Long Term Stability in CW Cavity Ring-Down Experiments

Haifeng Huang and Kevin Lehmann

64th OSU Symposium on Molecular Spectroscopy

June 26, 2009

Cavity Ring-Down Spectroscopy

shotpercm

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1

2

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2

N

i

iABiyN

0 200 400 600 800 10000

200

400

600

800

1000

1200

1400

Ring down decay signal

154.66±0.08µs, χ2 = 0.99

(µs)

(mV)

Laser DetectorCavityModulator

τ insensitive to laser power fluctuations

long effective absorption length >10 km

Cavity-Ringdown Spectroscopy, ACS and Oxford University Press, 1999

Experimental Setup

Lens

He-Ne laser

DFB diode laser

Laser control board

AOM

AOM driver

DetectorIsolator

Computer

3PZTs

Flat mirror Curved mirror

Mode matching opticsCavity

Trigger signal

Trace Methane Detection

Methane R4 lines, 2ν3 band, fitted with HITRAN

Total pressure 15.71 torr, methane concentration 33.0 ± 0.6 ppb

Trace Methane Detection

Drift of τ

Cavity was under vacuum. Ring-down data was recorded in 7 hours. The ring-down rate is 4.9 Hz for both panels. Δν is 23 GHz.

Sensitivity in CRDS

The minimum measurable difference (k – k0) determines the sensitivity.

)(1

0kkc

Absorption coefficient:

The signal averaging process is limited by the drift of CRDS system, caused by slow changes of experimental environment.

Differential measurement between k and k0 can improve the sensitivity by the cancellation between the drift of k and that of k0.

Long term stability of CRDS setup determines the final sensitivity in real experiments.

Differential Measurement

)(1

0kkc

Absorption coefficient:

CCBAD

D

FI

PC

PZT

SM1 SM2

CV

TS

MM

MOPM

MML

AOM

OS

Laser 2Laser 1

C

D/A

Δν adjustable

Switching time 13 ms

ν1 ν2

Drift of τ

Cavity was under vacuum. Ring-down data was recorded in 7 hours. The ring-down rate is 4.9 Hz for both panels. Δν is 23 GHz.

Allan Variance

D. W. Allan, Preceedings of the IEEE 54, 221 (1966); P. Werle et al., Applied Physics B 57, 131 (1993)

1

1

21

2

21

2,

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12

)(

21

)(

1

)]()([)1(2

1)(

11,)]()([2

1)(

1,1

)(

21

m

iiiA

jjjA

pn

npiin

Np

pA

pp

pA

p

pApAm

p

mjpApAp

mnxp

pA

xxxxxx

Normal algorithm, m = integer part of N/p

For white noise dominated signal, Allan plot slope is -1.

For linear drift dominated signal, Allan plot slope is +2.

Allan plot gives the optimal signal average size p.

The minimum of Allan variance gives final sensitivity in real experiments.

Allan plot: log-log plot of Allan variance versus average size p

Modified Algorithm

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Np

pA

ppp

pA

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Npp

pA

ppp

pA

p

pm

p

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Modified algorithm, m = N – 2p + 1

Drift Cancellation

Four hour data

Pressure < 0.1 torr

Both λ on peak

Ring-down rate 6.9 Hz for each laser

For each k, optimal p ~500

For k1 – k2, optimal p is 16890.

Optimal integration time increased from 72 sec to 41 min.

Final sensitivity: 2.8 × 10-12 cm-1

However…

Cavity pressure ~20 mtorr

Optical interference?

Optical Feedback

Detector not tilted

Short term Stdτ about 0.6 μs

With an isolator between output mirror and the detector, short term Stdτ ~0.07 μs

Effect of Ambient Pressure Change

Panel A: lab pressure change

Cavity under vacuum

Data averaged by every successive 20 decays

Seven hour data

The cell temperature 31.30 ± 0.03 °C

No correlation between decay rate drift and the lab temperature change has been observed.

Mechanical deformation of the cavity by pressure change. Mirror reflectivity is not spatially uniform.

Other Noise

Partial cancellation

Possible reason: mechanical vibration

Sensitivity: 8.1 × 10-12 cm-1 in 8.7 min

Noise in Baseline

Δν = 23 GHz, pressure < 0.1 torr

Average size ~ 25, platform in one of the Allan variance

Ring-down rate 8.3 Hz for each channel

Noise period ~ 200 decays

Noise reason still unknown

Methane Detection Limit

Low thermal expansion: Invar plate

Mechanical vibration isolation

Stable baseline of decay transients

Alignment minimizing optical interference

Sensitivity: 5.6 × 10-12 cm-1 in 15.4 min

Methane detection limit (3σ) at 1652nm:

0.3 ppbv at 20 torr

37 pptv at 760 torr

Conclusions

Low concentration (~0.2 ppb) methane in N2 has been measured in our lab.

Allan variance is used to characterize the drift of CRDS systems. Long term stability determines the final sensitivity in CRDS.

Noise factors include ambient pressure, optical interference, mechanical vibration, thermal stability and baseline noise. Further studies are needed.

With differential measurement, very high sensitivity in CRDS, e.g., 5.6×10-12 cm-1with 15.4 min averaging time, can be realized.

Acknowledgements

Funding:

Princeton Institute for the Science & Technology of Materials (PRISM), PU

University of Virginia

Prof. Brooks Pate and group members

Other Lehmann group members

Charles Lam of machine shop

Thank you!

Methane Detection Limit

4.4 × 10-12 cm-1

Stable time 32.8 min

N2 flow 20 sccm

Δν = 23 GHz

PZT mod 80 MHz

Pressure 1 atm

Single Shot Sensitivity Limit of CRDS

Detector noise limited CRDS:

18)(2

32

tNkifA

Pkk N

ideal

Shot noise limited CRDS:

1)( 32

tNkif

QA

hkk SNL

K. K. Lehmann and H. Huang, Frontiers of Molecular Spectroscopy, Chapter 18, Elsevier 2008

ktN Ae

Q

hPt

22 )(Noise density:

SOA as Light Modulator

Semiconductor optical amplifier (SOA):

Advantages:

Highest extinction ratio (> 80 dB) when used as light modulator

Fast speed: ns or sub ns

Broadband gain media: ~70 nm

Optical fiber connected, no extra alignment needed when λ tuning

M. J. Connelly, Semiconductor Optical Amplifier, Kluwer, Boston, 2002.

0th order

1st order to cavity

RF power 80 MHz

AOM crystal

Isolator

Output coupler

SOA

Fiber coupler95%

5%

1512 nm laser diodeOptical fiber

Trigger signal

Current source

Modified Setup

Lens

He-Ne laser

DFB diode laser

Laser control board

AOM

AOM driver

DetectorIsolator

Computer

3PZTs

Flat mirror Curved mirror

Mode matching opticsCavity

Trigger signal

IsolatorPolarizer or

Pockel’s cell

λ/2 plate