The present invention relates to a duplex or bidirectional optical communication device.
FIG. 1 schematically illustrates a duplex optical communication device. The duplex optical communication device is comprised of a connector plug 2 including a couple of optical fibers for reception and transmission (not shown), an optical fiber cable 1 being extended from the connecting plug 2; a light transmission module unit; a light reception module unit (not shown); and a connector receptacle 3 for receiving the connector plug 2. The transmission optical fiber and the reception optical fiber (not shown) are respectively held in place by metal ferrules 4 and 5. When the connector plug 2 is fitted into the connector receptacle 3, the end faces of the transmission and reception optical fibers face a light emitting element (not shown) in the transmission module unit and a light receiving element (not shown) in the reception module unit, respectively. In such a duplex optical communication device, the rays of light emitted from the light emitting element in the transmission module unit are received at the end face of the transmission optical fiber and are transmitted through the transmission optical fiber. The rays transmitted through the transmission optical fiber are received by the light receiving element in the reception module unit, through the end face of the transmission optical fiber.
In FIG. 2, only a module device 6, a transmission optical fiber 26 and a reception optical fiber 27 are illustrated, for the sake or simplicity.
In the figure, a transmission module unit 12 and a reception module unit 13 are both provided on a multi-layered interconnection ceramic substrate 11. The transmission module unit 12 contains a light emitting element 14, a driver integrated circuit 15 for driving the light emitting element 14, and a resistive element 25, which are formed on the ceramic substrate 11. The reception module unit 13 is comprised of a PIN-PD (photo diodide) 16, a receiver integrated circuit 17 for amplifying the PIN-PD 16, and a by-pass capacitor 18, which are also formed on the ceramic substrate 11. The transmission module unit 12 is further provided with a shell 20 made of Kovar to be used in covering the light emitting element 14, the driver integrated circuit 15 and the resistive element 25. Similarly, the reception module unit 13 is provided with a shell 22 made of Kovar to be used in covering the PIN-PD 16, the receiver integrated circuit 17 and the by-pass capacitor 18. A transparent window member 19, such as a glass plate is provided at the portion of the shell 20 facing the light emitting element 14. Likewise, a transparent window member 21, such as a glass plate is provided at the portion of the shell 22 facing the PIN-PD 16. The shells 20 and 22 are seam-welded to a seal ring 24 provided on a ring-like ceramic member 23. With this welding, the transmission module unit 12 and the reception module unit 13 are hermetically sealed. Disposed above the transparent window member 19 in the transmission module unit 12 is a transmission optical fiber 26 of which the light receiving end face confronts with the light emitting element 14. Similarly, disposed above the transparent window member 21 in the reception module unit 13 is a reception optical fiber 27 of which the light emitting end face faces the PIN-PD 16. The optical fibers 26, 27 are each provided with a core forming a light transmission path and a cladding surrounding the core.
In the duplex optical communication device with such a structure, the rays of light emitted from the light emitting element 14 in the transmission module unit 12 travel at a fixed radiating angle, pass through the transparent window member 19, and come to be incident on the light receiving end face of the transmission optical fiber 26. In this way, the rays from the light emitting element 14 are optically coupled with the transmission optical fiber 26. The rate of the total amount of light emitted from the light emitting element 14 and the amount of light transmitted through the transmission optical fiber 26 is referred to as the optical coupling efficiency .eta..sub.E. One of the factors to determine the light coupling efficiency .eta..sub.E of the reception module unit is a distance l between the light emitting surface of the light emitting element 14 and the light receiving end face of the transmission optical fiber 26, and the distance l is mathmetically related with the light coupling efficiency .eta..sub.E by the following equation ##EQU1##
As may be seen from the formula (A), to improve the light coupling efficiency .eta..sub.E, the distance l between the light emitting surface of the light emitting element 14 and the light receiving end face of the transmission optical fiber 26 must be minimized. Actually, there is a limit in shortening the distance between the light receiving surface of the light emitting element 14 and the light receiving end face of the transmission optical fiber 26. The reason for this is that the duplex optical communication device involves some inherent problems with structural features, such as with loop of the bonding wire attached to the light emitting element 14, the thickness of the resistive element 25 and the thickness of the transparent window member 19. The shortest possible distance l attainable is 0.7 mm-1.0 mm, at most. For example, for the light emitting surface of the light emitting element of 0.3 mm.times.0.3 mm and the transmission optical fiber 26 having a core of 0.5 mm .phi. and 0.4 of numeral aperture, the light coupling efficiency .eta..sub.E is approximately 5%.
Another factor to be considered in determining the light coupling efficiency .eta..sub.E is the radiation angle .theta..sub.E. The relationship of the radiation angle .theta..sub.E to the light coupling efficiency .eta..sub.E is given by relationship (B), as follows: ##EQU2##
Relationship (B) indicates that, in improving the light coupling efficiency .eta..sub.E, the radiation angle .theta..sub.E must be minimized.
The optical fiber allows only the light rays having an incident angle within a maximum light receiving angle .theta..sub.Fmax (=sin.sup.-1 NA), as given by the numeral aperture NA, to pass therethrough. Therefore, it is essential to make the radiation angle .theta..sub.E small, to increase the light coupling efficiency .theta..sub.E ; .theta..sub.Fmax (=sin.sup.-1 NA).gtoreq..theta..sub.Emax.
A conventional structure capable of decreasing the radiation angle .theta..sub.E may now be described with reference to FIG. 3. In the figure, a transmission module unit 40 has a light emitting element 33 formed on a metal header 31 with a copper plate 34 interposed therebetween. A ball glass or lens 32 is placed on the light emitting element 33. Three lead pins 35-37 are coupled with the metal header 31. The lead pin 36 is electrically connected to the copper plate 34. The lead pin 37 passes through the metal header 31 to connect to the light emitting element 33 through a bonding wire 38. The lead pin 35 is connected to nothing, that is, a dead pin. A metal shell 39 is mounted on the metal header 31 to hermetically seal the components on the metal header 31. A plate glass window 41 is set at that portion of the metal header 31 which confronts the light emitting element 33. With such a structure, the rays of light emitted from the light emitting element 33 enter the ball glass 32, where they are inclined toward each other in such a way as to decrease the radiation angle .theta..sub.E. Then, the rays are incident on the light receiving surface of the optical fiber 42, through the plate glass window 41.
The transmission module unit 40 with the structure shown in FIG. 3 needs the ball glass 32 fixed to the light receiving surface of the light emitting element 33, and is therefore costly to manufacture. This structure can indeed increase the light coupling efficiency .eta..sub.E in the transmission module unit 40, but can little increase that .eta..sub.D in the reception module unit when the ball glass 32 is fixed to the light receiving element PIN-PD. A total light coupling efficiency .eta..sub.T meaning a rate of the rays of light emitted from the transmission module unit to the rays reaching the reception module unit through the optical fiber, is given by EQU .eta..sub.T =(.eta..sub.E).times.(.eta..sub.D) (C)
As may be seen from the above equation, in increasing the total light coupling efficiency .eta..sub.T, it is necessary to increase both the light coupling efficiencies .eta..sub.E, .eta..sub.D on the transmission and reception sides. In this respect, the structure shown in FIG. 3 is not preferable in increasing the total light coupling efficiency .eta..sub.T.
A structure capable of increasing the light coupling efficiency and applicable on both the transmission and reception module units may now be described with reference to FIG. 4. For ease of explanation, the structure shown in FIG. 4 is applied for only the transmission module unit. In the transmission module unit 50 shown in FIG. 4, like reference numerals are applied to like or equivalent portions in FIG. 3, and the explanation of those portions will be omitted. The structure of FIG. 4 does not use the ball glass 32 and uses a convex lens 51 in place of the plate glass window 41. The rays emitted from the light emitting element 33 are collected by the convex lens 51 and incident on the light receiving end face of the optical fiber 52. The presence of the convex lens 51 improves the light coupling efficiency .eta..sub.E on the transmission side by the light collecting function of the lens. When the structure is applied to the reception module unit, the rays coming from the light emitting surface of the optical fiber are collected by the convex lens and applied to the light receiving element PIN-PD. The light collecting action by the convex lens improves the light coupling efficiency .eta..sub.D on the reception side. Thus, when the structure under discussion is applied to both the transmission and reception module unit, the total light coupling efficiency .eta..sub.T is improved with the light collecting action of both of the lens. Further, since the ball glass 32 is not used, the cost to manufacture is reduced.
FIG. 5 shows a duplex optical communication device in which the structure shown in FIG. 4 is applied to both transmission and reception module units. The duplex optical communication device 60 of FIG. 5 has the same structure as the duplex optical communication device of FIG. 2, except that the transparent window members 19 and 21 are replaced by convex lenses 61 and 62, respectively. The total light coupling efficiency .eta..sub.T of the duplex optical communication devices of FIG. 5 and FIG. 2 were measured. The results of measurement showed that a difference in the total light coupling efficiency .eta..sub.T between these optical communication devices is indistinctive and is smaller than the value expected. To improve the total light coupling efficiency .eta..sub.T, the distance between the light emitting surface of the light emitting element 14 and the light receiving end face of the optical fiber 26 in the transmission module unit 12, and the distance between the light receiving surface of the PIN-PD 16 and the light emitting surface of the reception optical fiber 27 in the reception module unit 13 are respectively set to the imaging lengths of the convex lenses 61, 62 in the transmission and receiving module units. The result was merely a several % point improvement of the total light coupling efficiency .eta..sub.T, which improvement is insufficient, as a general rule.
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