Embed Size (px)
Citation preview
Grating
SphericalMirror
Deformable mirror
14. Pulse Shaping
What do we mean by pulse shaping and why do we care about it?
Methods of pulse shapingFourier synthesis
Spatial-light modulatorsAcousto-optic modulators
Deformable mirrorsAcousto-optic shaping
Phase-only pulse shapingGenetic algorithmsSimulated annealing
Adaptive pulse-shaping
gratinggrating
outE inE Masks
Why pulse-shape?
To compress pulses with complex phase
To generate pulses that control chemical reactions or other phenomena
To generate trains of pulses for telecommunications
To precompensate for distortions that occur in dispersive media
gratinggrating
outE inE Masks
Pulse Shaping: A loose definition
Loosely defined: Pulse shaping includes anything that changes the pulse shape.
i tE t I t e iE S e
Altering any of pulse’s parameters changes the pulse.
Recall that a pulse is defined by its intensity and phase in either the time or frequency domain.
What do we really mean by pulse shaping?
Tailoring a pulse shape in a specific controlled manner.
Pulse Shaper
Experiment
Pulse Results
1 45
2 37
3 12
4 80
By changing the pulse shape we can alter the results of an experiment.
How do we modulate an ultrashort pulse?
We could try to modulate the pulse directly in time.
Unfortunately, modulators are too slow.
Alternatively, we can modulate the spectrum.
out inE H E
So all we have to do is to frequency-disperse the pulse in space and modulate the spectrum and spectral phase by creating a spatially varying transmission and phase delay.
out inE t h t E t
An All-Optical Fourier Transform:The Zero-Dispersion Stretcher
How it works:The grating disperses the light, mapping color onto angle.
The first lens maps angle (hence wavelength) to position.
The second lens and grating undo the spatio-temporal distortions.
The trick is to place a mask in the Fourier transform plane.
( )xx
Fourier Transform
Plane
f
ff f
f
f
gratinggrating
John Heritage, UC DavisAndrew Weiner, Purdue
The Fourier-Synthesis Pulse-shaper
We can control both the amplitude and phase of the pulse.
The two masks or “spatial light modulators” together can yield any desired pulse.
Amplitude maskTransmission = T(x) = T()
Phase maskPhase delay = (x) = ()
Fourier Transform Plane
( ) exp[ ( )]H T i
f
ff f
f
f
gratinggrating
outE inE
Some common spatial light modulators.
Early pulse shapers used masks created using lithographic techniques and that couldn’t be modified once created.
More recent shapers use “spatial light modulators,” which can be programmed on the fly.
Types of spatial light modulators
Liquid crystal arrays
Acousto-optic modulators
Deformable mirrors
Liquid-crystal spatial light modulators
Liquid crystals orient along a an applied dc E-field. They yield a phase delay (or birefringence) that depends on an applied voltage. They can yield both phase and amplitude masks.
Liquid crystal arrays
Front view
Liquid crystal modulators (LCMs) consist of two liquid crystal arrays at 90° to each other and at 45° to the incoming light.
The first array rotates the polarization of the light in one direction and the second in the opposite direction.
Rotating each the same amount (in opposite directions) yields a phase only modulation.
Rotating one more than the other yields an amplitude and phase modulation of the light.
PixelDead Space
The pixels in LCMs limit the resolution of the modulation. The finite width covers a range of wavelengths, reducing the fidelity of the shaping.The dead spaces (gaps between electrodes) also add artifacts to the pulse train (effectively an unshaped pulse).
[1] A.M.Weiner et. al., IEEE J. Quantum Electron., 28 (1992) 908.[2] K. Takasago et. al., IEEE J. Select. Topics in Quantum Electron., 4 (1998) 346.
SLM: 128 pixels (pixel width: 97 m, pixel gap 3 m)Groove interval of the grating d-1=651 lines/mm,Input angle: 6.5 deg (100 nm bandwidth)Focal length of the achromatic lens f = 145 mm
Parameters
Spatial-light-modulator pulse shaper: details
grating
lens
grating
lens
f f f f
spatial light modulator
(SLM)input pulse shaped pulse
Takasumi Tanabe, Kimihisa Ohno,
Tatsuyoshi Okamoto, Fumihiko Kannari
Spatial Light Modulator Example
FROG trace
A sinusoidal spectral phase
Spectrum and spectral phase
Omonetto and coworkers, LANL
Pulse illumination of SLM
Acousto-optic spatial light modulators
Acousto-optic modulators (AOM) offer a method of modulating the light.
<100 fs laser pulse in
Shaped laser pulse out
Acousto-optic Modulator (TeO2, InP)
Amplitude/frequency modulated R.F. pulses (200
MHz carrier, 50MHz bandwidth, 1W peak)
10 µs (4 cm in crystal)
1o
Undiffracted beam
<100 fs laser pulse in
Shaped laser pulse out
Acousto-optic Modulator (TeO2, InP)
Amplitude/frequency modulated R.F. pulses (200
MHz carrier, 50MHz bandwidth, 1W peak)
10 µs (4 cm in crystal)
1o
Undiffracted beam
AOMs offer both phase and amplitude modulation.
The strength of the sound wave is directly related to the intensity of the diffracted light.
The phase of the sound wave is also written directly onto the diffracted light.
AOMs have a very high number of effective “pixels,” the number of sound waves that fit across the aperture of the crystal.
AOM efficiency is less than other methods since it relies on the diffracted light.
Warren Warren and coworkers, Princeton
Grating
SphericalMirror
Deformable mirror
Deformable-Mirror Pulse-Shaper
A. Efimov, and D. H. Reitze, Proc. SPIE 2701, 190 (1996)K. F. Wong, D. Yankelevich, K. C. Chu, J. P. Heritage, and A. Dienes, Opt. Lett. 18, 558 (1993)
2( ) 2 ( )x z x
x
z
This modulates the phase but not the amplitude.
Micro-Machined Deformable Mirror (MMDM)
• 600 nm Silicon Nitride Membrane
• Gold or Silver Coated• 1 ms Response Time• ~280 V Drive Voltage• Computer Controlled• 3x13 or 1x19 Actuator
Layout
G.V. Vdovin and P.M. Sarro, ``Flexible mirror micromachined in silicon'', Applied Optics 34, 2968-2972 (1995)
E. Zeek, et. Al., “Pulse compression using deformable mirrors”, Opt. Lett. 24, 493-495 (1999)
Advantages and disadvantages of the various types of spatial light modulators
Liquid-Crystal ArraysPhase and amplitude
modulation
Pixellated with dead spaces
EfficientAcousto-Optic Modulators
Phase and amplitude modulation
No dead spaces
Small pixels
Inefficient
Deformable MirrorsPhase-only modulation
No dead spaces
Large pixels
Efficient
All spatial-light-modulator pulse-shapers induce spatio-temporal distortions in the pulse, which are proportional to the magnitude of the shaping.
Acousto-Optic Pulse-Shaping
This method works without the zero dispersion stretcher and hence without spatio-temporal pulse distortions.
It launches an acoustic wave along the beam in a birefringent crystal.
The input polarization is diffracted to the other by the sound wave. The frequency that has its polarization rotated depends on the acoustic-wave frequency. Its relative delay at the crystal exit depends on the relative group velocities of the two polarizations.
different from the acousto-optic SLM!
Acousto-optic pulse shaping: theory
a b c
( )z
y
x
z
a b c
Input optical wave
Interaction length
Acoustic wave
(ordinary axis)
(extraordinary axis)
Diffracted optical wave
S(z)
E1(t)
E2(t)
( ( )[ ( )2
( ) { ) ]( }) eo nn z L z
The extra phase delay seen by each wavelength depends on how far into the crystal the acoustic wave takes on that wavelength and the ordinary and extraordinary refractive indices.
The strength of the acoustic
wave at each wavelength
determines the amplitude of
the output wave at that wavelength.
[1] F. Verluise et. al., Opt. Lett. 8 (2000) 575.[2] K. Ohno et. al., J. Opt. Soc. Am. B, 19 (2002) in press
ParametersRF signal: center frequency:52.5 MHz, Bandwidth > 10 MHz dynamic range > 50 dBCrystal: TeO2 Crystal length: 25 mm (corresponds to 3 ps)Operation frequency: 1 kHzComplex programming (control data 4096×16 bits)
Acousto-Optic Pulse-Shaping: details
Takasumi Tanabe, Kimihisa Ohno,
Tatsuyoshi Okamoto, Fumihiko Kannari
Commercial device: the “Dazzler”
Acousto-optic pulse shaping yields intensity-and-phase shaping, it induces no spatio-temporal pulse distortions, and it is available commercially.
Results using the Dazzler
Compensating the phase of an ultrashort pulse
The resulting pulse length is reduced from 30 fs to 17 fs.
Phase-only pulse shaping is more efficient. But can it achieve the desired pulse shape?
Recall that the spectral phase is more important than the amplitude for determining E(t). So can we generate a given pulse with only a phase mask? Mostly. But calculating a phase-only mask is difficult.
Generally we’re given a target wave-form.Direct calculation of H() requires a phase and amplitude mask.
We must calculate the best possible phase-only mask.
out
in
EH
E
There now exist a whole class of optimization algorithms that specialize in such difficult (or impossible) problems.
The most common are Evolutionary (also called “Genetic”) Algorithms
Evolutionary Algorithms: What are they?
Evolutionary algorithms base their optimization on a simple axiom:
Survival of the fittest.Evolutionary algorithms perform a pseudo random search.
1. Start with a set of parents (initially random).
2. Make a set of children. Using crossover to combine parts of parents.
3. Add random mutations.
4. Evaluate the fitness of the individuals. If we keep the parents from the last generation, it’s called “elitism.”
5. Select the parents for the next generation.
Initial Parents
Mutate
Check Fitness
Select Parents
Make Childrenwith cross over Children
Elitism
Parents
MutatedChildren
Evolutionary algorithms provide a simple and robust optimization method.
Second GenerationFirst Generation
Evolutionary Algorithm Example
Parameter 1
Par
am
eter
2
1. The first generation evenly samples the parameter space. From this we select the parents for the next generation
2. The second generation is concentrated around the first set of parents, and from this we select the next set of parents.
Evolutionary algorithms are very reliable, but they are slow.
Okay, so we can pulse-shape. But what if we want to amplify, too?
Amplification will distort the pulse shape.
So amplify first and shape second.
But shaping is inefficient (remember the gratings…).
So shape first and amplify second.
Hmm… Worse, we may not actually know the input pulse shape.
The solution is Adaptive Pulse Shaping.
450 mW @ 1 kHz0 = 800 nm, = 20 nm~40 fs pulse length
1.88 ps
Takasumi Tanabe, Kimihisa Ohno,
Tatsuyoshi Okamoto, Fumihiko Kannari
Adaptive pulse-shaping with amplification…
Pulse-shape, then amplify, then measure. Feed back on the FROG or SI (TADPOLE) trace.
Amplifier
Target
Shaped pulseInitial FROG traceOptimization
1 10-5
2 10-5
3 10-5
4 10-5
5 10-5
6 10-5
7 10-5
0
2 10-6
0 1000 2000 3000
Iteration #
Simulated Annealing
Calculate the difference(No waveform reconstruction in each loop)
Adaptive pulse shaping using the FROG trace
0
1
-5
0
5
10
15
-300 -150 0 150 300
Time [fs]
targetshaped
Iterate on the FROG trace.
0
0.2
0.4
0.6
0.8
1
1.2
-4
-3
-2
-1
0
1
2
3
4
-300 -150 0 150 300
Time [fs]
Initial FROG trace
Calculate the difference(No waveform reconstruction in each loop)
Target FROG
Optimized FROG trace
0
0.2
0.4
0.6
0.8
1
1.2
-2
0
2
4
6
8
10
-300 -150 0 150 300
Time [fs]
Shaped pulse
Simulated Annealing
Wave-form reconstruction
Optimization
1 10-5
2 10-5
3 10-5
4 10-5
5 10-5
6 10-5
7 10-5
0
2 10-7
4 10-7
6 10-7
8 10-7
1 10-6
0 1000 2000 3000
Iteration #
-20
-15
-10
-5
0
5
10
0 20 40 60 80 100 120
Pixel #
Mast Function
Adaptive pulse shaping: a double pulse
Target pulse
Target FROG trace
ShapedFROG trace
Phase-only pulse-shaping cannot achieve a perfect square pulse.
Phase-only adaptive pulse shaping: a square pulse
Shaped in 3000 iterations
FROG error: 0.6% (128×128)
Target trace was obtained by adding only phase modulation.
Pulse shaping with TADPOLE feedback: 300 fs double pulse
Red: shaped pulseBlue: target pulse
Fast optimizationEqual peak intensityShaped phase mask agrees well with targetBut not quite as reliable as FROG
Temporal waveform Mask function
Conclusions
Pulse-shaping for telecommunications
The goal is to create multiple pulses with variable separations.
A shaped pulse for telecommunications
Ones and zeros…
Andrew Weiner and coworkers
