Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-08-02
2001-06-12
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S199200, C359S199200
Reexamination Certificate
active
06246514
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a system with optical amplifiers and in particular to a system where the transmitted signals are wavelength division multiplexed (WDM), and to a control process in this system.
THE STATE OF THE ART
Wavelength division multiplexing (WDM) is a technology which permits transmission of a number of optical signals through an optical fibre by using different separated light wavelengths. In this manner, the information carrying capacity can be increased significantly. The capacity depends on the band width of the fibre, the number of the wavelength channels used and how close the wavelength channels can lie to each other in practice. The signal at each wavelength travels through the fibre independently of the other signals, so each signal represents a discrete channel of large band width.
When signals are sent long distances, the signals must be regenerated or amplified at repeated intervals. In the latter alternative, it is possible to use optical amplifiers for example. The optical amplifier differs from its electrical counterpart in that the optical amplifier attempts to keep the output power constant, while the electrical amplifier has a constant amplification.
The fact that the output power is constant presents problems if the number of channels is changed, intentionally or because of faults. It can therefore be desirable to be able to control the output power. It is previously known to control the amplification in an optical amplifier with the aid of pilot tones (see EP 0 637 148 A1). An identifying pilot tone is modulated on each multiplexed wavelength. Each amplifier in the system determines from the pilot tone the total number of wavelengths which are sent and provides a corresponding regulation of the amplification by the amplifier. Either feed-forward control or feedback control can be used. The patent document also indicates that, in addition to counting the number of channels, the amplitude of the pilot tones on the channels can be measured to provide better regulation.
It is also previously known (see GB 2 294 170) to control, in various ways, the amplifier by measuring the total optical power at the amplifier output and comparing it to a reference voltage. This provides a feedback signal which controls the amplifier. Additionally, the number of channels can be counted and the regulation be adjusted with the aid thereof.
It is also known (see U.S. Pat. No. 4,991,229) to control an optical amplifier by measuring the power of only one wavelength channel. This is done by extracting the channel by filtering with the aid of a WDM coupler and detecting it. Otherwise the feedback control is effected as described above.
DESCRIPTION OF THE INVENTION
One problem with previously known technology is that it is difficult to make the feedback sufficiently rapid.
A problem with controlling the output power from an optical amplifier by only counting the number of channels, is that the number of channels is only a rough measure of the output power of the amplifier.
The purpose of the present invention is to solve these problems by using a feed-forward with a reading of the total optical input power to the amplifier. An output signal is formed as a non-linear function of this input power. The output signal then controls the amplifier, for example by controlling a pump laser which is coupled to the optical amplifier.
In order to increase the reliability it is possible in one embodiment of the invention to combine the feed-forward with negative feedback. This provides a rough adjustment with the feed-forward and a fine adjustment with the feedback.
The feedback is effected by measuring the optical channel output power and feeding it into a controller. There is also coupled to the controller a set value which indicates the desired channel output power. The controller emits a signal which is added or multiplied by the output signal from the feed-forward and controls the amplifier.
Measurement of the power in the channels can be done in various ways. Firstly, it is possible to count the number of channels, measure the total power in all of the channels and divide the power by the number of channels. Another method is to allocate one, or preferably two, channels solely for power measurement. In these channels, there will then essentially only be some type of known check signal, for example a pilot tone or a digital signal. The power is then measured only on these channels. Alternatively, the amplitude of the check signal can be measured. If several check signals are used, the mean value is taken.
The advantages of the present invention are that the measurements will be both rapid and reliable in an inexpensive and simple manner.
REFERENCES:
patent: 4918396 (1990-04-01), Halemane et al.
patent: 4991229 (1991-02-01), Nelson et al.
patent: 5467219 (1995-11-01), Ushirozawa
patent: 5703711 (1997-12-01), Hamada
patent: 5870217 (1999-02-01), Itou et al.
patent: 5875054 (1999-02-01), Onoda et al.
patent: 0 637 148 (1995-02-01), None
patent: 2 294 170 (1996-04-01), None
Bonnedal Dag
Forsberg Gunnar
Johansson Bengt
Lagerstrom Bo
Burns Doane Swecker & Mathis L.L.P.
Hellner Mark
Telefonaktiebolaget LM Ericsson (publ)
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