Andrew J. Benedick

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 ∼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 (∼1-Å) 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.

Highly stable ultrabroadband mid-IR optical parametric chirped-pulse amplifier optimized for superfluorescence suppression.

We present a 9 GW peak power, three-cycle, 2.2 microm optical parametric chirped-pulse amplification source with 1.5% rms energy and 150 mrad carrier envelope phase fluctuations. These characteristics, in addition to excellent beam, wavefront, and pulse quality, make the source suitable for long-wavelength-driven high-harmonic generation. High stability is achieved by careful optimization of superfluorescence suppression, enabling energy scaling.

Visible wavelength astro-comb

We demonstrate a tunable laser frequency comb operating near 420 nm with mode spacing of 20-50 GHz, usable bandwidth of 15 nm and output power per line of ~20 nW. Using the TRES spectrograph at the Fred Lawrence Whipple Observatory, we characterize this system to an accuracy below 1m/s, suitable for calibrating high-resolution astrophysical spectrographs used, e.g., in exoplanet studies.

Nonintrusive phase stabilization of sub-two-cycle pulses from a prismless octave-spanning Ti:sapphire laser.

Carrier-envelope (CE) phase-stabilized sub-two-cycle pulses are generated from a 500 MHz compact prismless octave-spanning laser without extracavity nonlinear optical processes distorting the laser output. The necessary f and 2f spectral components are generated intracavity and coupled out independently from the main pulse through specially designed cavity mirrors, resulting in a 55 dB CE beat note (100 kHz resolution bandwidth). The in-loop CE phase error (integrated from 2.5 mHz to 10 MHz) is 67 mrad, equivalent to a timing jitter between carrier and envelope of 28 as at 790 nm.

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.

Dynamics of Dispersion Managed Octave-Spanning Titanium:Sapphire Lasers

An extensive one-dimensional laser model based on dispersion managed mode locking is presented that accurately describes the pulse dynamics of octave-spanning titanium:sapphire lasers generating sub-two-cycle pulses. By including detailed characteristics for the intracavity elements (mirrors and output coupler), it is demonstrated that the spectral output and temporal pulse shape of these lasers can be predicted quantitatively in very good agreement with experimental results.

Broadband dispersion-free optical cavities based on zero group delay dispersion mirror sets

A broadband dispersion-free optical cavity using a zero group delay dispersion (zero-GDD) mirror set is demonstrated. In general zero-GDD mirror sets consist of two or more mirrors with opposite group delay dispersion (GDD), that when used together, form an optical cavity with vanishing dispersion over an enhanced bandwidth in comparison with traditional low GDD mirrors. More specifically, in this paper, we show a realization of such a two-mirror cavity, where the mirrors show opposite GDD and simultaneously a mirror reflectivity of 99.2% over 100 nm bandwidth (480 nm - 580 nm).

Octave-spanning, dual-output 2.166 GHz Ti:sapphire laser

A self-referenced octave-spanning Ti:sapphire laser with 2.166 GHz repetition rate is demonstrated. The laser features both direct generation of octave-spanning spectra and a dual-output design for non-intrusive carrier-envelope (CE) phase-stabilization. Only a few percent of total power containing 1f and 2f spectral components is coupled out through a specially designed laser mirror and generates a >50 dB CE beat note in 100 kHz resolution bandwidth without perturbing the main output that still delivers octave-spanning spectra and 750 mW of output power.

Diode-pumped, high-average-power femtosecond Cr3+ :LiCAF laser.

We demonstrate a high-average-power continuous wave (cw) and cw mode-locked Cr3+ :LiCAF laser pumped by broad-area laser diodes. In cw lasing experiments, up to 580 mW of output was obtained with 4.35 W of incident pump. A semiconductor saturable absorber mirror was used to initiate stable, self-starting, mode locking. In the cw mode-locked regime, the Cr3+ :LiCAF laser produced nearly transform-limited, 67 fs long pulses near 800 nm with an average output power of 300 mW. The pulse repetition rate was 120 MHz, with a pulse energy of 2.5 nJ.

Octave Spanning 1 GHz Ti:sapphire Oscillator For HeNe CH4-based Frequency Combs and Clocks

In this paper, we demonstrate a greatly improved octave spanning 1 GHz Ti:sapphire laser (Fortier, 2006) using the most broadband double-chirped mirror pairs, optimized Kerr-Lens modelocking (KLM) and an optimized output coupler. As a result the laser generates, at 9 W of pump power, 0.6 W-1 W of output power with an output spectrum of more than one octave as measured on a linear scale. The spectrum corresponds to a Fourier limited pulse of 3.5 fs duration. Second harmonic generation with this output in 1mm BBO directly generates 1f-2f beatnotes for carrier-envelop phase stabilization with >55 dB signal-to-noise (SNR) in 100 kHz bandwidth, and by difference frequency generation (DFG) in a 5 mm long PPLN radiation at 3.39 mum is generated. The 3.39 mum radiation is strong enough to result in a beatnote with a single frequency HeNe reference laser of 30 dB. This laser will serve as the clockwork of a HeNe CH4-based molecular clock (Foreman, 2005) with a measured Allan variance approaching 10~14 in 100 s and as an absolute femtosecond laser frequency comb for an optical arbitrary waveform generator.