Supercontinuum Broadband Laser Absorption Spectroscopy

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Introduction / Motivation

Current and future emission regulations for internal-combustion engines, gas tubines or even aero engines require further research into combustion principles and different approaches for optimization of the combustion processes. To allow such research feasible means of detection and measurement of several species have to be developed in order to provide the necessary data at high sampling rates (for high temporal resolution) and high resolution within the actual combustion environment. The necessity to measure within the actual process environment (IC engine, power plant, aero engine, etc.) requires measurement systems, which are ruggedized to withstand the harsh conditions within such environments. Key factors to withstand harsh environments include fiber-based sensors and field-proven equipment (such as telecommunication equipment, i.e. fiber-coupled DFB lasers, etc.)

Method and Theory

SCL emission spectrum dispersed in space by grating
SCL emission spectrum dispersed in space by grating

To fulfill this need for high precision concentration and temperature measurements of species in combustion environments it is necessary to develop novel approaches to obtain these fluid properties. Several very promising approaches utilize super continuum laser light sources (SC). In principle a super continuum laser light source (SC) uses a very short, but powerful laser pulse at a specific wavelength and directs this light pulse into a highly non-linear fiber (i.e. PCF = photonic crystal fiber). During the propagation of the light pulse through the non-linear fiber a combination of effects broadens the spectrum of the pulse in the frequency domain. Current setups allow the broadening of a pulse at 1064 nm to a continuous spectrum from 400 nm to 2400 nm. When leaving the PCF, the pulses of a SC are still temporally short (in the order of tens of ps), but contain a very broad spectrum. Furthermore these systems still behave like classical lasers in regards to intensity and coherence of the emitted light. In addition these laser light sources provide repetition rates for individual pulses of up to 80 MHz.

Such broadband laser light sources allow for multi-species detection in various environments at very high temporal resolutions, since with each individual pulse, a complete spectrum can be sampled and evaluated to determine all required parameters, i.e. species concentrations and temperature. To utilize these high sampling rates and broadband emission it is necessary to detect the spectrum at such high sampling rates. One approach in this area utilizes telecommunication components such as high-speed high-bandwidth photo detectors (InGaAs) and dispersion compensating fibers to transform the frequency information into the time domain and record the spectrum in the time domain. This technique requires very expensive equipment and thereby yields an opportunity for improvement.

Proceeding

Based on research conducted during the Master thesis, the current measurement setup including a SC laser source, high-speed photo detectors and dispersion compensation will be optimized, further improved and developed to ease the use of such systems and improve the overall performance of the system. Key elements for improvement include automation of the measurement process, full integration of all components and different spectral resolutions. This measurement setup will be then employed to measure combustion properties within real harsh combustion environments, such as IC engines.