Abstract:
The invention provides a method for manufacturing a optical subassembly even when the assembly involves the positional deviation of the components that induces the discrepancy in the direction of the optical output beam from the optical device. The method first determines the position (x 1 , y 1 , z 1 ) of the stub where the optical coupling between the stub and the optical device becomes the maximum. Next, the direction of the optical output beam from the optical device is calculated based on the position above, and, finally, the inclined direction of the end surface of the stub is aligned with the direction of the optical output beam evaluated in the previous step.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for manufacturing an optical module, where the optical module provides an optical device with a semiconductor laser diode and a lens for concentrating the light output from the laser diode. In particular, the method for optically coupling the optical device with a stub including a coupling fiber whose end surface facing the lens is inclined with respect to the optical axis of the coupling fiber.  
         [0003]     2. Related Prior Arts  
         [0004]     For the optical module that combines the optical device including the semiconductor optical device and the lens with the optical receptacle that includes a sleeve and a stub, an optical alignment between the optical device and the optical receptacle is inevitable, because a misaligned module with an inferior optical coupling therebetween degrades the signal-to-noise ratio to shorten the transmission range.  
         [0005]     The United States Patent, U.S. Pat. No. 5,963,696 has disclosed a method for optically aligning the optical receptacle with the optical device. That is, the method disclosed therein first calculates an optimum incident angle α for the inclined angle θ of the tip surface of the coupling fiber. Next, assuming the gap between the lens and the LD to be a, the displacement x 0  is calculated as x 0 =tan −1  (α). Subsequently, the LD chip is mounted on the package so as to target a position displaced by x 0  from the center S of the package, while the lens is mounted on the package so as to align the center of the lens with respect to the center of the package. Finally, the process fixes the optical fiber as aligning the direction of the inclined end surface thereof with the direction of the displacement and obtaining a position where the maximum coupling efficiency is realized by sliding the optical receptacle within a plane perpendicular to the optical axis.  
         [0006]     The LD module, which is often called as a transmitter optical sub-assembly (TOSA), generally integrates a stub with an inclined end surface by 6° to 8° to the optical axis to eliminate the light returning the LD. On the other hand, it is necessary for optically coupling the light output from the optical device with a good coupling efficient to incline the optical axis of the light with respect to the axis of the stub. This peculiar configuration between the optical receptacle and the optical device may be realized, for example, by assembling the LD chip as inclining the axis thereof, or by offsetting the lens in horizontally, namely, perpendicular to the optical axis, with respect to the LD.  
         [0007]     Thus, it is necessary for the optical alignment between the optical receptacle with the stub and the optical device to align the inclined direction of the end surface of the stub with the direction of the light beam from the optical device, which is inevitable to realize the optimum optical coupling therebetween and the superior performance for the wiggle characteristic, where the wiggle is the fluctuation of the optical coupling efficiency depending of the suspended state of the optical fiber.  
         [0008]     Conventionally, the direction of the light beam output from the optical device was determined primarily based on the designed displacement between the LD and the center of the lens. However, the manufacturing process inevitably involves a substantial tolerance in physical dimensions of components and the positional accuracy thereof. Thus, the direction of the light beam output from the optical device strongly depends on the relative position of the lens to the LD. Paradoxically, to control the relative position of the lens to the LD may adjust the direction and the angle of the output beam from the optical device.  
         [0009]     However, it is practically hard to control the relative position of the lens to the LD, that is, to suppress the positional deviation of the lens from the designed point within a desired range. The current assembling process using the image recognition technique for the LD chip realizes the positional deviation of the chip within a desired range. The aligning process for the lens with respect to the lens holder also realizes the deviation within an acceptable range. However, the welding of the lens holder to the stem of the package involves the positional deviation of about thirty and forty micron meters. In addition to this positional deviation of the lens holder, the misalignment and the inclination of the LD chip, and the inclination of the sub-mount may also generate the discrepancy between the practical direction and the designed direction of the optical output from the optical device. Accordingly, the direction of the output beam from the optical device scatters from 90° to 120°.  
         [0010]     Thus, the present invention is to provide a method for manufacturing an optical module even when the components therein involves the positional deviation and the optical output appears an inconsistent direction with the designed parameter.  
       SUMMARY OF THE INVENTION  
       [0011]     A method according to the present invention is for an optical module that comprises an optical device and an optical receptacle. The optical device includes a semiconductor laser diode to emit light and a lens for concentrating the light from the laser diode. The optical receptacle includes a stub with a coupling fiber in a center thereof. The stub together with the coupling fiber provides a surface to receive the concentrated light from the lens, and the surface is inclined with respect to an optical axis of the coupling fiber to suppress the light from being reflected to the incoming direction.  
         [0012]     The method of the invention comprises steps of: (a) determining a position where the maximum coupling efficiency between the optical device and the stub is realized, (b) estimating a direction of the light output from the optical device, and (c) aligning a direction of the inclined surface of the stub with the direction of the light output from the optical device.  
         [0013]     Because the method determines the direction of the optical output from the optical device after the completion of the device, and the inclined direction of the end surface of the stub is aligned with this direction of the output beam from the optical device, the optical coupling efficient between the optical device and the optical receptacle may become maximum independent of the positional displacement if the lens holder to the laser diode induced in the assembly process.  
         [0014]     The method may apply a position sensing device (PSD) for determining the position where the maximum optical coupling is realized, and calculates this position by the result sensed by the PSD. The method using the PSD may shorten the setup time for determining the position where the maximum optical coupling is obtained. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  is a cross section view showing the optical module according to the present invention;  
         [0016]      FIG. 2  explains a positional relation between the laser diode, the lens, and the tip of the coupling fiber for the optical coupling of the optical device with the optical receptacle;  
         [0017]      FIG. 3  illustrates a method for deciding the direction of the optical beam output from the optical device according to an embodiment of the present invention;  
         [0018]      FIG. 4  illustrates the optical alignment between the optical receptacle and the optical device;  
         [0019]      FIG. 5  explains another method for deciding the direction of the optical beam according to another embodiment of the present invention; and  
         [0020]      FIG. 6  illustrates the position sensing device applicable to the method shown in  FIG. 5 . 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]     Next, preferred embodiments of the present invention will be described as referring to accompanying drawings. In the description of drawings, the same symbols or numerals will refer to the same elements without overlapping explanations.  
         [0022]      FIG. 1  is a cross section of an optical module according to the present invention. The optical module  11  comprises an optical device  13 , an optical receptacle  31  and a joint holder  17  physically connecting the optical device  13  with the optical receptacle  31 . The optical device  13  includes a semiconductor optical device  19  such as semiconductor laser diode (LD) mounted on a sub-mount  27 , a lens  25  supported by a lens holder  23  and a photodiode (PD)  39  for monitoring a portion of light output from the LD  19 . In this embodiment, the lens  25  is a type of a spherical lens. The lens holder  23  is welded on a stem  21  of the optical device  13 . The optical device  13  further provides a plurality of lead pins, T 1  to T 3 , to carry an electrical signal and a power supply for the LD  19 . In  FIG. 1 , a symbol B 1  denotes laser light output from the front facet of the LD  19 , while, another symbol B 2  denotes light passing through the lens  25 .  
         [0023]     The optical receptacle  31  includes a stub  15  that secures a coupling fiber  14  in a center thereof, a sleeve cover  33 , a sleeve  35  and a bush  37 . The bush  37  is welded, in a bottom surface thereof, on the top surface of the joint holder  17 . The stub  15  with the tip of the coupling fiber  14  has an end surface inclined with respect to an optical axis of the coupling fiber, which faces the joint holder  17  and prevents the light B 2  from returning the LD  19 . In  FIG. 1 , the symbol B 3  denotes light output from the tip of the coupling fiber  14 .  
         [0024]      FIG. 2  explains an optical system of the optical module  11  shown in  FIG. 1 , which illustrates the positional relation between the LD  19 , the lens  25  and the stub  15 , exactly the coupling fiber  14 . The LD  19  includes an active layer  19   a  for emitting the laser light B 1 , where the laser  19  is mounted on a surface  19   b  of the sub-mount  27 . As shown in  FIG. 2 , the vertical position of the LD  19 , which is denoted as R 1  in  FIG. 2 , the center of the lens  25  denoted as R 2  and the optical axis of the coupling fiber  14  denoted as R 3  are shifted to each other. Further, the end surface  16  of the stub  15 , with the tip of the coupling fiber  14 , is inclined by an angle θ with respect to the optical axis of the coupling fiber  14 .  
         [0025]     As for the dimensional accuracy of the optical module  11 , the LD  19  is mounted on the sub-mount  27  and the sub-mount  27  is fixed to the stem  21  with enough accuracy. The lens  25  is also held by the lens holder  23  with enough accuracy. However, the lens holder  23  is fixed to the stem  21  by the welding with substantial tolerance of some thirty of forty micron meters. Accordingly, the direction of the light B 2  passing through the lens  25  may be widely scattered.  
         [0026]     A conventional optical module decided the position of the stub  15  and the end surface  16  thereof based only on the designed parameter, which degraded not only the optical coupling efficiency between the LD  19  and the coupling fiber  14  but also the wiggle characteristic, which is the fluctuation of the optical coupling efficiency when the fiber receives a suspension stress. On the other hand, the present optical module  11  determines the position of the stub  15 , namely, the position of the coupling fiber, after welding the lens holder  23  to the stem  21  by practically operating the LD  19  and monitoring the light B 2  passing through the lens  25 .  
         [0027]      FIG. 3  explains the practical optical system of the module  11 , in particular,  FIG. 3  shows the trace of the light B 2  passing through the lens  25 . First, the origin (X 0 , Y 0 , Z 0 ) of the co-ordinate system is set in the center of the lens  25 , which is common the optical device  13  and the optical receptacle  31 . Next, by sliding the optical receptacle horizontally on an X-Y plane apart from the origin (x 0 , y 0 , z 0 ) along the z-axis, a position (x 1 , y 1 , z 1 ), which is the location of the tip  16  of the coupling fiber  14 , is detected at which the intensity of the light beam B 2  becomes maximum as practically operating the LD  19  and monitoring the intensity of the light output from the tip of the coupling fiber  14  opposite to the receiving tip  16  thereof. Based on a relative position between the detected point (x 1 , y 1 , z 1 ) and the origin (x 0 , y 0 , z 0 ) the propagating direction of the light beam B 2  may be determined. More than single verification at the plane (x 1 , y 2 ) may enhance the accuracy of the propagating angle of the laser beam B 2 , that is, the verification at the plane (x 1 , y 1 ) and the other plane (x 2 , y 2 ). The latter xy-plane is far apart from the origin (x 0 , y 0 , z 0 ) compared to the former xy-plane. Finally, by aligning the inclined direction of the end surface of the stub  15  together the tip of the coupling fiber  14 , the optical receptacle  31  may be precisely aligned with the optical device  13 .  
         [0028]      FIG. 4  is a cross section of the optical module  11  and illustrates the mechanism of the optical alignment between the optical receptacle  31  and the optical device  13 . First, the inclined surface  16  of the stub  15  turns the direction, which is denoted by Q in  FIG. 4 . This direction may be obtained by the positions, (x 0 , y 0 , z 0 ) and (x 1 , y 1 , z 1 ), those explained in the previous drawings. Next, as aligning the inclined angle of the surface  16  with the direction Q, the optical receptacle  31  is slid on the top surface of the joint holder  17  and the joint holder  17  is slid along the optical axis on the outer surface of the optical device  13  to obtain a position of the joint holder  17  and the horizontal position of the optical receptacle  31  where the maximum coupling efficiency between the optical receptacle  31  and the optical device may be obtained. Finally, the bush  37  in the optical receptacle  31  may be welded on the top surface of the joint holder  17 , and the outer flange of the joint holder is welded to the side surface of the lens holder  23 . Thus, the optical receptacle  31  may be optically aligned with the optical device  13 .  
         [0029]     The process to obtain the propagating direction of the light B 2  in several xy-planes to enhance the accuracy of the alignment occasionally requires a lot of time. Then, a position sensing device (PSD) may be applicable without sliding the optical receptacle on a xy-plane. That is, a PSD made of GaInAs/InP based photodiode (PD) with a wide optically sensitive surface and plurality of photo-carrier correcting electrodes is used for the LD with the emission wavelength thereof within a region from 1.30 μm to 1.55 μm. Such PSD is placed in front of the optical device  13  and receives the light beam B 2 . By differentiating the magnitude of the photo-carrier corrected by each electrode, the position of the light beam B 2  may be estimated, which may determines the direction of the light beam B 2 .  
         [0030]      FIG. 5  explains the process for obtaining the direction of the beam B 2  by using the PSD, and  FIG. 6  describes the configuration of the PSD  50 . Moving the PSD  50  along the z-direction as maintaining the angle of the primary surface thereof in substantially right angle with respect to the z-axis, the position of the light beam B 2  at each level is calculated. Thus, the direction of the light beam B 2  may be obtained, and the alignment process of the optical module  11  may set the optical receptacle  31  on the optical device  13  based on the direction of the light B 2  thus determined.  
         [0031]     As shown in  FIG. 6 , the PSD  50  is a type of photodiode with the wide optically sensitive surface. The PSD  50  provides a plurality of electrodes, four electrodes,  51   a  to  51   d , arranged in respective sides in an example shown in  FIG. 6 . Assuming the photocurrent detected by respective electrodes to be Ix 1 , Ix 2 , Iy 1 and Iy 2 , the position (x, y) of the optical beam within the sensing surface may be obtained by equations:
 
x=Ax (Ix 1 −Ix 2 )/(Ix 1 +Ix 2 ),
 
y=Bx (Iy 1 −Iy 2 )/(Iy 1 +Iy 2 ),
 
 where A and B are correction factors, and the center of the optically sensing surface is origin. The trace of the light beam B 2  using the PSD  50  may omit the sliding of the optical receptacle  31  in a xy-plane, which saves the process time even when the process increases the number of z-points where the evaluation of the light beam B 2  is carried out. Moreover, the PSD  50  is a type of photodiode. Accordingly, the process may evaluate the field pattern and the optical output power of the light beam B 2  that corresponds to the optical output from the LD  19 .