Spectral terahertz imaging based on a quantum-cascade laser and a fast scanning mirror

Abstract

We demonstrate a transmission setup for fast THz spectral imaging based on a 3.4 THz continuous-wave multimode QCL, a fast scanning mirror (FSM), a gas cell, and a single pixel Ge:Ga detector. The THz radiation emitted by the QCL is focused on a THz transparent poly-4-fluorethylen (PTFE) tube located inside a gas cell that contains gaseous methanol. By rapid movement of the FSM, the beam is spirally scanned across the gas cell. The transmitted radiation is detected by the photoconductive Ge:Ga detector. At the same time, the QCL frequency is rapidly modulated by sweeping its current. The resulting field of view covers an elliptical area of 18.2x17.8 mm² dimension and is acquired within 1.5 s, including at least four spectra per pixel. Thus, we obtain information about the peak absorption and the line width. In addition to an imaging-only system which allows for fast screening of hidden objects, the present approach reveals the participating gases by spectral analysis. Furthermore, variations in the gas concentration can be recorded with a frame rate of up to 3 Hz since the measurement time can be further reduced by adjusting the field of view and the spectral range to the point of interest. This is demonstrated by using a tube with a hole of 0.2 mm diameter. During methanol injection, the inflowing gas is continuously imaged, and the leakage from the PTFE tube into the evacuated gas cell is observed. In addition to the relative absorption, the local pressure is mapped by exploiting the effect of pressure broadening. Both representations offer the unambiguous localization of the exiting gas. In summary, this system is a very promising approach for leakage detection and sensitive identification of hazardous substances.