Quantum cascade lasers (QCLs) are semiconductor lasers based on intersubband transitions in quantum wells [1]. Semiconductor lasers in ring format do not need cleaved facets or gratings to achieve optical feedback required for lasing and could support either bidirectional or unidirectional operation [2].In this paper, we report on QCL in this ring format for unidirectional operation and coupled ring tuning. A key advantage of unidirectional lasers (ULs) is that in the preferred emission direction, they can have up to double the quantum efficiency of bidirectional lasers. This behaviour was first reported in interband lasers by Hohimer and Vawter [3], by incorporating an "S-crossarm" waveguide into the ring in a manner that introduces an non reciprocal loss and gain in the counterclockwise (CCW) and clockwise (CW) direction respectively. Using similar technique, we recently reported on a unidirectional quantum cascade ring laser (UQCRL) with 90 % emission in the preferred CW direction [4]. We will report on the competing mode behaviour that gives rise to unidirectional operation. Shown in Fig.1(a) and (b) are an SEM image of the processed UQCRL and the plot of CW/CCW L-I respectively.Also we report an investigation into wavelength tuning characteristics of coupled ring QCLs. Coupled ring lasers have the potential of achieving M times the tuning range of a single ring laser, where M is the Vernier effect tuning enhancement factor associated with coupled waveguide [5]. Fig.1(c) and (d) are the SEM image of the processed coupled ring and the plot of wavelength versus DC current injected into ring 2 respectively. Current in ring 1 was held constant at 400 mA (pulsed).
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