The invention refers to the field of optical power equalizers. Such devices are widely used in telecommunications via optical light guides.
The invention is based on a priority application EP 00 440 301.0 which is hereby incorporated by reference.
In optical telecommunications signals, especially signals transmitting digital information, consist of an optical carrier wave of a certain wavelength travelling in an optical fiber and being modulated betweenxe2x80x94ideallyxe2x80x94one high and one low power level. Unfortunately there are many phenomena like several stray mechanisms, absorption etc. contributing to a degradation of the signal""s quality on its way from the transmitter terminal to the receiver terminal. Especially the high and the low levels of the modulated signal tend to vary around the one ideal high and the one ideal low power level respectively. Thus, it is necessary to regenerate the signal by means of regeneration modules, which are positioned at certain distances within the fiber line.
Opto-electronic devices are known, which perform an opto-electronic conversion of the optical signal into an electrical signal, e.g. by means of a fast photodiode. The electrical signal is amplified and electronic decision elements distinguish the high from the low power levels in order to create a new and regenerated electrical signal. The regenerated electrical signal is then fed into the control unit of a light source, e.g. a fast laser diode. The light source converts the electrical signal back into a fresh, regenerated optical signal. However, this way of regeneration involves several drawbacks. A rather complicated multi-component setup is needed involving elements like a photodiode, a broadband electrical amplifier and a broadband laser-driver or modulator driver. Such a setup is rather expensive and the large energy consumption is a further disadvantage. Additionally the speed of such systems is limited.
Another known means of signal regeneration works on an all-optical basis and uses the wavelength conversion properties of Mach-Zehnder interferometry. Such systems are even more complex than the opto-electronic ones described above, as they need expensive elements like CW-lasers and optical filters.
U.S. Pat. No. 5,781,326 discloses a two stage system, of which the second stage works on an interferometric basis, involving the disadvantages cited above. The first stage, however, which, for itself, represents an optical power equalizer according to the first portion of claim 1, is based on the use of a gain-clamped semiconductor optical amplifier (GC-SOA) according to the teaching of EP 0639 876 A1. A GC-SOA overcomes the drawback of high non-linearity of classical semiconductor amplifiers (SOA). To this end an internal laser oscillation is set up in the amplifier, which keeps constant the density of electronic charge carriers in the amplifier""s active guide. The wavelength of the clamping oscillation is slightly different from the wavelength of the signal to be amplified. The power of the clamping oscillation is maximal, when no input signal is fed into the amplifier and it is reduced according to an increase of the input signal. The density of the electronic charge carriers, which depends on the total optical power in the active guide, is, thus, kept constant over a wide range of input power. As a consequence, the gain of an GC-SOA, which is, at first approximation, a linear function of this density, is constant, too. The linear behaviour comes to an end as soon as the input power becomes as high as the maximal power of the clamping oscillation. At this point the equilibrium between carrier injection by the pump current and recombination due to stimulated emission no longer holds and the carrier density and, consequently, the gain drops dramatically. This range is called the saturation range of the GC-SOA. The point, where the drop of gain starts, is called the saturation point.
The optical power equalizer according to the teaching of U.S. Pat. No. 5,781,326 uses a GC-SOA working about its saturation range, i.e the gain and/or the amplifier input signal are dimensioned such that the high input power levels are beyond the saturation point, while the low input power levels are below the saturation point in the linear range of the amplifier""s gain. The amplifier input signal consists of two portions: a) a modulated portion of wavelength xcexe, which is identical to the main input signal, and b) a CW-laser portion of a slightly different wavelength xcexb. The total optical power of the amplifier input signal is dimensioned such, that the GC-SOA is still in the linear range, when the modulated part of the amplifier input signal is at a low power level and that it is beyond the saturation point, when the modulated part of the amplifier input signal is at a high power level. As a result the gain of the GC-SOA is at a fixed high level, when the modulated part of the amplifier input signal is at a low power level and it is at a low level depending on the actual input power, when the modulated part of the amplifier input signal is at a high power level. The non-modulated portion of the amplifier input signal is amplified by this modulated gain and selected from the amplifier output signal by means of a notch filter. Thus, the optical power equalizer emits a main output signal of wavelength xcexb representing the inverse of the main input signal, where the set of high power levels is equalized to one high power level, while the set of low power levels still varies about the ideal one low power level.
The main disadvantage of this prior art is the essential need of an CW-laser which is expensive and energy consuming. Furthermore the conversion of wavelength, which is unfavorable in certain applications, is inevitable. Finally, the main output signal is necessarily the inverse of the main input signal. This might be useful in certain cases, but for the main part of applications, which need an un-inverted signal, this feature is undesirable.
It is an object of the invention to provide an optical power equalizer, in which the restricting disadvantages cited above do not occur at all or at least in a reduced way and not in the inevitable combination as in the prior art. As an effect, the desired device should be less complicated and more compact in its setup, more flexible in use and, hence, more cost efficient.
The concrete shape of the GC-SOA""s gain as a function of the power of the amplifier input signal and especially its slope beyond the saturation point is largely accessible to design by people skilled in the art. According to the invention, the shape of the curve beyond the saturation point has to be adjusted such, that the high power levels of the amplifier input signal provoke a gain, which is suitable of amplifying this high input power level to one high output power level, which is the same for any of the occurring high input power levels. In other words, the dependence of the GC-SOA""s gain on the power of the amplifier input signal has to be adjusted in such a way, that the high input power levels of the amplifier input signal are directly converted into the one high output power level of the amplifier output signal.
Consequently, the amplifier output signal is the true amplified and equalized version of the amplifier input signal and not, like in the prior art, a replacement signal of a different wavelength, the behaviour of which is somehow related to the amplifier input signal. Especially, no inversion of the signal occurs.
The invention is explained in further detail by the dependent claims, the further specification and the drawings.
Like in the prior art, the main output signal of this most simple version of the invention is equalized in only one of the two sets of high and low power levels. In order to achieve a completely equalized main output signal, i.e. a main output signal, in which the high as well as the low power levels of the main input signal are equalized to the one high and the one low output power level of the main output signal, a more elaborated device is necessary, joining more components to the basic device. Different variations of such devices are treated by further dependent claims, the further specification and further drawings.