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Course Instructor: Dr Mushtaq Khan Specialization: Additive Manufacturing/ Laser Aided

Manufacturing Institute: Loughborough University UK

Laser Material Processing (DME-822)

1. Laser Material Processing (Fourth Edition)

By Willian M Steen and J Mazumder

Text Book

Background to Laser Design and General Applications

Basic Laser Optics ◦ The Nature of Electromagnetic Radiation

◦ Interaction of Electromagnetic Radiation with Matter

◦ Reflection or Absorption

◦ Refraction

◦ Interference

◦ Diffraction

◦ Laser Beam Characteristics

◦ Focusing with a Single Lens

◦ Optical Components

Laser Cutting, Drilling and Piercing

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Laser Welding

Laser Surface Treatment

Rapid Prototyping and Low-volume Manufacture

Laser Ablative Processes – Macro- and Micromachining

Laser Safety

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Background to Laser Design and General Applications

(Chapter -1)

The word “laser” is an acronym for Light Amplification by Stimulated Emission of Radiation. It was first proposed by Schawlow and Townes [6].

For references please consult relevant chapter of the text book

Stimulated Emission Phenomenon

For atomic systems in thermal equilibrium with their surrounding, the emission of light is the result of:

Absorption and subsequently, spontaneous emission of energy

There is another process whereby the atom in an upper energy level can be triggered or stimulated in phase with the an incoming photon. This process is:

◦ Stimulated emission: It is an important process for laser action

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1. Absorption

2. Spontaneous Emission

3. Stimulated Emission

Therefore 3 process of light emission:

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In 1917 Einstein [7] predicted that:

under certain circumstances a photon incident upon

a material can generate a second photon of

Exactly the same energy (frequency), Phase,

Direction of propagation

In other word, a coherent beam resulted.

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In fact it was not until 1928 that Ladenburg first confirmed it

by observing negative absorption in his spectroscopic work.

Through Einstein’s analysis of radiation from hot objects, he

postulated that there must be a radiant term based on a

photon of radiation striking an excited species and causing it

to release the energy of excitation.

This has since been shown to be true.

The stimulated photons are found to be in phase and

travelling in the same direction as the stimulating photons.

The amplification of light by stimulated emission is a fundamental concept in the basic understanding of laser action.

This lecture explains how laser amplification occurs starting from spontaneous emission of the first photon to saturation of the laser cavity and the establishment of a dynamic equilibrium state.

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Fully silvered mirror Partially

silvered mirror

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Several light waves spontaneously appear to simulate the process of stimulated emission from an external energy source.

The waves propagate back and forth through the laser cavity, with the intensity increasing at each pass (more waves are formed),

but some of the light is passed through the partially reflecting mirror (output mirror) on the right side of the laser cavity.

Eventually the laser reaches an equilibrium state where the cavity is filled with oscillating waves and pumping a continuous flow of light through the mirror.

Many materials can be made to show this stimulated emission phenomenon,

But only a few have significant power capability, since a further condition is that a population inversion is necessary,

population inversion means more atoms or molecules in the excited state than in the lower-energy state, so as to allow amplification as opposed to absorption.

The main lasers used in material processing are

◦ carbon dioxide (CO2), carbon monoxide (CO), Neodymium-doped Yttrium Aluminium Garnet (YAG; Nd:YAG), Neodymium Glass (Nd:glass), Ytterbium-doped YAG (Yb:YAG), Erbium-doped YAG (Er:YAG), Excimer (KrF, ArF, XeCl) and diode (Gallium arsenide (GaAs), Aluminium gallium arsenide (GaAlAs), Indium gallium arsenide (InGaAs), Gallium nitride (GaN) and others being developed) lasers.

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A laser must have the following three basic components (see also Section 1.2 on laser construction):

1. Active medium, which serves as a means to amplify light (see Figure below)

2. Pumping source, which is a means to excite the active medium to the amplifying state

3. Optical resonator, which is a means to provide optical feedback.

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The active medium may be any material that is solid, liquid, gas or plasma.

Any energy source can be used as a pumping source. The common pumping sources include flash lamps (incoherent light), lasers (coherent light), electron beam, chemical reactions, ion beams and X-ray sources.

Output power is proportional to the power of the pumping source and the amount of active medium;

therefore, power can be controlled by controlling either the pumping source or the active medium, e.g., fast axial flow CO2 provides 0.7 kW m−1, whereas a slow flow or sealed tube design provides 0.05 kWm−1

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An optical resonator causes the light generated by the active medium, parallel to its axis, to be reflected back and forth through the medium

If the light is amplified owing to this action, and if the gain equals the round trip losses in the resonator, then the combination of the amplifier and the resonator is at the threshold for lasing.

The lasing threshold is the lowest excitation level at which a laser's output is dominated by stimulated emission rather than by spontaneous emission.

Light in the excited resonator travelling parallel to the axis is amplified several times.

Only part of it is released in each pass through a partially transmitting window as a laser beam (Figure 1.3).

Therefore, the optical resonator is a cavity defined by a 100% reflective mirror at one end and a partially transmitting mirror at the other end.

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For a laser, the cavity is filled with an active medium and a pumping source, such as an electromagnetic field, is provided to excite the active medium.

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The basic laser consists of two mirrors which are placed parallel to each other to form an optical oscillator

Between the mirrors is an active medium which is capable of amplifying the light oscillations by the mechanism of stimulated emission

There is also some system for pumping the active medium so that it has the energy to become active. This is usually a DC or RF power

supply, for gas lasers such as CO2, excimer and He–Ne lasers, or a focused pulse of light for the Nd:YAG and solid-state lasers or an electric current for a semiconductor or free electron lasers or a chemical reaction, as with the iodine laser.

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One of the two mirrors is partially transparent to allow some of the oscillating power to emerge as the operating beam

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Kogelnik and Li [11] showed by geometric arguments that the mirror curvatures at either end of the cavity could only fall within certain values or the cavity would become “unstable” by losing the power around the edge of the output mirror.

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The wavelength, λ, of a laser is decided by the energy difference as the excited species is stimulated to a lower energy level [E = hc/λ, where h is Planck’s constant (6.626 × 10−34 J s) and c is the velocity of light (3 × 108 ms−1)].

In general, the quantum states refer to:

◦ molecular vibration levels for long-wavelength lasers,

◦ to electron orbit levels for visible laser radiation and

◦ to ionisation effects with ultraviolet lasers.

For material processing, CO2, Nd:YAG and fibre lasers are the most popular systems.

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Lases can classified into:

◦ Gas Lasers

◦ Solid State Lasers

◦ Dye Lasers

◦ Free Electron Lasers

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the CO2 molecule, it can take on various energy states depending upon some form of vibration and/or rotation

These states are quantised, that is, they can only exist at particular energy levels or not at all

The gas mixture in a CO2 laser is subject to an electric discharge causing the low-pressure gas (usually around 35–50 Torr) to form a plasma

In the plasma the molecules take up various excited states as expected from the Boltzmann distribution [ni = C exp(−E/kT), where ni is the number of molecules in energy state i, E is the energy of state i, k is the Boltzmann constant (1.3805 × 10−23 J K−1), T is the absolute temperature and C is a constant]

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Some will be in the upper state (0001), which represents an asymmetric oscillation mode.

By chance this (0001) molecule may lose its energy by collision with the walls of the cavity or by spontaneous emission.

Through spontaneous emission the state falls to the symmetric oscillation mode (1000) and a photon of light of wavelength 10.6 μm is emitted travelling in any direction dictated by chance

One of these photons, again by chance, will be travelling down the optic axis of the cavity and will start oscillating between the mirrors.

During this time it can be absorbed by a molecule in the (1000) state, it can be diffracted out of the system or it can strike a molecule which is already excited, in the (0001) state.

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At this moment it will stimulate that excited molecule to release its energy and fall to a lower energy state, thus emitting another photon of identical wavelength, travelling in exactly the same direction and with the same phase.

The two photons travelling in the same direction with the same phase now sweep back and forth within the cavity generating more photons from other excited molecules.

The excited state be comes depleted and so by the Boltzmann distribution more and more of the energy is passed into that state, giving a satisfactory conversion of electrical energy into the upper state.

It is necessary for another condition to hold, and that is that the excited state which lases should be slow to undergo spontaneous emission and the lower state should be faster in losing its energy.

This allows an inversion of the population of excited species to exist and thus makes a medium which is more available for the stimulated emission process (amplification) than for absorption.

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The CO2 laser is helped by a quirk of nature Nitrogen, which can only oscillate in one way (being

made of two atoms), has an energy gap between the different quanta of oscillation which is within a few hertz of that required to take cold CO2 and put it into the asymmetric oscillation mode : the upper laser level (0001).

Thus, by collision with excited nitrogen, cold CO2 can be made excited.

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Since only cold CO2 will undergo this reaction with nitrogen, the efficiency is a function of the gas temperature.

Thus, the design of a CO2 laser, in common with that of all lasers, is built around the requirement of cooling; in this case to have cool CO2 gas.

Firstly, the gas mixture in the laser is around 78% He for good conduction and stabilisation of the plasma, 13% N2 for this coupling effect and 10% CO2 to do the work.

Secondly, the gas is cooled by conduction through the walls for slow flow lasers or by convection in the fast axial flow and transverse flow lasers. ◦ Slow flow lasers ◦ Fast axial flow lasers ◦ Transverse flow lasers

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The main radiation from the CO2 laser is at 10.6-μm wavelength from the transitions of (001) → (100) (asymmetric to symmetric), or

to a lesser extent at 9.6μm from (001) → (020) (asymmetric to double bending).

the emission can vary between 9 and 11 μm; which one is made to lase depends on whether the cavity has a selective mechanism within it, such as a grating

See table 1.2 – 1.4 for properties

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The CO laser is constructed in a similar way to the CO2 laser – as are all gas lasers.

It has the advantage of a quantum efficiency of near 100% and

thus promises to have a wall plug efficiency twice that of the CO2 laser,

although this is seriously reduced by the cooling requirements.

For high-powered lasers, an improvement in efficiency could be significant since a 100-kW CO2 laser would require a power supply of at least 0.83MW.This is getting near to being a small power station

However, the CO laser currently operates best at very low temperatures at around 150K (liquid nitrogen temperatures) and requires extensive power for refrigeration, which may affect this potential efficiency advantage

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Higher-temperature operation has been achieved by adding xenon to the gas mixture.

Currently, operating efficiencies of around 19% have been reported, but these are reduced to nearer 8% when the power for cooling is considered.

These lasers emit at 5.4-μm wavelength, which is an interesting absorption area for water and therefore of medical interest.

Small sealed CO lasers are sold for that area of application.

High-powered CO lasers for material processing are not currently commercially available but the designs being considered are similar to those for a fast axial flow system with added cooling from liquid nitrogen or special refrigeration

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Some designs include substantial cooling by pressure expansion (Joule–Thompson cooling) but these tend to be noisy.

The CO laser output power is very sensitive to temperature. Therefore, it is an added expense compared with the CO2

laser.

The excimer laser has the energy diagram shown in Figure 1.22.

The name derives from the excited dimer molecules (strictly excited complex molecules), which are the lasing species.

There are several gas mixtures used in an excimer laser, usually noble gas halides; they are shown in Table 1.6.

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These lasers are slightly different in that the gain is so strong that they do not need an oscillator, although their performance is improved with one.

The pulses are usually very short, around 20 ns (a piece of light around 6m long!),

but very powerful, typically around 35MW (the energy per pulse is thus 0.7 J).

The construction of an industrial excimer laser is illustrated in Figure 1.23.

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Excitation is by a 50–100-ns duration 35–50-kV pulse across the electrodes with peak current densities of around 1 kA cm−2.

Preionisation to some 108 cm−3 is needed to avoid electron avalanching (sparking).

This is achieved by flooding the cavity with ultraviolet light from small spark discharges in the cavity.

The high photon energy of ultraviolet light and the potential of very high peak power densities of around 10MWcm−2 to 200 TWcm−2 (thermo fusion experiments) makes a niche for ultraviolet laser processing which can be achieved by excimer or frequency-quadrupled Nd:YAG radiation.

The optics of the excimer laser are made of fused silica, crystalline CaF2 or MgF2.

One of the cavity mirrors has a highly reflective aluminium coating on the rear surface to protect it from the corrosive atmosphere of the halogen gases.

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The output wavelength can be changed by changing the gases (see Table 1.6) and sometimes the optics, if coated optics have been used.

The size of the output beam is determined by the aperture from the cavity, normally a rectangle (e.g., 20–30×10mm2)

The gas mixture is typically 4–5-mbar halogen gas, 30–500-mbar argon, krypton or xenon as required and the rest is 4–5-bar helium or neon

The halogen gas is usually supplied as a 5% mixture in helium to reduce the danger.

The gas slowly degrades through corrosive action, forming dust particles which are filtered out of the gas stream.

This gives lifetimes of around 106 pulses per fill. The running costs are high, at $10–30

per hour (2009 prices), due mainly to the maintenance and equipment costs.

Fitting cryogenic traps can improve the lifetime and reduce the costs to as little as $1–2 per hour [12].

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