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Bradley M. Gibson Department of Chemistry, University of Illinois at Urbana-Champaign

Development of a Frequency-Stabilized Mid-Infrared External Cavity-QCL Cavity Ringdown Spectrometer. Bradley M. Gibson Department of Chemistry, University of Illinois at Urbana-Champaign Benjamin J. McCall Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign.

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Bradley M. Gibson Department of Chemistry, University of Illinois at Urbana-Champaign

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  1. Development of a Frequency-Stabilized Mid-Infrared External Cavity-QCL Cavity Ringdown Spectrometer Bradley M. Gibson Department of Chemistry, University of Illinois at Urbana-Champaign Benjamin J. McCall Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

  2. Overview of EC-QCLs • Initial Performance • Output Frequency Stabilization • Passive stabilization • Vibrational isolation • Active stabilization • Side-of-fringe locking • Automated re-locking • Tilt tuning Outline

  3. Emission in the mid-IR (4-10 μm) • Small minimum tuning steps, <15 MHz • Wide tuning range, ~100 cm-1 • Relatively high output power, ~10 mW • Laser chips can be swapped for additional tuning range Why use an EC-QCL?

  4. What is an EC-QCL? Figure from: Wysocki et al., Appl. Phys. B: Lasers Opt. (2008), 92, 305.

  5. What is an EC-QCL?

  6. What’s the tuning range?

  7. Tuning elements: • QCL injection current / temperature • External cavity length (piezo) • Cavity grating angle (piezo) How can we control the output?

  8. How do we control the output?

  9. (~226 MHz) How stable is the output? Time (m)

  10. Isolate the laser from vibrations / acoustic noise • Float the laser table • Decrease water chiller pulsing • Acoustically isolate the laser • Improve injection current / thermal stability • Actively lock the laser to an external reference How can we stabilize the frequency?

  11. Vacuum Chamber / Table Isolation Water Chiller Damping How can we isolate the laser?

  12. How well did the isolation work? Standard deviation: 0.00437 cm-1 (~131 MHz)

  13. Germanium Etalon Side-of-Fringe Locking • Simple, low-cost • Limited by laser power fluctuation, etalon temperature stability • In practice, both effects are small • Tuning is difficult; how can we make steps smaller than the FSR? • Use tilt tuning! How can we actively lock the laser? Fringe Position with Varying Angle of Incidence Figure from: RP Photonics Encyclopedia, Tilt Tuning of Etalons

  14. How can we actively lock the laser? Spectrometer QCL Beamsplitter Detector Lens Piezo Mirror Etalon Control Electronics

  15. How well did the locking work?

  16. How well did the tuning work?

  17. Wavemeter Readings During Automated Re-Locking How well did the re-locking work?

  18. PC Beaglebone QCL Current Mod. Can we do all-digital locking? Mirror EC-QCL Rot. Piezo 2x Amp ADC DAC 1.8x Amp EC-QCL Length Piezo 2.5x Amp • Single-frequency locking comparable to analog approach (15-30 MHz St. Dev.) • Automatic re-locking generally fails • Maintaining lock / acquiring new lock competing for computational resources?

  19. McCall Group • - Jacob Stewart • Gerard Wysocki Acknowledgements

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