Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-02-03
2002-10-01
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S015000, C385S031000, C385S011000
Reexamination Certificate
active
06459829
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier employed in optical fiber communications, optical measurements, or laser processing, and to the excitation light source for the laser thereof, the present invention providing a means for obtaining a higher output product.
The present specification is based on a patent application filed in Japan (Japanese Patent Application No. Hei 11-33524), a portion of the aforementioned application being incorporated herein by reference.
2. Description of the Related Art
FIG. 10A
is a schematic structural diagram showing an example of a conventional long distance optical fiber communications system. As shown in this figure, in this conventional communications system, it is necessary to perform regenerative repetition every several tens km along optical fiber
3
joining transmitter
1
and receiver
2
, in which an optical/electrical conversion, and then an electrical/optical conversion, are performed on the optical signal by regenerative repeaters
4
,
4
.
FIG. 10B
shows an example of the structure of this regenerative repeater
4
. The optical signal from optical fiber
3
on the transmitter
1
side undergoes optical/electrical conversation by passing through light detector
5
, waveform shaping circuit
6
, laser driver
7
and laser
8
. The signal then undergoes electrical/optical conversion, and is sent to optical fiber
3
on the receiver
2
side.
Due to progress in optical technology, it has become possible to obtain a high output laser inexpensively in recent years. Thus, as shown in
FIG. 11A
, an optical amplifier
10
is inserted along optical fiber
3
, in between transmitter
1
and receiver
2
, to realize a long distance optical fiber communications system in which the light signal is directly amplified by optical amplifier
10
.
FIG. 11B
shows an example of the structure of optical amplifier
10
. In optical amplifier
10
, a rare-earth doped optical fiber
11
is the active media in which the amplification is actually carried out. The optical signal is amplified by inputting excitation light from laser
14
(excitation light source) to rare-earth doped optical fiber
11
, via an optical fiber
13
which is connected to optical multiplexing element
12
provided at the front of rare-earth doped optical fiber
11
. This amplified optical signal is then output from rare-earth doped optical fiber
11
.
Isolator
15
, which is toward the rear of rare-earth doped optical fiber
11
, is provided to stabilize the operation of laser
14
by preventing feedback light.
By realizing an optical amplifier in this way, the attenuated optical signal is directly amplified, so that transmission without regenerative repetition is possible, even in the case of transmission over several thousand kilometers.
In current optical fiber communications, a 1.5 &mgr;m band amplifying erbium doped optical fiber known for its high efficiency is primarily used for rare-earth doped optical fiber
11
.
The absorption spectrum of the rare-earth element for forming rare-earth doped optical fiber
11
will differ depending on the type of rare-earth element employed. For example, as shown in
FIG. 12
, an erbium doped optical fiber has absorption spectrums of a comparatively broad wavelength width near 980 nm and 1480 nm. Thus, in a 1.5 &mgr;m band amplifying erbium doped optical fiber amplifier (denoted as “EDFA” hereinafter), a 1.5 &mgr;m band optical signal typically can be amplified using excitation light near 0.98 &mgr;m or 1.48 &mgr;m.
Typically, a semiconductor laser is employed as a laser
14
for oscillating the excitation light. Of these, a Fabry-Perot semiconductor laser (referred to as a “Fabry-Perot laser”, hereinafter) is mainly used in which power can be obtained relatively inexpensively.
On the other hand, a wavelength multiplex mode optical fiber communications system has been realized for multiplex transmission of signal lights having a plurality of wavelengths. Thus, it has become possible to further increase the amount of information which can be transmitted by one optical fiber.
When carrying out wavelength multiplex communications in the optical communications system shown in
FIG. 11A
, the output required of optical fiber amplifier
10
is greater than in the case where transmitting a single wavelength. For this reason, the power of the excitation light supplied from laser
14
is also required to be greater.
As a method for increasing the total power of the excitation light, a method may be considered in which the output of the laser is increased, for example. There is a limit to this approach, however, since the output of a typical laser is limited. As a result, sufficient effects cannot be obtained.
Thus, the following method may be considered.
Namely, a plurality of lasers is prepared. These lasers oscillate light in a wavelength band capable of exciting the rare-earth element in the rare-earth doped optical fiber, and have oscillation wavelengths which differ slightly from one another. The lights output from these lasers are multiplexed, and this multiplexed light is used as the excitation light.
For example, as shown in
FIG. 12
, the wavelength width in the excitation wavelength band around 1.48 &mgr;m in an EDFA is on the order of 1.45~1.49 &mgr;m, and the excitation wavelength width around 0.98 &mgr;m is on the order of several nm. These excitation wavelength widths are comparatively broad. Thus, when a plural lights having different wavelengths respectively within this wavelength band are multiplexed, the total of the various powers of the light becomes the power of the excitation light.
In other words, when n lasers are prepared, it is theoretically possible to obtain n-fold greater power as compared to the case where employing just one laser (assuming no loss when multiplexing, etc.).
However, as shown in
FIG. 13
, numerous vertical modes are present in the oscillation wavelengths of the Fabry-Perot laser that is typically used as laser
14
. These oscillation wavelengths have a broad wavelength width on the order of 15~20 nm. In general, multiplexing a plurality of lights having this type of broad wavelength width is difficult. However, polarized waves are present in the light output from a Fabry-Perot laser. For this reason, a method is performed for obtaining excitation light of a two-fold greater power by multiplexing two polarized waves that are perpendicular to one another. In theory, however, in this method as well, it is not possible to obtain excitation light having a power in excess of two-fold greater than normal.
Moreover, when lights having too broad oscillation wavelength widths are multiplexed, the width of the wavelength band of the multiplexed light is crowded out of the excitation wavelength band, decreasing efficacy as excitation light.
Accordingly, the following method may be considered.
First, as shown in
FIG. 14A
, a reflecting element
20
b
(external resonator) for reflecting light in a specific narrow wavelength band at a low reflection coefficient is attached to the rear of Fabry-Perot laser
20
a.
The combination of Fabry-Perot laser
20
a
and reflecting element
20
b
is formed to have a structure identical to the so-called Distributed Bragg Reflector laser (DBR laser), in which one of the reflecting surfaces of a Fabry-Perot laser is substituted with a DBR (distributed Bragg reflector). In other words, a laser oscillating element
20
is formed which oscillates only light in the wavelength band that is selectively reflected by reflecting element
20
b.
As a result, the light obtained via reflecting element
20
b
is rendered into a narrow spectrum as shown in FIG.
14
B.
FIG. 15A
shows an example of a light source for multiple wavelength excitation in wavelength multiplex mode employing a laser oscillating element consisting of this type of Fabry-Perot laser and reflecting element, and shows the design of an optical amplifier incorporating the aforementioned light source.
FIG. 15A
is a schematic structural diagram showing an
Nishide Kenji
Wada Akira
Yamasaki Shigefumi
Yamauchi Ryozo
Bovernick Rodney
Burns Doane Swecker & Mathis L.L.P.
Kang Juliana K.
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