Broadband time-domain absorption spectroscopy with a ns-pulse supercontinuum source

Abstract

A Q-switched laser based system for broadband absorption spectroscopy in the range of 1390-1740 nm (7200-5750 cm(-1)) has been developed and tested. In the spectrometer the 1064 nm light of a 25 kHz repetition-rate micro-chip Nd:YAG laser is directed into a photonic crystal fiber to produce a short (about 2 ns) pulse of radiation in a wide spectral range. This radiation is passed through a 25 km long dispersive single-mode fiber in order to spread the respective wavelengths over a time interval of about 140 ns at the fiber output. This fast swept-wavelength light source allows to record gas absorption spectra by temporally-resolved detection of the transmitted light power. The realized spectral resolution is about 2 cm(-1). Examples of spectra recorded in a cell with CO(2):CH(4):N(2) gas mixtures are presented. An algorithm employed for the evaluation of molar concentrations of different species from the spectra with non-overlapping absorption bands of mixture components is described. The uncertainties of the concentration values retrieved at different acquisition times due to the required averaging are evaluated. As an example, spectra with a signal-to-noise ratio large enough to provide species concentrations with a relative error of 5% can be obtained in real time at a millisecond time scale. Potentials and limitations of this technique are discussed.

Temperature and multi-species measurements by supercontinuum absorption spectroscopy for IC engine applications

“…Comparison of experimental and theoretical peak ratios. Figure 2 shows two SC absorption spectra for the ns-pumped [19] and ps SC source for a water volume fraction of 4.9 vol.-%, both averaged over 2500 consecutive single pulses. The spectral resolution ∆λ of a dispersion-swept SC spectrometer is given by ∆λ = τ m /|D|, where D is the total group-velocity dispersion of the employed dispersive element [9].…”

“…Through the interplay of various nonlinear optical effects, directed radiation with high brightness is generated, covering a wide spectral range from the ultraviolet, near-infrared up to the mid-infrared [12][13][14][15][16]. Typically combinations of passive mode-locked fiber lasers with repetition rates in the MHz regime and highly nonlinear waveguides, such as photonic crystal fibers (PCF) are used [1,9,[17][18][19]. For ps-and ns-pump sources the nonlinear spectral broadening of the pump pulse is dominated by four-wave mixing, stimulated Raman and Brillouin scattering [20].…”

“…This offers the possibility of time-domain based broadband spectroscopy by detection of the dispersed SC pulses with a high bandwidth photodiode and oscilloscope. Wavelength sweeps lasting several hundreds of ns ranging from about 1100 nm to 1700 nm facilitate high-speed multi-species optical sensing at atmospheric conditions [1,9,[17][18][19][22][23][24]. The capability of SC spectroscopy for minor species (e.g., NO 2 , NO 3 ) detection in the atmosphere by a cavity enhanced approach was presented in [25][26][27][28].…”

“…Furthermore, a comparatively cheap passively Q-switched ns-based SC source was compared to the employed mode-locked ps-pumped SC source regarding their applicability for time-domain based SC absorption measurements. In general, the ns-pumped system features higher spectral power densities per pulse compared to the ps-system because of the lower repetition rate of 25 kHz [19]. However, the achievable spectral resolution must be also considered, which in turn influences the signal-to-noise ratio.…”

Half mJ Supercontinuum Generation in a Telecommunication Multimode Fiber by a Q-switched Tm, Ho:YVO4Laser

“…SC generation in optical fibers has been the subject of extensive research in recent years due to the great fundamental interests and its potential applications in atmosphere analysis, spectroscopy, free-space communications, biology and medicine [1][2][3][4][5], especially in the mid-IR spectral region. With the recent development of various special optical fibers, including microstructure fibers (MF) [6,7], fiber tapers [8], highly nonlinear fibers (HNLF) [9,10], fluoride fibers [11][12][13], tellurite fibers, and chalcogenide fibers, broadband SC source can be efficiently generated by pumping with high power pulsed lasers (femtosecond, picosecond, and nanosecond pulses) or even with continuous-wave lasers.…”