Lecture 3 Ultrashort pulse parametric devices

1
Lecture 3Ultra-short pulse parametric devices

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

2
Lecture Outline

• General features and attractions of ultrashort
  pulse parametric devices
• Synchronously Pumped OPOs (SPOPOs) general
  considerations
• Specific examples of SPOPO performance
• Optical Parametric Amplifiers (OPA), Optical
  Parametric Chirped Pulse Amplifiers (OPCPA)
  Optical Parametric Generators (OPG)
• Carrier Envelope Phase considerations

3
Attractions of parametric processes in the
ultrashort pulse regime

• High gain damage intensity behaves 1/(pulse
  duration)½
• Broad gain bandwidth
• Wavelength flexibility (eg different from
  TiSapphire!)
• Reduced ASE, reduced background, good contrast
• High Quantum efficiency
• Low thermal effects
• Good beam quality
• Scalability

4
Some disadvantages of parametric processes

• Small aperture dimensions available
• No energy storage
• Synchronisation requirements
• High pump brightness required

5
Some general features of ultra-short pulse
parametric devices

• High gain and wide bandwidth can be obtained in a
  single pass of a parametric amplifier lasers
  require regenerative amplification
• For the shortest pulses, ensure a large enough
  gain-bandwidth good temporal overlap between
  the interacting waves over the NL medium
• Short crystal length can ensure the above, but
  places limits on the achievable gain
• Alternative ways to increase the gain bandwidth
  include
• near-degenerate operation
• non-collinear phase-matching
• Double refraction effects are reduced for shorter
  crystals
• Non-collinear phase-matching can contribute to
  group-velocity-matching

6
Dependence of double-refraction effects on
crystal length

• For a given double-refraction walk-off angle ?,
  and beam diameter D, the effect of walk-off in a
  crystal of length is insignificant if
• ?L/D ltlt 1
• For confocal focussing, 2pw02n/? L, i.e., D
  2w0 2L?/np½
• so ?L/D ?pnL/2?½
• Hence, for shorter crystals, as required for
  shorter pulses,
• confocal focussing is less compromised by double
  refraction
• 10x shorter pulse ?10x shorter Xtal ?
  toleratev10x greater ? value

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

• 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.

8
SPOPO pump requirement versus crystal length

• If length L is determined by the allowable Group
  Delay Difference,
• then, L ? T
• and if confocal focussing is used,
• then, gain ? LP LE/T ? E
• Hence, threshold is specified by an energy,
  independent of pulse duration,
• for a given repetition rate,
• threshold average power is then independent of
  pulse duration.
• But Self Phase Modulation is more problematic for
  shorter pulses, since
• effect of SPM ( fractional spectral broadening)
  ? IL ? PL/L ? E/T
• (T,P,E,I are, respectively, pump pulse duration,
  power, energy, intensity)

9
Some Attractions of SPOPOs

• Low threshold average power (amenable to diode
  pumping)
• Power scalable, eg via fibre-pumped SPOPOs
• Very wide tuning
• 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

10
SPOPO facts and figures

• Average output power gt 20 W
• Shortest pulses 13 fs
• Tuning range 0.45 9.7 micron
• Efficiency
• (diode ? laser ? OPO) 25
• Slope efficiency gt100 (170 observed)

11
Crystal length constraint for a SPOPO

• Require enough signal gain bandwidth for a
• signal pulse duration pump pulse duration T

(away from degeneracy)
Use higher order terms in Taylor expansion if the
vg are nearly equal

• Require signal ( idler) pulse not to walk away
  from pump pulse

Signal case
12
Typical resonator arrangement for SPOPO
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How to tune a QPM OPO

• Angle tuning may not be an option, so
• Fixed pump tune crystal temperature (fine
  tune)
• change grating period (coarse tune)
• Tune pump wavelength
• Fixed pump tune across gain-bandwidth via
  intra-cavity filter, or diffraction grating
  reflector.

14
SPOPO slope efficiency of gt 100
L.Lefort, et al., Optics Communications Vol.152
pp.55-58 (1998)
15
Order of magnitude pulse compression in a PPLN
SPOPO
4ps pump, 250fs signal, 20mm PPLN 100fs/mm
pump/signal Group delay difference
Lefort et al. Opt Letts, 24(1),28,1999
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Other features of SPOPO

• Cavity length change can change signal
  wavelength
• not a good technique for tuning as pulse
  characteristics will change
• Oscillation tolerates cavity length changes of
  many pulse widths.
• Stabilise cavity length via stabilising the
  output frequency
• Tuning through the gain profile can lead to
  higher
• order transverse modes of the signal
• Tuning elements involving angular dispersion, eg
  grating, produce tilted pulses
• In QPM materials, many additional outputs may be
• seen (2?s, 2?i, ?s?p, ?i?p).

17
PPLN SPOPO with feedback via diffraction grating
Tilted signal pulse is cleaned up in PPLN
amplifier before exiting the cavity
Hanna et al J Phys D Appl Phys,34,2440, (2001)
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Tilted pulses produced by diffraction grating
From Hanna et al.J Phys D, Appl Phys., 34,2440,
(2001)
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CdSe tandem-pumped SPOPO
M.A.Watson, M.V.O'Connor, D.P.Shepherd, D.C.Hanna
Optics Letters 28 (20) pp.1957 (2003)
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CdSe SPOPO
Non-critical (? 90o ) type-II phase-matching
curves in CdSe, for pump-wavelength tuning. The
pump wavelength range has been limited at the
long end to the signal range from the pump OPO
and at the short end by twice the band gap
wavelength, where two-photon absorption would
become significant. Inset diamonds indicate
experimental idler tuning points.
M.A.Watson, M.V.O'Connor, D.P.Shepherd, D.C.Hanna
Optics Letters 28 (20) pp.1957 (2003)
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Infrared absorption edge of Lithium Niobate
Sato et al Appl. Optics 38, 2560, 1999
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SPOPO with idler absorption (1)
Signal gain, if small, is
For large aL this is
i.e. threshold is increased by aL/4
Lowenthal IEEE JQE, 34, 1356 (1998) Lefort et
al APL, 73 (12), 1610 (1998) Watson et al
Opt.Letts 27 (23), 2106 (2002)
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SPOPO with idler absorption (2)

• Photon conversion efficiency to idler output

(D is pump depletion, R is signal round-trip loss)
Output idler power is that generated in last
extinction length of the crystal Strategy for
efficient idler generation Increase Ip until D
0.5 and make R as small as possible (eg use ring
resonator). But avoid excessive (damaging) signal
intensity
M.A.Watson et al. A.P.L.73 (12), 2108,(2002)
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SPOPO with idler absorption (3)
M.A.Watson et al, Optics Letters Vol.27(23)
pp.2106-8 (2002)
25
SPOPO pumped by femtosecond mode-locked fibre
laser
OConnor et al Opt Letts., 27 (12), 1052, (2002)
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High power femtosecond fibre feedback SPOPO
19W av o/p_at_ 1450nm, 7.8W _at_3570nm
Südmeyer et al. Opt Letts. 29, 1111, (2004)
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Fibre feedback SPOPO insensitivity of output
power to resonator length changes
Südmeyer et al. Opt Letts., 29,1111,(2004)
28
Femtosecond (down to 13fs) visible OPOvia
non-collinear phase-matching in BBO
Gale et al. JOSA B, 15, 792, (1998)
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Coupled NL equations for signal idler in the
pump pulse frame
Gale et al. JOSA B 15, 792, (1998)
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Non-collinearly phase-matched femtosecond OPA
with a 2000cm-1 bandwidth
Shirakawa and Kobayashi Appl. Phys. Letts.,
72(2),147, 1998
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Matching of group velocities by spatial walk-off
in collinear three-wave interaction with tilted
pulses
Danielius et al., Opt. Letts., 21, 13, 973, (1996)
32
Pulse-front matched OPA for sub-10-fs pulse
generation
Shirakawa et al. Opt. Letts., 23,16,1292, (1998)
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Visible pulse compression to 4fs by OPA
programmable dispersion control
Prism P3 imparts tilt (angular dispersion) to the
SH (ie pump) beam
Baltuska et al., Opt Letts., 27,306, (2002)
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Visible compression to 4fs by OPA programmable
dispersion control
Dashed curve is for monochromatic pump. Inset
shows spectrum of SH used as pump
Baltuska et al., Opt. Letts., 27, 306, (2002)
35
Yet more OPA designs

• OPCPA multiple pumps, at different wavelengths,
  to increase the gain bandwidth.
• Wang et al., Opt Commun., 237,169, (2004)
• Use of chirped broadband pump operation near
  degeneracy.
• Limpert et al., Opt. Express, 13, 19, 7386,
  (2005)
• Ultrabroadband (octave-spanning) OPCPA, using
  angularly
• dispersed signal
• Arisholm et al., Opt. Express, 12, 518, (2004)

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Efficiency-enhanced soliton OPA

• Pump, signal and idler are mutually trapped in a
  spatial soliton
• This requires a phase-mismatch whose ideal value
  depends on the mix of pump, signal and idler
  powers
• These powers evolve through the amplifier, hence
  ideally one needs a longitudinally varying
  phase-mismatch through the medium
• SOLUTION Use aperiodic QPM medium

Rodriguez et al JOSA B,19, 1396, (2002)
37
Tandem-chirped OPA grating design for
simultaneous control of group delay and gain
control

• Chirped grating 1 produces idler with
  frequency-dependent group delay
• Idler from grating 1 acts as signal for grating
  2, hence idler from 2 has frequency of
  original signal
• Grating 2 compensates group delay dispersion of
  grating 1

Charbonneau-Lefort et al., Opt. Letts.,
30,634,(2005)
38
Cavity-enhanced OPCPA

• Cavity acts as a reservoir and amplifier for
  the pump
• Long pump pulse avoids cavity dispersion issues
  
• Need to minimise optical Kerr effect in cavity

Ilday Kärtner, Opt . Letts.,31, 637, (2006)
39
Generation of few cycle terawatt light pulses via
OPCPA
CEP stabilised pulses from TiS oscillator
maintain their CEP in OPA compressor
Witte et al., Opt. Express, 13, 4903, (2005)
40
Carrier Envelope Phase (CEP)

• Carrier phase offset between
• carrier peak and envelope peak can vary from
  pulse to pulse
• This has significant effects in high field
  experiments using
• few-cycle pulses

Brabec and Krausz Rev. Mod. Phys., 72,545,2000
41
Self-stabilisation of CEP via parametric processes

• In an OPA, with signal only as input, the phase
  relation, fp-fs-fi -p/2 ,
• applies through the medium if ?k 0
• If the signal is derived from the pump, eg as in
  generation of supercontinuum, signal and pump
  have the same phase behaviour.
• So, using the pump to amplify this signal in an
  OPA leads to a CEP stable idler even if the pump
  is not CEP stable.
• If this CEP stable idler does not have the
  desired power it can be used as the input signal
  to a second amplifier, OPA2
• Since this amplified signal has its phase
  preserved in OPA2 one now has a high power pulse
  that is CEP stable

Baltuska et al., Phys Rev Letts. 88, 133901,
(2002)
42
Generation of high energy self-phase-stabilised
pulses via DFG and OPA
DFG between spectral components of the
supercontinuum produced in the fibre gives a CEP
stable pulse whose stability is maintained in OPAs
Manzoni et al. Opt Letts., 31, 963, (2006)
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Concluding remarks

• OPAs are widely seen as a preferred alternative
  to TiS for amplification of ultrashort pulses to
  high powers
• Much needs to be done to establish power-scaling
  limits of OPOs, and OPAs.
• Designs for OPAs are numerous and new proposals
  keep appearing. Not yet a mature field work is
  in progress.
• Different circumstances, e.g. pulse energy,
  duration, wavelength, call for different designs.
  Not a case of one size fits all
• Numerical calculations need to include transverse
  effects. Plane-wave models are ignoring vital
  aspects