1. Supercontinuum Fiber-Loop Cavity Enhanced Absorption Spectroscopy of H2O and D2O samples
Mingyun Li & Kevin Lehmann
Department of Chemistry and Physics
University of Virginia

2. Introduction
Supercontinuum (SC) is a broadband light source generated from a laser source.
A SC is generated by passing the laser through a photonic crystal fiber (PCF) to make super-broad in its wavelength.
125 𝜇m
5 𝜇m
SiO2

3. Introduction
Cavity Enhanced Absorption Spectroscopy (CEAS) is an optical sensing method to perform high sensitivity detection.
In order to achieve liquid phase sensing, a fiber- loop is build with a 500-meter-long single mode fiber. By using it as a resonant cavity, a multiple- pass measurement is achieved to perform lower detection limit than a single pass cell.

4. Previous Work
Last year we presented a setup of fiber-loop CEAS system to measure H2O/D2O samples.
Evanescent wave spectroscopy is performed by using a side-polished-fiber (SPF), but only 0.02% is used as evanescent wave.
In order to improve this, we want to change the sensing method to make more light into use.
The basic idea is to use free-space-coupling system so 100% of the light can be used.

5. Photonic Crystal Fiber
Experimental Setup
1064nm
Pulsed laser
700mW
10kHz
20ns
Small aperture
Optics
Objective
Output coupler
Input coupler 1%
99%
Resonant Fiber Loop
InGaAs array spectrometer
Photonic Crystal Fiber
Sensing Region
Sample flow system
Liquid in
Liquid out
pump
High NA transition fiber

6. SC light source
The SC we generated can cover from 600 to 1600nm

7. Sensing Region: SPF Evanescent wave Cladding core Light in 17 mm
Fiber out
Fiber in
Liquid in
Liquid out

8. Free-space coupling sensing
Focal length of the mirror is 9mm.
Single-pass of about 2cm is reached with this setup.
If all light can be used we can assume at least times increase in the signal level.

9. Results: SPF
Pure H2O samples and a comparison with 10% and 6% H2O samples in D2O with SPF sensor, pure D2O as reference.

10. Results: Free-space reflection
1% and 2% H2O solution samples in D2O with free-space coupling sensing, pure D2O as reference. Compared in each figure is the pure H2O sample with SPF sensor.

11. Conclusions
By switching from SPF to free-space reflection, signal level is significantly improved because much more of the light source can be used.
A detection limit of H2O concentration less than 0.1% is reached on free-space refection sensing.
The tests on H2O show the system is reliable to be used as liquid phase CEAS detection, and we will try to use it in more detections in organics.

12. Conclusions
Noise level is also higher than before, possible reasons are larger setup, and exposed fiber ends are easy to fluctuate during measurements.
Two peaks are overlapped, further measurements on organics can solve this.
More liquid samples are used each time, a further design is needed to improve this.
Organic samples are tested, issue with very high noise level is observed.

13. Future Work
The work we are doing now is to stabilize the spectra, so to improve the noise level.
The setup will be expand to organic samples mainly on H2O detection in different organics.
Another setup of an incoherent halogen light source is set using a single-pass fiber cavity. We are using it as a comparison of our SC setup.

14. Acknowledgements
Dr. Helen Waechter and Ryan Matz from Tiger Optics in building SC fiber-loop system
Dr. Florian Adler from Tiger Optics in helping design the free-space coupling system
Dr. Andrea Armani and Victoria Sun from the University of Southern California in building taper pulling system

15. Thank you!