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  b Input  b 6 meter radius of curvature P- polarization G. Engel et al., in Laser Spectroscopy XIV International Conference, Eds. R. Blatt et al. pgs (World Scientific, 1999).

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, (2009) Advantages of Prism Cavity

What are loses of Prism Cavity? Deviation from Brewster’s Angle 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.

Surface Scattering Losses Surfaces super polished to  = 1 Å rms Loss for each total internal reflection: – (4  n cos(  )  / 0 ) 2 = 0.15 ppm for 0 = 1  m Loss for each Brewster Surface: 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 ~  -4 –loss of ~ 1 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)

Source for Broad Bandwidth Coherent Radiation: Supercontinuum Photonic Crystal Fibers Material: Pure Silica Core diameter: µ m Cladding diameter: µ m Zero dispersion wavelength: nm Nonlinear Coefficient at 1060 nm: 11 (W · Km) -1

Supercontinuum Generation Fiber –Length = 12 m Input –Average power: nm –Rep rate: kHz –Pulse energy: 34  J –Peak power: 3.4 kW Output –Average output power: 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: 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, (2008)

Observed Cavity Loss Model:

Cavity enhanced spectroscopy Measure time integrated intensity 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.

O 2 SPECTRUM IN AIR

Fifth Overtone Spectrum of C 2 H 2

Allan Variance Read CCD every 10 sec for ~8 hrs Successive CCD readings were binned for time intervals of  t. Variance calculated for ratio of spectra for each  t pair. Minimum noise point: 90 min 650 nm

Current Status Absorption Sensitivity 5.88x10 -9 cm -1 –Equivalent to 1.6 x 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. CaF 2 prisms should allow extension into the UV, BaF 2 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

Acknowledgments Dr. Paul Rabinowitz Tiger Optics research team University of Virginia, National Science Foundation, and the Petroleum Research Fund.

White light sources