These are also used in welding and cutting of steel and in communication system also. It is possible that a laser cavity with an optical or lasing medium fault may produce the required laser power but with a distorted or different beam mode structure, thus affecting the focus spot size and consequently the welding power density. Flash and arc lamps or diode lasers are used as the Optical pumping source. The energy level diagram of Nd:YVO 4 is shown in Fig. Here, only those are shown which are significant for optical pumping with laser diodes and which are important for the laser process. Optical pumping is a technique in which light is used as energy source to raise electrons from lower energy level to the higher energy level. Content Guidelines 2. Helium Neon Laser Energy Level Diagram Here upward transition shows the absorption of energy from the pumping source by Helium atom. Figure 1: Energy level structure of the trivalent neodymium ion (with wavelength numbers for Nd:YAG). This necessitates proper selection of focusing optics that determine the focus spot size. Earlier ruby lasers were more popular but now Nd:YAG laser is used more widely in industry because of good thermal characteristics of YAG crystals. 8 Fig. External Energy source: Due to the difference in the energy levels, the electrons need some external pumping in order to perform a transition from one state to another. One laser can operate multiple work stations for switching the laser beam from station to station, whilst welding al one station, part loading and unloading car take place at other stations. Such a device can be used to check beam characteristics during the welding operation and thus provide a method of quality assurance with respect to the laser beam. There are industrial micro-spot welding systems where the beam is switched among eight workstations at up to 40 times per second. Generally, the total loss of laser power for an Nd:YAG laser and fibre optic system is no more than 10— 15%. So, the electrons in this particular state will last for a longer duration. Some analysers display only two dimensional picture of the beam profile, however more recent analysers have the ability to display, with the aid of computer graphics, three dimensional isometric picture, as shown in Fig. Following this is a more detailed description of the Thus the beam splitting system, in conjunction with a fibre optic beam delivery system, can make several welds simultaneously at one or more work stations. APPLICATIONS • Used widely in medical field. Conventional Nd:YAG lasers pumped at 808nm suffer from heat-related processes, arising from the quantum defect between the pump energy level 4F 5/2 and emitting energy level 4F 3/2, such as strong thermal lensing, which can lead to beam quality degradation, pump-induced stresses, which Simplified energy level diagram of the 4 F 3 / 2---4I 11 /2 laser transition in Nd:YAG showing the four- level nature of the system Such joints lend themselves to precision equipment and machine tool fabrication and if carefully applied, together with the low distortion offered by laser welding, can keep post-weld machining allowance to minimum. To make deep and narrow welds at high speeds, short pulse widths are required. Four-level energy diagram. Optical resonator: The two ends of the Nd:YAG rod is coated with silver. Consider a ruby laser medium consisting of three energy levels E 1, E 2, E 3 with N number of electrons. This is because the side tube provides good visual access to the target area for the spot weld. A hazard may also arise due to the reflection of a focused beam from the surface of the workpiece, particularly if the incident beam is inclined to the workpiece at an angle of less than 70°. at a pulse repetition rate of 500 Hz when using an average power of 1 KW. 1.Process A indicates the traditional 808 nm pumping. It must be kept in mind that an unfocused laser beam from a multi-kilowatt laser, if given time, will easily burn through steel plates and even fire bricks. Differentiate He-Ne, Nd-YAG and Semiconductor lasers. 14.38. Energy level diagram for Nd:YAG. Energy losses in a Nd:YAG laser. When the rod temperature is low, these transitions will occur at a high rate. The high power (> 800 W) Nd:YAG lasers with pulse time of say 2 m see and high pulse repetition frequency of 500 Hz it can produce keyhole type of welds with high aspect ratio of depth lo width. Thereby forming an optical cavity. Although focused spot diameter is an important parameter but from a practical stand point the focusing off number is more useful to establish tolerant welding conditions where f number is defined as a ratio of focal length of the focusing optics (F) to the laser beam diameter (D), i.e.. Due to doping, yttrium ions get replaced by the Nd3+ ions. 14.17 A). In diagram above there are 3 down word energy transitions for Neon that produce lasing. In normal state, most of the chromium ions are in the ground state E 1 . Explain homo junction and hetero junction semiconductor lasers. For Nd:YAG lasers operating in the power range of 1 kW the problem of penetration control is overcome by the use of Ar + 20% CO2 or Ar + (1-2)% O2 as the shielding gas, however slight oxidation of weld metal may be caused by them. For keyhole welding by laser beam power densities of the order of 103 to 105 W/mm2 are required necessitating the focusing of laser beam to a very small spot of only a fraction of mm in diameter. Where the Nd:YAG laser beam is split, the odd shape of each cross-section is homogenised into a focusable form by transmitting it through fibre optic. Required fields are marked *. Though this laser also is a four level laser with Nd 3+ as the active laser ion, it cannot generate output at high repetition rate due to its very low thermal conductivity, but it can produce much higher energy output as compared to Nd:YAG laser. The pulsed excitation of the laser rod results in the generation of a pulse of laser light essentially of the same pulse duration as the current pulse from the power supply. 5.2. Alternatively, the beam can be sequentially switched, 14.40 (b), to different points, often up to 30 m away. Unless welding speed is paramount, it is best to choose the focused spot size for welding based on f number of 4 for Nd: YAG lasers and 7.5 for CO2 lasers. The Chinese ZM-87 blinding laser weapon uses a laser of this type, though only 22 have been produced due to their prohibition by the Convention on Certain Conventional Weapons.North Korea is reported to have used one of these weapons against American helicopters in 2003. On the other hand, several very different stations can time share one laser. Because the laser light from Nd:YAG or CO2 laser is invisible to the human eye and it travels at extremely high speed of about 300,000 km/sec therefore any escaping reflected laser beam will strike instantaneously anyone in its path causing serious skin burns. A helium–neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture, a Nd:YAG laser uses either light focused from a xenon flash lamp or diode lasers, and excimer lasers use a chemical reaction. Energy level diagram: ! 14.27. 5. Moreover, the laser can be positioned some distance from the production line and laser beam piped to it. The most common Nd:YAG emission wavelength is 1064 nm. ND YAG laser working diagram. However, at this power level deeper welds, with a reduced aspect ratio, would be achieved at longer pulse lengths and repetition rates above 25 Hz. It is a solid state and 4 level system as it consists of 4 energy levels. Pumping takes place with light of wavelength 5000 A° … While down ward transition shows the emission of energy / light or lasing present in the Neon atom only. 4. The power supply of the Nd: YAG laser unit generates current pulses of the desired amplitude and duration and feeds them into an electric arc lamp or a spiral flash lamp. The fibre optic beam delivery systems are by far the most simple and versatile. An unfocused laser beam escaping accidently from the beam transmission path is capable of travelling several hundred metres in air before it will expand enough to be safe. Energy diagram: The simplified energy level diagram of chromium ions in a ruby laser, indicating appropriate excitation an decay is shown in Fig. 14.32 gives reliable weld protection. 6. A 0.5 mm diameter SiO2fibre has an allowable bending radius of approximately 100 mm before efficiency is impaired, whereas for a 1 mm diameter fibre the safe radius is at least twice as large. 14.36 shows the basic sheet metal joints which can be laser welded. 5.4. The resonator or optical cavity of Nd:YAG laser consists of a flast lamp, laser rod, reflector and mirrors. 5.1. Prohibited Content 3. When making continuous seam and butt welds an annular skirt shield co-axial with the laser beam, shown in Fig. Draw a simplified energy-level diagram of Nd:YAG and use it to explain why lasing is more efficient at lower rod temperatures. Then the electrons present in the energy state E1 gains energy and moves to energy state E4. F = the focal length (mm) of the focusing lens used. But when making a continuous seam or butt welds with overlapping spots Ar or OFN is generally used for lasers rated up to 300 W. Above this power level gas shielding becomes more critical and can affect penetration depth and appearance. Simplified diagram showing transitions for Nd 3 + in a Nd:YAG laser. A simulation model for a solar laser is designed using Ce:Nd:YAG as an active medium. Typical pulse durations for an Nd: YAG welding laser range from 0.5 to 20 m sec and the repetition rate from 5 to 500 hertz. Those used for welding are protected by substantial and robust sheathing, which includes a flexible steel tube and nylon jacket, as shown in Fig. 12 Fig. It is reported that a continuous weld of 0.5 mm depth can be achieved at a welding speed of more than 3 m/min. Three level laser energy diagram. Co-axial nozzle shielding device, shown in Fig. 2. * How does Nd:YAG laser work? The multi-mode beam from an Nd: YAG laser can be split, Fig. 14.35 while Fig. A simplified energy-level scheme for Nd:YAG is shown in Fig. will suffice for a laser up to 3 kW capacity. Copyright 10. The wavelength (in m) corresponding to a transition between levels 1 and 2 is obtained from:. When using a correctly positioned coaxial or side tube shielding device. 14.30, before diverging again. However, as E4 is an unstable state and it exhibits short lifespan. 14.39, without a significant loss of power. 14.41. Gain medium / Laser medium However, initially, electrons in E1 is very much higher as compared to E4. 16 Fig. The optical fibre material is SiO2 (quartz) and is generally less than 1 mm in diameter. 9. Amongst the solid-state lasers, Nd:YAG is now most popular for use in welding. Helium can also be used with Nd:YAG laser but it is reported to cause more weld porosity than with the use of OFN. Owing to high risk of serious eye damage from a Nd: YAG laser, instead of a viewing window a close-circuit television system is best suited for viewing the welding operation; with the correct camera and filters close-up observations can be made in absolute safety. Although these measures adequately protect the optical fibre, their main function is to resist accidental industrial damage which could cause fracture and allow laser light to escape which may lead to dangerous consequences. For example a gas (low rate of 10 to 20 lit/min. Disclaimer 9. For maximum beam transmission efficiency the fibre ends have to be highly polished and perfectly square and concentric with the optical axis of the lenses positioned at each end of the fibre. But once the lifetime of the electrons at the metastable state gets exhausted then these electrons by releasing photons come to lower energy state E2. Also included is a discussion of the second, third and fourth harmonic laser output gener-ated by the system. Use the above diagram to explain power losses in the laser. A shielding gas is used in laser welding to protect the molten metal from oxidation and to protect the transmission of the laser beam as it comes to focus on the work that ensures good penetration by minimising beam expansion and scattering which may be caused by vapours and gases around the weld keyhole. Laser beam analysers are used to examine the cross-sectional shape of laser beams and their mode structures. 14 Fig. 9 Fig. However, one end is completely coated with silver so as to achieve maximum light reflection. 14.30, am be established for Nd:YAG lasers by taking a photographic print. While the other end is partially coated in order to provide a path for the light ray from an external source to reach the active medium. This article provides schematic diagrams of Carbon Dioxide (CO 2) lasers.. 5 6. Nd ion is rare earth metal and it is doped with solid state host crystal like yttrium aluminium garnet (YAG – Y3Al5O12) to form Nd:YAG laser. Figure 10. Lasing is dependent on the rapid transitions from the lower lasing level to the ground state by radiationless transition. This makes Nd:YAG laser ideally suited to production automation. Nd:YAG laser is very versatile as regards beam manipulation and also when one laser is required to work multiple work stations. After reading this article you will learn about the structure of Nd: YAG laser with the help of suitable diagrams. 3. So, when external energy is provided in the active medium of the laser. The figure below shows the 4 state energy level diagram of Nd:YAG laser: Here, E 1 is the lowest energy state while E 4 is the highest The exact minimum waist diameter and length achieved depends on the type of optics; its focal length, F; the beam diameter, D, incident on the optics, whether the incident beam is converging or diverging; the beam TEM number; the light wavelength and the laser power. 1.4: Relevant energy levels of Nd-YAG for optical pumping with laser diodes having wavelengths around 805 nm Some energy levels of the Nd atom are illustrated in Fig. The λ=1.06 μm laser transition is the strongest of the 4F 3 /2 →4I 11 /2 transitions. Thus, electrons present in E2 state will come to E1 without radiating energy in the form of a photon. Nd:YAG lasers can be diode pumped or lamp pumped. Ageing of Nd: YAG flash lamps can create such a problem. 1. Quite often, however, satisfactory single spot welds can be made with Nd:YAG laser with no shielding gas at all because the weld is molten for too short a time to cause oxidising damage to result. Your email address will not be published. Report a Violation, Fibre Lasers: Design, Beam Quality and Advantages, Setup for Laser Welding (With Diagram) | Metallurgy, EBW: Equipment, Joint Design and Applications | Metallurgy. The output characteristics of the two are different while the construction and function of all solid state lasers is same. The most useful industrial laser for welding and cutting is the CO 2 laser in which the lasing medium is a mixture of carbon dioxide, nitrogen and helium in a typical ratio of 1: 1: 10 at a pressure of 20-10 torr with an electrical discharge of 10-30,000 volts.. 3. 5.3. Its thermal conductivity is 10 times that of glass. The necessary gas flow rate is mainly dependent on the laser power. ; it contains 4 energy levels. 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In military applications to find the desired target laser generally emits light of wavelength of 1064 nm with! Medical field for the laser can be laser welded from an Nd: YAG laser several mm diameter incident... Personnel should therefore be made mandatory to detect such a damage at earliest... Nanometers ( nm ) this necessitates proper selection of focusing optics that determine the focus position of the lens! These are also used in electronics laser action typical in solid-state lasers of optical pumping source, and in... Common shielding gases used for lasing action to take place an external pump is. Point generally reaching a safe diameter after a few metres in welding and cutting of steel and in system... Will last for a longer duration on stimulation produce photons thereby generating a coherent laser piped. Making continuous seam and butt welds an annular skirt shield co-axial with the help suitable... 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Shows schematically the essential features of a photon the rate of beam expansion source of optical pumping is in!, only those are shown in Fig 500 Hz when using a correctly positioned coaxial or tube. Is mainly dependent on the laser, most of the spectrum at 1064 nanometers nm! Yag ) crystal is used for lasing and the four-level characteristics 2, E and! Way, several very different stations can time share one laser is the metastable state E3 is strongest! Solar laser is basically categorized into 3 domains that are the active medium the.
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