IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 12, JUNE 15, 2014 1207 Simultaneous QPSK-to-2×BPSK Wavelength and Modulation Format Conversion in PPLN Francesco Da Ros, Kjeld Dalgaard, Yutaka Fukuchi, Jing Xu, Michael Galili, and Christophe Peucheret Abstract— Phase-sensitive cascaded second-harmonic gener- ation and difference-frequency generation in a periodically poled lithium niobate waveguide allow converting two orthog- onal quadratures of an optical field to different wavelengths, thus enabling simultaneous quadrature phase-shift keying-to- 2×binary phase-shift keying modulation format and wavelength conversions. Static phase-sensitive extinction ratios above 20 dB are obtained for both quadratures, resulting in error-free dynamic operation with low penalty (BER 10 -9 ) at 10 Gbaud. Index Terms— Phase shift keying, nonlinear optics, periodically poled lithium niobate (PPLN), phase-sensitive amplification. I. I NTRODUCTION I N RECENT years, the renewed interest in phase-sensitive amplification for all-optical signal processing has been combined with efforts towards improving the spectral effi- ciency through the use of higher order modulation for- mats such as quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM), leading to various schemes for phase regeneration of QPSK [1], 8-QAM [2], and 16-QAM signals [3] being demonstrated. Among demon- strations of all-optical signal processing targeting advanced modulation formats, R. P. Webb et al. have proposed the use of phase-sensitive four-wave mixing (FWM) for converting the two complex quadratures of an optical signal to different wave- lengths. This functionality is suitable for QPSK-to-2×BPSK modulation format and wavelength conversion [4] and could be employed to enhance the phase noise tolerance of a conven- tional QPSK balanced receiver [5], or to provide simultaneous Manuscript received January 29, 2014; revised March 20, 2014; accepted April 15, 2014. Date of publication April 18, 2014; date of current version May 16, 2014. This work was supported by the Danish Research Council for Technology and Production Sciences under Project 09-066562. F. Da Ros, K. Dalgaard, and M. Galili are with the Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark (e-mail: fdro@fotonik.dtu.dk; kdal@fotonik.dtu.dk; mgal@fotonik.dtu.dk). Y. Fukuchi is with the Department of Electrical Engineering, Fac- ulty of Engineering, Tokyo University of Science, Tokyo 125-8585, Japan, and also with the Department of Photonics Engineering, Tech- nical University of Denmark, DK-2800 Kgs. Lyngby, Denmark (e-mail: fukuchi@ee.kagu.tus.ac.jp). J. Xu was with the Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark. She is now with the School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China (e-mail: jing_xu@hust.edu.cn). C. Peucheret was with the Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark. He is now with FOTON Laboratory, University of Rennes 1, Lannion 22305, France (e-mail: christophe.peucheret@univ-rennes1.fr). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2318992 quadrature demultiplexing and phase regeneration in a scenario of a middle node in a network where the two binary phase- shift keying (BPSK) quadratures are to be directed to different destinations. In contrast with another recently demonstrated phase-sensitive technique relying on orientating the phase- sensitive gain axis to demultiplex the desired quadrature of a QPSK signal [6], this method enables the simultaneous recovery of both BPSK quadratures. Static operation (i.e. using continuous wave (CW) signals) of this scheme has also been demonstrated using semiconductor optical amplifiers (SOAs) as nonlinear media [4]. Furthermore, dynamic operation has been reported in SOAs for a 10.65-Gbaud QPSK signal [7], even though the lack of a phase-locking scheme limited the investigations of the performances. In spite of the operation of the scheme having been numer- ically predicted at a symbol rate as high as 40 Gbaud, and positive conversion efficiencies having been reported with pump spacings of 600 GHz [8], pattern effects need to be addressed when processing high bit rate signals in SOAs, due to a relatively slow carrier recovery time. To fully exploit the benefits of all-optical signal processing, bit rate transparent operation is a desirable condition. Along such a direction, we have shown the potential for implementing this scheme using the Kerr nonlinearity in highly nonlinear optical fibers (HNLFs) and experimen- tally investigated its performances statically [9] and dynam- ically [10]. However, HNLFs are limited by a relatively low nonlinear coefficient, which results in long fibers being required to achieve the necessary nonlinear phase shift. Fur- thermore, suppressing stimulated Brillouin scattering (SBS) in silica HNLFs is a major challenge when implement- ing phase-sensitive functionalities. Among other nonlinear media, periodically-poled lithium niobate (PPLN) waveguides offer a more compact implementation, as well as a better SBS immunity, and have been proven effective for achieving phase-sensitive processing exploiting cascaded second-order nonlinearities [3], [11]. In this work, we experimentally investigate the imple- mentation of the orthogonal quadratures converter in PPLN waveguides, showing static phase-sensitive extinction ratios (ERs) in excess of 20 dB for both converted idlers. Furthermore, dynamic operation is also successfully demon- strated, converting the orthogonal quadratures of a 10-Gbaud QPSK signal to two 10-Gbps BPSK signals located at different wavelengths. The converted quadratures can be recovered error-free (i.e. with a bit-error-ratio (BER) better than 10 -9 ) with low power penalty compared to standard demodulation 1041-1135 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.