Photodiode

Details Photodiode Photodiode

2002-12-06

2004-02-24

Whitehead, Jr., Carl (Department: 2813)

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

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S460000

Reexamination Certificate

active

06696740

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the structure of a photodiode used in an optical transceiver module for an optical communication system in which one optical fiber is used for carrying out transmission and reception of signals by using two different wavelengths, &lgr;1 and &lgr;2 (&lgr;1<&lgr;2). The photodiode can detect light having a shorter wavelength, &lgr;1, by dexterously eliminating the influence of outgoing light having a longer wavelength, &lgr;2. It should be noted that the photodiode is for detecting light having a shorter wavelength, not for detecting light having a longer wavelength. To be blocked is the light having a longer wavelength, not the light having a shorter wavelength.
2. Description of Related Arts
When one optical fiber is used for both transmission and reception of signals, a laser diode (LD), which emits outgoing light, and a photodiode (PD), which receives incoming light, are placed usually in the same housing or on the same platform. Similarly, when one optical fiber is used for transmitting signals uni-directionally by using two or more different wavelengths, two or more PDs are placed usually on the same platform. A PD is a device having high sensitivity. An LD emits intense outgoing light to transfer signals to a distant place. Although the PD and LD act at different wavelengths, the PD has sensitivity to the outgoing light and detects it. When an LD and a PD are placed on the same terminal, if the PD detects the outgoing light emitted by the LD, this phenomenon is called optical crosstalk. The outgoing light acts as noise for the PD. When the PD detects the outgoing light, incoming light cannot be detected accurately. Therefore, it is necessary to minimize the crosstalk between the PD and LD. Undoubtedly, there can be electrical crosstalk between a transmitter and a receiver caused by the magnetic coupling between their electric circuits. However, to be solved here is the problem of optical crosstalk.
Researchers and engineers have devised various types of transceivers that carry out transmission and reception of signals over one optical fiber. Those transceivers employ different methods for separating the outgoing light and incoming light. The most popular method uses an optical wavelength demultiplexer to branch the path for the outgoing light and the path for the incoming light. Such a method in which the two paths are separated spatially can solve the problem of optical crosstalk relatively easily. There is a rather special transceiver module in which a PD and an LD are arranged in a straight line. Such a method in which almost the same path is used for both transmission and reception makes it difficult to solve the problem of optical crosstalk.
FIG. 1
shows a system of simultaneous bidirectional optical communication in which one optical fiber connects a central office and a subscriber for carrying out bidirectional signal transmission by using two different wavelengths, &lgr;1 and &lgr;2. This is a system of wavelength division multiplexing (WDM) bidirectional communication. A central office generates a signal using LD
1
and sends it to PD
2
at a subscriber via an optical fiber
1
, an optical wavelength demultiplexer
2
, an optical fiber
3
, an optical wavelength demultiplexer
4
, and an optical fiber
5
. The subscriber generates another signal using LD
2
and sends it to PD
1
at the central office via an optical fiber
6
, the optical wavelength demultiplexer
4
, the optical fiber
3
, the optical wavelength demultiplexer
2
, and an optical fiber
7
. Thus, signals can be transmitted in opposite directions at the same time over one optical fiber.
At the central office, the optical wavelength demultiplexer
2
is connected to the optical fibers
7
and
1
to separate an upstream signal and a downstream signal according to their wavelengths. The outgoing light carrying downstream signals has the wavelength &lgr;2 and the incoming light carrying upstream signals has the wavelength &lgr;1. The downstream and upstream signals travel over the one optical fiber
3
at the same time.
At the subscriber, the optical wavelength demultiplexer
4
is connected to the optical fibers
5
and
6
to separate incoming light and outgoing light. The photodiode PD
2
receives incoming light having the wavelength &lgr;2, and LD
2
generates outgoing light having the wavelength &lgr;1. Although not shown, there are individual electric circuits beyond PD
2
and LD
2
.
The present invention addresses problems related to a transceiver at a central office. Problems at a central office are different from those at a subscriber because the aspect related to wavelength is reversed In the example shown in
FIG. 1
, the optical wavelength demultiplexers
2
and
4
spatially separate the optical paths of the outgoing light and the incoming light. Therefore, the problem of optical crosstalk can be solved by the improvement of the performance of the optical wavelength demultiplexer, for example. The problem to be solved by the present invention is the optical crosstalk between the PD and LD at a central office.
FIG. 2
shows another system in which two signals are transmitted unidirectionally from a central office to a subscriber by using different wavelengths, &lgr;1 and &lgr;2. This is a system of WDM unidirectional communication. A central office generates two signals by using two different LDs. They are combined at an optical wavelength multiplexer
8
and transmitted from the central office to a subscriber over one optical fiber. At the subscriber, the two signals are separated according to their wavelengths by an optical wavelength demultiplexer
4
. The photodiodes PD
1
and PD
2
selectively receive the signals. Here, crosstalk between PD
1
and PD
2
also poses a problem.
FIG. 3
shows a typical example of a conventional PD module used as a receiver in an optical communication system in which the optical paths are separated as shown in
FIGS. 1 and 2
. This type of PD module is still mainly used. Lead pins
9
are provided at a circular metal stem
10
, at the center of which a submount
11
supports a PD chip
12
. A lens
13
is attached to a cylindrical cap
14
welded to the stem
10
in alignment. A cylindrical sleeve
15
is placed on the cap. A ferrule
16
is inserted into the mandrel hole of the sleeve
15
. The ferrule
16
supports the end of an optical fiber
17
. The tip of the ferrule
16
is polished on the skew. The sleeve
15
is covered with a bend limiter
18
to protect the optical fiber
17
. This explains the structure of a PD module currently in use. The systems shown in
FIGS. 1 and 2
also include LD modules. In an LD module, only the PD shown in
FIG. 3
is replaced by an LD. Therefore, an LD module has a structure similar to that of a PD module, and no explanation about it is provided here. PD modules and LD modules now in use employ a metal case, so that optical fibers are arranged stereoscopically (three-dimensionally). Although high in performance, those modules require centering work at the time of assembly. The centering is a time-consuming job and increases the manufacturing cost. Because of the high price, those modules are not suitable in achieving widespread application.
Researchers and engineers have been energetically studying surface-mounted types for use as less costly PD modules, LD modules, or PD-LD modules.
FIG. 4
shows an example of a conventional surface-mounted-type module that uses a back-illuminated-type PD. A rectangular silicon platform
19
is provided with a longitudinal, V-shaped groove
20
at the center. The groove is formed by etching. A slanted mirror face
21
is provided at the end of the V-shaped groove
20
. The etching work simultaneously forms the mirror face
21
also. A PD chip
23
is fixed directly above the end portion of the V-shaped groove
20
. The PD chip
23
, a back-illuminated-type PD, is provided with a photo-sensitive area
24
at the upper zone. The light emerging from an optical fiber
22
propagates in the V-sh

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