Combustion Diagnostics Using Polarized FTIR Spectroscopy

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Combustion is a crude yet effective method of releasing energy from the environment. This important natural process is seen in gas burners and forest fires every day and is mostly taken for granted. Despite extensive investigation, there remains a major deal that is yet to be understood about combustion, however, IR polarizers, provided by Specac, can be used to probe the mysteries of combustion chemistry.

Even simple combustion processes include a complex series of rapid chemical reactions between reactive species which are not completely understood. Improving the effectiveness of combustion plays a vital role in reducing pollution and waste, meaning there is a great demand to attain a stronger fundamental understanding of the combustion process.

Laser Based Combustion Diagnostics

Since the mid-1980s, laser diagnostic techniques for applied combustion research have been in use for measuring important parameters, such as species concentration, temperature, velocity, and particle characteristics, with the potential to provide in-depth data in real-time (<10 ns) within particular spatial resolution constraints (10-50 µm). Laser-based IR techniques are non-intrusive allowing measurement without the necessity of any physical probe, for instance, a thermocouple, which could influence the combustion process by affecting both the chemical and physical environment.

Other advantages are because of the nature of laser radiation itself. Laser diagnostics have no upper limit in terms of temperature measurements and they are also capable of the two-dimensional visualization of turbulent flows, such as those that happen in flames. Additionally, this measurement can also be carried out in-situ.

Laser diagnostics is made up of non-linear and linear techniques. Linear techniques include those recognizable to most chemists - laser-induced fluorescence, absorption spectroscopy, and Rayleigh scattering.

Non-linear laser techniques are more specialist, as they employ high laser powers, and they include polarization spectroscopy (PS), laser-induced grating spectroscopy (LIGS) and coherent anti-Stokes Raman scattering (CARS). Non-linear techniques all use four-wave mixing processes in connection with non-linear interactions between matter and light.

The key advantages of using non-linear laser methods are high sensitivity and a high temporal and spatial resolution. Even though these benefits come at a price, non-linear methods are more difficult to set up and need more complex data analysis.

Using Polarization Spectroscopy for Combustion Analysis

Polarization Spectroscopes are capable of detecting minor species in harsh environments. Polarization spectroscopy (PS) was initially developed by Wieman and Hänsch ^[1] in 1976 as a Doppler-free spectroscopic method, related to saturation spectroscopy but offering a significantly better signal-to-background noise ratio. Species detection and the temperature measurements required mean that laser spectroscopy usually relies on finding resonant absorption lines for a particular molecule and employing a laser of this wavelength to probe the molecule. The problem is that many species involved in combustion applications, such as CH₄, C₂H₄, C₂H₆, HCN, and NH₃, have no accessible electronic transitions.

In such cases, polarization spectroscopy is considered to be useful, as these species do have strong fundamental absorption bands in the mid-infrared spectral region, despite the fact that the background IR in this region from the ‘hot’ combustion process can be problematic to suppress. This is where polarization spectroscopy becomes useful as the signal is generated as a coherent, laser-like beam, which makes it easy to collect the signal and then filter out any background radiation. PS is essentially an absorption-based technique with no background.

The Advantage of Four-Wave Mixing

Polarization spectroscopy is one of the simplest four-wave mixing techniques ^[3]. Four-wave mixing (FWM) is an intermodulation phenomenon in non-linear optics, whereby interactions between two or three wavelengths generate one or two new wavelengths.

Four-wave mixing is applied in polarization spectroscopy where two input waves produce a detected signal with a slightly higher optical frequency. With a variable time delay between the input beams, it is possible to measure excited-state lifetimes and dephasing rates for detected species.

Experimental Systems for Polarization Spectroscopy

Polarization spectroscopy for combustion analysis usually uses a strong pump beam and a weak probe beam from a tunable laser. It is necessary to tune the laser to the optical transitions of the target species with a common ground or excited state. The two laser outputs are ‘crossed’ as they pass through the sample and the optical pumping of the target species by the polarized pump beam produces birefringence and dichroism. The effect of this is to induce detectable polarization changes in the weak probe beam.

This change is identified by monitoring the introduced leakage of the weak probe beam via two crossed polarizers (this should block all signal). In essence, the probe beam should not be able to pass the second polarizer unless the species in the sample has efficiently changed the characteristics of the probe beam.

The polarizers/polarizing filters employed are vital to the success of this technique and Specac is able to supply the finest polarizing elements.

Infrared Polarizers from Specac

A polarizer is an optical filter that allows light waves of a specific polarization pass and blocks light waves of other polarizations. Specac polarizers are manufactured from extremely fine conducting parallel elements or a grid placed upon an ideal transparent base material.

When the grid spacing is much smaller than the wavelength of light, the light with a wave vector parallel with the grid will be reflected and only the component with perpendicular vector is transmitted. The overall transmission characteristic of the polarizer relies upon the substrate, but the polarization efficiency relies upon the line width, period and other design parameters of the polarizer.

Infrared Polarizers from Specac

In the mid-infrared range, mainly for combustion analysis, the most practical and frequently used polarizers are ruled or holographic wire grid structures. The polarization effect comes from the very same principle as the free-standing wire grid, except the fine wires are developed on the surface of an infrared transmitting optical window material.

Specac offers a variety of holographic infrared wire grid polarizers for general-purpose optical physics applications. These IR polarizers are offered from on substrates including barium fluoride (BaF₂), zinc selenide (ZnSe), calcium fluoride (CaF₂), and thallium bromoiodiode (KRS-5).

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These polarizers have an operating range of 2 μm to 35 μm (5000 cm^-1 to 285 cm^-1) and are manufactured with a grid periodicity of 2500 lines/mm, to offer exceptional extinction coefficient and transmission.

References and Further Reading

1. C. Wieman; T. W. Hänsch, Phys. Rev. Lett. 36 (20) (1976) 1170---1170
2. M. Aldén, J. Bood, Z.S. Li and M. Richter, Visualization and understanding of combustion processes using spatially and temporally resolved laser diagnostic techniques, Proceedings of the Combustion Institute 33, (2011) 69-97.
3. D. Nodop et al., “Efficient high-power generation of visible and mid-infrared light by degenerate four-wave-mixing in a large-mode-area photonic-crystal fiber”, Opt. Lett. 34 (22), 3499 (2009)

This information has been sourced, reviewed and adapted from materials provided by Specac.

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