Mid-infrared external-cavity two-segment quantum-cascade laser

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Abstract
An external-cavity mid-infrared quantum-cascade laser allows broad and continuous wavelength tuning and modulation when controlled with a combination of grating position and cavity phase on a two-segment optical amplifier chip. The electrically controlled phase segment allows fine wavelength tuning to complement the coarse grating tuning. The 4.8-μm laser was tested with CO gas absorption and wavelength modulation spectroscopy, showing a spectral resolution limited by the laser linewidth of ∼550 MHz, but also allowing sublinewidth modulation of ⩽200 MHz. The tuning range from 4.855 to 4.9 μ m (0.57 THz) was limited by the device intrinsic gain.