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.