Weak Index Guiding of Interband Cascade Lasers

A weakly index-guided interband cascade laser (ICL), comprising:

• a silicon substrate;
• a high-index layer disposed on an upper surface of the substrate;
• an n+ contact layer disposed on an upper surface of the high-index layer;
• and
• an inner laser ridge disposed on an upper surface of the n+ contact layer, the inner laser ridge comprising:
• a plurality of active ICL stages formed on an upper surface of the n+ contact layer;
• an n-type upper cladding layer formed on an upper surface of the active ICL stages;
• and
• an n+ top contact layer;
• wherein a refractive index of the n-type upper cladding layer is lower than a refractive index of the high-index layer;
• wherein the high-index layer is sufficiently thick that a sufficient fraction of the fundamental lateral optical mode profile lies below the etch depth of the inner laser ridge to impose weak index guiding on at least one predetermined optical mode in the laser waveguide;
• and
• wherein the inner laser ridge is not so wide that significant lasing occurs in higher-order lateral modes.

A weakly index-guided interband cascade laser (ICL) comprising:

• a Si-based substrate comprising a silicon layer and a high-index separate confinement layer (SCL) formed on top of the silicon layer;
• a III-V gain chip disposed epitaxial-side-down on top of the Si-based substrate, the III-V gain chip comprising:
• a GaSb or InAs substrate;
• an etch-stop layer disposed on an upper surface of the GaSb or InAs substrate;
• a sacrificial layer disposed on an upper surface of the etch-stop layer;
• a first n+ contact layer disposed on an upper surface of the sacrificial layer;
• an n-type cladding layer disposed on an upper surface of the first n+ contact layer;
• a plurality of active ICL stages disposed on an upper surface of the n-type cladding layer;
• and
• a second n+ contact layer;
• a portion of the GaSb or InAs substrate of the III-V gain chip, the etch-stop layer, and the sacrificial layer being removed by a combination of mechanical lapping and chemical etches, followed by etching of an inner laser ridge to a depth near to a bottom or below a bottom of the active ICL stages;
• wherein the active ICL stages have a refractive index higher than a refractive index of the n-type cladding layer;
• wherein the high-index SCL has a refractive index higher than that of the n-type cladding layer of the III-V gain chip and of at least one underlying layer in the Si-based substrate;
• wherein the high-index SCL is sufficiently thick that a sufficient fraction of the fundamental lateral optical mode profile lies below the active ICL stages to impose weak index guiding on at least one predetermined optical in the laser waveguide;
• and
• wherein the inner laser ridge is not so wide that significant lasing occurs in higher-order lateral modes.