laser cavity. The FBG F-P etalon discriminates and selects the laser longitudinal modes efficiently. The spatial hole burning effect is restrained by using fiber Faraday rotator. The output power is more than 50 mW and slope efficiency is 27%. The linewidth of the fiber laser is less than 10 kHz. The temperature tuning results indicate the laser exhibits good stability. The fiber laser has a number of potential applications for high resolution fiber sensor. REFERENCES 1. M. Horowita, R. Daisy, and B. Fischer, et al, Narrow-linewidth, single-mode Er-doped fibre laser with intracavity wave mixing in saturable absorber, Electron Lett 30 (1994), 648 – 649. 2. Y. Cheng, J.T. Kringlebotn, and W.H. Loh, et al, Stable single- frequency traveling-wave loop laser with integral saturable absorber- based tracking narrow-band filter, Opt Lett 20 (1995), 875– 877. 3. S. Huang, Y. Feng, and J. Dong, et al, 1083 nm single frequency ytterbium doped fiber laser, Laser Phys Lett 2 (2005), 498 –501. 4. 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Kaivola, Laser linewidth measurements using self-homodyne detection with short delay, Opt Commun 155(1998), 180 –186. © 2007 Wiley Periodicals, Inc. A BAND-PASS FILTER USING VIA- HOLE-WALL CAVITY Ruey Bing Hwang and Jun Liang Pan Department of Communication Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China Received 23 October 2006 ABSTRACT: In this paper, we presented a band-pass filter consisting of a post-wall cavity resonator. This band-pass filter was fabricated in a two-layered printed circuit board, which includes an input and an out- put substrate integrated waveguides as the feeding structures in the top layer and a cavity in the bottom layer. The interconnection of the signal between different layers is achieved via the apertures etched on the in- put and output waveguides and cavity. We found the bandwidth of such a class of band-pass filter could be altered by tuning the length of the coupling apertures. Besides, the pass-band frequency can be estimated by the resonant frequencies of the cavity. We have fabricated the band- pass filter on a low-loss dielectric substrate and measured its S-parame- ters, including the return- and insertion- loss. In addition, the numerical simulation by the CST microwave studio, a full-wave time-domain numeri- cal method, was also carried out. A good agreement between the measured and simulation results was obtained. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1456–1459, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop. 22438 Key words: via-hole-array waveguide; periodic structures; band-pass filter 1. INTRODUCTION Recently, the via-hole-wall (or post-wall) technique [1– 8], or the substrate integrated waveguide technique [1– 4], was developed to fabricate an equivalent rectangular metallic waveguide or cavity on a PCB (printed circuit board) with low-loss dielectric substrate. Such a class of waveguides has been proved [2] to be able to preserve the well-known advantages of commonly used closed rectangular waveguide, such as high-Q factor. Besides, the sub- strate integrated waveguide is based on PCB fabrication process; therefore, it is easy for being integrated with the micro-strip, coplanar waveguide, or the other planar circuits, to design a microwave/mm wave sub-system. In addition to the substrate integrated waveguide, the cavity based on the via-hole-wall tech- nology was also well developed [3]. The novel substrate integrated waveguide cavity filter with defected ground structure was inves- tigated to provide a high stop-band rejection and low insertion loss. The V-band 3-D multilayer cavity resonators and three-pole (three coupling cavities) band-pass filters using slot excitation with quar- ter guided-wave wavelength open stub was demonstrated [9]. In this paper, we developed a band-pass filter using a cavity resonator with two coupling apertures. The cavity resonator was fabricated using via-hole array to approximate the metal wall. As was well known, when the pitch between two via holes is small enough, the via-hole array can act as a metallic wall to reflect the incident wave. Therefore, the electromagnetic field energy can be preserved within the cavity. Furthermore, the cavity feeding struc- ture is a substrate integrated waveguide connecting with a taper micro-strip transition. The input/output substrate integrated waveguide is on the top of the cavity with their apertures overlap- ping to each other. Thus, the electromagnetic field coupling is taking place between the substrate integrated waveguide and the cavity resonator. Since the band-pass filter is designed based on the cavity resonator, the pass-band frequency can be roughly estimated by its Figure 7 Wavelength tuning characteristics of the fiber laser 1456 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 6, June 2007 DOI 10.1002/mop