Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1998-03-18
2001-09-11
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S199200, C359S199200, C359S341300
Reexamination Certificate
active
06288836
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to wavelength division multiplexing (WDM) using a plurality of optical signals having different wavelengths, and more particularly to an optical amplifier applied to WDM and an optical communication system including the optical amplifier.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical transmission system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for amplifying signal light has been put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 &mgr;m band or a 1.48 &mgr;m band, a gain band including a wavelength of 1.55 &mgr;m can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
Accordingly, by combining an optical amplifier and WDM, the span and capacity of an optical transmission system can be increased.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by a gain characteristic (wavelength dependence of gain) which is represented by a gain deviation or gain tilt. For example, in an EDFA, a gain deviation is produced at wavelengths in the vicinity of 1.55 &mgr;m. When a plurality of EDFAs are cascaded to cause accumulation of gain deviations, an optical SNR (signal-to-noise ratio) in a channel included in a band giving a small gain is degraded.
Automatic gain control (AGC) is effective in maintaining the gain characteristic of an optical amplifier constant. In an EDFA, for example, an optical input level and an optical output level are monitored, and the power of pump light is controlled so that the ratio or difference between the optical input level and the optical output level is maintained constant. By presetting a target value of the pump light power to a suitable value, a flat gain characteristic is maintained.
However, in the case of performing AGC only, the optical output level in each channel of the WDM signal light is not always maintained constant. For example, if the optical output level in a certain channel exceeds a tolerable range, transmission characteristics in this channel are degraded by the influence of nonlinear effects occurring in an optical fiber transmission line. The influence of nonlinear effects is remarkable especially in the case of high-speed transmission at rates over 10 Gb/s.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical amplifier which can maintain constant the optical output level in each channel of WDM signal light and can maintain the gain characteristic constant.
It is another object of the present invention to provide a novel optical communication system including such an optical amplifier.
Other objects of the present invention will become apparent from the following description.
According to one aspect of the present invention, there is provided an optical amplifier comprising an optical amplifying medium to which WDM (wavelength division multiplexed) signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths is supplied; means for pumping the optical amplifying medium so that the optical amplifying medium gives a gain to the WDM signal light; means for controlling the pumping means according to an input level and an output level of the optical amplifying medium so that the gain is maintained constant; and an optical attenuator operatively connected to the optical amplifying medium for giving a variable attenuation to the WDM signal light.
With this configuration, gain control is performed according to the input level and the output level of the optical amplifying medium so that the gain is maintained constant, thereby maintaining the gain characteristic constant. Further, since the optical attenuator for giving a variable attenuation to the WDM signal light is adopted, the optical output level in each channel of the WDM signal light can be maintained by manual or automatic adjustment of the attenuation of the optical attenuator.
In accordance with another aspect of the present invention, there is provided an optical communication system comprising an optical fiber transmission line for propagating WDM signal light, and at least one optical amplifier provided in the optical fiber transmission line. This optical amplifier has the configuration of the optical amplifier according to the present invention.
In this specification, the wording that an element and another element are operatively connected includes the case that these elements are directly connected, and also includes the case that these elements are so provided as to be related with each other to such an extent that an electrical signal or an optical signal can be mutually transferred between these elements.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.
REFERENCES:
patent: 5664131 (1997-09-01), Sugiya
patent: 5703711 (1997-12-01), Hamada
patent: 5818629 (1998-10-01), Kinoshita
patent: 6025947 (2000-02-01), Sugaya et al.
patent: 782289 (1997-07-01), None
patent: 2294170 (1996-04-01), None
patent: 8-264871 (1996-10-01), None
Kinoshita et al, OSA Trends in Optics and Phatonics, vol. 5, pp. 49-52, Jul. 13, 1996.*
Desurvire, E; Physics Today, pp. 20-27, Jan. 1994.
Kawasaki Yumiko
Nishimoto Hiroshi
Okano Satoru
Tsuda Takashi
Yamane Kazuo
Fujitsu Limited
Moskowitz Nelson
Staas & Halsey , LLP
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