Spatially coherent surface-emitting, grating coupled quantum cascade laser with unstable resonance cavity

A surface emitting semiconductor laser having longitudinal and lateral dimensions, and a transverse dimension perpendicular thereto, a wide active lasing region perpendicular to the transverse dimension for producing a spatially coherent output beam comprising:

• a semiconductor resonance cavity producing a laser mode of diverging counter-propagating traveling wave beams of light derived from unipolar inter-subband transitions within an active region of the semiconductor laser and along a centralized axis parallel to the longitudinal dimension;
• and
• a diffraction grating for resonantly coupling the diverging counter-propagating traveling wave beams while also coupling a portion of the traveling wave beams into an output beam transverse to the centralized axis;
• and
• a semiconductor layer for coacting with the traveling wave beams and having an effective index of refraction that varies quadratically in a direction transverse to the centralized axis, with lowest values thereof at centralized portions proximate or on the centralized axis of the laser and higher values thereof at noncentralized portions of the laser.

A unipolar surface emitting semiconductor laser having longitudinal and lateral dimensions, and a transverse dimension perpendicular thereto, and a wide active lasing region perpendicular to the transverse dimension for producing a spatially coherent output beam comprising:

• a semiconductor resonance cavity for producing a laser mode of diverging counter-propagating traveling wave beams of light derived from unipolar inter-subband transitions within an active region of the semiconductor laser and along a centralized axis parallel to the longitudinal dimension;
• a diffraction grating having grooves extending perpendicular to the longitudinal axis for resonantly coupling the diverging counter-propagating traveling wave beams while also coupling a portion of the traveling wave beams into an output beam perpendicular to the centralized axis;
• and
• a semiconductor layer for coacting with the traveling wave beams and having an effective index of refraction that varies quadratically in a direction perpendicular to the centralized axis, with the lowest value thereof at centralized portions of the laser and the higher values thereof at noncentralized portions of the laser proximate the centralized axis.

A unipolar semiconductor laser, comprising:

• a multilayer semiconductor structure comprising:
• doped semiconductor material of only one conductivity type;
• a plurality of nominally identical active regions, each active region separated from an adjoining active region by a carrier injection/relaxation region, and comprising one or more quantum wells having a higher energy state and a lower energy state for carriers of the one conductivity type;
• the energy relaxation region selected to provide for energy relaxation of the carriers of the one conductivity type when a current source or electrical bias is coupled to the semiconductor laser, some of the carriers of the one conductivity type being introduced into the energy relaxation region from the active region;
• some of the carriers of the one conductivity type undergoing a radiative transition from the higher energy state to the lower energy state;
• a diffraction grating for providing feedback in a portion of the multilayered semiconductor structure to enhance the radiative transitions;
• and
• a reflecting surface disposed relative to the grating such that light directed in one direction by the grating and reflected from the reflecting surface that forms a portion of the output light from the laser is in phase with other light directed in a different direction by the grating that forms another portion of the output light.

A quantum cascade (QC) semiconductor laser comprising a core region including a QC active region comprising a plurality of unipolar radiative transition regions interleaved with unipolar injection/relaxation regions;

• upper and lower cladding regions bounding the core region;
• the core region and the cladding regions comprising an optical waveguide;
• a diffraction grating adapted to couple light within the optical waveguide as output light;
• a reflecting surface spaced from the grating by an optical path such that light coupled from the waveguide to the reflecting surface has substantially the same phase upon return to the grating as light coupled from the waveguide by the grating but not reflected from the reflecting surface.

A quantum cascade (QC) laser comprising:

• a substrate;
• core and cladding regions grown on the substrate;
• a QC active region disposed within the core region;
• a grating disposed within one of the cladding regions to couple light propagating within the laser to output light;
• and
• a reflecting surface spaced from the grating such that light diffracted by the grating and reflected from the reflecting surface is combined substantially in-phase with other light just diffracting from the grating and not reflecting from the reflecting surface as the output light.

A surface emitting unipolar quantum cascade semiconductor laser for producing a spatially coherent single-mode laser output beam substantially perpendicular to a semiconductor substrate surface of the laser, the laser comprising:

• a multilayer semiconductor structure having doped semiconductor material of only one conductivity type;
• the multilayer semiconductor structure comprising:
• a core region of larger effective refractive index between cladding regions of smaller effective refractive index, the core region having a plurality of repeat units, each repeat unit having a nominally identical active region and a carrier injection and relaxation region, wherein the plurality of repeat units are for quantum cascade generation of a lasing resonance mode within a lasing resonance cavity defined by the multilayer semiconductor structure,
• a diffraction grating structure for resonantly coupling diverging counter-propagating traveling wave beams of the laser resonance mode together while diffracting light into an upward direction perpendicular to a grating plane of the grating structure and toward the substrate surface, and diffracting other light into a downward direction opposite said upward direction,
• a reflecting surface for reflecting the downward coupled laser light in the upward direction, wherein the distance of the reflecting mirror from the grating structure is selected to cause the phase of the reflected light to match that of the upward coupled laser light so that the reflected light and the upward coupled laser light combine to form the output beam, and
• a variable index of refraction semiconductor layer structure having a variable effective refractive index profile in a direction transverse to the laser resonance cavity and parallel to the semiconductor substrate surface, the lowest value of the effective refractive index profile being located at a centralized portion of the laser and having an increasing value when moving away from the centralized portion to form a lens-like media for interacting with, the counter-propagating wave beams to discriminate in favor of one resonance mode over other resonance modes.