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

Abstract

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.

Visible wavelength astro-comb

“…1. A recently developed scanned cavity technique [16] can be used to identify and remove these systematic shifts from the final calibration which is critical since any dielectric mirror coating will contain both small errors and a systematic divergence of the dispersion at the edges of its working range.…”

“…Systematics of this calibration system including dispersion, higher order transverse modes of the cavity filter and alignment of the frequency comb and cavity filter transmission resonances have been characterized. Using a scanned-cavity technique [16], the accuracy of the astro-comb system has been compared to the underlying laser frequency comb. This type of broadband characterization of an astro-comb system is necessary to ensure that the optical filtering process has not left unevenly suppressed source comb lines on either side of each astro-comb line.…”

Conjugate Fabry–Perot cavity pair for improved astro-comb accuracy

“…Several prototype astro-combs have recently been developed and tested with astrophysical spectrographs [5][6][7][8][9]11]. These tests have shown that the mode-filtering systems must be improved to provide adequate suppression of unwanted spectral lines ("side modes") in the presence of nonlinear processes in optical fibers.…”

“…where f A is the astro-comb line frequency, δf A is the shift of the astro-comb line as measured on the spectrograph, I k is the intensity of the kth side mode (the source comb line k repetition frequencies away from the astro-comb line), and c is the speed of light [11,12]. To calibrate the spectrograph with an accuracy of 100 kHz with an astrocomb, the intensity of the side modes must be suppressed or controlled to <10 −4 of the astro-comb peak intensity [1,11].…”

“…To calibrate the spectrograph with an accuracy of 100 kHz with an astrocomb, the intensity of the side modes must be suppressed or controlled to <10 −4 of the astro-comb peak intensity [1,11]. This suppression is nominally achievable with FPCs of moderate finesse, but can be significantly degraded in the presence of nonlinear optical elements, as discussed below.…”