Abstract
High peak-intensity laser pulses are desired for the generation of appreciable amounts of nonlinear optical signals in a light-matter interaction. Especially for spectroscopic applications, one of pulsed laser sources involved in the nonlinear optical interaction is necessarily frequency-tunable or spectrally broad over some frequency ranges of interest. Broadband sources are more preferable to narrowband frequency-tunable ones for the situations in which single-shot multiplex measurements are necessary. In particular an efficient generation of broadband ultraviolet (BBUV) pulses is important to get the multiplex spectra of molecular species which have electronic transition lines in an UV region1 We report in this paper the generation of BBUV pulses from the second harmonic generation of 7 ns broadband dye laser using two thin BBO crystals of 0.5 mm. The dye laser is pumped by frequency-doubled Q-switch Nd:YAG laser operating at 532 nm. The crystals are cut about 41 degree from optic axis to make the SHG efficiency maximized near 600 nm fundamental wavelength. The total second harmonic output of UV is measured 0.2 mJ/pulse from input dye laser energy of 6.4 mJ/pulse. We find that the BBUV pulses obtained in our experiment have an enough power for the multiplex nonresonant pump four wave mixing (NRP-FWM) spectroscopy of OH radicals in combustion reaction and a wide spectral width (~ 0.5 nm FWHM) enough to cover several R1 and R2 branch electronic transition lines of those chemical species.2 The experimental result of the UV spectral profile is compared to the theoretical calculation as shown in Fig. 1(a), in which the dispersion curves of BBO crystal for ordinary and extraordinary waves are used to get the phase mismatch factor between second harmonic and fundamental beams as a function of wavelength. The beam propagation angle of 40.5 degree from optic axis is used to calculate the refractive index of the second harmonic extraordinary wave. Fig. 1(b) shows the NRP-FWM spectrum of OH radicals. To measure the spectrum, the BBUV beam around 306.4 nm and Nd:YAG laser at 532 nm are focused on an air-propane premixed flame and the generated signal is spatially fdtered and directed into 1 m single grating monochromator to be dispersed on an intensified CCD. The spectral lines get broadened instrumentally and adjacent narrow lines overlap to each other in our present setup.
© 2007 IEEE
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