We have demonstrated a hyper coherent spectral linewidth evaluation for a frequency stabilized 405nm GaN violet laser diode (LD) based on the delayed self-heterodyne beat. The laser light source was stabilized to a reference confocal Fabry-Perot (CFP) cavity by negative electrical feedback to the injection current of the LD under the Pound-Drever-Hall technique. In addition, by introducing optical feedback from another tilted CFP cavity, the spectral linewidth has been efficiently narrowed. In this scheme, we have achieved 1.65x10(-11) estimated with Allan variance for the feedback error signal from CFP cavity. We also measured the linewidth directly by a separate FP interferometer, which resulted in close coincidence with above Allan variance estimations. In this work, we have tried to measure our narrowed linewidth by the delayed self heterodyne technique which consists of optical fiber of about 1km similar to 3km length to have delay time the light of our stabilized violet laser. In conclusion, we have achieved a practical and inexpensive linewidth control for a high power violet LD to attain hyper coherent conditions of the semiconductor lasers.