7 Dec 2017

Polarized spectroscopy for combustion analysis | Spectroscopy Solutions

Combustion is a crude but effective method of releasing energy from the environment. Even in more technical applications, such as inside automobile engines, gas turbines and power plants combustion is the central process.

Polarized FTIR spectroscopy can filter out background radiation when measuring paramters in applied combustion. Specac's Wire Grid FTIR Polarizers are a great choice for this application.

FTIR Wire Grid Polarizers are ideal for monitoring and analyzing applied combustion using spectroscopy.

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Despite extensive investigation, there remains a great deal that is not yet understand about combustion. Even simple combustion processes include a complicated series of rapid chemical reactions between reactive species which are not fully understood. Improving the efficiency of combustion plays an important role in reducing pollution and reducing waste, meaning there is a great need to achieve a stronger fundamental understanding of the combustion process.

Laser Based Combustion Diagnostics

Laser diagnostic techniques for applied combustion research have been in use since the mid-1980’s for the measurement of important parameters, such as temperature, species concentration, velocity and particle characteristics, with the ability to provide detailed data in real-time (<10 ns) within specific spatial resolution constraints (10-50 µm).

For monitoring and measuring applied combustion, polarised spectroscopy using wire grid FTIR polarizers is a fantastic method.

Laser based IR techniques are non-intrusive allowing measurement without the necessity of any physical probe, for example a thermocouple, which might influence the combustion process by affecting both the physical and chemical environment.

Other advantages are due to 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 occur in flames. In addition to this measurement can also be performed in-situ.

Laser diagnostics consists of linear and non-linear techniques. Linear techniques include those recognisable to most chemists - absorption spectroscopy, laser-induced fluorescence and Rayleigh scattering.

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

The major advantages of using non-linear laser methods are a high temporal and spatial resolution, and a high sensitivity.

Although these benefits come at a price, non-linear methods are more complex to set up and require more complex data analysis.

Using Polarization Spectroscopy for Combustion Analysis

Polarization spectroscopes can detect minor species in harsh environments. Polarization spectroscopy (PS) was first developed by Wieman and Hänsch [1] in 1976 as a Doppler-free spectroscopic method, related to saturation spectroscopy but offering a considerably better signal-to-background noise ratio.

Species detection and the temperature measurements required mean that laser spectroscopy generally depends on finding resonant absorption lines for a specific molecule and using a laser of this wavelength to probe the molecule. The problem is that many species involved in combustion applications, such as CH4, C2H4, C2H6, NH3 and HCN, have no accessible electronic transitions.

In these cases, polarization spectroscopy is useful, as these species do have strong fundamental absorption bands in the mid-infrared spectral region, although the background IR in this region from the ‘hot’ combustion process can be difficult 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 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 produce one or two new wavelengths.

Four-wave mixing is applied in polarization spectroscopy where two input waves generate 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 generally uses a strong pump beam and a weak probe beam from a tunable laser. The laser should be tuned 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 a detectable polarization changes in the weak probe beam.

This change is detected by monitoring the introduced leakage of the weak probe beam through 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 effectively changed the characteristics of the probe beam.

The polarizers/polarizing filters used are crucial to the success of this technique and Specac are able to supply the finest polarizing elements.

Infrared Polarizers from Specac

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

FTIR Wire Grid Polarizers are perfect for applied combustion analysis.

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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 depends upon the substrate, but the polarization efficiency depends upon the period, line width and other design parameters of the polarizer.

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

Specac offer a range of holographic infrared wire grid polarizers for general-purpose optical physics applications. These IR polarizers are offered from on substrates including: calcium fluoride (CaF2), barium fluoride (BaF2), zinc selenide (ZnSe), thallium bromoiodiode (KRS-5). These polarizers have an operating range of 2 microns to 35 microns (5000 cm-1 to 285 cm-1) and are manufactured with a grid periodicity of 2500 lines/mm, to offer excellent extinction coefficient and transmission.

References

  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)