Optical Parametric Devices

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Optical Parametric Devices

• David Hanna
• Optoelectronics Research Centre
• University of Southampton
• Lectures at Friedrich Schiller University, Jena
• July/August 2006

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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

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Lecture 1Optical Parametric Devices an overview

• David Hanna
• Optoelectronics Research Centre
• University of Southampton
• Lectures at Friedrich Schiller University, Jena
• July/August 2006

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Peter Alden Franken
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Optical parametric amplification
3 wave interactions
SFG
Energy conservation, ??1, ??2 annihilated,
??3 created
DFG
??3 annihilated, ??2, ??1 created
input ?2, wave is amplified (parametric
amplification)
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10-8 photon conversion efficiency, 10-6 ,
3x10-10 /W
2006 Capability 1000/W?13 orders in 45 years
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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
• n3?3 n1?1n2?2 (co-linear)

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Parametric amplification and parametric noise
?3
?1
Pump
Idler (generated)


?2
?2
Signal
Signal (amplified)


Transparent nonlinear (?(2)) dielectric
?3
Pump

?1


Amplified noise
?2


Input pump spontaneously generates pairs of
photons ??1, ??2 (parametric noise) which are
then amplified.
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Optical parametric oscillation
Pump
Signal
Idler
Doubly-resonant oscillator (DRO)
Pump
Signal
Idler
Singly-resonant oscillator (SRO)
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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

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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

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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 provided OP
Generator no input signal, output generated by
amplification from parametric noise
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Synchronously-pumped OPO


Signal and idler output pulse train
Mode-locked pump pulse separation matches round
trip of OPO
N.L.Xtal

• 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.

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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

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Quasi-Phase-Matching Proposed
Armstrong, Bloembergen, Ducuing, Pershan, Phys
Rev 27,1918,(1962)
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Periodic-poling scheme (e.g. as in PPLN)
4lc
Period 2lc 1st order phase-matching
ESH
Phase-matched
3lc
c -c c -c c -c
Quasi- phase- matched
2lc
lc
lc
lc
2lc period
2lc
3lc
4lc
ESH 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.
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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

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Periodically Poled Lithium Niobate Crystal
Acknowledgements to Peter Smith, Corin Gawith and
Lu Ming ORC, University of Southampton
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Frequency-conversion efficiency and parametric
gain in PPLN
SHG conversion efficiency, confocal focus (l b
2p wo2n1/?) (?1? 2?1) 16p2P(?1)d2eff
l/c?0n1n2 ?13
SHG, 1064nm ? 532nm or Parametric gain
532nm ? 1064nm
2/ Wcm (deff 17pm/V)
(Waveguide enhancement by l?/2nw2 102 -103
1000/ Wcm2) Parametric gain, 1µm ? 2µm,
0.25 / Wcm (PPLN) 2µm ? 4µm, 0.5 / Wcm
(GaAs)
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Minimum pump power/energy for 1µm pumpedPPLN
parametric devices

• cw SRO 1-3W
• Nanosecond-pumped OPO 5 µJ
• Synchronously-pumped OPO 100pJ
• (10 mW _at_ 100 MHz)
• Optical parametric generator 100nJ (fs/ps)
• 100µJ (1 nsec)

130 dB gain
All power/energy values scale as (d2/n2?3)-1
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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 _at_
  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 _at_ 2.1µm
  (2.5w pump)
• Fibre-laser-pumped Gross et al. O.L., 27, 418
  (2002)
• 1.9w idler _at_ 3.2µm for 8.3w pump

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Some results from PPLN ps/fs parametric devices

• Low threshold SPOPO
• 7.5 mW (av), 1047nm pump, 4ps, _at_120 MHz
• 21mW, pumped by Yb fibre laser
• High gain devices (at mode-locked rep. rate)
• Widely-tuned SPOPO, idler 7µm
• OPCPA, 40 dB gain, mJ output
• OPG operated at 35 MHz, 0.5W signal
• High average power femtosecond SPOPO
• 19W (av) signal _at_ 1.45 µm, 7.8W _at_ 3.57 µm

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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)

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PPLN Waveguide Optical Parametric Generator
2ps, 200pJ, (100W) pump _at_ 780nm gives 100dB
gain _at_1550nm 10dB needs a pump Power of 1W
Xie
Xie et al JOSA B, 21, 1397, (2004)
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Two spatial-mode waveguide parametric amplifier
OPG threshold 300pJ , 2ps _at_ 780nm Xie
Fejer, Optics Letters, 31, 799, (2006)
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OPCPAOptical Parametric Chirped Pulse
Amplification
Butkus et al Applied Physics B, 79, 693 (2004)
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The OPCPA march towards Petawatts
Dubietis et al IEEE J Sel Topics in QE,12, 163,
(2006)
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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)
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Angular acceptance of pump I
Angular acceptance determined by the
phase-mismatch, ?k, that can be tolerated
?k
kp
ki
?
ks
ki
kp

• ?kL p sets limit to ?
• Next relate ?k to ?

?k
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Concluding remarks

• ?(2) Parametric processes now have the pump
  sources
• they need and deserve.
• ?(2) Parametric devices are very versatile cw 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
  ?(2) nonlinear media.