What is The Laser Resonator?

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The instrument that produces laser source light is called a laser resonator, which includes gas laser, liquid laser, solid-state laser, semiconductor optical device, and other lasers. Among them, the more typical lasers are CO₂ gas lasers, semiconductor lasers, YAG solid-state lasers, and fiber lasers.

Basic composition and development of laser

The basic composition of laser

Although there are many kinds of lasers, they all produce lasers through excitation and stimulated radiation. Therefore, the basic composition of lasers is fixed, usually composed of working materials (that is, working media that can produce population inversion after being excited), excitation sources (The energy that can cause the working substance to invert the number of particles, also known as the pump source) and the optical resonant cavity are composed of three parts.

Working substance

The production of the laser must choose a suitable working material, which can be gas, liquid, solid, or semiconductor. In this medium, the number of particles can be reversed to create the necessary conditions for obtaining laser light. The existence of metastable energy levels is very beneficial to the realization of population inversion. There are nearly a thousand kinds of working materials and the laser wavelengths that can be generated cover a wide range of vacuum ultraviolet bands to far-infrared bands.

Excitation source

In order to make the number of particles in the working substance reverse, a certain method must be adopted to excite the particle system and increase the number of particles at high energy levels. The gas discharge method can use electrons with kinetic energy to excite the working substance, which is called electrical excitation; pulse light source can also be used to irradiate the working substance to produce excitation, which is called optical excitation; there are thermal excitation, chemical excitation and so on. Various incentive methods are vividly called pumping or pumping. In order to continuously obtain the laser output, it must be continuously pumped to maintain the number of particles in the excited state.

Optical cavity

With a suitable working material and excitation source, the population inversion can be achieved, but the intensity of the stimulated radiation generated in this way is very low and cannot be applied. So people thought that an optical resonant cavity could be used to amplify the stimulated radiation. The optical resonant cavity is composed of two mirrors with a certain geometric shape and optical reflection characteristics combined in a specific way. Its main functions are as follows.

Provide optical feedback capability to make the stimulated emission photons go back and forth in the cavity multiple times to form a coherent continuous oscillation.

Limit the direction and frequency of the oscillating beam in the cavity to ensure that the output laser has a certain directionality and monochromaticity.

The development of lasers

The laser is one of the indispensable core components in modern laser processing systems. With the development of laser processing technology, lasers are also constantly moving forward, and many new lasers have appeared.

Early laser source processing lasers were mainly high-power CO₂, gas lasers and lamp-pumped YAG solid-state lasers. From the perspective of the development history of laser processing technology, the high-capped CO₂ and lasers that appeared in the mid-1970s have developed diffusion-cooled CO₂ lasers. Table 2.1 shows the development status of CO₂ lasers.

Laser type	Sealed-off type	Slow axial flow type	Cross flow type	Fast axial flow type	Turbo fan Fast axial flow	Diffusion cooling type SLAB
Age of appearance	Mid 1970s	Early 1980s	Mid 1980s	Late 1980s	Early 1990s	20th Century Mid-90s
Power/W	500	1000	20000	5000	10000	5000
Beam quality (M2 factor	Unstable	1.5	10	5	2.5	1.2
Beam quality (Kf/mm• mrad)	Unstable	5	35	17	9	4.5

Early CO₂ lasers tended to develop in the direction of increasing laser power, but when the laser power reached a certain requirement, the beam quality of the laser was paid attention to, and the development of the laser shifted to improving the beam quality. Recently, the diffusion-cooled slab CO₂ laser, which is close to the diffraction limit, has good beam quality and has been widely used once it is launched, especially in the field of laser cutting, and is favored by many companies.

The CO₂ laser resonator has the disadvantages of large volume, complex structure, and difficult maintenance. Metal cannot absorb the laser with the wavelength of 10.6чm well, cannot use optical fiber to transmit the laser, and the welding time-induced plasma is serious and other shortcomings. Later, the YAG solid-state laser with a wavelength of 1.06 чm made up for the shortcomings of the CO₂ laser to a certain extent. Early YAG solid-state lasers used lamp pumping methods, which had problems such as low laser efficiency (about 3%) and poor beam quality. With the continuous advancement of laser technology, YAG solid-state lasers continued to make progress, and many new lasers appeared. The development status of YAG solid-state lasers is shown in Table 2.2.

Laser type	Lamp pumped	Diode pumped	Fiber pumped	Flake DISC	Semiconductor end-pumped	fiber laser
Age of appearance	1980s	Late 1980s	Mid-1990s	Mid-1990s	Late 1990s	Early 21st century
Power/W	6000	4400	2000	4000（prototype）	200	10000
Beam quality (M2 factor)	70	35	35	7	1.1	70
Beam quality (Kf/mm• mard)	25	12	12	2.5	0.35	25

It can be seen from Table 2.1 and Table 2.2 that in addition to continuously improving the power of the laser, another important aspect of the development of the laser is to continuously improve the beam quality of the laser. The laser beam quality often plays a more important role in the laser processing process than laser power.

The development of manufacturing laser with laser power and beam quality is shown in Figure 2.1.

At the beginning of the 21st century, another new type of laser-semiconductor laser appeared. Compared with traditional high-power CO₂ lasers resonator and YAG solid-state lasers, semiconductor lasers have obvious technical advantages, such as small size, lightweight, high efficiency, low energy consumption, long life, and high absorption rate of metal to semiconductor lasers. With the continuous development of semiconductor laser technology, other solid-state lasers based on semiconductor lasers, such as fiber lasers, semiconductor-pumped solid-state lasers, and sheet lasers, have developed rapidly. Among them, fiber lasers are developing rapidly, especially rare-earth-doped fiber lasers, which have been widely used in fiber communications, fiber sensing, laser material processing, and other fields.

From CO₂ gas laser to fiber laser

CO₂ gas laser

A laser that uses CO₂ as the main working substance is called a CO₂ laser. A small amount of N² and He needs to be added to its working substance to improve the gain, heat resistance efficiency, and output power of the laser. CO₂ laser has the following characteristics.

• The output power is large. The general closed-tube CO₂ laser can have a continuous output power of tens of watts, which is far more than other gas lasers. The lateral flow electrically excited CO₂ laser can have a continuous output of tens of kilowatts.
• High energy conversion efficiency. The energy conversion efficiency of CO₂ lasers can reach 30%~40%, which exceeds other gas lasers.
• The CO₂ laser uses the transition between the energy levels of the CO₂ molecular vibration and has a relatively rich spectrum. There are dozens of spectrum lines in the laser output near the wavelength of 10 чm. The high-pressure CO₂ laser developed in recent years can achieve continuously tunable output from 9 to 10 чm.
• The output band of the CO₂ laser is exactly the atmospheric window (that is, the transparency of the atmosphere to this wavelength is relatively high)
• In addition, CO₂ lasers also have the advantages of high output beam quality, good coherence, narrow linewidth, stable operation, etc., so they have been widely used in industry and national defense.

The structure of CO₂ laser

A typical sealed-off longitudinal electrically-excited CO₂ laser resonator consists of a laser tube, electrodes, and a resonant cavity (Figure 2.2). The most critical component is a laser tube made of hard glass, which generally adopts a layered sleeve structure. The innermost layer is a discharge tube, the second layer is a water-cooled casing tube, and the outermost layer is a gas storage tube.

The discharge tube is located in the positive column area of ​​the glow discharge in the gas discharge. This region is rich in energy-carrying particles, such as electrons, ions, metastable particles, and photons, which is the gain region of the laser. For this reason, there are certain requirements for the diameter, length, roundness, and straightness of the discharge tube. Most of the equipment below 100W is made of hard glass. Medium power (100~500W) devices are usually made of quartz glass tubes to ensure the stability of power or frequency. The diameter of the tube is generally about 10mm, and the tube length can be slightly thicker.

There is a cold water jacket next to the discharge tube, its function is to reduce the temperature of the working gas in the tube, to ensure that the device realizes the population inversion distribution, and to prevent the discharge tube from being heated and cracked during the discharge excitation process. The purpose of adding a water-cooled casing is to cool the air and gas so that the output power remains stable. The discharge tube is connected to the gas storage tube at both ends. One end of the gas storage tube has a small hole communicating with the discharge tube, and the other end is connected to the discharge tube through the spiral return tube so that the gas can circulate in the discharge tube and the gas storage tube. The gas in the pipe can be exchanged with the gas in the gas storage pipe at any time.

The function of the outermost gas storage tube is to reduce the change of the working gas composition and pressure during the discharge process and to enhance the mechanical stability of the discharge tube.

The air return tube is a thin spiral tube connecting the two spaces of the cathode and the anode, which can improve the unbalanced distribution of the pressure between the electrodes caused by the electrophoresis phenomenon. The value of the diameter and length of the return pipe is very important. It not only enables the gas at the cathode to quickly flow to the anode area to achieve a uniform gas distribution but also prevents the discharge phenomenon in the return pipe.

The electrodes are divided into anode and cathode. The cathode material requires the ability to emit electrons, a low sputtering rate, and the ability to reduce CO₂. At present, most CO₂ and laser resonators use nickel electrodes, and the electrode area is determined by the inner diameter of the discharge tube and the working current. The electrodeposition is coaxial with the discharge tube. The size of the anode can be the same as that of the cathode, or it can be slightly smaller.

The resonant cavity is composed of a total mirror and an output mirror. The total reflection mirrors of medium and low-power CO₂ lasers resonator generally use gold-plated glass mirrors, because the gold film has a high reflectivity of 10.6 чm light and is chemically stable. However, glass substrate mirrors have poor thermal conductivity, so high-power CO₂ lasers often use metal mirrors, such as copper mirrors or molybdenum mirrors, or mirrors coated with gold and dielectric film on a polished oxygen-free copper stainless steel substrate. The output mirror usually uses a material that can transmit a 10.6um wavelength as the substrate, and a multilayer film is plated on it to control a certain transmittance to achieve the best coupling output. Commonly used materials are potassium chloride, sodium chloride, aluminum, arsenic, zinc selenide, cadmium telluride, and so on.

The resonant cavity of the CO₂ laser is usually flat and concave. The total mirror is made of K8 optical glass or optical quartz, which is processed into a concave mirror with a large radius of curvature. The mirror surface is coated with a high-reflectivity metal film-a a gold-plated film, at a wavelength of 10. 6чm The reflectivity at the place reaches 98.8%, and the chemical properties are stable.

The light emitted by carbon dioxide is infrared light, so all-reflection mirrors need to use materials that transmit infrared light. Because ordinary optical glass is not transparent to infrared light, it is required to open a small hole in the center of the total mirror, and then seal a piece of infrared material that can transmit 10.6 чm lasers to seal gas, which makes the laser in the resonant cavity apart is an output from the small hole outside the cavity to form a beam of laser light or light knife.

The discharge current of the sealed-off CO₂ laser resonator is relatively small. The cold electrode is used, and the cathode is made of a molybdenum sheet or a nickel sheet into a cylindrical shape. The working current is 30~40MA, the area of ​​the cathode cylinder is 500cm², in order not to pollute the lens, a light barrier is added between the cathode and the lens. The pump is excited by a continuous DC power supply.

Output characteristics of CO₂ laser system

Crossflow CO₂ laser resonator. The gas flow is perpendicular to the axis of the cavity. The CO₂ laser with this structure has low beam quality and is mainly used for surface treatment of materials, and is generally not used for cutting. Compared with other CO₂ lasers, cross-flow CO₂ lasers have high output power, low beam quality, and low prices.

Cross-flow CO₂ lasers can use direct current (DC) excitation and high frequency (HF) excitation, and the electrodes are placed on both sides of the plasma zone parallel to the axis of the cavity. The ignition and operating voltage of the plasma are low, the gas flows through the plasma zone perpendicular to the beam, and the passage of the gas flowing through the electrode system is very wide, so the flow resistance is very small, the cooling of the plasma is very effective, and the power of the laser is not too great. Many restrictions.

The length of this type of laser is less than 1m, but it can generate 8KW of power. However, due to the lateral flow of gas through the plasma, this type of laser blows the plasma away from the main discharge circuit, causing the plasma area on the beam section to deviate more or less into a triangle, the beam quality is not high, and high-order modes appear. If a circular hole is used to limit the mode, the symmetry of the beam can be improved to a certain extent.

Fast axial flow CO₂ laser resonator. The structure is shown in Figure 2.3. The flow of laser gas of this kind of CO₂ laser is along the axis of the resonator. The output power of CO₂ laser with this structure ranges from hundreds of watts to 20KW. The output beam quality is better, and it is the mainstream structure currently used in laser cutting.

Fast axial flow CO₂ lasers can use direct current (DC) excitation and radio frequency (RF) excitation. The shape of the plasma between the electrodes is a slender column. In order to prevent the plasma from dispersing in the surrounding area, this type of discharge area is often in a hollow cylindrical glass tube or ceramic tube. The plasma can be ignited and maintained at both ends of the two ring electrodes. The ignition and operation voltage depends on the electrode. The maximum voltage used in practical applications is 20~30KV.

The cooling of the circulating gas adopts the form of rapid axial flow. In order to ensure effective heat conduction, Roots blowers or adjustable wheel fans are commonly used to achieve this high-speed flow, but the flow resistance of this geometric shape is relatively high, and the output laser power is subject to certain limitations, such as the laser output of only a few hundred watts of the DC exciter. The output power of the laser is limited, so several axial flow cooling discharge tubes are often connected in the optical form to provide sufficient laser power.

Since the output power of the CO₂ laser resonator mainly depends on the electrical power input per unit volume, the RF excitation is higher than the DC excitation and the plasma density is higher. The RF excitation axial flow laser in which several axial cooling discharge tubes are connected in an optical form, continuous The output power can reach 20KW. Axial CO₂ lasers, due to the axial symmetry of the plasma, are easy to operate in the fundamental mode and produce high beam quality.

Slat-type diffusion cooling CO₂ laser. Diffusion-cooled CO₂ lasers are similar to the early sealed-off CO₂ lasers. The working gas of the sealed-off CO₂ laser is enclosed in a discharge tube and cooled by heat conduction. Although the outer wall of the discharge tube is effectively cooled, the discharge tube can only generate 50W of laser energy per meter, and it is impossible to make a compact, high-energy laser. Diffusion-cooled CO₂ lasers also use gas-enclosed methods, but the lasers are compact structures, the gas discharge excited by radiofrequency occurs between two copper electrodes with a larger area. The electrodes can be cooled by water cooling, and the narrow gap between the two electrodes can dissipate heat from the discharge cavity as much as possible so that a relatively high output power density can be obtained.

The diffusion-cooled CO₂ laser resonator adopts a stable resonant cavity composed of cylindrical mirrors. Since the optically unstable cavity can easily adapt to the geometry of the excited laser gain medium, the slab-type diffusion-cooled CO₂ laser can produce high-power-density laser beams, and the laser beam quality High, but the original output beam of this type of laser is rectangular, and a water-cooled reflected beam shaping device is required to shape the rectangular beam into a circular symmetrical laser beam. At present, the output power range of this type of laser is 1~5KW.

Compared with gas flow CO₂ lasers, slab diffusion cooling CO₂ lasers have the characteristics of compact and sturdy structure and have an outstanding advantage, that is, in practical applications, they do not need to be fresh as gas flow CO₂ lasers. Laser working gas, but a small about 10L cylindrical container is installed in the laser head to store the laser working gas. This can be achieved through an external laser working gas supply device and a water permanent gas tank exchanger. This kind of executive agency has been working for more than one year.

A semiconductor laser

Semiconductor laser refers to a type of laser with semiconductor as its working material. Compared with other lasers, semiconductor lasers have the advantages of small size, high efficiency, simple and robust structure, and direct modulation. Semiconductor lasers have important applications in communications, ranging and information processing.

Semiconductor foundation

Pure semiconductors without impurities are called intrinsic semiconductors. If impurity atoms are doped into intrinsic semiconductors, impurity levels are formed below the conduction band and above the valence band, which are called donor level and acceptor level, respectively. Figure 2.4 shows the impurity levels of Si single crystal semiconductors.

Semiconductor materials are mostly crystalline structures. When a large number of atoms are regularly and tightly combined into a crystal, those valence electrons in the crystal are all in the crystal energy band. When an external electric field is applied, the electrons in the valence band transition to the conduction band, and can move freely in the conduction band to conduct electricity. The loss of an electron in the valence band is equivalent to the appearance of a positively charged hole, which can also conduct electricity under the action of an external electric field. Therefore, the holes in the valence band and the electrons in the conduction band have a conductive effect, which is collectively called carriers.

A semiconductor with a donor level is called an n-type semiconductor; a semiconductor with an acceptor level is called a p-type semiconductor. At room temperature, most of the donor atoms of n-type semiconductors are ionized by thermal energy, and electrons are excited to the conduction band and become free electrons. Most of the acceptor atoms of p-type semiconductors capture electrons in the valence band and form holes in the valence band. Therefore, n-type semiconductors are mainly conducted by electrons in the conduction band; p-type semiconductors are mainly conducted by holes in the valence band.

In a piece of semiconductor material, the sudden change from the p-type region to the n-type region is called the p-n junction. A space charge zone is formed at the interface. The electrons in the conduction band of the n-type semiconductor diffuse to the p region, and the holes in the valence band of the p-type semiconductor diffuse to the n region. The n-type region near the junction region is positively charged because it is a donor, and the p-type region near the junction region is negatively charged because it is an acceptor. At the interface, an electric field directed from the n zone to the p zone is formed, which is called the built-in electric field (or self-built electric field). This electric field prevents the continued diffusion of electrons and holes.

If a forward bias is applied to the semiconductor material that forms the p-n junction, the p area is connected to the positive electrode and the n area is connected to the negative electrode. The electric field of the forward voltage is opposite to the built-in electric field of the p-n junction, which weakens the built-in electric field’s hindrance to the diffusion of electrons in the crystal so that the free electrons in the n-zone are constantly under the action of the forward voltage.

Diffusion to the p region through the p-n junction. When there are a large number of electrons in the conduction band and holes in the valence band at the same time in the junction zone, they recombine in the injection zone. When the electrons in the conduction band transition to the valence band, the excess energy are emitted in the form of light. come out. This is the mechanism of semiconductor electroluminescence, and this spontaneous recombination luminescence is called spontaneous emission.

To make the p-n junction generate laser light, a particle inversion distribution must be formed in the junction area, a heavily doped semiconductor material must be used, and the current injected into the p-n junction must be large enough (such as 30KA/cm²). In this way, in the local area of ​​the p-n junction, a reversed distribution state of more electrons in the conduction band than holes in the valence band can be formed, thereby generating stimulated radiation and emitting laser light.

The optical resonant cavity of a semiconductor laser resonator is composed of a cleavage plane (110 faces) perpendicular to the p-n junction plane. It has a reflectivity of 35%, which is enough to cause laser oscillation. If it is necessary to increase the reflectivity, a layer of SiO₂ can be plated on the crystal surface, and then a layer of metallic silver film can be plated to obtain a reflectivity of more than 95%.

Once a forward bias is applied to the semiconductor laser, the population inversion occurs in the junction area and recombination occurs.

Conditions for semiconductor stimulated emission

Semiconductor lasers work by injecting carriers, and emitting lasers must meet the following three basic conditions.

• It is necessary to produce sufficient population inversion distribution, that is, the number of particles in the high-energy state is sufficiently larger than the number of particles in the low-energy state.
• There is a suitable resonant cavity that can play a feedback role so that the photons of the stimulated radiation are proliferated to produce laser oscillation.
• A certain threshold condition must be met to make the photon gain equal to or greater than the photon loss.

Injection type homojunction semiconductor laser

The injection-type homojunction GaAs semiconductor laser resonator is the first semiconductor laser to be successfully developed. Homogeneous junction refers to a p-n junction composed of p-type and n-type semiconductors of the same matrix material (such as GaAs), and injection type refers to a pumping method that directly energizes the semiconductor laser and injects current to excite the working substance.

Figure 2.5 (a) shows the typical appearance structure of this laser. There is a small window on the tube shell to output the laser, and the electrode at the lower end of the tube is used for the external power supply. Inside the shell is the laser die, as shown in Figure 2.5(b). There are many shapes of the die, Figure 2.5(c) is a schematic diagram of the structure of the mesa-shaped die. The thickness of the p-n junction is only tens of microns. Generally, a thin layer of p-type GaAs is grown on the bottom of the n-type GaAs village to form the p-n junction.

The resonant cavity of the laser generally directly utilizes two end faces perpendicular to the p-n junction. The refractive index of GaAs is 3.6, and the reflectivity of light perpendicular to the end surface is 32%. In order to increase the output power and reduce the operating current, one of the reflective surfaces is generally plated with gold.

Heterojunction semiconductor laser

Studies have shown that it is difficult for homojunction semiconductor lasers to obtain low threshold currents and achieve continuous operation at room temperature. Therefore, people have developed heterojunction lasers on this basis. Heterojunction lasers are also single heterojunction (SH) lasers and double heterojunction (SH) lasers. Mass junction (DH) laser.

Single heterojunction semiconductor laser. Figure 2.6 shows the structure of a single heterojunction laser (GaAs-P-Ga_1-xAl_xAs) and a schematic diagram of the energy band change, refractive index change, and light intensity distribution of each region. It can be seen that after adding the heterogeneous material GaAs-P-Ga_1-xAl_xAs to the P-GaAs side, the interface electron energy barrier makes the electrons injected into P-GaAs from N-GaAs can only be confined in the P zone to recombine and generate photons. Because of the change of refractive index at the interface of P-GaAs and P-Ga_1-xAl_xAs, the photons generated by the recombination in the active area are reflected and confined in the P-GaAs layer.

The confinement effect of the heterojunction on electrons and photons reduces their loss so that the threshold current density of the single heterojunction laser at room temperature is reduced to 8KA/cm².

In a single heterojunction laser source, the heterojunction plays a role in limiting the diffusion of carriers, but it is not used for injection, so the value of x is generally chosen to be relatively large, such as 0.3<x<0.5. In a semiconductor laser resonator, the thickness d of the active region is critical. If d is too large, it will lose the meaning of carrier limitation, and if d is too small, it will increase the loss. In single heterojunction lasers, d≈2чm is generally adopted.

Double heterojunction semiconductor laser source. Liquid phase epitaxy was used to sequentially grow N-Ga_1-xAl_xAs, P-GaAs, P-Ga_1-xAl_xAs, As single crystal thin layers on the N-GaAs village bottom. There are N- Ga_1-xAl_xAs, as layers and P- Ga_1-xAl_xAs as layers on both sides of the active area P-GaAs, forming N-Ga_1-xAl_xAs /P-GaAs and P-GaAs/P-Ga_1-xAl_xAs two heterojunctions of N-Ga_1-xAl_xAs and P-Ga_1-xAl_xAs are shown in Figure 2.7.

Figure 2.8 shows the energy band, refractive index, and light intensity distribution of a double heterojunction laser. The active region P-GaAs is sandwiched between two wide-bandgap Ga_1-xAl_xAs layers. For this structure, due to its symmetry, it is no longer limited to only electron injection. The double-heterojunction structure allows both electron injection and hole injection to be effectively utilized. If the width of the active region is smaller than the diffusion length of carriers, most of the carriers can diffuse to the active region before recombination. When they reach the heterojunction, they are repelled by the potential barrier and stay in the active region. If the thickness d of the active region is much smaller than the diffusion length of the carriers, the carriers will evenly fill the active region. For this kind of laser, recombination occurs almost uniformly in the active region.

Because both sides of the active area are broadband materials, the effective refractive index jumps in the hierarchy, so that the photons are confined in the active area, and the distribution of the light field is also symmetrical. The double heterojunction can effectively limit the carriers and photons, so the threshold current density of the laser is significantly reduced, and the continuous operation of the laser at room temperature is realized.

After the double heterojunction laser achieves continuous operation at room temperature, the outstanding problem is how to improve the life of the device, which can start from solving the problem of active area structure and heat dissipation. With the different requirements, there are multiple structures of double heterojunction lasers, the more typical one is the bar double heterojunction (DH) laser. In GaAs/ Ga_1-xAl_xAs DH lasers, the bandgap of GaAs corresponds to a laser wavelength of about 0.89um. InP/InGaAsP DH lasers cover a range of 0.92~1.65чm. Since the lowest loss of optical fiber is 1.3~1.6чm, InP/InGaAsP DH lasers have important applications for long-distance optical fiber communication systems, while GaAs/ Ga_1-xAl_xAs DH lasers are often used in short-distance optical fiber communication systems.

YAG solid-state laser

The core of the laser emission is the laser working substance (that is, the working substance containing the metastable energy level) in the laser that can realize the population inversion, such as the laser whose working substance is crystalline or glass, which is called crystal laser and glass laser, respectively. Usually, these two types of lasers are collectively referred to as solid-state lasers. Among the lasers, the solid-state laser was the first to develop. This kind of laser has a small size, high output power, and convenient application. There are three main working materials for solid-state lasers; neodymium-doped yttrium aluminum garnet (Nd: YAG), with an output wavelength of 1.06 чm, which is white and blue; neodymium glass, with an output wavelength of 1.06 чm, which is purple-blue; ruby, the output wavelength is 0.694чm, which is red.

YAG lasers are the most common type of solid-state lasers. YAG lasers came out later than ruby ​​and neodymium glass lasers. In 1964, YAG crystals were successfully developed. After several years of hard work, the optical and physical properties of YAG crystal materials have been continuously improved, and the preparation process of large-size YAG crystals has been overcome. By 1971, large-size Nd: YAG crystals with a diameter of 40mm and a length of 200mm were able to be drawn, which provided high-quality crystals at a moderate cost for the development of YAG lasers, and promoted the development of the YAG lasers.

In the 1970s, the development of lasers ushered in an upsurge in the research and application of YAG lasers. Research institutions in many industrially developed countries invested a lot of manpower and financial resources to study how to improve the efficiency, power, and reliability of YAG lasers and solve engineering problems. Some application results have been achieved in the fields of laser ranging, laser radar, laser industrial processing, and laser medical treatment. For example, the YAG Laser Precision Tracking Radar (PATS system) was successfully used in the missile measurement range in 1971 by Silvania Company of the United States. In the 1980s, the research and application of YAG lasers have matured and entered a period of rapid development, becoming the mainstream of the development and application of various lasers.

The structure of YAG laser

Generally speaking, the YAG laser refers to the Nd: YAG laser doped with trivalent Nd³⁺ in the yttrium aluminum garnet (YAG) crystal. It emits a near-infrared laser source of 1.06 чm and is a solid-state laser that can work continuously at room temperature. In the small and medium-power pulsed lasers, Nd: YAG lasers are currently used in quantities far more than other lasers. The single pulse power emitted by this laser can reach 107W or higher, which can process materials at extremely high speeds. YAG lasers have high energy, high peak power, compact structure, firmness and durability, reliable performance, safe processing, simple control, etc. Features, it is widely used in industry, national defense, medical treatment, scientific research, and other fields. Nd: YAG crystal has excellent thermal properties and is very suitable for making continuous and repetitive laser devices.

YAG laser includes YAG laser source rod, xenon lamp, condenser cavity, Q switch, polarizer, total mirror, semi-feedback, etc., the structure is shown in Figure 2.9

The working medium of the YAG micro-optical device is Nd: YAG rod, the sides are roughened, the two ends are ground into a plane, and the antireflection coating is plated. The frequency doubling crystal adopts potassium tetany oxide (KTP) crystal with an anti-reflection coating on both sides. The laser spectroscopy cavity adopts a plano-concave stable cavity, the cavity length is 530mm, and the radius of curvature of the plano-concave total mirror is 2m. Please use high-transmittance and high-reflection quartz lenses for the galvanometer mirror, and the modulation frequency of the Q switch device is adjustable.

The laser resonant cavity is a three-mirror folded cavity with 1.3mm spectral line resonance, including two semiconductor laser pump modules, each module is composed of 20W continuous-wave semiconductor laser arrays (LD) with a center wavelength of 808nm, and the total spectral line width Less than 3nm, the laser crystal is 3mm×75mm Nd: YAG, the doping concentration is 1.0%, and a 1.319nm laser 90° quartz rotator is inserted between the two LD pump modules to compensate for the thermally induced birefringence effect.

The stable areas of the resonant cavity of the radially polarized light and the radially polarized light overlap each other, which is beneficial to increase the output power and improve the beam quality. The acousto-optic Q switch with high diffraction loss is used to generate Q-switched pulse output, and the repetition frequency can be adjusted in the range of 1~50kHz. The designed resonant cavity produces a real focus on the folded arm to increase the power density, which is beneficial to nonlinear frequency conversion.

Plano mirror M₁ is coated with 1319nm, 659. 4nm double high-reflection film system, plano-concave mirror M₂ is an output coupling mirror, and plano-concave mirror M₃ is 1319nm, 659nm, 440nm three-wavelength high-reflection film. Since the 1064nm spectral line intensity of the Nd: YAG crystal is three times that of the 1319nm wavelength, the M₁, M₂, M₃, cavity mirror design requires the transmittance of the 1064nm wavelength to be greater than 60%, which is very important to suppress the 1064nm laser oscillation. of.

In order to reduce the insertion loss in the cavity, all components in the cavity should be coated with an anti-reflection coating. The semiconductor laser does not add any shaping measures or optical imaging components, and the Nd: YAG crystal is pumped from the adjacent 120° directions. By optimizing the pumping parameters, a relatively uniform and Gauss-like gain profile can be obtained. This design is simple, compact, and practical, and can be better matched with the Eigenmode of the resonator, which is beneficial to improve the energy extraction efficiency and beam quality.

Because lithium tribemate (LBO) crystal has a high damage threshold, low absorption of fundamental frequency light, and frequency-doubled light, it can achieve 1319nm double frequency and triple frequency phase matching and has the advantages of suitable effective nonlinear coefficients, so choose two LBO crystals are used as crystals for intracavity frequency doubling and intracavity sum-frequency.

Output characteristics of YAG laser

• Lamp-pumped Nd: YAG laser. The structure is shown in Figure 2.10 and Figure 2.11. The gain medium Nd: YAG is rod-shaped, and it is often placed on the focal line of the double-sugar circle reflection condenser cavity. The two pump lamps are located on the two outer focal lines of the double ellipse, and the cooling water flows between the pump lamp and the laser rod with a glass tube sleeve.
• In high-power lasers, the thermal effect of the laser rod limits the maximum output power of each laser rod. The heat inside the laser rod and the cooling of the surface of the laser rod cause the temperature gradient of the crystal so that the maximum power of the pump must be lower than to cause damage. The stress limit. The effective power range of a single rod Nd: YAG laser is 50~800W. Higher power Nd: YAG lasers can be obtained by connecting Nd: YAG laser rods in series.
• Diode-pumped Nd: YAG laser. The structure of a diode-pumped Nd: YAG laser is shown in Figure 2.12, and a GaAlAs semiconductor laser is used as the pump light source.
• Using a semiconductor laser as the pump source increases the life of the components and eliminates the requirement of regular replacement of the pump lamp when using lamp pumping. The diode-pumped Nd: YAG laser has higher reliability and longer working time.
• The high conversion efficiency of the diode-pumped Nd: YAG laser comes from the good spectral matching between the emission spectrum of the semiconductor laser and the absorption of Nd: YAG. GaAIAs semiconductor laser emits a narrow-band wavelength. By precisely adjusting the Al content, it can emit light at 808nm, which is in the absorption band of Nd³⁺ particles. The electro-optical conversion efficiency of semiconductor lasers is approximately 40%-50%, which is the reason that diode-pumped Nd; YAG lasers can achieve a conversion efficiency of more than 10%. While the lamp is excited to produce white light, the Nd: YAG crystal only absorbs a small part of the spectrum, which leads to its low efficiency.

Fiber laser

Classification of fiber lasers

Fiber lasers are lasers that use optical fibers as the laser source medium. According to the incentive mechanism, it can be divided into the following four categories.

• Rare-earth-doped fiber laser source, through doping different rare-earth ions in the fiber matrix material to obtain the laser output of the required wavelength band.
• Fiber lasers made using the nonlinear effects of fibers, such as stimulated Raman scattering (SRS), etc.
• Single-crystal fiber lasers, including ruby ​​single-crystal fiber lasers, Nd: YAG single product fiber lasers, etc.
• Dye fiber laser, by filling the plastic core or cladding with dye to realize laser output.

Among these types of fiber lasers, fiber lasers and amplifiers doped with rare-earth ions are the most important and have the fastest development. They have been applied in the fields of fiber communication, fiber sensing, and laser material processing, this type of laser.

Waveguide principle of fiber laser

The geometric structure of a single-layer fiber laser source is shown in Figure 2.13. Compared with solid-state lasers source, fiber lasers have at least one free beam path formed in the laser resonator, and beam formation and introduction into fiber lasers are realized in optical waveguides. Generally, these optical waveguides are based on rare-earth-doped optoelectronic dielectric materials. For example, silicon, phosphate glass, and fluoride glass materials show an attenuation of about 10dB/km, which is several orders of magnitude less than solid-state laser crystals. Compared with crystalline solid materials, the absorption and emission bands of rare-earth ions show a broadened spectrum. This is because the interaction of the glass substrate reduces the frequency stability and the required width of the pump light source. Therefore, it is necessary to choose a laser diode pump source with a suitable wavelength for fiber lasers.

The optical fiber contains a rare-earth-doped active core with a refractive index of n₁, usually surrounded by a layer of pure silica glass cladding, and the refractive index of the cladding is n₂<n₁. Therefore, based on the total reflection inside the interface between the core and the cladding, the waveguide is generated in the core layer. For pump radiation and laser radiation, the core layer of the fiber laser is both an active medium and a waveguide. The entire optical fiber is protected from external influences by a polymer outer layer.

For optically excited fiber lasers, the pump radiation is coupled to the laser core through the fiber surface. However, if it is axially pumped, the pump radiation must be coupled into a waveguide of only a few microns. Therefore, a highly transparent pump radiation source must be used to excite the multi-mode fiber, and the current output power of the radiation source is limited to about 1W. In order to amplify the pump power proportionally, it is necessary to match the beam parameters of the large-opening fiber with the high-power semiconductor laser array. However, the enlarged fiber active core allows higher transverse mode oscillations, which will result in reduced beam quality. At present, a double-clad design is used, that is, an isolated core layer is used to pump and emit lasers, and good results can be obtained.

Double-clad fiber laser

Double-clad doped fiber consists of four parts: core, inner cladding, outer cladding and protective layer.

The function of the fiber core is to absorb the incoming pump light and confine the radiated laser light in the core; as a waveguide, confine the laser light to transmit in the core and control the mode.

The role of the inner cladding layer is to wrap the core and confine the radiated laser light within the core; as a waveguide, the multimode transmission of the pump light coupled to the inner cladding layer makes it reflect back and forth between the inner cladding layer and the outer cladding layer. Pass through the single-mode fiber core and be absorbed

For double-clad fiber lasers, the pump radiation is not directly emitted to the active core layer, but into the surrounding multimode core layer. The pump core layer is also like the cladding layer. In order to realize the optical waveguide characteristics of the pump core layer to the active core layer, the surrounding coating must have a small refractive index. Usually, fluorine-doped silica glass or a highly transparent polymer with a low refractive index is used. The typical diameter of the pump core is several hundred microns, and its numerical aperture NA≈0.32~0.7, as shown in Figure 2.14.

The radiation emitted to the pump core is coupled into the laser core over the entire length of the fiber, where it is absorbed by the rare-earth ions, and all high-level light is excited. Using this technology, multi-mode pump radiation can be effectively converted from high-power semiconductor lasers into laser radiation, and it has excellent beam quality.

Technical characteristics of fiber laser source

Fiber lasers provide the possibility to overcome the limitation of the calibrated output power of solid-state lasers while maintaining the beam quality. The quality of the final laser beam depends on the refractive index profile of the fiber, and the refractive index profile of the fiber ultimately depends on the geometric size and the numerical aperture of the activated waveguide. When the fundamental mode is propagated, the laser oscillation has nothing to do with external factors. This means that compared with other (even semiconductor pumped) solid-state lasers, fiber lasers do not have thermo-optical effects.

The prism effect caused by heat and the birefringence effect caused by pressure in the active zone will cause the beam quality to decrease. When the pump energy is transported, the fiber laser does not observe a decrease in efficiency even at high power.

For fiber laser source, the thermal load caused by the pumping process will expand to a longer area. Because of the larger surface area to volume ratio, the thermal effect is easier to eliminate. Therefore, the temperature rise of the fiber laser core is small compared to solid semiconductor pump lasers. Therefore, when the laser is working, the quantum efficiency is attenuated due to the increasing temperature, which plays a secondary role in fiber lasers.

Taken together, fiber lasers source have the following main advantages.

• Optical fiber as a guided wave medium has high coupling efficiency, small core diameter, high power density is easily formed in the core, and can be easily connected to the current optical fiber communication system efficiently, and the formed laser has high conversion efficiency and low laser threshold., The output beam quality is good and the line width is narrow.
• Because the optical fiber has a large surface-to-volume ratio, the heat dissipation effect is good, and the ambient temperature is allowed to be between -20~+70℃, without a huge water cooling system, only simple air cooling.
• It can work in harsh environments, such as high impact, high vibration, high temperature, and dusty conditions.
• Because the optical fiber has excellent flexibility, the laser can be designed to be small and flexible, compact in appearance, easy to system integration, and cost-effective.
• Has quite a lot of tunable parameters and selectivity. For example, a Bragg fiber grating with appropriate wavelength and transmittance is directly written on both ends of a double-clad fiber to replace the resonant cavity formed by mirror reflection. The all-fiber Raman laser is composed of a unidirectional fiber ring, a circular waveguide cavity. The signal in the cavity is directly amplified by the pump light without population inversion.