Unbalanced Mach-Zehnder interferometer and modulator based thereupon

A Mach-Zehnder interferometer comprising:

• an optical splitter/combiner for receiving an input optical signal on an input port of said optical splitter/combiner and splitting said optical signal into a first and a second light portion on respective first and second ports of said optical splitter/combiner and recombining, on an output port of said optical splitter/combiner, said first and second light portions returning to said optical splitter/combiner on said first and second ports to mutually interfere;
• a loop comprising at least a first and a second cascaded segments of polarization-maintaining fibers in the optical path between said first and second ports of said optical splitter/combiner such that said first and second light portions propagate concurrently in mutually opposite directions in said loop;
• and
• a first optical arrangement between the first segment and the second segment of said loop for rearranging the axes of propagation of said first and second light portions such that, on at least said first segment, said first and second light portions propagate along respective mutually orthogonal polarization axes of the polarization-maintaining fiber, whereby birefringence of said at least said first segment where the light portions propagate on mutually orthogonal polarization axes provides a differential path length of said interferometer;
• wherein said first optical arrangement is a first non-reciprocal polarization-transforming arrangement, and wherein said interferometer further comprises a second non-reciprocal polarization-transforming arrangement disposed along said loop such that, on said second segment, said first and second light portions propagate along a same polarization axis of the polarization-maintaining fiber.

A method for effecting self-interference of an optical signal in a Mach-Zehnder interferometer, the method comprising:

• splitting said optical signal into a first and a second light portion, using an optical splitter/combiner;
• coupling said first and second light portions to a fiber-optic loop comprising at least a first and a second cascaded segments of polarization-maintaining fiber such that said first and second light portions propagate concurrently in mutually opposite directions in said loop;
• rearranging the axes of propagation of said first and second light portions between the first segment and the second segment of polarization-maintaining fiber of said loop using a first optical arrangement such that said first and second light portions propagate along respective mutually orthogonal polarization axes on at least said first segment, whereby birefringence of said at least said first segment where the light portions propagate on mutually orthogonal polarization axes provides a differential path length;
• and
• interfering together the propagated light portions using said optical splitter/combiner to provide a Mach-Zehnder interferometer having said differential path length;
• wherein the axes of propagation of said first and second light portions are rearranged between said first and said second segment using a non-reciprocal polarization-transforming arrangement such that they propagate along a same polarization axis on said second segment of polarization-maintaining fiber of said loop.