Optical waveguides – With disengagable mechanical connector – Optical fiber to a nonfiber optical device connector
Reexamination Certificate
2002-01-28
2003-12-16
Font, Frank G. (Department: 2877)
Optical waveguides
With disengagable mechanical connector
Optical fiber to a nonfiber optical device connector
C385S014000, C257S082000
Reexamination Certificate
active
06663295
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmitter-receiver module used in an optimal communication system, and more particularly to an optical transmitter-receiver module which has an integration of a transmitter circuit and a receiver circuit with a reduced crosstalk between the transmitter circuit and the receiver circuit.
2. Description of the Related Art
The optical transmitter-receiver module is suitably applicable to various data communication systems, typically, local area networks and wide area networks. The optical transmitter-receiver module has requirements for further improvement in high speed performance, further cost reduction, and further side reduction or shrinkage. For satisfying those requirements, it is essential that an optical transmitter-receiver device is packaged in compact over a single substrate, wherein the optical transmitter-receiver device has an integration of the optical transmitter circuit and the optical receiver circuit. The optical transmitter circuit includes light emitting devices, whilst the optical receiver circuit includes light receiving devices. In general, a minimum output current from the light receiving device is much smaller than a driving current for driving the light emitting device. For example, the driving current for driving the light emitting device is 100 mA, whilst the minimum output current from the light receiving device is 10 micro-A, so that a difference is 80 dB.
The optical transmitter-receiver device often uses a standard connector MT-RJ. In this case, a distance between the light emitting device and the light receiving device should be narrow, for example, 750 micrometers. If the optical transmitter-receiver device is required to perform a high bit rate, for example, 10 Gbps or higher, then an undesirable crosstalk between the transmitter circuit and the receiver circuit becomes remarkable.
In order to reduce the crosstalk between the transmitter circuit and the receiver circuit, it is effective to provide a shielding plate between the transmitter circuit and the receiver circuit. This idea is disclosed in 2000 Electronics Information Communication Society SC-3-7, entitled “Analysis of Crosstalk for MT-RJ Optical Sub-Assembly”.
FIG. 1
is a schematic perspective view illustrative of a conventional optical transmitter-receiver module having an integration of the optical transmitter circuit and the optical receiver circuit over a single platform substrate. The conventional optical transmitter-receiver module has a silicon platform substrate
101
. A silicon oxide film
102
overlies the silicon platform substrate
101
. Interconnections
103
and
106
are selectively provided over the silicon oxide film
102
in an optical transmitter circuit region and an optical receiver circuit region respectively. A light receiving device
104
and a receiver LSI circuit
105
are further provided in the optical receiver circuit region. A light emitting device
107
and a transmitter LSI circuit
108
are further provided in the optical transmitter circuit region. Further, a shielding plate
109
is provided between the optical transmitter circuit region and the optical receiver circuit region. The above-described literature reported that the shielding plate reduces the crosstalk by about 20 dB at 1 GHz.
Both the light emitting device and the light receiving device are optically coupled through a ferrule to optical fibers.
FIG. 2
is a plane view illustrative of the optical transmitter-receiver module of FIG.
1
. The light receiving device
104
and the light emitting device
107
are optically coupled through a ferrule
114
to optical fibers
118
respectively. The ferrule
114
include short optical fibers
115
which are optically coupled to the light receiving device
104
and the light emitting device
107
. The optical fibers
118
are further optically coupled to the short optical fibers
115
of the ferrule
114
. The ferrule
114
may be made of a resin material. The ferrule
114
is aligned to the silicon platform substrate
101
, so that the short optical fibers
115
which are aligned to the light receiving device
104
and the light emitting device
107
respectively.
The optical fibers
118
are supported by an optical connector
117
which is mechanically coupled with the ferrule
114
, wherein the ferrule
114
has plural engaging projections
114
a
, whilst the optical connector
117
has plural engaging holes
117
a
which are engagable with the engaging projections
114
a
of the ferrule
114
. This engagement mechanism aligns the optical connector
117
to the ferrule
114
, whereby the optical fibers
118
are aligned to the short optical fibers
115
.
The ferrule
114
is made of an optical shielding resin material which contains an light-absorbing additive such as a black pigment, in order to prevent that a stray light generated in the transmitter side undesirably enters into the receiver side. The interpose of the ferrule
114
between the optical connector and the transmitter circuit and the receiver circuit is disclosed in 2000 Electronics Information Communication Society S-3-140, entitled “SM-Fiber MT-RJ Optical Transceiver Module”.
For integrally packaging the light emitting device and the light receiving device over a single substrate, silicon may often be selected for the substrate material because of its low cost and high beat conductivity. Silicon has a high heat conductivity of 150 W/mk whilst alumina has a high heat conductivity of 20 W/mk. Since silicon is relatively high in electrical conductivity as compared to insulators, as shown in
FIG. 1
, the light emitting device and the light receiving device are electrically coupled through the silicon platform substrate
101
but weakly, however, a relatively large cross talk appears through the silicon platform substrate
101
between the transmitter circuit and the receiver circuit.
Silicon is much lower in electrical conductivity than metal materials. Silicon has a specific resistivity of about 1E4 ohms cm, whilst copper has a specific resistivity of about 1.6E-64 ohms cm. Even if the silicon platform substrate
101
is grounded, the electrical coupling is still present between the transmitter circuit and the receiver circuit through the silicon platform substrate
101
. It was confirmed that it is difficult to reduce the crosstalk to about −80 dB at 10 GHz.
It was proposed that in order to reduce the crosstalk, the silicon substrate is divided into the transmitter side and the receiver side for preventing the electrical coupling between the transmitter circuit and the receiver circuit through the silicon substrate. Separate packaging processes of the light emitting device and the light receiving device over the divided silicon substrate and separate alignment processes in optical axis are necessary. This increases the fabrication processes and also the final product cost.
As described above, the ferrule
114
is made of the optical shielding resin material which shields the stray light but does not shield electromagnetic waves. The resin ferrule
114
undesirably allows formation of an electromagnetic wave propagation route
116
at a confronting edge of the silicon platform substrate
101
to the ferrule
114
. This electromagnetic wave propagation route
116
allows propagation of electromagnetic wave from the transmitter side to the receiver side, resulting in a possible generation of the undesirable crosstalk between the transmitter circuit and the receiver circuit.
If the ferrule
114
is made of a metal which is capable of shielding the electromagnetic wave for suppressing any formation of the electromagnetic wave propagation route
116
, then the following difficulty is alternatively raised. As described above, the ferrule
114
has holes for incorporating the short optical fibers
115
, wherein the holes have a diameter which is slightly larger than a diameter of the short optical fibers
115
, and further the holes are distanced at a pitch exactly identical with a pi
Kami Nobuharu
Kurata Kazuhiko
Oda Mikio
Font Frank G.
NEC Corporation
Valentin II Juan D
Young & Thompson
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