title: Comment on "double-lens extended fractional Fourier transform".
abstract: An inadequate interpretation in an article [Appl. Opt. 45, 8315 (2006)] on complex order extended fractional Fourier transform is pointed out. We also give our analysis of the optical system and demonstrate that it can implement only the fractional Fourier transform with a quadratic phase modulation at the output plane.
authors.affiliation: Department of Physics, Harbin Institute of Technology, China.
authors.forename: Zhengjun
authors.initials: Z
authors.lastname: Liu
authors.forename: Shutian
authors.initials: S
authors.lastname: Liu