resin. Solidifying the former one may be carried out by irradiating with ultraviolet rays, while by heating for the latter one. Thus, the solidified resin J 2  for fixing the second lens  16   a  is formed. 
     Steps S 1  to S 10  align other first lenses,  13   b  to  13   d , with respective LDs,  12   b  to  12   d , and steps S 11  to S 17  align and fix other second lenses,  16   b  to  16   d . However, the steps to position the auxiliary component against the housing  1   b  are replaceable to the process to position the auxiliary component in the memorized positions. Thus, the first lenses,  13   a  to  13   d , and the second lenses,  16   a  to  16   d , are optically aligned with respective LDs,  12   a  to  12   d.    
     While there has been illustrated and described what are presently considered to be example embodiments, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the invention. Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims.