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Supercontinuum Fiber-Loop Cavity
Enhanced Absorption
Spectroscopy for H2O/D2O samples
Mingyun Li & Kevin Lehmann
Department of Chemistry and Physics
University of Virginia
Introduction
• Liquid phase Cavity Enhanced Absorption
spectroscopy (CEAS) can perform liquid
sensing at low concentrations.
• A single mode fiber-loop is made to work as a
resonant cavity because of its simple and
robust setup. Input
l
Resonant Cavity
Output
detector
Introduction
• A Supercontinuum (SC) is a broadband light
source that can be generated from a narrow
band laser.
• Based on nonlinear optical effects, a SC can
cover from ultraviolet to infrared.
• In this work a Photonic Crystal Fiber (PCF) is
used as the medium to generate a SC.
Introduction
polarization-maintaining Photonic Crystal Fiber From
NKT Photonics
SiO2
125 𝜇m
5 𝜇m
http://www.nktphotonics.com/product/nonlinear-photonic-crystal-fibers/
Introduction
• Side-Polished-Fiber (SPF) from Phoenix
Photonics
Cladding
Evanescent wave
Light in
core
17 mm
Cladding Diameter 125 𝜇m
Core diameter 8 𝜇m
Side-polished 1-2 𝜇m to the core
http://www.phoenixphotonics.com/website/technology/side-polished-fibers.html
Experimental Setup
Optics
Optics Objective
Pin hole
1064nm
Pulsed laser
700mW
20kHz
20 ns
Photonic
Crystal Fiber
InGaAs array
spectrometer
High NA transition fiber
1%
99%
Output coupler
1%
Input coupler
Resonant Fiber Loop
Side-Polished-Fiber
pump
Liquid in
Sample flow system
Liquid out
99%
Experimental Setup
• The SC spectrum and its light spot.
70000
60000
50000
Intesnity
40000
30000
20000
10000
0
-10000
800
1000
1200
1400
wavelength / nm
1600
1800
Experimental Setup
Interference filters (IFs) are used to separate
wavelengths and fit ring-down time
3.0
0.018
0.016
2.5
0.014
peak area
Intensity
0.012
0.010
0.008
0.006
0.004
2.0
1.5
1.0
0.002
0.000
0.5
-0.002
-10
0
10
20
time / s
30
40
50
4
6
8
10
12
14
time / s
16
18
20
22
Experimental Setup
Ring-down time comparison • FWHM for IFs is 2nm
• Each wavelength has
similar ring-down
times in H2O and D2O
• Based on ring-down
times, a round trip
loss of 18% and 20%
are got for 1400nm
and 1500nm.
18
H2O
17
16
D2O
ringdown time / s
15
14
13
12
11
10
9
8
7
6
5
1300
1350
1400
1450
wavelength / nm
1500
1550
Results
0.020
pure H2O
0.020
0.015
6% H2O
10% H2O
Absorption
0.015
absorption
peak shift
0.010
0.005
0.010
0.005
0.000
0.000
-0.005
1100
1200
1300
1400
wavelength / nm
1500
1600
-0.005
1100
1200
1300
1400
wavelength / nm
Figures on pure H2O/D2O ratio and a comparison of
pure peak with 10% and 6% H2O solutions in D2O.
1500
1600
Results
• The pure H2O/D2O comparison peak shows a
signal to noise of more than 100.
• Evanescent wave exposure on the sidepolished-fiber is 0.02% and a detection limit of
lower than 3% H2O solution is achieved.
• Experiments on 10% and 6% H2O show a
distinguishable peak in determining lower
concentrations of H2O.
Future Work
• Tapers with 25 𝜇m waist give almost the same
evanescent exposure as SPFs. With waist of 5
𝜇m, tapers can have more than 1% exposure.
• Only Side-Polished-Fibers give positive results,
experiments on tapers showed high noise
level and instability in SC spectra.
Light out
Light in
Tapered region
Light out
Future Work
• More tests will be done on tapers. Tapers have
thinner waist than SPFs to provide more
evanescent wave exposure.
• Other organic sample pairs will be tested to
confirm the setup can be used in
measurement of H2O in organic solutions.
Acknowledgements
• Dr. Helen Waechter and Ryan Matz from Tiger
Optics
• Dr. Andrea Armani and Victoria Sun from the
University of Southern California in building
taper pulling system
Thank you!