Abstract
This work theoretically investigates the frequency noise (FN) characteristics of quantum cascade lasers (QCLs) through a three-level rate equation model, which takes into account both the carrier noise and the spontaneous emission noise through the Langevin approach. It is found that the power spectral density of the FN exhibits a broad peak due to the carrier noise induced carrier variation in the upper laser level, which is enhanced by the stimulated emission process. The peak amplitude is strongly dependent on the gain stage number and the linewidth broadening factor. In addition, an analytical formula of the intrinsic spectral linewidth of QCLs is derived based on the FN analysis. It is demonstrated that the laser linewidth can be narrowed by reducing the gain coefficient and/or accelerating the carrier scattering rates of the upper and the lower laser levels.
Highlights
Quantum cascade lasers (QCLs) are intersubband semiconductor light sources emitting in the spectral range of mid-infrared (MIR) and terahertz (THz) [1,2]
We theoretically investigate the frequency/phase noise (FN) characteristics of QCLs through a three-level rate equation model, which includes all the Langevin noise sources for the carriers, the photon, and the phase of the electric field
The FN peak disappears once the carrier noise F1,2,3 in Eq (9) is removed. This phenomenon suggests that the FN peak is due to the carrier noise in the upper and the lower laser levels, since the carrier noise in the bottom level does not contribute to the FN
Summary
Quantum cascade lasers (QCLs) are intersubband semiconductor light sources emitting in the spectral range of mid-infrared (MIR) and terahertz (THz) [1,2]. In comparison with interband semiconductor lasers, the relative intensity noise of QCLs does not exhibit any resonance peak owing to the ultra-fast carrier lifetimes (around 1.0 ps) [12]. It decreases more slowly with increasing optical power than that in interband lasers [13,14,15]. QCLs usually exhibit near-zero LBFs, leading to narrow intrinsic linewidth in the range of 0.1–1.0 kHz [21,22,23] The latter flicker noise arising from the current source, the thermal fluctuation, and the internal electrical noise considerably broadens the total spectral linewidth of QCLs to the sub-MHz or MHz range [24,25,26]. It is proved that the intrinsic linewidth can be narrowed by reducing the gain coefficient or/and increasing the carrier scattering rates of the upper and the lower laser levels
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