use of fringe-resolved autocorrelation for the diagnosis ... · use of fringe-resolved...
Post on 21-Apr-2018
228 Views
Preview:
TRANSCRIPT
Use of Fringe-Resolved Autocorrelation for
the Diagnosis of the Wavelength Stability
of KU-FEL
and single-shot measurement of mid-IR laser spectra
Institute of Advanced Energy
Kyoto University
Takashi Nakajima, Yu Qin, Xiaolong Wang,
Heishun Zen, Toshiteru Kii,
and Hideaki Ohgaki
Outline
• Introduction
• Autocorrelation
intensity autocorrelation (IAC) fringe-resolved autocorrelation (FRAC)
• Idea to estimate the shot-to-shot change of
laser wavelength
spectrally unresolved method using IAC and FRAC
• Single-shot measurement of mid-IR FEL spectra
sum-frequency mixing
• Summary
Introduction
KU-FEL: Oscillator-type FEL for the mid-IR (5-14 μm)
time
macro pulse
micro pulse
~ µs
~ 350 ps
pulse structure of KU-FEL
~ ps
0.2 μm
Shot-to-shot change of laser wavelength
If 1 ps pulse and
1 % change of λ0
scan
12 11.5 13 (μm)
FEL wavelength
1.0
0.5
0
Inte
ns
ity
λ0
different macropulse
different micropulse
scanning-type spectrometer
array-type mid-IR photodetector
For the detection of single-micropulse spectra
Autocorrelation methods
Nonlinear crystal
filter
Photo
detector
τ dttEtE
4
21 )()(
FEL pulse
Movable
stage
FRAC signal
ω
2ω
【Fringe-resolved autocorrelation (FRAC)】
dttItI )()( τ
2ω
Photo
detector
Inter-
ferometer
【Intensity autocorrelation (IAC)】
FEL pulse
Movable
stage
Nonlinear crystal
IAC signal
Delay time τ
τp 2
IAC
sig
nal
Techniques to measure the ps~fs pulse duration
Delay time τ
FRA
C s
ign
al
ω
Idea to estimate the shot-to-shot change of laser wavelength
How does it work? λ(k) λ(k+1)
train of
micropulses time
interference smeared out
for larger delay if λ(k)≠λ(k+1)
c/ period noscillatio
λ(k) λ(k+1) λ(k)+ λ(k+1)
+ = narrowing
replica
4)1()1(
4)()(
)()(
)()()(
tEtE
tEtEI
kk
kk
FRAC
λ(k)
𝒄
λ(k+1)
𝒄
)(exp1
1)1(2ln2exp)( )()(
0
)(
3
2
0
)(
2
)(
0
)(
1
)( ttiaia
tktIatE kkk
k
kkk
M
k
k tEtI1
2)( )()(
IAC and FRAC signals with various fluctuations
autocorrelation
signals
dttItII IAC )()()(
dttEtEIN
k
kk
FRAC
1
4)()( )()()(
Intensity of the macropulse
Field amplitude of the kth micropulse
micropulse k the of
frequency central and duration, intensity, of nfluctuatio :, ,
micropulse kth the offrequency central :
duration micropulse
th
(k)
0
)(
3
)(
2
)(
1
:
kkk
p
aaa
0
21 p
0% 0.5%
1% 1.5%
Delay time (ps) Delay time (ps)
1.53ps 1.36ps
1.09ps 0.89ps
Narrowing of FRAC signals by wavelength fluctuation
After averaging over thousands of shots,
IAC signals : no change
FRAC signals: narrowing by shot-to-shot wavelength change
FRAC signal narrowing
Wavelength fluctuation
For 1 ps pulse,
Qin et al., Jpn.J.Appl.Phys. (in press)
2
2
0
22
0
2
2
22
02
2222
0
2
2
22
4
2ln2exp2
2cos2)1(2ln2
exp
cos2
2lncos
22
)3(2lnexp4
1)()(
p
p
pp
dttEtE
Retrieval of pulse duration from FRAC signals
If we can retrieve the pulse duration from the FRAC signals,
we may only perform the FRAC measurement !
Expectation values of
0
0
1.0
ps p
delay (ps)
Pulse duration is retrieved by the
time-integration of FRAC signals p
1)( FRACI
d
d
d
pFRACId
2ln
21)(
1)( FRACI
Experimental results
step size = 2 μm (= 1/150 ps)
FRAC measurement
exp.envelope
fitted envelope
delay (ps)
exp.envelope
fitted envelope
delay (ps)
step size = 10 μm (= 1/30 ps)
IAC measurement
delay (ps)
exp.data
Gaussian fitting
%4.1
0
66.0
ps p
%0
52.1
66.0
ps p
ps 64.02
9.0 p
0.9 ps
• pulse duration 0.64~0.66 ps by
both IAC and FRAC methods
• wavelength stability is < 1.4 %
Qin et al. (in preparation)
Toward single-shot measurement of Mid-IR FEL pulse spectra
Mid-IR : MCT-based array photodetector (very expensive)
Near–IR: Si-based array photodetetor (cheap!)
Convert the MIR laser pulse into the NIR pulse by sum-frequency mixing !
ω1 (11 μm) + ω2 (1064 nm) → ω3(970 nm)
KU-FEL Nd:YAG or
microchip lasers
SFM signal
(near-IR)
Signal intensity
ISFM ∝ IMIR x I1064nm
< 1 ps 20 ns or
0.8 ns
< 1 ps
Wang et al. (in preparation)
spectrometer
shortpass
filter
nonlinear
crystal
(AgGaS2)
SFM signal
~970 nm
Schematics of sum-frequency mixing
SFM signal detectable?
Experimental setup for sum-frequency mixing
2 mm
5 mJ/pulse for YAG
(20 ns)
100μJ/pulse
for microchip laser
(0.8 ns)
KU-FEL
@11 μm
5mJ/macropulse at 1Hz
~5000 micropulses
YA
G / m
icroch
ip
spectro -meter
SP filter
Spectral resolution
~0.8 nm
KU-FEL
macropulse
control
delay 1064 nm
pulse
λ/2
AgGaS2
Shot-to-shot change of the SFM spectra by 20 ns Nd:YAG laser
Sum of the micropulse spectra in the same macropulse
𝟐𝟎 𝒏𝒔
𝟑𝟓𝟎 𝒑𝒔= 𝟓𝟕
SF
M s
ign
al
966 968
Wavelength (nm)
972 974 970
1064nm pulse
macro pulse
micropulse separation
20 ns
350ps
SFM
Wang et al. (in preparation)
𝛌𝐅𝐄𝐋 = (𝟏
𝛌𝐒𝐅𝐌−
𝟏
𝛌𝟏𝟎𝟔𝟒𝐧𝐦)−𝟏
retrieved
FEL spectrum
Retrieval of the FEL spectra
Wavelength (μm)
raw spectrum
SF
M s
ignal (a
.u.)
Wavelength (nm)
deconvolved
spectrum
FEL spectrum by
monochrometer
FE
L s
ignal (a
.u.)
Wang et al. (in preparation)
Time (μs) wavelength (μm)
Inte
nsi
ty (
a.u
.)
Inte
nsi
ty (
a.u
.) raw
deconvolved
Macropulse shape FEL spectrum
Time (μs) wavelength (μm)
Inte
nsi
ty (
a.u
.)
Inte
nsi
ty (
a.u
.) raw
deconvolved
Macropulse shape FEL spectrum
Time (μs) wavelength (μm)
Inte
nsi
ty (
a.u
.)
Inte
nsi
ty (
a.u
.) raw
deconvolved
Macropulse shape FEL spectrum
Summary
Measurement of the pulse duration and wavelength stability
of KU-FEL using fringe-resolved autocorrelation
Spectrally unresolved detection !
Periodicity of the fringe wavelength information
micropulse duration = 0.64~0.66 ps
wavelength instability <1.4%
Single-shot measurement of the laser spectra of KU-FEL for temporally selected micropulse(s)
Sum-frequency mixing to generate near-IR signals
top related