title: Direct generation of 12.5-GHz-spaced optical frequency comb with ultrabroad coverage in near-infrared region by cascaded fiber configuration.
abstract: We generated a 12.5-GHz-spacing optical frequency comb that can be resolved over 100 THz, from 1040 to 1750 nm, without spectral mode filtering. To cover such a broad spectrum, we used electro-optic modulation of single frequency light and line-by-line pulse synthesis to produce a clear pulse train and subsequent spectral broadening in highly nonlinear fibers (HNLFs). We numerically and experimentally investigated a configuration of the HNLFs and find that a two-stage broadening through different HNLFs is required when using limited pulse energy at a high repetition rate. We designed and fabricated solid silica-based HNLFs with small zero-dispersion wavelengths to obtain strong spectral broadening, especially at the shorter wavelengths. The individual lines of the proposed frequency comb are resolvable with high contrast over the entire spectral range. The results described in this paper should lead to the development of multicarrier sources for wavelength-division-multiplexing communication and super-multi-point frequency calibration for spectrometers, especially in astrophysics.
authors.forename: Ken
authors.initials: K
authors.lastname: Kashiwagi
authors.forename: Takashi
authors.initials: T
authors.lastname: Kurokawa
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Okuyama
authors.forename: Takahiro
authors.initials: T
authors.lastname: Mori
authors.forename: Yosuke
authors.initials: Y
authors.lastname: Tanaka
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Yamamoto
authors.forename: Masaaki
authors.initials: M
authors.lastname: Hirano