laser offset stabilization for terahertz (thz) frequency generation kevin cossel dr. geoff blake...
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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
Kevin Cossel
Dr. Geoff Blake
California Institute of Technology
What is Terahertz Spectroscopy?What is Terahertz Spectroscopy?
~1x10~1x101111-1x10-1x101313 Hz or ~0.1-10 Hz or ~0.1-10 Terahertz (THz)Terahertz (THz)
~3 - 300 cm~3 - 300 cm-1-1
~3000 - 30 µm~3000 - 30 µm Also known as far-infrared Also known as far-infrared
(FIR) or sub-millimeter (FIR) or sub-millimeter spectroscopyspectroscopy
Study low-energy processes Study low-energy processes both in the laboratory and in both in the laboratory and in remote sensing applicationsremote sensing applications
~1x10~1x101111-1x10-1x101313 Hz or ~0.1-10 Hz or ~0.1-10 Terahertz (THz)Terahertz (THz)
~3 - 300 cm~3 - 300 cm-1-1
~3000 - 30 µm~3000 - 30 µm Also known as far-infrared Also known as far-infrared
(FIR) or sub-millimeter (FIR) or sub-millimeter spectroscopyspectroscopy
Study low-energy processes Study low-energy processes both in the laboratory and in both in the laboratory and in remote sensing applicationsremote sensing applications
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Why study Thz region?Why study Thz region? Many usesMany uses High-resolution spectroscopyHigh-resolution spectroscopy
Vibration-rotation couplingVibration-rotation coupling Lower spectral density expectedLower spectral density expected
Remote sensingRemote sensing
AstronomyAstronomy:: Matched to emission from cold dust cloudsMatched to emission from cold dust clouds Characterize organic material (especially amino acids) Characterize organic material (especially amino acids)
present in the interstellar mediumpresent in the interstellar medium Lower spectral density expectedLower spectral density expected SOFIA & HerschelSOFIA & Herschel Need lab data firstNeed lab data first
Many usesMany uses High-resolution spectroscopyHigh-resolution spectroscopy
Vibration-rotation couplingVibration-rotation coupling Lower spectral density expectedLower spectral density expected
Remote sensingRemote sensing
AstronomyAstronomy:: Matched to emission from cold dust cloudsMatched to emission from cold dust clouds Characterize organic material (especially amino acids) Characterize organic material (especially amino acids)
present in the interstellar mediumpresent in the interstellar medium Lower spectral density expectedLower spectral density expected SOFIA & HerschelSOFIA & Herschel Need lab data firstNeed lab data first
THz sourcesTHz sources Existing sources have problemsExisting sources have problems Solid-state electronic oscillators Solid-state electronic oscillators
Power drops above 200 MHzPower drops above 200 MHz Doubling/tripling not good above 1 THzDoubling/tripling not good above 1 THz
LasersLasers Low frequency = long lifetime, no direct bandgap lasersLow frequency = long lifetime, no direct bandgap lasers Quantum cascade lasers – >3 THz, 10 Kelvin, narrow Quantum cascade lasers – >3 THz, 10 Kelvin, narrow
tunabilitytunability
THz Time Domain SpectroscopyTHz Time Domain Spectroscopy Probe with sub-picosecond pulsesProbe with sub-picosecond pulses Gate detector with laserGate detector with laser Limited resolutionLimited resolution
Optical-heterodyneOptical-heterodyne
Existing sources have problemsExisting sources have problems Solid-state electronic oscillators Solid-state electronic oscillators
Power drops above 200 MHzPower drops above 200 MHz Doubling/tripling not good above 1 THzDoubling/tripling not good above 1 THz
LasersLasers Low frequency = long lifetime, no direct bandgap lasersLow frequency = long lifetime, no direct bandgap lasers Quantum cascade lasers – >3 THz, 10 Kelvin, narrow Quantum cascade lasers – >3 THz, 10 Kelvin, narrow
tunabilitytunability
THz Time Domain SpectroscopyTHz Time Domain Spectroscopy Probe with sub-picosecond pulsesProbe with sub-picosecond pulses Gate detector with laserGate detector with laser Limited resolutionLimited resolution
Optical-heterodyneOptical-heterodyne
PurposePurpose Develop a spectrometer that can be used to characterize Develop a spectrometer that can be used to characterize
the spectra of molecules in the range of ~0.5-10 the spectra of molecules in the range of ~0.5-10 Terahertz (THz)Terahertz (THz)
Need THz sourceNeed THz source InexpensiveInexpensive Multiterahertz bandwidthMultiterahertz bandwidth AccurateAccurate Low linewidth (<10 MHz)Low linewidth (<10 MHz) High-stabilityHigh-stability
Develop a spectrometer that can be used to characterize Develop a spectrometer that can be used to characterize the spectra of molecules in the range of ~0.5-10 the spectra of molecules in the range of ~0.5-10 Terahertz (THz)Terahertz (THz)
Need THz sourceNeed THz source InexpensiveInexpensive Multiterahertz bandwidthMultiterahertz bandwidth AccurateAccurate Low linewidth (<10 MHz)Low linewidth (<10 MHz) High-stabilityHigh-stability
Frequency ModulationFrequency Modulation
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Change current = change Change current = change laser frequencylaser frequencyChange current = change Change current = change laser frequencylaser frequency The same as adding The same as adding
frequency componentsfrequency componentsThe same as adding The same as adding frequency componentsfrequency components
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Then scan the laserThen scan the laserThen scan the laserThen scan the laser
What’s happening?What’s happening?What’s happening?What’s happening?
Frequency Modulation Spectroscopy of HDOFrequency Modulation Spectroscopy of HDO
Diode laser lockingDiode laser locking
Use feedback to reduce wavelength Use feedback to reduce wavelength
fluctuations (reduce linewidth)fluctuations (reduce linewidth) FMS signal is error signalFMS signal is error signal Negative error increases wavelengthNegative error increases wavelength Use PID controller:Use PID controller:
Feedback = P + I + DFeedback = P + I + D
P = proportional to error signalP = proportional to error signal
I = integrate error (remove offset)I = integrate error (remove offset)
D = derivative (anticipate movement)D = derivative (anticipate movement)
Use feedback to reduce wavelength Use feedback to reduce wavelength
fluctuations (reduce linewidth)fluctuations (reduce linewidth) FMS signal is error signalFMS signal is error signal Negative error increases wavelengthNegative error increases wavelength Use PID controller:Use PID controller:
Feedback = P + I + DFeedback = P + I + D
P = proportional to error signalP = proportional to error signal
I = integrate error (remove offset)I = integrate error (remove offset)
D = derivative (anticipate movement)D = derivative (anticipate movement)
0
Locking Range
Error
Wavelength
Tunable lockingTunable locking Lock laser 1 to HDO lineLock laser 1 to HDO line Generate offset between Generate offset between
laser 1 and laser 2laser 1 and laser 2 Lock offsetLock offset Lock laser 3 to different Lock laser 3 to different
HDO lineHDO line Output is difference Output is difference
between laser 2 & laser 3between laser 2 & laser 3 Narrow tune = offset Narrow tune = offset Wide tune = lock to Wide tune = lock to
different linesdifferent lines
Lock laser 1 to HDO lineLock laser 1 to HDO line Generate offset between Generate offset between
laser 1 and laser 2laser 1 and laser 2 Lock offsetLock offset Lock laser 3 to different Lock laser 3 to different
HDO lineHDO line Output is difference Output is difference
between laser 2 & laser 3between laser 2 & laser 3 Narrow tune = offset Narrow tune = offset Wide tune = lock to Wide tune = lock to
different linesdifferent lines
FMS LockingFMS Locking
Electro-optic modulator provides frequency modulationElectro-optic modulator provides frequency modulation Photodetector varying intensity beat notePhotodetector varying intensity beat note Mix with driving RF DC outputMix with driving RF DC output Feedback DC error signal to PID controllerFeedback DC error signal to PID controller Controls piezo which adjust wavelengthControls piezo which adjust wavelength
Electro-optic modulator provides frequency modulationElectro-optic modulator provides frequency modulation Photodetector varying intensity beat notePhotodetector varying intensity beat note Mix with driving RF DC outputMix with driving RF DC output Feedback DC error signal to PID controllerFeedback DC error signal to PID controller Controls piezo which adjust wavelengthControls piezo which adjust wavelength
Offset LockingOffset 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
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 lockingResults – 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
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
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 lockingResults – Offset locking
Difference frequencyDifference frequencyTwo free-running (blue, left):Two free-running (blue, left):
300 MHz drift300 MHz drift5 MHz RMSE5 MHz RMSE
One laser PID locked (red)One laser PID locked (red)PID + offset lockingPID + offset locking
1.3 MHz standard deviation (over 75 seconds)1.3 MHz standard deviation (over 75 seconds) Mean accurate to 260 kHzMean accurate to 260 kHz <1x10<1x10-6-6 MH/second drift (stable for 15 hours) MH/second drift (stable for 15 hours)
Difference frequencyDifference frequencyTwo free-running (blue, left):Two free-running (blue, left):
300 MHz drift300 MHz drift5 MHz RMSE5 MHz RMSE
One laser PID locked (red)One laser PID locked (red)PID + offset lockingPID + offset locking
1.3 MHz standard deviation (over 75 seconds)1.3 MHz standard deviation (over 75 seconds) Mean accurate to 260 kHzMean accurate to 260 kHz <1x10<1x10-6-6 MH/second drift (stable for 15 hours) MH/second drift (stable for 15 hours)
DiscussionDiscussion 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
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
ConclusionConclusion 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)
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)
AcknowledgementsAcknowledgements
Dr. Geoff Blake
Rogier Braakman
Matthew Kelley
Dan Holland
NSF Grant
Dr. Geoff Blake
Rogier Braakman
Matthew Kelley
Dan Holland
NSF Grant