Detecting Next to Nothing: Spectroscopy in Optical Cavities Kevin - - PowerPoint PPT Presentation

Detecting Next to Nothing: Spectroscopy in Optical Cavities

Kevin Lehmann

Cavity Ring Down Spectroscopy

Kevin Lehmann Departments of Chemistry & Physics University of Virginia

Collaborators

• Daniele Romanini
• Joan Gambogi
• John Dudek
• Peter Tarsa
• Iris Scheele
• Haifeng Huang (UVa)

Cavity Ring Down Spectroscopy

• Greg Engel
• Wilton Virgo
• Paul Johnston (UVa)
• Paul Rabinowitz

Wen-Bin Yan, Calvin Kruzen, Bob Augustine, Chris Wu, Yu Chen, Lisa Bergson

Generic Absorption Spectroscopy Instrument

Lock-In

L Cavity Ring Down Spectroscopy Source Filter Sample Chopper Detector

Lock-In

DAS L

Beer’s Law

• Iout(ν) = Iin(ν) exp ( - σ(ν) N L)

− σ(ν): Absorption cross section – N: Number density of absorber

Cavity Ring Down Spectroscopy

– N: Number density of absorber (Concentration) – L: Optical pathlength through sample

• Minimum Detectable Concentration:

Nmin = (∆I/I)min / (σ(ν) L)

Review of Optical Cavities (aka etalons)

Cavity Ring Down Spectroscopy

(aka etalons)

Radii of Curvature: R1, R2 Length of Cavity: L detector Simple Linear Optical Cavity Cavity Ring Down Spectroscopy Length of Cavity: L Mirror Transmission: T Mirror Reflectivity: R Mirror Loss: A = 1 - R - T

Stable Optical Cavities

• For R = 1, modes exist which exactly reproduce

themselves upon round trip.

• 0 < L < R1 or R2 < L < R1+R2 (R1 < R2)

– If R2 - R1, << L, then R1 < L < R2 only weakly unstable

• Optic axis defined by line through centers of curvature of

Cavity Ring Down Spectroscopy

• Optic axis defined by line through centers of curvature of

mirrors

• Light rays will oscillate around optic axis.
• Spot size of mirror

• Cavity Transmission as function of Mirror Reflectivity

Cavity Ring Down Spectroscopy

• Ring-down Time

Peak Transmission of Cavity

• Peak Intracavity Gain
• Cavity Ring Down Spectroscopy

Mirrors with T,A ~ 5 ppm are available in near IR and red Intracavity power gain of ~105 can be realized

Transverse resonance modes of Cavities

• Cavity Ring Down Spectroscopy

If δg = M/N, then we have rational cavity with periodic transmission spectrum. Arbitrary pulse inside Cavity will exactly reshape after N round trips

• - such cavities are used for Herriott Cells.

Some Uses for Optical Cavities in Spectroscopy

• Control frequency and linewidth of lasers
• Monitor laser scan for calibration
• Laser linewidth is reduced by locking on to the

transmission peak of a cavity

Cavity Ring Down Spectroscopy

transmission peak of a cavity

• Cavities are used to build up intensity

– External c.w. second harmonic generators – Pump extremely weak transitions

• Used to enhance optical absorption

Cavity Ring-Down Absorption Spectroscopy

R>99.99% R>99.99%

Absorption Cavity Ring Down Spectroscopy

Time Intensity

Where: c : speed of light L : length of cavity R : mirror reflectivity σ : absorption cross section N : number density (concentration) Leff: effective pathlength

Absorption I(t) = I0 exp[−t( c Lln R+ c⋅σ(λ)⋅ N)] Leff=L/(1-R)

Ring Down Cavity Technique

First Developed by O’Keefe and Deacon

• Rev. Sci. Instr. 59, 2544 (1988)

Theory: Romanini and Lehmann

• J. Chem. Phys. 99, 6287 (1993)

Cavity Ring Down Spectroscopy

• Use a passive optical cavity formed from two

high reflective mirrors (T~1-100 ppm)

• Excite cavity with a pulsed laser to ‘fill’ with photons
• Detect exponential decay of light intensity inside resonator
• Decay rate reflects:

– Loss due to mirrors (slowly changing with wavelengths) – Absorption of gas between mirrors

Advantages of CRDS Method

• Allows much longer pathlengths than traditional multipass cells
• Only sensitive to absorption and scattering between mirrors
• Beer’s Law holds for all pathlengths; pathlengths determined by time

– if resolution exceeds width of absorption lines – Calibration samples are not needed

Cavity Ring Down Spectroscopy

– Calibration samples are not needed

• Cell is very compact; light contained in narrow spot of ~ 1 mm2
• Cell insensitive to vibration since it is a stable optical cavity
• Amplitude noise of laser not important
• Can use low power optical sources

Continue growth- publications per year

1980 1 1992 1 2001 60 1993 5 2002 77 1984 2 1994 12 2003 69 1985 1 1995 18 2004 99 1996 20 2005 95

Cavity Ring Down Spectroscopy

1996 20 2005 95 1988 3 1997 30 2006 144 1998 40 2007 130 1990 3 1999 68 1991 2 2000 48

Based upon searches of Web of Science data base for CRDS, CRLAS, CEAS, ICOS, NICE-OHMS

Early CRDS Work @ Princeton

• Used to detect high overtone transitions
• f HCN and other molecules

– Provided way to determine absolute

Cavity Ring Down Spectroscopy

– Provided way to determine absolute absorption strength of extremely weak transitions. – Could be done with very simple experimental set-up compared to intracavity photoaccoustic spectroscopy.

Cavity Ring Down Spectroscopy

• D. Romanini and KKL, J. Chem. Phys. 99, 6287-301 (1993)

HCN (106) overtone band L(eff) = 24 km Cavity Ring Down Spectroscopy This spectrum is now on the cover of a Spectroscopy text by Hollis

Diode Laser Advantages

• Low cost, compact, all solid state
• Low power requirements

Cavity Ring Down Spectroscopy

• Wide electronic frequency tuning
• Single mode diodes in the near-IR are

becoming available for sensing apps.

– H2O, C2H2, CH4, CO2, NO2, NH3, etc.

Faraday Isolator Mirror Acousto-Optical Modulator Diode Laser HR Mirror

Experimental Setup

Mode Matching Optics

Cavity Ring Down Spectroscopy

Cavity Ring-Down Absorption Cell HR Mirror HR Mirror

3 PZTs

Trigger Computer Photodiode Mirror

• J. B. Dudek et al., Analytical Chemistry 75, 4599-4609 (2003).

0.3 0.4 0.5

Cavity Ring-Down Decay

τ = 295.82+0.20 µsec

nal (Volts) Cavity Ring Down Spectroscopy

200 400 600 800 1000 1200 1400 1600 0.0 0.1 0.2 0.3

τ = 295.82 0.20 µsec data points fit

Signa

Time (µsec)

125 130 135 140

7161.5 cm-1

Water Scan with Lorentz fit

Cavity Ring Down Spectroscopy

7160.8 7161.0 7161.2 7161.4 7161.6 7161.8 7162.0 105 110 115 120

7161.5 cm-1 S=1.5×10-20 ~70 ppb

Wavenumber

How stable is the ring down time?

Laser power upon entering the cavity: 1.397 µm ~ 3 - 5 mW Ensemble Standard Deviation:

Cavity Ring Down Spectroscopy

Ensemble Standard Deviation: 10 pts: 201.384 ±0.139 µs 0.069% 100 pts: 201.378 ±0.165 µs 0.082% 1000 pts: 201.304 ±0.146 µs 0.073%

Allan variance and the detection limit in CRDS

In CRDS the accurate measurement of decay time constant can be limited by slow drift of setup. Allan variance can be used to analyze the stability of instrument. For a N time-series data xi the Allan variance is given by:

• k is the subgroup size and m+1 is the number of subgroups. The integration time T equals to k/f, where f is sample
• rate. When white noise is dominant in the system (uncorrelated decays), Allan variance is proportional to 1/T and

averaging data can improve the signal to noise ratio. When the drift appears Allan variance will become larger. The longest T during which the instrument can be regarded stable is determined by the drift of the system. The minimum

• f Allan variance gives the smallest detectable change during the longest integration time period.

Applied Physics B 57, 131- 139 (1993)

Cavity Ring Down Spectroscopy

• f Allan variance gives the smallest detectable change during the longest integration time period.

What does this mean for water detection?

• Noise equivalent to 68 pptv divided by

the square root of the number of ring

Cavity Ring Down Spectroscopy

the square root of the number of ring down events averaged to get signal, i.e. ~2 pptv for averaging 1000 decays.

CRDS in Practice

Cavity Ring Down Spectroscopy

MTO-1000-H2O

Size (14”x 19”x 26”) Weight (45 kg)

MTO Response to Moisture Addition

Data used with permission from Air Products and Chemicals Inc., Allentown, PA

1.5 2 2.5

ing (ppb)

Cavity Ring Down Spectroscopy

0.5 1 6/15/02 3:36 6/15/02 6:00 6/15/02 8:24 6/15/02 10:48 6/15/02 13:12 6/15/02 15:36 6/15/02 18:00

Time Readin

Variations on CRDS Method

• CRDS (a.k.a. CRLAS, RDCS, cavity leak-out

spectroscopy)

– pulsed CRDS – cw CRDS – phase shift CRDS – Fourier Transform CRDS

Cavity Ring Down Spectroscopy

– Fourier Transform CRDS – broad band CRDS – evanescent wave CRDS – fiber optic CRDS, fiber loop CRDS – Cavity Ring-down polarimetry – Optical feedback CRDS

• Cavity Enhanced Absorption Spectroscopy

(CEAS)-Engleln, Meijer, et al.

– a.k.a Integrated cavity output spectroscopy (ICOS) - O’Keefe – Frequency chirped CEAS

• Noise Immune Intracavity optical

Cavity Ring Down Spectroscopy

• Noise Immune Intracavity optical

heterodyne method (NICE-OHMS)

• Intracavity laser absorption spectroscopy

(ICLAS)

• Intracavity photoaccoustic spectroscopy

– attractive with optical locking!

Excellent Review: C. Vallance, New J. of Chem. 29, 869 (2005)

SUPERCONTINUUM BASED BROADBAND CAVITY ENHANCED ABSORPTION SPECTROSCOPY

Cavity Ring Down Spectroscopy

Paul S. Johnston Kevin K. Lehmann Department of Chemistry University of Virginia

Broad Bandwidth

• Light sources: broad bandwidth dye lasers, Free

electron lasers, fs-lasers, LEDs, arc-source

• Engeln & Meier, Fourier transform CRDS,

1996

Cavity Ring Down Spectroscopy

1996

• Thorpe & Ye, 2007

– Mode lock sources with cavities a multiple of the laser repetition rate allows much improved transmission – Cavity dispersion a challenge

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

Cavity Ring Down Spectroscopy

www.crystal-fibre.com

Supercontinuum parameters

• Input

– Average power: 1.0 W – Rep rate: 30 KHz

Cavity Ring Down Spectroscopy

– Rep rate: 30 KHz – Pulse energy: 34 µJ, 10 ns – Peak power: 3400 W

• Output

– Average output power: 0.29 W

! "##$$

Supercontinuum Output

Cavity Ring Down Spectroscopy

White light sources

Cavity Ring Down Spectroscopy

%&&'

Prism Ring-down Resonator

Output

θb

Input

θb

P- polarization

Cavity Ring Down Spectroscopy

b

6 meter radius

• f curvature

P- polarization

• G. Engel et al., in Laser Spectroscopy XIV International Conference,
• Eds. R. Blatt et al. pgs. 314-315 (World Scientific, 1999).

• Wide spectral coverage - Simultaneous

detection of multiple species

• Compact ring geometry (optical isolation)

Advantages of Prism Cavity

Cavity Ring Down Spectroscopy

• Compact ring geometry (optical isolation)
• No dielectric coatings (harsh environments)
• Coupling can be optimized for broadband

Broadband system using white light from photonic crystal fiber

Mode Matching Mirrors 20x objective

Fiber Output

Collimating mirror PC Fiber

Cavity Ring Down Spectroscopy

()"* %#*'"**+,- (%."* " .**/'"& 01%2/ Time

λ

CCD array Sample Cell Gas inlet Mirrors Nd:Vanadate laser

Loss Due to Dispersion changing Brewster’s Angle in Fused Silica

ppm R 1 ) 1 . , 46 . 1 ( = °

2 4 ) 1 (

6 2 4

) , ( δθ δθ

n n

n R

−

=

• Cavity Ring Down Spectroscopy

ppm R ppm R 7 . 98 ) . 1 , 46 . 1 ( 1 ) 1 . , 46 . 1 ( = ° = °

**Only Brewster’s angle loss**

Modeling Cavity Loss

Cavity Ring Down Spectroscopy

• Model:

loss angle s Brewster' scattering Loss + =

High Equivalent Reflectivity

Ring-down Test for Fused Silica Prism

Fused Silica Prisms (built in 43cm cavity) ring down tau/ppm loss vs. wavelength

40 50 60 70 icrosecond) 90 110 130 150

loss

Tau Trend measure Tau ppm loss Trend Measure PPM Loss

Near-IR Prism Cavity Loss Measurements (Tiger Optics) Cavity Ring Down Spectroscopy

10 20 30 40 1300 1350 1400 1450 1500 1550 1600 1650 1700

Wavelength(nm)

Ringdown(tau-mic 10 30 50 70 90

ppm lo

Tau measurenment at 1310, 1368, 1377,1392,1522,1531, 1578,1635,1671nm. Every Diode laser Temp scan from (40

• r ) 35~0 Celsisus

degree.

Cavity enhanced spectroscopy

• Measure time integrated intensity
• Cavity Ring Down Spectroscopy
• 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.

Atmospheric Oxygen

Cavity Ring Down Spectroscopy

Current Status

• Collaborating with Tiger Optics to

commercialize a detector of multiple chemical species.

• Will try near-IR spectroscopy with InGaAs

array detector

• Will use FT-IR for dispersion

Cavity Ring Down Spectroscopy

• Will use FT-IR for dispersion
• Have begun building mode-locked (80 MHz)

super-continuum source that we expect > 10 W average power.

– Frequency comb of source can be matched to frequency comb of cavity transmission to greatly improve transmission – Potentially can use “Vernier” principle to improve upon resolution of spectrograph

Applications of Broad band CRDS

• Combustion and Plasma diagnostics

– parallel detection improves S/N ratio if we have unstable sample – “single shot” determination of temperature Cavity Ring Down Spectroscopy – “single shot” determination of temperature

• Breath Analysis Applications

– both species and isotopic composition studies

• Could be combined with optical comb technology for

high accuracy metrology.

Other Cavity Spectroscopy related projects

• Methane Isotope Ratio Instrument

– 13C/12C and D/H ratios – CH4 in oceans, air , and emitted from permafrost. – Possible mission to Mars

• Detect NO and other molecules in human breath

Cavity Ring Down Spectroscopy

• Detect NO and other molecules in human breath
• Fabricate and test Prisms from CaF2 (UV) and

BaF2 (IR)

• Collaboration with group looking for WIMPS

with liquid Ar detector

– Need to detect H2O, O2, and N2 impurities < 1 ppb – Plan to try to detect N2 via 3Σ state produced in discharge.

Contact Information

• Phone 243-2130
• Office: Rm. 124 in Chemistry Building
• Email: Lehmann@virginia.edu

Cavity Ring Down Spectroscopy

• Email: Lehmann@virginia.edu