Contents I. Introduction
Optical signal processing of ultrashort pulses in various frequency domains has been extensively studied using a lens pair and diffraction gratings in a 4- alignment, as shown in Fig. 1(a) [1], [2]. In last decade, on the other hand, the performance of waveguide types of grating, namely, arrayed-waveguide gratings (AWGs), have been improved; thus, optical signal processing has become possible using AWGs [3]–[6], as shown in Fig. 1(b). The frequency decomposed parallel optical signal processing using an AWG is very suitable for recent high-capacity dense wavelength division multiplexing (DWDM) optical communication systems because the pulse width of such a system is in the range of several picoseconds. Furthermore, an AWG can be integrated with other optical waveguide elements, sufficiently stable, and compatible with fiber optics. The generation of the femtosecond optical pulse train using an AWG was reported by Muralidharan et al. [7]. The operation principle was different, and it showed other possibilities of the AWG for high-speed photonic signal processing. A virtually imaged phased-array (VIPA) is also applied to several optical signal processing experiments, especially to the dispersion compensation [8]–[10]. The advantages of the VIPA are low-loss characteristics and negligible phase error because it is based on very uniform glass plate.
