Laser Technology or Why is My Ray Gun So Expensive ?

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Laser TechnologyorWhy is My Ray GunSo
Expensive ?
Graeme HirstSTFC Central Laser Facility

• Cockcroft Institute Laser Lectures
• April 2008

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Ray Guns

• Yoda, Hirst et al, Empire,Issue 122 p 56, (Aug
  1999)

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Lecture 4 Plan

• Pump sources
• Gain media
• Cavity issues
• Laser amplifiers
• Optical parametrics
• High harmonic generation

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Pump Sources
To date lasers have been pumped optically,
electrically (e-beam,conventional current and
gas discharge), chemically, by nuclearreactors
and explosions and, perhaps, mechanically
(impulsivecompression). Optical and electrical
pumping now dominate.
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Pump Sources - Optical
Optical pumping is used across the widest range
of lasers.Advantages include selectivity, high
peak power, precise beamcontrol and cleanliness.
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Pump Sources - Electrical
Diode lasers are electrically pumped(low
voltage, high current). Peakpower limits are
only 10 timesaverage power ones so low duty
cycle operation is rare. Diodesare quite
expensive and very, very electrically fragile so
suppliesare either dull (good) or exciting
(maybe very, very bad).
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Gain Media Nd3
One of the earliest laser media to be discovered
was the Nd3 ion, doped into yttrium aluminium
garnet (YAG) crystals.
YAG is a robust material available inlarge
pieces with good optical quality. Otherhosts
include YLF, YVO4, glass ...
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Gain Media Nd3
Nd3 is still a very widespread material today.
Its use rangesfrom megajoule glass lasersto the
battery-powered, cwDP vanadate chips at
theheart of green laser pointers.
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Gain Media Nd3

• A high pulse energy (gt100J/pulse), high average
  power(few kW) Q-switched nanosecond system based
  on Ndglasswas developed in the 1990s.
• It is now in regular use for aerospace materials
  processing(laser peening).
• The severe optical problems arising from glasss
  poor thermalproperties have been managed by (at
  least) two approaches
• sophisticated cavity design to minimise
  amplitude variations at vulnerable points
• the use of an intracavity optical phase
  conjugator to reverse thermal phase aberrations
  and to lock the beams transverse phase across
  multiple apertures

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Gain Media TiS
The Ti3 ion doped into sapphire (TiAl2O3 or
TiS) is one of thenewer laser materials (mid
1980s). The Ti3 energy levels are split by
strongvibronic interactions with
phononsleading to a very broad NIR
fluorescencefollowing visible excitation. A
transition cross-section of 3-410-19 cm2gives
reasonable laser gain.
Careful crystal growth is important toavoid
optical inhomogeneities andabsorption by lattice
defects.
The upper state lifetime is 3.2ms so laser
pumpingis required (flash pumping has
beendemonstrated but is heroic).
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Gain Media TiS
The Ti3 ion doped into sapphire (TiAl2O3 or
TiS) is one of thenewer laser materials (mid
1980s). The Ti3 energy levels are split by
strongvibronic interactions with
phononsleading to a very broad NIR
fluorescencefollowing visible excitation. A
transition cross-section of 3-410-19 cm2gives
reasonable laser gain.
Careful crystal growth is important toavoid
optical inhomogeneities andabsorption by lattice
defects.
The upper state lifetime is 3.2ms so laser
pumpingis required (flash pumping has
beendemonstrated but is heroic).
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Gain Media TiS
TiS lasers deliver the shortest pulses currently
available. Oscillators are capable of
sub-10fsbut gain narrowing makes this hardto
maintain at mJ energy (seespectrum and pulse
profile). Re-broadening in rare-gas
filledhollow-core fibres is effective butlimits
the pulse energy.
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Gain Media
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Optical Cavities
In the steady state a laser will oscillate if the
round trip gainexceeds the cavity loss
(primarily due to beam out-coupling) Losses are
minimised by using high quality bulkmedia (gain
elements, intracavity crystals)andmultilayer
dielectric coatings (Rgt99.8 and lt0.2are both
possible) or Brewster-cut surfaces
Other issues include thermaland mechanical
stability,aberration control and damage In short
pulse cavities the compensation of group
velocity dispersion is vital (chirped
mirrors are the latest technology). Gain
flatteningwith filters is also used.
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Laser Amplifiers

• Amplifiers are used when
• High power or energy is needed but is not
  compatible with, forexample, the pulse rate or
  stability or efficiency of an oscillator
• To stage the gain e.g. for isolation, beam
  resizing orconditioning

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Laser Amplifiers

• Amplifiers are used when
• High power or energy is needed but is not
  compatible with, forexample, the pulse rate or
  stability or efficiency of an oscillator
• To stage the gain e.g. for isolation, beam
  resizing orconditioning

The two common types are multi-passand
regenerative. Multi-pass amplifier beams may
beseparated by angle or position orthey may be
polarisation-switched. Scaling to large aperture
is possible. If efficiency is important
thentop-hat profiles need to bereconciled with
beam transport.Image-relaying may be used.
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Laser Amplifiers
Regenerative amplifiers have the gain medium
inside an opticalcavity, maintaining good mode
quality over very many passes. Polarisation
switching is needed so regens are inherently
pulsed,up to a few hundred kilohertz. The
Pockels cell(s) can beleaky, lossy,
dispersiveand fragile. Mode volume
restrictionslimit the pulse energy (e.g.to
millijoules with TiS). But very high gains
arepossible with a compactlayout.
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Chirped Pulse Amplification
Nonlinear effects in materials can self-phase
modulate andself-focus high intensity laser
beams. To control this theadditional phase along
the beams path, (2p/l)?n2Idl, should bekept
below 1 rad. One way to reduce I is to stretch
the pulse.
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Optical Parametric Processes
Take place in nonlinear crystals where the
inducedpolarisation, P, depends nonlinearly on
the optical field, E,generating field components
at different frequencies The simplest processes
are 3-wave, where a pump photonsplits into
signal and idler photons (usually ws gt wi)
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Optical Parametric Issues
TUNING Both angle-tuning and temperature-tuning
are used.
EFFICIENCY Limited first by photon energy ratio.
Other factorsinclude intensity variations (ideal
beams are top-hat in timeand space), phase
matching (real beams have bandwidth
anddivergence), back conversion (OP processes
are two-way)and walk-off (Poynting vector not
parallel to wavevector).
POLARISATION Nonlinear crystals are typically
birefringent.This can be vital for phase
matching but is also constraining.
DAMAGE NLO necessarily involves extremes of
intensitywhich can cause both surface and bulk
damage.
BEAM SHAPE Process asymmetries can distort the
beam.
STABILITY Shot-to-shot pump intensity noisecan
be amplified by NLO.
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Practical OP Devices
The market leaders in OPGare Light Conversion Ltd
The LC TOPAS 800-fsdelivers 130fs pulsesfrom
0.2-30mm whenpumped with 2.7mJ100fs TiS pulses
The TOPAS White NOPAcan generate 10-30fs
pulseswith a narrower tuning range
Seeding the OP stages from a white light
continuum sourcegenerates the maximum bandwidth
and ensuressynchronisation and even phase
coherencebetween multiple OPAs
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OPCPA
Despite challenges withphase matching,
OPAsoffer very wide bandwidth The very high gain
andlow loss minimise thermaland nonlinear
problems
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High Harmonic Generation
HHG can be understoodusing the 3-step
modeltunnel ionisation, classicalacceleration
and recollision
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High Harmonics - Spectrum
The spectrum consists of oddharmonics whose
energy fallsrapidly to a broad plateauwith a
sharp cutoff Each plateau harmonic canhave 10-6
of the drive energy
Conversion efficiency is affected by target
ionisation, absorptionand phase matching. The
spectrum can be tailored by control ofthe
driving E-field and quasi phase matching in a
target capillary
E-field control can also fine tune the harmonic
wavelengths tofill spectral gaps above, say,
40eV. Reasonably efficientpulse-length
preserving monochromators based onsagittal
grating technology are nowbecoming available
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High Harmonics - Prospects

• AVERAGE POWER 10-6 conversion of a 10mJ laser
  pulse should already give 109 hn/pulse at 50eV
  which exceeds the pulse performance of
  undulators by a factor of 104. Kilowatt class
  lasers should deliver gt1014 hn/s which is 1-2
  orders of magnitude below the average power from
  undulators.

• PHOTON ENERGY Raising the cutoff energy will
  involve raising the electron energy. Options
  include using ions (higher Ip) or longer drive
  wavelengths (higher Up, but with more time for
  core wavepacket expansion hence
  lower recollision probability)

• SURFACE HARMONICS Laser scattering from a
  rapidly oscillating dense plasma delivers keV
  harmonics with higher conversion efficiency than
  HHG in gases but requires a national
  scale drive laser

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Conclusions

• A laser is a combination of three conceptually
  separateelements - gain medium, pump source and
  optics
• In most lasers the operating point of each
  element (more exactly, the combination of
  operating points)is close to the technological
  limit(s)
• The most effective laser designs are based
  ontreating the system as an integrated whole
• As the field matures progress becomes dependent
  on technical advances as well as scientific
  discovery

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Thank you !