The present invention relates to optoelectronic devices and more particularly concerns such a device, simple in construction, and where the coupling wavelength between two waveguides is tunable. 
Optical devices such as wavelength add/drop filters, bandbass filters, directional couplers, etc.  are crucial elements of optical communication systems.  They are mainly used in DWDM (Dense Wavelength Division Multiplexing) applications, where efficient adding and dropping of channels is essential.  In this context, a wavelength tunable add/drop/filter is very advantageous since it allows network reconfiguration.  Such a device is also useful for wavelength routing of the signal.  This characteristic is even more important for metro or access DWDM optical networks where reconfigurations are constant.  The market of wavelength tunable bandpass filters is also important, where there is a great advantage to use a tunable filter with fast response time, integrated and with no moving parts (electronic control).  An even more advantageous feature of a such a wavelength tunable device is that it may serve as the main building block of an integrated OADM (Optical Add/Drop Multiplexer) if it is combined with, or integrated to, the proper wavelength converter. 
A wavelength tunable device is mentioned in U.S. Pat.  No.  5,887,089 (Deacon et al).  Deacon teaches a structure made of a ferroelectric material having good optoelectronic properties provided with channel waveguides therein.  In one embodiment, shown in FIG.  10 of the above mentioned patent, where two adjacent waveguides lie in the structure and are provided with a periodically poled structure extending over both of them.  Electrodes are provided on either side of the coupling region.  When an electric field is applied between the electrodes, the refractive index grating defined by the poled structure is turned on, and coupling is allowed between the two waveguides for light of a given wavelength, determined by the propagation constants of the waveguides and the period of the grating. 
In the above-mentioned patent, Deacon explores at length the possibility of tuning the coupling wavelength of such a device.  To achieve such a result, one must operate an average refractive index change in the coupling region.  To this end, Deacon suggests several techniques, such as using, in the periodic structure, alternate domains of optoelectronic and non-optoelectronic material, using an asymmetric grating to obtain a duty cycle different than 50%, depositing an additional optoelectronic layer over the basic structure, etc.  All of the proposed solutions however involve a more complex and costly manufacturing process for the resulting device. 
It is therefore an object of the present invention to provide a simple wavelength tunable optoelectronic device. 
It is a secondary object of the present invention to provide such a device also allowing a tuning of the signal bandwidth. 
Accordingly, the present invention provides a wavelength tunable optoelectronic device, including an optoelectronic structure made of a ferroelectric material.  First and second channel waveguides are provided in the optoelectronic structure.  The two waveguides are adjacent at least along a coupling region.  A periodically poled structure is provided in this coupling region, and at least one of the first and second channel waveguides is untouched by this periodically poled structure. 
The device also includes means for generating an electric field of selectable amplitude in the coupling region, through both the first and second waveguides.  The electric field enables a coupling of light of a coupling wavelength between the first and second waveguides in said coupling region.  The amplitude of the electric field in the waveguide untouched by the periodically poled structure determines the coupling wavelength. 
According to an alternative embodiment of the present invention, there is also provided a wavelength and a bandwidth tunable optoelectronic device. 
The device includes an optoelectronic structure made of a ferroelectric material.  First, second and third channel waveguides are provided in this structure. 
The first and second waveguides are adjacent at least along a first coupling region.  A first periodically poled structure is provided in the first coupling region.  At least one of the first and second channel waveguides is however untouched by the first periodically poled structure. 
Similarly, the second and third waveguides are adjacent at least along a second coupling region, a second periodically poled structure being provided in the second coupling region, at least one of the second and third channel waveguides being untouched by said periodically poled structure. 
Means are provided for generating a first electric field of selectable amplitude in the first coupling region, through both the first and second waveguides.  The first electric field enables a coupling of light of a first coupling wavelength and first bandwidth between the first and second waveguides, in said first coupling region.  The amplitude of the first electric field in the waveguide untouched by the first periodically poled structure determines the first coupling wavelength. 
Means for generating a second electric field of selectable amplitude, in the second coupling region, through both the second and third waveguides, are also provided.  The second electric field enables a coupling of light of a second coupling wavelength and a second bandwidth between the second and third waveguides in the second coupling region.  The amplitude of the second electric field in the waveguide untouched by the second periodically poled structure determines the second coupling wavelength. 
In this manner, the device enables a coupling of light of a tunable wavelength and tunable bandwidth from the first to the third waveguides. 
Other features and advantages of the present invention will be better understood upon reading the description of preferred embodiments thereof with reference to the appended drawings.