Mid-infrared supercontinuum generation spanning 1.8 octaves using step-index indium fluoride fiber pumped by a femtosecond fiber laser near 2 µm.

A nearly two-octave wide coherent mid-infrared supercontinuum is demonstrated in a dispersion-engineered step-index indium fluoride fiber pumped near 2 µm. The pump source is an all-fiber femtosecond laser with 100 fs pulse width, 570 mW average power and 50 MHz repetition rate. The supercontinuum spectrum spans from 1.25 µm to 4.6 µm. Numerical modelling of the supercontinuum spectra show good agreement with the measurements. The coherence of the supercontinuum is calculated using a numerical model and shows a high degree of coherence across the generated bandwidth allowing it to be used for frequency comb applications.

In-situ determination of astro-comb calibrator lines to better than 10 cm s(-1).

Improved wavelength calibrators for high-resolution astrophysical spectrographs will be essential for precision radial velocity (RV) detection of Earth-like exoplanets and direct observation of cosmological deceleration. The astro-comb is a combination of an octave-spanning femtosecond laser frequency comb and a Fabry-Pérot cavity used to achieve calibrator line spacings that can be resolved by an astrophysical spectrograph. Systematic spectral shifts associated with the cavity can be 0.1-1 MHz, corresponding to RV errors of 10-100 cm/s, due to the dispersive properties of the cavity mirrors over broad spectral widths. Although these systematic shifts are very stable, their correction is crucial to high accuracy astrophysical spectroscopy. Here, we demonstrate an in-situ technique to determine the systematic shifts of astro-comb lines due to finite Fabry-Pérot cavity dispersion. The technique is practical for implementation at a telescope-based spectrograph to enable wavelength calibration accuracy better than 10 cm/s.

Generation of low-timing-jitter femtosecond pulse trains with 2 GHz repetition rate via external repetition rate multiplication.

Generation of low-timing-jitter 150 fs pulse trains at 1560 nm with 2 GHz repetition rate is demonstrated by locking a 200 MHz fundamental polarization additive-pulse mode-locked erbium fiber laser to high-finesse external Fabry-Perot cavities. The timing jitter and relative intensity noise of the repetition-rate multiplied pulse train are investigated.

A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s(-1).

Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s(-1) (ref. 1), which is sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. To find a 1-Earth-mass planet in an Earth-like orbit, a precision of approximately 5 cm s(-1) is necessary. The combination of a laser frequency comb with a Fabry-Pérot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration, with recent encouraging results. Here we report the fabrication of such a filtered laser comb with up to 40-GHz (approximately 1-A) line spacing, generated from a 1-GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb, or 'astro-comb', is well matched to the resolving power of high-resolution astrophysical spectrographs. The astro-comb should allow a precision as high as 1 cm s(-1) in astronomical radial velocity measurements.