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

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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