15-laser techniques qswitching
TRANSCRIPT
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Qswitching, Frequency-
Doubling & Diode-pumping
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Ruby Laser
A ru by laser em its i ts laserenergy at a wavelength o f
694.3 nm
In a short pulse of energy over
a duration of around 250 s
Typical output energies forruby lasers range from a few
millijoules to several hundred
joules
Beam divergences about 5
mrad
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Q-switching
The mode of operation we have previously described for solidstate lasers is known as
f ree-running
also known as "burst", "relaxation oscillation" or "normal" mode.
There is an alternative mode of operation,
known as Q-switched(QS),
enables much higher peak powers to be obtained for a given pumpenergy.
energy emitted in short, intense bursts
In the QS mode an optical shutter is placed in the cavity,
usually between the back mirror and the laser medium,
feedback from the mirror is prevented while shutter is closed.
even although a population inversion is still building up
Laser oscillation in the cavity cannot take place.
In this way the ability of the cavity to store energy is increasedover its normal value
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Q-factor
The ability of a resonant cavity to store
energy is usually defined in terms of its Q-
factor.
The Q of a cavity is a measure of how much energy
can be stored in it against the power loss from it.Q 1/(energy dissipated per cycle)
= 2x (energy stored at resonance/energy
dissipated per cycle)
Lasers typically have Q's of a few million compared toa few hundred for electrical oscillators.
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Time- or irradiance-dependent loss in cavity
While shutter is closed the Q is low
At high lossno laser oscillations possible
gain due to population inversion increases far above threshold value
high loss prevents laser action while energy pumped into excited state
When shutter opened
Q switches rapidly to high valuescavity losses are low
threshold gain drops immediately to normal (high Q) value
Laser oscillations build up rapidly
emission of energy proceeds in one short, sharp burst
Upper laser level rapidly depopulated below threshold, lasing stops
known as Q-switched or giant pulse
pulse durations as short as a few nanoseconds are obtainable
giving rise to average powers of megawatts and higher.
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Qswitching Operations
Pumping rate must be
faster than
spontaneous emission
rate from upper laser
level
upper level will
empty faster than it
is being filled
Populationinversion prevented
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Q-switching Mechanisms
Rotating Mirror
One of the simplest, and earliest used, methods was
the replacement of the total reflector, the back mirror,
with a rotating mirror.
Laser oscillation is prevented until the mirror comes
into alignment with the optic axis of the cavity,
until front and back mirrors are momentarily parallel.
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Dye Q-switch
A bleachable organic dye is placed in the cavity
Dye is a saturableabsorber
absorption decreases with increasing irradiance
such as cryptocyanine
better control of the operation with,
shorter pulses and higher power,
The dye is initially opaque and withholds laser oscillation until a
certain power is reached,
the dye becomes transparent (bleaches) and laser action proceeds.
The particular value of power threshold depends on type and
thickness of dye and the wavelength of laser light. The main disadvantage of the dye is its limited lifetime and the
variability in the timing of the onset of laser action.
Such dyes also tend to be carcinogenic!
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Pockels Cell Q-switch
Overcomes some of shortcomings of dye QS
A Pockels cell is an electro-optic crystal
Only passes light in a preferential mode of polarisation
When a high voltage is placed across a Pockel's cell the plane
of polarisation of the light beam is rotated.
Cavity oscillations cannot take place between beams of differentpolarisations thus laser action is suspended.
On removal of the voltage the plane of polarisation reverts to its original
state and oscillation proceeds.
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Pockets Cell QS
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Frequency Doubling
In most solid materials when light is incident uponthem The electric field induces dipole moments in the molecules (or
nuclei and associated electrons)The molecules line up along the field direction
Electric Polarisation (different from polarisation of E-field vector )
The -field causes the dipoles to oscillate & re-
radiate light If the amplitude of oscillation is small
The magnitude of polarisation is linearly proportional to appliedfield
P E [since E= E0cos(t)]
Incident light (E-field) enters & leaves at same frequency
- + - +- + - +
E
P E
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Frequency Doubling
Some materials exhibit a non-linearresponse to
incident light
When the incident irradiation is sufficiently intense
The oscillation amplitude becomes large & non-linear
P E+ E2+ higher terms
P E0cos(t) + (E0cos(t))2+ higher terms
P E0cos(t) + ((1/2)E02(1-cos2t)
P + 2+ higher terms
Incident light of frequency leaves as twocomponents One at frequency and one at frequency 2(2) i.e /2 Second Harmonic Generation (SHG)or Frequency Doubling
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SHG is low efficiency
50% max conversion efficiency between incident
power at fundamental & output power at second
harmonic
The second harmonic is not significant until the E-field
is about 106V/m (about 109W/m2)
Typical materials
Quartz, ammonium dihydrogen phosphate (ADP),
potassium dihydrogen phosphate (KDP)
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Diode Pumping of Lasers
Diode lasers can be used as pump sources for solidstate lasers
Diode pumped solid state laser (DPSS)
Wavelength of diode laser can be tuned by temperature ordoping
Made to coincide with absorption peak of solid state laser
e.g. 807 nm for Nd-YAG High efficiency
up to 25% of diode output to laser output
Diode laser
Coupling lens
Nd-YAG rod
Output coupler
Dielectric coating to reflect YAG light
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Diode-pumped frequency-doubled Nd-YAG laser
Diode pumping coupling withfrequency-doubling
Now commonly applied to Nd-YAG lasers
Diode-pumped frequency-doubled Nd-YAG
laser
Efficient, compact, reliable
Available in c.w. and pulsed models