Advanced Manufacturing Processes: Laser

Laser

Index

Laser ................................................................................................................................................................. 1

Basics of Lasers ............................................................................................................................................. 1

Laser sources ................................................................................................................................................ 6

Laser optics................................................................................................................................................... 7

Laser-material interaction .......................................................................................................................... 12

Laser cutting ............................................................................................................................................... 13

Laser welding.............................................................................................................................................. 17

Safety ......................................................................................................................................................... 18

Basics of Lasers

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. It is essentially a coherent,

convergent, and monochromatic beam of electromagnetic radiation with wavelength ranging from UV to IR.

When the oscillations of the electric field vector are

in particular order, the light is polarized. In

⃗

particular, in a plane-polarized wave, oscillates in

⃗

a single plane as the wave travels. In contrast, in

the completely unpolarised light can assume any

possible directions (i.e., it oscillates randomly in

more than one plane).

⃗ = 2( − )

=

Intensity is defined as the energy per unit area perpendicular to the direction of motion of the wave and is

proportional to the square of amplitude of the wave.

In 1960, Maiman came up with the first working ruby laser. The three processes required to produce the

high-energy laser beam are population inversion, stimulated emission and amplification.

 Population inversion. Without this process,

there will be net absorption of emission

instead of stimulated one. According to the

1

Boltzmann law , the higher energy states

decreases exponentially with energy.

Population inversion corresponds to a non-

equilibrium distribution of electrons such that

the higher energy states have a large number

of them than the lower energy states. The process of achieving that by exciting the electrons to the

higher energy states is referred to as pumping. In most of the lasers, population inversion involves

three or four levels. Radiation decay is really rapid – thus the population inversion is achieved.

−

2 1

1 = exp⁡[− ]

2 1 1

Pumping can be:

o Optical: glass or quartz tubes filled with

gases such as Xe or Kr where some

wavelength of the flash matches with the

absorption characteristic of the active

laser medium, facilitating population

inversion. This is used in solid-state lasers

like ruby and Nd:YAG. Recently, diode-

pumped solid-state laser have been

developed, offering significant

advantages over conventional flashlamps

such as better match between the output

spectrum of the pumping laser and

absorption characteristics of laser

medium, increased efficiency, and

compact and lighter laser systems.

o Electrical: used in gas lasers, is achieved

by passing a high-voltage electric current directly through the mixture of active gas medium.

The collision of discharge electrons of sufficient kinetic energy excites one of the gases to

high energy levels, which subsequently transfer its excitation to the second gas through

collision. There is a minimum population inversion, referred to as threshold condition,

required for lasing. −

 2 1

=

Stimulated emission. The incoming photon of energy interacts with the excited atom of

2

ℎ

active laser medium with population inversion between the two states. Thus the incoming photon

triggers the emission of radiation by bringing the atom to the lower energy state. The resulting

radiations have the same frequency, direction and phase as the incoming photon, giving rise to a

stream of photons.

 Amplification. Since the stimulated

photons are in the same phase and state of

polarization, they add constructively to the

incoming photon, resulting in an increase in

amplitude. The active laser material is placed in

a resonant cavity, consisting of a set of well-

2

2

aligned highly reflecting mirrors at

the ends, perpendicular to the cavity

axis (a). When the laser is off, the

optical cavity contains all the laser

material in its unexcited state. The

excitation of atoms is soon achieved

by pumping (b), followed by initiation

of stimulated emission (c). The

intensity is increased as it travels to the end of the mirrors. Further amplification is accomplished by

reflecting the photon into the active medium (d). The photons travel the long path back and forth

through the lasing medium stimulating more and more emissions, resulting in a high-intensity laser

beam output (e).

Properties of laser radiation  Monochromaticity. The laser

output consists of very closely spaced,

discrete and narrow spectral lines, which

satisfies the resonance condition given by:

= /2 where d is the cavity length. These

discrete lines, called laser modes or cavity

modes, spread over a range of frequencies

/2.

separated by Frequencies in this range

are amplified if the gain is higher than losses.

Δν

# =

/2

Δ is the range of frequency, also called spectral width.

The number of axial modes may exceed hundreds of

modes: monochromaticity is due to narrow spectral

widths of individual modes. A laser can be constructed to

operate in only

one longitudinal

mode to give

better results.

 Collimation. It is related with the directional nature of the

beam. Highly collimated beams can be focused on a very small

area even at longer distances, hence energy can be efficiently

collected on a small area without much loss in the intensity.

2 One of the mirrors has some transmission to allow laser output. 3

≈ ⁡

The divergence angle of a diffraction limited beam can be expressed as: where is the

0

0

beam waist, i.e. the smallest value of sideways spread of the beam, therefore the minimum spot.

Laser beams are characterized by very small divergence angle (0.2-10 mrad).

 Coherency. It is the degree of orderliness of waves. Spatial coherence correlates the phases at

different points in space at a single moment in time; temporal coherence correlates the phases at a

single point in space over a period of time.

 Brightness or radiance. It is the amount of power emitted per unit area per unit solid angle. Laser

-6

beams have divergence angles in the range of 10 steradians, hence they can be focused on a very

small area: high brightness is achieved. It is a very important factor in material processing and

3

determines the intensity (power density) or fluency (energy density) of the laser beam . It cannot be

increased by the optical system; however, it is possible to operate in Gaussian mode with minimum

divergence angle.

 Spatial (transverse) modes. The cross section of laser beams exhibit distinct spatial profiles termed

as transverse modes

and are represented

as the transverse

electromagnetic

mode, TEM , where

mn

m and n are small

integers representing

the number of nodes

in direction

orthogonal to the beam propagation direction. The fundamental mode TEM has Gaussian spatial

00

distribution and is the most commonly used mode in laser machining applications. The intensity

distribution in the Gaussian beam can be expressed as:

2

2

() = exp⁡[− ]

0 2

Where r is the radius of the beam , I the intensity at r = 0 and

0 −2

= = 0.135

w is the radius of the beam at which . The

0 0 quality of a beam is expressed in

terms of the beam quality factor or

2

beam propagation ratio M ,

comparing the divergence with

that of a pure Gaussian beam for

2

which M = 1.

2

=

Therefore, we can express every

divergence angle as:

3 = ⁡[]; = [ ]; = [ ]

Beam energy: fluency: intensity: .

2 2

4

2

=

0

Often, for solid-state lasers, the beam parameter product is preferable:

2

= =

0

 Temporal modes. The output can be continuous,

constant amplitude (CW mode) or periodic (pulsed beam

mode). The former discharges constant energy,

uninterruptedly for a long time. The latter stores pumped

energy until a threshold is reached. Once it happens, the

stored energy is rapidly discharged into short duration

4

pulses of high energy density . Most of the gas lasers,

Nd:YAG and semiconductor lasers are operated in CW

mode, whereas solid-state lasers such as ruby Nd:glass

lasers are primarily operated in pulsed mode. In this case

a fundamental parameter is the pulse repetition time.

Then, pulsing can be carried out in various ways:

o Normal pulsing/ free running: variation of

inductance and capacitance in the circuit of the

flashlamp. Properties of the resonator are kept constant.

Typical pulses are μs-ms (pulse frequency of 0.01 Hz).

o Q-switching: pulsing is obtained by changing

through different methods the Q value of the cavity (i.e.

the measure of its ability to store the radiant energy).

-9

Short (10 s) and intense pulse of radiation is achieved.

Typical pulse frequency is 100 kHz. -12 -15

o Mode Locking: a train of extremely short (10 -10

s) and equally spaced pulses is produced. Interaction

among the longitudinal modes results in oscillatory

behaviour of the laser output. This makes the longitudinal modes

maintain fixed phase relationship. Typical pulse frequencies are 1

MHz – 1 GHz.

4 = /

The peak power of pulsed laser is always higher than CW laser. 5

Laser sources Lasers are generally classified into four

main types depending on the physical

nature of the active medium used:

solid-state lasers, gas lasers,

semiconductor lasers, and dye lasers.

Solid-state Lasers

In solid-state lasers, active medium

consists of a small percentage of

impurity ions doped in a solid host

material. Nd:YAG is the most commonly used laser. Crystalline YAG is the host material and it is a four-level

system. Its wavelength is 1.06 μm, with efficiency of 5% and good thermal stability. To increase efficiency,

diode pumping is preferred.

Gas Lasers

The active laser medium is a gas. With respect to solid-state lasers, gases:

 Act as homogeneous laser medium

 Can be easily transported for cooling and replenishment

 Are relatively inexpensive

 But low density requires huge amounts to achieve population inversion, so the sys