Optical amplification apparatus and optical transmission system

Optical waveguides – With optical coupler – Plural

Reexamination Certificate

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Details

C359S334000, C359S345000, C359S341300, C359S341400

Reexamination Certificate

active

06636659

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical amplification apparatus and an optical transmission system capable of realizing long distance transmission with higher stability and reliability by compensating for a signal light distorted in an EDFA through Raman amplification so as to become the most adequate input power for the EDFA.
BACKGROUND OF THE INVENTION
There has been a problem on increasing data traffic caused by increased number of callings in communications and a larger amount of contents data such as moving pictures under such situations as recent rapid penetration of the Internet and sharply increased accesses between intra corporate LANs. Therefore, a WDM (Wavelength Division Multiplexing) system is remarkably progressing and becoming rapidly widespread in order to prevent reduction of communication performance due to increased data traffic.
The WDM system has realized high-capacity transmission that is 100 times as high as conventional transmission through a line of fiber by superposing a plurality of optical signals on different wavelengths. Particularly, the existing WDM system uses an erbium-doped fiber amplifier (hereafter EDFA) to enable broadband long distance transmission. The EDFA mentioned here indicates an amplifier to which the principle as follows is applied. This principle is that light having a wavelength of 1550 nm-band as a transmission signal is amplified in a specified fiber when a pumping laser having a wavelength of 1480 nm or 980 nm is passed through the specified optical fiber obtained by doping an element such as erbium into the fiber.
FIG. 9
is a block diagram showing a schematic structure of the conventional WDM system. As shown in
FIG. 9
, the conventional WDM system has EDFAs (
100
,
110
) in each predetermined zone on a transmission line
99
based on an optical fiber as a transmission medium. The signal light passing through the transmission line
99
is amplified by these EDFAs to maintain the lowest possible power required only for being recognized as information.
Each of the EDFAs (
100
,
110
) generally comprises an erbium-doped fiber, pumping laser for pumping the erbium-doped fiber, optical isolator, and the optical filter (not shown).
The EDFAs (
100
,
110
) for performing such multiple wavelength amplification are required to have a flat gain profile over a multiplexed wavelength band so that the degree of amplification is prevented from being different in each wavelength. That is, the EDFAs (
100
,
110
) are desired to minimize a gain deviation of each signal light in the multiplexed wavelength band.
Therefore, generally, in the EDFAs (
100
,
110
), gain specification is in many cases optimized by a gain equalization filer or the like so as to show the flattest gain profile with respect to a signal light having a specified signal light power.
FIG. 10
shows a diagram for explaining a gain profile in the conventional EDFA. In
FIG. 10
, gain profiles in a case where the signal light power is −17 dBm and a case where it is −25 dBm are shown. Particularly, this EDFA is adjusted so that the uniform gain can be obtained over wavelengths of 1540 nm to 1580 nm when the signal light power of −17 dBm is received. On the other hand, when the signal light power of −25 dBm is received, a gain deviation on the short wavelength side is large as compared to the case where the signal light power of −17 dBm is received. Therefore, uniform gain cannot be obtained.
Accordingly, the WDM system using the EDFA is desired to design the power of the signal light to be input into the EDFA so that the gain profile of the EDFA becomes the flattest.
However, the WDM system for an ultra long distance such that the number of repeaters exceeds 100 has a problem that a gain band is narrowed because a gain deviation is accumulated as the number of repeating stages increase even if the gain deviation in the EDFA is a small amount.
FIG.
11
A and
FIG. 11B
are diagrams for explaining the above-mentioned problems.
FIG. 11A
shows an output spectrum at an output port PA of the EDFA
100
in the first stage shown in
FIG. 9
, and
FIG. 11B
shows an output spectrum at an output port PB of the EDFA
110
in the following stage shown in FIG.
9
. As shown in
FIGS. 11A and 11B
, even the signal light having the same information is output as the signal light having a different power distribution between the outputs of continuously disposed EDFAs. This is because the signal light power is not amplified perfectly flatly over multiple wavelengths due to the fine gain deviation, and in addition, the signal light is deviated from the most appropriate power by the gain deviation, so that the signal light cannot undergo amplification by a flat gain profile in the next EDFA.
The EDFA in particular cannot avoid production of ASE (Amplified Spontaneous Emission) noise. Therefore, as shown in
FIG. 11A
, the signal light spectrum together with the ASE component
120
undergoes amplification by the same gain profile. Accordingly, as shown in
FIG. 11B
, the ASE component
130
is also affected by the gain deviation.
The EDFA is a lumped amplifier in which parts pumping the optical signal are concentrated. Therefore, this lumped amplifier has such restriction that it undergoes propagation loss along an optical fiber as a transmission line leading to accumulation of noise, and non-linearity that may cause signal distortion and noise. Further, the EDFA enables optical amplification in a wavelength band defined by band gap energy of erbium. Therefore, the EDFA has difficulty in working on a wider band required for further multiplexing.
To solve the problem, a Raman amplifier has been focused on as an optical amplifier instead of the EDFA. The Raman amplifier is a distributed optical amplifier that does not require a specific fiber such as an erbium-doped fiber like the EDFA and uses an ordinary optical fiber for a transmission line as a gain medium.
However, a WDM system using this Raman amplifier also has problems as follows because at least two pumping light sources provided in the respective Raman amplifiers are always operated in constant output power.
(1) In Raman amplification, a transmission line through which a signal light is transmitted is used as a medium for amplification. Therefore, amplification characteristics of the medium depend on a type of transmission line (optical fiber). For example, an SMF (Single Mode optical Fiber) has an efficiency (Raman gain/pumping light power) by one-half as compared to that of a DSF (Dispersion Shifted Optical Fiber). Therefore, when the type of optical fiber forming the transmission line is changed while output power of the pumping light source is kept constant, the amplification characteristics of the transmission line are changed to be incapable of maintaining sufficient transmission quality if the characteristics remain changed.
(2) In Raman amplification, a transmission line through which a signal light is transmitted is used as a medium for amplification. Therefore, amplification characteristics of the medium also depend on transmission loss of the transmission line (optical fiber). Therefore, when the transmission loss of the optical fiber forming the transmission line fluctuates while output power of the pumping light source is kept constant, the amplification characteristics of the transmission line also fluctuate to be incapable of maintaining sufficient transmission quality. Further, when the transmission loss of the transmission line becomes larger, not only the signal light undergoes a large loss but also the pumping light undergoes a large loss, which makes the Raman gain decreased. Therefore, the fluctuation in the signal light power becomes larger than the fluctuation in the loss of the transmission line.
(3) When a wavelength multiplexed optical signal is subjected to Raman amplification, the amplification gain depends on the number of wavelengths (number of channels) of the signal light and optical power for each signal light in each wavelength. Therefore,

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