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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.

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broadband cavity enhanced absorption spectroscopy with a supercontinuum source

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


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
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







P- polarization

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
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
Surface Scattering Losses
  • Surfaces super polished to s = 1 Å rms
  • Loss for each total internal reflection:
    • (4 n cos()  / 0)2 =0.15 ppm for 0 = 1 mm
  • Loss for each Brewster Surface:
  • Total < 2 ppm/prism at 1 mm.
  • Finite angular spread of beam leads to <0.02 ppm loss
bulk loss
Bulk Loss
  • For fused silica, scattering loss dominates absorption for l < 1.8 mm.
    • small residual [OH] absorption near 1.4 mm.
    • Prisms made of Suprasil 3001 which has [OH] < 1 ppm
  • Rayleigh scattering loss ~ l0-4
    • loss of ~ 1 ppm/cm @ 1 mm.
    • Intracavity pathlength of 3.8 cm for our prisms (16 mm length on short side)
source for broad bandwidth coherent radiation supercontinuum photonic crystal fibers
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

supercontinuum generation
Supercontinuum Generation
  • Fiber
    • Length = 12 m
  • Input
    • Average power: 1.0 W @ 1064 nm
    • Rep rate: 29.41 kHz
    • Pulse energy: 34 mJ
    • 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
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
cavity enhanced spectroscopy
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.

allan variance
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
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
  • 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
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
  • Dr. Paul Rabinowitz
  • Tiger Optics research team
  • University of Virginia, National Science Foundation, and the Petroleum Research Fund.
white light sources
White light sources