Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2002-03-20
2003-12-30
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341300
Reexamination Certificate
active
06671083
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a Raman amplifier and an optical transmission system for supplying excitation lights to an amplifying medium on an optical transmission path, and particularly, to a technique for reducing characteristic deterioration caused due to excitation lights mixedly existing within a wavelength band of signal light.
(2) Related Art
In conventional long distance optical transmission systems, there has been adopted an optical regenerating repeater for converting an optical signal into an electrical signal, and for reshaping, retiming and regenerating the electrical signal, to thereby perform an optical transmission. However, with the recent development of the practical use of optical amplifiers, there has been investigated an optical amplifying-and-repeating transmission system for utilizing an optical amplifier as a linear repeater. Namely, by replacing the optical regenerating repeater by an optical amplifying repeater, it is expected that the number of constituent parts within the repeater are drastically decreased, and the cost is drastically reduced while ensuring reliability.
As one of methods for realizing a large capacity of optical transmission system, attention has been directed to a wavelength division multiplexing (WDM) optical transmission system for multiplexing optical signals of two or more different wavelengths to transmit to one transmission path.
In a WDM optical amplifying-and-repeating transmission system combining the aforementioned optical amplifying-and-repeating transmission system with the WDM optical transmission system, it is possible to collectively amplify WDM signal lights by utilizing an optical amplifier, thereby enabling to realize a large capacity and long distance transmission with a simple configuration (economical merit).
FIG. 25
is a view showing an exemplary configuration of a typical WDM optical amplifying-and-repeating transmission system.
The system of
FIG. 25
comprises: for example, an optical sender station
101
; an optical receiver station
102
; an optical transmission path
103
that connects between the optical sender station and the optical receiver station; and a plurality of optical repeaters
104
arranged on the optical transmission path
103
at required intervals. The optical sender station
101
includes: a plurality of optical senders (E/Os)
101
A that output a plurality of optical signals of different wavelengths, respectively; a multiplexer
101
B that wavelength division multiplexes the plurality of optical signals into a WDM signal light; and a post-amplifier
101
C that amplifies the WDM signal light from the multiplexer
101
B to a required level, to thereby output the amplified WDM signal light to the optical transmission path
103
. The optical receiver station
102
includes: a pre-amplifier
102
C that amplifies the WDM signal light of respective wavelength bands, that has been transmitted via the optical transmission path
103
, to a required level; a demultiplexer
102
B that demultiplexes an output light from the pre-amplifier
102
C into a plurality of optical signals corresponding to the wavelengths thereof; and a plurality of optical receivers (O/Es)
102
A that receives to process the plurality of optical signals, respectively. The optical transmission path
103
includes a plurality of repeating sections that connect between the optical sender station
101
and the optical receiver station
102
. The WDM signal light sent from the optical sender station
101
is propagated through the optical transmission path
103
, is optically amplified by each optical repeater
104
arranged in each repeating section, and then, is propagated through the optical transmission path
103
, and is transmitted up to the optical receiver station
102
, while repeating the above propagation and amplification.
For each of the optical repeaters
104
of the aforementioned WDM optical amplifying-and-repeating transmission system, there is typically adopted an erbium-doped fiber amplifier (EDFA). A gain wavelength band of such an EDFA is a 1.55 &mgr;m band (C-band), while a gain wavelength band of a GS-EDFA (Gain shifted-EDFA) obtained by shifting a gain band of the EDFA to a longer wavelength side, is a 1.58 &mgr;m band (L-band). Each of the EDFA and the GS-EDFA has a gain wavelength bandwidth of 30 nm or above. Thus, by utilizing a multiplexing and demultiplexing device corresponding to both of the C-band and L-band to thereby use together the two signal light wavelength bands, it is possible to realize the amplifying-and-repeating transmission of WDM signal light having a wavelength bandwidth of 60 nm or above.
Recently, it has been also tried to apply Raman amplification to the aforementioned optical transmission system. The Raman amplification has a characteristic with a gain peak at the frequency lower than the frequency of excitation light by 13.2 THz, in a case where a silica (SiO
2
) based optical fiber doped with germanium (Ge) for example, is adopted as an amplifying medium. Therefore, a Raman gain is caused at a longer wavelength side than a wavelength of excitation light. For example, a peak wavelength of the Raman gain is 1.55 &mgr;m, which is shifted to the longer wavelength side by about 100 nm relative to an excitation light wavelength of 1.45 &mgr;m. Thus, it is possible to freely set a gain wavelength band and a bandwidth of Raman amplification, by selecting a plurality of excitation light wavelengths to adjust an excitation light power. Namely, in the Raman amplification, in order to realize an amplifying function for a required signal light wavelength, it is important to be able to set an excitation light wavelength taking account of the shift frequency in the Raman gain. It is also possible to flatten a gain wavelength characteristic of Raman amplification, by using a plurality of excitation lights having different oscillation center wavelengths.
Specifically, in a Raman amplifier such as shown in Y. Emori, et al., “100 nm bandwidth flat gain Raman amplifiers pumped and gain equalized by 12-wavelength-channel WDM high power laser diodes”, OFC'99, PD19, 1999, excitation light powers and oscillation wavelengths thereof are adjusted to ensure about 100 nm as a gain wavelength bandwidth of Raman amplifier. As shown in one example of
FIG. 26
, typically, such a conventional Raman amplifier is constituted so that excitation lights from an excitation light source
202
are supplied to be propagated through an optical fiber
201
acting as an amplifying medium, in a direction opposite to the propagation direction of signal lights. In this exemplary configuration, for a multiplexer
203
that supplies excitation lights to the optical fiber
201
, a wavelength multiplexer (WDM coupler) having ports for transmitting lights of different wavelengths, respectively. In the aforementioned Raman amplifier, as shown in
FIG. 27
, a plurality of excitation lights P
1
to P
K
of different wavelengths and a plurality of signal lights S
1
to S
L
of different wavelengths are arranged corresponding to the shift frequencies of Raman gains, to form such a wavelength arrangement that the wavelength band &lgr;
P1
to &lgr;
PK
of the excitation lights and the wavelength band &lgr;
S1
to &lgr;
SL
of the signal lights are separated into different regions.
Meanwhile, there has been proposed a hybrid amplifier combining a Raman amplifier with an EDFA, such as in “Consideration of SRS loss and compensating method in 3-band WDM transmission” (Society Conference 2000, B-10-167, by Institute of Electronics, Information and Communication Engineer) by Yano et al. This article described a hybrid amplifier having a constitution as shown in
FIG. 28
, in which an optical circulator
204
is adopted as a multiplexer that supplies excitation lights for Raman amplification to an amplifying medium.
Moreover, it is important to further broaden a wavelength bandwidth of signal lights, in order to realize a large capacity and long distance transmission system.
Naito Takao
Tanaka Toshiki
Torii Ken-ichi
Fujitsu Limited
Hellner Mark
Staas & Halsey , LLP
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