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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