Symmetrical waveguide coupler

DETAILED DESCRIPTION It is an object of the present invention to provide a multiplexer or demultiplexer, respectively, for signals of different wavelengths which is of simple design for the above-described applications.
The above and other objects are accomplished in accordance with the invention wherein a symmetrical waveguide coupler is provided having two signal channels for coupling a first signal having a wavelength .
lambda.
.
sub.
1 and a second signal having a wavelength .
lambda.
.
sub.
2 which is longer than .
lambda.
.
sub.
1.
The coupler includes a carrier substrate and first and second dielectric strips embedded in the substrate to form two identical dielectric waveguides which together with the substrate define a coupling region.
The first waveguide has first and second arms connected to the coupling region and the second waveguide has a third arm connected to the coupling region.
The second and third arms are oriented in a direction facing away from the first arm.
The dielectric strips are so dimensioned and arranged within the substrate as to present an effective coupling length c.
sub.
1 L.
sub.
1 equal to .
pi.
/.
sub.
2 at a wavelength of .
lambda.
.
sub.
1 for coupling the first signal between the first and third arms and an effective coupling length c.
sub.
1 L.
sub.
1 equal to .
pi.
at a wavelength of .
lambda.
.
sub.
2 for coupling the second signal between the first and second arms, with c.
sub.
1 being the coupling coefficient between the fundamental modes of the signals in the strips and a function of wavelength, and L.
sub.
1 being the length of the coupling region.