with a lumped amplifier and a high-output coupler. This implementation not only increases the pulse energy, but also improves the spectrum quality. High-quality rectangle-shaped spectra without the 1 st -order sidebands will be very attractive for numerous applications such as amplification, frequency doubling, and supercontinuum generation.
ACKNOWLEDGMENTSThis work was supported by the National Science Foundation of China (90201011). The authors thank Dr. F.O. Ilday from Cornell University, for his help. During the past decade, photonic crystals have received growing attention because of their novel scientific and potential engineering applications [1][2][3], such as narrowband filters, high-quality resonant cavities, laser reflectors, antenna substrates, strongly guiding devices, and so on. These artificial materials, which are periodic arrangements of discrete dielectric or metallic objects, have the very interesting nature of stop-bands in which any electromagnetic-wave propagation is forbidden. This property is crucial in the development of photonic-crystal devices applying to microwave, millimeter-wave, and optical systems. If we introduce a defect on a perfect photonic crystal, it is possible to create localized electromagnetic modes. Therefore, electromagnetic waves in a certain frequency band can locally be trapped inside the defect area. This important property can be widely used in various applications. The frequency response in reflectance, the frequency range, and the mode propagation have been developed using the cylindricalharmonic expansion method [4], the T-matrix and lattice sums' method [5][6][7], the Fourier series method [8 -10], the scattering matrix and mode-matching method [11], the finite-element method [12], the differential method [13], and time-domain techniques [14]. Among those approaches, the T-matrix and lattice sums' method is a rigorous and effective technique with a very simple formulation. In order to extend the scope of applications and to improve the computing accuracy, an improved T-matrix and lattice sums' method [15] was recently proposed, which successfully solved various periodic problems with complex structures.In this paper, we describe a very simple and effective formulation to analyze 2D photonic crystals with periodic defects. This method is an extension of the improved T-matrix and lattice sums' method [15]. The proposed formula is in S-matrix form and is defined by space harmonics, so that it is very suitable for multilayered structures. The problem of coupled-cavity waveguides, which consist of chains of defects in a perfect photonic crystal, is a very important issue, because they allow more freedom to engineer the dispersive properties of propagating modes than channel waveguides do [2,3,16]. For these problems, the improved T-matrix and lattice sums' method [15] is a very effective technique, but we propose another very simple formula for this kind of special structure, because the proposed method has a much simpler formula, much faster convergence, and much ...