Fourier Transform Spectrometer For THz Applications

Terahertz (THz) technology has been developing very fast in recent two decades. It has broad applicable prospects in physics, biomedicine, material sciences, non-destructive testing, communication and military. High sensitivity THz detectors and stable THz sources are the important part of terahertz technology. To evaluate the performance of detectors and source, we built a Fourier transform spectrometer for terahertz applications in this paper. The system can measure the frequency of up to30THz, with the highest resolution being750MHz and the working frequency ranging from0.3THz to1.5THz.In this paper, the main work is as follow:(1)Using superconducting Niobium Nitride (NbN) Hot Electron Bolometer (HEB) mixer as the most important components of a spectrometer-detector, we conduct a large number of experiments to obtain the intermediate frequency (IF) characteristics of superconducting NbN HEB mixer by the system. Moreover, we also measured the system response using multiple local oscillation (LO) frequencies, got the response of the HEB chip as a function of the frequency of local oscillator, compared the results with the calculated values and analysed the reason of measurement error which is caused by the tilt of moving mirror in scanning. We analyzed the causes of error and conducted the simulation with matlab.(2)In order to minimize the error occurred in measurement, we improved the performance of the system by introducing automatic alignment system which used piezoelectric ceramic brake and laser to guarantee the stability of FTS moving mirror. We gave the algorithm of the automatic alignment system and built it.(3)The improved system was subsequently tested many times using a solid terahertz source and the HEB detetor. The improved system are much more stable and have higher signal to noise ratio (SNR) after such an introduction by comparing the measured results after and before.