Patent ID: 7162108

Claim:
A variable optical attenuator formed of a planar lightwave circuit that includes a waveguide structure embedded in a cladding layer supported on a substrate, the lightwave circuit containing a cross-configured π (pi)-shifted Mach Zehnder interferometer comprising: a first optical coupler having a device input and a first output and a second output; a second optical coupler having a first input and a second input and a device output; a first channel waveguide optically coupling the first output of the first optical coupler to the first input of the second optical coupler; a second channel waveguide optically coupling the second output of the first optical coupler to the second input of the second optical coupler; and respective heaters disposed on the first and second channel waveguides for selectively modifying the index of refraction of at least one of the first and second channel waveguides, thereby controlling a transmission power for selectively attenuating an optical signal; and wherein: a portion of the cladding layer on both sides of the first and second channel waveguides has been selectively removed for a predetermined length to form a channel waveguide support structure, said channel waveguide support structure comprising: a main pillar of width w to support each of said first and second channel waveguides on the substrate and exposed to air on three sides over the predetermined length, thereby reducing heat loss by conduction and power consumption of the variable optical attenuator, and a plurality of stress relief pillars of width t less than w, integral with the main pillar, and spaced regularly at common points on opposite sides of the main pillar over the predetermined length in order to provide support to the main pillar, thereby reducing polarization dependent phase (PDP) of said interferometer that is independent of the power applied to the heaters, and thereby reducing the polarization dependent loss of the variable optical attenuator.