An optical device (100, 200, 300, 400), comprising: a) a first optical coupler (131, 231, 331) configured to receive a light signal and provide a first output and a second output; b) a first optical waveguide (141, 241, 341) in optical communication with the first output and configured to provide a first optical path for a first portion of the light signal; and c) a second optical waveguide (142, 242, 342) in optical communication with the second output and configured to provide a second optical path for a second portion of the light signal, d) wherein at least the first optical waveguide (141, 241, 341) is configured to provide a phase differential (150, 250, 350) between the first optical path and the second optical path, e) wherein the second optical waveguide (142, 242, 342) is positioned within a lateral thermal diffusion length associated with the first optical waveguide (141, 241, 341), and f) wherein the lateral thermal diffusion length is a spreading distance of a thermal effect in a direction about perpendicular to the first optical path; g) wherein the first optical waveguide (141, 241, 341) is constructed from a silicon material, wherein to provide the phase differential, the first optical waveguide (141, 241, 341) is doped with dopants to cause injection of free carriers into the silicon material, and wherein the injection of the free carriers causes the thermal effect; and h) wherein the first optical waveguide (141, 241, 341) is disposed on a silicon substrate layer, and a part of the silicon substrate layer is removed such that air cavities are formed between the first optical waveguide (141, 241, 341) and the silicon substrate layer to thermally insulate the waveguides (411, 412) from the silicon substrate layer. a) a first optical coupler (131, 231, 331) configured to receive a light signal and provide a first output and a second output; b) a first optical waveguide (141, 241, 341) in optical communication with the first output and configured to provide a first optical path for a first portion of the light signal; and c) a second optical waveguide (142, 242, 342) in optical communication with the second output and configured to provide a second optical path for a second portion of the light signal, d) wherein at least the first optical waveguide (141, 241, 341) is configured to provide a phase differential (150, 250, 350) between the first optical path and the second optical path, e) wherein the second optical waveguide (142, 242, 342) is positioned within a lateral thermal diffusion length associated with the first optical waveguide (141, 241, 341), and f) wherein the lateral thermal diffusion length is a spreading distance of a thermal effect in a direction about perpendicular to the first optical path; g) wherein the first optical waveguide (141, 241, 341) is constructed from a silicon material, wherein to provide the phase differential, the first optical waveguide (141, 241, 341) is doped with dopants to cause injection of free carriers into the silicon material, and wherein the injection of the free carriers causes the thermal effect; and h) wherein the first optical waveguide (141, 241, 341) is disposed on a silicon substrate layer, and a part of the silicon substrate layer is removed such that air cavities are formed between the first optical waveguide (141, 241, 341) and the silicon substrate layer to thermally insulate the waveguides (411, 412) from the silicon substrate layer.