Low Threshold Distributed Feedback Quantum Cascade Lasers with Widely Spaced Emission Wavelengths

We report lasing properties of distributed feedback quantum cascade lasers (DFB QCLs) including a double-phonon-resonance active region, at wavelength of about 8.4μm. A broad gain spectrum is generated due to the coupling between the lower laser level in the active region and the levels in the injector, and is demonstrated by the lasing spectrum of the corresponding Fabry--Perot QCLs whose width is 0.5μm at 1.5 times of the threshold current. As a result, the DFB QCLs employing different grating periods exhibit a wavelength span of 0.18μm at room temperature and total wavelength coverage of 0.28μm at various heat sink temperatures. A high side mode
suppression ratio of about 30dB and a low threshold current density of 1.78kA/cm² are achieved as the lasers operate at room temperature in pulsed mode.