Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-04-26
2002-12-31
Buczinski, Stephen C. (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S345000
Reexamination Certificate
active
06501592
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based on, and claims priority to, Japanese application number 10-117043, filed Apr. 27, 1998, and which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier which reflects spontaneous emission light back into the optical amplifier to improve excitation efficiency of the optical amplifier.
2. Description of the Related Art
Wavelength division multiplexing (WDM) is being used to increase transmission capacity of optical communication systems. With WDM, a plurality of wavelengths are multiplexed together and transmitted through a single fiber.
Moreover, in an optical communication system employing WDM, an optical amplifier can be used as a repeater. The transmission capacity of the optical communication system can be increased by expanding the gain wavelength band of the optical amplifier, to thereby increase the number of wavelengths which can be multiplexed together.
An Erbium (Er) doped fiber amplifier (EDFA) is a common optical amplifier which is widely used in optical communication systems employing WDM. In a typical EDFA, an excitation wavelength in a 1.48 &mgr;m band or a 0.98 &mgr;m band is used, and gain is obtained in a wavelength band which includes 1.53 &mgr;m to 1.56 &mgr;m (hereinafter referred to as the 1.55 &mgr;m band).
However, with such an EDFA, the gain wavelength band is limited to the 1.55 &mgr;m band. Thus, to realize a WDM optical communication system which can provide much larger capacity as will be required in the future, it is necessary to exploit a new gain wavelength band.
There is a known technique for realizing a gain wavelength band different from the 1.55 &mgr;m band in an EDFA. In this technique, a wavelength band from 1.56 &mgr;m to 1.62 &mgr;m (hereinafter referred to as the 1.58 &mgr;m band) is used. For example, there is reported a technique in which a gain of 25 dB is realized at a wavelength band from 1.57 &mgr;m to 1.61 &mgr;m, while adopting an excitation wavelength in the 1.55 &mgr;m band. See, J. F. Massicott et el., ELECTRONICS LETTERS, Vol. 26, No. 20, pp. 1645-1646,27th September 1990.
Further, it has been lately reported that a gain at a wavelength band from 1.56 &mgr;m to 1.62 &mgr;m can be obtained by utilizing a laser diode of 1.48 &mgr;m band or 0.98 &mgr;m band as an excitation light source. See, H. Ono et el., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 5, pp. 596-597, May 1997. This technique is advantageous in that current techniques also use a laser diode excited at the 1.48 &mgr;m or the 0.98 &mgr;m band, and an Erbium doped optical fiber (EDF).
The following is a brief explanation about an operational principle of 1.55 &mgr;m band and 1.58 &mgr;m band EDFAs. As an example of a fiber amplifier doped with rare earth element, there are considered here 1.55 &mgr;m band and 1.58 &mgr;m band EDFAs utilizing an excitation light source of 1.48 &mgr;m band or 0.98 &mgr;m band.
FIGS.
1
(A) and
1
(B) are diagrams showing energy levels of Erbium atom. As shown in FIG.
1
(A), in a conventional 1.55 &mgr;m band EDFA in which the gain is obtained between 1.53 &mgr;m and 1.56 &mgr;m, Erbium atom is excited by an excitation light source of 1.48 &mgr;m band or 0.98 &mgr;m band, and there is caused induced emission between energy levels
4
I
13/2
and
1
I
15/2
. By utilizing this induced emission, there is obtained a gain at 1.55 &mgr;m band.
Further, as shown in FIG.
1
(B), those energy levels, which are utilized in a 1.58 &mgr;m band EDFA, are also energy levels
4
I
13/2
and
4
I
15/2
. For example, in a 1.58 &mgr;m band EDFA, Erbium atom is excited by an excitation light source of 1.48 &mgr;m band or 0.98 &mgr;m band, and 1.55 &mgr;m band spontaneous emission will generate and lead to amplified spontaneous emission (ASE) within a front half part of the fiber. See, H. Ono et al., TECHNICAL REPORT OF IEICE, OCS97-5(1997-05), pp. 25-30. This 1.55 &mgr;m band ASE is resorbed at a rear half part of the fiber, thereby causing induced emission of 1.58 &mgr;m band. In this 1.58 &mgr;m band EDFA, since the cross sectional area for induced emission at 1.58 &mgr;m band is smaller than that of the 1.55 &mgr;m band EDFA, and since a sufficiently strong 1.55 &mgr;m band ASE is to be generated, it is necessary to provide an EDF of sufficient fiber length.
However, in the aforementioned 1.58 &mgr;m band optical amplifier, it is difficult to confine an excitation light or 1.55 &mgr;m band ASE within the EDF efficiently, thereby resulting in a conversion efficiency which is not high.
The following is an explanation of a state of spontaneous emission which is generated within an EDF of a conventional forward excitation type 1.58 &mgr;m band EDFA.
FIG. 2
is a diagram showing various lights travelling through an EDF, and
FIG. 3
is an enlarged view of the EDF in FIG.
2
.
As shown in
FIGS. 2 and 3
, an excitation light source
1
A of an excitation part
1
produces excitation light Tp. Excitation light Lp is provided to EDF
2
via a WDM coupler
1
B. Erbium atoms within EDF
2
are excited by excitation light Lp, so that spontaneous emission lights are generated. The spontaneous emission light generated from the Erbium atoms is composed of lights advancing in random directions, and only the lights directing into those modes, through which the lights can be propagated within EDF
2
, will propagatingly advance within EDF
2
. The spontaneous emission lights directing into these propagation modes will be amplified, during propagation through EDF
2
in an excited state, to become 1.55 &mgr;m band ASE, and will be finally emitted from EDF
2
.
Since the propagation modes should exist in both of the fore and aft directions, the 1.55 &mgr;m band ASE will be emitted from both ends of EDF
2
. On the other hand, those spontaneous emission lights, which do not direct into any propagation modes, will be emitted outwardly from EDF
2
via an outer surface of a cladding. In
FIGS. 2 and 3
, the 1.55 &mgr;m band ASE in the backward propagation mode is indicated by an ASE light Lab, and the 1.55 &mgr;m band ASE in the forward propagation mode is indicated by an ASE light Laf.
According to the above cited reference H. Ono et al., TECHNICAL REPORT OF IEICE, the 1.55 &mgr;m band ASE, which has been generated at the front half part of EDF
2
, is resorbed at the rear half of EDF
2
, leading to generation of induced emission (optical amplification) at the 1.58 &mgr;m band. Thus, the amount of ASE in the forward propagation mode, which is outwardly emitted from the output end of the signal light Ls, has a smaller value. However, the amount of ASE in the backward propagation mode, which is emitted outwardly from the input end of the signal light Ls, has a larger value since this mode does not contribute so much to the amplification at the 1.58 &mgr;m band. Further, those spontaneous emission lights, which do not direct into any propagation modes, do not contribute to optical amplification at 1.58 &mgr;m and are outwardly emitted via outer surface of cladding. As a result, a part of the energy of excitation light Lp supplied from the excitation light source
1
A is wastefully consumed, thereby reducing excitation efficiency.
Meanwhile, conventional optical amplifiers aiming at improving excitation efficiency include one described in U.S. Pat. No. 5,138,483, and one described in Japanese Unexamined Patent Publication No. 3-135081. Such optical amplifiers have a configuration as shown in FIG.
4
.
Referring now to
FIG. 4
, the optical amplifier, of which excitation efficiency has been improved, includes an excitation light reflector
4
at a rear side of EDF
2
(i.e., at an outer side of the other end opposite to an input end of excitation light Lp). Excitation light reflector
4
reflects the excitation light Lp and transmits the signal light at 1.55 &mgr;m band. Via excitation light reflector
4
, the excitation light Lp is reflected to make one reciprocation within EDF
2
, thereby improving excitation efficien
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