A single oscillation-mode laser employing the asymmetric waveguide structure is designed and analyzed. The mode selection mechanism is realized by using an asymmetric Bragg reflection waveguide (BRW) and shown to be effective to achieve high side-mode suppression ratio (SMSR). As an example, a silicon-based quasi-one-dimensional BRW with Er-doped Si-nanocrystal in the silicon oxide core is considered and illustrated for the laser structure. Guidance properties and threshold conditions are examined to verify the design procedure and performance feasibility for the single oscillation mode laser.

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The single mode Fabry-Perot (FP) semiconductor lasers are investigated systematically by a rigorous time-domain theoretical model based on the transfer matrix method. Static and high-speed dynamic performances under direct modulation and strong external optical feedbacks are simulated for both symmetric and asymmetric longitudinal structures of the lasers. Comparisons with the DFB and conventional FP lasers are made to confirm its effectiveness in achieving single-mode lasing with high spectrum purity under modulation and feedback conditions. Structural optimization is also carried out with respect to the key design parameters.

In this paper, we propose an external optical feedback resistant distributed feedback (DFB) laser diode (LD) by exploiting parity-time symmetric complex coupling. With its complex refractive index followed a parity-time symmetry, the grating shows a strongly asymmetric reflection to the contra-propagating light inside the DFB cavity, which effectively rejects the returning light from one end. Consequently, the DFB LD is much less sensitive to external optical feedback. On the contrary, the transmissivity of such grating is still symmetric so that the output light of the DFB LD is not affected. Numerical simulation result shows that the lasing wavelength drift can be less than 0.2 nm with a SMSR exceeding 45 dB under a coherent external optical feedback as high as -10 dB.

Distributed feedback lasers comprised of a reflection section and an active section have been proposed for high direct-modulation bandwidth. The reflection section has the same core layer as the active section so butt-joint re-growth is avoided. Without current injection the reflection section will be pumped to near transparency by the emission from the laser itself so high reflection (> 0.75) can still be achieved as confirmed by the simulation. Therefore a short (150 µm) active section can be used, which enables a low threshold current (~5 mA) and a high direct modulation bandwidth (>30 GHz) as demonstrated by the simulation.