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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

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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

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

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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 OKeefe (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)

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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

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

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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

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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)

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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

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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

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Results: Side-polished Fibers Flow system with fast response time:

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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

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

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Experimental Setup: Core-only Fiber Multimode fiber used for loop, core diameter = 100 m 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 m step-index fiber to 100 m core-only fiber

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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

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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

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Acknowledgements Kevin Lehmann

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Thank you!