Photodiode

Details Photodiode Photodiode

2000-05-09

2003-02-11

Flynn, Nathan J. (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S461000

Reexamination Certificate

active

06518638

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a photodiode for an LD/PD module for bidirectional optical communication system with signal light of two wavelengths &lgr;
1
and &lgr;
2
via a single optical fiber which can suppress the influence of transmitting light upon the photodiode. The single-fiber bidirectional optical communication system using of a single fiber both for transmitting and receiving signals employs an LD/PD module. The LD/PD module has a substrate, a package, an LD emitting transmission signals and a PD receiving signals mounted upon the common substrate in the common package. This is called a multiwavelength bidirectional LD/PD module.
Another optical communication system transmits more than one signal by the light of plural wavelengths in one direction via a single fiber and receives the multiwavelength signals by a PD module having a plurality of photodiodes. The PD module has more than one photodiode on a common substrate in a common package. This is a unidirectional multiwavelength PD module.
A photodiode is a sensor which converts light power (light signals) to a photocurrent (electric signals) in proportion to the light power. The PD is sometimes called an O/E transducer or O/E sensor. The photodiode is a highly sensitive sensor. The LD generates strong light power for transmitting optical signals to a far distanced port. Although the wavelengths are different for transmitting signals and for receiving signals, the photodiode which has sensitivity also to the transmission wavelength has a possibility of sensing the transmission power yielded by the laser diode mounted on the same package.
This application claims the priority of Japanese Patent Applications No.11-201519 (201519/1999) filed on Jul. 15, 1999 and No.11-260016 (260016/1999) filed on Sep. 14, 1999 which are incorporated herein by reference.
The phenomenon that the PD senses the transmission signal emitted from the LD which is stored in the same package is called “optical crosstalk”. The LD light of the same port is noise for the PD. Sensing the transmission light at the same port hinders the PD receiving the transmission light from the counterpart port (e.g., central station). It is important to suppress the crosstalk from the LD (transmitting device) to the PD (receiving device) at the same package. There are two interactions between the transmitting device and the receiving device on the same substrate. One is the optical crosstalk which is optical coupling between the PD and the LD. The other is the electric crosstalk which is conveyed from the LD to the PD by electromagnetic waves. Both kinds of crosstalk are difficult for the LD/PD module to conquer. This invention aims at solving the optical crosstalk.
There are several versions of the bidirectional LD/PD module for making use of a single fiber both for transmission and reception with regard to the modes of signal separation of the transmitting light and the receiving light. A typical signal separation device is a WDM (wavelength division multiplexer) which divides the common path spatially to a transmission path and a reception path by the difference of the light wavelengths. The WDM separation alleviates the difficulty of the optical crosstalk, since the WDM allots different paths to the LD and the PD for separating them spatially. A special disposition is a serial alignment of the PD and the LD on the same straight line. In this case, most of the transmission path is common with the reception path. The common path type module is suffering from more serious crosstalk problem.
This application claims the priority of Japanese Patent Applications No.11-201519(201519/19990) filed on Jul. 15, 1999 and No.11-260016(260016/1999) filed on Sep. 14, 1999, which are incorporated herein by reference.
2. Description of Related Art
FIG. 1
shows a typical multiwavelength bidirectional optical communication system having LD/PD modules at a central station and at a subscriber port. At the station, an LD
1
generates downward signals. The downward signals travel through an optical fiber
1
, a WDM
2
, an optical fiber
3
, another WDM
4
and an optical fiber
5
to a PD
2
at the subscriber site. The PD
2
converts the downward optical signals to electric (received) signals. At the ONU (optical network unit) terminal, a subscriber LD
2
generates upward optical signals. The upward signals spread through an optical fiber
6
, the WDM
4
, the fiber
3
, the WDM
2
and the fiber
7
and attain at a PD
1
at the station. The PD
2
converts the light signal from the subscriber into electric signal. The single fiber
3
enables both the upward signals and the downward signals to spread in both directions between the central station and the ONU terminal. The WDM
2
at the station alternatively allocates the downward signals and the upward signals into the fiber
1
or fiber
7
by the difference of the wavelengths. The downward light wavelength is denoted by &lgr;
2
. The upward light wavelength is designated by &lgr;
1
. Both signals are propagating in both directions in the same fiber
3
. The WDM
4
at the subscriber port (ONU) alternatively allocates the downward signals and the upward signals into the fiber
5
or fiber
6
by the difference of the wavelengths. The ONU receives the downward (receiving) signals &lgr;
2
by the PD
2
. The LD
2
generates the transmission (upward) signals &lgr;
1
at the ONU. Electric circuits following the PD
2
and the LD
2
are omitted in FIG.
1
.
The words of “transmitting” or “receiving” signals have reverse directions (or inverse flows) at the station and at the ONU. In the description, the words should be defined at the ONU site. Thus, the upward light &lgr;
1
corresponds to the transmitting signals. The downward light &lgr;
2
carries the receiving signals. The prior art (multiwavelength bidirectional communication) of
FIG. 1
separates the PD
2
and the LD
2
spatially by dividing the light paths by the WDM.
FIG. 2
shows a prior multiwavelength unidirectional optical communication system for transmitting various signals from a central station in a downward direction to a subscriber port. At the station, an LD
1
and an LD
2
generate different downward signals of &lgr;
1
and &lgr;
2
. The downward signals travel through an optical fiber
1
or
7
, a WDM
8
, an optical fiber
3
, another WDM
4
and an optical fiber
5
or
6
to a PD
1
or a PD
2
at the subscriber site. The WDM
4
separates two different signals by the difference of the wavelengths &lgr;
1
and &lgr;
2
. The PD
1
senses &lgr;
1
. The PD
2
detects &lgr;
2
. Crosstalk occurs between PD
1
and PD
2
also in the multiwavelength unidirectional system.
FIG. 3
is a sectional view of a prior art PD module which has widely been used as a receiving device in the optical communication network having the spatially separated paths as shown in
FIG. 1
or FIG.
2
. The PD module has a metallic bottom circular stem
10
with lead pins
9
extending downward. A PD chip
12
is mounted via a submount
11
onto the stem
10
. A thin metal cap
14
having a lens
13
is adjusted and welded on the stem
10
. A cylindrical sleeve
15
is adjusted and welded on the stem
10
above the cap
14
. A ferrule
16
is inserted, adjusted and fixed in an axial hole of the sleeve
15
. The ferrule
16
clamps an end of an optical fiber
17
. The end of the ferrule
16
is slantingly polished. An elastic bend-limiter
18
caps the top end of the sleeve
15
for protecting the fiber
17
from overbending. FIG.
1
and
FIG. 2
include LD modules in addition to the PD modules explained by FIG.
3
. The LD module is omitted to describe, since it is simply obtained by replacing the PD chip with an LD chip in the module of FIG.
3
.
This invention is applicable to the spatially separating LD/PD or PD/PD modules as shown in
FIG. 1
or FIG.
2
. The prior PD module of
FIG. 3
has a three-dimensional structure making use of a metallic package. The expensive metal package hermetically seals the PD device and shields the PD from

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