A high power semiconductor laser diode with a tapered and cascaded active multimode interferometer (MMI) cavity was designed and demonstrated.An output power as high as 32 mW was obtained for the novel laser diode with a tapered and cascaded active MMI cavity,being much higher than the 9.8 mW output power of the conventional single ridge F-P laser with the same material structure and the same device length due to the larger active area;and also being higher than the 21.2 mW output power of the rectangular and cascaded active MMI laser diode with nearly the same structure,except for the shape of the MMI area.In addition,the tapered and cascaded active multimode interferometer laser showed stable single mode outputs up to the maximum output power. A high power semiconductor laser diode with a tapered and cascaded active multimode interferometer (MMI) cavity was designed and demonstrated. An output power as high as 32 mW was obtained for the novel laser diode with a tapered and cascaded active MMI cavity, being much higher than the 9.8 mW output power of the conventional single ridge FP laser with the same material structure and the same device length due to the larger active area; and also being higher than the 21.2 mW output power of the rectangular and cascaded active MMI laser diode with nearly the same structure, except for the shape of the MMI area. In addition, the tapered and cascaded active multimode interferometer laser showed stable single mode outputs up to the maximum output power.