Knowledge Bank

Various registration methods for ultrasensitive diode laser based absorption spectroscopy are discussed. The sensitivity of a single-beam wavelength modulation technique is limited by excess laser noise and wavelength-dependent effects in optical system. A dual-beam logarithmic detection allows to reduce excess laser noises but actually shot-noise level is not usually achieved and sensitivity is limited by low-frequency transmittance fluctuations and a flicker noise from atomizer. Simultaneous using a double-beam logarithmic detection and a wavelength-modulation technique allows to minimize a low-frequency noise contribution. Absorbance of $7\times 108$/(Hz)$\mathrm{<mrow\; data-mjx-texclass="ORD">1<mrow\; data-mjx-texclass="ORD">/</mrow>2</mrow>}$ in such a system is demonstrated. Applications to trace measurements of chlorine in glow-discharge atomizer and lithium in graphite tube atomizer are described.