Electro-optic intensity modulator with improved linearity

A method for improving linearity of an optical direction coupler for coupling a first optical waveguide with a second optical waveguide, the method which comprises:

• providing a directional coupler waveguide structure having two optical waveguides with a length of at least two coupling lengths;
• forming three electrode pairs over said waveguide structure;
• defining an active region with a first electrode pair being at least one coupling length long;
• defining two passive sections with a second electrode pair and a third electrode pair having a combined total length of at least one coupling length;
• applying a first bias voltage and an input RF signal to said first electrode pair to induce a mismatch in propagation constants of two optical channels being coupled, one channel corresponding to each of said first and said second optical waveguides;
• applying a second bias voltage to said second electrode pair and a third bias voltage to said third electrode pair;
• and
• adjusting said first bias voltage, said second bias voltage and said third bias voltage to minimize at least one of second order and third order nonlinearities introduced by said active section.

A method for reducing third order non-linearity detected at a receiver during modulation of a fundamental signal in a Mach-Zehnder modulator having an active electrode, the method which comprises:

• applying a first bias voltage to said active electrode;
• observing said third order non-linearity at said receiver;
• varying said first bias voltage to minimize said third order nonlinearity at a low bias optical modulation depth while maintaining a predetermined optical modulation depth of said fundamentals signal;
• and
• filtering out electrically second order nonlinearities at said receiver.