1. Stabilizing the Carrier-Envelope Phase of the Kansas Light Source
Eric Moon
Zuoliang Duan

2. Outline Theoretical Description of the CE phase
Why do we care about the CE phase?
Can we control it? Yes! Here’s how it’s done for the KLS and why it works.
Single-Shot CE Phase Measurement Setup
Results
Future Plans

3. Why do we care about controlling the change of the carrier-envelope phase?
Important for experiments utilizing few-cycle laser pulses, e.g. High Harmonic Generation
Can use a stabilized frequency comb to perform spectroscopy.
Related to this year’s Nobel prize!
More applications to come!

4. Results from Others
Fortier et al1, have reported phase coherence times of 326 s.
Witte et al2, have observed coherence times of 500 s.
Our group has observed coherence times of 85 s.
The main goal is to achieve long term, on the order of hours, for running experiments.
[1] Fortier et al, IEEE Journal Topics Quantum Electron, Vol. 9, , 2003
[2] Witte et al, App. Physics B, 78, 5-12, 2004

5. Theory1 Mode-locked lasers emit a regular train of pulses.
For a single laser pulse:
Envelope-function
Carrier-frequency
Carrier-envelope phase
[1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp ,2003.

6. Theory1 Time-Domain Description of the Mode-Locked Pulse Train
[1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp ,2003.

7. Theory1
Due to material dispersion inside the laser cavity, the CE phase changes.
The laser cavity length:
[1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp ,2003.

8. Theory1 Mode-Locked Pulse Train in the Time Domain:
Mode-Locked Pulse Train in the Frequency Domain:
[1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp ,2003.

9. Frequency Comb and Laser Spectrum1
[1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp ,2003.

10. Theory How? Can use a self-referencing technique!
The regular spacing of the frequency comb allows access to the change of the carrier-envelope phase.
How?
Can use a self-referencing technique!

11. Theory1
The self-referencing technique requires an octave-spanning spectrum of the laser.
Beating the second harmonic and fundamental frequency combs of the laser yields a frequency proportional to the change of the carrier-envelope phase.
[1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp ,2003.

12. Theory
The CE phase change can be controlled by locking the offset frequency, f0, to a known frequency.
In the case of the KLS, f0 is set equal to one-quarter of the repetition rate of the oscillator.

13. Experiment
The KLS utilizes a Kerr-Lens Mode locked Ti:Sapphire Oscillator emitting a ~77 million pulses per second.
The pulses are roughly 12 fs at the output of the laser and carry nJ energy per pulse.
The oscillator is the starting point for the self-referencing technique.

14. Why not use the amplifier output?
One reason: Spectrum too narrow!

15. KLS Oscillator Cavity Pump M5 Lens Ti:S A1 M1 ECDC-Module M0 M6 M7 CPOC M8 M2
M10
M4E
Ultrashort
Pulse
Output M4

17. Stabilization Experimental Setup
offset frequency
photodiode
APD
collimating Lens
f=30mm
focusing Lens
f=30mm
focusing Lens
f=30mm
HR1064nm
mirror
HR532nm
mirror
BBO
crystal
λ/2
half wave plate 1064nm
polarizing
beam-splitter
532nm
filter RG715
HR532nm mirror
λ/2
half wave plate
532nm
λ/2
HR532nm mirror
half wave plate
532nm
dichroic beam splitter
HR 532nm,HT1064nm
polarizing
beam-splitter
532nm
out-coupling
objective
f=8.55mm
in-coupling
objective
f=7.5mm
IR mirror
Silver
mirror
PCF
λ/2
half wave plate 800nm
Chirped
mirror
Chirped
mirror
grating
900lines/mm
From fs Laser

18. 532 nm
1064 nm

19. ~1064 nm, Doubled in BBO Crystal

20. Offset Frequency while Phase Locked

21. Observation of Beat Note and Frequency Comb
frep-f0
f0=19.375MHz

22. CE Phase Stability After Pulse Amplification2
A second f-2f interferometer after the KLS amplifier provides a means for quantifying the CE phase stabilization stability.
10% of the KLS amplifier output is sent to the experimental setup.
White-light is generated in a sapphire plate and a BBO crystal provides second-harmonic generation.
[2] Baltuska et al.,IEEE J. Select. Topics Quantum Electron., vol. 9, pp , 2003.

23. Theory2
Interference between the white light and second harmonic pulses:
Phase of the Interference Signal:
The shot-to-shot change of this phase can be monitored by the second f-2f setup.
[2] Baltuska et al.,IEEE J. Select. Topics Quantum Electron., vol. 9, pp , 2003.

24. Experiment compressor amplifier stretcher nonlinear interferometer
Pump
locking
electronics
AO modulator  M5
Lens
Ti:S A1 M1 M0 M6 M7
HR IR mirror BS
50:50 CP M3 OC M8 M2
M4E M4
nonlinear
interferometer
spectral
broadening
HR IR mirror
BS 9:1
compressor
amplifier
stretcher
1kHz fs laser
HR IR mirror
Single-shot phase measurement

25. f-2f Interferometer after KLS Amplifier
1kHz fs laser
concave silver
Mirrors: f=100mm
half wave plate
half wave plate
spectrometer
FCWL
two silver mirrors
SHG
VNA
VNA
sapphire
d=2.3mm
FCWL: fundamental
Continuum white light
silver mirrors
silver mirror
f=70mm
BBO
polarizer
SHG
∆T=0.265ps
FCWL
532nm HR mirror
532nm HR mirror
f=75mm

26. Spectrum of the Second Harmonic generated in the BBO Crystal

27. Single-Shot: Not Locked

28. Line-Out of the Interference Pattern

29. 1 pulse
Phase-Locked
Not Phase-Locked

30. 51 pulses
Phase-Locked
Not Phase-Locked

31. 101 pulses
Phase-Locked
Not Phase-Locked

32. 200 pulses
Phase-Locked
Not Phase-Locked

33. 1000 pulses
Phase-Locked
10000 pulses
phase-locked
pulses
Phase-locked

35. Summary
The change of the carrier-envelope phase of the KLS has been stabilized.
A technique for observing the carrier-envelope phase change shot-to-shot has been utilized.
CE phase coherence times of up to 85 seconds have been observed.

36. Future
Send a slow CE phase drift signal from the second f-2f interferometer back to the locking electronics to achieve longer locking times.

37. Thanks! Dr. Zenghu Chang Al Rankin
KLS Members: Mahendra Shakya, Shambhu Ghimire, Chris Nakamura, Chengquan Li, and Steve Gilbertson
Zuoliang Duan for being a great partner on this project.
Dr. Corwin and Dr. Washburn