Since its inception, laser absorption spectroscopy has become an indispensable analytical technique for environmental monitoring, security, and medical diagnostics applications, particularly in the gas phase.
Recently, optical frequency combs (OFCs) have attracted considerable attention as a promising source for multi-species air quality monitoring. This is dictated by the European Union’s effort to reduce air pollution, which necessitates continuous monitoring of pollutant concentrations. The new National Emissions Ceilings (NEC) Directive (2016/2284/EU) has set commitments for member states on several important air pollutants such as nitrogen oxides (NOx), sulfur dioxide (SO2), ammonia (NH3), non-methane volatile organic compounds (NMVOCs), and persistent organic pollutants (POPs). Although the first four can be detected using existing electrochemical sensors or marketed tunable laser spectrometers, selective detection of volatile organic compounds often remains elusive for broadband optical instruments, which in turn often suffer from low signal-to-noise ratios.
This niche can be perfectly filled by OFC spectroscopy. Not only does it allow for probing broad features of NMVOCs with high brightness, but also it is well suited for simultaneous multi-species detection.
Using digital signal processing platforms, quasi-real-time coherent averaging of spectroscopic signals has been obtained in an unstabilized dual-comb spectrometer (without any moving parts) for sensing molecular species. This lifts the requirement of user supervision or manual processing of acquired spectroscopic data.
Next, nonlinear interactions in laser oscillators have been studied. They often lead to poor noise performance and corrupt spectroscopic signal. It has been found that incorporation of optical filters into the cavity, while reducing some amounts of intensity and phase noise, leads to the creation of dispersive structures and spectral sidebands. A reasonable filter bandwidth was found to improve lower the timing jitter and intensity noise of a typical 1550 nm oscillator. A numerical model to simulate the effects of intracavity spectral filtering has also been developed.
Another interesting aspect explored during the project are chip-scale OFC sources emitting natively in the mid-infrared region with battery-operation capabilities. Diode laser frequency combs are generally easier to fabricate than more sophisticated cascade-type lasers. The fact that it is possible to employ these devices for unstabilized dual-comb systems in the spectroscopically-relevant 3 micron region makes them attractive for future portable air quality monitoring systems.
