Tilman Rhode-Jüchtern Friedrich-Schiller-Universität Jena Institut für Geographie
Optical Parametric Devices David Hanna Optoelectronics Research Centre University of Southampton...
Transcript of Optical Parametric Devices David Hanna Optoelectronics Research Centre University of Southampton...
Optical Parametric Devices
David HannaOptoelectronics Research Centre
University of Southampton
Lectures at Friedrich Schiller University, JenaJuly/August 2006
Outline of lecture series: Optical parametric Devices
• Lecture1: Optical parametric devices: an overview
• Lecture2: Optical parametric amplification and oscillation:
Basic principles
• Lecture3: Ultra-short pulse parametric devices
• Lecture4: The role of Quasi-Phase Matching in parametric devices, PLUS Brightness enhancement via parametric amplification
Lecture 1Optical Parametric Devices: an overview
David HannaOptoelectronics Research Centre
University of Southampton
Lectures at Friedrich Schiller University, JenaJuly/August 2006
Peter Alden Franken
Optical parametric amplification
3 – wave interactions
...)3(0
)2(0
)1(0 EEEEEEP
input ω2, wave is amplified (parametric amplification)
321
123
Energy conservation, ћω1, ћω2 annihilated, ћω3 created
SFG
DFG
ћω3 annihilated, ћω2, ћω1 created
10-8 photon conversion efficiency, 10-6 % , 3x10-10 %/W
2006 Capability: ~1000%/W→13 orders in 45 years
Parametric gain: key information needed
Magnitude of gain, and its dependence on crystal length,
pump intensity, crystal nonlinearity
Gain bandwidth, ie range of signal wavelengths that
experience amplification
For significant gain, need phase-matching
k3 = k1 + k2
n3ω3 = n1ω1+n2ω2 (co-linear)
Parametric amplification and parametric noise
Input pump spontaneously generates pairs of photons ћω1, ћω2 (parametric noise) which are then amplified.
Transparent nonlinear(χ(2)) dielectric
Pump
Pump
Signal Signal (amplified)
Idler (generated)
Amplified noise
> >
> >
ω3
ω2
ω1
ω2
>ω3
>>
ω1
ω2
>>
Optical parametric oscillation
Doubly-resonant oscillator (DRO)
Singly-resonant oscillator (SRO)
Pump
PumpSignal
Idler
Signal
Idler
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
Parametric gain vs laser gain
Gain only present while pump is present. No storage of gain/energy
No equivalent of Q-switching.Few OPO round trips if nsec Q-switched
pump 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
Parametric devices
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
Pump SignalIdler
>>>
Synchronously-pumped OPO
Mode-locked pump: pulse separation matches round trip of OPO
Signal and idler
output pulse train
OPO gain corresponds to the peak power of the pump pulse
Crystal length must be short enough so that group velocity dispersion does not separate pump, signal and idler pulses in the crystal.
> >
>
>>N.L.Xtal
Attractions of SPOPO
Low threshold average power
Synchronised outputs at two wavelengths
(e.g. for CARS)
Very high gain possible, can oscillate even with
very high idler loss
Very high efficiency,
e.g. makes the tandem OPO practical
Quasi-Phase-Matching Proposed
Armstrong, Bloembergen, Ducuing, Pershan, Phys Rev 27,1918,(1962)
Periodic-poling scheme (e.g. as in PPLN)
- - -
Period = 2lc
1st order phase-matching
lc
2lc period
ESH after each lc is /2 smaller than for perfect phase-matching over the same length of medium.
So, effective nonlinear coefficient reduced by /2.
2lc
lc
4lc
3lc
ESH
2lclc 3lc 4lc
Phase-matched
Quasi-phase-matched
Some benefits of QPM
Access materials having too low a birefringence for
phase-matching, e.g. LiTaO3, GaAs
Ability to phase-match any frequencies in the transparency range,
freedom to choose ideal pump for an OPO
Non-critical (90°) phase-matching,
allows tight (confocal) focussing
Access to largest nonlinear coefficient,
e.g. d33 in LiNbO3
Periodically Poled Lithium Niobate Crystal
Acknowledgements to Peter Smith, Corin Gawith and Lu MingORC, University of Southampton
Frequency-conversion efficiency and parametric gain in PPLN
(Waveguide enhancement by lλ/2nw2 ~102 -103 ; >1000%/ Wcm2)
Parametric gain, 1µm → 2µm, ~0.25% / Wcm (PPLN) 2µm → 4µm, ~0.5% / Wcm (GaAs)
SHG, 1064nm → 532nm orParametric gain 532nm → 1064nm
~2%/ Wcm(deff = 17pm/V)
SHG conversion efficiency, confocal focus (l = b = 2π wo2n1/λ)
(ω1→ 2ω1)
~ 16π2P(ω1)d2eff l/cє0n1n2 λ1
3
Minimum pump power/energy for 1µm – pumpedPPLN parametric devices
cw SRO ~1-3W
Nanosecond-pumped OPO ~5 µJ
Synchronously-pumped OPO ~100pJ (~10 mW @ 100 MHz)
Optical parametric generator ~100nJ (fs/ps) ~100µJ (1 nsec)
130 dBgain
All power/energy values scale as (d2/n2λ3)-1
CW singly-resonant OPOs in PPLN
First cw SRO: Bosenberg et al. O.L., 21, 713 (1996)
13w NdYAG pumped 50mm XL, ~3w threshold, >1.2w @ 3.3µm
Cw single-frequency: van Herpen et al. O.L., 28, 2497 (2003)
Single-frequency idler, 3.7 → 4.7 µm, ~1w → 0.1w
Direct diode-pumped: Klein et al. O.L., 24, 1142 (1999)
925nm MOPA diode, 1.5w thresh., 0.5w @ 2.1µm (2.5w pump)
Fibre-laser-pumped: Gross et al. O.L., 27, 418 (2002)
1.9w idler @ 3.2µm for 8.3w pump
Some results from PPLN ps/fs parametric devices
● Low threshold SPOPO;7.5 mW (av), 1047nm pump, 4ps, @120 MHz21mW, pumped by Yb fibre laser
● High gain devices (at mode-locked rep. rate)Widely-tuned SPOPO, idler >7µmOPCPA, 40 dB gain, mJ outputOPG operated at 35 MHz, ~0.5W signal
● High average power femtosecond SPOPO19W (av) signal @ 1.45 µm, 7.8W @ 3.57 µm
SPOPO facts and figures
Average output power > 20 W
Shortest pulses 13 fs
Tuning range 0.45 – 9.7 micron
Efficiency (diode laser OPO) 25%
Slope efficiency >100% (170% observed)
PPLN Waveguide Optical Parametric Generator
~2ps, 200pJ, (100W) pump @ 780nm gives ~100dB gain @1550nm
10dB needs a pumpPower of 1W
Xie
Xie et al JOSA B, 21, 1397, (2004)
Two spatial-mode waveguide parametric amplifier
OPG threshold: 300pJ , 2ps @ 780nm
Xie & Fejer, Optics Letters, 31, 799, (2006)
OPCPAOptical Parametric Chirped Pulse Amplification
Butkus et al Applied Physics B, 79, 693 (2004)
The OPCPA march towards Petawatts
Dubietis et al IEEE J Sel Topics in QE,12, 163, (2006)
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)
Angular acceptance of pump I
ΔkL = π sets limit to θ
Next: relate Δk to θ
isp kkkk k
Angular acceptance: determined by the phase-mismatch, Δk,
that can be tolerated
kp
ks
kp
ki
ki
Δk
Δk
θ
Concluding remarks
• Χ(2) Parametric processes now have the pump sources
they need and deserve.
• Χ(2) Parametric devices are very versatilecw to femtosecond
UV to TeraHertz
mW→TW→PW
• Absence(?) of heat generation in active medium is of growing interest.
• Caveat: There is not an abundance of suitable χ(2) nonlinear media.