The linewidth enhancement factor (LEF) of terahertz quantum cascade lasers is measured using an optical feedback self-mixing technique. As expected, a much lower LEF is found than is common for interband lasers, but instead of the predicted value of zero, the LEF depends on the laser conditions and can be as high as 0.5. The measured value tends to increase with increasing current. Cross absorption within the laser active region is suggested as a possible cause for the nonzero LEF observed. (C) 2008 American Institute of Physics
Tunable quantum cascade lasers operating in the terahertz frequency range are demonstrated. By using an external cavity based on the reflection from a movable mirror, both broad and fine tuning of the frequency are achieved by varying the cavity length. Coarse tuning up to 3cm−1 is obtained near the center frequency of 4.8THz (∼160cm−1), and continuous mode-hop-free tuning is observed over ∼ 0.4cm−1, nearly corresponding to the cavity free spectral range.
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