A Fabry Perot based instrument for biomarkers detection

Observations of nearby terrestrial exoplanets atmospheres may reveal if those planets show evidence of bioactivity. Molecular oxygen, along with methane, has been identified as the leading biomarkers in the Earth atmosphere [1]. Our capability to detect O2 in planets outside of our solar system is limited even when considering the next generation of Extremely Large Telescopes (ELTs). Due to refraction effects in an exoplanet atmosphere, transmission spectroscopy, the preferred detection technique, does not probe the high pressure layers of the atmosphere close to the exoplanet surface [2,3]. Hence, the observed O2 transition lines in an exoplanet atmosphere are narrower than the equivalent telluric lines observed in the Earth atmosphere, and are partially unresolved even when observing at ℛ ≈ λ/δλ ~105), typical of modern high resolution spectrographs. The transition lines become fully resolved at ℛ~3 − 5 ∙ 105), at which detection becomes favorable [4,5]. Most modern high resolution spectrographs are echelle spectrographs. These instruments utilize echelle gratings with low line densities blazed for high diffraction orders. For echelle spectrographs on ELTs at seeing limited conditions, the collimator and echelle grating dimensions needed to achieve spectral resolutions in excess of ℛ~105) become impractical. While several techniques can increase the resolution obtained by an echelle spectrograph, none of them seems to offer the desired resolution increase for seeing limited observations at visible wavelengths [6,7]. We conclude that a novel approach for extremely high resolution spectroscopy with ELTs is in order. In this paper, we propose a concept instrument capable of extreme high resolutions on ELTs under seeing limited condition. The instrument is based on the interference properties of Fabry Perot Interferometers (FPIs). A single FPI of modest dimensions can achieve a resolving power well in excess of ℛ~10). In contrast to the high spectral resolving power, the sampling frequency of an FPI tends to be quite low in most practical cases. In addition, the interference orders of the FPI will overlap. Hence, a single etalon can not generate a continuous spectrum. We circumvent these problems by chaining several FPIs, each one with a slightly different thickness. The beam reflected from each FPI is feeding the next FPI in the chain, while the beam transmitted by each FPI is imaged onto the slit of an external spectrograph capable of separating interference orders. The thickness difference between FPIs in the chain dictates the instrument target resolution, while the absolute thickness of the FPIs in the chain dictates the resolution required from the external spectrograph to separate the overlapping interference orders. We begin our discussion with an analysis of an ideal FPI array instrument. We than present a practical design and discuss its expected performance when taking into account manufacturing and alignment errors. We conclude with a summary and future plans. Throughout the paper, we focus our discussion on the O2 A-band (λ~755 − 775nm), the favorable detection waveband for O2 using the transmission spectroscopy technique [4]. In addition, we assume the FPIs are fused silica etalons1. A detailed analysis of an FPI instrument in the context of molecular oxygen detection can be found in [5].