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Future high-contrast imaging spectroscopy with a large segmented telescopewill be able to detect atmospheric molecules of Earth-like planets around G- orK-type main-sequence stars. Increasing the number of target planets willrequire a coronagraph with a small inner working angle (IWA), and wide spectralbandwidth is required if we enhance a variety of detectable atmosphericmolecules. To satisfy these requirements, in this paper, we present acoronagraphic system that provides an IWA less than 1$\lambda_0 / D$ over amoderate wavelength band, where $\lambda_0$ is the design-center wavelength and$D$ denotes the full width of the rectangular aperture included in thetelescope aperture. A performance simulation shows that the proposed systemapproximately achieves a contrast below $10^{-10}$ at 1$\lambda_0 / D$ over thewavelengths of 650--750nm. In addition, this system has a core throughput$\geq$ 10\% at input separation angles of $\sim$ 0.7--1.4$\lambda_0 / D$; toreduce telescope time, we need prior information on the target's orbit by otherobservational methods to a precision higher than the width of the field ofview. For some types of aberration including tilt aberration, the proposedsystem has a sensitivity less than ever-proposed coronagraphs that have IWAs ofapproximately $1\lambda_0/D$. In future observations of Earth-like planets, theproposed coronagraphic system may serve as a supplementary coronagraphic systemdedicated to achieving an extremely small IWA.

A Coronagraph with a Sub-$\lambda/D$ IWA and a Moderate Spectral Bandwidth