Frequency modulation upon nonstationary heating of the p–n junction in high-sensitive diode laser spectroscopy

A special type of modulation of the injection current of a diode laser is proposed at which the frequency modulation of radiation is not accompanied by the residual amplitude modulation. This method considerably reduces the influence of the diode laser radiation instability on the recorded absorption spectra. This allows a prolonged monitoring of small amounts of impurities in gas analysis by retaining a high sensitivity. Prolonged measurements of absorption spectra are performed at a relative absorption of 8×10^-7. By using a 50-cm multipass cell with the optical length of 90 m, the absorption coefficient of 1.2×10^-10 cm^-1 was detected. As an example, the day evolution of the background concentrations of NO₂ molecules was measured in the atmosphere.