Frequency-modulation spectroscopy provides ultrasensitive absorption measurements. The technique is especially adaptable to diode lasers, which can be modulated easily, and has been used extensively in the near-infrared and infrared spectral regions. The availability of blue diode lasers now means that the accessible wavelength region can be increased. We successfully demonstrate wavelength-modulation spectroscopy and two-tone frequency-modulation spectroscopy for the weak second resonance line of potassium at 404.8 nm and for the transition at 405.8 nm in lead, starting from the thermally populated 6<i><sub>p</sub></i><sup>2 3</sup><i>P</i><sub>2</sub> metastable level. Information on the modulation parameters is obtained with a fitting procedure. Experimental signal-to-noise ratios at different absorption levels are compared with theoretical signal-to-noise ratios and show good agreement. Detection sensitivities of 2 × 10<sup>−6</sup> and 5 × 10<sup>−6</sup> for wavelength and two-tone frequency-modulation spectroscopy, respectively, for a 120-Hz bandwidth are demonstrated.
© 2000 Optical Society of America
Ulf Gustafsson, Gabriel Somesfalean, Janis Alnis, and Sune Svanberg, "Frequency-Modulation Spectroscopy with Blue Diode Lasers," Appl. Opt. 39, 3774-3780 (2000)