Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-03-31
2001-07-10
Buczinski, Stephen C. (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S199200, C359S199200, C359S349000
Reexamination Certificate
active
06259556
ABSTRACT:
The present invention relates to an optical amplifier based on erbium-doped fibers and also to a fiber optical network including such an optical amplifier.
BACKGROUND
Optical fibers are presently widely used for communicating information such as in large telecommunication systems, primarily owing to their large reliability, their insensitivity to electrical interference and their high capacity. Of course, there is a desire in the existing telecommunication networks to use the available optical fibers in their networks as efficiently as possible, in particular for communication over long distances, since such fibers obviously have high installation costs. By introducing wavelength division multiplexing WDM in existing communication systems using optical fibers and in new communication systems to be built a plurality of individual wavelength channels can be transmitted on the same optical fiber and thus the information transmitted over the fiber can be multiplied.
In optical fiber networks for long distance communication there may be a need for amplifying the optical signals. Such amplification can of course be achieved by a repeater built in a straight-forward way, including components converting the optical signals to electrical signals, amplifying the electrical signals and converting the electrical signals to optical signals. For WDM signals this will require one optoelectrical and one electrooptical converter per wavelength channel used in the WDM transmission and also one filter or demultiplexer for filtering out the different wavelengths in the incoming signal. This will obviously be very costly and also results in reliability problems owing the large number of components, both electronic and optical, which are required.
Another type of amplifier comprises optical fiber amplifiers based on optical fibers doped with rare-earth metals, primarily erbium-doped fiber amplifiers. Such amplifiers have great advantages when used in optical fiber systems owing to e.g. their compatibility with the optical fibers and their high gain, and they are in particular advantageous when used in wavelength multiplexed transmission systems, since they are capable of simultaneously amplifying a number of WDM channels and only require a limited amount of electronic components. The basic design of an erbium-doped fiber amplifier includes one length of an active, erbium-doped optical fiber, connected at its input end to the output of a 2-to-1 optical coupler, the coupler receiving on one of its inputs the signal to be amplified and on the other input more energetic light providing the power for amplifying the signal. This more energetic input light is called the pump light and is obtained from an optical power source, called the optical pump. The pump light has a shorter wavelength than that of the signal and is generally more energetic and is capable of lifting erbium ions from lower energy states to higher energy states in the erbium-doped fiber. Light is then generated in the fiber when the ions return to lower energy levels.
In a typical WDM system the number of used wavelength channels normally varies at random. The gain of an erbium-doped fiber amplifier operated in the conventional saturated way is dependent on the number of wavelength channels, see the diagram shown in FIG
1
, owing to the total constant output power of the amplifier. In
FIG. 1
the gain for a typical erbium-doped fiber amplifier is plotted as a function of wavelength of the case of only using one WDM wavelength channel, see the upper curve, and for the case of all channels being active or used, see the lower curve. The gain has a difference of at least about 10 dB for the important wavelength band of 1540-1560 nm. This means that also the gain will vary at random when the erbium-doped fiber amplifier is used in a typical WDM system used for telecommunication.
In WDM systems it is obviously important to have a constant and flat output gain in the used wavelength band, since this will allow e.g. an optimization of other components. Also, it is inefficient to use gain flattening filters, see e.g. the article by Paul F. Wysocki et al., “Broad-band Erbium-Doped Fiber Amplifier Flattened Beyond 40 nm Using Long-Period Grating Filter”, IEEE Photonics Techn. Lett., Vol. 9, 10, Oct. 1997, since such filters must be designed according to the actual gain curve of the amplifier. A possible solution would be to control the pump power provided to the active fiber, but this includes disadvantages resulting from the long lifetime of excited erbium ions in the active fiber.
In the published European patent application 0 777 346 an optical amplifier used in a communication system is disclosed comprising an optical amplifying medium, i.e. a length of an erbium doped optical fiber, a pump light source and a probe light source, which sources both inject light into the amplifying medium. The probe light has a wavelength included in the amplification band of the optical amplifying medium and is preferably different from the wavelength of the signal light. Control means maintain the gain for the input signal constant. In the control the power of the probe light and the power of the input signal light are added to each other, giving the powers suitable weights in the adding operation. The result of the weighted addition is controlled to be constant by adjusting the power of the probe light.
SUMMARY
It is an object of the invention to provide an optical fiber amplifier having a gain curve which is independent of the number of active wavelength channels.
It is another object of the invention to provide an optical fiber amplifier having a gain for different wavelength channels, which for each considered channel is approximately constant in time being affected as little as possible and/or during as short time as possible by another channel becoming inactive or active.
The problem solved by the invention is thus how to provide an optical fiber amplifier used for amplifying light signals of different wavelengths and having substantially the same gain in time for all incoming light signals independently of the number of input signals present and how to achieve that the gain for wavelength channel is as little as possible affected by other channels.
When designing an erbium-doped fiber amplifier the gain of the amplifier is dimensioned for the case of all WDM-channels being active. The gain of the amplifier depends on the number of excited erbium ions and this number is approximately constant for a sufficient input pump light power, the amplifier then being operated in a saturated state. This results in an approximately constant output power of the amplifier irrespective of the number of input signals to be amplified. This in turn results in that the gain will increase when one or more WDM-channels become/becomes inactive. The number of excited erbium ions can be changed by altering the pump light power provided to the amplifier which results in a change of the gain characteristic. For a suitable control the amplifier can be made to have a substantially constant gain independent of the number of active WDM-channels, this being a commonly used method of controlling optical fiber amplifiers. However, the response time of such a control can be too long, as indicated above, since excited erbium ions in an active fiber have too long lifetimes, of about 10 ms.
Instead, in the optical fiber amplifier the switched-off channel or channels can be replaced with a dummy or idling channel having a wavelength outside the wavelength band used by the WDM-channels or generally having a wavelength different from those used the WDM-channels such as at wavelength between two wavelength bands. In practical cases the wavelength used for the idling channel can be a wavelength larger than the wavelength band used for signal transmission, such as larger than the commonly used wavelength band around 1530-1550 nm. A laser diode supplying the idling wavelength channel can be rapidly controlled to modify its output power so that the output of the total amplifier is kept substanti
Blixt Peter
Lutz Dirk
Buczinski Stephen C.
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM Ericsson (publ)
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