LD/PD module and LED/PD module

Details LD/PD module and LED/PD module LD/PD module and LED/PD module

1999-02-23

2001-05-22

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular active media

Semiconductor

C372S050121

Reexamination Certificate

active

06236669

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an LD/PD (laser diode/photodiode) module or an LED/PD (light emitting diode/photodiode) module as a sending/receiving apparatus used at base ports (broadcasting station) and subscribers in a unidirectional or bidirectional optical communication system which transmits optical signals of different wavelength in a unidirectional direction or bidirectional directions. In particular, this invention relates to an LD/PD or an LED/PD module which ensures facile attachment to and detachment from an optical connector. The word “LD/PD module” is adopted to signify both an “LD/PD module” and an “LED/PD module” hereafter for simplicity. At a subscriber port, an incoming light is called “receiving signal light” and an outgoing light is called “sending signal light”.
This application claims the priority of Japanese Patent Application No.10-58806(58806/1998) filed Feb. 23, 1998 which is incorporated herein by reference.
2. Description of Related Art
[EXPLANATION OF BIDIRECTIONAL OPTICAL COMMUNICATION]
Recent development of technology has reduced transmission loss of optical fibers and has improved the properties of semiconductor laser diodes (hereafter indicated as LDs) and semiconductor photodiodes (hereafter indicated as PDs). The improvements of fibers, LDs and PDs enable us to transmit various types of information by light signals. The transmission is called “optical communication”, since light signals carry information. The types of information for sending or receiving at stations or at subscriber ports are, for example, telephones, facsimiles or televisions. In particular, people have vigorously tried optical communication based on long wavelength light (near infrared), for example, of a 1.3 &mgr;m wavelength or of a 1.55 &mgr;m wavelength. Recently bidirectional transmission attracts attention, since only a single optical fiber can send signals both in a forward direction and in a backward direction at the same time. The system of the communication is called a “bidirectional communication system”. Fortunately the bidirectional system saves one optical fiber.
FIG. 1
schematically indicates a multiwavelength bidirectional optical communication system which adopts a plurality of wavelengths for sending signals simultaneously both in a forward direction and in a backward direction.
One station is connected to a plurality of subscribers (ONUs) by optical fibers. Although
FIG. 1
shows only a single subscriber for drawing convenience, many subscriber ports are connected to the central station. The fiber from the station branches at many bisecting points into a plurality of fibers linking with individual subscribers.
The central station amplifies the signals of telephones or televisions as digital signals or analog signals and drives a semiconductor laser (LD
1
) which produces &lgr;1 light responsive to the amplified signals. The light of &lgr;1 emitted from the LDI (P
1
) enters an optical fiber
1
as light signals of &lgr;1. A wavelength division multiplexer (WDM)
2
introduces the &lgr;1 light into an intermediate optical fiber
3
. Another wavelength multiplexer (WDM)
4
allocates the &lgr;1 light to an optical fiber
5
. A photodiode (PD
2
) senses the &lgr;1 signals for converting the optical signals to electric signals (P
3
). A receiver apparatus on the subscriber side amplifies and processes the electric signals (P
3
) for reproducing a voice or image. The signals flowing from the station to the subscribers are called “downward signals”. The direction is called a “downward direction”.
On the contrary, a subscriber converts electric signals of a facsimile or a telephone into &lgr;2 light signals by a semiconductor laser diode (LD
2
) which oscillates at a wavelength &lgr;2 (P
4
). Going into a fiber
6
, the &lgr;2 light passes the WDM
4
, propagates in the intermediate optical fiber
3
to the station. The WDM
2
allocates the &lgr;2 light into a fiber
7
. A photodiode (PD
1
) senses the &lgr;2 light for converting into electric signals (P
2
). Converters or signal processing circuits on the station side regenerate telephone voice or facsimile images. The direction of the signal flow from the subscribers to the station is called an “upward direction”. The signals are called “upward signals”.
The above system appropriates &lgr;1 to downward signals and &lgr;2 to upward signals exclusively. Another system uses only one wavelength in common for both upward and downward signals. A further system doubly allocates two wavelengths &lgr;1 and &lgr;2 both for upward and downward signals. Separation of two wavelengths is an important problem in the optical communication system which carries different wavelength signals in an optical fiber.
[Explanation of wavelength division multiplexer]
Both the station and the subscribers require a device for discriminating wavelengths and separating one wavelength from others. A WDM is a device having such a function. In
FIG. 1
, the WDMs
2
and
4
play the role of distinguishing and separating different wavelengths. A WDM either joins &lgr;1 to &lgr;2 for introducing them into a fiber or extracts only one wavelength light from two wavelengths propagating in a fiber. WDMs play an important role in multiwavelength bidirectional optical communication systems.
Various types of wavelength division multiplexers have been suggested.
FIG. 2
indicates a WDM constructed by optical fibers or optical waveguides. Two optical paths
8
and
9
lie in a close relation at a part
10
for allowing the exchange of power. The distance D and the length L of the close portion
10
determine the modes of coupling. In the example, when &lgr;1 enters the path
8
(P
1
), the same wave appears in a path
11
(P
3
). Going into a path
12
(P
4
), &lgr;2 appears in a path
9
instead of the path
8
(P
2
). The coupling portion
10
gives wavelength selectivity to the device.
FIG. 3
shows another WDM which uses a multilayered mirror. The WDM consists of two rectangular isosceles triangle columns
13
and
14
, and a dielectric multilayer mirror
15
formed on the slanting plate of the columns. The two columns are glued at the dielectric mirror
15
for making a square column. Selection of the refractive index and the thickness gives the dielectric multilayer the wavelength selectivity of allowing one wavelength &lgr;1 shooting at 45 degrees to the multilayer to pass through and of reflecting another wavelength &lgr;2 at a right angle. This dielectric layer type WDM can be the WDMs
2
and
4
in the optical communication system of FIG.
1
. The WDM is sometimes called a wave-division-integration device. Fiber-type WDMs and glass block type WDMs are already on sale.
An example of an LD/PD module on a subscriber side is explained by referring to
FIG. 16. A
single mode optical fiber
16
spreading from the central station is connected by an optical connector
17
to an optical fiber
18
of a subscriber (ONU) module. The ONU module has a fiber-type WDM
21
which couples the fiber
18
to a fiber
19
with wavelength selectivity. A contact portion
20
exchanges light power. An optical connector
22
couples the fiber
18
to an LD module
25
in the ONU. Another optical connector
23
joins the fiber
19
to a PD module
27
.
The LD module
25
and the fiber
24
are part of an upward system. 1.3 &mgr;m light carries signals from the subscriber to the station through the upward system. The fiber
26
and the PD module
27
are part of a downward system. The station sends 1.55 &mgr;m light carrying signals to subscribers through the downward system. The PD module
27
converts the optical signals into electric signals. The LD module
25
, a signal sending device, includes an electric circuit for amplifying and modulating the signals of telephones and facsimiles, and a laser diode (LD) for converting electric signals into optical ones. The PD module
27
, a receiving device, contains a photodiode for converting optical signals from the station into electric signals, an

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