This document describes a near-infrared spatial heterodyne Raman spectroscopy system and its capabilities and limitations. The system uses a 785nm laser, filters, and diffraction gratings to isolate and analyze Raman scattered photons. It produces Raman spectra with high light throughput without moving parts, making it suitable for extraterrestrial and medical applications. Experiments showed the system can maintain signal quality over a 3mm beam and detect samples across a 2.5mm beam, though sensitivity drops at the edges. The coherence length limits resolving power and the Raman bandwidth is currently around 500cm-1 instead of the expected 1200cm-1. Future work aims to address these limitations through equipment upgrades and testing a 532nm system.

1. Near-Infrared Spatial Heterodyne Raman Spectroscopy
Overview
Raman spectroscopy has potential applications in extraterrestrial
and medical work due to its nondestructive nature as well as its
ability to identify specific chemical species. By using a near-infrared
(NIR) excitation source, we reduce fluorescence significantly, which
can be a problem when trying to see low intensity Raman signals.
More specifically, spatial heterodyne spectrometers (SHS) are of
interest in performing Raman work due to their lack of moving parts
and high light throughput. No moving parts makes the SHS system
ideal for extraterrestrial research, where it has less chance to fail.
The high light throughput allows for smaller integration times, so that
the system is useful in medical applications such as quickly scanning
tissue samples for cancerous cells.
Current System
A 300 mW, 785 nm laser diode is used as the excitation source. We
are interested in collecting the stokes Raman scattered photons,
which have a higher wavelength than the incident light, but only
occur in approximately one in every million photons. To isolate this
weak signal, a dichroic mirror, a notch filter and a long-pass filter are
used to filter out the Rayleigh and the anti-stokes Raman scattering
that emit photons of 785 nm or lower wavelength.
The stokes Raman signal is then sent to the diffraction
gratings, reflected and recombined by a 50/50 beamsplitter to
produce an interference pattern in the form of fringes. The angle of
reflectance is wavelength-dependent to produce fringes of different
spatial frequencies, and the gratings are set up such that they
produce zero frequency at the incident 785 nm source. The fringe
pattern produced by the interference is then captured by the CMOS
sensor to be processed to produce a Raman spectrum.
Spectral Results
The fringe pattern captured by the CMOS sensor returns a Raman
spectrum following the process illustrated in Fig.2. The raw image
(Fig. 2A) is first processed by stacking several short exposure
images taken continuously to increase the integration time without
saturating the sensor. Background images are then subtracted to
reduce the thermal noise.
By performing a one dimensional fast fourier transform (FFT) on
each row across the resultant image with a hann windowing function
and averaging over a total of 2048 rows, a spectrum is produced (Fig.
2B) with its horizontal axis being the spatial frequency on the camera.
Several known sources can then be used to calibrate the spatial
frequency to the Raman shift, which produces the Raman spectrum
(Fig. 2C). The Raman spectra of sulfur (Fig. 2C) and CCl4 (Fig. 3) are
consistent with known literature values 1
.
Experiments
Fringe dependence on beam size
Using a pair of simple lenses allowed us to change the size of the
beam shone onto the sample in the excitation path without affecting
the Raman collection path. We observed that the fringe visibility and
the intensity of the Raman signal did not exhibit significant degradation
as the excitation beam was gradually defocused to a diameter of
approximately 3 mm, as shown in Fig. 4. Hence, the system is capable
of scanning larger areas in a short amount of time using a large beam
diameter. The power per unit area delivered to the sample is thus
reduced, which is especially important for delicate biological samples.
Lateral variation of sample within the excitation beam
A piece of 1.5 mm diameter sulfur was translated through an excitation
beam of roughly 2.5 mm diameter from edge to edge. The result in
Fig. 5 shows that the intensity of the Raman signal drops off as the
sample moves away from the center of the beam. Hence, for the
system’s application in the microscopic level, this significant drop-off in
the sensitivity at the edge of the beam needs to be taken into account.
A scan across the entire bulk sample at the center of the beam is
required for a thorough detection of any microscopic species of interest
inside the sample.
Limitations
- Coherence length limits the size of the interference envelope as
shown in Figure 6A. The width of the sulfur rayleigh interference
envelope (Fig. 6B) compared to that of the Raman (Fig. 6C)
suggests that the coherence length of Raman scattering decreases
significantly, which limits the width of the visible fringe pattern, thus
limiting the resolving power of the system 2
;
- Bandwidth of the Raman signal is limited to around 500 cm-1
as
opposed to the expected 1200 cm-1
, most likely due to the extremely
low Raman efficiency with a NIR excitation source.
Future Work
- Building a 532 nm SHS Raman device to compare results directly to
the 532 nm dispersive system created last summer in EVIL;
- Implementing an external cavity attachment to vary the coherence of
a 532 nm diode laser to examine its impact on the Raman coherence;
- Adding on a Michelson-Morley interferometer to measure the
coherence length of the Raman signal;
- Moving from 300 grooves/mm to 150 grooves/mm gratings to extend
the Raman bandwidth;
- Changing the gratings to a back-to-back configuration to assure equal
angles and path lengths, which are the conditions for interference;
Acknowledgments
We would like to acknowledge Prof. Lyzenga, Dr. Storrie-Lombardi for
their insights and advice, Fall 2015 & Spring 2016 Kinohi Clinic Team
for their previous work on the project, and, Annie Atiyeh and Daniel
Guerra for their support in facilitating our experiments.
References
1. RRUFF. “Raman spectrum of Sulfur, RRUFF ID: R050006.3.” Web. 22 July 2016.
2. Lenzner, Matthias, and Jean-Claude Diels. "Concerning the Spatial Heterodyne Spectrometer." Optical
Society of America. OSA Publishing, 25 Jan. 2016. Web. 1 July 2016.
Fig. 5 A plot of the intensity of the Raman signal
versus the lateral position of the sample within the
beam.
Fig. 4 A plot of the intensity of the Raman signal
versus the diameter of the beam.
Fig. 2A Fringe pattern produced by the
sulfur Raman scattering. Fig. 2B Spatial frequency spectrum obtained by
performing 1D-FFT on Fig. 2A after processing.
Fig. 2C Raman spectrum of sulfur.Fig. 3 Raman spectrum of CCl4.
Fig. 1A The current system is comprised of an
excitation path, a Raman collection path and
an imaging path.
Fig. 1B A diagram of the NIR spatial heterodyne
Raman spectrometer.
Fig. 6A An illustration of how the
coherence length limits the width
of the interference envelope 2
.
Fig. 6C The width of the envelope
over which the sulfur Raman
interference produces visible fringes.
Fig. 6B Fringe pattern produced by
the sulfur rayleigh scattering, which
is visible across the entire beam.
coherence
length
Gregory Lyzenga, Luis Martinez, Michael Storrie-Lombardi, Jiaxin Yu, Willie Zuniga
Extraterrestrial Vehicle Instrumentation Lab (EVIL): Harvey Mudd College, 301 Platt Blvd, Claremont, CA 91711
evil.physics.hmc.edu