Modified window function for optically thick samples measured by a terahertz time-domain spectroscopic system using a multimode laser diode

Abstract

A low-cost type terahertz time-domain spectroscopic system can be constituted using a multimode laser diode instead of a pulse laser. To suppress noise, a window function is usually used in the Fourier transformation. When this system is used to measure an optically thick sample, the obtained refractive index shows sinusoidal structures. This is caused by ingress of a signal fragment into the measured range and egress of another signal fragment out of the measured range. In addition, a broad positive background component appears in the imaginary part of the refractive index. This is because of the decreased amplitude of the central structure of the signal, which results from the time shift under the window function caused by sample insertion. These false structures can be eliminated by using a modified window function that is shifted with the signal when the sample is inserted.