Laser Offset Stabilization for Terahertz (THz) Frequency Generation

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 gt3 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 (lt10 MHz)
• High-stability

6
Frequency Modulation
Whats 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
0
Error
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
• lt1x10-6 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 gt3 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 lt10 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