Ultra-High-Speed, Time-Resolved0 pages

Application Note
em

IMAGING GROUP

Ultra-High-Speed, Time-Resolved
Spontaneous Raman Scattering
Spectroscopy in Combustion
© 2013 Princeton Instruments, Inc. All rights reserved.

Introduction

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