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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