Multiplex Coherent Anti Stokes Raman Spectroscopy (MCARS) has been shown to generate a complete Raman spectrum of a material on a millisecond time scale which allows for rapid identification of a wide variety of molecular targets. Along with the desired resonant spectrum due to the vibrational Raman spectroscopy of the analyte, MCARS is known to simultaneously generate a nonresonant spectrum that can obscure the desired Raman spectrum which hinders detection. Extracting the desired resonant Raman signal analytically from the overall MCARS signal has proven difficult without having prior knowledge of the analyte. We have developed an algorithm that utilizes a combination of the maximum entropy method in conjunction with advanced Fourier filtering to analytically remove the nonresonant background from our MCARS spectra without having prior knowledge of the vibrational spectrum of the analyte. In this report, we will report on the theoretical background for this algorithm as well as our experimental work testing this algorithm under various nonresonant spectra conditions for a number of analytes. We will systematically vary the amount of nonresonant background generated in the sample by changing the temporal overlap of the two beams necessary to generate the MCARS signal. Additionally, we place the analyte into increasing concentrations of water to generate increasing amounts of nonresonant background spectra to test the algorithm’s effectiveness. Finally, we compare the analyte vibrational spectral output from the algorithm to the Raman spectrum measured with the spontaneous Raman system in the laboratory of the same sample in an effort to ascertain accuracy of the output spectra.
There is a significant need for the development of optical diagnostics for rapid and accurate detection of chemical species in convoluted systems. In particular, chemical warfare agents and explosive materials are of interest, however, identification of these species is difficult for a wide variety of reasons. Low vapor pressures, for example, cause traditional Raman scattering to be ineffective due to the incredibly long signal collection times that are required. Multiplex Coherent Anti-Stokes Raman Scattering (MCARS) spectroscopy generates a complete Raman spectrum from the material of interest using a combination of a broadband pulse which drives multiple molecular vibrations simultaneously and a narrow band probe pulse. For most species, the complete Raman spectrum can be detected in milliseconds; this makes MCARS an excellent technique for trace material detection in complex systems. In this paper, we present experimental MCARS results on solid state chemical species in complex systems. The 40fs Ti:Sapphire laser used in this study has sufficient output power to produce both the broadband continuum pulse and narrow band probe pulse simultaneously. A series of explosive materials of interest have been identified and compared with spontaneous Raman spectra, showing the specificity and stability of this system.
There is a significant need for the generation of highly stable continuum beams for a wide variety of optical diagnostic techniques. Of particular interest to this group are those techniques being used for chemical detection, such as Multiplex Coherent Anti-Stokes Raman Scattering (MCARS), stimulated Raman scattering, two-photon absorption spectroscopy, and techniques involving ultrafast optical parametric amplifiers (OPAs). While photonic crystal fibers (PCFs) are popular and provide an ample method for continuum generation under very specific conditions, they are not particularly stable in unfavorable conditions and can exhibit energy fluctuations and lack of coherence. Bulk solid materials, commonly sapphire or YAG crystals, can provide incredibly broad and smooth spectra with better temporal and spatial coherence. In this study, we present an in-depth analysis of femtosecond continuum generation in sapphire and YAG crystals using a 40fs Ti:Sapphire laser. Beam size, pump pulse energy, beam profile, and a variety of focusing conditions are considered. In addition, an analysis of the thick lens theory required for collimation of the continuum beam has been conducted and experimentally verified.
There is a need for rapid and accurate detection and identification of complex aerosol particles in a number of fields
for countless applications. Full identification of these particles has been hampered by the inability to use an
information-rich spectroscopic method such as Raman scattering in a flowing aerosol environment due to the time
needed to generate a Raman spectrum. Multiplex coherent anti-Stokes Raman spectroscopy (MCARS) has been
shown to generate a complete Raman spectrum from the material of interest using a single ultrabroadband pulse to
coherently drive multiple molecular vibrations simultaneously. When used in conjunction with a narrow probe
pulse, a complete Raman spectrum is created that can be detected in milliseconds. We will report on the MCARS
spectra obtained from materials of interest at a distance of 1 m from the sample location. A limit of detection study
of the MCARS spectrum of various materials of interest will be also reported in with the nonresonant background
both present and removed. Additionally, a limit of detection study as a function of the number of pulses used to
comprise the CARS spectrum of the materials of interest will be presented.
Cascade pumping schemes that utilize single-QW gain stages enhanced both the power conversion efficiency and the output power level of GaSb-based diode lasers that emit near and above 3 μm at room temperature. The cascade lasers discussed in this work had densely stacked type-I QWs gain stages characterized by high differential gain. The 3 μm emitting devices demonstrated CW threshold current densities near 100 A/cm2, a twofold improvement over the previous world record, that resulted in peak power conversion efficiencies increasing to 16% at 17°C. Comparable narrow ridge two-stage devices generated more than 100 mW of CW power with ~10% power conversion efficiencies. Three-stage multimode cascade lasers emitted 960 mW of CW output power near 3 μm and 120 mW CW near 3.3 μm.
Conference Committee Involvement (1)
Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XXV
22 April 2024 | National Harbor, Maryland, United States
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