Polarization-independent electro-optically switched directional coupler

A polarization-independent electro-optically switched directional coupler utilizing reverse differential propagation constant control comprising:

• an electro-optic material having at least a pair of wave guide channels within an interaction region;
• and
• at least a pair of electrodes for selectively producing an electric field across the wave guide channels which electro-optically alters an optical propagation constant of the wave guide channels so as to vary the effects of an optical coupling between the wave guide channels,
• the wave guide channels having a curvature region defined by a constant radius that is selected so as to coincide a set of operating voltages of a cross state of the directional coupler for a transverse magnetic (TM) polarization mode with a set of operating voltages of a cross state of the directional coupler for a transverse electric (TE) polarization mode by shifting the cross state of the TM polarization mode relative to the cross state of the TE polarization mode due to the relative difference between the magnitude of the electro-optic effect for the TM polarization mode light signal and the magnitude of the electro-optic effect for the TE polarization mode light signal such that an effect of the optical coupling between the wave guide channels is, for the cross state, effectively independent of the polarization of a light signal incident to the directional coupler means for creating at least one differential optical path length comprises:
• a curvature region of the wave guide channels having a radius selected so as to shift the set of operating voltages of the optical switch state of the TM polarization mode relative to the set of operating voltage of the optical switch state of the TE polarization mode due to the relative difference between the magnitude of the electro-optic effect for the TM polarization mode light signal and the magnitude of the electro- optic effect for the TE polarization mode light signal.

A method for selecting design parameters for a polarization-independent directional coupler utilizing reverse differential propagation constant control and formed of an electro-optic material having at least a pair of wave guide channels within an interaction region and at least a pair of electrodes for selectively producing an electric field across the wave guide channels which electro-optically alters an optical propagation constant of the wave guide channels so as to vary an optical coupling between the wave guide channels, the method comprising the steps of:

• (a) for a given electro-optic material and wavelength of light, providing a desired set of operating voltages for the electrodes for the directional coupler;
• (b) choosing a maximum optical power loss for the directional coupler;
• (c) choosing a set of wave guide fabrication conditions for the directional coupler such that an optical power loss of the directional coupler is at or below the maximum optical power loss;
• (d) choosing a coupler length (L) based on the desired set of operating voltages;
• (e) determining an effective coupling length (l) for both a TM polarization mode and a TE polarization mode of the wavelength of light for the values given by steps (a)-(d);
• (f) choosing an average inter-wave guide channel spacing (G) such that the coupler length (L) is about twice the effective coupling length (l) for the TM polarization mode;
• (g) if the ratio of the coupler length (L) to the effective coupling length (l) for the TE polarization mode is not between about (1+4 m) and (3+4 m), where m is an integer greater than or equal to zero, returning to step (b) to modify the maximum optical power loss chosen;
• (h) determining a required offset of the set of operating voltages of the electrodes for an optical switch state as to shift the optical switch state for a TE polarization mode onto the optical switch state for a TM polarization mode;
• (i) determining a radius of curvature (R) for the pair of wave guide channels in terms of a differential length (.DELTA.l) of the pair of wave guide channels necessary to achieve the required offset of the operating voltages;
• (j) if the radius of curvature (R) results in an optical power loss of the directional coupler that is more than the maximum optical power loss chosen in step (b), returning to steps (b)-(d) to modify one or more of the choices in those steps;
• (k) using the coupler length (L), the average inter-wave guide channel spacing (G) and the radius of curvature (R) to design the polarization-independent directional coupler.