1 Introduction

Single-mode fibers are useful, in the focal plane of seeing limited telescopes, for their spatial filtering capability. The waveguides transform wavefront phase corrugations into intensity fluctuations, thus providing a stable, well-defined output stellar beam with a minimum étendue ($\lambda^2$). This property can be used in high resolution spectroscopy (Ge et al. 1998), or to combine high angular and spectral resolution behind adaptive optics, and finally in stellar interferometry (Foresto et al. 1997). The spatial filtering capability, however, comes at a price: from the incoherent wavefront, only part of the light can be coupled into the single-mode waveguide. Thus a key issue with single-mode fibers is the efficiency of their coupling to starlight. Point source to fiber coupling theory is reviewed and illustrated in Sect. 2, for two cases: The wavefront is diffraction (Sect. 2.1) and seeing (Sect. 2.2) limited. It is shown how the coupling coefficient is linked to the instantaneous Strehl ratio. Thus, a single-mode fiber associated to a photometer can be used as a "fast Strehlmeter'' to monitor the rapid image quality fluctuations of a dynamic optical system. This is illustrated by experimental data obtained in the K band (2.0$\mu$m $\, \leq \lambda \leq 2.4\,\mu$m) with a 3.6 m telescope corrected by the ADONIS adaptive optics system (Sect. 3).