Journal
JAPANESE JOURNAL OF APPLIED PHYSICS
Volume 47, Issue 8, Pages 6320-6326Publisher
IOP PUBLISHING LTD
DOI: 10.1143/JJAP.47.6320
Keywords
quantum cascade laser; mid-infrared; subband; linewidth enhancement factor; nonparabolicity
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Funding
- Hamamatsu Photonics K.K.
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We report the experimental linewidth enhancement factor (alpha-factor) results on mid-infrared InGaAs/InAlAs quantum cascade lasers (QCLs) obtained using the self-mixing method. The waveforms appearing under self-mixing conditions were different from those originally predicted, which assumed that the pin was symmetric around the gain peak and that alpha = 0. The signs of the QCL alpha-factors were identified to be negative and asymmetric to the pin peak by using the nonsymmetrical mirror drive, and by comparing their wave patterns to those of conventional 1.55-mu m laser diodes. The nonparabolicity of the subbands was a possible cause of the alpha-factor's asymmetry. The subbands are no longer parallel if the lasing energy is restricted by the nonparabolicity, which produces inhomogeneous broadening and an asymmetric optical gain spectrum with a long wavelength tail. Taking the nonparabolicity into account, we determined the QCL gain spectrum and the dependence of the alpha-factor on the spectral position relative to the gain peak. The dependence of the alpha-factor on detuning was in reasonable agreement with the calculation that accounts for the asymmetry in gain.
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