Tunable Ultraviolet Light Source for Combustion Experiments

John H. Glenn Research Center, Cleveland, Ohio

An ultraviolet light source that comprises a tunable diode laser and associated optics provides the optical excitation needed for simultaneous wavelength-modulation-absorption spectroscopy (WMS) and laser-induced-fluorescence (LIF) measurements in experiments on combustion. These measurements are needed for determining the spatially and temporally resolved concentrations of molecular and radical species that play important roles in flames. Other uses for measurements of this type could include general detection and quantitation of trace gases in the atmosphere, including toxic gases emitted by industrial facilities. Instruments that incorporate light sources like the present one and that will perform these measurements are undergoing development. In comparison with prior ultraviolet lasers and with prior WMS and LIF instruments, the present light source and the developmental instruments are compact and rugged and consume less power. As a result, the developmental instruments are expected to be suitable for use, not only in laboratories, but also in diverse harsh environments, including those of drop towers, aircraft, and spacecraft.

Heretofore, WMS and LIF measurements have been performed separately. WMS enables highly sensitive detection and quantitation of trace molecular and radical species. However, because it involves integrated absorbance along a laser beam, it yields no information on the spatial distribution of species. LIF measurements taken along lines of sight perpendicular to a laser beam can be used to map relative densities of species as functions of position along the line of sight. However, it is extremely difficult to obtain absolute density information from LIF data because of a need for careful calibration of laser intensity and geometric factors and mathematical modeling of fluorescence quenching by collision partners. By suitable processing of WMS and LIF data acquired simultaneously by the developmental instruments, it should be possible to eliminate the difficulties associated with geometric factors and laser-intensity fluctuations, thereby facilitating the determination of absolute density distributions.

The Tunable Ultraviolet Light Source has been incorporated into a laboratory apparatus used to demonstrate the feasibility of simultaneous WMS and LIF measurements to determine distributions of CH radicals along a line defined by a laser beam.

The figure is a simplified schematic depiction of the present light source as part of a prototype apparatus for probing a hydrocarbon flame with simultaneous WMS and LIF measurements in a wavelength band centered about 426 nm. This wavelength band is associated with CH radicals, which play a major role in combustion of hydrocarbons. Pump light at a wavelength of ≈852 nm, with a power of ≈120 mW, is generated by a distributed-Bragg-reflector (DBR) diode laser. The laser beam is collimated and circularized by an anamorphic prism pair. The beam is then focused into a 7-mm-long A-cut KNbO₃ crystal, which is optically nonlinear and thus serves to convert some of the laser power to the second harmonic, which is at the desired wavelength of ≈426 nm. An optical isolator between the diode laser and the frequency-doubling KNbO₃crystal minimizes feedback of reflected light to the laser diode. A half-wave plate, also between the diode laser and the KNbO₃ crystal, matches the polarization of the pump beam with the orientation of the crystal.

The KNbO₃ crystal is mounted on a thermoelectric cooler in an N₂-purged housing for temperature-tuned noncritical phase-matching operation at a temperature near 10 °C. The 426-nm beam is collimated and separated from the residual 852-nm pump beam by use of a CaF₂prism. The frequency of the residual pump beam is monitored to within 0.01 cm^-1by use of a scanning interferometer. When the 852-nm power input to the crystal is 85 mW, the 426-nm output power is 100 μW. This output ultraviolet power is stable as long as the temperature of the crystal is stabilized to within±0.1 °C.

The wavelength of the beam generated by the laser diode is tuned by adjustment of its temperature and injection current. The injection current is modulated at a frequency of 50 kHz to produce wavelength modulation of the frequency-doubled beam. A linear current sweep is also applied to the diode laser, to tune the center frequency across the absorption spectral line of interest during wavelength modulation.

For laboratory feasibility measurements, the 426-nm beam is directed along the flame front of a slot burner. For the purpose of WMS, the intensity of the beam after transmission through the flame is monitored with a silicon positive/intrinsic/negative (PIN) photodiode, the output of which is sent to a lock-in amplifier for detection at a frequency of 100 kHz. A blue-pass filter (not shown in the figure) in front of the photodiode reduces the incident visible and near-infrared flame luminescence. Second-harmonic CH absorption spectra are acquired by detection of the transmitted beam while the frequency is repetitively swept across the absorption spectral line. The second-harmonic signals are calibrated against direct (unmodulated) absorbance measurements.

For LIF measurements, a lens images a 2-mm segment of the region illuminated by the laser beam onto a photomultiplier tube (PMT). A 431±1-nm band-pass filter suppresses flame-luminescence light while passing CH-fluorescence light to the photomultiplier. The output of the photomultiplier is sent to a lock-in amplifier for phase-sensitive detection of LIF at a sampling frequency of 100 kHz. (Additional experimental details and measurement results are found in K. A. Peterson and D. B. Oh, Optics Letters, vol 24, pp 667-669.)

This work was done by Daniel B. Oh and Kristen A. Peterson of Southwest Sciences, Inc., for Glenn Research Center.

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