Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2001-07-26
2003-03-11
Moskowitz, Nelson (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S337120, C359S341410, C359S341420
Reexamination Certificate
active
06532103
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical fiber amplifier for an optical transmission system and an optical transmission system using an optical fiber amplifier. More particularly, the invention relates to an optical fiber amplifier for wavelength multiplexing and a wavelength multiplexing optical transmission system.
By the appearance of an optical fiber amplifier, a light signal having a weak light intensity can be amplified to light having a high output power with little noise. As a result, the optical transfer distance can be made much longer.
Further, since the optical fiber amplifier has a wide gain wavelength range from 1530 nm to 1565 nm, a wavelength multiplexing optical transmission in which a plurality of signal lasers within the amplification wavelength range are simultaneously amplified and transferred can be realized. For example, according to “2.6 Terabit/s WDM Transmission Experiment using Optical Duobinary Coding” (22nd European Conference on Optical Communication—ECOC '96 Postdated Line Paper Th. 3.1), it is realized that lasers of 132 wavelengths from 1529 nm to 1564 nm at the modulation rate of 20 Gb/s per wavelength are simultaneously transmitted over 120 km. In the announcement, an optical fiber amplifier for transmission compensates an optical loss occurring in a wave multiplexing part when the wavelength multiplexing is performed and has a function of increasing the output in order to make the transfer distance longer. An optical fiber amplifier on the transmission side obtains a light output of 21 dBm when the light of 132 wavelengths are simultaneously outputted.
In the wavelength multiplexing transmission, it is necessary to set an optical output of each signal wavelength between a lower limit optical output for keeping a signal-to-noise ratio at a necessary level and an upper limit optical output which does not cause a waveform distortion by a non-linear effect in the transmission line. On the other hand, in the optical fiber amplifier, a gain usually has wavelength dependency (gain deviation) and an output range between wavelengths is accumulated every relay and amplification. Since a signal error occurs when the range exceeds a permissible width of the optical output, it is necessary to suppress the gain deviation between wavelengths by the optical fiber amplifier.
As a method of controlling the optical output of the optical fiber amplifier at the time of wavelength multiplexing, there is a method of adjusting an optical output by an optical attenuator on the output side by executing a gain flattening control so that the optical output of every wavelength becomes constant irrespective of the degree of multiplexing as disclosed in “Er:Doped Fiber Amplifier for WDM transmission Using Fiber Gain Control” (Technical Report of IEICE, OCS94-66, p. 31). In order to satisfy a gain flat condition, however, when an optical input increases, a large optical output from an optical fiber for amplification is requested and a strong pump optical power is accordingly necessary. In order to set the optical output within a predetermined optical output range, the increased optical output is decreased by an optical attenuator to an optical output equivalent to that at the time of low input level, so that it is not efficient. In “Configuration Design of Multi-wavelength Er-doped Fiber Amplifier for WDM transmission System” (Technical Report of IEICE, OCS95-36, p. 21), another configuration of the wavelength multiplexing Er-doped fiber amplifier is shown. With the configuration of two-stage amplifier, gain flattening control is performed by an amplifier at the front stage to thereby keep the gain constant irrespective of an optical input. In the optical fiber amplifier, since the wavelength strongly depends on gain, by executing the gain flattening control, it can be controlled so that the gain dependency of the wavelength does not depend on the optical input. In a post-stage amplifier, an optical attenuator is arranged in an input part and it is controlled so that the value of an optical input to the amplifying part is constant. In this manner, while maintaining the whole light gain to be constant, it is controlled so that the optical output is constant. Further, by designing so that gain tilt at the front and rear stages is cancelled, the whole gain flatness can be obtained without using an optical filter. The optical output is set so that the total optical output of all of the wavelengths is constant by decreasing an output of 7 dBm of each channel at the time of four-channel multiplexing to
1
dBm at the time of 16-channel multiplexing.
The optical fiber amplifier is used not only for amplifying a light output in the event of wavelength multiplexing but also for compensating a loss in a functional optical component. As the distance of the optical fiber transmission line having wavelength dispersion is increased, a dispersion value becomes larger. In order to eliminate an influence caused by the dispersion, it is necessary to compensate the wavelength dispersion. In “Dispersion-Compensator-Incorporated Er-Doped Fiber Amplifier” (Optical Amplifiers and Their Applications 994 Technical Digest Series Vol. 14, p. 130), it is described that a wavelength dispersion compensator is incorporated as an optical function component in the center of an amplifier to compensate dispersion. When an optical component in which a loss occurs is arranged in the central part of an optical fiber amplifier which is divided into two parts, while keeping low-noise performance of the optical amplifier, a loss of the optical function component is seemingly reduced and a pumping efficiency can be increased.
In Japanese Patent Application Laid-Open No. 7-281219, an optical amplifier in which a variable optical attenuator is inserted into the front stage and an output distortion of an optical fiber amplifier is reduced is described. Further, in U.S. Pat. Nos. 5,500,756 and 5,555,477, a supervisory optical control is described.
When light of a number of wavelengths enters an optical fiber amplifier and an output level is controlled so as to be constant, an optical gain expressed by a ratio of an optical output and an optical input changes when the optical input changes. Since the wavelength dependency of gain of the optical fiber amplifier changes when the gain changes, the wavelength dependency of gain changes with the change in the optical input. There is no problem if interval lengths of a transmission line as intervals of providing optical fiber amplifiers are the same and a fixed optical input is supplied to each of the optical fiber amplifiers. In reality, however, there are various interval lengths and various optical input levels. Consequently, an optical fiber amplifier in which the wavelength dependency of gain does not change even if an optical input level changes is necessary. For example, in a reception optical fiber amplifier in which the maximum gain is 30 dB and the optical output is 0 dBm, the dynamic range has to correspond to a range from −30 dBm to −9 dBm. In a relay optical fiber amplifier, the gain of about 40 dB is necessary. In a silica erbium-doped optical fiber amplifier, a gain deviation between 1535 nm and 1542 nm is every large and there is a change corresponding to the gain difference of about 6 dB between the minimum and maximum optical inputs.
A problem of the wavelength dependency will now be described with reference to FIG.
1
.
FIG. 1
shows a wavelength dependency of an optical output of an EDFA (erbium-doped fiber amplifier) when a total output level is controlled to be constant. A, B, and C in the diagram are arranged in accordance with the order from lower optical inputs. As the optical input increases, the optical output is markedly reduced around 1530 nm and is increased around 1560 nm. Even if an optical filter for correction is inserted so as to flatten the wavelength dependency of the optical output with a specific optical input, since the wavelength dependency characteristic of the gain changes when the o
Kosaka Junya
Sakano Shinji
Suzuki Takayuki
Yokota Ichiro
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