1. Field of the invention
The invention is in the field of integrated opto-electronic or photonic devices that can be used for transmitting or switching optical digital data. The invention is more particularly concerned with components having a saturable amplifying medium, i.e. exhibiting a carrier depletion phenomenon, for use as wavelength converters or stabilized gain amplifiers.
2. Description of the prior art
In optical transmission systems information is usually in the form of binary data represented by pulses modulating an optical carrier wave. To carry out wavelength multiplexing and demultiplexing operations, for example in space-time switching matrices, it is often necessary to be able to carry out wavelength conversion. Wavelength conversion consists in replacing the wavelength conveying received information with another, usually different wavelength to form an output wave with the same modulation as the input wave. This conversion is advantageously accompanied by amplification and reshaping of the signal.
To enable transmission at high bit rates it is desirable for the wavelength conversions to be carried out by exclusively optical means, i.e. without any optical-electronic conversion or vice versa.
A first constraint on wavelength converters is that they must supply a monomode output wave independent of the characteristics of the input wave. They must therefore be independent of the wavelength conveying the input signals. It is also desirable for their wavelength to be tunable electrically and independently of the polarization of the received waves. In optical transmission, signals are usually conveyed by optical fibers which always introduce some indeterminacy as to the polarization of the waves appearing at their output. A wavelength converter sensitive to the polarization of the input signal would therefore require a polarization sensitive device between the end of the fiber and the input of the converter. This solution would therefore be difficult to apply on an industrial scale.
A first way to avoid this problem is to exploit the saturable nature of semiconductor optical amplifiers. A constant power monomode auxiliary wave having a wavelength different from that of the input signal carrier is introduced into the amplifier at the same time as the input signal. The modulation of the input signal modulates the gain of the amplifier with the result that the auxiliary wave is modulated correspondingly, with the opposite phase. Independence of the polarization of the input signal can be obtained by making the amplifier insensitive to polarization, for example by the choice of appropriate geometrical dimensions for the active layer (substantially square cross-section) or using a strained quantum well structure. However, this solution allows only limited reshaping of the signals. Furthermore, the amplifier must be associated with a laser, which makes the system more complex and reduces its performance: additional coupling losses, higher cost.
Another possibility is to use the carrier depletion effect in a distributed Bragg reflector (DBR) laser. The wavelength of this component can be tuned but the component is sensitive to the polarization because of the active structure employed: strongly rectangular waveguide or polarization-sensitive quantum well.
Consideration could also be given to using a distributed feedback (DFB) laser oscillator, employing a substantially square cross-section amplifying medium to make it insensitive to polarization. The wavelength of this component cannot be tuned electrically, however, and since the amplifying medium is insensitive to polarization, the polarization of the laser emission is indeterminate. An output polarizer would therefore be required to obtain a transversely monomode wave. This type of component, already used as a stabilized gain amplifier, nevertheless offers improved performance in terms of control power.
An aim of the invention is to remedy the drawbacks of the previously mentioned solutions by proposing an integrated opto-electronic component offering the wavelength tunable laser source function and that can be used as a wavelength converter or as a stabilized gain amplifier insensitive to the polarization of the input wave.