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Laser Offset Stabilization for Terahertz (THz) Frequency Generation

Laser Offset Stabilization for Terahertz (THz) Frequency Generation. Kevin Cossel Dr. Geoff Blake California Institute of Technology. What is Terahertz Spectroscopy?. ~1x10 11 -1x10 13 Hz or ~0.1-10 Terahertz (THz) ~3 - 300 cm -1 ~3000 - 30 µm

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Laser Offset Stabilization for Terahertz (THz) Frequency Generation

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  1. Laser Offset Stabilization for Terahertz (THz) Frequency Generation Kevin Cossel Dr. Geoff Blake California Institute of Technology

  2. What is Terahertz Spectroscopy? • ~1x1011-1x1013 Hz or ~0.1-10 Terahertz (THz) • ~3 - 300 cm-1 • ~3000 - 30 µm • Also known as far-infrared (FIR) or sub-millimeter spectroscopy • Study low-energy processes both in the laboratory and in remote sensing applications

  3. Why study Thz region? • Many uses • High-resolution spectroscopy • Vibration-rotation coupling • Lower spectral density expected • Remote sensing • Astronomy: • Matched to emission from cold dust clouds • Characterize organic material (especially amino acids) present in the interstellar medium • Lower spectral density expected • SOFIA & Herschel • Need lab data first

  4. THz sources • Existing sources have problems • Solid-state electronic oscillators • Power drops above 200 MHz • Doubling/tripling not good above 1 THz • Lasers • Low frequency = long lifetime, no direct bandgap lasers • Quantum cascade lasers – >3 THz, 10 Kelvin, narrow tunability • THz Time Domain Spectroscopy • Probe with sub-picosecond pulses • Gate detector with laser • Limited resolution • Optical-heterodyne

  5. Purpose • Develop a spectrometer that can be used to characterize the spectra of molecules in the range of ~0.5-10 Terahertz (THz) • Need THz source • Inexpensive • Multiterahertz bandwidth • Accurate • Low linewidth (<10 MHz) • High-stability

  6. Frequency Modulation What’s happening? Change current = change laser frequency The same as adding frequency components Then scan the laser

  7. Frequency Modulation Spectroscopy of HDO

  8. Diode laser locking • Use feedback to reduce wavelength fluctuations (reduce linewidth) • FMS signal is error signal • Negative error increases wavelength • Use PID controller: • Feedback = P + I + D • P = proportional to error signal • I = integrate error (remove offset) • D = derivative (anticipate movement) Locking Range Error 0 Wavelength

  9. Tunable locking • Lock laser 1 to HDO line • Generate offset between laser 1 and laser 2 • Lock offset • Lock laser 3 to different HDO line • Output is difference between laser 2 & laser 3 • Narrow tune = offset • Wide tune = lock to different lines

  10. FMS Locking • Electro-optic modulator provides frequency modulation • Photodetector varying intensity beat note • Mix with driving RF DC output • Feedback DC error signal to PID controller • Controls piezo which adjust wavelength

  11. Offset Locking • Laser 1 locked to HDO • Lasers 1 and 2 combined on fast (40 GHz) photodetector • Output difference frequency • Mix with tunable RF source Output 0-1 GHz • Send to source locking counter • Feedback to laser 2, offset locking up to ±20 GHz

  12. Results – FMS locking • 2 hours • Free-running (blue) • 47 MHz standard deviation • 4.9 MHz RMSE • 2 MHz/second drift • Locked (red) • Mean 20 kHz • 3.5 MHz standard deviation • 5x10-5 MHz/second drift • 10 seconds • Free-running (blue) • 30 MHz peak-peak deviations • 5.5 MHz standard deviation • Locked (red) • 10 MHz peak-peak • 3 MHz standard deviation

  13. Results – Offset locking • Difference frequency • Two free-running (blue, left): • 300 MHz drift • 5 MHz RMSE • One laser PID locked (red) • PID + offset locking • 1.3 MHz standard deviation (over 75 seconds) • Mean accurate to 260 kHz • <1x10-6 MH/second drift (stable for 15 hours)

  14. Discussion • Currently: • PID lock • 20 kHz accuracy • 3 MHz linewidth • Low drift • Offset (Lasers 1 & 2) • ±20 GHz (easily changed to ±40 GHz) • 300 kHz accuracy • Very stable • High spectral density of HDO • Predicted: >3 THz bandwidth, 8 MHz linewidth, 300 kHz accuracy • Work to lower linewidth/improve accuracy

  15. Conclusion • Developed a technique for generating a tunable THz difference between two lasers with a final linewidth of <10 MHz • Combine lasers on ErAs/InGaAs photomixer to generate THz radiation • Other techniques could provide higher stability at the cost of tunability or wide bandwidth but limited resolution • Compromise system • Working on improving linewidth (hopefully 1 MHz) and bandwidth (up to 15 THz) • Tunability/linewidth combination already useful for spectroscopy (developing Fourier transform terahertz spectrometer)

  16. Acknowledgements Dr. Geoff Blake Rogier Braakman Matthew Kelley Dan Holland NSF Grant

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