Application Noteem

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 opticalgating 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 opticalbackground 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, whilea high-speed mechanical shutter with a rotary opticalchopper 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 notsufficient 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 ina 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 orthogonallypolarized excitation lasers, the technique described abovemeasures single-shot vibrational Raman scattering thatis minimally contaminated by optical background noise.
Nonetheless, its relatively long gating (~5 µsec) still leaves
room for improvement in terms of optical backgroundrejection.
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

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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 fastergating (<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 camerathat keeps up with the latest 10 kHz lasers.
Reﬂector	  Lens	  Nd:YAG	  Pulsed	  Laser	  (10	  kHz)	  Prism	  Burner	  Lens	  Beam	  Dump	  Lens	  Lens	  To enhance the SNR, the image intensifier was operated
at a 10 kHz rate to keep pace with the high-speed laser
and gated at 90 nsec to cut out the optical flame emission
background, while the EMCCD was operated at a rate of
1 kHz (i.e., 10 laser-shot accumulation) using a special
feature within Princeton Instruments LightField® softwarethat enables customization of CCD size and readout
speed. This custom detection setting effectively enabled
the diagnostic system to achieve the highest signal levelever achieved in the NASA facility without sacrificing thenecessary kHz data rate.

Enabling Technology

New PI-MAX4:512EM (see Figure 3) and PI-MAX4:512EMB
cameras from Princeton Instruments leverage the key
advantages of both EMCCDs and ICCDs by fiberopticallycoupling an EMCCD to an image intensifier. This innovative
emICCD technology lets the new cameras deliver anunrivaled combination of precision, true single-photon
detection, intelligence, and speed.

PI-­‐MAX4:	  512EM	  Op9cal	  Fiber	  Figure 2. Experimental apparatus showing a high-speed laser Raman
diagnostic system employing an Nd:YAG pulsed laser (532 nm, 88
nsec pulse width, 10 kHz repetition rate) and a Princeton Instruments
PI-MAX4:512EM camera coupled to a lens spectrograph. Courtesy ofJ. Kojima (OAI/NASA).

Here, a second-harmonic Nd:YAG pulsed laser was used as
an excitation source (200 W) and operated at a 10 kHz
repetition rate to interrogate a flame. The scattering light
was collected by fiber-coupled lens optics and transmitted
to a volume-transmissive lens spectrograph equipped with aPI-MAX®4:512EM camera from Princeton Instruments.

2emSpectrograph	  Figure 3. The new Princeton Instruments PI-MAX4:512EM is the firstscientific camera on the market to utilize revolutionary emICCD
technology. The PI-MAX4:512EM uses a front-illuminated EMCCD; thePI-MAX4:512EMB uses a back-illuminated EMCCD.

The PI-MAX4:512EMB camera’s back-illuminated EMCCD
boasts 95% QE at 532 nm, the wavelength typically usedfor SRS spectroscopy in combustion. Furthermore, coupling
the EMCCD to an image intensifier via fiberoptics delivers
6x higher light throughput between the image intensifier andthe detector than lens-coupled configurations. Fiberoptic
bonding also provides much better SNR than lens-coupleddevices.

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The exceptional linearity and dynamic range of these newemICCD 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-likeuser interface (see Figure 4) even remembers complete
experimental setups. In addition, thanks to the ability toacquire 10,000 spectra/sec when operated in a special
custom chip mode, Princeton Instruments’ emICCD cameras
can capture every pulse from next-generation lasers.

Figure 4. An oscilloscope-like user interface enhances the utility of
Princeton Instruments’ emICCD cameras.

Results

Observed Raman Bands

High-speed
laser
spectroscopy
measurements
were performed using the diagnostic apparatus
shown in Figure 2 at the Atmospheric Pressure
Combustion Diagnostics (APCD) lab, NASA Glenn ResearchCenter, in Cleveland, Ohio. Figure 5a shows a close-up ofa flame. Figure 5b presents data for temporal variationof spontaneous Raman scattering of combustion species(oxygen, nitrogen, and water vapor) measured at the tipof a fuel-lean hydrogen-air flame. Pixel region (1 to 512)
corresponds to the wavelength region of 486 to 680 nm.

Pure Rotational

Figure 5. (a) Close-up of flame.
(b)Time-series Stokes Raman
scattering spectra recorded ata sampling rate of 1 kHz over
one second in an oscillating
fuel-lean hydrogen-air flameusing the high-speed Raman
diagnostic
apparatus
with
PI-MAX4:512EM. Courtesy ofJ. Kojima and D. Fischer (OAI/NASA).

(a)

(b)

3

O2

N2H2O

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The signal visibility in Figure 5b is significantly higher thanpreviously reported data of this kind. It is clearly seen fromthe data that the flame oscillates at a certain frequency(here, ~46 Hz) due to an interaction of the flame withambient air entrainment.
A careful observation reveals that the pure rotationalband is in inverse correlation with the O2 and N2 spectra.
This is explained by the fact that the pure rotationalband is a flame marker (high temperature) in contrast to
the two species, which decrease their peak intensity athigher temperature. Higher concentration of water vapor(H2O), the combustion product, appears when the flameis observed. It is significant that flame dynamics werecharacterized in a species-resolved fashion using highspeed Raman spectroscopy.

Future Directions

The recent use of a new diagnostic apparatus to measure
the dynamics of each individual molecular species, asopposed to simply acquire bulk information (e.g., pressure),
points to the possibility of performing temperature andfrequency analyses of species in combustion. Ultimately,such potential applications could become diagnostic tools
for fuel-air ratio dynamics at different temperatures andpressures.

Advances in scientific detector technology, such as thePI-MAX4:512EMB camera’s ability to gate out all opticalbackground noise at <1 nsec and thus improve SNR fortime-resolved spontaneous Raman scattering spectroscopy
in combustion, continue to extend the horizons of researchin this area.

Resources

To learn more about combustion research at NASA
Glenn, please visit:
http://facilities.grc.nasa.gov/erb/capabilities.html
For more information about Princeton Instruments’new emICCD technology, please visit:www.emICCD.com

Reference

1. J. Kojima, D. Fischer, and Q.-V. Nguyen, Opt. Lett.
35, 9 (2010).

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Rev A14