QC laser active regions with multiple transitions from strongly coupled upper laser states to lower laser states were
designed to achieve broad gain spectra as well as high performance. Two broad gain QC laser designs in the 7-9 μm
wavelength region were demonstrated based on the continuum-to-bound design. The first embodiment of one design at
~8 μm enables external cavity tuning over 190 cm<sup>-1</sup> (7.5 - 8.8 μm) in pulsed mode operation at 0°C. We also
demonstrated a QC laser structure based on a continuum-to-continuum active region in the 4-5 μm wavelength region
with a gain spectrum of ~ 430 cm<sup>-1</sup>. External cavity tuning over 340 cm<sup>-1</sup> (4.4 - 5.2 μm) was achieved with this design in
pulsed mode operation at 15°C. In spite of the broad gain spectrum, a low threshold current density (1.6 kA/cm2), large
slope efficiency (4.5 W/A), good temperature performance (T0=160 K), high peak power (up to 5 W) and high wall plug
efficiency (WPE, up to 20%) were achieved for ridge lasers with as cleaved facets, in pulsed mode operation at 295 K,
demonstrating that it is possible to make a wide gain spectrum compatible with high power and efficiency performance.
We present ring-cavity surface-emitting lasers (ring-CSELs) based on quantum cascade structures as an elementary building block for two-dimensional quantum cascade laser arrays. The devices operate at temperatures of 380 K and above. A reduction in threshold current density and enhanced radiation efficiency are observed as compared to Fabry-Pérot lasers. The devices exhibit single-mode emission at a wavelength around 8 µm with a side-mode suppression ratio of 30 dB at room temperature. Tuning of the resonance is achieved by variation of the grating period or change in temperature. The emitters exhibit a low-divergence ring-shaped beam pattern with a lobe separation of 1.5 deg. Based on a direct coupling scheme, phase locking of two ring-CSELs is demonstrated with a fringe visibility of 60%. Coherent operation of ring-type lasers results in a narrow spectral line and thus in an enhancement of the spectral brightness.
The ν<sub>1</sub>+ν<sub>3</sub> combination band of uranium hexafluoride (UF<sub>6</sub>) is targeted to perform analytical enrichment measurements
using laser absorption spectroscopy. A high performance widely tunable EC-QCL sources emitting radiation at 7.74 μm
(1291 cm<sup>-1</sup>) is employed as an UF<sub>6</sub>-LAS optical source to measure the unresolved rotational-vibrational spectral
structure of several tens of wavenumbers (cm<sup>-1</sup>). A preliminary spectroscopic measurement based on a direct laser
absorption spectroscopy of methane (CH<sub>4</sub>) as an appropriate UF<sub>6</sub> analyte simulant, was demonstrated.
The voltage tuning of gain spectra in three types of Quantum Cascade laser designs is investigated. The gain spectra of
the luminescence device are tunable over the whole voltage operation range for all designs. The lasers are as tunable as
the electroluminescence below threshold, while a reduced tunability is observed in all lasers above threshold. This is
attributed to the decrease of resistance across the laser active region as the photon density increases. A resumed
tunability high above threshold occurs in all lasers based on the anti-crossed designs. Lasers based on the anti-crossed
diagonal transition are tunable above threshold, with a tuning range of about 40 cm<sup>-1</sup> (~4% of the laser emission
wavenumber) at room temperature, i.e. a tuning rate of 800 cm-1 per volt per period of active region and injector.