Broadband Cavity Enhanced Absorption Spectroscopy With a Supercontinuum Source Paul S. Johnston Kevin K.
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Broadband Cavity Enhanced Absorption Spectroscopy With a Supercontinuum Source Paul S. Johnston Kevin K. Lehmann Departments of Chemistry & Physics University of Virginia In the past decade, the use of low loss optical cavities have become widely used to achieve high sensitivity absorption spectroscopy. Multiple variants, including Cavity Ring-Down Spectroscopy NICE-OHMS Cavity Enhanced Absorption Spectroscopy Dielectric Super Mirrors ( < 100 ppm reflection loss) have been key to the sensitivity enhancements of these methods. Limitations of Super Mirrors • High reflectivity bandwidth of dielectric mirrors limited to a few % in wavelength – They are 1-dimensional photonic crystals. • One can extend bandwidth by chirping the coatings of the layers but this increases loss and dramatically reduces damage threshold • High reflectivity (>99.9%) is only available for far less than an octave in the spectrum. • Need to use reflectors that do not depend upon interference. Brewster Angle Prism Retroreflector Ring-down Resonator Output Input qb P- polarization qb 6 meter radius of curvature G. Engel et al., in Laser Spectroscopy XIV International Conference, Eds. R. Blatt et al. pgs. 314-315 (World Scientific, 1999). Advantages of Prism Cavity • Wide spectral coverage - Simultaneous detection of multiple species • Compact ring geometry (no optical isolation required) • No dielectric coatings (harsh environments) • Coupling can be optimized for broadband • Analysis: Paul S. Johnston & KKL, Applied Optics 48, 2966-2978 (2009) What are loses of Prism Cavity? • Deviation from Brewster’s Angle R q n 4 1 4n 6 2 2 96ppmq / deg 2 • Surface Scattering at optical surfaces – Need super polishing • Bulk Absorption and Scattering Losses – Rayleigh Scattering Dominates for fused silica prisms • Birefringence which converts P -> S polarization – Strain must be minimized. dq B 1 n 2 dT 1 dn 40rad/ K dT Surface Scattering Losses • Surfaces super polished to s = 1 Å rms • Loss for each total internal reflection: – (4 n cos(q) s / 0)2 = 0.15 ppm for 0 = 1 m • Loss for each Brewster Surface: s n 2 1 2 2 0.8 ppm/ surface 2 0 n 1 • Total < 2 ppm/prism at 1 m. • Finite angular spread of beam leads to <0.02 ppm loss Bulk Loss • For fused silica, scattering loss dominates absorption for < 1.8 m. – small residual [OH] absorption near 1.4 m. – Prisms made of Suprasil 3001 which has [OH] < 1 ppm • Rayleigh scattering loss ~ 0-4 – loss of ~ 1 ppm/cm @ 1 m. – Intracavity pathlength of 3.8 cm for our prisms (16 mm length on short side) Loss of Prism Cavity in near-IR (Tiger Optics) Fused Silica Prisms (built in 43cm cavity) ring down tau/ppm loss vs. wavelength Tau Trend measure Tau 70 150 ppm loss Trend 60 130 50 110 40 90 30 70 20 50 10 30 1310, 1368, ppm loss Ringdown(tau-microsecond) Measure PPM Loss Tau measurenment at 0 1300 1350 1400 1450 1500 1550 Wavelength(nm) 1600 1650 1377,1392,1522,1531, 1578,1635,1671nm. 10 Every Diode laser 1700 Temp scan from (40 or ) 35~0 Celsisus degree. Source for Broad Bandwidth Coherent Radiation: Supercontinuum Photonic Crystal Fibers • • • • • Material: Pure Silica Core diameter: 4.8 + 0.2 µm Cladding diameter: 125 + 3 µm Zero dispersion wavelength: 1040 + 10 nm Nonlinear Coefficient at 1060 nm: 11 (W·Km)-1 www.crystal-fibre.com Supercontinuum Generation • Fiber – Length = 12 m • Input – – – – Average power: 1.0 W @ 1064 nm Rep rate: 29.41 kHz Pulse energy: 34 J Peak power: 3.4 kW • Output – Average output power: 0.270 W (at input polarizer) – Loss of ~50% power through polarizer Higher Power Supercontinuum from mode lock Nd:YAG laser • Input: – Spectra Physics Vanguard. – 80 MHz/ 30 psec pulse train – Average power: 9.5 W @ 1064 nm – Peak power: ~10 kW • Supercontinuum Output – Average output power: 3.2 W – With optimized fiber, we expect higher conversion Broadband system using white light from photonic crystal fiber Paul S. Johnston and KKL, Optics Express, 16, 15013-23 (2008) Observed Cavity Loss Model: Loss scattering Brewster's angle loss 4 2 A (n 1) 2 2 Loss 2 q q 1 2 6 4 4n Cavity enhanced spectroscopy • Measure time integrated intensity Io ( ) 1 R ( ) 1 I( ) l I( ) time integrat ed int ensity with absorbing species I o ( ) time integrat ed int ensity of empty cavity • Advantages – Relatively high sensitivity – Simpler set up • Sensitivity limitations – Residual mode structure – Laser noise Berden, G.; Peeters, R.; Meijer, G. Int. Rev. Phys. Chem. 2000, 19, 565. O2 SPECTRUM IN AIR Fifth Overtone Spectrum of C2H2 Allan Variance • Read CCD every 10 sec for ~8 hrs • Successive CCD readings were binned for time intervals of Dt. • Variance calculated for ratio of spectra for each Dt pair. • Minimum noise point: 90 min 650 nm Current Status • Absorption Sensitivity 5.88x10-9 cm-1 – Equivalent to 1.6 x 10-9 cm-1 Hz-1/2 – Shot noise limited – Shot noise limit extends to ~90 min integration. • Resolution ~0.05 cm-1 (2 GHz) – Close to diffraction limit for 25 cm grating used • Bandwidth vs. resolution limited by CCD Improvements.... • Expand simultaneous spectral coverage – Plan to use FTIR to cover entire spectral range of super continuum. – Considering construction of Echelle Spectrograph which will allow efficient use of most of CCD pixels. • CaF2 prisms should allow extension into the UV, BaF2 prisms into the mid-IR. I admit it; I’ve got comb envy! • As already discussed by Jun Ye, a vastly higher power can be coupled in with a frequency comb • Hansch’s group has shown how a vernier principle can be used to get single comb resolution with a modest resolution spectrograph • Dispersion limits the spectral width that can be simultaneously coupled into the cavity 1 D dFSR D d ~ 100GHz 8 FSR d 2 c Acknowledgments • Dr. Paul Rabinowitz • Tiger Optics research team • University of Virginia, National Science Foundation, and the Petroleum Research Fund. White light sources http://www.crystal-fibre.com