Sensitive detection of NO₂ using high-frequency heterodyne spectroscopy with a GaAlAs diode laser

Optical heterodyne spectroscopy has been performed by modulating the injection of a GaAlAs diode laser at 250 MHz. Sensitive and fast detection of NO2 was accomplished using phase-sensitive detection electronics. By proper adjustment of the local oscillator phase the heterodyne beat signal induced by NO2 absorption lines could be detected against zero background. A minimum NO2 absorption of 1×10-6 was measured with an effective bandwidth of 6 Hz. Increasing the detection bandwidth and scanning the diode laser very rapidly permitted detection of 5×10-4 absorption on a 100-ns time scale. The obtained results demonstrate that optical heterodyne spectroscopy with current modulated diode lasers offers the high sensitivity and fast time response required for the detection of gaseous species in pollution studies, combustion control, and industrial process monitoring.