Name: high-speed continuous-wave stimulated brillouin scattering spectrometer for material analysis
Uploaded: 2017-09-22T12:30:00
Description: Watch this Scientific Journal Video about High-speed Continuous-wave Stimulated Brillouin Scattering Spectrometer for Material Analysis at JoVE.com

IR is grateful to the Azrieli Foundation for the PhD fellowship award.

Note: Unless stated otherwise, (i) connect all mounts to post holders and tighten the post bases with a clamping fork or mounting base to the optical table, and (ii) use output laser powers of 2 - 10 mW for all alignment procedures.
Note: Turn on all electrical/optoelectronic devices in the setup and allow 30 min of warmup time prior to use.
1. Prepare the Probe Beam Optical Path
<ol>
	<li>Mount and align the fiber collimator of the probe laser.
	...

<ol>
	<li>Koski, K. J., Akhenblit, P., McKiernan, K., Yarger, J. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Non-invasive+determination+of+the+complete+elastic+moduli+of+spider+silks.">Non-invasive determination of the complete elastic moduli of spider silks.</a> Nat. Mater. 12 (3), 262-267 (2013).</li><li>Palombo, F., Madami, M., Stone, N., Fioretto, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanical+mapping+with+chemical+specificity+by+confocal+Brillouin+and+Raman+microscopy.">Mechanical mapping with chemical specificity by confocal Brillouin and Raman microscopy.</a> Analyst. 139 (4), 729-733 (2014).</li><li>Scarcelli, G., Yun, S. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vivo+Brillouin+optical+microscopy+of+the+human+eye.">In vivo Brillouin optical microscopy of the human eye.</a> Opt. Exp. 20 (8), 9197-9202 (2012).</li><li>Scarcelli, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Noncontact+three-dimensional+mapping+of+intracellular+hydromechanical+properties+by+Brillouin+microscopy.">Noncontact three-dimensional mapping of intracellular hydromechanical properties by Brillouin microscopy.</a> Nat. Methods. 12 (12), 1132-1134 (2015).</li><li>Traverso, A. J., Thompson, J. V., Steelman, Z. A., Meng, Z., Scully, M. O., Yakovlev, V. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dual+Raman-Brillouin+microscope+for+chemical+and+mechanical+characterization+and+imaging.">Dual Raman-Brillouin microscope for chemical and mechanical characterization and imaging.</a> Anal. Chem. 87 (15), 7519-7523 (2015).</li><li>Antonacci, G., Foreman, M. R., Paterson, C., T&#246;r&#246;k, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectral+broadening+in+Brillouin+imaging.">Spectral broadening in Brillouin imaging.</a> Appl. Phys. Lett. 103 (22), 221105 (2013).</li><li>Antonacci, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantification+of+plaque+stiffness+by+Brillouin+microscopy+in+experimental+thin+cap+fibroatheroma.">Quantification of plaque stiffness by Brillouin microscopy in experimental thin cap fibroatheroma.</a> J. R. Soc. Interface. 12 (112), 20150483 (2015).</li><li>Grubbs, W. T., MacPhail, R. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High+resolution+stimulated+Brillouin+gain+spectrometer.">High resolution stimulated Brillouin gain spectrometer.</a> Rev. Sci. Instrum. 65 (1), 34-41 (1994).</li><li>Ballmann, C. W., Thompson, J. V., Traverso, A. J., Meng, Z., Scully, M. O., Yakovlev, V. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stimulated+Brillouin+scattering+microscopic+imaging.">Stimulated Brillouin scattering microscopic imaging.</a> Sci Rep. 5, 18139 (2015).</li><li>Remer, I., Bilenca, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Background-free+Brillouin+spectroscopy+in+scattering+media+at+780+nm+via+stimulated+Brillouin+scattering.">Background-free Brillouin spectroscopy in scattering media at 780 nm via stimulated Brillouin scattering.</a> Opt. Lett. 41 (5), 926-929 (2016).</li><li>Remer, I., Bilenca, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-speed+stimulated+Brillouin+scattering+spectroscopy+at+780+nm.">High-speed stimulated Brillouin scattering spectroscopy at 780 nm.</a> APL Photonics. 1 (6), 061301 (2016).</li><li>She, C. Y., Moosm&#252;ller, H., Herring, G. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coherent+light+scattering+spectroscopy+for+supersonic+flow+measurements.">Coherent light scattering spectroscopy for supersonic flow measurements.</a> Appl. Phys. B-Lasers O. 46 (4), 283-297 (1988).</li><li>Fiore, A., Zhang, j., Peng Shao, ., Yun, S. H., Scarcelli, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-extinction+virtually+imaged+phased+array-based+Brillouin+spectroscopy+of+turbid+biological+media.">High-extinction virtually imaged phased array-based Brillouin spectroscopy of turbid biological media.</a> Appl. Phys. Lett. 108 (20), 203701 (2016).</li></ol>

The system, shown in Figure 1, was designed to be built on an 18&#39;&#39; x 24&#39;&#39; breadboard that can be mounted vertically on an optical table, facilitating placement of watery samples. As a result, it is important to strongly tighten all optical and mechanical elements and ensure that the pump and probe beams are collinear and concentric with the various elements prior to illuminating the sample in off-axis geometry.
Difficulties in observing the stimulated Brillouin...

Spontaneous Brillouin spectroscopy has been established, in recent years, as a valuable approach for the mechanical analysis of soft materials, such as liquids, real tissue, tissue phantoms and biological cells1,2,3,4,5,6,7. In this approach, a single laser illuminates the sample and light that is inelastically scattered from spontaneous thermal acoustic waves in the medium is collected by a spectrometer, providing useful information on the viscoelastic properties of the sample. The spontaneous Brillouin spectrum includes two Brillouin peaks at the acoustic Stokes and anti-Stokes resonances of the material, and a Rayleigh peak at the illuminating laser frequency (due to elastically scattered light). For a Brillouin backscattering geometry, the Brillouin frequencies are shifted by several GHz from the illuminating laser frequency and have spectral width of hundreds of MHz.
While scanning Fabry-Perot spectrometers have been the systems-of-choice for acquiring spontaneous Brillouin spectra in soft matter1,2, recent technological advances in virtually imaged phase array (VIPA) spectrometers have enabled significantly faster (sub-second) Brillouin measurements with adequate spectral-resolution (sub-GHz)3,4,5,6,7. In this protocol, we present the construction of a different, high-speed, high spectral-resolution, accurate Brillouin spectrometer based on the detection of continuous-wave-stimulated-Brillouin-scattering (CW-SBS) light from non-turbid and turbid samples in a nearly back scattering geometry.
In CW-SBS spectroscopy, continuous-wave (CW) pump and probe lasers, slightly detuned in frequency, overlap in a sample to stimulate acoustic waves. When the frequency difference between the pump and probe beams matches a specific acoustic resonance of the material, amplification or deamplification of the probe signal is provided by stimulated Brillouin gain or loss (SBG/SBL) processes, respectively; otherwise, no SBS (de)amplification occurs8,9,10,11. Thus, an SBG (SBL) spectrum can be acquired by scanning the frequency difference between the lasers across the material Brillouin resonances and detecting the increase (decrease), or gain (loss), in the probe intensity due to SBS. Unlike in spontaneous Brillouin scattering, elastic scattering background is inherently absent in SBS, enabling excellent Brillouin contrast in both turbid and non-turbid samples without any need for Rayleigh rejection filters as required in VIPA spectrometers10,11,13.
The main building blocks of a CW-SBS spectrometer are the pump and probe lasers and the stimulated Brillouin gain/loss detector. For high spectral-resolution, high speed CW-SBS spectroscopy, the lasers need to be single-frequency (&#60; 10 MHz linewidth) with sufficiently wide wavelength tunability (20 - 30 GHz) and scanning rate (&#62; 200 GHz/s), long-term frequency stability (&#60;50 MHz/h) and low intensity noise. Furthermore, linearly polarized and diffraction-limited laser beams with powers of few hundreds (tens) of mW on the sample are required for the pump (probe) beam. Finally, the stimulated Brillouin gain/loss detector should be designed to reliably detect weak backward stimulated Brillouin gain/loss (SBG/SBL) levels (10-5 - 10-6) in soft matter. To meet these needs, we selected distributed feedback (DFB) diode lasers coupled to polarization-maintaining fibers along with a stimulated Brillouin gain/loss detector combining an ultra-narrowband atomic vapor notch-filter and a high-frequency single-modulation lock-in amplifier as illustrated in Figure 1. This detection scheme doubles the intensity of the SBG signal while significantly reducing noise in the probe intensity, where the desired SBG signal is embedded11. Note that the role of the atomic vapor notch-filter used in our SBS spectrometer is to significantly reduce the detection of unwanted stray pump reflections rather than to decrease the elastic scattering background as in VIPA spectrometers that detect both spontaneous Rayleigh and Brillouin scattered light. Using the protocol detailed below, a CW-SBS spectrometer can be constructed with the capability of acquiring transmission spectra of water and tissue phantoms with SBG levels as low as 10-6 at &#60;35 MHz Brillouin-shift measurement precision and within 100 ms or less.
<img alt="Figure 1" src="/files/ftp_upload/55527/55527fig1.jpg" /> 
Figure 1: Continuous-wave Stimulated Brillouin Scattering (CW-SBS) Spectrometer. Two continuous-wave pump and probe diode lasers (DL), frequency detuned around the Brillouin shift of the sample, are coupled into polarization-maintaining single-mode fibers with collimators C1 and C2, respectively. The pump-probe frequency difference is measured by detecting the beat frequency between beams peeled from the pump and probe lasers using a set of fiber splitters (FS), a fast photodetector (FPD), and a frequency counter (FC). The S-polarized probe beam (light red), expanded using a Keplerian beam expander (L1 and L2), is right circularly polarized by a quarter-wave plate (&#955;1/4) and focused on the sample (S) by an achromatic lens (L3). For effective SBS interaction and optical isolation, the pump beam (deep red), expanded using a Keplerian beam expander (L5 and L6), is first P-polarized using a half-wave plate &#955;2/4), then transmitted through a polarizing beam splitter (PBS), and is finally left circularly polarized by a quarter-wave plate (&#955;2/4) and focused on the sample with an achromatic lens (L4; same as L3). Note that the pump and probe beams nearly counter-propagate in the sample and that an S-oriented polarizer (P) was used to prevent the P-polarized pump beam (coming out of &#955;1/4) from entering the probe laser. For lock-in detection, the pump beam is sinusoidally modulated at fm with an acousto-optical modulator (AOM). The SBG signal, manifested as intensity variations at frequency fm (see inset), is demodulated with a lock-in amplifier (LIA) following detection by a large-area photodiode (PD). For significant elimination of stray pump reflections in the photodiode, a narrowband Bragg filter (BF) and an atomic notch filter (85RB) around the pump wavelength are used alongside with a light-blocking iris (I). Data is recorded by a data acquisition card (DAQ) connected to a personal computer (PC) for further analysis of the Brillouin spectrum. All folding mirrors (M1-M6) are used to fit the spectrometer on a 18&#39;&#39;&#215;24&#39;&#39; breadboard that is vertically mounted on the optical table for facilitating placement of watery samples. <a href="http://cloudflare.jove.com/files/ftp_upload/55527/55527fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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	<tbody>
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			<td>Probe diode laser head and controller</td>
			<td>Toptica Photonics</td>
			<td>SYST DL-100-DFB</td>
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			<td>Pump amplified diode laser and controller</td>
			<td>Toptica Photonics</td>
			<td>SYST TA-pro-DFB</td>
			<td>Quantity: 1</td>
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			<td>FC/APC fiber dock</td>
			<td>Toptica Photonics</td>
			<td>FiberDock&#160;</td>
			<td>Quantity: 3</td>
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			<td>High power single mode polarization maintaining FC/APC fiber patchcord</td>
			<td>Toptica Photonics</td>
			<td>OE-000796</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>FC/APC fiber collimation with adjustable collimation optics</td>
			<td>Toptica Photonics</td>
			<td>FiberOut</td>
			<td>Quantity: 1</td>
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			<td>FC/APC fiber fixed collimator</td>
			<td>OZ Optics</td>
			<td>HPUCO-33A-780-P-6.1-AS</td>
			<td>Quantity: 1</td>
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			<td>Single mode polarization maintaining fiber splitter 33:67</td>
			<td>OZ Optics</td>
			<td>FOBS-12P-111-4/125-PPP-780-67/33-40-3A3A3A-3-1</td>
			<td>Quantity: 1</td>
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			<td>Single mode polarization maintaining fiber splitter 50:50</td>
			<td>OZ Optics</td>
			<td>FOBS-12P-111-4/125-PPP-780-50/50-40-3S3A3A-3-1</td>
			<td>Quantity: 1</td>
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			<td>f=25 mm, &#216;1/2&#34; Achromatic Doublet, SM05-Threaded Mount, ARC: 650-1050 nm</td>
			<td>Thorlabs</td>
			<td>AC127-025-B-ML</td>
			<td>Quantity: 1</td>
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			<td>f=30 mm, &#216;1&#34; Achromatic Doublet, SM1-Threaded Mount, ARC: 650-1050 nm</td>
			<td>Thorlabs</td>
			<td>AC254-30-B-ML</td>
			<td>Quantity: 2</td>
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			<td>f=50 mm, &#216;1&#34; Achromatic Doublet, SM1-Threaded Mount, ARC: 650-1050 nm</td>
			<td>Thorlabs</td>
			<td>AC254-50-B-ML</td>
			<td>Quantity: 1</td>
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			<td>f=100 mm, &#216;1&#34; Achromatic Doublet, SM1-Threaded Mount, ARC: 650-1050 nm</td>
			<td>Thorlabs</td>
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AC254-100-B-ML</td>
			<td>Quantity: 1</td>
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			<td>f=200 mm, &#216;1&#34; Achromatic Doublet, SM1-Threaded Mount, ARC: 650-1050 nm</td>
			<td>Thorlabs</td>
			<td>
AC254-200-B-ML</td>
			<td>Quantity: 1</td>
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			<td>&#216;1/2&#34; Broadband Dielectric Mirror, 750-1100 nm</td>
			<td>Thorlabs</td>
			<td>
BB05-E03</td>
			<td>Quantity: 4</td>
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			<td>&#216;1&#34; Broadband Dielectric Mirror, 750-1100 nm</td>
			<td>Thorlabs</td>
			<td>
BB1-E03</td>
			<td>Quantity: 2</td>
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			<td>1&#34; Polarizing beamsplitter cube, 780 nm</td>
			<td>Thorlabs</td>
			<td>
PBS25-780</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>&#216;1&#34; Linear polarizer with N-BK7 protective windows, 600-1100 nm</td>
			<td>Thorlabs</td>
			<td>
LPNIRE100-B</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>Shearing Interferometer with a 1-3 mm Beam Diameter Shear Plate</td>
			<td>Thorlabs</td>
			<td>SI035</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>6-Axis Locking kinematic optic mount</td>
			<td>Thorlabs</td>
			<td>
K6XS</td>
			<td>Quantity: 4</td>
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			<td>Compact five-axis platform</td>
			<td>Thorlabs</td>
			<td>
PY005</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>Pedestal mounting adapter for 5-axis platform</td>
			<td>Thorlabs</td>
			<td>
PY005A2</td>
			<td>Quantity: 1</td>
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			<td>Polaris low drift &#216;1/2&#34; kinematic mirror mount, 3 adjusters</td>
			<td>Thorlabs</td>
			<td>
POLARIS-K05</td>
			<td>Quantity: 4</td>
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			<td>Lens mount for &#216;1&#34; optics</td>
			<td>Thorlabs</td>
			<td>
LMR1</td>
			<td>Quantity: 5</td>
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		<tr>
			<td>Adapter with external SM1 threads and Internal SM05 threads, 0.40&#34; thick</td>
			<td>Thorlabs</td>
			<td>
SM1A6T</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>Rotation mount for &#216;1&#34; optics</td>
			<td>Thorlabs</td>
			<td>
RSP1</td>
			<td>Quantity: 2</td>
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		<tr>
			<td>1&#34; Kinematic prism mount</td>
			<td>Thorlabs</td>
			<td>
KM100PM</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>Graduated ring-activated SM1 iris diaphragm</td>
			<td>Thorlabs</td>
			<td>
SM1D12C</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>Post-mounted iris diaphragm, &#216;12.0 mm max aperture</td>
			<td>Thorlabs</td>
			<td>ID12</td>
			<td>Quantity: 2</td>
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		<tr>
			<td>1/2&#34; translation stage with standard micrometer</td>
			<td>Thorlabs</td>
			<td>MT1</td>
			<td>Quantity: 3</td>
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		<tr>
			<td>&#216;1&#34; Pedestal pillar post, 8-32 taps, L = 1&#34;</td>
			<td>Thorlabs</td>
			<td>RS1P8E</td>
			<td>Quantity: 1</td>
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			<td>&#216;1&#34; Pedestal pillar post, 8-32 taps, L = 1.5&#34;</td>
			<td>Thorlabs</td>
			<td>RS1.5P8E</td>
			<td>Quantity: 2</td>
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		<tr>
			<td>&#216;1&#34; Pedestal pillar post, 8-32 taps, L = 2&#34;</td>
			<td>Thorlabs</td>
			<td>RS2P8E</td>
			<td>Quantity: 4</td>
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		<tr>
			<td>&#216;1&#34; Pedestal pillar post, 8-32 taps, L = 2.5&#34;</td>
			<td>Thorlabs</td>
			<td>RS2.5P8E</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>&#216;1&#34; Pedestal pillar post, 8-32 taps, L = 3&#34;</td>
			<td>Thorlabs</td>
			<td>RS3P8E</td>
			<td>Quantity: 4</td>
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		<tr>
			<td>Short clamping fork</td>
			<td>Thorlabs</td>
			<td>CF125</td>
			<td>Quantity: 12</td>
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		<tr>
			<td>Mounting base</td>
			<td>Thorlabs</td>
			<td>BA1S</td>
			<td>Quantity: 8</td>
		</tr>
		<tr>
			<td>Large V-Clamp with PM4 Clamping Arm, 2.5&#34; Long, Imperial</td>
			<td>Thorlabs</td>
			<td>VC3C</td>
			<td>Quantity: 1</td>
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		<tr>
			<td>&#216;1/2&#34; Post holder, spring-loaded hex-locking thumbscrew, L = 1&#34;</td>
			<td>Thorlabs</td>
			<td>PH1</td>
			<td>Quantity: 2</td>
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		<tr>
			<td>&#216;1/2&#34; Post holder, spring-loaded hex-locking thumbscrew, L = 1.5&#34;</td>
			<td>Thorlabs</td>
			<td>PH1.5</td>
			<td>Quantity: 2</td>
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			<td>&#216;1/2&#34; Post holder, spring-loaded hex-locking thumbscrew, L = 2&#34;</td>
			<td>Thorlabs</td>
			<td>PH2</td>
			<td>Quantity: 6</td>
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		<tr>
			<td>&#216;1/2&#34; Optical post, SS, 8-32 setscrew, 1/4&#34;-20 tap, L = 1&#34;</td>
			<td>Thorlabs</td>
			<td>TR1</td>
			<td>Quantity: 2</td>
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		<tr>
			<td>&#216;1/2&#34; Optical post, SS, 8-32 setscrew, 1/4&#34;-20 tap, L = 1.5&#34;</td>
			<td>Thorlabs</td>
			<td>TR1.5</td>
			<td>Quantity: 2</td>
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		<tr>
			<td>&#216;1/2&#34; Optical post, SS, 8-32 setscrew, 1/4&#34;-20 tap, L = 2&#34;</td>
			<td>Thorlabs</td>
			<td>TR2</td>
			<td>Quantity: 6</td>
		</tr>
		<tr>
			<td>Aluminum breadboard 18&#34; x 24&#34; x 1/2&#34;, 1/4&#34;-20 taps</td>
			<td>Thorlabs</td>
			<td>MB1824</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>12&#34; Vertical bracket for breadboards, 1/4&#34;-20 holes, 1 piece</td>
			<td>Thorlabs</td>
			<td>VB01</td>
			<td>Quantity: 2</td>
		</tr>
		<tr>
			<td>Si photodiode, 40 ns Rise time, 400 - 1100 nm, 10 mm x 10 mm active area</td>
			<td>Thorlabs</td>
			<td>
FDS1010</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Waveplate, zero order, 1/4 wave 780nm</td>
			<td>Tower Optics</td>
			<td>Z-17.5-A-.250-B-780</td>
			<td>Quantity: 2</td>
		</tr>
		<tr>
			<td>Waveplate, zero order, 1/2 wave 780nm</td>
			<td>Tower Optics</td>
			<td>Z-17.5-A-.500-B-780</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Fiber coupled ultra high speed photodetector</td>
			<td>Newport</td>
			<td>1434</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Gimbal optical miror mount</td>
			<td>Newport</td>
			<td>U100-G2H ULTIMA</td>
			<td>Quantity: 3</td>
		</tr>
		<tr>
			<td>linear stage with 25 mm travel range</td>
			<td>Newport</td>
			<td>&#160;M-423&#160;</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Lockable differential micrometer, 25 mm coarse, 0.2 mm fine,11 lb. load</td>
			<td>Newport</td>
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high-speed continuous-wave stimulated brillouin scattering spectrometer for material analysis

Recent years have witnessed a significant increase in the use of spontaneous Brillouin spectrometers for non-contact analysis of soft matter, such as aqueous solutions and biomaterials, with fast acquisition times. Here, we discuss the assembly and operation of a Brillouin spectrometer that uses stimulated Brillouin scattering (SBS) to measure stimulated Brillouin gain (SBG) spectra of water and lipid emulsion-based tissue-like samples in transmission mode with &lt;10 MHz spectral-resolution and &lt;35 MHz Brillouin-shift measurement precision at &lt;100 ms. The spectrometer consists of two nearly counter-propagating continuous-wave (CW) narrow-linewidth lasers at 780 nm whose frequency detuning is scanned through the material Brillouin shift. By using an ultra-narrowband hot rubidium-85 vapor notch filter and a phase-sensitive detector, the signal-to-noise-ratio of the SBG signal is significantly enhanced compared to that obtained with existing CW-SBS spectrometers. This improvement enables measurement of SBG spectra with up to 100-fold faster acquisition times, thereby facilitating high spectral-resolution and high-precision Brillouin analysis of soft materials at high speed.

Figures 2b and 3b display typical point SBG spectra of distilled water and lipid-emulsion tissue phantom samples (with 2.25 scattering events and an attenuation coefficient of 45 cm-1) measured within 10 ms and 100 ms, respectively. For comparison, we measured the SBG spectra in 10 s as shown in Figures 2a and 3a. In these measurements, the rubidium-85 vapor cell was heated to 90 &#176;C for attenuating stray p...

Watch this Scientific Journal Video about High-speed Continuous-wave Stimulated Brillouin Scattering Spectrometer for Material Analysis at JoVE.com

High-speed Continuous-wave Stimulated Brillouin Scattering Spectrometer for Material Analysis

We describe the construction of a rapid continuous-wave-stimulated-Brillouin-scattering (CW-SBS) spectrometer. The spectrometer employs single-frequency diode-lasers and an atomic vapor notch-filter to acquire transmission spectra of turbid/non-turbid samples with high spectral-resolution at speeds up to 100-fold faster than those of existing CW-SBS spectrometers. This improvement enables high-speed Brillouin material analysis.

Recent years have witnessed a significant increase in the use of spontaneous Brillouin spectrometers for non-contact analysis of soft matter, such as ...

The goal of this experiment is to construct and operate a continuous-wave stimulated Brillouin scattering spectrometer to acquire transmission stimulated Brillouin spectra of turbid and non-turbid samples with high spectro resolution and speed. This method can advance the use of Brillouin spectroscopy and imaging for investigating the mechanics of biomaterials such as cells and tissue. The main advantage of this technique is that it can provide rapid acquisition of Brillouin spectra of turbid and non-turbid matter.To begin the experiment, verify that the components of the CW-SBS spectrometer are securely mounted on an optical board. Check that the custom data acquisition software is receiving data from the microwave frequency counter, the lock-in amplifier, the pump and probe laser controllers, and the function generator. Then fill with distilled water a sample chamber made of two 25 millimeter diameter 0.17 millimeter thick glass cover slips and 500 micrometer thick polytetrafluoroethylene tape.Mount a chamber holder on a three-axis motorized translation stage. Secure the water sample in the chamber holder. Translate the sample to the common focus point of the probe and pump focusing lenses.Then adjust the current knob of the tapered amplifier pump laser controller until the pump laser power is above 250 milliwatts. Set the lock-in amplifier time constant to one second, the low-pass filter to 24 decibels per octave, and the sensitivity to one millivolt RMS. Set the phase shift between the amplifier reference and signal inputs to zero.While monitoring the stray pump reflections on channel one, slowly adjust the pump laser current until the observed reflections are at a minimum. Then set the rubidium vapor cell heating assembly power supply to 17 volts DC.Once the rubidium cell stabilizes at 90 degrees Celsius, place the power meter before the probe laser focusing lens. Slowly adjust the probe laser current until the measured power is over 10 milliwatts.Next, move the power meter behind the rubidium cell. Adjust the probe laser temperature until the measured power is at a minimum. Then adjust the probe laser current until the measured power stabilizes above 10 milliwatts.Alter the probe laser current to set the frequency detuning between the pump and probe lasers to approximately the Brillouin shift of the sample. Set the lock-in amplifier sensitivity to 100 microvolts RMS and set the phase shift back to zero. Then carefully turn the pitch and yaw screws of the pump beam folding mirror kinematic mount and transfer the pump focusing lens along the optical axis of the system to optimize the crossing efficiency of the pump and probe lasers.Block the probe beam and note the levels of the stray pump laser reflections while monitoring channel one of the lock-in amplifier. It is critical to properly reject pump stray reflections from the chamber and sample. If needed, slightly close the iris placed in front of the rubidium cell and slightly increase the cross angle between the pump and probe laser to improve the rejection levels.Unblock the probe beam and continue adjusting the pump laser mirror and focusing lens until the stimulated Brillouin gain signal is at a maximum above two microvolts RMS and the stray pump reflections are at a constant minimum. Once the system has been optimized for the sample, create a calibration curve from a plot of probed modulation current versus pump-probe frequency detuning. Set the lock-in amplifier time constant to greater than or equal to 100 microseconds.Set the function generator connected to the laser controller current modulation input to channel one. Select a triangle waveform, set the amplitude to a peak to peak voltage of 150 millivolts, and set the frequency to 50 Hertz. Set the data acquisition unit sampling rate to less than or equal to 100, 000 samples per second per channel.Using the custom data acquisition software, record the SBG and probe laser modulation current signals for at least 10 milliseconds. Import the acquired data into computational software. Use the calibration curve to convert the measured probe laser modulation current values to the pump-probe frequency detuning values.Subtract the average noise floor. Plot the SBG measurements against the pump-probe frequency detuning values to generate the SBG spectrum. Fit the spectrum with a Lorentzian curve using the amplitude, frequency position, and full width at half of the highest point of the spectrum as initial values.Calculate the Brillouin shift and the line width from the frequency positions of the maximum and the full width at half maximum of the Lorentzian fit respectively. Use values from repeated Lorentzian fitted spectra to generate a corresponding histogram. SBG spectra of distilled water and lipid emulsion tissue phantom samples were generated using this method with a spatial resolution of approximately eight micrometers.The water samples were acquired in 10 seconds and 10 milliseconds and the lipid samples were acquired in 10 seconds and 100 milliseconds. The mean Brillouin shifts determined from the rapidly acquired spectra were 5.08 gigahertz for water and 5.11 gigahertz for the tissue phantom. These values were consistent with those calculated from the spectra recorded over 10 seconds and with literature values.200 successive measurements were performed on each sample and the standard deviations of the distributions of estimated Brillouin shifts were evaluated. The standard deviations were 8.5 megahertz and 33 megahertz for the water and tissue samples respectively. After watching this video, you should have a good understanding of how to assemble and operate a stimulated Brillouin scattering spectrometer to rapidly measure stimulated Brillouin gained spectra of water and tissue-like samples in transmission mode.

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