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SHORT PULSES
AS INTRODUCTION TO FOURIER TRANSFORMS
SPACE TIME ANALOGY:
What applies to pulses in time can be transposed tobeams in space
In time: dispersionIn space: diffraction
Pulse description --- a propagating pulse
A Bandwidth limited pulse No Fourier Transform involved
Actually, we may need the Fourier transforms (review)
Construct the Fourier transform of
Pulse Energy, Parceval theorem
Slowly Varying Envelope Approximation
Complex representation of the electric field
A Bandwidth limited pulse
Some (experimental) displays of electric field versus time
-6 -4 -2 0 2 4 6
-1
0
1
-20 -10 0 10 20
Delay (fs)
A Bandwidth limited pulse
Some (experimental) displays of electric field versus time
-20 -10 0 10 20
Delay (fs)
Shift
Derivative
Linear superposition
Specific functions: Square pulse Gaussian Single sided exponential
Real E(E*(-
Linear phase
Product Convolution
Derivative
Properties of Fourier transforms
Construct the Fourier transform of
Pulse Energy, Parceval theorem
Poynting theorem
Pulse energy
Parceval theorem
Intensity?
Spectral intensity
Description of an optical pulse
Real electric field:
Fourier transform:
Positive and negative frequencies: redundant information Eliminate
Relation with the real physical measurable field:
Instantaneous frequency
Instantaneous frequency
In general one chooses:
And we are left with
0 2-2 44
Time (in optical periods)
-1
1
0
-1
Field (Field)7
0 2-2 44
Time (in optical periods)
1
0
-1
Field(Field)7
Frequency and phase – CEP – is it “femtonitpicking”?
0 2-2 44 0 2-2 44
Time (in optical periods)
-1-1
0 2-2 44 0 2-2 44
Time (in optical periods)
1
0
-1
1
0
-1
1
0
-1
Two pulses of 2.5 optical cycle. The blue line is the electric field.
The green dotted line is the seventh power.
T
Traditional CEP measurement through high order nonlinear interaction
High order effects depend on the CEP
Pulse description --- a propagating pulse
A Bandwidth limited pulse No Fourier Transform involved
Actually, we may need the Fourier transforms (review)
Construct the Fourier transform of
Pulse Energy, Parceval theorem
Frequency and phase – CEP – is it “femtonitpicking”?
Slowly Varying Envelope Approximation
Complex representation of the electric field
Maxwell’s equations, linear propagation
Propagation of the complex field
Maxwell’s equations, nonlinear propagation
Pulse broadening, dispersion
Maxwell’s equations, nonlinear propagation
Maxwell’s equation:
Since the E field is no longer transverse
Gadi Fibich and Boaz Ilan PHYSICAL REVIEW E 67, 036622 (2003)
Is it important?
Only if
20 0
02
n nE P
z c t z c t t
2 2 2 20
02 2 2 2
nE P
z c t t
22
2F FP P
t
Study of propagation from second to first order
From Second order to first order (the tedious way)
( ) ( )kz kz
2 2 2 20 i t i t
02 2 2 2
ne P e
z c t t
2 2 22
2 2 2 2 2
22
0 0 02
1 2ik 2ik
c z c t c t z
P i P Pt t
01 i cP
z c t 2
(Polarization envelope)
time0
Electric fieldamplitude
Pulse broadening, dispersion
z = ct
Spectral phase
z = v2t(slow)
z = v1t(fast)
Spectral phase
time
Electric fieldamplitude
z = v1t(fast)
z = v2t(slow)
time
E(t)
z = ct
Pulse broadening, dispersion
Broadening andchirping
Solution of 2nd order equation
22
02
( ) ( , ) 0E zz
0( ) (1 ( ))
( )( , ) ( , ) ik zE z E 0 e
( ) ( )2 20k
0( )P E Propagation through medium
No change in frequency spectrum
To make F.T easier shift in frequencyExpand k value around central freq l
l
( )( , ) ( , ) lik zz 0 e ε εz
Z=0
1( , ) ( , ) ( )
2i tE t z E z e d
1
0gz v t
ε ε
Study of linear propagation
Expand k to first order, leads to a group delay:
Expansion orders in k(Material property
l
l
2| 22
1( , ) ( ,0) (1 | ( ) ) ( )
2l
dkiik z i td d k
t z e e e i z dd
ε ε
( )( , ) ( , ) lik zz 0 e ε εll
| ( )| ( )( , )
22
2 l
1 d kdk i zi z ik z2d d0 e e e
ε
l
l
| ( )( , ) ( | ( ) ) l
dk 2i z 2 ik zd2
1 d k0 e 1 i z e
2 d
ε
22
2
( ) 1( ) ( )
2ixtt
x x e d xt
ε ε
2 2
2 2
10
2g
i d k
z v t d t
ε ε ε
Study of linear propagation
Propagation in dispersive media: the pulse is chirped and broadening
Propagation in nonlinear media: the pulse is chirped
Combination of both: can be pulse broadening, compression,Soliton generation
DISPERSION
n()or
k()() ()e-ikz
Propagation in the frequency domain
Retarded frame and taking the inverse FT: