Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2000-09-01
2002-09-24
Tarcza, Thomas H. (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
Reexamination Certificate
active
06456428
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier for collectively amplifying signal levels in a predetermined wavelength band.
2. Related Background Art
Due to social needs with the advent of advanced information society, research and development have been booming concerning large-capacity, high-speed optical communications and long-haul optical communications utilizing optical fiber transmission line networks. Here, wavelength division multiplexing (WDM) transmission systems are systems which carry out high-speed, large-capacity optical communications by transmitting WDM signals (a plurality of optical signals having different wavelengths) through an optical fiber line, and their development and introduction have been in progress in order to respond to the surge in demands for communications due to Internet and the like.
In such a WDM transmission system, optical amplifiers are typically installed as repeater stations at predetermined intervals in order to compensate for the transmission loss in WDM signals caused by long-haul transmissions. Each optical amplifier comprises an optical amplification section, such as an optical fiber doped with a fluorescent material excitable with pumping light, for amplifying optical signals inputted thereto; and a pumping light source for supplying the pumping light to the optical amplification section. A known example thereof is an erbium-doped fiber amplifier (EDFA).
Also, OXC (Optical Cross Connect) and OADM (Optical Add/Drop Multiplexer) are often employed in WDM transmission systems for their efficient operation and improved reliability. In recent years, in particular, research on large-capacity signal switching and acceleration in switching speed has been under way concerning such OXC, ODAM, and the like along with the increase in transmission capacity.
When an optical amplifier is employed in a transmission system equipped with optical devices having a signal switching function, such as the above-mentioned OXC and OADM, there is a possibility that a problem such as one that follows may occur. Namely, it is important for the optical amplifier to amplify WDM signals in a bundle with a constant gain (amplification factor). If the optical signals are partly switched at a high speed, so that the number of optical signals (number of channels) fed into the optical amplifier is changed abruptly, then transient gain fluctuations may be generated in the optical amplifier due to cross saturation. With respect to such a problem, it has been proposed to speed up an automatic gain control scheme for suppressing the gain fluctuation in the optical amplifier.
For example, as for the gain control in the optical amplifier, gain control techniques utilizing a part of noise light outputted from the optical amplification section that is called ASE (Amplified Spontaneous Emission) are described in Japanese Patent No. 2778438 (document 1) and H. Yoon et al., IEEE Photon. Technol. Lett., vol. 11. no. 3, pp. 316-318,1999 (document 2).
SUMMARY OF THE INVENTION
The inventors have studied the above-mentioned conventional techniques and, as a result, have found problems as follows. Namely, the optical amplifiers shown in the above-mentioned documents 1 and 2 have been problematic in that the power proportion of the component utilized for automatic power control (APC) in the noise light included in the amplified light outputted from the optical amplifier cannot fully be secured in terms of structure.
For example, a part of noise light tapped by an optical coupler is utilized as monitor light in the optical amplifier shown in document 1, whereas a part of noise light tapped by an arrayed waveguide grating (AWG) is utilized as monitor light in the optical amplifier shown in document 2, whereby none of the optical amplifiers can fully enhance the noise light power to be detected.
Conversely, it means that a considerable amount of noise light not utilized as monitor light is included in the amplified light outputted to the outside (fiber transmission line) from the optical amplifier. Thus, the conventional optical amplifiers have been problematic in that they lack a structure for fully eliminating noise light, thus degrading the noise characteristics, thereby failing to yield a sufficient transmission quality.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical amplifier which carries out automatic gain control by automatic power control utilizing noise light, and comprises a structure for enhancing the noise light power utilized for the automatic power control, thereby effectively preventing its transmission quality from degrating, and realizing control for constantly maintaining the amplification gain for each optical signal (constant gain control) with a higher speed and a higher accuracy.
For achieving the above-mentioned object, the optical amplifier according to the present invention comprises, at least, an optical amplification section such as an optical fiber doped with a rare earth element or the like, an optical circulator, an optical filter, a photodetector device, and a gain control system, wherein noise light separated from amplified light outputted from the optical amplification section is guided to a branch line different from a main transmission line through which amplified optical signals propagate, and then the noise light is utilized for constant gain control of the optical amplification section.
Specifically, the optical amplification section is an optical component for amplifying optical signals inputted together with pumping light, and includes, for example, an amplification optical fiber doped with a rare earth element such as Er. The pumping light source is an optical component such as LD (Laser Diode), and supplies pumping light to the optical amplification section. The optical circulator is an optical component for separating the noise light from the amplified light outputted from the optical amplification section, and has a first port for inputting the amplified light including the amplified optical signals and the noise light, a second port for outputting the amplified light from the first port to the branch line, and a third port for outputting, of the amplified light, the optical signals re-inputted by way of the second port toward the outside of the optical amplifier (toward the main transmission line). The optical filter is an optical component disposed on the second port of the optical circulator, passes therethrough the noise light of the amplified light outputted from the second port, and reflects the optical signals of the amplified light toward the second port. The photodetector device is an optical component such as PD (Photo Diode) for detecting the noise light power having passed through the optical filter. The gain control system controls the pumping power by monitoring the noise light power to control the gain in the optical amplification section.
The optical amplifier according to the present invention carries out automatic power control by utilizing most of the ASE noise light separated from the amplified light, thus enabling more efficient automatic gain control of optical amplification. Since most of the noise light is utilized by the above-mentioned configuration, the automatic gain control scheme, specifically, the circuit configuration of a gain control circuit or the like included in the above-mentioned gain control system, can be simplified, and also the automatic gain control can be accelerated. (It is important for the noise light utilized as monitor light to have a higher detection level in order to realize a higher speed in automatic gain control, since it is necessary for the PD to enhance its load resistance if the power of noise light to be detected is low, which inevitably hinders the control for the pumping light source and the like from following fluctuations in amplification gain.)
Further, in the optical amplifier according to the present invention, without part of noise light
Kakui Motoki
Nakaji Haruo
Hughes Deandra
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
Tarcza Thomas H.
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