Chirp Properties of Soa-based Wavelength Converters for Fsk/im Combined Modulation Format

The frequency chirp properties of both SOA-Mach-Zehnder interferometer and nonlinear polarization rotation wavelength converters are studied and their feasibility for supporting combined frequency shift keying/ intensity modulation (FSK/IM) labeling scheme are experimentally assessed. Introduction Labelled optical burst switching (LOBS) is considered a promising technique for efficient IP over wavelength division multiplexing (WDM) networking. In a LOBS network, bursts of data are composed by assembling several IP packets at the ingress nodes and providing them with an optical label that is used by the core nodes for forwarding and routing decisions. A promising approach for labeling of optical signals is to use orthogonal modulation formats [1]. One important issue in an optical label swapping (OLBS) core node is the ability to perform label swapping including the erasure of the old FSK label, insertion of the new FSK label to deliver the packet to its next hop, and wavelength conversion, often used for routing purposes. This paper presents an assessment of the performance of two semiconductor optical amplifiers (SOAs) based wavelength converter structures for label swapping of FSK/IM combined modulation formats, namely a SOA-MZI wavelength converter [2] and a wavelength converter based on polarization rotation in a single SOA [3]. Specifically, we focus on the frequency chirp properties of the two SOA-based wavelength converters as any added chirp is of crucial importance for the performance of the FSK modulation format. MZI Wavelength Conversion The employed chirp measurement method is based on a Fabry-Perot interferometer, as a frequency discriminator [4]. The measurement setup for the chirp measurement is shown in figure 2, where we used an SOA-MZI structure as wavelength converter. Chirp measurements on the MZI wavelength converter were carried out at 2.5 and 10 Gbit/s. For non-inverting conversion we found a red shift (negative frequency shift) on the rising edge and a blue shift (positive frequency shift) on the falling edge. For inverting operation, the chirp is opposite in sign [5]. The peak-to-peak chirp measured is varying between 3.5 and 5 GHz. For both considered bit rates, no substantial differences in peak-to-peak chirp were observed. No dependence of the chirp peaks on the extinction ratio of the output or on the modulation speed was observed. Two modulation techniques with different chirp properties are used for the modulation of the wavelength converter input signal. Namely direct modulation of a distributed feedback (DFB) laser and the usage of an external LiNbO3modulator. The results show no substantial difference in the chirp of the output signal, as the chirp of the wavelength-converted signal is primarily determined by the intensity of the modulated input signal and the device properties, and not by the chirp of the input signal [6]. The input CW power (probe) for 10 Gbit/s is 1.7 dBm and the input signal power (pump) is Pin=11.0 dBm for non-inverting operation and Pin=-10 dBm for inverting operation. Fig. 1 SOA-MZI chirp measurement for 10 Gbit/s and non-inverting operation. The Dashed line is the pulse shape corresponding to the left y-axis, and the solid line is the chirp (GHz) corresponding to the right yaxis. Table 1 Chirp peaks for co-propagating operation of the SOA-MZI wavelength converter. Bit Rate (Gbit/s) Chirp falling edge (GHz) Chirp rising edge (GHz) Inv/Non-inv