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14/06/2015
1
412 PHYS
Lasers and their Applications
Department of Physics
Faculty of Science
Jazan University
KSA
Pulsed laser operation
Lecture-8
Modes of operations
- CW operation
- Pulsed operation: pulsed mode operation can be
categorized into three types:
1- Free running laser pulse mode
2- Q-switching mode
3- Mode-locking mode
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1- Free running laser pulse mode
In this mode laser is designed mainly as pulsed laser due to its active medium and pumping
scheme requirements such as: Short life time of the upper laser level
- difficulties in depopulating laser lower level- heat build up by pumping- fast
pumping scheme or pulsed pumping scheme
- In this mode the width of the laser pulse is controlled by the pumping pulse
Pulse width of this operation mode
100 s few ms
2- Q-switched operation
In this type a device is inserted in the laser cavity for the purpose of making a fast
variation of the oscillation feedback from a minimum to a maximum values and vise
versa and that is described by the quality factor (Q-factor)
The Q-factor is used to describe any resonant cavity ; It defines the ability of a
cavity to store the radiant energy
In the process of Q-switching: the Q-facto blocks (switched-off) the feedback
between the laser mirrors until a maximum energy is pumped to the active medium
and then switched –on to allow for a maximum feedback to occur….As a result very
short laser pulses with very high peak powers are generated
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Concept of Q-switching
Let’s assume a laser cavity as
shown in figure which is a
Fabry-Perot resonator
The Q-factor is expressed as
Q
Assuming that the reflection is the main source of losses and neglecting the
losses due to absorption and diffraction, we can define the Q-factor as the
ratio between the stored energy to the dissipated energy per cycle in the
cavity
Totalstored Energy 22
Energydissipated per cycle
WQ
W
The total stored energy
The rate of dissipated energy
Frequency of oscillation
Frequency band width of the laser output
W h
W
Note: The photon has to make passes per cavity length before it leaves
the cavity → this means that it has to traverse a distance of in a velocity
of light
and hence the life time of the photon in the cavity equals
R11
RL
12
2
1L
c R
Therefore, the rate of energy dissipation 1
2
h c RW
L
The Q-factor is then given by
4
2 21 1
h LQ L
h c R R
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( 1 )L mAssume
m 6328.0 0.95R
84 10Q
And hence the bandwidth 610 1Hz MHz
More analysis
The output Power of Q-switched laser ( ) /L c cP h V n
p c L p c
p
c c
n V h n V h cP
The output Peak Power of Q-switched laser
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Example: A Q-Switched Nd:YAG laser with a wavelength of 1064 nm. Its active medium is a
cylindrical solid of length of 20 cm and cross-sectional diameter of 5 cm and refractive
index of (1.8). Assume the two mirrors of reflectivities of 95% & 100% are attached to
the ends of the medium. If the peak density of the oscillated photons is 1020 m-3
Find the output peak power?
Sol. Let’s first calculate the photon life time inside the cavity
8
8
1 2
2 2 1.8 0.25 10 50
ln( ) 3 10 ln(0.95)c
n Ls ns
c R R
2 2 2 4 3( / 2) 3.14(2.5 10 ) 0.2 4 10cV d L m The effective volume
20 4 34 8
9 9
5
10 4 10 6.63 10 3 10
1064 10 50 10
1.5 10
p c L p c
p
c c
n V h n V hcP
W
The output peak power
Methods of Q-switching
1. Rotating prism or mirror
Rotating mirrors or prisms
w ~ 10,000 rpm
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2. The electro-optic Q-switching
3. Acousto-optic Q switching
4. Photochemical Q-switching
5. Exploding film Q-switching
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Mode Locking
Resonant cavity modes and the gain
spectrum of a laser.
Part (a)
shows single-mode lasing, where the gain
envelope overlaps with just one cavity
resonance.
Part (b)
shows multimode lasing, where the gain
envelope overlaps
with several cavity resonances.
(a) No phase coherence between the
multiple modes (80 modes
simulated),
(b) 5 phase coherent modes, and
(c) 80 phase coherent modes.
The pulse train emerges as ultrashort intensity spikes (fs scale) spaced by
longer ns intervals (roundtrip time of the cavity).