The HORATIO project, funded by the German Federal Ministry of Education and Research (BMBF, project code 13N13178) and carried out from 2015 to 2018, aimed to create a compact, continuous‑wave terahertz (THz) sensor for the detection of toxic gases. The consortium comprised the Heinrich‑Hertz‑Institut (HHI) in Berlin, Toptica Photonics AG in Germany, Bruker Optik GmbH in Ettlingen, and the associated partner Analytic Task Force (ATF) of the Mannheim fire department. HHI and Toptica supplied the THz source and detector components, Bruker integrated the system into a demonstrator, and ATF performed the final field tests.
The technical core of the system is a cw‑THz transmitter and receiver based on photomixers driven by distributed‑feedback (DFB) lasers at 1.5 µm. The photomixers use InGaAs/InGaAsP for the transmitter and InGaAs/InAlAs for the receiver, enabling operation in the 1.1–1.3 THz band. The THz beam is directed through a 4‑meter optical path inside a gas cell that can be filled at low pressure (≈ 1 mbar). At this pressure the Doppler‑broadened absorption lines of target gases have widths of about 10 MHz, far narrower than the 10–100 GHz widths at atmospheric pressure. The cw‑THz source has an emission linewidth of only a few megahertz, allowing the system to resolve individual spectral lines with separations as small as 10 MHz and with less than 1 % overlap between lines.
During laboratory tests the system successfully detected five gases at the concentrations specified in the project brief: hydrogen chloride (HCl), ammonia (NH₃), hydrogen sulfide (H₂S), hydrogen cyanide (HCN) at 100 ppm each, and water vapor (H₂O) at 20 000 ppm. The detection limits reported in the conference paper presented at the 41st International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW‑THz) in 2016 were in the parts‑per‑million range, demonstrating the high sensitivity of the approach. A key innovation was a new numerical analysis method that significantly increased the effective spectral resolution, enabling reliable detection of gases at the low‑pressure conditions required for field deployment.
The demonstrator was built by Bruker and integrated into a VERTEX 80V vacuum spectrometer. It was designed for easy removal and insertion of the gas cell, allowing the system to be filled with sample gas on site and then re‑inserted into the demonstrator via a self‑centering mechanical interface. Field tests conducted by ATF in 2017 revealed challenges in transferring gas into the cell, which were addressed through mechanical redesign and improved sealing procedures.
Patents covering the improved numerical method and the mechanical integration of the gas cell are currently pending. The project’s results were disseminated through conference proceedings and journal articles, including a 2017 Optics Express paper by Vogt and Leonhardt that described a similar data‑processing technique.
In summary, HORATIO delivered a validated, high‑resolution cw‑THz sensor capable of detecting toxic gases at ppm levels under low‑pressure conditions, with a robust mechanical design suitable for deployment by emergency responders. The collaboration among academic, industrial, and fire‑department partners ensured that the system met both scientific and operational requirements within the three‑year project timeline.