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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).

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1. Active Mode-Locking

Methods of Q-Switching

2. Passive Mode-Locking