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 Laser Detector Cavity Modulator 154.66±0.08µs, χ2 = 0.99 (µs) (mV) 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 Detector Isolator Computer 3PZTs Flat mirror Curved mirror Mode matching optics Cavity 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 signal averaging process is limited by the drift of CRDS system, caused by slow changes of experimental environment. The minimum measurable difference (k – k0) determines the sensitivity. Absorption coefficient: 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 Absorption coefficient: CCB AD D FI PC PZT SM1 SM2 CV TS M OPM MML AOM OS Laser 2 Laser 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 Allan plot: log-log plot of Allan variance versus average size p 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. D. W. Allan, Preceedings of the IEEE 54, 221 (1966); P. Werle et al., Applied Physics B 57, 131 (1993)

Modified Algorithm 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 Noise density: Detector noise limited CRDS: Shot noise limited CRDS: K. K. Lehmann and H. Huang, Frontiers of Molecular Spectroscopy, Chapter 18, Elsevier 2008

SOA as Light Modulator Semiconductor optical amplifier (SOA): 0th order 1st order to cavity RF power 80 MHz AOM crystal Isolator Output coupler SOA Fiber coupler 95% 5% 1512 nm laser diode Optical fiber Trigger signal Current source 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.

Modified Setup Lens He-Ne laser DFB diode laser Laser control board AOM AOM driver Detector Isolator Computer 3PZTs Flat mirror Curved mirror Mode matching optics Cavity Trigger signal Polarizer or Pockel’s cell λ/2 plate