OPTICAL MODULE AND MANUFACTURING METHOD OF OPTICAL MODULE

Abstract

An optical module according to the present invention has a stem having an interconnection terminal; an optical device positioned on the stem and electrically connected to the interconnection terminal; a cap fixed onto the stem so as to enclose the optical device within the cap; and a lens for optically coupling the optical device with an optical member positioned outside the cap. As a feature of this optical module, the cap is manufactured with a high degree of precision so that the height of the cap with respect to the stem can be used as a positional reference in the direction of the optical axis of the optical system, which consists of the optical device, the lens, and the optical member extending outside the cap. In addition, the shape of the side wall of the cap is designed so that it can function as a reference for maintaining the coaxiality between the photo detector and the lens. The stem is fixed to the cap at the proper position a high precision.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] This invention relates to an optical module used to transmit and receive optical signals in optical communication systems.

[0003] 2. Description of Related Art

[0004] In general, optical signals which have been transmitted through optical fibers are received by the input ports of receivers, and converted into electric signals by photo detector modules using, for example, photo diodes (PD).

[0005] The size of the detection area of the photo detector used in such a photo detector module is very small having a diameter of about 80 &mgr;m. In order to efficiently receive the optical beam emitted from the optical fiber into the photo detector, a collecting lens is typically used. For example, a technique of attaching a ball lens directly to the cap of the photo detector is known as a low-cost light-collecting method.

[0006] FIG. 5 illustrates an example of this type of photo detector module. This photo detector module comprises a commercially available stem 2, which is generally referred to as a TO-46 type stem, and a photo detector 1 mounted on the stem 2. The stem 2 is fixed to a cap 3 so that the photo detector 1 is accommodated in the cap 3. A ball lens 4 made of low-temperature-melting glass is attached to the middle of the cap. The cap 3 is fixed to a holder 5 by a bond 5, such as adhesive or solder.

[0007] An optical fiber 7 is connected to the holder 5 via a sleeve 9. To be more precise, the optical fiber 7 is inserted in a ferrule 8, and the ferrule 8 is inserted in and fixed to the sleeve 9 by YAG welding. As a result of the YAG welding, nuggets 10 are formed at the boundaries between the holder 5 and the ferrule 8, and at the boundaries between the ferrule 8 and the sleeve 9.

[0008] In assembling the conventional photo detector, first, the photo detector 1 is bonded to the stem 2. Then, the stem 2 is fitted into the cap 3, to which the ball lens 4 has already been fixed, and the stem 2 is fixed to the cap 3 by resistance-welding.

[0009] Then, the holder 5 is fixed to the cap 3 at the end opposite the stem 2 by a bond 6. The cap 3 can not be directly YAG welded because the cap 3 is press-processed.

[0010] Finally, the optical fiber 7 is positioned in the optimal position by an optical alignment technique, and then fixed to the holder 5, via the sleeve 9, by YAG welding.

[0011] When this photo detector module is used for high-speed transmission in a STM system, the optical signal which has been propagated through the optical fiber 7 must be prevented from being reflected by the surface of the photo detector 1 back to the transmission path. For this reason, the end surfaces of the ferrule 8 and the optical fiber 7, which surface the ball lens 4, are slanted by grinding or polishing for the purpose of inclining the incident angle of the beam onto the photo detector 1, and of reducing the return light to the optical fiber 7.

[0012] In particular, if the inclination of the end surface of the optical fiber 7 is set to about 12 degrees, the incident angle of the beam on the photo detector 1 becomes about 6 degrees, which can reduce the reflected component of the signal light to less than −40 dB.

[0013] However, this arrangement causes a large variation in the quantity of the reflected light because the coaxiality between the photo detector 1 and the ball lens 4 is lost.

[0014] FIGS. 6(1) through 6(3) show the optical coupling and the coaxiality between the photo detector 1 and the ball lens 4.

[0015] FIG. 6(1) illustrates the optical connection at the optimal position with little separation at the ball lens 4. In this arrangement, the light emitted from the optical fiber 7 strikes the photo detector 1 at an optimal incident angle, and the amount of the reflected light back to the optical fiber 7 via the ball lens 4 is greatly reduced. In this arrangement, the orientation of the end surface of the optical fiber 7 does not affect the physical properties of the optical connection between the ball lens 4 and the photo detector 1.

[0016] However, if the optical axes of the photo detector 1 and the ball lens 4 are offset from each other, as shown in FIGS. 6(2) and 6(3), the quantity of reflected light increases, while the coupling efficiency decreases.

[0017] For example, if the separation between the photo detector 1 and the ball lens 4 and the orientation of the slanted end surface of the optical fiber 7 satisfy particular conditions (that is, if the orientation of the end surface of the optical fiber 7 is 12 degrees, and if the separation is about 0.11 mm, as shown in FIG. 6(2)), then a problem arises. In the example of FIG. 6(2), if the optical fiber 7 is aligned to the optimal position, the light which exits obliquely from the optical fiber 7 strikes the surface of the photo detector 1 at a normal angle. The normal incident light is reflected by the photo detector, and returns along the same path as the incident light to the optical fiber 7. The amount of reflected light may reach a maximum in this arrangement.

[0018] On the other hand, as shown in FIG. 6(3), if the orientation of the inclined end surface of the optical fiber 6 is opposite the orientation shown in FIG. 6(2), the quantity of reflected light is reduced. However, in this arrangement, the light emitted from the optical fiber 7 passes through the ball lens 4 at a position offset from the center axis of the ball lens 4. As a result, the spherical aberration of the ball lens 4 adversely affects the system, which reduces the optical coupling efficiency and causes the light-collecting ability of the photo detector to deteriorate.

[0019] In addition to these problems, the coaxiability between the photo detector 1 and the ball lens 4 of the conventional photo detector module may be offset by 0.25 mm at most due to the following factors:

[0020] (1) Because there is no target or alignment mark on the stem 2, the photo detector 1 can not be precisely bonded;

[0021] (2) Because the cap 3 is press-processed, its dimensional stability is insufficient and, accordingly, a large clearance is required at the portion for receiving the stem 2, which causes the optical axes of the stem 2 and the cap 3 to be offset from each other during the resistance welding step; and

[0022] (3) It is mechanically difficult to accurately fix the ball lens 4 in the center of the cap 3.

[0023] If the separation between the photo detector 1 and the ball lens 4 is large, the amount of reflected light and the coupling efficiency vary greatly with the slanting orientation of the end surface of the optical fiber 7. In order to achieve a predetermined desired performance, employing an optical alignment technique for setting the orientation of the slanting end surface of the optical fiber 7 at the optimal angle, while monitoring the reflected light and the coupling efficiency, is indispensable. However, such an optical alignment technique requires a number of steps, and the production yield is lowered as a result.

[0024] In addition, it is also difficult in the conventional photo detector module to precisely position the holder 5 with respect to the cap 3 along the optical axis (i.e., the z-axis) because of the existence of adhesive or solder. Since the optical fiber 7 must be precisely positioned both in the direction perpendicular to the optical axis and in the direcxtion parallel to the optical axis, the ferrule 8 and the holder 5 need to be secured at two positions via the sleeve 9 by YAG welding.

[0025] Furthermore, an adhesive or solder is also used to fix the holder 5 to the cap 3, which is not preferable for maintaining the reliability of the optical module for a long period of time.

SUMMARY OF THE INVENTION

[0026] Therefore, it is an object of the invention to overcome these problems in the prior art, and to provide an optical module which comprises a stem having an interconnection terminal; an optical device positioned on the stem and electrically connected to the interconnection terminal; a cap fixed onto the stem so as to enclose the optical device within the cap; and a lens for optically coupling the optical device with an optical member positioned outside the cap. As a feature of this optical module, the cap is manufactured with a high degree of precision so that the height of the cap with respect to the stem can be used as a positional reference in the direction of the optical axis of the optical system, which consists of the optical device, the lens, and the optical member extending outside the cap.

[0027] In addition, the shape of the side wall of the cap is designed so that it can function as a reference for maintaining the coaxiality between the photo detector and the lens. The stem is fixed to the cap at the proper position a high precision.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The above and other objects and features of the invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings, wherein:

[0029] FIG. 1 is a cross-sectional view of the optical module according to an embodiment of the invention;

[0030] FIG. 2 is an exploded perspective view of the optical module, which is used to explain the process of assembling the optical module of the invention;

[0031] FIG. 3 illustrates how the stem is fixed to a base;

[0032] FIG. 4 illustrates how the stem is fixed to the cap;

[0033] FIG. 5 illustrates a conventional photo detector module; and

[0034] FIG. 6 illustrates the relationship between the optical coupling state and the coaxiability between the photo detector and the ball lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The preferred embodiments of the invention will now be described in detail with reference to the attached drawings. FIG. 1 is a cross-sectional view of the optical module according to a preferred embodiment of the invention, and FIG. 2 is an exploded perspective view of the optical module of this embodiment, which shows how this optical module is assembled.

[0036] In this embodiment, a photo detector 11 consisting of a photo diode (PD) is used as the optical device. The photo detector 11 is mounted on a pad provided to the surface of a base 13 made of ceramic. The base 13 is fixed to a metallic stem 12 which is commercially available under the trade name of “TO-46 type”. The base 13 is capable of receiving a simple circuit including, for example, a pre-amplifier 14 and a capacitor 15, in addition to the photo detector 11. In this embodiment, these circuitry elements are electrically connected to the interconnection terminal of the stem 12, whereby a photo detector module having a built-in pre-amplifier is formed.

[0037] A metallic cap 16 is fixed to the stem 12 by resistance welding, so that the photo detector 11 on the stem 12 is enclosed within the cap 16. A ball lens 17 is fixed in the middle of the cap 16 by low-temperature-melting glass. The ball lens 17 is used to optically couple the photo detector 11 with an optical member positioned outside the cap 16.

[0038] The cap 16 is formed with a high degree of precision by cutting processes, unlike the conventional cap which is formed by press processes. The height of the cap 16 with respect to the stem 12 is used as a positional reference in the z-direction along the optical axis of the system extending from the ball lens 17 to the external optical member. In particular, the top wall of the cap 16 is shaped so that the top surface of the cap 16 is positioned a predetermined distance from the ball lens 17. In other words, as shown in FIG. 1, the thickness of the top wall of the cap 16 is adjusted so that the focal point of the ball lens 17 is positioned below the top surface of the cap 16.

[0039] The external optical member is an optical fiber 18 in this embodiment. The end surface of the optical fiber 18 is optically coupled to the cap 16. The optical fiber 18 is held by a ferrule 19 which functions as a fiber holder. The end surface of the ferrule 19 is fixed to the top surface of the cap 16 by YAG welding, whereby the optical fiber 18 is also secured to the cap 16. The end surface of the optical fiber 18 projects from the end surface of the ferrule 19 by a predetermined length along the optical axis, such that the focal point of the ball lens 16 is located within the plane of the end surface of the optical fiber 18. As a result of the YAG welding of the ferrule 19 to the cap 16, a nugget 20 is formed at the boundary between the ferrule 19 and the cap 16.

[0040] Next, the process of assembling this photo detector module will be described. Many techniques for achieving highly precise alignment between the photo detector 11 and the ball lens 17 are employed in this invention.

[0041] First, the base 13, on which a pad for receiving the photo detector 11 is formed, is mounted on the stem 12. Then, the photo detector 11 is bonded onto the pad. The stem 12 is fitted into the cap 16, to which the ball lens 17 has already been secured, and then the stem 12 is fixed to the cap 16 by resistance welding. Finally, after the optical fiber 18 held by the ferrule 19 is optically aligned with the photo detector 11, the ferrule 19 is secured to the cap 16 by YAG welding.

[0042] The factors affecting the coaxiability between the photo detector 11 and the ball lens 17 are listed below.

[0043] (1) Precision of the patterning of the pad on the base 13

[0044] (2) Positional accuracy of mounting the base 13 on the stem 12

[0045] (3) Accuracy of bonding the photo detector 11 onto the base 13

[0046] (4) Accuracy in resistance-welding the cap 16 to the stem 12

[0047] (5) Coaxiality between the cap 16 and the ball lens 17

[0048] The separations due to these factors are cumulated, which results in a total separation between the photo detector 11 and the ball lens 17.

[0049] Each of the above-listed factors will now be explained in more detail.

[0050] (1) Precision of the patterning pad on the base 13

[0051] The base 13 is made of ceramic, and a pad for receiving the photo detector 11 is formed on the base 13 by etching using a photomask. The precision of the patterning of the pad is determined by the precision of the alignment of the photomask.

[0052] (2) Positional accuracy of mounting the base 13 on the stem 12

[0053] In order to mount the base 13 on the stem 12 at the proper position with a high degree of precision, the base 13 and the stem 12 are coupled using a coupling device 21 shown in FIG. 3, which illustrates the coupling steps. The upper half of FIG. 3 shows cross-sectional views, while the lower half shows the corresponding perspective views.

[0054] The coupling device 21 has a guide hole 21-1 whose diameter is slightly larger than the outer diameter of the base 13, and a guide hole 21-2 whose diameter is slightly larger than the diameter of the component-mounting area of the stem 12. The guide holes 21-1 and 21-2 are concentric.

[0055] In coupling the base 13 to the stem 12, the base 13 is set into the guide hole 21-1 of the coupling device 21, as shown in FIG. 3(1), and a bond (not shown), such as solder, is put on the top surface of the base 13.

[0056] Then, the stem 12 is fitted into the guide hole 21-2 so that the component-mounting area of the stem 12 comes into contact with the top surface of the base 13 with the bond therebetween, as shown in FIG. 3(2).

[0057] The coupling device 21, in which the base 13, the bond, and the stem 12 are set, is heated up, as shown in FIG. 3(3), whereby the base 13 and the stem 12 are bonded to each other.

[0058] Finally, the base 13 and the stem 12, which are now bonded into one unit, are removed from the coupling device 21, as shown in FIG. 3(4).

[0059] In this process, the coaxiality between the base 13 and the stem 12 depends on the gap between the guide hole 21-1 and the base 13, and the gap between the guide hole 21-2 and the stem 12.

[0060] In this embodiment, a commercially available TO-46-type stem is used. This stem 12 is press-processed, and the outer diameter of the component-mounting part, including the systematic error (or tolerance), is &phgr;4.2±0.025 mm. The base 13 is made of ceramic, and its outer diameter tolerance depends on the amount of contraction during the baking process. The actually measured tolerance of the base 13 is ±0.03 mm. The diameters of the guide holes 21-1 and 21-2 of the coupling device 21 are set slightly larger than the maximum tolerances of the base 13 and the stem 12, respectively. Accordingly, the maximum separation caused in the worst case is about 0.065 mm. Since the circumference of the base 13 must not stick out beyond the circumference of the stem 12 even in the worst case, the outer diameter of the base 13 is set slightly smaller than the outer diameter of the stem 12.

[0061] (3) Accuracy of bonding of the photo detector 11 onto the base 13

[0062] The photo detector 11 is mounted on the pad formed on the base 13 by an ordinary bonding process. The uncertainty of the positioning of the photo detector 11 with respect to the pad is about 0.05 mm.

[0063] (4)Accuracy in resistance-welding the cap 16 to the stem 12

[0064] FIG. 4 illustrates how the cap 16 is fixed to the stem 12. The stem 12 is fitted into the cap 16 so that the side wall of the component-mounting part of the stem 12 comes into contact with the inner surface of the edge of the cap 16. The coaxiality between the cap 16 and the stem 12 depends on the gap between the outer diameter of the component-mounting part of the stem 12 and the inner diameter of the cap 16.

[0065] As has been explained above, the outer diameter of the component-mounting part of the stem 12 is &phgr;4.2±0.025 mm. The cap 16 is manufactured by cutting processes, and the uncertainty in its inner diameter is ±0.025 mm or less. Accordingly, if the inner diameter of the cap 16 is set to &phgr;4.25±0.025 mm, the maximum separation between the cap 16 and the stem 12 is only 0.050 mm in the worst case.

[0066] The flange 23a of the stem 12 and the flange 23b of the cap 16 are fixed to each other by resistance welding. The inner corner of the flange 23b of the cap 16 is chamfered, as indicated by the numerical reference 22 in FIG. 4. In general, the boundary between the flange 23a and the side wall of the press-processed stem 12 becomes distort during the resistance welding. However, the chamfer 22 can prevent such distortion from adversely affecting the bonding of the stem 12 and the cap 16. The shape of the chamfer 22 is not limited to the example shown in FIG. 4, and any shapes can be selected as long as the flange 23b of the cap 16 does not come into contact directly with the distortion of the stem 12.

[0067] In the example shown in FIG. 4, the diameter of the flange 23b of the cap 16 is set larger than the diameter of the flange 23a of the stem 12. The flange 23b of the cap 16 is also thicker than the flange 23a of the stem 12 because the cap 16 is formed by cutting processes. During the resistance welding, the flange 23b of the cap 16 and the flange 23a of the stem 12 are pressed against each other at an appropriate pressure, and the flange 23b of the cap 16, which is thicker and has a greater diameter than the flange 23a of the stem 12, can reliably receive and hold the stem 12 during the resistance welding.

[0068] (5) Coaxiality between the cap 16 and the ball lens 17

[0069] The ball lens 17 is secured to the cap 16 by low-temperature-melding glass using an ordinary alignment device. A conventional alignment device is sufficient to reduce errors in the coaxiality and the position along the optical axis.

[0070] As has been described, each component is positioned and aligned with a high degree of precision, and the overall coaxiality between the photo detector 11 and the ball lens 17 is greatly improved as compared with the conventional art.

[0071] In this embodiment, the coaxiality between the photo detector 11 and the ball lens 17 is within 0.1 mm. It was confirmed by experiment that, with this arrangement, the amount of reflected light and the coupling efficiency do not greatly change with variation of the slanting angles and the orientations of the end surfaces of the optical fiber 18 and the ferrule 19. The troublesome steps performed in the prior art of adjusting the orientation of the ferrule 19 to the optimal angle, while monitoring the physical properties of the optical system, can be eliminated, and the work efficiency can thereby be improved.

[0072] The accuracy of positioning along the optical axis is also improved because the cap 16 is manufactured by cutting processes, and the height of the cap 16 can be set to the most appropriate value so as to satisfy the predetermined optical-coupling conditions. In other words, the cap 16 processed by cutting and having a desired height in accordance with the predetermined conditions is bonded directly to the stem 12 by resistance welding, without requiring further positioning or adjustment along the optical axis. This is a great advantage over the conventional press-processed cap.

[0073] Thus, the cap 16 functions as a positional reference along the optical axis of the optical system, which consists of the optical device formed on the stem, the ball lens, and the external optical member (i.e., the optical fiber). This facilitates the optical alignment of the optical fiber, and the reflected light from the top surface of the photo detector can stably be reduced.

[0074] In addition, adhesive or solder used in the prior art is eliminated, and the reliability and mechanical accuracy of the entire module can be improved.

[0075] Although a photo detector module and its manufacturing method have been described, the present invention can also be applied to a light-emitting module. In this case, a ball lens is incorporated into a cap processed by cutting, and the cap is resistance-welded to a stem using the cap's height as a positioning reference along the optical axis. Accordingly, a light-emitting module using a laser diode (LD) is also included in the scope of the invention.

Claims

1. An optical module comprising:

a stem having an interconnection terminal;

an optical device positioned on the stem and electrically connected to the interconnection terminal;

a cap fixed onto the stem so as to enclose the optical device within the cap; and

a lens for optically coupling the optical device with an optical member positioned outside the cap, wherein,

a height of the cap with respect to the stem can be used as a positional reference in a direction of an optical axis of an optical system.

2. An optical module according to

claim 1, wherein the stem is fixed to the cap and shape of a sidewall of the cap is designed so that it can function as a reference for maintaining a coaxiality between the optical device and the lens.

3. An optical module according to

claim 1, wherein a top surface of the cap is positioned a predetermined distance from the lens.

4. An optical module according to

claim 3, wherein a focal point of the lens is positioned below the top surface of the cap.

5. An optical module according to

claim 3, the optical member comprising:

an optical fiber having an end surface which is optically coupled to the optical device;

a fiber holder for holding the optical fiber and has an end surface which is fixed to the top surface of the cap; and

the end surface of the optical fiber is aligned from the end surface of the fiber holder by a predetermined length along the optical axis.

6. An optical module according to

claim 3, wherein the stem is a metallic stem, the cap is a metallic cap, and the metallic cap is fixed to the stem by resistance welding.

7. An optical module according to

claim 1, wherein the optical device is a photo detector.

8. An optical module according to

claim 1, wherein the optical system consists of the optical device, the lens, and the optical member.

9. A method of manufacturing an optical module comprising the steps of:

positioning an optical device on a stem which has an interconnection terminal and electrically connecting the optical device to the interconnection terminal;

positioning an lens in a predetermined position of a cap which has a predetermined height; and

fixing the cap onto the stem so as to enclose the optical device within the cap,

so that the height of the cap with respect to the stem can be used as a positional reference in a direction of an optical axis of an optical system.

10. A method of manufacturing an optical module according to

claim 9, wherein the stem is fixed to the cap and shape of a side wall of the cap is designed so that it can function as a reference for maintaining a coaxiality between the optical device and the lens.

11. A method of manufacturing an optical module according to

claim 9, further comprising the steps of:

holding an optical fiber which has an end surface by a holder, so that the end surface of the optical fiber is located a predetermined position from an end surface of the holder; and

fixing the end surface of the holder to the top surface of the cap.

12. A method of manufacturing an optical module according to

claim 11, wherein a focal point of the lens is positioned below the top surface of the cap.

13. A method of manufacturing an optical module according to

claim 9, wherein the optical device is set on a base and the base is bonded to the stem, and the step of positioning the optical device on the stem comprises fitting the base and the stem into a predetermined guide hole of a coupling device and heating up the coupling device for coupling the base and the stem.

14. A method of manufacturing an optical module according to

claim 9, wherein an inner corner of the cap is chamfered.

15. A method of manufacturing an optical module according to

claim 11, wherein the stem is a metallic stem, the cap is a metallic cap, and the metallic cap is fixed to the stem by resistance welding.

16. A method of manufacturing an optical module according to

claim 15, wherein the stem has a first flange for the resistance welding, the cap has a second flange for the resistance welding, and a diameter of the second flange is set larger than a diameter of the first flange.