Wavelength Conversion of a 160 Gb/s RZ OTDM Signal in a PPLN Waveguide at Room Temperature M. V. Drummond 1 , J. D. Reis 1 , R. N. Nogueira 1,2 , P. P. Monteiro 1,2 , A. L. Teixeira 1,3 , S. Shinada 4 , N. Wada 4 and H. Ito 5,6 1: Instituto de Telecomunicações, 3810-193 Aveiro, Portugal 2: Nokia Siemens Networks Portugal S.A., RT TAF ON, 2720-093 Amadora, Portugal 3: Nokia Siemens Networks Portugal S.A., IE WSM, 2720-093 Amadora, Portugal 4: National Institute of Information and Communications Technology, Tokyo 184-8795, Japan 5: RIKEN, Sendai, 519-1399, Aramaki Aoba, Aoba-ku, Sendai 980-0845, Japan 6: Research Institute of Electrical Communication (RIEC), Tohoku University, Sendai 980-8577, Japan Abstract: We demonstrate 160 Gb/s wavelength conversion on a PPLN waveguide at room temperature. A maximum power penalty of 2.1 dB for a bit error rate of 10 -9 was achieved over a wavelength range of 29 nm. Keywords: Wavelength conversion, periodically poled lithium niobate waveguide, optical time division multiplexing signals. Introduction Ultrafast all-optical signal processing in compact waveguides has been recently receiving considerable interest, mainly due to the possibility for integration [1, 2]. Nonlinear signal processing techniques such as optical time division signal multiplexing [3], demultiplexing [2], add/drop [2] and wavelength conversion [4] have been reported using periodically poled lithium niobate (PPLN) waveguides at bitrates up to 320 Gb/s. All these applications take advantage of the ultrafast response and high efficiency offered by such devices. In PPLN waveguides, tunable wavelength conversion is achieved through cascaded sum frequency generation / difference frequency generation (cSFG/DFG), enabled by the second order nonlinearity of the lithium niobate. The main limitation of PPLN waveguides is that SFG only occurs within a limited bandwidth, defined by the quasi- phase matching condition (QPM). There are two ways of increasing such bandwidth. One is to trade maximum efficiency for bandwidth on the PPLN design. Other is to resort to pump depletion techniques [2]. Although the latter approach has enabled operation at 320 Gb/s, it presents additional complexity since it requires a clocked pump synchronized with the input signal. On the other hand, the problem of having a broad bandwidth and low efficiency is that high pump powers are required. Furthermore, in order to reduce photorefractive damage induced by high pump powers on the waveguide, operation temperatures higher than 150 ºC are usually required. This is a major drawback due to high power consumption needed to heat the device. In this paper, we demonstrate wavelength conversion of a 160 Gb/s optical time division multiplexing (OTDM) signal by means of cSFG/DFG on a 45 mm long PPLN waveguide at 25 ºC. The PPLN waveguide presents a broad bandwidth of about 5 nm in the C-band, and a low second harmonic generation (SHG) efficiency of 80 %/W. Error-free operation was achieved with a maximum power penalty of 2.1 dB over a wavelength range of 29 nm, thus proving that photorefractive damage was effectively mitigated when using high pump powers. Experiment The experimental setup is depicted in Fig. 1. A mode- locked laser diode (MLLD) provided an optical clock signal at 10 GHz with central wavelength of 1547 nm and a full- width at half maximum (FWHM) of 2.2 ps. The clock signal was intensity modulated with a pseudorandom binary sequence (PRBS) using a Mach-Zehnder modulator. An optical time domain multiplexer (OTDM) was used to increase the bitrate from 10 to 160 Gb/s. The modulated signal and two CW pumps (P1 and P2) were inputted on the PPLN. The wavelength of the signal and P1 were fixed at 1547 and 1560.92 nm, respectively. P2 was set at 1572.3, 1568.1 and 1543 nm in order to achieve a converted signal at 1536, 1540 and 1565 nm, respectively. As such, a wavelength range spanning 29 nm was tested. The average powers of the modulated signal, P1 and P2 at the input of the PPLN were of 21.6, 27.8 and 30.3 dBm, respectively, totaling an input power of 32.6 dBm. Each channel of the 160 Gb/s converted signal was demultiplexed through four-wave mixing with control pulses on a HNLF. The control pulses were achieved by intensity modulating a wavelength tunable CW laser at 10 GHz. Two cascaded electroabsorption modulators (EAMs) were used with this end. Pulses with a FWHM of 4.5 ps were obtained. The demultiplexed channel was detected using an optically pre-amplified receiver. The 45 mm long PPLN waveguide was fabricated using the proton-exchange technique on a 5 mol% MgO doped LiNbO3 wafer. Such doping enabled a high photorefractive damage threshold, which in turn allowed operating the device at room temperature with high input powers. Results The experimental results are shown in Fig. 2. The spectrum of the signals at the output of the PPLN (Fig. 2 (a)) shows that a conversion efficiency of –25 dB was obtained. This value was expected considering the used pump powers and a SHG efficiency of about 50 %/W, found to be in the