Double heterojunction semiconductor diode lasers operating in an external cavity can be actively and passively mode-locked to produce continuous trains of picosecond pulses of around 1 W peak power. These lasers, which are also frequency tunable, provide a convenient and cheap source of coherent ultra-short laser pulses for timedomain spectroscopy of semiconductor and molecular materials. With developments in laser diode processing techniques the spectral range could be extended to cover the visible to near i.r. region (up to 4 pm). Intracavity spectroscopy, with increased sensitivity, is also possible, particularly for the study of semiconductor carrier dynamics. Other applications include the study of coherent pulse propagation, two-photon spectroscopy and high-resolution spectroscopy when the diode lasers are operated in a single mode.

Valuable achievements in differential optical absorption spectroscopy (DOAS) for monitoring atmospheric trace gas have been made in the past decades. However, in flue gas monitoring, it is difficult to monitor the pollutant gases at low optical density. In this paper, two improved inversion algorithms based on the genetic algorithm and Kalman filter, respectively, were developed to accurately calculate the concentrations of SO 2 and NO 2 from flue gas. Experimental results show that the improved algorithms have the detection limit of 10 ppm and have a zero drift of 1 ppm at a light path length of 0.5 m. Additionally, the improved algorithms of DOAS combined with the traditional DOAS algorithm can broaden the measurement range of pollutant gases by appropriately setting critical values for the maximum value of the differential optical density.