A Compact Laser Absorption Spectroscopy Tomographic System With Short Spectral Scanning Time and Adjustable Frame Rate

Abstract

A compact hardware system for laser absorption spectroscopy tomography is introduced to monitor dynamic flame evolution with a short and fixed spectral scanning time and an adjustable frame rate. The system mainly consists of two parts, a pentagon sensor and a reconfigurable laser control and data acquisition hardware. In the sensor, 12 photodetectors are installed in each of the five sides of the pentagon. Fan-shaped laser beams from each vertex illuminate the opposite side and half of its two adjacent sides. Compared with the traditional illuminating scheme from each vertex to only the opposite side, this design reduces the sensor size and improves the spatial resolution. In the hardware, the distributed feedback lasers are integrated with the controller and modulated to generate a programmed waveform and implement spectral scanning. The spectral scanning time is short and fixed, and an adjustable frame rate is achieved by changing the interval, i.e., the waiting time, between two successive scans. The sampling rate of each channel can reach up to 20 MHz and is synchronized to the laser emission. A DDR2 memory is used for temporary data storage for four channels, enabling a full frame rate of up to 2 kHz. In the experiment, water vapor in a flat flame burner was chosen as the target absorbing species. Local integral absorbance and temperature distributions in the case of a moving steel ruler across the burner were reconstructed and agreed well with the observed distributions.