Wide shoulder, high order mode filter for thick-silicon waveguides

A method for filtering higher-order transverse modes in a ridge waveguide, the method comprising:

• transmitting a beam of light, comprising a fundamental mode and one or more higher-order transverse modes, along a direction of beam propagation in a first ridge waveguide, wherein the first ridge waveguide includes:
• a waveguide shoulder slab, formed of a first material disposed atop a substrate material, the first material having a higher index of refraction than the substrate material, and
• a waveguide ridge, formed of the first material, disposed atop the waveguide shoulder slab and having an input waveguide ridge width, wherein a combination of the waveguide ridge and the waveguide shoulder slab is capable of guiding the fundamental mode and the one or more higher-order transverse modes;
• coupling the beam of light along the direction of beam propagation into an optical filter, wherein the optical filter comprises:
• a filter shoulder slab, formed of the first material and disposed atop the substrate material, and
• a filter ridge, formed of the first material, disposed atop the filter shoulder slab and having a filter ridge width, wherein:
• the filter ridge width matches the input waveguide ridge width where the beam of light couples from the first ridge waveguide into the optical filter, and
• the filter ridge width decreases by adiabatically tapering along the direction of beam propagation such that the fundamental mode continues to be guided by the filter ridge, while at least a portion of the one or more higher-order transverse modes are pushed into the filter shoulder slab, attenuating the one or more higher-order transverse modes in the filter ridge;
• and
• coupling the fundamental mode into a second ridge waveguide, after the fundamental mode has passed through the optical filter and the one or more higher-order transverse modes are attenuated in the filter ridge.

An optical filter for reducing higher-order transverse modes in an output of a semiconductor laser, the optical filter comprising:

• a shoulder slab, wherein:
• the shoulder slab is formed of a first material having a first index of refraction and is disposed atop a second material having a second index of refraction, the first index of refraction being higher than the second index of refraction;
• and
• a waveguide ridge disposed atop the shoulder slab, wherein:
• the waveguide ridge is formed of the first material;
• the waveguide ridge forms an input width in a first waveguide section along a direction of beam propagation, wherein:
• a first filter section, formed by the shoulder slab and the waveguide ridge in the first waveguide section, is capable of guiding light of a fundamental mode and the higher-order transverse modes along the direction of beam propagation;
• and
• the waveguide ridge tapers adiabatically inward from the input width to a reduced width, in a second waveguide section that adjoins the first waveguide section along the direction of beam propagation, wherein:
• a second filter section, formed by the shoulder slab and the tapering waveguide ridge in the second waveguide section, is capable of guiding the fundamental mode, wherein the higher-order transverse modes are pushed into the shoulder slab where they are less strongly guided than the fundamental mode,
• and wherein the optical filter is operably coupled within an optical cavity of the semiconductor laser, and increases a proportion of energy in the fundamental mode, to energy in the higher-order transverse modes, in the output of the laser.