High Speed Sub-GHz Spectrometer for Brillouin Scattering Analysis

Summary

Here we present a protocol to build a rapid Brillouin spectrometer. Cascading virtually imaged phase array (VIPA) etalons achieve a measurement speed more than 1,000 times faster than traditional scanning Fabry-Perot spectrometers. This improvement provides the means for Brillouin analysis of tissue and biomaterials at low power levels in vivo.

Abstract

The goal of this protocol is to build a parallel high-extinction and high-resolution optical Brillouin spectrometer. Brillouin spectroscopy is a non-contact measurement method that can be used to obtain direct readouts of viscoelastic material properties. It has been a useful tool in material characterization, structural monitoring and environmental sensing. In the past, Brillouin spectroscopy has usually employed scanning Fabry-Perot etalons to perform spectral analysis. This process requires high illumination power and long acquisition times, making the technique unsuitable for biomedical applications. A recently introduced novel spectrometer overcomes this challenge by employing two VIPAs in a cross-axis configuration. This innovation enables sub-Gigahertz (GHz) resolution spectral analysis with sub-second acquisition time and illumination power within the safety limits of biological tissue. The multiple new applications facilitated by this improvement are currently being explored in biological research and clinical application.

Introduction

Brillouin scattering, first described by Leon Brillouin¹ in 1922, is the inelastic scattering of light from the thermal acoustic modes in a solid and from the thermal density fluctuations in a liquid or gas. The spectral shift of the scattered light, usually in the sub GHz-range, provides information about the interaction between the incident light and the acoustic phonons in the sample. As a result, it can provide useful information regarding the viscoelastic properties of the examined material.

In its spontaneous version, Brillouin scattering generally has cross-sections in the order of Raman scattering, resulting in a very wea....

Protocol

Note: Brillouin spectral analysis requires a single-longitudinal mode laser (~10 mW at the sample). For aligning purposes, use a strongly attenuated portion of this laser beam (<0.1 mW).

1. Initial Setup of Fiber and the EMCCD (Electron Multiplied Charge Coupled Device) Camera

1. Identify about 1,600 mm free aligning space for the spectrometer on an optical table.
2. Mount the EMCCD camera at the end of the free aligning space.
   1. Use set screws to attach the camera to posts. Tighten the posts in post holders at the desired optical axis height. Tighten the post holders onto the optical table using table clamps.

Results

Figure 3 shows representative Brillouin spectra and their fits for different materials. The VIPAs both have a thickness of 5 mm which results in a FSR of approximately 20 GHz. The integration time for these measurements was 100 msec. 100 measurements were taken and averaged. One calibration measurement was taken prior to acquiring the spectra.

Discussion

A key design feature of this spectrometer configuration is that the two stages can be aligned independently. When a VIPA etalon is slid out of the optical path, the remaining lenses of the spectrometer stage form a 1:1 imaging system, so that the spectral pattern from each stage is imaged onto the CCD camera. Therefore, it is straightforward to go back to either one of the stages to improve its performance without affecting the alignment of the other stage. The set of translational stages and degrees of freedom suggested.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
OPTICS
VIPA (virtual image phase array)	LIGH MACHINERY		Quantity: 2
Bundle of Three 423 Linear Stages with SM-25 Micrometers	NEWPORT	423-MIC	Quantity: 1
SS Crossed-Roller Bearing Translation Stage, 0.5 in., 8-32, 1/4-20	NEWPORT	9066-X	Quantity: 1
Vernier Micrometer, 13 mm Travel, 9 lb Load Capacity, 50.8 TPI	NEWPORT	SM-13	Quantity: 1
Adjustable Width Slit	NEWPORT	SV-0.5	Quantity: 2
Compact Dovetail Linear Stage, 0.20 in. Z Travel, 1.57x1.57x1.38 in.	NEWPORT	DS40-Z	Quantity: 2
Slotted Base Plate, 25 or 40 mm to 65 mm Stage, 1.1 in. Range	NEWPORT	B-2B	Quantity: 2
Ø1/2" Optical Post, 8-32 Setscrew, 1/4"-20 Tap, L = 2", 5 Pack	THORLABS	TR2-P5	Quantity: 2
Ø1/2" Post Holders, Spring-Loaded Hex-Locking Thumbscrews, L = 2", 5 Pack	THORLABS	PH2-P5	Quantity: 1
Ø1/2" Post Holders, Spring-Loaded Hex-Locking Thumbscrew, L = 3", 5 Pack	THORLABS	PH3-P5	Quantity: 1
Imperial Lens Mount For 2" Optics, 8-32 Tap	THORLABS	LMR2	Quantity: 2
f=200.0 mm, Ø2" Achromatic Doublet, ARC: 400-700 nm	THORLABS	AC254-200-A	Quantity: 2
Kinematic Mount for up to 1.3" (33 mm) Tall Rectangular Optics, Right Handed	THORLABS	KM100C	Quantity: 2
Fixed Cylindrical Lens Mount, Max Optic Height: 1.60" (40.6 mm)	THORLABS	CH1A	Quantity: 2
f=200.00 mm, H=30.00 mm, L=32.0 mm, N-BK7 Plano-Convex Cylindrical Lens, Antireflection Coating: 350-700 nm	THORLABS	L1653L1-A	Quantity: 2
Right-Angle Post Clamp, Fixed 90° Adapter	THORLABS	RA90	Quantity: 1
Adapter with External C-Mount Threads and Internal SM1 Threads	THORLABS	SM1A9	Quantity: 1
Studded Pedestal Base Adapter, 1/4"-20 Thread	THORLABS	PB4	Quantity: 2
Spacer, 2" x 3", 1.000" Thick	THORLABS	Ba2S7	Quantity: 2
543 nm, f=15.01 mm, NA=0.17 FC/APC Fiber Collimation Pkg.	THORLABS	F260APC-A	Quantity: 1
SM1-Threaded Adapter for Ø11 mm collimators	THORLABS	Ad11F	Quantity: 1
Translating Lens Mount for Ø1" Optics, 1 Retaining Ring Included	THORLABS	LM1XY	Quantity: 1
Single Mode Patch Cable, 450 - 600 nm, FC/APC, 2 m Long	THORLABS	P3-460B-FC-2	Quantity: 1
1:1 Matched Achr. Pair, f1=30 mm, f2=30 mm, BBAR 400-700 nm	THORLABS	MAP103030-A	Quantity: 1
SM1 Lens Tube…length to adjust depend on CCD, we have 3.5 inches	THORLABS	SM1LXX	Quantity: 1
Base Adapters for Ø1/2" Post Holders and Ø1" Posts	THORLABS	BE1	Quantity: 8
Clamping Forks for  Ø1/2" Post Holders and Ø1" Posts	THORLABS	CF125	Quantity: 8
HW-KIT5 - 4-40 Cap Screw and Hardware Kit for Mini-Series	THORLABS	HW-KIT5	Quantity: 1
D20S - Standard Iris, Ø20.0 mm Max Aperture	THORLABS	D20S	Quantity: 2
FOR ENCLOSURE
25 mm Construction Rail, L = 21"	THORLABS	XE25L21	Quantity: 6
1" Construction Cube with Three 1/4" (M6) Counterbored Holes	THORLABS	RM1G	Quantity: 8
Right-Angle Bracket for 25 mm Rails	THORLABS	XE25A90	Quantity: 12
25 mm Construction Rail, L = 15"	THORLABS	XE25L15	Quantity: 4
25 mm Construction Rail, L = 9"	THORLABS	XE25L09	Quantity: 8
High Performance Black Masking Tape, 2" x 60 yds. (50 mm x 55 m) Roll	THORLABS	T743-2.0	Quantity: 1
Low-Profile T-Nut, 1/4"-20 Tapped Hole, Qty: 10	THORLABS	XE25T3	Quantity: 1
1/4"-20 Low-Profile Channel Screws (100 Screws/Box)	THORLABS	SH25LP38	Quantity: 1
60" (W) x 3 yds. (L) x 0.005" (T) (1.5 m x 2.7 m x 0.12 mm) Blackout Fabric	THORLABS	BK5	Quantity: 1
CAMERA, LASER and MICROSCOPE
EMCCD camera	ANDOR	iXon Ultra 897	Quantity: 1
400 mW single mode green laser	LASER QUANTUM	torus 532	Quantity: 1
Research Inverted System Microscope	OLYMPUS	IX71	Quantity: 1