Optical fiber based supercontinuum (SC) light sources combine the brightness of lasers with the bandwidth of incandescent lamps and thus are promising candidates for sources in spectroscopic applications requiring high brightness and broad bandwidth. Near-infrared (IR) Fourier transform (FT) spectrometry with an SC light source has been demonstrated for long path absorption spectroscopy of trace gas species. Spectra of these trace gases illustrate the advantage the SC source has over traditional incandescent sources in that it can propagate in a collimated fashion over long distances, thus providing a sensitivity advantage. The spectra also demonstrate the disadvantage of the SC source with respect to the lamp in the increased level of amplitude noise.
This project was initiated in an effort to assess the utility of SC lasers as light sources for FTIR spectrometry. FTIR is a widely used spectroscopic technique that provides information on the chemical composition of a specimen. The light source utilized in this technique is a blackbody source, chosen for its low cost, stability and broad bandwidth; however, because it radiates in all directions, its brightness (power per steradian) is relatively low. This limits its utility in certain applications such as long pathlength absorbance spectroscopy (e.g., standoff detection) and focusing to diffraction limited spots (e.g., IR chemical imaging). The effort to assess the efficacy of SC sources in these applications will provide key information to practitioners considering implementation of these new sources in fields such as defense/homeland security, process analytical, polymers and pharmaceuticals.
There were three major goals of this project. The first was to interface a commercial near-IR SC laser source with a long pathlength, multipass gas cell and a commercial FTIR spectrometer. The second was to characterize the baseline noise on the SC source and compare it to that exhibited by the conventional incandescent sources. This noise comparison was to be done as a function of spectral resolution and extent of signal averaging. Finally, spectra of a series of trace gases were to be acquired using the SC source and the conventional source to allow a head-to-head comparison. These comparisons were intended to form the basis of an assessment of the utility of the SC laser source in FT spectrometry and a guide to the conditions under which the SC source might prove superior to the incandescent source.
Light from the SC laser was aligned through an 18 m multipass cell and a commercial FT bench interferometer. The power level reaching the detector was sufficiently high to require significant attenuation to avoid detector saturation. The bandwidth of the source easily extended over the entire near-IR region (4500 to 9000 cm-1). The combination of broad bandwidth and long distance collimated propagation akin to a laser is a clear demonstration of the anticipated benefits of an SC as a source for spectroscopy. The noise characteristics of the source were investigated. The spectrum of the source was quite constant with excellent reproducibility exhibited in consecutive mirror scans. The RMS noise on 100% lines was approximately a factor of 10 higher than that exhibited in comparable data acquired with a tungsten lamp, indicating a significantly higher level of amplitude noise on the SC output than that present on the incandescent source. This ultimately could limit the S/N ratio attainable with this source as fluctuation noise is multiplicative in FT spectrometry. Near-IR FT spectra of methane, shown in Figure 1, and methyl salicylate were acquired with both sources to demonstrate the reasonable spectral quality that can be obtained with the SC source. This work demonstrates the feasibility of implementing supercontinuua as light sources for applications of FT spectrometry in which high brightness and broad bandwidth are a necessity.
Figure 1. FTIR spectra of 23 overtone band of methane acquired at 2.0 cm-1 resolution with the tungsten lamp source (right y-axis scale) and with the supercontinuum source (left y-axis scale). The total pressure was nominally 760 Torr of 0.2% methane in N2 in a multipass cell of pathlength 2 m (tungsten lamp) or 18 m (supercontinuum).
• Demonstrated long path propagation of NIR SC laser.
• Demonstrated use of SC laser for FTIR spectrometry in long path absorbance spectroscopy of trace gases.
• Benchmarked noise performance of SC laser against incandescent sources.