Brian Siller, Ryan Matz, and Helen Waechter Recent Progress in Developing a Commercial Fiber-Loop Cavity Ring-Down System International Symposium on Molecular Spectroscopy 2014

Overview  Introduction to Tiger Optics’ CRDS platforms  Fiber-loop CRDS – Ideal for Liquids  Principles of operation  Features and benefits  Target applications  Experimental Setups and Results  Sensing element types  Sampling systems  Conclusions and Outlook

Versatility of CW-CRDS: Major Platforms Mirrors FibersPrisms Continuous-Wave Cavity Ring-Down Spectroscopy

Fiber-Loop Background  Primary inventors circa 2001  Kevin Lehmann (fiber loop, tapers)  Peter Loock (fiber loop, gap in loop)  Markus Sigrist (gold-coated fiber ends as mirrors)  Anthony O’Keefe (fiber Bragg gratings as mirrors)  Brian Culshaw (fiber amplifier in fiber loop)  Initial applications  Chemical, biological sensing (absorption and refractive index)  Strain sensing, temperature sensing (C.J. Wang)

Fundamentals of Fiber-loop CRDS  Optical cavity is fiber loop  Measure ring-down times as with mirror cavities  Sensing element allows light to interact with sample Sensing Elements Couplers

Features and Benefits of Fiber-Loop CRDS  Measures very small volumes of liquid samples (~µL)  Requires very little space  Low cost of components  Handles turbid or absorbing matrices, bubbles, and particles  Withstands harsh environments  Temperatures ranging from -270 to 600°C  Harsh chemical solutions

Potential Fiber-Loop Applications  Cryogenic liquid purity: NSF Small Business Innovation Research (SBIR) grant  Process Monitoring: Wet-etching solutions and cleaning baths, liquid chemical streams, micro-reactors  Quality Monitoring: Contaminants in chemicals, composition of mixtures  Composition of biomedical and biochemical samples

Fiber-Loop = Broadband Cavity  Limited only by the transparency range of fiber and fiber components  Fiber-loop offers a versatile platform:  Laser at a single wavelength (single analyte)  Multiple lasers at different wavelengths (multiple analytes)  Broadband light source (spectra of complex mixtures)

Fiber-Loop Sensor Element Types Standard fiber  Light mostly confined to the core  Small amount of light extends into cladding  No evanescent field outside the cladding Fiber sensing elements  Majority of the light is confined in the fiber core and cladding  A small amount of light extends outside of the fiber Cladding Core n core > n cladding Tapered Side-polished Core-only

Experimental Setup: Side-polished Fibers  Two fiber couplers (99:1) for coupling light in and out  Fiber-loop length: 2km  Static sample monitor or use in flow system Side-polished region

Results: Side-polished Fibers  Flow system with fast response time: <1 minute (10% to 90%) with flow rate of 13 µL/min  Average over 400 ring-downs (6.6 seconds)  Very little long-term drift; can be compensated with off-peak measurements  Limit of detection: 1% H 2 O in ethanol

Experimental Setup: Tapers  Much more fragile than side-polished fibers  Mounted on ceramic support structures  Only used for static sample measurement Taper Waist Human Hair

Results: Tapers  Sensitivity ~0.3% H 2 O in D 2 O  Usefulness limited by fringing Transmission [dB] Wavelength [nm]

Experimental Setup: Core-only Fiber  Multimode fiber used for loop, core diameter = 100 µm  Sensing element length: 0.1 to 2 meters  Tested both with fiber-loop and single-pass with spectrometer  Long sensing element: potential for very low detection limits, e.g. for cryogenic liquid contaminants Splice of 100/125 µm step-index fiber to 100 µm core-only fiber

Preliminary Results: Core-only Fiber  Spectrometer shows ice absorption peak  Very small shift in ring-down time when inserting sensing element into liquid nitrogen Liquid Nitrogen Air Spectrometer Signal: Single pass through core-only fiber

Summary  Side-polished fiber  Sensitivity not as good as tapers (1% H 2 O in ethanol)  Much more robust than tapers  Tapers  Good sensitivity (0.3% H 2 O in D 2 O)  Fragility and fringing limit usefulness in commercial system  Core-only fiber  Longest sensing elements; potentially highest sensitivity  Currently too lossy for sensitive detection  Ideal for cryogenic liquids  Optimization continues for commercial use

Acknowledgements Kevin Lehmann

Thank you!