5 Basics You Should Know About Solid State Lasers

Ruby, neodymium glass and neodymium doped yttrium aluminum garnet lasers are the most practical and mature solid-state lasers.

The solid-state laser used in laser welding is mainly Nd∶ YAG (neodymium: ytrium, aluminumgar-net) laser.

Neodymium doped yttrium aluminum garnet laser can be pulsed or continuous.

This kind of laser is characterized by high output power, small volume and firm structure;

Its disadvantage is that the coherence of light and the stability of frequency are poor, which is not as good as gas laser.

1. Basic structure of solid state laser

Solid state laser is mainly composed of laser working material (ruby, YAG or neodymium glass rod), concentrator, resonator (full mirror and output window), pump light source, power supply and control equipment.

Working material is the core of laser, which is used to produce stimulated radiation of light.

The working material of the laser is divided into matrix and activating material.

The activating substance is luminous, and the matrix is inlaid with the activating substance. Activators are generally ions of transition metals (such as Cr, Co, Ni, etc.), rare earth metals (such as Nd, Sm, Ce, Er, etc.), actinides (such as Ae, Th, U, etc.).

It is generally required that the radius of the ions mixed in the matrix is similar to that of the matrix, and the valence state is the same, so that materials with better properties can be obtained.

Nd3 + doped yttrium aluminum garnet is a kind of crystal, which is obtained by adding a small amount of Nd2O3 into yttrium aluminum garnet crystal.

The molecular formula of yttrium aluminum garnet is X3Al5O12, which is called YAG for short. Nd3 + is the activator and YAG is the matrix.

Nd3 + replaces some yttrium atoms in the crystal.

Generally, the content of Nd3 + is about 1% atomic ratio, so the molecular formula is Y2 97Nd30 +. 03Al5O12, abbreviated as Nd3 + YAG.

There are four energy levels of the activating substance Nd3 +, belonging to the four level system.

The working principle of the four level system is shown in Fig. 1.

It can be seen from the figure that when the xenon lamp is excited, some neodymium ions absorb light energy and excite from the ground state to E4.

The particles excited to E4 quickly return to E3 in a non radiative transition.

E3 is a metastable energy level with a long lifetime. Particles can accumulate on E3.

As long as the light system is strong, the particle number inversion between E3 and E2 can be realized.

Therefore, as long as the optical pump excites a small number of particles on the energy level E4, the particle number inversion between E3 and E2 can be realized.

2. Pump light source

The pump light source, also known as the excitation source, is used to excite the working material to obtain the particle number inversion distribution.

Solid-state lasers are excited by light, so they are also called light sources, generally xenon lamps, krypton lamps, etc.

working principle of four level system

Fig. 1 Working principle of four level system

(1) Pump light

For solid-state lasers, the most common pumping method is optical pump.

In pulsed solid-state lasers, pulsed xenon lamps are generally used as optical pumps.

Several shapes of common xenon lamps are shown in Fig. 2.

Straight tubes are now used most.

The problem of spectral matching should be considered in use.

In continuous solid-state lasers (i.e. Nd3 + YAG lasers), krypton lamps, iodine tungsten lamps and continuous emitting xenon lamps are used as continuous light sources.

At low power, the output power of iodine tungsten lamp is high;

Krypton lamp and xenon lamp can be used at high power. Krypton lamp is better than xenon lamp.

several shapes of xenon lamp

a) Spiral

b) Straight tube

c) II shape

Fig. 2 Several shapes of xenon lamp

(2) As the pump source of solid-state laser, diode laser is used to excite high-power Nd∶YAG crystal.

Using direct diode array to excite the laser with the output wavelength in the near-infrared region, the average power has reached 1kW and the photoelectric conversion efficiency is close to 50%.

The diode also has a longer service life (10000h), which is conducive to reducing the maintenance cost of laser equipment.

3. Condenser

In order to make better use of the light emitted by the optical pump and reflect the light emitted by the optical pump in all directions back to the working material, a concentrator is also used in the solid-state laser.

A concentrator is a reflector of light.

It can make 80% of the light emitted by the light pump converge on the working material.

The purpose of using the concentrator is to concentrate the discrete light on the working material through the reflection of the concentrator, so as to improve the efficiency.

Therefore, the shape of the concentrator is required to be conducive to gathering more light from the light pump to the working material, and the reflective coating plated on the inner surface of the concentrator should have high reflectivity to the light at the absorption peak of the working material, and the inner surface should be polished to reduce the scattering of light.

There are many shapes of concentrators, including elliptical cylindrical (including double elliptical cylindrical) and cylindrical (including double cylindrical), as shown in Fig. 3.

As can be seen from the figure, in the elliptical concentrator, the working material is placed on one focus of the ellipse and the neon lamp is placed on the other focus.

For the concentrator composed of double ellipses, the four mirrors have a common focus, and the working material is placed on this common focus.

In this way, the working material concentrates the light emitted from the four xenon lamps, and the efficiency is greatly improved.

The material of the concentrator is generally required to be the material with dense quality, easy polishing, good heat dissipation and small thermal deformation.

At present, aluminum, copper and glass are used, of which copper is the most.

The coating in the concentrator mostly adopts metal coating.

Commonly used metals include gold, silver and aluminum to improve reflectivity.

two types of concentrators

a) Cylindrical

b) Elliptic cylinder

Fig. 3 Two types of concentrators

4. Resonator

The resonator is generally composed of two parallel multilayer dielectric film plane mirrors.

It can make the photons along the axial direction reflect back to the working material to produce stimulated radiation, and get many photons with the same frequency, propagation direction, phase and polarization.

Even if the photons along the axial direction produce oscillation amplification, the photons in other directions disappear quickly after reflection.

In this way, on the one hand, it plays the role of oscillation amplification, on the other hand, it also improves the output directivity and monochromaticity.

There are many kinds of resonators. The most commonly used in solid-state lasers is the plane resonator composed of two parallel plane mirrors.

The parallel error angle between the two planes shall not exceed 10″.

There are also two ways for planar mirrors:

One is to apply metal film or multi-layer dielectric film on both ends of the processed crystal working material, one end of which is made into total reflection, and the other end is made into semi reflection mirror.

Sometimes, a small hole is opened in the coating to replace the semi reflection mirror;

Another way is to coat a layer of reflective film on the substrate of optical glass to make an interchangeable mirror, that is, the working material is separated from the plane mirror.

These two methods are applied in practice. However, multilayer dielectric films are mostly used, and films with different reflectivity can be made according to needs (such as wavelength).

5. Water cooling system

The common method is to cool the optical pump, electrode, working material and cavity with water.

The cooling mode can be divided into full cooling type or sub cooling type.

In order to make the light pump emit light, a set of power supply circuit is also needed, which constitutes a complete solid-state laser.

Figure 4 shows the structure of the pulsed solid state laser.

structure of pulsed solid state laser

Fig. 4 Structure of pulsed solid state laser

The simple working process of pulsed solid-state laser is: 

When the capacitor is charged with high voltage, use a pulse high voltage of tens of thousands of volts to form sparks in the lamp tube, release the electric energy stored in the capacitor, and make the xenon lamp glow.

One part is directly irradiated on the working material, and the other part is concentrated on the working material after one or more reflections of the concentrator.

Part of the light energy gathered on the working material is absorbed by the working material, which excites the low-energy particles to the high-energy level, so that the working material is in the state of particle number inversion.

Under the action of the resonator, when the input energy is strong enough and the amplification exceeds the loss, oscillation can be generated and the laser can be output.

Fig. 5 shows the structure of Nd∶YAG laser.

Fig. 5 Structure of typical Nd∶YAG laser

The main advantage of Nd3 +: YAG is that it is easy to realize particle number inversion and the minimum excitation light intensity is small.

At the same time, neodymium doped yttrium aluminum garnet crystal has good thermal conductivity and small linear expansion coefficient.

It is suitable for working in three states of pulse, continuity and high repetition rate. It is the only solid-state laser working material that can work continuously at room temperature.

Its optical pump adopts xenon lamp. Because the spectral lines emitted by xenon lamp with wavelengths of 0.75 μm and 0.8 μm are the strongest, this just matches the strong absorption band of Nd3 +.

The wavelength of the YAG laser output laser is 1.06, which is 1 / 10 of that of the CO2 laser.

Shorter wavelength is conducive to laser focusing, optical fiber transmission and metal surface absorption, which is the advantage of YAG laser.

However, YAG laser adopts optical pump, with many energy conversion links, the total efficiency is 3% ~ 4%, which is lower than CO2 laser, and the service life of the pump lamp is shorter.

In addition, YAG lasers generally output multimode beams with irregular modes and large divergence angles.

When the Nd3 +: YAG continuous laser works, the xenon lamp is powered on and emits strong light, which irradiates on the laser working material (YAG laser rod) to make the particle number reverse.

The process of light generated by stimulated radiation is superior to the process of light absorption.

The light of stimulated radiation oscillates and amplifies in the resonant cavity and emits the laser through the window.

In order to improve the continuous output power of Nd3 +: YHG laser, several Nd∶YAG rods can be connected in series to obtain a high-power laser beam.

The Nd∶YAG laser system can realize 8 cavities in series, and the output power has reached more than 5kW.

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