Post on 03-Jun-2018
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Quasi Phase Matching
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Quasi Phase Matching
Periodicmodification ofproperties of NLmedium
Correct relative phaseat regular intervalwithout matchingphase velocity
QPM does not rely on BPM, only polarization
can be used, including cases where all waves
parallelly polarized with respect to one another
Largest deff found in ferroelectrics LiNbO3,
LiTaO3, and isotropic media GaAs where BPM is
not possible
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Domain Reversal
Electric Field poling in bulk crystals of ferroelectric families, KTP families Patterning a periodic electrode on one surface of the crystal
Applying voltage to produce a field exceeding coercive field to reversedomains in electrode region
LiNbO3/LiTaO3:Transparent near UV
LiNbO3-28pm/V, absorption edge:220nm, shortest shg:386 nm LiTaO3-16pm/V, absorption edge:277nm, shortest shg:325 nm
Mid-infrared: Multi-photon Absorption-4-5 m-> limit high poweroperation
CW interaction:6.6m
Short pulse operation:7.25 m Photorefractive effect->MgO doping
GRIIRA
Mg-LN: Grating period-1.4m, thickness-1mm
Challenge in increasing thickness due to pulsed poling, diffusion bonding
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Other CrystalsKTP families: d33-17,15,16 pm/V
QPM SHG wavelength: PPKTP: minimum-359 nm, maximum 2.74 m
Length- 8mm
Resistant to photo-refraction
Gray tracking-long lived color centers under intense visible radiation
Depends on ->intensity, spot diameter , pulse repetition rate, method of crystal growth.
Draw back:
Multi-photon absorption
Difficulty in average power operation
Semiconductor families: III-V, II-VI mid-IR applicationGaAs: 95 pm/V, transparent upto 3 phonon peak-12m
Thermal Conductivity-52W/m-k
Coherence length-10-100 m
Challenge- in fabrication, corresponds to high loss
Grating period achieved-40m in.5mm thick crystal, 20m in 200 m thick crystal
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Grating Transversely varying grating Multi grating,->array of grating
fan grating->continuous tuning of QPMbased on wavelength
angled grating->Tilt domain pattern atsmall angle to o/p facet of crystal
Radially varying grating
Uniformely patterned grating->conversionwill vary radially, leaving a radially varyinglength of medium unpoled
deff vary radially, with radially varyingduty cycle of QPM grating
Longitudinally varying grating Multigrating->two or more grating in sequencein same crystal->phase matching SHG in 1stgrating ->combine with fundamental andproduce 3rdharmonic in 2ndgrating->reducedno of interface, simpler anti-reflection coatingrequirement
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Analytical expression for grating
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Analytical expression for grating
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Different gratingsLinear-chirped gratings
Apodized gratings
Fibbonacci gratings
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Different gratingsPhase reversed gratings
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Periodically chirped gratings
Summed component aperiodic gratings
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Numerical modeling of errors in 1D QPM
gratings and its effect on parametric process
Numerical modeling of stochastic domain errors
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effect of periodicity
Duty cycle fluctuations
Numerical modeling of errors in 1D QPM
gratings and its effect on parametric process
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External factors
Numerical modeling of errors in 1D QPM
gratings and its effect on parametric process
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LiNbO3-properties
Photorefractive Effect
Pyro-electric Effect
Piezo electric Effect
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Photorefractive Effect
Extrinsic defect- presence of Fe+. Cu+,Rh+ in
LN->intermediate energy level between
conduction and valence band ->electrons in
that level get optically excited for longexposure to laser beam->electrons migrate to
Conduction band leaving holes and diffuse
through crystal to low-intensity region->generation of local electric field changing
refractive index
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Reduction of Photorefractive effect
Application of external Electric field tocancel photo-induced internal Electricfield at 170C->Limit operation in visiblerange
Dope with Mg,Zn, Hf which are photo-resistant, beyond Optical DamageThreshold which enhance optical damage
Below ODT- dopant occupying Nb5+Li+
Above ODT-dopant replaces Nb/Li Non-dopant LN-tolerate 10KW/sq.cm
Near stoichiometry-1000KW/sq.cm dueto fewer intrinsic defects, no impurities
ODT @ MG-LN- 4.6 mol% MgO->100 foldincrease in Optical damage resistance
ODT@Zn-LN--6 mol% Zn->Zn replacesNb5+Li below ODT, Zn replaces Li+ aboveODT
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Pyro-electric effect
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FDTD in 2D Lithium Niobate