Multistage spot size converter in silicon photonics

A method, for converting optical mode spot size between a semiconductor waveguide and an optical fiber, the method comprising:

• receiving a light beam at a waveguide comprising a waveguide taper region, the waveguide taper region comprising a shoulder portion and a ridge portion, the ridge portion disposed directly above the shoulder portion and oriented along a direction of beam propagation, the ridge portion having a width that tapers to meet a width of the shoulder portion, the waveguide taper region comprising a first material;
• expanding an optical mode of the light beam in the waveguide taper region;
• coupling the light beam from the waveguide taper region to a mode converter, wherein:
• the mode converter comprises a plurality of stages;
• each of the plurality of stages tapers in a direction similar to a direction of taper of the waveguide taper region;
• and
• the mode converter is made of a second material different from the first material;
• expanding the light beam through each of the plurality of stages in the mode converter;
• and
• coupling the light beam to the optical fiber.

A device for optical mode spot size conversion to optically couple a semiconductor waveguide with an optical fiber, the device comprising:

• a waveguide comprising a waveguide taper region and a waveguide extension region coupled to the waveguide taper region, wherein:
• the waveguide taper region comprises a shoulder portion and a ridge portion above the shoulder portion, the ridge portion having a width that tapers to meet a width of the shoulder portion;
• and
• the waveguide taper region is made of a first material;
• the waveguide extension region is made of the first material;
• the waveguide extension region has a thickness equal to a combined thickness of the shoulder portion and the ridge portion of the waveguide taper region;
• and
• the waveguide extension region and the shoulder portion of the waveguide taper region have a common width;
• and
• a mode converter coupled to the waveguide extension region, wherein:
• the mode converter comprises a plurality of stages;
• each of the plurality of stages tapers in a direction similar to a direction of taper of the waveguide taper region;
• the mode converter comprises a first stage and a second stage of the plurality of stages, the first stage comprising a first portion that does not taper and a second portion that tapers;
• and
• the second stage extends over the first portion and the second portion of the first stage;
• and
• the mode converter is made of a second material different from the first material.