Wavelength-Conversion Free Optical Phase Conjugator Based on Difference Frequency Generation - Yasuhiro Okamura; Shunsuke Noami; Atsushi Takada (2019 24th OptoElectronics and Communications Conference (OECC) and 2019 International Conference on Photonics in Switching and Computing (PSC), Fukuoka, Japan, July 7-11, 2019, 2019)