Photodiode and photodiode module

Details Photodiode and photodiode module Photodiode and photodiode module

2000-03-14

2002-01-22

Wilson, Allan R. (Department: 2815)

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

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S233000

Reexamination Certificate

active

06340831

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a photodiode as a light receiving device and photodiode module combining a photodiode and a light emitting device for bilateral or unilateral optical communication of information between a base station and a plurality of subscribers by transmitting two light signals having different wavelengths &lgr;1 and &lgr;2 in a unilateral direction or bilateral directions passing through an optical fiber.
This application claims the priority with respect to Japanese Patent Application No.256107/1997 filed on Sep. 3, 1997 which is incorporated herein by reference.
2. Description of the Related Art
Explanation of Optical Bilateral Communication
Recently semiconductor laser diodes (LD) and semiconductor photodiodes (PD) have enhanced their properties and the transmission loss of optical fibers has been lowered. Therefore, the transmission of various sorts of information by light have been feasible. Transmission by using light is referred to as “optical communication”. Telephones, facsimiles, television image signals and so on are various known media for transmitting information. Various attempts to utilize light with long wavelengths, for example, a wavelength of 1.3 &mgr;m and wavelengths of 1.5 to 1.6 &mgr;m, are succeeding in the optical communication field. Nowadays, a new system for sending and receiving signals is being developed, in which light signals can be sent in bilateral directions simultaneously through a single optical fiber. This system is called “bilateral communication”, because signals can be transmitted in bilateral directions. The most noticeable advantage is that the bilateral communication requires only one optical fiber.
FIG. 1
explains the principle of the bilateral multicommunication using two light signals of different wavelengths. An optical fiber connects one base station to one of many subscribers. In
FIG. 1
, only one subscriber is shown for simplicity, but actually, there are many branch points, and an optical fiber from the base station is branched into a plurality of optical fibers that are connected to respective subscriber devices.
In the base station, the signal from a TV, a phone or the like is amplified and processed to be a digital or an analog signal, and such an amplified signal drives a laser diode (LD
1
) of &lgr;1. The light signal having a wavelength &lgr;1 enters an optical fiber
1
, and then to an intermediate optical fiber
3
via a wavelength division multiplexer (WDM)
2
. The signal introduced in the optical fiber
3
is transmitted to an optical fiber
5
via a wavelength division multiplexer (WDM)
4
set up on the subscriber side, and is received by a photodiode (PD
2
), whereby the signal is changed to an electric signal (P
3
). The electric signal (P
3
) is amplified and processed by a device on the subscriber side, and is regenerated as the voice of a phone or television images. The signal going from the base station to the subscriber side is called a “downward signal”, and the direction of the flow is called a “downward direction”.
On the other hand, on the side of a subscriber, a laser diode LD
2
converts a signal (P
4
) from a phone or a facsimile into a light signal having a wavelength of &lgr;2. The &lgr;2 light enters an optical fiber
6
, is introduced into the intermediate optical fiber
3
by the wavelength division multiplexer (WDM)
4
, and enters photodiode PD
1
passing through the wavelength division multiplexer
2
of the base station. A device equipped in the base station converts the &lgr;2 light signal to an electric signal using photodiode PD
1
. This electric signal is appropriately converted and/or processed. The direction from the subscriber side to the base station is called an “upward direction”.
Explanation of Wavelength Division Multiplex
Both the base station and the subscriber side must be able to distinguish and separate two sorts of light having different wavelengths for the implementation of optical bilateral communication using a single optical fiber. The wavelength division multiplexers
2
and
4
shown in
FIG. 1
provide that capability. Such a wavelength division multiplexer combines two sorts of light having different wavelengths of &lgr;1 and &lgr;2 and couples the combined light into only one optical fiber or selects one sort of light from light having different wavelengths, and couples the light of one wavelength to only one optical fiber. Therefore, the wavelength division multiplexer plays an extremely important role in carrying out multiwavelength bilateral communication.
There are several kinds of wavelength division multiplexers, which have been proposed. These wavelength division multiplexers will be explained by referring to
FIG. 2
to
FIG. 4. A
wavelength division multiplexer shown by
FIG. 2
is made from optical fibers or an optical waveguide. Two optical paths
8
and
9
are close to each other at a part
10
of the paths, and the exchange of optical energy is carried out here. Various kinds of couplings can be realized by changing gap D and length L of the neighboring part
10
.
When the light having a wavelength &lgr;1 enters the optical path
8
and the light with a wavelength &lgr;2 enters the optical path
9
, both the &lgr;1 wavelength light and the &lgr;2 wavelength light exit via an optical path
11
, and no light enters an optical path
12
. Hence, the &lgr;1 light from a port P
1
and the &lgr;2 light from a port P
2
appear in a port P
3
. No light appears in a port P
4
. The &lgr;1 light never enters the neighboring optical fiber, but all of the &lgr;2 light enters the neighboring optical fiber because the &lgr;2 satisfies the condition of phase. Since such a wavelength division multiplexer is made from optical fibers or an optical waveguide, there is an advantage of little polarization dependency.
The route of light passing through an optical fiber or a waveguide has reversibility.
FIG. 3
shows a practical device for the wavelength division multiplexer shown in
FIG. 2
in the bilateral communication. The &lgr;1 light from P
1
enters the optical fiber
8
, and exits via P
3
. The &lgr;2 light entering P
3
exits via P
2
. Here, this WDM can be used as the WDMs
2
and
4
shown in FIG.
1
.
FIG. 4
shows a wavelength division multiplexer provided with a multi-layer mirror. This multi-layer mirror is made of two glass blocks
13
and
14
shaped like isosceles triangle columns and a dielectric multi-layer
15
formed on the slanting surfaces of the glass blocks
13
and
14
. All of the &lgr;1 light passes through but the all of the &lgr;2 light is reflected on the multi-layer mirror by adjusting and combining the refractive index and the thickness of the dielectric multi-layer. Here, there exists the dependency of polarization because the incoming light is reflected at an angle of 45°. This wavelength division multiplexer is utilized as the wavelength division multiplexers
2
and
4
shown in FIG.
1
. Such a kind of wavelength division multiplexer is called a “wavelength dividing or combining device” or called a “WDM”. The optical fiber type WDM and the glass block type WDM mentioned above have already been on the market.
The light sending and receiving module on the subscriber side will be explained by referring to FIG.
5
. In
FIG. 5
, an end of an optical fiber
16
leading from the base station to each subscriber is connected to an indoor optical fiber
18
by an optical connector
17
. An indoor ONU module equipped on the subscriber side is provided with an optical fiber WDM (Wavelength Division Multiplexer)
21
. The WDM has another optical fiber
19
which is connected to the fiber
18
at a coupling portion
20
via evanescent waves. The optical fibers
18
and
19
are wavelength-selectively coupled with modules in the WDM
21
. The optical fiber
18
is coupled with an LD module
25
by an optical connector
22
. The optical fiber
19
is coupled with a PD module
27
via an optical connector
23
.
The LD
25
and the optical fiber
24
are of the upward-direction ty

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