Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2002-09-30
2004-05-25
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
Reexamination Certificate
active
06739727
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is used for optical communication and concerns especially, an optical amplifier used for Raman amplification.
2. Description of Related Art
In a wavelength division multiplex (WDM) optical transmission system, signal channels having a plurality of different wavelengths are simultaneously transmitted in one optical fiber. This means that in a WDM transmission system, increasing the number of wavelengths (the number of signal channels) expands the transmission capacity for each optical fiber thereby enabling greater economy in the number of optical fibers.
A WDM optical transmission system is comprised mainly of: (1) an optical transmitter installed in a transmitting terminal, (2) an optical receiver installed in a receiving terminal and (3) an optical transmission line connecting the optical transmitter and receiver. Usually, an optical transmission line is comprised of: (A) a plurality of optical amplifiers such as erbium doped optical fiber amplifiers, for amplifying WDM optical signals and (B) a plurality of transmission optical fibers connecting between the optical amplifiers.
The signal quality of signal light output from an optical transmitter deteriorates as it is transmitted through an optical transmission line. The primary factors of this deterioration are degradation through amplified spontaneous emission (ASE) and degradation due to nonlinear effects. ASE refers to a constant amount of light with random phases. Nonlinear effects include various kinds of effects, among which distortion due to self phase modulation group velocity dispersion (SPM-GVD) is the most typical one. Distortion due to SPM-GVD refers to a wave distortion caused by time-dependent nonlinear refractive index changes in the transmission fiber induced by signal light pulses. The distortion increases with the signal light power.
Signal deterioration caused by ASE has a tendency to increase as signal light power output from transmission fiber decreases. This is because the output terminal of a transmission fiber is normally connected to the input terminal of an optical amplifier which usually generates a constant amount of optical noise. Therefore, if signal power output from transmission fiber decreases, the ratio of signal power to ASE, that is to say, the signal-to-noise ratio (SNR) decreases leading to signal degradation. In other words, to reduce the effects of ASE, higher signal light power is preferable at the output of a transmission fiber.
Signal deterioration from nonlinear wave distortion caused by SPM-GVD has a tendency to increase as signal light power input into transmission fiber increases. This is because the nonlinear refractive index change that causes SPM-GVD nonlinear waveform distortion increases together with signal light power. Accordingly, to reduce the effects of nonlinear wave distortion caused by SPM-GVD, lower signal light power is preferable at the input of a transmission fiber.
Accordingly, due to these two kinds of signal deterioration, higher signal light power is preferable at the output of a transmission fiber while lower signal light power is preferable at the input of a transmission fiber. However, because the output signal light power and the input signal light power is related uniquely though the optical transmission fiber loss, both those conditions cannot be satisfied simultaneously. Therefore, signal light power is usually set to minimize the overall deterioration from the both kinds.
Even with signal light power at optimum conditions, both these deterioration cannot be completely avoided. Further, both these kinds of deterioration accumulate and worsen as signals pass through a plurality of optical amplifiers and transmission fibers. Therefore, the maximum transmission distance for an optical transmission system is usually restricted by the accumulation of deterioration through both causes.
For relaxing such restrictions, methods and systems using Raman amplification are proposed. Raman amplification refers to a phenomenon wherein amplification of signal light inside transmission fiber is achieved via the stimulated Raman effect. This occurs as pumping light, light of specific wavelengths different from the wavelengths of the signal lights, is input into the transmission fiber simultaneously with the signal light. There are three configurations for a system using Raman amplification: the “backward pumping” configuration where backward pumping light is transmitted in the opposite direction to the signal light; the “forward pumping” configuration where forward pumping light is transmitted in the same direction as the signal light; and the “bidirectional pumping” configuration where both forward pumping light and backward pumping light are transmitted in the same direction and the opposite direction to the signal light, respectively. By using Raman amplification of any of these three configurations, it is possible to achieve better signal quality than provided by a configuration not applying Raman amplification, if signal light power of at the input of the transmission fiber is appropriately set.
Improved signal quality through forward pumping will now be described with reference to FIG.
13
. In
FIG. 13
distance along the transmission fiber is plotted on the horizontal axis and average signal light power is plotted on the vertical axis. Average signal light power in transmission fiber for a system not applying Raman amplification is shown by (a) of FIG.
13
. Here, if forward pumping light is injected from the input terminal of the transmission fiber, the signal light is amplified within the transmission fiber and signal power inside the transmission fiber changes as shown by (b) of FIG.
13
. If, here, input signal power is lowered so that average signal light output from the transmission fiber is the same as before Raman amplification is applied, the result would be as shown by (c) of FIG.
13
. This means that as it is possible to decrease input signal light power while maintaining the output signal light power from the transmission fiber constant (keeping deterioration caused by ASE constant). This means that nonlinear waveform distortion degradation from SPM-GVD can be reduced, leading to improved signal quality.
Four wave mixing (FWM) is another typical kind of nonlinear effect. FWM refers to the effect of power conversion between four lightwaves via the third order optical nonlinear effect of a transmission fiber.
This effect occurs whether the wavelengths of the four lightwaves are all the same, are all different or whether some are the same and some are not. Usually, the field of optical communication is concerned with the case where all four of the lightwaves are of different wavelengths or where two of the four lightwaves have the same wavelengths while the other 2 have different wavelengths. In the former case, if the wavelengths of the four lightwaves in ascending order of wavelength are &lgr;
1
, &lgr;
2
, &lgr;
3
, &lgr;
4
, power conversion between these four lightwaves through FWM arises when for example the following conditions are met:
&lgr;
4
=&lgr;
1
×&lgr;
2
×&lgr;
3
/(&lgr;
1
×&lgr;
2
−&lgr;
2
×&lgr;
3
+&lgr;
3
×&lgr;
1
)
If &lgr;
1
=1530 nm, &lgr;
2
=1535 nm, &lgr;
3
=1580 nm for example, power conversion between the four lightwaves occurs when &lgr;
4
=1585 nm. This results in the power elements of wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
to be mixed into the lightwave of wavelength &lgr;
4
. (If there was no wave of wavelength &lgr;
4
originally, a new lightwave would arise). Because these four lightwaves represent different wavelength channels, mixture of signal light leads to deterioration of signal quality (For example, for wavelength channel of &lgr;
4
, power elements &lgr;
1
, &lgr;
2
, &lgr;
3
are noise). Thus, FWM should be avoided. A means for avoiding signal quality degradation caused by FWM is to allocate signal light wavelengths avoiding the conditions of the above expre
Hellner Mark
McGinn & Gibb PLLC
NEC Corporation
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