Presented in IEEE LEOS 16th Annual Meeting
Tucson, Arizona, Oct. 28, 2003
Extremely Sensitive Ultrashort Pulse
Measurement with Chirped Aperiodically
Poled Lithium Niobate (A-PPLN) Waveguides
Shang-Da Yang, and Andrew M. Weiner
Purdue University
Krishnan R. Parameswaran, and Martin M. Fejer
Stanford University
Sponsored by: NSF (9900369-ECS), DARPA (MDA972-03-1-0014), Air Force (F49620-02-1-0240)
Highlight
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Goal: Substantially lower the minimum power
requirements for nonlinear pulse duration
measurements, especially for optical
communications using femtosecond pulses.
Nonlinear autocorrelation of optical pulses with 220
fs duration, and 52 aJ coupled pulse energy (400
photons) is carried out.
By chirping the poling period of LiNbO3
waveguides, we broaden the SHG bandwidth to
get distortion-free measurements.
Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
Autocorrelation Sensitivity Comparison
Measurement Technology
Sensitivity [*]
Ref.
Bulk crystal, SHG
0.1~10 (mW)2
[†]
Silicon avalanche photodiode, TPA
1.5×10 -3 (mW)2
[‡]
InGaAsP laser diode, TPA
1.5×10 -4 (mW)2
[†]
Chirped A-PPLN waveguide, SHG
3.2 ×10 -7 (mW)2
[*] Sensitivity is defined as ppeak· pavg of the minimum detectable input,
the lower the better!
† J D. Harvey, J M. Dudley, L P. Barry, OFC, 1999
‡ C. Xu, J M. Roth, W H. Knox, K. Bergman, Electronics Letters. 38(2), 2002
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Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
Intensity Autocorrelation Measurement
ω0
e(t)
< PSHG >
BS
2ω 0
τ
τ
BS
eout(t)
SHG
Crystal
eSHG(t)
PSHG (t)
Filter
Power
Meter
Variable Delay Stage
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Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
Waveguides Improve SHG Efficiency
ð
Aeff
Smaller Aeff and longer L permit higher SHG
efficiency and better measurement sensitivity
Bulk
Crystal
L
Aeff
L
Waveguide
Aeff
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L
Bulk
Crystal
Gaussian beam diffraction
prevents the coexistence of
small Aeff and long L
n
Waveguides enable both
small Aeff and long L
n
QPM technique for SHG in
waveguides is the best
available one in telecom.
Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
SHG Bandwidth— Concept
vg1
PNL (f ) = A(f ) ⊗ A(f )
z
SHG
SHG PM
spectrum
Nonlinear
polarization
SHG
vg2
z
2/Ts
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n
GVM: Ts ∝ ∆ (v g− 1 ) ⋅ L
n
Thick crystal inherits large GVM
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f− 2f o
SHG BW: (∆ f ) ∝ 1 L
Insufficient SHG BW causes
measurement distortion
Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
SHG Bandwidth— Improved by Engineering
Nonlinear
polarization
Unchirped PPLN
waveguides
Nonlinear
polarization
Chirped A-PPLN
waveguides
f-2f o
Λ=Λ
0
f-2f o
Λ>Λ
0
Λ=Λ
0
Λ<Λ
Broadening SHG spectrum by longitudinally
chirping the poling period [†]
† G. Imeshev et. al, J. Opt. Soc. Am. B, 17(2), 2000
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Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
0
Efficiency and Bandwidth Trade-off
(Long PPLN)
1
ηSHG ∼ηo
PNL(f)
∆f
(Short PPLN)
(Long A-PPLN)
1/Ν
f-2fo
ηSHG ∼ηo
ηSHG ∼ηo/N
2
(1/Ν)
Ν ∆f
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f-2fo
Ν ∆f
f-2fo
Short PPLN improves SHG BW, but degrades efficiency
Long A-PPLN improves SHG BW, and preserves efficiency
Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
SHG Spectra Characterizations
S H G efficiency
Tunable
CW Laser
λ~1550
SMF
A-PPLN
waveguide
nm
Collimator
Lens
Phase matching spectrum
unchirped
PPLN wg:
(∆ λ ) ~0.17(nm)
Chirped A-PPLN wg:
(∆ λ )
PM
1538
1538.2
1538.4
λ
Power Meter
Oven
Phase matching spectrum
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λ
(nm)
1538.6
1538.8
1525
PM
~25(nm)
1530
1535
1540
λ
1545
1550
(nm)
Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
1555
Collinear Autocorrelator Setup
SMF
Polarization
Controller
Mode-locked fiber laser:
∆t~220(fs), frep=50(MHz)
λo=1545(nm), ∆λ=13(nm)
Collimator
Fiber
Laser
DCF
SMF
Chopper
PBS
Iris
Non-polarizing
Beamsplitter
HR Mirror
Variable
Delay Stage
(step resolution=0.1µm)
Iris
A-PPLN
Waveguides
PMT &
Lock-in
Amp.
Output
Coupler
10
Oven
Half-Wave
Plate
Input
Coupler
Fixed Arm
Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
Experimental Result — New Sensitivity Record
Intensity autocorrelation functions
1
0.8
U=12fJ, ∆ t=215fs
U=52aJ, ∆ t=214fs
Input pulse: (∆ λ )~13nm
A-PPLN: (∆ λ )PM~25nm
(a.u.)
0.6
0.4
0.2
0
-0.8
-0.6
-0.4
-0.2
0
τ
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0.2
0.4
0.6
0.8
(ps)
Coupled input powers: 290nW, 1.3nW
Corresponding sensitivities:
1.6×10–3 (mW)2, 0.32 ( µW)2
Data acquisition time < 3min.
Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
Experimental Result — Comparison with
Traces by Thin LiIO3 Bulk Crystal
Intensity autocorrelation functions
1.2
Input pulse:
∆ λ =40nm):
∆ t=221fs
∆ t=221fs
LiIOLiIO
(∆ λ (=40nm):
3 3
(∆ λ )~13nm
A-PPLN (∆ λ =25nm): ∆ t=222fs
1
(a.u.)
0.8
0.6
0.4
0.2
0
-1
12
-0.8
-0.6
-0.4
-0.2
τ
0
0.2
0.4
0.6
0.8
1
(ps)
Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory
Summary
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We experimentally demonstrate the ultra-sensitive
nonlinear autocorrelation using A-PPLN waveguides.
The sensitivity is 0.32 ( µ W)2 , about 500 times better
than the previous record.
Chirped A-PPLN waveguides with phase matching
bandwidth comparable with that of input pulse is used
to avoid SHG efficiency and bandwidth trade-off.
The A-PPLN waveguides can be further applied to
cross-correlation, FROG and SPIDER measurement
techniques.
Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory