Abstract

Intracavity absorption in a multimode laser is one of the most sensitive spectroscopic techniques [1], in particular when a long cw dye laser is used [2]. Although the fundamental limitation of the sensitivity is due to spontaneous emission, usually the factual limitation results from nonlinear redistribution of light among the modes. It is characterized by the rate of perturbing the photon occupation of a particular mode which is X = 5 × 10⁶ s⁻¹ with an available dye laser. The corresponding minimum constant of absorption is k= 3 × 10⁻¹⁰ cm⁻¹ when the signal-noise ratio S/n ≥ 10 is admitted. When applied to a strong atomic line, say, one of the D-lines of Na, c. 50 atoms are detectable in an atomic vapour at room temperature. This number decreases with the Doppler line broadening being reduced. Thus it seems appropriate to consider the absorptive detection of a single trapped and laser-cooled ion, Ba', e.g. [3]. So far, the alternative technique of modulation spectroscopy has been capable to detect some 60 Ba' ions with signal/noise ≃ I [3], and a single Hg‘ with slightly better S/n (4].

© 1996 IEEE