Tunable astigmatic π/2 converter of laser modes with a fixed distance between input and output planes

The scheme of a tunable astigmatic π/2 mode converter is proposed in which the distance between input and output planes is fixed. The converter is tuned only by rotating the cylindrical components of optical quadrupoles used in the converter around its optical axis. The Gouy phase difference in the orthogonal planes of the astigmatic π/2 converter required for mode conversion was achieved for the first time by using the scaled fractional Fourier transforms of the appropriate orders.