Method for producing spot-size convertor

A method for producing a spot-size convertor, comprising the steps of:

• preparing a substrate having a principal surface including a first area and a second area;
• forming a stacked semiconductor layer on the principal surface of the substrate, the stacked semiconductor layer including a cladding layer, a first core layer, a core-separation layer, and a second core layer;
• forming a first mask on the stacked semiconductor layer;
• forming a mesa structure on the first area and the second area by etching the stacked semiconductor layer from an upper face of the stacked semiconductor layer to a middle of the first core layer using the first mask as an etching mask, the mesa structure including a side surface and a bottom portion of the first core layer, the mesa structure extending along a waveguide axis;
• forming a protective mask covering the side surface of the mesa structure;
• etching the bottom portion of the first core layer using the protective mask to form a top mesa until an upper surface of the core-separation layer is exposed;
• forming a second mask having a stripe-shaped pattern with a larger width than a mesa width of the top mesa;
• and
• forming a bottom mesa on the second area by etching the core-separation layer and the second core layer using the second mask,
• wherein, in the second area, the top mesa is located on the bottom mesa,
• the top mesa includes the cladding layer, the first core layer, and a portion having the mesa width gradually reduced in a first direction of the waveguide axis, and
• the bottom mesa includes the core-separation layer, the second core layer, and a portion having a mesa width gradually reduced in a second direction opposite to the first direction of the waveguide axis.