Real time imaging analysis using a terahertz quantum cascade laser and a microbolometer focal plane array

Abstract

It is widely published that the terahertz (THz) spectral range has potential for imaging in the fields of military and security applications. The Sensors Research Laboratory previously achieved real-time imaging of concealed objects using a 1mW quantum cascade laser (QCL) and an uncooled vanadium oxide/silicon nitride based microbolometer. This thesis introduces an amorphous silicon based microbolometer with improved NETD in the 8-12 micrometer infrared spectral range. The QCL is usually operated in pulsed mode with rate in the hundreds of kHz which is much higher than the cut-off frequency of microbolometers of about tens of Hz. This indicates that neither camera should be able to detect the individual pulses of the THz beam. A detailed analysis showed that microbolometers can only detect the average power. Earlier experiments were then reproduced using the amorphous silicon based camera to assess the image quality but found it to be inferior to the silicon nitride based camera. These observations indicate that the absorption of THz in amorphous silicon is much weaker than silicon nitride. Other materials used to conceal military assets were analyzed and imaged to prove in principal the possibility of active THz imaging detection at a distance in narrow atmospheric windows.