Gas Sensing with a 9 µm Peltier-cooled Quasi-CW Distributed Feedback Quantum Cascade Laser Damien Weidmann, Gerard Wysocki, Anatoliy Kosterev, and Frank K. Tittel Rice University, 6100 Main Street, Houston, TX, 77005, USA Thierry Aellen, Mattias Beck, Daniel Hofstetter, and Jerome Faist Université de Neuchâtel, Rue A. L. Bréguet 1, 2000 Neuchâtel, Switzerland Stéphane Blaser Alpes Laser SA, 1-3 Max.-de-Meuron, CP 1766, 2000 Neuchâtel, Switzerland

http://www.ece.rice.edu/lasersci

Towards CW Mode Operation of DFB QCLs

Direct Absorption Based Gas Sensor Architecture

„ „ „

SO2 and NH3 Direct Absorption Spectra

Direct Absorption Sensitivity Determination Calibrated NH3:N2 Mixture

• NH3:N2 mixture

• Pure SO2 5 torr

• Main drawbacks of QCL pulsed operation

San Francisco, California, USA May 16 – 21, 2004

• Flow configuration

Mixture NH3 in N2 1038 ± 21 ppm

Pulse to pulse intensity variation Linewidth broadening by thermal chirp Requirement of nanosecond electronics

• Retrieved concentration

• Efforts towards achieving quasi-RT CW DFB QCLs „ „

„

• CW QCL Characteristics • CW QCL Structure

80 cm cavity

Quasi-CW Operation of 9.1 µm QCL In CW mode, laser threshold is close to maximum current ⇒ Hence limited wavelength tuning range • Quasi CW: Square signal

Quantum Cascade Laser Linewidth Study Study with pure SO2 at 1114.1741 cm-1

„ „

∆νobs=72 ± 7 MHz

Active zone temperature

τ = 30 µs

Wavelength tuning is ensured by self heating

Detected Signals – Wavelength Calibration

„ „ „

Emissions from burning fossil fuel Emissions from volcanoes Precursor of acid rains Precursor of sulfate aerosols formation

σ = 0.045 mV

• In a Gaussian lineshape approximation: ∆νobs2

= ∆νDop + 2

∆νLas2

⇒ ∆νLas = 47 MHz

• Detector noise is insignificant • Residual Etalon noise • Current source noise ⇒ Laser linewidth contribution

recognition with HITRAN 2k database

Effective tuning rate: ∆σ/∆i = 17.5 = 525 MHz/mA And ∆i = ± 0.05 mA ⇒ ∆ν = 52 MHz

• NH3 monitoring „ „

„

„

„

Toxic industrial chemicals Agricultural emissions 3rd most abundant nitrogen containing compound Precursor of ammonium aerosols formation Study of possible environmental impact Study of nitrogen cycle

• Application of a 1038 ppm NH3:N2 mixture

• Demonstration of quasi CW Peltier cooled QCL operation for trace gas monitoring „ „ „ „ „

⇒ Intensity noise enhancement on absorption line edge

⇒ Improvement by a factor of 3 compared to direct absorption spectroscopy

Conclusions

Noise Sources of QCL Based Gas Sensor

• Direct absolute calibration using spectral

„

Modulation depth 4.2 mA

1σ extrapolated sensitivity 6 ppm per meter of absorption

cm-1/A

„

Wavelength Modulation Spectroscopy

Quasi CW + Wavelength modulation

NH3 SO2

• SO2 monitoring

σ = 2.8×10-3

• Current applied:

Frequency 1-10 Hz with 10 to 50% duty cycle

Example: 5 Hz, 50% 5 torr SO2

HITRAN 2K survey

18 ppm per meter of absorption 1 scan, 25 ms acquisition time

• Fit based on HITRAN data and Levenberg Marquardt fitting routine

Peltier cooled operation

• Selection from

• Extrapolated 1σ sensitivity

• SNR variation due to laser power variation

„

Potential Target Molecules

Potential sources of discrepancy ŠHITRAN

• Limiting noise: unexpected etalon fringes

⇒ Reduces laser threshold from 750 to 520 mA ⇒ 3 cm-1 total tuning range

T. Aellen et al., Applied Physics Letters, 83, 1929, 2003

„

ŠNH3 stickiness

M. Beck et al., Science, 295, 301-305, 2002 T. Aellen et al., Applied Physics Letters, 83, 1929, 2003 A. Evans, et al., Applied Physics Letters, 84, 314, 2004 [NON DFB QCL]

9.1 µm Peltier Cooled CW DFB-QCL

1163 ppm

P = 99.4 torr L = 21”

No requirement for cryogenic cooling No need for nanosecond electronics Smaller QCL linewidths: ~ 47 MHz Compact size Etalon effects are the main limitation

• Application to SO2 for wavelength calibration • Application to NH3 detection „ „

Direct Absorption Spectroscopy : 18 ppm.m (25 ms) Wavelength Modulation Spectroscopy : 82 ppb.m/√Hz