Presentation on theme: "Optical Parametric Devices"— Presentation transcript:
1 Optical Parametric Devices David HannaOptoelectronics Research CentreUniversity of SouthamptonLectures at Friedrich Schiller University, JenaJuly/August 2006
2 Outline of lecture series: Optical parametric Devices Lecture1: Optical parametric devices: an overviewLecture2: Optical parametric amplification and oscillation:Basic principlesLecture3: Ultra-short pulse parametric devicesLecture4: The role of Quasi-Phase Matching in parametric devices, PLUS Brightness enhancement via parametric amplification
3 Lecture 1 Optical Parametric Devices: an overview David HannaOptoelectronics Research CentreUniversity of SouthamptonLectures at Friedrich Schiller University, JenaJuly/August 2006
6 10-8 photon conversion efficiency, 10-6 % , 3x10-10 %/W 2006 Capability: ~1000%/W→13 orders in 45 years
7 Parametric gain: key information needed Magnitude of gain, and its dependence on crystal length,pump intensity, crystal nonlinearityGain bandwidth, ie range of signal wavelengths thatexperience amplificationFor significant gain, need phase-matchingk3 = k1 + k2n3ω3 = n1ω1+n2ω2 (co-linear)
12 Parametric gain vs laser gain Gain peak can be tuned, by tuning the phase-match condition (change tilt of crystal, or temperature, or QPM grating period). Very wide signal-idler tuning is possible.Gain is produced at two wavelengths – two outputs.Choice of resonator (DRO or SRO).Coherent relation between interacting waves;restriction on relative direction of the waves.No analogue of side-pumped laser.Finite range of allowed pump wave directions can amplify single signal wave. Multimode pump can be used. brightness enhancement
13 Parametric gain vs laser gain Gain only present while pump is present.No storage of gain/energyNo equivalent of Q-switching.Few OPO round trips if nsec Q-switchedpump pulses are used.Gain is determined by peak pump intensity:very high gain with intense ultrashort pump pulses.No energy exchange with nonlinear medium – only exchange between the interacting waves.No heat input to the medium
14 Parametric devices Pump > Signal > > Idler Oscillators: SRO or DRO,pump: single-pass, double-pass or resonated,cw or pulsed.long pulse (many round trips),or train of short pulses,SPOPO (synchronously pumped OPO)OP Amplifier: input signal providedOP Generator: no input signal, output generated by amplification from parametric noise
15 Synchronously-pumped OPO >>Signal and idleroutput pulse trainMode-locked pump: pulse separation matches round trip of OPON.L.Xtal>>>OPO gain corresponds to the peak power of the pump pulseCrystal length must be short enough so that group velocitydispersion does not separate pump, signal and idler pulses in the crystal.
16 Attractions of SPOPO Low threshold average power Synchronised outputs at two wavelengths(e.g. for CARS)Very high gain possible, can oscillate even withvery high idler lossVery high efficiency,e.g. makes the tandem OPO practical
18 Periodic-poling scheme (e.g. as in PPLN) 4lcPeriod = 2lc1st orderphase-matchingESHPhase-matched3lcc -c c -c c -cQuasi-phase-matched2lclclc2lc periodlc2lc3lc4lcESH after each lc is p/2 smaller than for perfect phase-matching over the same length of medium.So, effective nonlinear coefficient reduced by p/2.
19 Some benefits of QPMAccess materials having too low a birefringence forphase-matching, e.g. LiTaO3, GaAsAbility to phase-match any frequencies in the transparency range,freedom to choose ideal pump for an OPONon-critical (90°) phase-matching,allows tight (confocal) focussingAccess to largest nonlinear coefficient,e.g. d33 in LiNbO3
20 Periodically Poled Lithium Niobate Crystal Acknowledgements to Peter Smith, Corin Gawith and Lu MingORC, University of Southampton
21 Frequency-conversion efficiency and parametric gain in PPLN SHG conversion efficiency, confocal focus (l = b = 2π wo2n1/λ)(ω1→ 2ω1)~ 16π2P(ω1)d2eff l/cє0n1n2 λ13SHG, 1064nm → 532nmorParametric gain 532nm → 1064nm~2%/ Wcm(deff = 17pm/V)(Waveguide enhancement by lλ/2nw2 ~ ; >1000%/ Wcm2)Parametric gain, 1µm → 2µm, ~0.25% / Wcm (PPLN)2µm → 4µm, ~0.5% / Wcm (GaAs)
28 OPCPA Optical Parametric Chirped Pulse Amplification Butkus et al Applied Physics B, 79, 693 (2004)
29 The OPCPA march towards Petawatts Dubietis et al IEEE J Sel Topics in QE,12, 163, (2006)
30 Brightness Enhancement via Parametric Amplification Although parametric amplification requires a high-brightness pump, this does not imply a perfect, diffraction-limited pump.A range of pump wave angles (modes) can effectively pump a SINGLE signal wave (mode).So the amplified signal wave can be brighter than the input pump.▼Brightness Enhancement(and no heat input)
31 Angular acceptance of pump I Angular acceptance: determined by the phase-mismatch, Δk,that can be toleratedΔkkpkiθkskikpΔkL = π sets limit to θNext: relate Δk to θΔk
32 Concluding remarks Χ(2) Parametric processes now have the pump sources they need and deserve.Χ(2) Parametric devices are very versatile cw to femtosecondUV to TeraHertzmW→TW→PWAbsence(?) of heat generation in active medium is of growing interest.Caveat: There is not an abundance of suitable χ(2) nonlinear media.