We report the first demonstration of a unidirectional ring diode laser based on the use of an active crossover waveguide. The crossover waveguide introduces both a nonreciprocal loss and gain into the ring cavity to promote unidirectional lasing in the clockwise ring direction. Up to 95% of the continuous-wave lasing output of 3.5 mW (at 150 mA ring current) is emitted from this preferred lasing direction. Unidirectional operation of the ring diode laser is also shown to increase the linearity of the light-vs-current curves, eliminating kinks otherwise occurring from gain competition in the ring cavity.
We report continuous-wave room-temperature operation of a semiconductor ring resonator diode laser with a single Y-junction outcoupling waveguide. This device with a 150 μm radius and 8-μm-wide etched-rib waveguide has a threshold current of 72 mA and emits up to 1 mW of single-frequency output. The side-mode-rejection ratio of this laser exceeds 22 dBm over a ≳50 mA current range corresponding to a stable operating mode of the laser. Studies of the emission behavior in this and other similar ring lasers suggest that the ring is lasing in ‘‘whispering gallery’’ modes, and also that the Y junction is affecting mode selection in the ring.
Results of a study of the electrical derivative characteristics of InGaAsP buried heterostructure lasers are presented. By considering current-leakage paths and electrical contact nonlinearities, an equivalent circuit model appropriate to the buried heterostructure laser has been developed. Solutions for the theoretical electrical derivatives of series-parallel-reducible networks have been obtained and applied to the equivalent circuit model of the buried heterostructure laser. Comparison of experimentally measured and calculated electrical derivative characteristics provides both qualitative and quantitative information concerning the distribution of current and voltage in buried heterostructure lasers. By combining the techniques of electrical-derivative measurement and equivalent-circuit modelling, an improved understanding of the properties of buried heterostructure lasers has been established.
The role of stress on the polarization of stimulated emission is discussed in this paper. The stimulated emission from injection lasers is usually polarized in the TE mode. However, in the presence of a sufficiently high stress in the active region, TM polarized stimulated emission at threshold can be observed. We find that for some InGaAsP buried heterostructure lasers with a kink in the light-current characteristic, the kink represents the onset of a TM polarized stimulated emission. X-ray diffraction measurements show that the active region of these devices are under stress. We show that the change in polarization characteristics associated with the light-current kinks can be due to (i) higher optical gain of the TM emission than TE in the presence of stress and (ii) larger reflectivity of the higher-order TM modes. This mechanism is supported by the observation of higher-order TM modes than TE and the observation that the TM gain is unsaturated above TE threshold. We also find that the TM kinks occur at lower currents as the external pressure is increased. In the absence of stress in the active region, stimulated emission in higher-order TE modes associated with light-current kinks is observed in buried heterostructure lasers with large active region. Our results show that the observation of stimulated TM emission in real index guided laser structures is indicative of internal stress in the active region of these devices.
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