Fourier Transform Spectrometer Combined with a Mid-Infrared Supercontinuum Source for Trace Gas Sensing

Trace gas detection has many applications in research and industry; such as breath analysis, environmental monitoring, plasma diagnostics, and fruit quality monitoring [1] . Amongst various methods used for trace gas detection, molecular absorption spectroscopy is a powerful technique, which provides fast, selective, sensitive, and non-destructive detection. The fundamental rotational-vibrational transitions of molecular species are mostly in the mid-infrared (MIR) "fingerprint" (2–20 μm) region, which are significantly stronger than the overtone or combination bands in the near-infrared region. Fourier transform spectroscopy (FTS) is one of the well-known optical techniques, which is capable of broadband detection in the MIR region. However, incoherent thermal light sources, traditionally used in FTS, are omnidirectional and have low spectral brightness. Thus, the maximum achievable interaction length with a gas sample is very limited, resulting in low detection sensitivity. On the other hand, recently developed MIR supercontinuum (SC) light sources can deliver very high spectral brightness compared to the thermal sources or even a synchrotron radiation source [2] . Here, we present a multi-species trace gas sensor based on a MIR SC source, a multi-pass absorption cell, and a compact home-built FTS. For enhancement of the detection sensitivity and reducing the averaging time, we utilized a balanced detection scheme to reduce the relative intensity noise. For this, we measured the two out-of-phase outputs of the interferometer by two detectors. The two interferograms are 180 degrees out-of-phase, but they have an in-phase intensity noise. By subtracting the two outputs, the interferograms are doubled, while the noise is significantly reduced.