High sensitive translational temperature measurement using characteristic curve of second harmonic signal in wavelength modulation spectroscopy

A high sensitive measurement system of translational temperature of plasma was developed. In this system, which is based on wavelength modulation spectroscopy, a peak value of second harmonic signal was measured as a function of modulation depth. The translational temperature was estimated by fitting the theoretically calculated curve to the measured characteristic curve. The performance of this system was examined using microwave discharge plasma. As a result of comparison with conventional laser absorption spectroscopy, both results show good agreement in the measurable region of the laser absorption spectroscopy. Next, the measurable limit of this system was investigated by decreasing the target number density. The detectable fractional absorption was as low as 3.7 × 10−5 in which condition the signal to noise ratio was the order of single digit at the averaging number of 40. This value is more than two orders of magnitude lower than that of the laser absorption spectroscopy.