[en] This paper presents a high efficiency Denisov quasi-optical mode converter for 140 GHz, TE_28,8 mode gyrotron. The mode converter includes a dimpled wall launcher and a mirror system. The mirror system consists of four mirrors, of which the first mirror is a quasi-parabolic one. A simulation code is developed based on the coupled mode theory and the vector diffraction theory. The simulation results show that the focal length of the parabolic mirror has a great effect on the vector Gaussian correlation coefficient. Therefore, it is important to select an appropriate focal length for the mirror system. The simulation results show that the scalar Gaussian correlation coefficient of the operating cavity mode to a fundamental Gaussian distribution on the window plane is about 98%, and the corresponding vector correlation coefficient is more than 90%. (authors)