Studies on Enhancement of the Fringe Visibility in Femtosecond Laser Interferometry

Abstract

A broadband femtosecond laser interferometry is widely used for ultrafast dynamics studies of laser-matter interactions. However, it is well known that fringe patterns of the broadband laser interferometry are blurrier than those of the narrowband case. So it is not easy to employ the broadband laser interferometry for fast dynamics studies. In this research, therefore, we studied a new method to improve the blurry fringe problem in the femtosecond laser interferometry. To enhance the visibility of the blurry interference patterns, the bandwidth of the probe beam in the interferometer was controlled by using narrowband pass filters in this research. This technique was applied for diagnostics of the laser-produced plasma in the air. For our research, we employed the pump-probe interferometry for air-plasma diagnostics. In the experiment, a Mach-Zehnder interferometer was used with various bandwidth filters and their differences were studied. Here, the Ti:sapphire regenerative amplifier laser with an energy of 3.3 mJ/pulse, a pulse duration of 34 fs, and a repetition rate of 1 kHz was used. The laser pulse was divided into two beams: the main beam with an energy of 2.3 mJ was focused in an air to produce a plasma and the other beam was sent to the Mach-Zehnder interferometer for plasma diagnostics. We found that the interference fringe with a narrower band-pass filter has a higher visibility and it can give a more accurate plasma density information. The proposed method in this research is simple, but it was demonstrated to be a very useful tool for better plasma diagnostics using the interferometry.