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tunability key, is loaded at each open-ended stub. Figure 12 illustrates a PCB of the resonator (Type D). The measured resonance characteristics of the PCB are provided in Figure 13. As the results of Figures 11 and 13 indicate, the measured results are in good accordance with the calculated results. CONCLUSIONWe have examined the behavior of the dominant resonant frequency and location of the attenuation poles of the tunable halfwavelength resonators. In this study, the calculation and experiment of the tunable end-coupling resonator and three types of tunable tapped resonators have been carried out. It has been confirmed that the proposed tunable resonators are essential for realizing tunable characteristics for the attenuation pole and the dominant resonant frequency.In these investigations, the characteristics of the overall resonators have been calculated using a circuit simulator under lossless conditions and ideal lumped elements, as well as the discontinuity effect. We plan to measure the proposed resonators' Q and estimate the resonator characteristics, including various losses such as each capacitor used, the varactor, and the transmission lines. Also, we plan to apply these resonators to tunable filters. ACKNOWLEDGMENTThis work was partially supported by a Grant-in-Aid for the 21 st Century COE Program from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japanese Government (Aoyama Gakuin University, Japan). and polarization-hole burning effects [5], and utilizing frequency shifters [6, 7]. Nevertheless, most of the above approaches are either inconvenient or costly for practical applications. In this paper, by utilizing the polarization-hole burning effect induced by intracavity birefringence, an L-band dual-wavelength erbium-doped fiber laser (EDFL) is presented and the operation principle of this EDFL is analyzed in detail. Moreover, by repeatedly scanning the laser spectrum, we find that this dual-wavelength laser has good stability at room temperature. Figure 1 shows the configuration of our L-band dual-wavelength EDFL. About 17.8-m of EDF with cutoff wavelength of 960 nm and an absorption coefficient of 15.2 dB/m at 979 nm serves as the gain medium. A 980-nm laser diode (LD) with pump power of 96.5 mW is used as the pump source. A section of high birefringence (HiBi) fiber with beat length of 3.1 mm at 1550 nm is combined with a fiber polarizer to constitute the fundamental elements of a Lyot filter. To minimize the high intracavity loss caused by the HiBi fiber and the polarizer, two 3-dB fiber-loop mirrors are placed on either side of the EDFL, serving as the cavity reflectors. The reflectivity of a 3-dB fiber-loop mirror (FLM) is theoretically nearly 100%. Factually, however, the coupling ratio of a 3-dB coupler is not vigorously 50% and the coupler has also inherent loss. Therefore, the 3-dB FLM can be used as a reflector with high reflectivity. In our experiment, reflectivity of the output FLM and the one splicing with the signal port of wavelength division...

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[2,5]. The advantage of the SA techniques is the ability to provide variations to the CLS method for differing sidelobe topographies. The edge brightening that appeared in the amplitude distributions obtained using the SA approach can be reduced by adding a corresponding term in the cost function of Eq. (3), thus penalizing that behavior. Nevertheless, this is of concern with regard to the implementation of other kinds of excitation distributions analogous to Taylor's (such as, for example, the one in a work elaborated upon by Rhodes [6]). In that case, the amplitude distributions behave very similarly to those obtained using the CLS implementation, with a central segment and flanking monotonic decaying curves, as was verified by the authors via inspection of some the results not shown here for brevity. As straightforward extensions of the methods, circular continuous sources and -symmetrical arrays are under consideration by some of the authors in a recent paper [7]. ACKNOWLEDGMENTSThis work was supported in part by the National Science Foundation NSF Grant ECS-0098547 and by the Spanish Ministry of Science and Technology under project no. TIC2002-04084-C03-02. As the natural extension for increasing the transmission capacity of a wavelength division multiplexing (WDM) system, long-wavelength-band erbium-doped fiber amplifiers (L-band EDFAs) has become the subject of extensive studies [1,2]. Nevertheless, since the L-band is far from the gain peak of silica-based erbium-doped fiber (EDF), the gain coefficient of the L-band is much lower than that of the conventional C-band by a factor of 10, approximately [3]. In recent years, much effort has been made to enhance the amplification characteristics in the L-band [4 -8]. One of the most promising amplifier schemes to realize high gain in the L-band is the double-pass configuration [6]. The double-pass L-band EDFA provides significant enhancement to the amplifier gain. And a reflective L-band EDFA with a simpler and economized structure was also proposed in [8]. However, in such a scheme the amplified spontaneous emission (ASE) is reflected back into the EDF together with the signal light, which leads to relatively high noise figure. GAIN-AND NOISE-FIGURE IMPROVEMENT OF A REFLECTIVE L-BAND EDFA BY USING A SHORT LENGTH OF PREAMPLIFICATION EDFIn this paper, a reflective L-band EDFA with improved noise figure is achieved by utilizing a section of forward-pumped shortlength EDF to increase the Er 3ϩ inversion level at the input part of the amplifier. The experimental results show that the L-band EDFA with preamplification EDF has much higher gain and lower noise figure than the conventional design without the first stage. Moreover, the saturation output power under different prestage pump powers is also investigated. CONFIGURATION AND OPERATION PRINCIPLEThe basic configuration of our novel L-band EDFA structure is shown in Figure 1. This amplifier is composed of two stages. In the first stage, a 980-nm laser diode (LD) is used as the pump source and EDF1 has an absor...