Solid-state lasers have the active medium held in an insulating dielectric crystal or amorphous glass.

The lasing action comes from energy jumps between discrete electronic energy levels of the dopant such as rare earth ions or transition ions with unfilled outer shells.

The main industrial solid-state lasers include Nd3+:YAG, Er3+:YAG, Yb3+:YAG, ruby (Cr3+:Al2O3), titanium sapphire (Ti3+:Al2O3) and alexandrite (Cr3+:BeAl2O4)

The host material for the neodymium or other rare earth element may be YAG (Y3Al5O12), yttrium lithium fluoride (YLF), yttrium aluminium perovskite (YAP; YAlO3), yttrium vanadate (YVO4) or phosphate or silica glass.

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Solid-state lasers have the advantage of relatively long lifetimes for the excited states.

which allows higher energy storage than for gas lasers and hence allows them to be Q-switched to give very high peak powers in short pulses.

Q Switching

Q switching is a technique for obtaining energetic short (but not ultrashort) pulses from a laser by modulating the intracavity losses and thus the Q factor of the laser resonator.

The technique is mainly applied for the generation of nanosecond pulses of high energy and peak power with solid-state bulk lasers.

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Initially, the resonator losses are kept at a high level. As lasing cannot occur at that time, the energy fed into the gain medium by the pumping mechanism accumulates there.

Then, the losses are suddenly (with active or passive means, see below) reduced to a small value, so that the power of the laser radiation builds up very quickly in the laser resonator.

Once the temporally integrated intracavity power has reached the order of the saturation energy of the gain medium, the gain starts to be saturated.

The energy of the generated pulse is typically higher than the saturation energy of the gain medium and can be in the milli-joule range even for small lasers.

In most cases, Q-switched lasers generate regular pulse trains via repetitive Q switching.

Lasers to which the Q-switching technique is applied are called Q-switched lasers

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Pure Y3Al5O12 is a colourless optically isotropic crystal with the cubic structure of garnet.

If around 1% of the yttrium rare earth is substituted by the alternative rare earth neodymium, the lattice will then contain Nd3+

ions. These ions can undergo the transitions shown in Figure 1.24.

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Other solid-state lasers for material processing include Nd:YVO4 operating at 1.06 μm, Yb:YAG operating at 1.03μm and Er:YAG operating in the “eye-safe” region of 1.54 μm.

“eye-safe” means the radiation will be absorbed on the cornea and not penetrate to the retina with a 105 times amplification in intensity.

The overall construction of a Nd:YAG laser is shown in Figure 1.27 for pumping by a lamp or diode.

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The Nd:glass lasers have the same energy diagram for Nd3+ as the YAG laser but the energy conversion is better in glass.

However, the cooling problems are more severe owing to the poor conductivity of glass

so the Nd:glass lasers are confined to slow repetition rates, approximately 1 Hz.

At higher repetition rates the beam divergence (or ease of focusing) becomes unacceptable for material processing.

The beam froma glass laser is more spiked than that froma YAG laser as seen in Figure 1.28. It is more prone to burst mode operation.

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There is a problem with flash-lamp-pumped Nd:YAG lasers, in that only a few percent of the flash lamp power is actually absorbed by the Nd3+ ions and so used in the lasing action;

the waste energy heats up the YAG rod, causing distortion and variations in the refractive index.

This leads to poor pulse-to-pulse consistency (approximately 10–15% variation) and low beam quality.

The lamps have a lifetime of a few hundred hours and require substantial power supplies to drive them.

These problems can be eliminated by using diode lasers instead of flash lamps to excite the Nd3+, as illustrated in Figure 1.27.

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The wall plug efficiency of diodes is around 30–40% and all the light is emitted centred on a strong absorption line of Nd3+ at 808 nm.

The power supply and cooling requirements are greatly reduced

The remaining problem is the cost of the high-powered diodes required to do the pumping.

As the size of market for laser diodes increases, so the price is likely to fall.

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One version of a diode-pumped solid-state laser is a disc geometry in which a coin shaped YAG crystal doped with ytterbium forms the lasing medium.

A disc, 0.3mmthick and 7mmin diameter, doped with ytterbium up to 25% can produce over 500Wof high-quality beam from the top surface of the “coin” at 1.03 μm wavelength.

The reason for the high power and quality is the superior cooling and higher dopant rates possible with this geometry, as well as the cavity design

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Meanwhile, diode-pumped fibre lasers are being developed, these lasers are doped plastic or glass fibres that are end- or side-pumped by diode lasers.

IPGPhotonics is marketing a 2-kW CW fibre laser based on ytterbium operating at 1,085 nm.

It has a beam quality 10 times better than that of a standard Nd:YAG laser.

The fibre can be very thin, 100 μm, and hence the only way oscillations can be contained is by wave guiding within the fibre as Gaussian beam.

The wall plug efficiency is stated as 20 %,whereas the lifetime for the pumping diodes is reckoned to be 100,000 h, indicating several years of maintenance-free operation.

The 700-W version of a fibre laser was able to cut through 50mm of steel. The 2-kW version could

weld steel from several metres distance. With this sort of performance these lasers appear to have much to

offer

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These lasers are becoming the most important laser material processing tool both for pumping solid-state lasers and for direct application to surface heating and welding.

They have the advantages of being compact, efficient, with a quick modulation response and reliability.

Diode lasers [17] are currently the most efficient devices for converting electrical into optical energy.

Their wall plug efficiency may reach up to 50%. In a diode laser the excited state is that of the electrons in the

conduction band. The two states, electrons and holes, come together in an active

region set at a p–n junction in a semiconductor material. A current flow induces electrons to jump from the conduction band

down to the valence band and give up the energy difference between these two Fermi levels as radiation (hv).

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Dye lasers are one of the most readily tunable lasers [16].They can also operate at high powers with pulse lengths from CW to femtoseconds.

They are, however, extremely inefficient.

They work through the absorption of a pumping laser and emit over a wide range of wavelengths, which can be selected by cavity tuning, changing the concentration of the dye and changing the pressure.

The range of wavelengths and some of the dyes that have been studied are shown in Figure 1.33.

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The free-electron laser does not depend on excited states but depends on synchrotron radiation.

Synchrotron radiation is emitted when an electron changes direction; the energy involved appears as radiation

The laser consists of a circuit for relativistic electrons streaming around in a ring.

As part of the circuit there is a magnetic wiggler, which is a short-wavelength magnetic field that causes the electrons to make a wiggly path, emitting photons on each turn in the same direction as the travelling electrons.

These machines can generate radiation from deep infrared to X-radiation.

Owing to the velocity distribution within the flow stream, there is considerable spectral spread in the output beam.

It is sometimes known as a “rainbow laser”.

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