370 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 5, MAY 2007 Relative Intensity Noise Performance of Wavelength Converters Based on Four-Wave Mixing in Semiconductor Optical Ampliﬁers Hercules Simos, Ioanna Stamataki, and Dimitris Syvridis Abstract—We present a numerical and experimental study of the relative intensity noise (RIN) induced by the four-wave mixing (FWM) based wavelength conversion process in a semiconductor optical ampliﬁer (SOA). The study is based on the RIN evaluation of the converted signal, under various operating conditions (input power levels and wavelength detuning) and input noise characteris- tics. A detailed numerical model is employed to simulate the FWM process, taking into account the ampliﬁed spontaneous emission (ASE) noise, the wavelength dependent gain, and the interaction of four waves in the SOA (two input waves and two product waves). It is shown that for low noise input signals, the output RIN is de- termined by the power levels of the pump and signal and the wave- length detuning. Operation under saturation allows reduction of the output RIN levels with respect to the input. Index Terms—Four-wave mixing (FWM), relative intensity noise (RIN), semiconductor optical ampliﬁer (SOA), wavelength conver- sion. I. INTRODUCTION A LL optical wavelength conversion is considered as a key function for the future WDM lightwave systems. High bit rate and efﬁcient conversion has been demonstrated using tech- niques like cross gain modulation (XGM), cross-phase modula- tion (XPM) and four wave mixing (FWM) in a variety of passive or active media [1]. FWM in traveling-wave semiconductor op- tical ampliﬁers (TW-SOAs) is probably the most favored and studied technique for -conversion, since it offers transparency to the modulation format and to the applied data rate up to tens of gigabits per second (Gb/s) [2]. The other major advantages of FWM in TW-SOAs are the tunability over a wide range of wavelengths and the capability for multiwavelength conversion [3], both exploiting the practically constant (for over 1 THz) gain spectrum of the SOA. The major drawbacks of the FWM process in SOAs, are the efﬁciency degradation at high detuning values and the optical signal-to-noise ratio (OSNR) degradation due to the ampliﬁed spontaneous emission (ASE). In general the spontaneous emission perturbs both the amplitude and phase of the converted signal, resulting to its intensity and phase noise degradation. The latter has already been investigated by mea- suring the increase in the linewidth of the converted signal, rel- ative to that in the input [4]. On the contrary the relative inten- sity noise (RIN) properties of the FWM converted signal have Manuscript received November 16, 2006; revised January 15, 2007. The authors are with the Department of Informatics and Telecommunications, University of Athens, Athens GR-15784, Greece (e-mail: simos@di.uoa.gr; istamat@di.uoa.gr; dsyvridi@di.uoa.gr). Digital Object Identiﬁer 10.1109/JQE.2007.893012 not been investigated up to now, neither theoretically nor exper- imentally. Such an investigation is very useful since RIN affects the performance of lightwave systems using intensity modula- tion [5]. In this paper, we report a numerical and experimental study on the RIN performance of the FWM in SOAs. The inﬂuence of the power and the wavelength detuning, as well as the intensity noise properties of the input signals, is investigated. Although in most cases a degradation of the converted signal is observed, it will be shown that improvement in the RIN performance is possible under certain operating conditions. The paper is orga- nized as follows. In Section II, the general aspects of our inves- tigation are presented, as well as the numerical modeling of the FWM in SOAs. The experimental setup for the RIN measure- ments is given in Section III. Finally, the numerical data and experimental results are given and discussed in Section IV. II. THEORY AND MODELING A. Methodology of Investigation Aim of this part of the work is to investigate the intensity noise performance of the wavelength converters based on the FWM process in a SOA by calculating the RIN of the FWM product at different cases. The operating conditions of the FWM process affect the noise performance of the conversion, due to the dependence of the conversion efﬁciency and ASE generation on the power levels and wavelength separation of the input sig- nals. The ﬁrst group of results obtained, relates the RIN of the converted signal ( ) with the input power levels ( , ) and the wavelength detuning. Moreover the intensity noise per- formance of the converter depends on the RIN level of the input signal. Since a pump wave is involved in the mixing process as well, it is reasonable to include in the study the effect of the pump noise characteristics. For different RIN values of the input pump ( ), the RIN of the converted signal ( ) is calcu- lated, as a function of the input signal RIN ( ). Special attention is paid to the statistical behavior of the in- tensity ﬂuctuations. It has been observed from both simulations and experimental measurements, that the statistical nature of ASE, adds an uncertainty in the evaluation of the RIN value of a FWM signal. To overcome the problems related to the uncer- tainty in the estimation of the RIN values, a long time series for each ﬁeld has been considered in the simulations for the calcu- lation of each RIN value. The ﬁnal RIN is the average of a large 0018-9197/$25.00 © 2007 IEEE