Noise Spectroscopy of K Atoms with a Diode Laser

Abstract

There has been much interest in the influence of the laser noise on the atomic system. So far theoretical works have been performed on calculation of the mean values and the variance of the fluctuated atomic populations, and verified through observation of the fluctuating resonance fluorescence from the atomic vapors. Yabuzaki et al. have observed the excess intensity noise of the resonant light from a diode laser propagated through alkali atomic vapor. They have measured the intensity power spectrum of the light when the laser was tuned near the D2 line of Cs and observed all corresponding hyperfine-splitting resonances in the excited state and Zeeman splitting resonances within a hyperfine level of the ground state. Walser et al. have theoretically studied the intensity fluctuation of laser light propagated through a weakly absorbing medium. The calculated power spectrum of a phase-diffusing field has been found qualitative agreement with the experimental results by Yabuzaki et al. on hyperfine-splitting resonances. Recently, Mitsui has reported a new type of noise spectroscopy with a highly stabilized diode laser. He has investigated the spontaneous noise appeared in optical detection of magnetic resonance of Rb atoms. In this note we report the first observation of hyperfine spectra in the ground state of K and K appearing as the intensity fluctuation at hyperfine splittings of 462MHz and 254MHz of circularly polarized light from a diode laser transmitted through a sample cell. We have also observed the Zeeman hyperfine spectra of K in a static magnetic field. We show that the spectra are strongly dependent of the direction of the static magnetic field. Their splittings as functions of the strength of static magnetic field are compared with the Breit–Rabi formula. The experimental apparatus is quite simple as shown in Fig. 1. The diode laser used was an ordinary Fabry–Perot type with output power of 5mW, operating near the D1 line (769.9 nm) of K atoms. The output of the laser was circularly polarized and applied to a cell containing K vapor at a temperature 90 C, corresponding atomic density to be about 1:15 10 cm 3. The transmitted light was detected by an avalanche photo-diode having frequency response up to 1GHz. The output of the photo-diode was applied to a rf frequency analyzer to obtain the power spectrum of the fluctuation of the transmitted light intensity. A static magnetic field is provided by two pairs of Helmholtz coils mutually perpendicular and about 90 cm in diameter. The strength of the magnetic field is calibrated by using magnetic resonance signals of optically pumped Rb atoms. Figure 2 shows the frequency spectrum of the intensity fluctuation of transmitted light observed when the laser was tuned to the transition from the state F 1⁄4 1 in the ground state to the state F0 1⁄4 1 in the excited state by adjusting the driving current of the laser diode. Sharp resonance signals can be seen at the hyperfine splittings of K and K, 462 and 254MHz, respectively, on the background of broad noise spectrum. When a weak static magnetic field H0 is applied to the K vapor, the hyperfine spectra of K splits into several lines (the effective gyro-magnetic ratio of K is known to be 1⁄4 2 700 kHz/G). Figures 3(a) and 3(b) show the observed Zeeman hyperfine spectra of K for