Optical module and a method of fabricating the same

An optical module which can reduce its size, and a method of making the same are provided. The optical module has a substrate, a carbon-coated optical fiber, a ferrule, and an optical device. The substrate has first, second, and third regions along an axis, and also has a ferrule support groove in the first region, an optical fiber support groove in the second region, and a device mount portion in the third region. The carbon-coated optical fiber is mounted in the optical fiber support groove. The ferrule covers the side face of the optical fiber and is secured to the ferrule support groove. The optical device is mounted at the device mount portion and is optically coupled to the carbon-coated optical fiber.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an optical module comprising an optical
 fiber whose cladding surface is coated with carbon, and a method of making
 the optical module.
 2. Related Background Art
 Employed in optical modules is a bared optical fiber stripped of a resin
 coated on the side face thereof, so that the cladding surface is exposed.
 The bared optical fiber is used in the optical module, while the bared
 optical fiber is inserted in a ferrule. The ferrule and the optical fiber
 whose side face is covered with the ferrule are mounted to an optical
 module substrate.
 SUMMARY OF THE INVENTION
 Having studied the prior art, the inventor has found the following
 problems.
 When the bared optical fiber is mounted on the optical module substrate,
 the bared optical fiber is bent within a permissible range of radius of
 curvature so that no excess bending stress is applied thereto. Since the
 bared optical fiber is bent within a predetermined range of radius of
 curvature, the part of bared optical fiber not covered with the ferrule
 cannot be shortened. Therefore, it has been impossible for the optical
 module substrate mounted with the bared optical fiber to reduce its size.
 The size of the optical module substrate has been one factor preventing
 the optical module from reducing its dimensions.
 It is thus an object of the present invention to provide an optical module
 that can reduce the dimensions thereof, and a method of making the optical
 module.
 The optical module in accordance with the present invention comprises an
 optical module substrate, a carbon-coated optical fiber, a ferrule, and an
 optical device. The carbon-coated optical fiber has the outer periphery of
 its cladding coated with carbon. The coating thickness of carbon is within
 the range of 0.03 .mu.m to 0.05 .mu.m, for example. The optical module
 substrate has first, second, and third regions along a predetermined axis.
 This substrate is provided with a ferrule support groove in the first
 region, an optical fiber support groove in the second region, and a device
 mount portion in the third region. The carbon-coated optical fiber is
 mounted on the optical fiber support groove. The side face of the
 carbon-coated optical fiber is covered with the ferrule. The ferrule is
 mounted on the ferrule support groove. In the device mount portion, the
 optical device is placed so as to be optically coupled with the optical
 fiber.
 When the carbon-coated optical fiber is employed as the optical fiber
 placed on the optical module substrate, then the radius of curvature
 permitted to the optical fiber in the placement can be lowered. As a
 result, the part of optical fiber not covered with the ferrule can reduce
 its length, whereby the optical fiber support groove can be shortened.
 Hence, the optical module substrate would attain a smaller size.
 The ferrule support groove supports the ferrule by two faces thereof,
 whereas the optical fiber support groove supports the carbon-coated
 optical fiber by two faces thereof. The device mount portion includes a
 position marker for determining the position at which the optical device
 is to be mounted.
 The optical module substrate can comprise the ferrule support groove and
 the optical fiber support groove on the same surface of the optical module
 substrate. In this case, the optical module substrate is preferably formed
 from a silicon substrate.
 The optical module substrate can have a connection groove formed so as to
 separate the first region and the second region from each other. The
 connection groove can have a portion deeper than the ferrule support
 groove.
 The optical module substrate can have a positioning groove formed so as to
 separate the second region and the third region from each other.
 The optical module substrate can have a base having the ferrule support
 groove and a platform having the optical fiber support groove and the
 device mount portion. The platform is mounted on the base.
 The method of making an optical module comprises the steps of: preparing a
 substrate having first, second, and third regions placed along a
 predetermined axis; mounting an optical device on a device mount portion
 of the substrate; placing a carbon-coated optical fiber in an optical
 fiber support groove, said carbon-coated optical fiber being inserted and
 secured to a ferrule; securing an end portion of the carbon-coated optical
 fiber placed in the optical fiber support groove to the substrate; and
 placing the ferrule with the carbon-coated optical fiber inserted therein
 in a ferrule support groove.
 If the carbon-coated optical fiber is used, the carbon-coated optical fiber
 can be bent with a curvature smaller than that permitted in the
 conventional bared optical fiber when the carbon-coated optical fiber is
 placed in the optical fiber support groove while bending. The part of the
 carbon-coated optical fiber extending from the ferrule can be placed in
 the short optical fiber support groove of the small-sized substrate.
 The method of making an optical module can comprise a step of forming the
 ferrule support groove in the first region, the optical fiber support
 groove in the second region, and a marker for mounting the optical device
 in the third region.
 This step of forming can have a step of collectively forming the ferrule
 support groove, optical fiber support groove, and marker along a
 predetermined axis. The ferrule support groove supports the ferrule by two
 faces thereof, whereas the optical fiber support groove supports the
 carbon-coated optical fiber by two faces thereof.
 Also, the method can have a step of forming a connection groove that
 separates the first region and the second region from each other and
 intersects the predetermined axis.
 Further, the method can have a step of forming a positioning groove that
 separates the second region and the third region from each other and
 extends in a direction intersecting the predetermined axis. The
 positioning groove can be used to define the position of an end portion of
 the carbon-coated optical fiber.
 The step of securing has a step of providing a UV curing resin between the
 substrate and a securing member which is ultraviolet(UV)-transparent, and
 a step of irradiating the UV-curing resin with ultraviolet ray so as to
 cure the UV-curing resin. Using the UV-transparent securing member and the
 UV-curing resin makes it easier to position the optical fiber with respect
 to the substrate and secure the optical fiber to the substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments of the present invention will be explained with reference to
 the accompanying drawings. Parts identical or equivalent to each other
 will be referred to with identical numerals or letters if possible,
 without repeating their overlapping descriptions. In the following
 explanation, an optical fiber means a carbon-coated optical fiber unless
 otherwise specified in particular.
 FIG. 1 shows an optical module substrate. Referring to FIG. 1, a first
 region 2a, a second region 2b, and a third region 2c are located on a
 substrate 2 along a predetermined axis 3.
 The first region 2a is provided with a ferrule support groove 4 for
 supporting a ferrule. The ferrule support groove 4 has two side faces 4a,
 4b for supporting the ferrule, and a bottom face 4c held between these two
 side faces. This groove has a trapezoidal cross section.
 The second region 2b is provided with an optical fiber support groove 5 for
 supporting an optical fiber. The optical fiber support groove 5 has two
 side faces 5a, 5bfor supporting the optical fiber. This groove has a
 V-shaped cross section.
 The ferrule support groove 4 has a tapered surface 4d at the boundary
 between the ferrule support groove 4 and the optical fiber support groove
 5. The optical fiber support 5 has a tapered surface 5c at one end
 thereof.
 The third region 2c has markers 7 for determining the position at which an
 optical device is disposed.
 For example, the ferrule support groove 4, the optical fiber support groove
 5, and the markers 7 are formed together as follows. A mask pattern is
 formed on the surface of the substrate 2 by a photolithography method. A
 silicon substrate is preferably used as the material for the substrate 2.
 Employed as the surface orientation of the main surface of the silicon
 substrate is (100) surface, whereas a KOH solution is used as an etchant.
 Since the etching rate varies depending on the surface orientation of
 silicon, it is possible to use a property that (111) surfaces with a slow
 etching rate appear. By adjusting the etching time and the mask pattern
 width depending on respective grooves to be formed, the grooves 4, 5 can
 have a V-shaped or trapezoidal cross section as in the embodiment shown in
 FIG. 1. The two side faces (surfaces equivalent to the (111) surface of
 silicon) constituting each of the grooves 4, 5 form obtuse angles with
 their corresponding surface 2a, 2b of the substrate 2.
 In the optical module substrate 2, the optical fiber support groove 5 and
 the ferrule support groove 4 are formed on the same substrate 2.
 Referring to FIG. 2, the substrate 2 is formed with a positioning groove 9
 and a connection groove 10.
 The substrate 2 has the positioning groove 9, formed so as to separate the
 second region 2b and the third region 2c from each other, for positioning
 the optical fiber. The positioning groove 9 is located at one end portion
 of the optical fiber support groove 5 and intersects the optical fiber
 support groove 5 at a predetermined angle, e.g., 90.degree.. The
 positioning groove 9 is a rectangular groove, deeper than the optical
 fiber support groove, having a side face 9a. The positioning groove 9 can
 be formed, for example, by dicing.
 The substrate 2 has the connection groove 10 formed so as to separate the
 first region 2a and the second region 2b from each other. The connection
 groove 10 is located between the optical fiber support groove 4 and the
 ferrule support groove 5. The connection groove 10 intersects the optical
 fiber support groove 4 and the ferrule support groove at an angle such as
 90.degree.. The connection groove 10 can be formed across the substrate 2
 from one of a pair of opposed side faces 2e, 2f to the other, for example,
 by dicing. The connection groove 10 is a rectangular groove, deeper than
 the ferrule support groove 5, having a side face 10a.
 When the optical fiber support groove 5 and the ferrule support groove 4
 are formed by etching, a tapered surface (4d in FIG. 1) is formed at the
 boundary between the optical fiber support groove 5 and the ferrule
 support groove 4. The connection groove 10 is formed so as to eliminate
 this tapered surface. As a result, the ferrule disposed in the ferrule
 support groove 4 can approach the optical fiber support groove 5. The
 optical fiber support groove 5 has a tapered surface (5c in FIG. 1) at an
 end portion thereof. The positioning groove 9 is formed so as to eliminate
 this tapered surface.
 The third region 2c has an optical device mount portion 6. The drawings
 show the substrate 2 preferable for optically coupling a semiconductor
 light-emitting device (11 in FIG. 3) to the optical fiber. A semiconductor
 light-emitting device such as semiconductor laser is mounted on the
 optical device mount portion 6. For monitoring the light-emitting state of
 the semiconductor laser, a monitor light-receiving device (12 in FIG. 3)
 such as photodiode can be provided. The optical device mount portion 6 has
 electrodes 8a, 8b, 8c, 8d for the optical device.
 The substrate 2 formed from a silicon substrate is also referred to as
 silicon bench.
 FIG. 3 shows a manufacturing step in which the optical device is mounted
 onto the substrate 2. Referring to FIG. 3, the optical device is arranged
 on the optical device mount portion 6 of the substrate 2. The optical
 device is optically coupled with the optical fiber. Such an optical device
 can be a semiconductor light-emitting device or a semiconductor
 light-receiving device. The following explanation relates to a
 semiconductor laser (LD) 11 and a monitor photodiode (PD) 12 mounted on
 the substrate as the semiconductor light-emitting device and the
 semiconductor light-receiving device, respectively.
 The LD 11 is die-bonded to the optical device mount portion 6 of the
 substrate 2. The die bonding is carried out after a marker on the LD and
 the markers on the substrate 2 are aligned with respect to each other by
 use of image recognition. For achieving reliable optical coupling with the
 optical fiber (single-mode optical fiber: SMF) disposed in the optical
 fiber support groove 4, a positioning accuracy of 2 .mu.m or shorter is
 required.
 Subsequently, the PD 12 is die-bonded onto the optical device mount portion
 6 of the substrate 2. The die bonding is carried out after a marker on the
 PD and the markers 7a, 7b on the substrate 2 are aligned with respect to
 each other by use of image recognition. The positioning accuracy required
 for this step is within about 10 .mu.m.
 FIG. 4 shows a step of securing an optical fiber to the substrate 2. An
 optical fiber 16 inserted in a ferrule 17 and a securing member 18 are
 prepared. In this step, the optical fiber 16 is provided in the optical
 fiber support groove 5 of the substrate 2 and is secured thereto by means
 of the securing member 18. For this purpose, a UV-curing resin is dropped
 on the second region of the substrate 2 so as to avoid the V-shaped groove
 for the optical fiber, whereby resin members 19a are formed. The optical
 fiber 16 is placed in the optical fiber support groove 5 and is covered
 with the securing member 18. The securing member 18 has a groove 18a for
 accommodating the optical fiber 16 and a bonding surface 18b facing the
 substrate 2. As a result, the thickness of the resin members 19a between
 the securing member 18 and the substrate 2 can be reduced. If temperature
 or humidity fluctuates, the resin members 19a will expand or contract.
 When the securing member 18 is provided with this structure, however, the
 strength for securing the optical fiber 16 can be maintained. In this
 case, the optical fiber 16 is supported by the three flat surfaces
 including the two side faces of the optical fiber support groove 5 and the
 bottom face of the groove 18a of the securing member 18. The securing
 member 18 is formed from UV transparent material, e.g., quartz.
 FIG. 5A shows a step of securing the ferrule 17. The ferrule 17 with the
 optical fiber 16 inserted therein is disposed at the ferrule support
 groove 4. The UV-curing resin is also applied to boundaries between the
 side face of the ferrule 17 and the first region 2a of the substrate 2 to
 form resin members 19b for securing the ferrule.
 Thereafter, the UV-curing resin members 19a, 19b are irradiated with
 ultraviolet rays. The optical fiber 16 is secured by the securing member
 18 and the resin members 19a, whereas the ferrule 17 is secured by the
 resin members 19b. Thus, an optical module principal portion 1 is
 completed.
 Since the UV-curing resin is employed for securing the ferrule 17 to the
 substrate 2, the handling of the optical module principal portion 1
 becomes easier after this step. A thermosetting resin can be also used in
 place of the UV-curing resin.
 FIG. 5B is a sectional view showing the optical fiber 16 secured to the
 optical fiber support groove 5. The optical fiber 16 comes into contact
 with the two side faces 5a, 5b so as to be supported thereby.
 FIG. 5C is a sectional view showing the ferrule 17 secured to the ferrule
 support groove 4. The ferrule 17 comes into contact with the two side
 faces 4a, 4b so as to be supported thereby.
 FIG. 6 shows a step in which the optical module principal portion 1 is
 assembled on a lead frame 30. The optical module principal portion 1 is
 bonded to a die pad 31 of the lead frame 30. After the die pad 31 is
 coated with silver paste, the optical module principal portion 1 is
 mounted to the die pad 31. The silver paste is thermally cured by heating
 under such conditions as 180.degree. C. and 20 minutes, for example. In
 securing, the lead frame 30 and the ferrule are positioned with respect to
 each other.
 Thereafter, the optical module principal portion 1 and the lead frame 30
 are wire-bonded to each other.
 FIG. 7 shows a potting step in which a resin is dropped on the optical
 module principal portion 1. It is desirable that the potting be carried
 out on the optical module principal portion at two positions.
 One of the two positions lies in a region including the LD 11, the PD 12,
 and the end portion of the optical fiber 16 optically connected to the LD
 11. The potting resin is transparent to the wavelength of light generated
 by the LD 11. A potting resin body 39a ensures an optical path between the
 LD 11, and the PD 12 and optical fiber 16.
 The other lies in a region covering the optical fiber 16 exposed on the
 substrate 2. Molding materials used in transfer molding contract upon
 curing. When the optical fiber is covered with a potting resin body 39b,
 then it is possible to reduce the influence of the contraction on the
 optical fiber
 Any of UV-curing resins and thermosetting resins can be used for forming
 the potting resin bodies 39a, 39b. They have lower moduli of elasticity
 and reduce the stress applied to the optical fiber 16 due to the
 contraction upon curing and due to the thermal expansion or contraction of
 the resins. It is desirable that the resin be degassed under reduced
 pressure prior to curing it so as to prevent bubbles from occurring within
 the resin.
 Thereafter, the optical module principal portion 1 is encapsulated with a
 resin body 38 formed by a transfer molding method and then lead forming is
 carried out, whereby an optical module 40 as shown in FIG. 22 is
 completed.
 In the following, a step in which the carbon-coated optical fiber is
 provided in the optical module substrate will be explained in detail with
 reference to FIGS. 8 to 13.
 A procedure in which the optical fiber and the ferrule are disposed in the
 substrate 2 will now be explained. FIGS. 8 to 11 are side views of the
 optical module in accordance with an embodiment during assembly, and FIG.
 13 is a prospective view of FIG. 10. The following explanation relates to
 a case where the optical fiber support groove 5 is a V-shaped groove, and
 the ferrule support groove 4 is a trapezoidal groove.
 Referring to FIG. 8, the substrate 2, the carbon-coated optical fiber 16 to
 be disposed in the optical fiber support groove 5 on the substrate 2, and
 the ferrule 17 are shown. The optical device 11 is mounted on the device
 mount portion on the main surface of the substrate 2 so as to face a first
 end part of the optical fiber support groove 5. Though the optical fiber
 support groove 5 and the ferrule support groove 4 do not actually appear
 in side view of FIG. 8, they are indicated by broken lines in FIG. 8.
 An end face of the optical fiber 16 is brought into contact with one side
 face 9a of the positioning groove 9. As a consequence, the end portion of
 the optical fiber 16 is positioned. The optical fiber 16 is bent such that
 a tip portion thereof having a length of L is arranged in the optical
 fiber support groove 5. FIG. 9 shows the optical fiber 16 thus arranged in
 the optical fiber support groove 5. In this arrangement, the end face of
 the optical fiber 16 is installed so as to face one end face of the
 optical device 11. According to FIG. 9, the part of optical fiber 16
 having a length of L is arranged in the V-shaped groove 5 without bending.
 A remaining part of optical fiber 16 is bent at a predetermined curvature
 R. The optical fiber 16 does not bend within the ferrule 17. As a
 consequence, these linear parts are tangent to the bent part at the point
 A (the boundary between the horizontal part of the fiber and the bent
 part) and the point B (the position of an end face of the ferrule) located
 at both ends of the bent part of the optical fiber 16, respectively.
 Referring to FIG. 9, the point B is positioned at a height .delta. above
 the surface of the substrate 2. Let the angle formed between the extension
 of the center axis of the optical fiber 16 within the ferrule 17 and the
 surface of the substrate 2 be .theta.. Let the length of the bent part of
 the optical fiber 16 as projected onto the surface of the substrate 2 be
 I.
 As explained above, the optical fiber 16 and the ferrule 17 are arranged
 such that the tangents at the points A and B coincide with the fiber
 center axis and ferrule center axis, respectively. In order to attain this
 arrangement, it is necessary for R, .delta., I, and .theta. to be set
 properly. Their relationships are approximately represented by:
EQU R=(3.delta.)/(2.theta..sup.2) (1)
EQU I=(3.delta.)/.theta. (2)
 when .theta. is very small. FIGS. 14 and 15 show the characteristics
 represented by equations (1) and (2). FIG. 14 is a graph showing
 relationships between I and .delta. with respect to R, whereas FIG. 15 is
 a graph showing relationships between .delta. and .theta. with respect to
 R. For example, in order to realize a bending of R=10 mm and a height of
 .delta.=0.1 mm, the angle .theta. can be determined with reference to FIG.
 15. As a result, it can be seen that .theta. is required for about
 7.degree.. The value of I at this condition can be determined with
 reference to FIG. 14 and is found to be about 2.5 mm.
 When the value of I can be made smaller, then the substrate 2 can be
 shortened in the groove-extending direction. For example, in the case
 where the substrate 2 is formed from a silicon wafer, the yield of the
 substrate 2 would increase.
 Lowering R and .delta. is effective in lowering I. On the other hand, it is
 preferable for characteristics of the optical fiber 16 that R be kept from
 too small value. In general, it is desirable that an R of 30 mm or greater
 be ensured for a normal silica fiber having a cladding diameter of 125
 .mu.m. In the carbon-coated optical fiber 16, however, an R is permitted
 in the order of 10 to 15 .mu.m because the carbon-coated optical fiber 16
 exhibits an excellent static fatigue characteristic.
 In the step shown in FIGS. 10 and 13, after the tip portion of the optical
 fiber 16 is arranged, the optical fiber 16 is secured to the substrate 2
 by means of the securing member 18 and the resin members 19a made of the
 UV-curing resin. Using the UV-curing resin makes it easier to position the
 optical fiber 16 with respect to the substrate 2 and secure them together
 in cooperation with the substrate 2 and the securing member 18.
 Subsequently, in the step shown in FIGS. 11 and 12, the ferrule 17 with the
 optical fiber 16 inserted therein is disposed at the ferrule support
 groove 4.
 According to such a method, the optical fiber 16 can be disposed on the
 substrate 2 having its reduced size in the direction of the optical fiber
 support groove 5 without bending the carbon-coated optical fiber 16 at a
 small R.
 The subsequent manufacturing steps will not be explained here because they
 are the same as the steps of making the optical module that have already
 been done with reference to FIGS. 1 to 7.
 Although an optical module principal portion 61 with the optical module
 substrate having two members consisting of a base and a platform will be
 explained in the following, the explanation made with reference to FIGS. 1
 to 7 is similarly applicable to the optical module principal portion 61 as
 well.
 FIG. 16 is a perspective view of the optical module principal portion 61.
 Referring to FIG. 16, the optical module principal portion 61 comprises a
 substrate 62, an optical fiber 16, a ferrule 17, an optical device 71, and
 a light-receiving device 72. The substrate 62 comprises a base 62a and a
 platform 62b. The base 62b comprises a trapezoidal groove 64, extending in
 one direction, for mounting the ferrule 17. The platform 62a comprises, on
 a main surface thereof, a V-shaped groove 65 extending in the same
 direction as the trapezoidal groove 64, and a positioning groove 69
 extending in a direction orthogonal to the V-shaped groove 65. The
 positioning groove 69 has a side face 69a for defining the position of the
 optical fiber disposed in the V-shaped groove. This side face 69a
 corresponds to the side face 9a of the positioning groove 9 in FIG. 2. The
 platform 62a is mounted on the base 62b. On the platform 62a, the optical
 device 71 is mounted adjacent to an end portion of the V-shaped groove 65.
 For example, the platform 62a is formed from a silicon (Si) substrate,
 whereas the base 62b is made of metal. The base 62b and the platform 62a
 are also secured such that the center axes of both grooves 64, 65 align
 with each other. The trapezoidal groove 64 and the V-shaped groove 65 are
 arranged along the same center axis.
 FIGS. 17 to 21 show the method of making an optical module explained with
 reference to FIGS. 8 to 13 with respect to the substrate 62 shown in FIG.
 16. FIGS. 17 to 21 correspond to FIGS. 8 to 12, respectively. In FIGS. 17
 to 21, since the substrate 62 constituted by the platform 62aand the base
 62b is employed, the ferrule support groove 64 and the optical fiber
 support groove 65 are formed in their distinct members. The ferrule
 support groove 4 and optical fiber support groove 5 shown in FIG. 2
 correspond to the ferrule support groove 64 and optical fiber support
 groove 65, respectively. In view of these facts, it is not necessary to
 mention that the carbon-coated optical fiber 16 and the ferrule 17 can be
 secured to the V-shaped groove 65 and the trapezoidal groove 64,
 respectively, in the same procedure as that shown in FIGS. 8 to 13.
 As explained in detail with reference to the drawings, the carbon-coated
 optical fiber is installed at the optical fiber support groove. As a
 consequence, the carbon-coated optical fiber can be bent with a curvature
 R smaller than that permitted in a bared optical fiber when being
 installed at the optical fiber support groove by bending. Therefore, the
 part of optical fiber extending from the ferrule can be shortened. Since
 the optical fiber support groove can be also shortened, the optical module
 substrate can attain a smaller size.
 In the method of making an optical module, it is not necessary to pay
 attention to the handling of the carbon-coated optical fiber as much as
 has been required for the bared optical fiber in conventional optical
 modules. The carbon-coated optical fiber also has durability against
 bending stress. As a consequence, more reliable optical module is provided
 a s compared with conventionally available module.
 Since the carbon coated optical fiber has a strength superior to that of
 the bared optical fiber, the handling of the optical fiber in inserting it
 into the ferrule becomes easier than that of bared optical fiber. If the
 optical fiber is inserted in the ferrule, the part of the optical fiber
 projected from the ferrule and the optical fiber at the position where the
 optical fiber projects from the ferrule are excellent in strength.