Lasers and fiber optics for astrophysics

Abstract

Optical spectroscopy has been and remains an important feature of astrophysical research. Huge telescopes are being built to collect radiation from the most distant stars in the Universe for subsequent study with unique astronomical spectrographs. The fundamental problem to be faced here is that of transferring the extremely weak radiation focused by a moving telescope to the entrance of a stationary spectrograph. The solution is to connect a telescope to a spectrograph through an optical fiber, and research and development of the system’s necessary components is currently a major task in the field of fiber optics. A key problem in astronomical spectroscopy is the precision measurement of Doppler line shifts in the spectra of stars to determine the velocity of a star movement along the observation line (ray velocity, RV). A remarkable feature of Doppler spectroscopy is that a precision measurement of quite small RV variations (in fact, accelerations) can last for long periods of time. Such star RV variations can be due, for example, to a planet orbiting the star; the action of the planet causes periodical changes in the motion of the star, thus leading to a Doppler shift of the star’s spectrum. The precise measurement of this shift has provided an indirect method for searching for and discovering planets outside the Solar System (exoplanets). The important particular problem of searching for habitable earthlike exoplanets requires a spectral measurement accuracy sufficient to detect RV variations at the level of a few centimeters per second per year. Ten to fifteen years of such measurements would provide a direct estimate of the hypothetical accelerated expansion of the Universe. However, the accuracy required for this is more than conventional spectroscopy techniques (iodine cell and spectral lamps) are capable of. This paper reviews approaches to radically improving Doppler spectroscopy techniques to achieve the required shift measurement accuracy. These approaches include the development of fiber-optic systems for connecting the telescope with the astronomical spectrograph and the development of precision calibrators of astronomical spectrographs based on the advances in laser physics and fiber optics.