Transcript Towards a Fourier Transform Far Infrared Spectrometer

Laser Offset Stabilization for
Terahertz (THz) Frequency
Generation
Kevin Cossel
Dr. Geoff Blake
California Institute of Technology
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
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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
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
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
Frequency Modulation
What’s happening?
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Change current = change
laser frequency
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The same as adding
frequency components
Then scan the laser
Frequency Modulation
Spectroscopy of HDO
Diode laser locking
 Use feedback to reduce wavelength
Locking Range
fluctuations (reduce linewidth)
 FMS signal is error signal
 Negative error increases wavelength Error 0
 Use PID controller:
Feedback = P + I + D
P = proportional to error signal
I = integrate error (remove offset)
Wavelength
D = derivative (anticipate movement)
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
FMS Locking
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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
Offset Locking
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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
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
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)
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
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)
Acknowledgements
Dr. Geoff Blake
Rogier Braakman
Matthew Kelley
Dan Holland
NSF Grant