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Based on a novel structure of waveguide, a broadband electroabsorption modulator (EAM) with low driving voltage and high extinction ratio has been demonstrated in this letter. The waveguide of InGaAsP-InP p-i-n layer structure is fabricated by two consecutive steps of selective undercut-wet-etching: 1)HCl : H3PO4 on p-InP (p- layer), and 2)H3PO4 : H2O2 : H2O on InGaAsP (active region), showing a wide ridge with a narrow undercut active region. Low capacitance and low cladding impedance can thus be simultaneously attained in such waveguides, leading to low microwave loss and high-speed electrooptical (EO) response. A ridge as wide as 8 mum with a 3-mum- wide active region and a 450-nm gap height in the undercut portion has been fabricated. A 350- mum -long waveguide of EAM is designed, revealing a high extinction ratio of > 30 dB (D.C.) and a modulation efficiency of > 20 dB/V (D.C.) with polarization-insensitive operation at a wavelength of 1550 nm. As high as 60 GHz of a 3-dB bandwidth is measured in the high-speed EO conversion. Calculations by an equivalent circuit model are quite fitted with the measurement, revealing that broadband performance is mainly attributed to the low microwave propagation loss in such waveguides.