Sum-frequency generation with a blue diode laser for mercury spectroscopy at 254 nm

Abstract

Blue diode lasers emitting 5 mW continuous-wave power around 400 nm have recently become available. We report on the use of a blue diode laser together with a 30 mW red diode laser for sum-frequency generation around 254 nm. The ultraviolet power is estimated to be 0.9 nW, and 35 GHz mode-hop-free tuning range is achieved. This is enough to perform high-resolution ultraviolet spectroscopy of mercury isotopes. The possibility to use frequency modulation in the ultraviolet is demonstrated; however, at present the ultraviolet power is too low to give advantages over direct absorption monitoring. Mercury detection at atmospheric pressure is also considered which is of great interest for environmental monitoring.

Detection of elemental mercury by multimode diode laser correlation spectroscopy

“…The violet and red laser powers entering the BBO crystal are measured to be 11 mW and 25 mW, respectively. From theoretical calculations, one order of magnitude larger UV power could be obtained by decreasing the laser spot size in the crystal [10]. However, no such attempts were made since it would increase the system sensitivity to vibrations, thus degrading the system stability.…”

“…For the TDLAS technique, at least 80-GHz (three times the absorption linewidth of mercury vapor) of mode-hop-free tuning range is required to extract the whole absorption feature of mercury out of the broadband attenuation due to absorption of other species or particulate scattering [10,12]. However, for the MDL-COSPEC method, there is no special request on mode-hop-free tuning range; instead, the mode hop is used to increase the wavelength coverage and thus obtain an off-resonant baseline.…”

“…An alternative technique for elemental mercury detection is the employment of laser absorption spectroscopy. Various techniques have been developed to generate laser radiation at 253.7 nm, including sum-frequency generation (SFG) by two diode lasers (DLs) [10][11][12], frequency doubling pulsed dye lasers or optical-parametric-oscillator lasers [13][14][15] and frequency quadrupling diode lasers or solid-state lasers [16,17]. By comparison, SFG-DL based systems possess the merits of agile tunability, simplicity and low-cost.…”

“…By comparison, SFG-DL based systems possess the merits of agile tunability, simplicity and low-cost. Diode-laser-based systems with mode-hop-free tuning in the range of 35-100 GHz have previously been developed [10][11][12]. For mercury sensing by tunable diode laser absorption spectroscopy (TDLAS) two requirements should be fulfilled: extremely accurate wavelength control and wide mode-hop free tuning range (> 80 GHz).…”

Environmental monitoring using optical techniques

“…With the fast development of the semiconductor laser technology, new wavelength regions are becoming available. We recently demonstrated first single-mode diode laser spectroscopy in the violet spectral range, frequency-modulation spectroscopy with blue diodes, and also their usefulness in sum-frequency generation to the UV region, also accessing the mercury line at 254 nm [28][29][30]. We are also, like many other groups, performing diode laser spectroscopy in the IR range, accessible by differencefrequency generation or directly by quantum cascade lasers.…”