A Compact Circular Multipass Cell for Millimeter-Wave/Terahertz Gas Spectroscopy

Millimeter-wave/terahertz (mmW/THz) spectros-copy is a promising tool for gas sensing applications, such as breath analysis or the detection of toxic chemicals, which require compact portable systems. Long gas cells that are needed for high sensitivities are often bulky and limit the portability of the system. We present a compact mmW/THz circular multipass gas cell with an optical path length of 1.9 m and an outer diameter of only 21.5 cm. The beam is refocused at each internal reflection as a consequence of the large divergence of the mmW/THz beams. We determined the losses of the cell to about 3 dB around 250 GHz and demonstrated sensitive gas spectroscopy with a detection limit of 14 ppm/√Hz for acetaldehyde in a mixture with methanol.