This paper examines recent progress in the use of semiconductor optical amplifiers for phase sensitive signal processing functions, a discussion of the world's first multi-wavelength regenerative wavelength conversion using semiconductor optical amplifiers for BPSK signals.
IntroductionAn impending capacity crunch has been widely discussed recently, arising from the inability of a single optical fiber strand to continue to support the exponentially growing traffic demands [1]. Many solutions have been proposed including novel optical amplifiers, installation of new fibers to allow spatial multiplexing and the re-emergence of optical regeneration [2]. Of these only optical regeneration offers a significant increase in the overall capacity of a fully populated link without significant increases in cost or power consumption. Conventional (Silica based) highly nonlinear fiber (HNLF) with high SBS threshold has proven effective in enabling many of the essential features of a future all optical regenerator, including⢠Operation with phase encoded signals, to ensure compliance with coherently detected signal formats [3,4] ⢠Multi-wavelength operation, to ensure cost competitiveness with optical amplifiers [5]⢠Simultaneous amplitude and phase regeneration, to give the prospects of regeneration of QAM signals [6] ⢠Quantum limited performance, to maximize performance benefits [7] Despite the excellent fiber performance [eg 8] these reports typically employ at least one high power (Watt class) optical amplifier, reducing the prospects for compact and low energy consumption devices. Several alternative material systems exist for the production of nonlinear devices based on four wave mixing processes including periodically poled lithium niobate [9], photonic crystal [10] and/or bismuth fiber [11,12], calcogenide planar waveguides [13], polymer clad silicon waveguides [14] and amorphous silicon [15], unfortunately when practical configurations are considered such devices either require higher pump powers than for the equivalent function in HNLF, or have power handling limitations such as two photon absorption or the Staebler-Wronski effect. In contrast to the materials above operating on variations of the Kerr effect, nonlinearity in SOA's is mediated by carrier excitation enabling signal processing with significantly lower optical power levels [16]. SOAs have been previously demonstrated in many FWM based devices including wavelength conversion [17], clock recovery [18],optical sampling [19] and optical regeneration [20], but very little work has been performed on phase sensitive applications [21,22]. In this paper, we report on the first ever multi-channel regeneration in a semiconductor optical amplifier [22]. This is achieved in a fully "black box", fiber in -fiber out configuration, operating with phase modulated data. The system summarized here employs semiconductor devices for all active functions, opening the possibility of a fully integrated optical subsystem. Optical power requirements are modest (< 4mW...