Quantum cascade lasers with quantum dot (QD) active regions have been developed to overcome the drawbacks of quantum-well-based cascade lasers. Low threshold current and enhanced bandwidth characteristics of QD lasers can be combined with promising energy-efficient features of vertical cavity surface emitting lasers (VCSELs) through introducing the novel, to the best of our knowledge, concept of a QD cascade VCSEL (QDC-VCSEL). Design steps and predicted features of a QDC-VCSEL operating at 4.5 µm wavelength are discussed in this paper, where the cavity and active region are optimized for TE mode using a high contrast grating design. Laser characteristics are investigated by developing rate equations for the cascade scheme. Threshold and L-I characteristics are obtained at different temperatures. Also, the dependency of threshold current, slope efficiency, and modulation bandwidth on design parameters, mainly the carrier tunneling time, is studied. Results exhibit output peak powers of
∼
2
and
∼
0.5
m
W
at 150 and 273 K, respectively. In addition, threshold current increment and modulation bandwidth reduction are observed with the increase in tunneling time.
The operation of an injection-locked quantum dot laser including the output characteristics, modulation response, and linewidth enhancement factor (LEF) has been investigated in detail considering the effect of homogeneous and inhomogeneous broadenings. Results demonstrate that a single-mode operation is guaranteed for a broad range of homogeneous broadening values (operation temperatures). Injection power-dependant reduction in the threshold current and a large increase in the slope efficiency and the output power have been achieved. Also, a near-zero and uniform LEF is obtained for the whole lasing spectra where drastic reduction from ∼60 to 0.76 is observed for non-injected modes. The device exhibits a reduced settling time and modified modulation response especially a reduced frequency overshoot and enhanced 3 dB modulation bandwidth. Obtained characteristics provide a design degree of freedom for spectral shaping and single/multi-mode operation which are favorable features for optical signal processing applications.
Relative intensity noise (RIN) behavior of a quantum dot laser (QDL) at free running and under optical injection locking (OIL) has been reported in detail. Considering the mutual effects of the inhomogeneous and homogeneous broadenings, different and also non-routine RIN characteristics of a QDL at low, intermediate, and high homogeneous broadening regimes have been discussed. Results demonstrate that an injection-locked quantum dot laser exhibit enhanced characteristic over free-running operation along with low-frequency noise behavior in a wide range of homogeneous broadening values. RIN reduction of about 25, 55, and 7 dB/Hz have been achieved for low, intermediate, and high values of homogeneous broadening which implies the spontaneous emission noise suppression even at low injection power. The promising impact of the OIL on side-mode suppression for the intermediate homogeneous broadening regime has also been discussed. Our results guarantee the single-mode and low-noise operation of the OIL-QDL at any homogeneous broadening value.
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