The ultimate instrumentation for trace gas analysis would be one that can simultaneously provide a sensitive, selective, and fast, multi-gas measurement with wide dynamic range all in a compact and robust system. One of the few such promising technologies that can deliver these requirements is laser based photoacoustic spectroscopy (PAS). In this application note, the PAS technique is presented using the Cobolt Odin™ tunable mid-IR laser.
Recent developments in technologies for mid-IR light sources allow cantilever-enhanced (CE) PAS instruments to now run with monochromatic and tunable wavelengths, broad spectral selectivity, and high output powers bringing sensitivity and selectivity with sub ppb levels. There are a number of laser technologies now available on the market capable of providing spectrally flexible narrow-band emission in the mid IR spectral range; quantum cascade lasers (QCLs), inter-band cascade lasers (ICLs), DFB diode lasers, and optical parametric oscillators (OPOs).
Figure 1: Cobolt Odin™. Complete OPO system with all-integrated laser head (pump and OPO) and driver unit.
In many aspects the OPO technology is the most ideal choice for driving CE-PAS instruments due to the broad spectral selectivity and tuning capability within the 2.8–3.6 μm range. This wavelength region is not readily reachable with QCLs and the available power levels of DFBs and ICLs are comparatively low. Advancements in OPO design and laser packaging technology mean the traditional drawbacks for OPOs of bulkiness and complexity has now been removed.
The Cobolt Odin™ is a compact pulsed OPO with center wavelength between 2–5 μm and tunable >60 nm where the linewidth can be tailored from <1.5 nm to <0.2 nm, depending on the selectivity and dynamic range requirements. The proprietary HTCure™ manufacturing technology ensures reliability and robustness and means the laser is essentially insensitive to varying ambient conditions (for example, resistant to 60 G shocks and exposure to -20 C to +70 C cycling) bringing reliability and ease-of-use to instrumentation for in-field trace gas detection.
Figure 3: OPO/CE-PAS measurement of EtOH, MeOH and CH4.
Experimental demonstration of the OPO/CE-PAS capability was performed using the Cobolt Odin™ operating between 3405–3463 nm (110 mW, 10 kHz). The OPO was coupled into one of Gasera's commercially available PA201 research photoacoustic detector for laser sources to measure ethanol, methanol, and methane.
Respective univariate detection limits obtained were (at 2x rms, with 1s CIT): for EtOH 7.7 ppb, for MeOH 11.4 ppb and for CH4 35 ppb. The total response time of the system was about 30 s, including the automatic gas exchange and the signal processing for a reduced tuning range.
The compact design and high mid-IR output power from the Cobolt Odin™, combined with Gasera's novel cantilever enhanced photoaccoustic measurement technology, allow the OPO/CE-PAS technology to offer extreme sensitivity and selectivity for compact, reliable, industrial gas analysis and monitoring.
(1) J. Peltola, M. Vainio, T. Hieta, J. Uotila, S. Sinisalo, M. Metsälä, et al. Optics Express21(8), 10240 (2013), doi:10.1364/OE.21.010240.
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