Theoretical and experimental analysis of unidirectionality of asymmetrically coupled semiconductor ring or disk lasers

We present analytical, numerical and experimental results about the unidirectional behaviour of semiconductor ring or disk lasers in which the coupling from the clockwise (CW) mode to the counterclockwise (CCW) mode is different from the coupling from the counterclockwise to the clockwise mode. The theoretical and numerical results show different regimes, depending on the gain suppression in the active layer. At very low power, the ratio of the powers of the CW and CCW modes depends mainly on the ratio of the coupling constants, while at high power gain suppression is dominant and the ratio of the powers depends on the ratio of the gain suppression and the weakest coupling coefficient. Our analytical expressions for the unidirectionality are in excellent agreement with numerical results obtained using coupled rate equations. Some experimental results are given for a microdisk laser coupled to a bus waveguide, on one side of which an almost 100% reflecting Bragg grating is designed