| Due to the three dimensional confinement for the carriers, quantum dot lasers are promised to have superior characteristics than quantum well lasers. The direct modulation is playing one of the key roles in ever increasing optical communications, so the characteristics of the semiconductor lasers are of great importance.;In this dissertation, the chirp characteristics of the quantum dot lasers are under investigated in details. The spectrum characteristics of the quantum dot lasers are interpreted to understand the reduced thermal conductivity in InGaAs/GaAs quantum dot active region. A thermal conductive model is employed to quantitatively analyze the thermal conductivity in the quantum dots, which is about one order of magnitude less than that in the quantum well lasers. A gain model is built to analyze the dependence of linewidth enhancement factor on duty cycle in quantum dot lasers. Because of the unusual thermal sensitivity of quantum dot active regions, a range of linewidth enhancement factors, from positive, near-zero to negative, could be expected on different duty cycles. A symmetric gain distribution should be approached to achieve near-zero linewidth enhancement factor. 1.3mum In(Ga)As/GaAs quantum dot DFB lasers fabricated by holography is demonstrated. Lloyd's mirror exposure system is used for the interference lithography. The laser structure is designed, the laser mode distribution is simulated, and the Bragg grating is analyzed. A single mode operation around 1.3mum is realized in the DFB quantum dot lasers. In DFB lasers, on the shorter wavelength side of the peak gain, the spontaneous emission becomes significant and plays an important role in affecting the spectral linewidth, so care should be taken in using linewidth enhancement factor alone as figure-of-merit to describe the linewidth across the whole spectrum. Finally, the correlation of linewidth to distributed coefficient is discussed and effective facet reflectivity is modeled. |
