All-Optical Wavelength Conversion and

Acknowledgement: this work was funded by Science Foundation Ireland grant 06/IN/I969.
978-1-4577-1473-3/12/$26.00 ©2012 IEEE
1 0.8 0.6 0.4 0.2 0 0
2
1.0
H 1B
8th IEEE, IET International Symposium on Communication Systems, Networks and Digital Signal Processing
All-Optical Wavelength Conversion and Regeneration in SOAs
R.P. Webb, J.M. Dailey and R.J. Manning
Tyndall National Institute & Department of Physics, University College Cork, Lee Maltings, Cork, Ireland rod.webb@tyndall.ie four-wave mixing products [9], a feature that can be exploited to obtain a range of novel phase sensitive signal processing functions. In the following two sections, we present two examples from our recent work that illustrate the use of SOAs to process phase encoded signals: generation of an 8-phaseshift keyed (8PSK) signal [10] and conversion of quaternary (Q)PSK to two binary (B)PSK signals by fourwave mixing [11]. II. GENERATION OF 21.3 GBAUD 8PSK
H1A
2
Phase Difference (radians / )
*
OOK Pump (1550 nm ) QPSK Input (1562 nm ) Holding Beam (1555 nm )

8PSK Output Bandpass Filter (1562 nm )
SOA 2
Figure 1. SOA-based MZI all-optical phase modulator. Control of the phase bias allows effective suppression of XGM
*
I. INTRODUCTION Nonlinear semiconductor optical amplifiers (SOAs) continue to offer potential benefits for optical signal processing in communication systems despite the widespread use of digital signal processing (DSP). Whereas DSP provides unsurpassed flexibility for the generation of advanced modulation formats and the compensation for transmission impairments, it does so at the cost of substantial power consumption [1]. Energy consumption is widely recognised as a major limitation to the future growth of the internet because telecommunication systems already account for 1% of the power requirements of a typical industrialised country [2]. At present, most of this power is consumed by routers, but as more efficient network architectures are adopted, pressure to improve the energy efficiency of transmission systems will follow. Consequently, opportunities to transfer some processing functions to the optical domain should continue to be sought if power can be saved by doing so. SOAs, having modest power requirements and small size, are excellent nonlinear devices for signal processing [3]. Their gain enables them to operate with low optical input powers and simplifies the cascading of devices. The use of differential operation [4] with various techniques to increase the effective speed of response [5-7] has enabled wavelength conversion and logic operations on amplitude shift keyed (ASK) signals at speeds far beyond the limit imposed by their inherent recovery time. Residual patterning, a feature of most SOA-based gates, can also be overcome with the aid of complementary signals [8]. Many of the advantages of SOAs are equally applicable to processing modern phase encoded data formats. Although a time-varying power envelope is required to stimulate the nonlinear response, such a power variation is readily obtained from a phase encoded signal by inserting an interferometer before the SOA. The strong cross-phase modulation (XPM) that results facilitates the generation of a phase encoded output signal. Placing the SOA inside an appropriately tuned Mach-Zehnder interferometer (MZI) can cancel the unwanted amplitude modulation, as shown in the next section. SOAs are also efficient generators of
A. Principle The 8PSK modulation format provides an extra bit per symbol compared to QPSK [12, 13], but retains the constant symbol amplitude and reduced susceptibility to nonlinear transmission impairments [14]. A QPSK signal can be converted to 8PSK by a further stage of phase modulation, and if the additional data is in optical form, then the modulation may conveniently be implemented by XPM in an SOA. The unwanted cross-gain modulation (XGM) can be cancelled by placing the SOA in an MZI (Fig. 1), whose normalised outputs for the two states of the binary channel are given by:
SOA 1 Phase Bias
Abstract—We argue that the use of nonlinear semiconductor optical amplifiers for all-optical processing of phase encoded signals offers potential benefits. To illustrate the point, we demonstrate QPSK to 8PSK conversion and decomposition of QPSK to two BPSK outputs at new wavelengths by fourwave mixing.