Coupler used to fabricate add-drop devices, dispersion compensators, amplifiers, oscillators, superluminescent devices, and communications systems

A communications system which comprises:

• an optical fiber transmission ring, the ring being comprised of a substantially single mode waveguide;
• one or more stations coupled to the optical fiber transmission ring, a station comprising:
• an evanescent wave coupler having a coupling region formed from two substantially single mode waveguides, wherein size and spacing of the waveguides in the coupling region are fabricated having dimensions determined by a condition that if the coupling region were sufficiently long, substantially complete evanescent field coupling of light from one waveguide to the other in a predetermined wavelength band would occur, the coupler having a length substantially equal to twice a length for complete evanescent field coupling of light from one waveguide to the other in a predetermined wavelength band to occur (which length is referred to as a half-beat-length) plus an amount which is substantially equal to N multiplied by the half-beat-length, where N is an even positive integer or zero;
• a first tunable Bragg grating disposed in the coupling region in each of the waveguides, wherein the Bragg gratings are positioned at a distance from an entrance end of the coupler region which is substantially equal to one-half of the half-beat-length plus an amount which is substantially equal to N1 multiplied by the half-beat-length, where N1 is a positive integer or zero;
• and
• a second tunable Bragg grating disposed in the coupling region in each of the waveguides, wherein the Bragg gratings are positioned at a distance from an end opposite the entrance end of the coupler region which is substantially equal to one-half of the half-beat-length plus an amount which is substantially equal to N2 multiplied by the half-beat-length, where N2 is a positive integer or zero.