Coherent Anti-Stokes Raman Spectroscopy in Combustion Diagnostics

Coherent Anti-Stokes Raman Spectroscopy (CARS) is a non-intrusive laser-based diagnostic technique that probes molecular energy transitions to yield spatially and temporally resolved data on temperature and species concentrations in flames. By employing pump, Stokes and probe pulses, CARS generates an anti-Stokes signal whose intensity and spectral shape reflect the population and coherence of rotational and ro-vibrational states. Hybrid femtosecond/picosecond implementations suppress nonresonant background and enhance spectral resolution, while ultrabroadband excitation and supercontinuum generation extend coverage across multiple molecular branches. Recent developments have enabled single-shot, planar thermometry and multispecies mapping in laminar and turbulent burners, even under elevated pressures. Key challenges include accounting for collisional dephasing at high density, fine-tuning probe delays to isolate resonant contributions and modelling complex spectra for accurate retrieval. As a result, CARS remains central to elucidating combustion kinetics, pollutant formation pathways and optimising practical combustion systems for improved efficiency and reduced emissions.

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Coherent Anti-Stokes Raman Spectroscopy in Combustion Diagnostics publication trend

The graph below shows the total number of articles in coherent anti-stokes raman spectroscopy in combustion diagnostics across all publications each year (not limited to Nature Index journals).

Technical terms

Coherent Anti-Stokes Raman Scattering (CARS): A four-wave mixing technique that uses pump, Stokes and probe laser pulses to generate an anti-Stokes signal indicative of molecular energy coherences.

Femtosecond (fs) and Picosecond (ps) Pulses: Ultrashort laser pulses of durations ~10⁻¹⁵ s and ~10⁻¹² s, respectively, used to excite and probe molecular vibrations and rotations with high temporal resolution.

Nonresonant Background: Undesired four-wave mixing contributions arising from electronic nonresonant interactions, which can obscure the resonant Raman signal.

Rotational and Ro-Vibrational Transitions: Energy level changes involving molecular rotation only, or combined rotational and vibrational motions, probed by CARS to determine temperature and species concentration.

Supercontinuum Generation: Creation of a broadband light source via nonlinear processes (e.g. filamentation) to cover multiple Raman transitions in a single excitation pulse.

Filamentation: Self-focusing propagation of intense ultrashort pulses in a medium, leading to a stable plasma channel and broadband spectral broadening.

Probe Delay: Controlled temporal offset between excitation and probe pulses, used to discriminate resonant signal decay from nonresonant background.

Thermometry: Measurement of temperature in a reacting flow, often achieved by analysing CARS spectral intensities or line shapes.

References

1. High-temperature rotational-vibrational O2 CO2 coherent Raman spectroscopy with ultrabroadband femtosecond laser excitation generated in-situ. Combustion and Flame (2022).
2. Coherent Raman spectroscopy on hydrogen with in-situ generation, in-situ use, and in-situ referencing of the ultrabroadband excitation.. Optics Express (2022).
3. 5 kHz single shot hybrid fs/ps-CARS thermometry in an atmospheric flame.. Optics Express (2020).