INAOE, 27/04/2011 1
Supercontinuum generation in optical
fibers
E.A.Kuzin
INAOE, Puebla, Mexico
INAOE, 27/04/2011 2
Outline
• Short historical background on
supercontinuum.
• Introduction to nonlinear effects involving in
the process of supercontinuum generation.
• Some latest results on supercontinuum and its
applications.
• Some results of the investigations in INAOE
• Conclusions
INAOE, 27/04/2011 3
What is supercontinuum?
Supercontinuum generation is a process where
laser light is converted to light with a very broad spectral
bandwidth, whereas the spatial coherence remains high.
Lasers:– Narrow range of frequencies– Very bright• Natural light:– Wide frequency range– Not very bright• Supercontinuum light:– Very bright– White
INAOE, 27/04/2011 4
First observation.
R.R.Alfano and S.L.Shapiro, 1970
filaments
The spectrum
400 nm to 800 nm.
1-GW, 532 nm, 5 ps pulse
Frequency doubled
Nd:Glass mode-locked
laser
Several filaments were formed in the sample and broad band light was detected
at the sample output. Four wave mixing and self phase modulation were supposed
to be responsible for the effect.
Glass
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Optical fiber vs bulk materials
L
S
Nonlinear effects ~L*P/S
D
Fiber is very effective
nonlinear material !!!
/SL
Several mm2 in fibers
INAOE, 27/04/2011 6
First experiment with fibers
(1976)
In 1976 Lin and Stolen reported a new nanosecond source that produced
continua with a bandwidth of 110-180 nm centered on 530 nm at output
powers of around a kW [Appl. Phys. Lett., v.28, pp. 216 -218 (1976).
The system used a 10-20 kW dye laser producing 10 ns pulses with 15-20 nm
of bandwidth to pump a 19.5 m long, 7 μm core diameter silica fiber.
INAOE, 27/04/2011 7
Super continuum in fibers is effective at anomalous
dispersion.
Telcom fibers >1300 nm
M.N.Islam, G.Sucha, I.Bar-Joseph, M.Wegener, J.P.Gordon, D.S.Chemla, 1989
NaCl color
center laser
λ = 1.5 µm
Pulse width 14 ps
INAOE, 27/04/2011 8
J. K. Ranka, R. S. Windeler, A. J. Stentz, Optics Letters (2000).
The 75-cm section of microstructure fiber with the 1.7 µm diameter of
nuclear. The dashed curve shows the spectrum of the initial 8-kW 100-fs
pulse from Ti-sapphire laser.
First SC experiment in Photonic Crystal Fibers
INAOE, 27/04/2011 9
T. A. Birks, W. J. Wadsworth, and P. St. J. Russell, Optical Letters (2000).
SC in tapered fibers
9 cm
2-µm waist
Ti:sapphire, 100-fs laser
380 W
210 W60 W
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Theoretical background
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Spectrum of pulses
DfDt
Dt Dt
t
Af
DD
A=0.46 for Gaussian pulse
A=0.32 for secant hyperbolic
if the pulse and spectrum width are measured
as the Full Width at Half Maxima (FWHM).
pulse spectrum
At Dt=1 ps D=4 nm
(1550 nm)
Propagation of pulsos, phase velocity, group
velocity
)( 00,,zti
etzAtzE
tzA ,
f0; se mueve con
velocidad de fase
Se mueve con
velocidad de grupo
n
c
phv
d
ndn
c
d
d
d
dvg
1
12INAOE, 27/04/2011
nc
2
INAOE, 27/04/2011 13
Dispersion relation,
group velocity dispersion
....)(2
1)()()( 2
02
2
00
grv
11
....)(2
)()( 2
02
010
d
vd gr/1
2
2
2
grph vv
0
;0 ;0
grv
0
;0 ;0
grv
0
An interval of frequencies
in our process
red)n faster tha islight (blue
dispersion anomalous 0;
blue)n faster tha islight (red
dispersion normal 0;
2
2
INAOE, 27/04/2011 14
Another definition of group velocity
dispersion
LDtt 2121
t=0
t2
t1
22
2
cD
2
1
fiber
Anomalous dispersions; D>0 (2<0)
INAOE, 27/04/2011 15
Super continuum in fibers is effective at
anomalous dispersion.
Telcom fibers >1300 nm
M.N.Islam, G.Sucha, I.Bar-Joseph, M.Wegener, J.P.Gordon, D.S.Chemla, 1989
NaCl color
center laser
λ = 1.5 µm
Pulse width 14 ps
INAOE, 27/04/2011 16
Nonlinear index refraction
Wmn
A
Pnnn
eff
/103~
~
220
2
20
At high power of light the refraction
index is changed
eff
NL
eff
A
Pn
c
A
Pnn
cn
c
2
20
~
~
D
At high power of light the wave
number is changed; phase and group
velocities are changed.
Equations for pulse propagation
Ait
tzAi
t
tzA
z
tzANL D
...
),(
2
),(),(2
2
21
;' 1ztt
A – amplitude of
field
fiberinput
't't
output
Retarded time frame
AAnc
iAit
tzAi
z
tzANL
2
22
2
2~),(
2
),( D
17INAOE, 27/04/2011
0 0
Effects of nonlinearity (n2) and
group velocity dispersion (2)
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INAOE, 27/04/2011 19
Nonlinear chirp in the pulse at 2=0
Self Phase Modulation, SPM
dt
tdP
An
cdt
td
eAeAeAtzE
eff
tiztizti
)(1~)(
,
20
)(
000000
D
Z
leading edge, dP/dt>0
ω< ω0
tailing edge, dP/dt<0
ω> ω0
Broadening of the spectrum because of chirp
1 ps pulse; 20 m fiber; D=0
INAOE, 27/04/2011
20
50 W100 W500 W
D, nm
1 ps
Low power pulse propagation in fiber;
1-W power pulse; D = 18 ps/nm-km
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0 meters5 meters10 meters20 meters50 meters100 meters
Time, ps
A 100 W power pulse propagation in fiber with
anomalous dispersion; D=18 ps/nm-km
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0 meters5 meters10 meters20 meters50 meters100 meters
Time, ps
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Soliton
00 //0
2)/(sec)(
TtTtee
TthtE
102
2
2
00
ñT
A
P
eff
The condition for the
pulse to be soliton
1-ps FWHM soliton
P = 44 W at D = 18 ps/nm-km
The shape of soliton is not changed in propagation,
dispersion must be anomalous
Propagation of a 20-W 50-ps pulse through
the fiber with anomalous dispersion
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input50-m length150-m length200-m length
Time, ps
200-m length input
output
Modulation instability (MI) at anomalous dispersion
causes the amplification of fluctuation and pulse
breakupgLeionamplificat
2/1
2
0max
2
P
0max 2 Pg
25INAOE, 27/04/2011
Propagation of a 20-W, 50-ps pulse through
the 200-m fiber with normal dispersion
INAOE, 27/04/2011 26
Pulse Spectrum
Behavior of the pulse in the fiber is affected
drastically by the sign of the dispersion.
D>0: normal dispersion
D<0: anomalous dispersion
Small anomalous dispersion is favorable for
supercontinuum !!!
INAOE, 27/04/2011 27
protective
polymer
sheath
Silica Glass Optical Fibers, dispersion tuning
Silica cladding
n ~ 1.45
more complex profiles
to tune dispersion
“high” index
doped-silica core
n ~ 1.46
“LP01”
confined mode
field diameter ~ 8µmPossibilities of dispersion tuning
are restricted by properties of
glasses
28INAOE, 27/04/2011
Calculated dispersion for small core fiber
INAOE, 27/04/2011 29
From J. K. Ranka, R. S. Windeler, A. J. Stentz, 2000
2 m core
diameter; Dn=0.3
1 m core
diameter; Dn=0.1
D>0, anomalous dispersion
Photonic Crystal Fiber (PCF)
Microstructured Fiber (MSF)
Microstructured Fiber (MSF)
Class 1
Glass
Holes
Large mode area
(LMA)
High Nonlinear
(HNL)
High Numerical Aperture
(HNA)
30INAOE, 27/04/2011
Some examples of PCF from
Universidad de Valencia
25 m5 m
35 m
31INAOE, 27/04/2011
INAOE, 27/04/2011 32
First Supercontinuum Experiment in
Photonic crystal fibers
From J. K. Ranka, R. S. Windeler, A. J. Stentz, 2000
Squares – measured PCF
Circles – standard fiber
Ti:supphiere 1.6 kW, 100 fs
pulses were used for pump
Dispersion for tapered fiber and
supercontinuum generation
INAOE, 27/04/2011 33
1 m1.5 m2.5 m
Standard fiber60 W210 W
380 W
Ti:supphiere 100 fs pulses were
used for pump
T. A. Birks, W. J. Wadsworth, and P. St. J. Russell, OL , 415 (2000)
Stimulated Raman scattering
Pump
Stokes
Depleted
pump
Amplified
Stokes
S>P
zA
Pg
SSeff
P
ePzP )0()(
g – Raman gain coefficient
34INAOE, 27/04/2011
New spectral lines shifted by appr. 12 THz
to “red” may appear as a result of SRS
INAOE, 27/04/2011 35
Raman self-frequency shift
PPPPP AAi
T
Ai
z
A 2
2
2
2
2
Equation for pulse propagation
P
P
RPPPPP A
T
ATAAi
T
Ai
z
A
2
2
2
2
2
2
Raman term
TR = 3 fs input2-m 4-m6-m8-m
LT 4
0
1D
This part
as pump
Spectrum of
the 100 fs
pulse
This part
as Stokes
INAOE, 27/04/2011 36
Modulation instability starts pulse break and soliton
formation. Solitons are shifted to the red by soliton
self frequency shift.
AT
ATAi
T
Ai
z
AR
2
2
2
2
2
2
Basic equation for
supercontinuum
Time, ps Wavelength shift, nm
The 20-W, 50-ps pulse after 1-km of fiber
INAOE, 27/04/2011 37
Four wave mixing
cnnnnk
kkkkk
/
0)(
22114433
2143
2143
D
D
ω
ω
4
ω
3
ω
1
ω
2
The phase matching conditions is the principal problem to solve for effective
FWM process.
The longer the fiber the less is tolerance to phase mismatching.
INAOE, 27/04/2011 38
Preliminary summary of nonlinear effects
involved to SC generation
Self phase modulation. It occurs at any dispersion; the pulse
duration has to be shorter than 1 ps.
Soliton self frequency shift. Anomalous dispersion is required.
It is effective for pulses shorter than 1 ps.
Modulation instability. It requires the anomalous
dispersion; it is effective for long
pulses including cw.
FWM. It is more effective for anomalous
dispersion nearly the point of zero
dispersion and for short fibers.
INAOE, 27/04/2011 39
Part2
Experimental results on SC
INAOE, 27/04/2011 40
Preliminary summary of nonlinear effects
involved to SC generation
Self phase modulation. It occurs at any dispersion; the pulse
duration has to be shorter than 1 ps.
Soliton self frequency shift. Anomalous dispersion is required.
It is effective for pulses shorter than 1 ps.
Modulation instability. It requires the anomalous
dispersion; it is effective for long
pulses including cw.
FWM. It is more effective for anomalous
dispersion nearly the point of zero
dispersion and for short fibers.
INAOE, 27/04/2011 41
First observation
R.R.Alfano and S.L.Shapiro, 1970
filaments
The spectrum
400 nm to 800 nm.
1-GW, 530 nm, 5 ps pulse
Several filaments were formed in the sample and broad band light was detected
at the sample output. Four wave mixing and self phase modulation were
supposed to be responsible for the effect.
Glass
INAOE, 27/04/2011 42
Soliton self-frequency shift in fibers
M.N.Islam, G.Sucha, I.Bar-Joseph, M.Wegener, J.P.Gordon, D.S.Chemla, 1989
NaCl color
Center laser
λ = 1.5 µm
Pulse width 14 ps
500-m fiber with
anomalous dispersion
100-fs solitons were generated and soliton self frequency shift
caused the spectrum broadening
INAOE, 27/04/2011 43
J. K. Ranka, R. S. Windeler, A. J. Stentz, 2000
First observation of super continuum in photonic a
crystal fiber
Ti:supphiere 100 fs pulses were used for pump
First supercontinuum obsevation in
tapered fiber
INAOE, 27/04/2011 44
Ti:supphiere 100 fs pulses were used for pump
60 W210 W
380 W
T. A. Birks, W. J. Wadsworth, and P. St. J.Russell (2000).
INAOE, 27/04/2011 45
Nanosecond supercontinuum in PCF
Experiments used a microchip laser
producing 0.8-ns-duration pulses at 532 nm
with an average output power of 5 mW.
the extent of the spectral broadening is
explained in terms of the interplay
between Raman scattering and FWM
ZDW = 770 nm
Long pulses and cw supercontinuum
INAOE, 27/04/2011 46
A. V. Avdokhin*, S. V. Popov, and J. R. Taylor, OL, 1353 (2003)
10 mW cw laser at 1065 nm was modulated
and amplified by Yb amplifier.
Pulse durations available are of 1 ns to
several tens ns, or continuous wave (CW)
Yb modelocked laser as a source of
pulses for SC generation
INAOE, 27/04/2011 47
60kW peak - 8W average-power 2.2 ps
duration and 40 MHz repetition rate pulses
were generated at the output
Yb amplifier Yb amplifier
A. B. Rulkov, M. Y. VyatkinS. V. Popov, J. R. Taylor, Optics Express 2005
INAOE, 27/04/2011 48
The Yb source was spliced to PCF with zero dispersion at 1040nm , mode field
diameter of 3.2μm and attenuation of less than 2dB/km
A. B. Rulkov, M. Y. VyatkinS. V. Popov, J. R. Taylor, Optics Express 2005
SC in 1550 nm region is also investigated
INAOE, 27/04/2011 49
Takashi Hori, Jun Takayanagi, Norihiko Nishizawa, and Toshio Goto, Optics Express, 2004
Pulse compression
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Possible configuration for SC generation - 1;
direct pumping
Ju Han Lee and Kazuro Kikuchi, Optics Express, 4848 (2005)
0.185 W
2.18 W
Possible configuration for SC generation - 2;
Er laser with high nonlinear (HNL) fiber in a cavity
INAOE, 27/04/2011 51
Ju Han Lee and Kazuro Kikuchi, Optics Express, 4848 (2005)
0.185 W
2.18 W
Er-doped fiber laser
Possible configuration for SC generation - 3;
Raman laser with high nonlinear (HNL) fiber in a
cavity.
INAOE, 27/04/2011 52
2.18 W
0.185 W
Ju Han Lee and Kazuro Kikuchi, Optics Express, 4848 (2005)
PCF at 1550 nm pumping
INAOE, 27/04/2011 53
J. W. Nicholson,1,* R. Bise,1 J. Alonzo et al, Optics Letters, 28 (2008)
High average power SC
INAOE, 27/04/2011 54
J. C. Travers, A. B. Rulkov, B. A. Cumberland, S. V. Popov and J. R. Taylor, Optics Express (2008)
Application of SC
INAOE, 27/04/2011 55
Ultra high resolution optical coherence tomography
(OCT) for bio-medical investigations
INAOE, 27/04/2011 56
Traditional OCT is based on superluminescent diode
light sources; resolution 10 – 15 m
Broadband modelocked Ti:Al2O3 lasers have been demonstrated
to extend the axial resolution to 1-5 μm in tissue. However these systems are
expensive
All fiber SC generator can be a good choice.
Low coherence interferometry
INAOE, 27/04/2011 57
Input light
detector
displacement
mirror
Signal appears
only if L1 = L2
within the coherence length
L1
L2
INAOE, 27/04/2011 58
Special calculations were done to choose the PCF
characteristics and obtain desirable SC spectrum
Aaron D. Aguirre, Norihiko Nishizawa, James G. Fujimoto, Wolfgang
Seitz, Max Lederer, Daniel Kopf, Optics Express, 2006
Theoretical
INAOE, 27/04/2011 59
800 nm 1300 nm
Example of the tomography of skin
INAOE, 27/04/2011 60
800-nm supercontinuum 1300-nm supercontinuum
INAOE, 27/04/2011 61
All Fiber Source of Supercontinuum for
Optical Tomography
N.Nishizawa, A.D.Aguirre, J.G.Gujimoto, 2006
INAOE, 27/04/2011 62
Experimental Results Obtained with Al-
fiber SC Source
Spectrum of the radiationHuman skin
Optical Frequency Measurements
INAOE, 27/04/2011 63
Examples of two different optical frequency standards:
657 nm (456 THz) standard using laser-cooled Ca atoms;
282-nm (1064-THz) standard using a single trapped and laser-cooled Hg ion.
How to measure the frequency with high accuracy?
Laser mode frequency comb may be used for very exact measurements
Th. Udem, J. Reichert, R. Holzwarth, and T. W. H¨ansch, Optics Letters (2001)
The use of the laser
INAOE, 27/04/2011 64
0
frequency
Beating between lines can be
measured very accurate
1 GHz
1 2 N
frequency
0
Unknown displacement
N
repN Nfff 0
The measurement of f0
65
frequency
0
Unknown displacement
N
repN Nfff 0Second harmonic
generationrepN Nfff 222 0
frequency
0
Unknown displacement
N
f0
Broad band laser pulses are required Supercontinuum
Laser system for presision frequency
measurement
INAOE, 27/04/2011 66
Leo Hollberg, Associate Member, IEEE, Chris W. Oates, E. Anne Curtis, Eugene N. Ivanov, Scott A. Diddams,
Thomas Udem, Hugh G. Robinson, James C. Bergquist, Robert J. Rafac, Wayne M. Itano, Robert E. Drullinger, and
David J. Wineland, IEEE QE 2001
INAOE, 27/04/2011 67
Investigations in INAOE
E.A.Kuzin, B. Ibarra-Escamilla, INAOE
R.Rojas-Laguna, University of Guanajuato
O.Pottiez, CIO
J.W.Haus, Dayton University
M.V.Andres, Valencia Spain
INAOE, 27/04/2011 68
Two stage amplifier of pump pulses
10-m EDF
1549-nm FBG
15-m EDF
980-nm pumpInput from
DFB laser
980-nm pump
1:99 coupler
output
monitor
circulator
0 10 20 30 400
5
10
15
20
25
Pow
er,
W
Time, ns
pulse curent, mA
8
7
6
5
4
INAOE, 27/04/2011 69
Development of the modulation instability.
The 210-m long fiber.
1535 1540 1545 1550 1555-40
-35
-30
-25
-20
-15
-10
-5
0
No
rma
lize
d P
ow
er,
dB
Wavelength, nm
Pplateau
= 4 W
Pplateau
= 10 W
1536 1540 1544 1548 1552 1556 1560-40
-35
-30
-25
-20
-15
-10
-5
0
No
rma
lize
d P
ow
er,
dB
Wavelength, nm
Ppeak
=10 W
Ppeak
=30 W
Plateau Peak
INAOE, 27/04/2011 70
Raman enhanced supercontinuum generation.
The 9.13-km long fiber, 18 pump power.
1550 1600 1650 17000
200
400
600
800
1000
Sp
ectr
al po
we
r, a
.u.
Wavelength, nm
3 ns
30 ns
1550 1600 1650 1700-4
-3
-2
-1
0
Log o
f pow
er,
a.u
.Wavelength,nm
T0=3 ns
T0=30 ns
The F8L with the polarization splitter
used for the output
71
0.0 0.2 0.4 0.6 0.8 1.0 1.20.0
0.1
0.2
0.3
0.4
0.5
Tra
nsm
issio
n
Pin/Pcr
Nonlinear dependence
for transmitted pulses
-75 -50 -25 0 25 50 750.0
0.2
0.4
0.6
0.8
1.0
35. 71 ps
Am
plit
ude, a. u.
Time, ps
72
The soliton transmission for
different EDFA amplifications
The maximum transmission moves
towards longer soliton durations as the
amplification increases. The
calculations were done for 40-m long
fiber in the loop
EDFA
TwistedFiber
NOLMOutput
50/50
α – QWR
angle
Input linear
solitons
50-m
QWR
Total transmission is shown
Basic configuration for investigation
of pulse breakup
Fiber 1
Fiber 2
EDFA PC
TwistedFiber
OutputPulse NOLM
Output
50/50
QWR
OpticalAttenuator
Dispersive fiber
73
T
AATAAi
T
Ai
z
AR
22
22
2
-20 -10 0 10 200
20
40
60
80
Po
we
r, W
Time, ps
T=0.8 ps
Pulses at the NOLM output and at the
end of the 500-m dispersive fiber
74
-20 -10 0 10 200
20
40
60
80
Po
we
r, W
Time, ps
T=0.8 ps
Output of the NOLMOutput of the
dissipative fiber
-20 -10 0 10 200
10
20
30
Pow
er,
W
Time, ps
Amplification
x 2
T=1.1 ps
-20 -10 0 10 200
40
80
120
160P
ow
er,
W
Time, ps
Amplification
x3
T=0.9 ps
-20 -10 0 10 20 300
5
10
15Atenuation = 2
Po
we
r, W
Time, ps
-40 -20 0 20 400
10
20
30
40Attenuation=3
Pow
er,
W
Time, ps
T=1.4 ps
Experimental setup used for
extraction of solitons
75
Effect the amplification of
EDFA-2
-15 -10 -5 0 5 10 150,0
0,2
0,4
0,6
0,8
1,0
Am
plit
ud
e,
a.u
.
Time, ps
Amplification of
EDFA 2
2 times
3 times
4 times
5 times
Autocorrelation traces
At the NOLM output
Transmission at
different amplifications
76
Autocorrelation functions at the
NOLM input and output
-40 -20 0 20 400
1
Time, ps-40 -20 0 20 400
1
Time, ps
-15 -10 -5 0 5 10 150
1
T=2.25 ps
Time, ps-15 -10 -5 0 5 10 150
1
Tac=1.86 ps
Time, ps
-30 -20 -10 0 10 20 300
1
T=2.01 ps
Time, ps
NOLM
input
NOLM
output
10 W pump 15 W pump 20 W pump
77
A red line shows an
autocorrelation
function of the pump
pulse
A red line shows an
Sech2 profile
-60 -40 -20 0 20 40 600
1
Time, ps
Autocorrelation functions at the NOLM
output and the dispersive fiber output
-15 -10 -5 0 5 10 150.0
0.2
0.4
0.6
0.8
1.0 (a)
Tac=2.25 ps
Sech2 profile
Am
plit
ude, a.u
.
Time, ps
-16 -12 -8 -4 0 4 8 12 160,0
0,2
0,4
0,6
0,8
1,0
Tac=1.53 ps
Am
plit
ud
e, a
.u.
Time, ps
Sech2 profile
The NOLM outputThe dispersive fiber output
78
Soliton power is estimated as 100 W, 5 times
higher than the input pulse power. This value
corresponds to theoretical estimations20-W input pulse was applied
Conclusions
INAOE, 27/04/2011 79
The SC can be generated using many source of light: fs lasers, ns
lasers, cw laser.
Extremely important is the choice of the fiber with appropriate
group velocity dispersion preferably in the anomalous dispersion
region.
Generally the mechanisms responsible for SC generation look to
be clear, however many details are still for understanding.
First commercial applications appear in last years.