down spectroscopy, wavelength modulation spectroscopy, off-axis integrated cavity output spectroscopy (OA-ICOS) [10,11] and photoacoustic spectroscopy [12,13]. For these spectroscopic methods specificity, costs and complexity depend highly on the used laser wavelength. For probing molecular gas species in mid-infrared molecular fingerprint region (2-20 μm) continuous wave quantum cascade lasers (QCL) [14] and interband cascade lasers [15] are mostly used, next to non-linear generated light via different frequency generation (DFG) [16], or optical parametric oscillators (OPO) [17].In the near-infrared wavelength region, compact diode lasers are designed to operate efficiently around the 1.5 μm wavelength region for telecommunications purposes, as optical losses in fibers are minimal at this wavelength. In recent years, an advance and sensitive optical spectrometer based on a near-infrared laser and cavity ring down spectroscopy (CRDS) has been successfully developed and used to measure HCN below the ppbv (part per billion volume, 1:10 9 ) level [18,19]. In the present study, a widely tunable telecommunication laser is used in combination with off-axis integrated cavity output spectroscopy (OA-ICOS) for trace gas detection. Off-axis-ICOS offers a compact, robust setup with fast detection and lower costs, but at similar sensitivity performance levels as compared to CRDS. As such, these features allow this to be considered as an easy-to-implement solution for real-world applications.Here, we will use the system for detection of HCN in exhaled breath. In vitro studies have shown that cultures of Pseudomonas aeruginosa bacteria produce the toxic hydrogen cyanide (HCN) [20,21]. Furthermore, HCN has been detected in sputum of P. aeruginosa infected cystic fibrosis (CF) patients [22,23]. In vivo studies using selected ion flow tube mass spectrometry (SIFT-MS) report that HCN can be identified as a biomarker for P. aeruginosa infection Abstract A compact, cost-effective sensor is developed for detection of hydrogen cyanide (HCN) in exhaled breath within seconds. For this, an off-axis integrated cavity output spectroscopy setup is combined with a widely tunable compact near-infrared laser (tunability 1527-1564 nm). For HCN a detection sensitivity has been obtained of 8 ppbv in nitrogen (within 1 s), equal to a noise equivalent absorption sensitivity of 1.9 × 10 −9 cm −1 Hz −1/2 . With this sensor we demonstrated the presence of HCN in exhaled breath; its detection could be a good indicator for bacterial lung infection. Due to its compact, cost-effective and user-friendly design, this laser-based sensor has the potential to be implemented in future clinical applications.