Optical module coupling device

In an optical module with a substrate having an optical unit mounted thereon which substrate is provided on the bottom of a package so that the optical unit is optically coupled with an end of an optical fiber extending into the package, a chip carrier, including the optical unit, is die-bonded to a conductive land formed on the substrate. A recess is formed in the bottom of the package which faces the land. Thus, a capacitance of a parallel-plate capacitor formed by the land and the bottom of the is made negligibly small.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to an optical module for use with an optical 
communication system such as data link or an optical LAN which uses light 
as an information transmission medium. 
2. Related Background Art 
In a known optical module, a semiconductor laser or a light emitting diode, 
which is a light emitting element, or a PIN photo-diode which is a light 
detecting element, is mounted as an optical unit and is optically coupled 
with an optical fiber in a package. However, a response frequency band of 
the known optical module is not sufficiently high. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical module 
having a sufficiently high response frequency band. 
In order to achieve the above object, the present invention provides an 
optical module with a substrate having an optical unit mounted thereon, 
which substrate is provided on the bottom of a package so that the optical 
unit is optically coupled with an end of an optical fiber extending into 
the package. A chip carrier including the optical unit is die-bonded to a 
conductive land formed on the substrate and a recess formed in the bottom 
of the package faces the land. 
The present invention will become more fully understood from the detailed 
description given hereinbelow and the accompanying drawings which are 
given by way of illustration only, and thus are not to be considered as 
limiting the present invention. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIGS. 1 and 2, a hybrid IC substrate 6 made of ceramics is 
mounted on an inner face of the bottom of a package 8 made of metal. A 
chip carrier 2 is mounted on the hybrid IC substrate 6. 
As shown in FIG. 3, the chip carrier 2 is of parallelepiped shape and made 
of ceramics. A PIN photo-diode 3 is mounted on a front plane of the chip 
carrier 2. Two terminals 5, which are to be connected to an anode and a 
cathode of the PIN photo-diode 3 on the chip carrier 2, are formed to 
extend from the front plane to a bottom plane. Those terminals 5 are 
formed by metallizing selected surface areas of the chip carrier 2. On the 
other hand, two conductive lands 7 which serve as electrodes are formed on 
a surface of the hybrid IC substrate 6. The terminals 5 of the chip 
carrier 2 are die-bonded to the lands 7. The lands 7 are designed to have 
wider areas than the terminals 5 assuming possible misalignment in 
mounting the chip carrier 2. With present technique, the area of the land 
7 is in the order of 1mm.times.2mm at minimum. 
The PIN photo-diode 3 mounted on the chip carrier 2 faces a leading end 
plane of an optical fiber 1 in the package 8. The leading end of the 
optical fiber 1 is inserted into the package 8 through an opening formed 
in a side wall of the package 8. A cover of the optical fiber 1 is striped 
off at the leading end thereof so that a glass fiber is exposed. The 
exposed area is metal-plated except for the leading end plane. The 
metal-plated area Ia is positioned on a fixed member 9 mounted on the 
bottom of the package 8 and fixed thereto by soldering. The metal-plated 
area 1a of the optical fiber 1 is positioned such that light emitted from 
the leading end plane of the optical fiber 1 is sufficiently directed to a 
photo-sensing area of the PIN photo-diode 3. In this manner, the optical 
fiber 1 and the PIN photo-diode 3 are optically coupled. 
A recess 10 is formed in the bottom of the package 8. The recess 10 is 
formed at a position which faces the lands 7 with the intervention of the 
hybrid IC substrate 6 when the hybrid IC substrate 6 is mounted in the 
package 8. Thus, the lands 7 and the bottom of the package 8 are separated 
from each other by a space. The recess 10 serves to render a capacitance 
of a parallel-plate capacitor, which is essentially formed by the lands 7 
and the bottom of the package 8, negligibly small. 
The parallel-plate capacitor which is essentially formed by the lands 7 and 
tbe bottom of the package 8 is now explained. FIG. 4 shows an embodiment 
of a light receiver which uses the PIN photo-diode 3. The parallel-plate 
capacitor formed by the lands 7 and the bottom of the package 8 function 
as an input capacitance C.sub.pal of an amplifier 12. A response frequency 
band of the light receiver is determined by a time constant T which is 
determined by a resistance R.sub.in of an input resistor 11, a junction 
capacitance C.sub.pd of the PIN photo-diode 3 and the input capacitance 
C.sub.pal. As the time constant T increases, the amount of information per 
unit time decreases. The time constant T is given by 
EQU T=C.sub.pd +C.sub.pal).multidot.Rin 
The junction capacitance C.sub.pd of the PIN photo-diode 3 used for high 
speed optical communication is very small, usually on the order of 
0.3.about.0.4 pF. Accordingly, the affect of the input capacitance 
C.sub.pal on the time constant T is very great and it should be as small 
as possible. 
If the recess 10 is not formed, the capacitance C.sub.pal of the 
parallel-plate capacitor is 0.46 pF assuming that the area of the land 7 
is 1mm.times.2mm, a dielectric constant .epsilon. of the hybrid IC 
substrate 6 is 9, and a thickness thereof is 350 .mu.m. It is 0.25 pF 
assuming that the thickness of the hybrid IC substrate 6 is 650 .mu.m. 
This capacitance is substantially equal to or larger than the capacitance 
of C.sub.pd (0.3.about.0.4 pF). It is thus seen that tbe affect on the 
time constant T is very great. 
In the optical module of the present embodiment, since the recess 10 is 
formed in the inner bottom of the package 8, the capacitance of the 
parallel-plate capacitor formed by the lands 7 and the bottom of the 
package 8 is small enough to permit neglection of the input capacitance 
C.sub.pal of the amplifier 12. As a result, the time constant T of the 
light receiver is reduced and the response frequency band is increased. 
This embodiment is a receiving optical module which uses the PIN 
photo-diode 3 as the optical unit which is optically coupled to the 
optical fiber 1. The same problem is encountered in a transmitting optical 
module which uses a light emitting element such as LED or laser diode as 
the optical unit, and the present invention is also applicable to such a 
transmitting optical module. 
From the invention thus described, it will be obvious that the invention 
may be varied in many ways. Such variations are not to be regarded as a 
departure from the spirit and scope of the invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
to be included within the scope of the following claims.