Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-11-19
2001-04-10
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S199200, C359S199200
Reexamination Certificate
active
06215583
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. An optical amplifier can consist of erbium doped optical fibre which is pumped by a high power semiconductor laser, called a pump laser in the following.
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 variants, the amplifier by measuring the total optical power at the amplifier output and compare 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 filtering out the channel 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 for 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. Another problem is that when a pilot tone is modulated on data in a wavelength channel, the system becomes much more sensitive to disturbances, since the “ones” and “zeros” are affected and data can possibly be lost.
In EP 0 637 148 A1 it is indicated that the amplitudes of the superimposed pilot tones could be measured. The problem with this measuring method is that it can be difficult to measure the amplitudes on all channels at the same time. Furthermore it can be difficult and take a long time to measure a small amplitude which the pilot tone has when it is superimposed on another signal.
One problem in measuring the total power at the output of the amplifier is that the number of channels must also be counted.
One problem with measuring the optical power on only one channel at the amplifier output is that an expensive WDM coupler is required to filter out the channel.
One purpose with the present invention is to solve these problem by designating at least one channel solely for power control. At least one known check signal is sent thereon and its amplitude is measured. If several check signals are used, the mean value is taken.
The amplitude can be measured in various ways. A simple and inexpensive manner is to tap light from the amplifier output with a common optical coupler. The check signal, which in this case is sine signal or the like can be extracted by filtering with the aid of a narrow band filter, whereafter the amplitude is measured. The check signal can also conceiveably be a digital signal for example.
The advantages of this are that the measurements will be more rapid, more reliable and the transmitted data will not be disturbed. Another advantage is that the check signal can be given a double purpose, by putting it to some other use at some other location.
An additional problem with the previously known solutions is that pump lasers are still expensive. This is solved in an embodiment of the present invention by using a pump laser to serve at least two different amplifiers, while the control of the power of the amplifiers is taken care of by a separate blast laser for each amplifier. The ballast laser is then coupled via a multiplexer to the amplifier input and thus affects the input power of the amplifier and thereby indirectly its output power. The advantage of this is that it is less expensive.
In one embodiment of the ballast laser concept, a redundancy coupling is used. This is achieved by two amplifiers sharing together two pump lasers. Each pump laser can control either one or both of the amplifiers at the same time. The advantage of this is significantly higher fault security.
REFERENCES:
patent: 4991229 (1991-02-01), Nelson et al.
patent: 5864423 (1999-01-01), Kosaka
patent: 5870217 (1999-02-01), Itou et al.
patent: 6023366 (2000-02-01), Kinoshita
patent: 0552937A1 (1993-07-01), None
patent: 0 637 148 A1 (1995-02-01), None
patent: 2 294 170 (1996-04-01), None
Bonnedal Dag
Forsberg Gunnar
Johansson Bengt
Lagerstrom Bo
Oberg Magnus
Burns Doane Swecker & Mathis L.L.P.
Hellner Mark
Telefonaktiebolaget LM Ericsson (publ)
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