Ultra-High-Speed, Time-Resolved
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IMAGING GROUP Ultra-High-Speed, Time-Resolved Spontaneous Raman Scattering Spectroscopy in Combustion © 2013 Princeton Instruments, Inc. All rights reserved. In combustion, until recently only two temporal optical gating schemes were available to increase signal-to-noise ratio (SNR) for time-resolved spontaneous Raman scattering (SRS) spectroscopy. Problematic optical background noise could be rejected either by electronic gating with an image intensifier or by using a mechanical shutter. Unfortunately, each of these traditional approaches has its shortcomings. Image intensifiers, for example, provide excellent optical background noise rejection via <2 nsec gating capability but carry several inherent limitations, such as lesser image quality and lower dynamic range. On the other hand, while a high-speed mechanical shutter with a rotary optical chopper is able to deliver wider dynamic range without diminishing the quantum efficiency (QE) of the detection system’s CCD, its 30 Hz speed and ~10 µsec gating are not sufficient for rejecting noise and can result in transmission losses of up to 50%. In 2010, Dr. Jun Kojima of the Ohio Aerospace Institute, working with Dr. David Fischer and Dr. Quang-Viet Nguyen (NASA Glenn Research Center), described an architecture* for SRS that employs a frame-transfer CCD operating in a subframe burst-gating mode to realize time-resolved combustion diagnostics.1 This patented technique enables all-electronic optical gating at microsecond shutter speeds (<5 µsec) without compromising optical throughput or image fidelity. Dr. Kojima uses a Princeton Instruments ProEM® electron-multiplying CCD (EMCCD) camera for this method. When utilized in conjunction with a pair of orthogonally polarized excitation lasers, the technique described above measures single-shot vibrational Raman scattering that is minimally contaminated by optical background noise. Nonetheless, its relatively long gating (~5 µsec) still leaves room for improvement in terms of optical background rejection. Recently, Dr. Kojima has developed another advanced technique for measuring time-resolved SRS spectroscopy in combustion (see Figure 1). An overview of this new approach, which offers even higher SNR and permits ultra-high-speed observation of combustion dynamics, is provided herein. Figure 1. Dr. Jun Kojima of the Ohio Aerospace Institute testing new technique at NASA Glenn Research Center. * U.S. Patent No. 8,310,671 B1

IMAGING GROUP New Way To Measure Time-Resolved Spontaneous Raman Scattering Dr. Kojima’s new experimental apparatus is shown in Figure 2. It measures Raman scattering with much faster gating (<2 nsec), wider dynamic range, and higher sensitivity in combustion than previously reported techniques, allowing observation of flame instability dynamics via a newly introduced Princeton Instruments intensified emICCD camera that keeps up with the latest 10 kHz lasers. Reector Lens Nd:YAG Pulsed Laser (10 kHz) To enhance the SNR, the image intensifier was operated at a 10 kHz rate to keep...

IMAGING GROUP The exceptional linearity and dynamic range of these new emICCD cameras, achieved by intelligently programming gains between the image intensifier and the EMCCD, are critical for quantitative imaging and spectroscopy applications such as combustion. Their true single-photon detection capability, meanwhile, ensures the high sensitivity needed for light-starved applications. An oscilloscope-like user interface (see Figure 4) even remembers complete experimental setups. In addition, thanks to the ability to acquire 10,000 spectra/sec when operated in a special custom chip mode,...

IMAGING GROUP The signal visibility in Figure 5b is significantly higher than previously reported data of this kind. It is clearly seen from the data that the flame oscillates at a certain frequency (here, ~46 Hz) due to an interaction of the flame with ambient air entrainment. A careful observation reveals that the pure rotational band is in inverse correlation with the O2 and N2 spectra. This is explained by the fact that the pure rotational band is a flame marker (high temperature) in contrast to the two species, which decrease their peak intensity at higher temperature. Higher...