Quantum cascade lasers (QCLs) are promising as compact light sources in the mid-infrared region. In order to put them into a practical use, their relatively high threshold currents should be reduced. Facet reflectivity increase by distributed Bragg reflector (DBR) is effective for this purpose, but there have been few reports on DBR-integrated QCLs (DBRQCLs). In this paper, we report a successful operation of a DBR-QCL in 7 μm wavelength region. With the fabrication, an n-InP buffer layer, a core region consisting of AlInAs/GaInAs superlattices, an n-InP cladding layer, and an n-GaInAs contact layer were successively grown on an n-InP substrate using OMVPE in the first growth. Then, the wafer was processed into a mesa-stripe, and it was buried by an Fe-doped InP current-blocking layer to form a buriedheterostructure (BH) waveguide. After that, a DBR in which semiconductor-walls and air-gaps were alternately arranged was formed at the front or end of the cavity by dry-etching the epitaxial layers of the air-gap regions, and thus a DBRQCL was fabricated. A DBR-QCL chip (Mesa-width:10 μm, Cavity-legth:2 mm) which had a DBR-structure consisting of 1 pair of a 3λ/4-thick semiconductor-wall/3λ/4-thick air-gap at the front end and a high reflective facet at the rear end oscillated successfully under continuous-wave condition at 15°C. This is the first report on the InP-based DBR-QCL to our knowledge. The facet reflectivity at the DBR was 66%, which was about two times larger than that of the cleaved facet. This result clearly shows that the DBR-structure is effective for threshold current reduction of QCL.
Quantum cascade lasers (QCLs) are promising light sources for real time high-sensitivity gas sensing in the mid-infrared
region. For the practical use of QCLs as a compact and portable gas sensor, their power-consumption needs to be
reduced. We report a successful operation of a low power-consumption distributed feedback (DFB) QCL. For the
reduction of power consumption, we introduced a vertical-transition structure in a core region to improve carrier
transition efficiency and reduced the core volume. DFB-QCL epitaxial structure was grown by low-pressure OMVPE.
The core region consists of AlInAs/GaInAs superlattices lattice-matched to InP. A first-order Bragg-grating was formed
near the core region to obtain a large coupling coefficiency. A mesa-strip was formed by reactive ion etching and a
buried-heterostructure was fabricated by the regrowth of semi-insulating InP. High-reflective facet coatings were also
performed to decrease the mirror loss for the reduction of the threshold current. A device (5x500μm) operated with a
single mode in the wavelength region from 7.23μm to 7.27μm. The threshold current and threshold voltage under CW
operation at 20 °C were 52mA and 8.4V respectively. A very low threshold power-consumption as low as 0.44 W was
achieved, which is among the lowest values at room temperature to our knowledge.