Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2001-03-16
2002-05-14
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341400
Reexamination Certificate
active
06388804
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical amplifier, and particularly to an optical amplifier used suitably for systems which adopt the optical transmission system based on wavelength division multiplex.
Recently, optical transmission systems which use optical amplifiers as repeaters are studied and put into practice vigorously. It is particularly beneficial for the multi-media service industry typified by the Internet to increase the communication capacity based on the WDM (Wavelength Division Multiplex) system for multiplexing individual signal lights of different wavelengths. In the optical communication system using the WDM technique, the optical repeating amplifier which amplifies all individual signal lights at once has an extremely crucial role for the extension of transmission distance.
For the optical amplifying medium which forms the optical amplifier, optical fiber, with rare earth being added thereto, is useful, and it is under study and about to be put into practice. Particularly, erbium-doped fiber (hereinafter, abbreviated as “EDF”), which is active for amplification in a wide range of wavelength in which the loss of optical fiber is small, is used extensively in optical fiber communication systems.
In order for the optical amplifying medium to be active for amplification in the band of signal light wavelengths, a pumping light which is shorter in wavelength than the signal light is inputted together with the signal light into the medium. The optical amplifying medium has at its end the connection of a WDM optical coupler so that the signal light and pumping light are incident efficiently to the medium.
However, the rare-earth-added optical fiber is narrow in its gain flatness region against signal wavelengths in amplifying the WDM signal light at once, and even with the imposition of limitation of the wavelength range, the gain flatness against signal wavelengths is deteriorated by the variation of input signal light power.
As a scheme of overcoming this problem, there is known a technique for suppressing the wavelength dependency of the gain at different input light power levels, in which the input power and output power of the optical amplifying medium are measured and the pumping light power is adjusted based on automatic gain control (AGC) so that the average gain evaluated from the measured power levels is constant. The optical amplifier based on this scheme further adopts constant output control by use of a variable optical attenuator.
However, the actual optical amplifier has its gain characteristics varied by the temperature variation in addition to the variation of input light power. Therefore, an optical amplifier, which is designed to have a minimal gain's wavelength dependency at a certain temperature, cannot be rid of the wavelength dependency based solely on the AGC due to the variation of operating temperature.
With the intention of overcoming this problem, there is known a technique of flattening the gain of optical amplifier throughout the wavelengths. For example, a technique disclosed in Japanese Patent Laid-open (Kokai) No. Hei 4-11794 implements constant temperature control for the optical amplifying medium by using a temperature control device such as a Peltier device, and the optical amplifier has a flat gain throughout the wavelengths. Another technique disclosed in Japanese Patent Laid-open (Kokai) No. Hei 10-335722 is designed to control the characteristics of the optical filter device in response to the temperature measured with a thermistor, thereby offsetting the gain's wavelength dependency.
SUMMARY OF THE INVENTION
However, the above-mentioned conventional techniques involve the following problems. A system which uses a temperature control device such as a Peltier device has inevitably an increased power consumption, and the system will be complex and bulky due to the need of heat dissipation of the Peltier device. A system which offsets the gain's wavelength dependency by using a thermistor and optical filter device encounters the difficulty in the availability of an optical filter device capable of rectifying sufficiently the gain's wavelength dependency of EDF attributable to the variation of input light power and temperature. Moreover, this system having the additional control parameter of temperature will inevitably have an increased component parts and intricate control operation.
A primary object of the present invention is to overcome the foregoing prior art deficiencies, and specifically provide an optical amplifier which is smaller in power consumption and number of component parts, simpler in structure, and rid of the gain's wavelength dependency against the variation of input light power and temperature.
A second object of the present invention is to provide an optical amplifier which has a flatter gain (smaller in dependency of gain on the input signal power, temperature of optical amplifier and input signal wavelengths).
A third object of the present invention is to offer a structure of optical amplifier which is smaller in power consumption than conventional counterparts.
A fourth object of the present invention is to provide an optical amplifier which has a smaller number of component parts as compared with conventional counterparts.
The inventors of the present invention have conducted experiments on rare-earth-added fiber as a kind of optical amplifier, and found the following facts. (1) The output light power vs. pumping light power proper curve, with the input light power and temperature being varied and with the gain's wavelength dependency being retained virtually constant, i.e., the gain tilt is virtually constant throughout the wavelengths, is a virtually simple increase function or a virtually linear function. (2) If the output light power and pumping light power are related to lie on the above-mentioned proper curve, gain characteristics with a virtually invariable gain's wavelength dependency against the variation of input light power and temperature can be obtained.
Accordingly, based on the use of a proper curve without or with practically a sufficiently small gain tilt and on the retention of the output light power and pumping light power on the proper curve, it is possible to accomplish an optical amplifier which operates virtually on the proper curve without or with practically a sufficiently small gain tilt, as will be explained in detail later.
In case the input WDM signal has a gain tilt, a proper curve of the opposite tilt for the input WDM signal is used so that the output light power and pumping light power are related to lie on the proper curve, and the EDF optical amplifier can produce an output light without a gain tilt.
The present invention is based on the foregoing findings, and the above-mentioned objectives are attained by the provision of a characteristic information table which contains data of the output light power vs. pumping light power proper curve of the optical amplifying medium at a virtually constant gain tilt of WDM signal light, and the provision of a pumping light controller which controls the pumping light power so that the output light power and pumping light power of the optical amplifying medium are related to lie virtually on the given proper curve. Adopting this scheme retains the relation of the output light power and pumping light power virtually on the proper curve, and attains gain characteristics without wavelength dependency or with practically a sufficiently small gain tilt against the variation of input light power and ambient temperature.
The pumping light controller is arranged, for example, as follows. The proper curve is obtained in advance. The relation between the pumping power and output power on condition that the gain tilt is virtually minimum, i.e., the variation of output power in response to the increase of pumping power, can be evaluated based on the experiment. The output power vs. pumping power relation in terms of an approximate curve provides the proper curve. Light detecting
Hellner Mark
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
LandOfFree
Optical amplifier does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical amplifier, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical amplifier will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2890900