The approximate focused spot size for these lasers is usually estimated from the following formula: θ = the divergence angle (radians) of the laser beam as it leaves the laser or collimator. Nd:YAG laser is very versatile as regards beam manipulation and also when one laser is required to work multiple work stations. 16 Fig. 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. Generally, the total loss of laser power for an Nd:YAG laser and fibre optic system is no more than 10— 15%. The former is used for oscillating continuous wave (CW) and the latter for the pulsed wave (PW) light. The energy state E3 is the metastable state and exhibits longer lifespan. 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. Because glass and acrylic are transparent to laser beam of 1.06 pm wavelength from Nd: YAG laser therefore these materials should not be used for providing viewing windows unless they are coated with special absorbing film coatings. Construction and working of Nd YAG laser - Duration: ... winnerscience 54,739 views. 14.30, before diverging again. 14.27. 2. 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.. This crystal was developed by Geusic et al in 1962. This laser generally emits light of wavelength of nearly 1.064 μm. Differentiate He-Ne, Nd-YAG and Semiconductor lasers. In this way, several electrons on stimulation produce photons thereby generating a coherent laser beam of 1.064 µm. Regular eye tests for laser personnel should therefore be made mandatory to detect such a damage at the earliest. ; it contains 4 energy levels. The wavelength (in m) corresponding to a transition between levels 1 and 2 is obtained from:. 14.36 The basic sheet metal joint configurations that can be laser welded. Reasons for using Nd: YAG Laser in medical field • It has very high depth of penetration in tissues (>5 mm) • Can be used as an endoscopic laser because the laser … The figure below shows the 4 state energy level diagram of Nd:YAG laser: Here, E1 is the lowest energy state while E4 is the highest energy level. A simulation model for a solar laser is designed using Ce:Nd:YAG as an active medium. ... (Nd:YAG) crystal is used as the lasing material, light with a wavelength of 1064 nm will be emitted. Apart from the joints shown in Fig. For the transition from energy level 4I 9/2 to 4F 5/3 energy level. 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. 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. Due to this more number of electrons will be present at the metastable state E3. 3. 14.41. Gain medium / Laser medium Laser light from Nd:YAG laser with its wavelength of 1.06 pm is particularly dangerous to the eye, because the lens in the eye can focus this wavelength to a very small spot on the retina and cause serious eye bums. Some typical joint configurations which assist laser beam to joint access and part location are shown in Fig 14.37; these are practical to engineer in thickness of 3 mm or more. Explain the terms "quantum efficiency" and "operating efficiency." So, the electrons in this particular state will last for a longer duration. Nd: YAG laser generates laser light commonly in the near-infrared region of the spectrum at 1064 nanometers (nm). Laser beam analysers are used to examine the cross-sectional shape of laser beams and their mode structures. Nd:YAG lasers can be diode pumped or lamp pumped. The laser rod is an Yttrium aluminium garnet (YAG) consisting of Y3A15O12 isometric crystal implanted with carefully distributed 1 % neodymium (Nd) ions. 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. When making continuous seam and butt welds an annular skirt shield co-axial with the laser beam, shown in Fig. In ND : YAG laser, Nd 3+ ion is an active medium. 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. Figure 8 shows the energy-level diagram of a Nd:YAG laser. 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°. Alternatively, the beam can be sequentially switched, 14.40 (b), to different points, often up to 30 m away. Simplified diagram showing transitions for Nd 3 + in a Nd:YAG laser. The essential component found in a Nd:YAG laser is the crystal responsible for 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 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. In normal state, most of the chromium ions are in the ground state E 1 . Copyright 10. The output characteristics of the two are different while the construction and function of all solid state lasers is same. Nd ions have two absorption band, and excitation is done by optical pumping, either by flash lamps for pulsed lasers, or by arc lamps for continuous wave lasers. Its thermal conductivity is 10 times that of glass. 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. Lasing is dependent on the rapid transitions from the lower lasing level to the ground state by radiationless transition. Establishing Conditions for Nd: YAG Lasers: Keyhole welding is usually not possible with Nd: YAG lasers with output power below 500 W. At low average powers (400W) and the related pulse timing of 4-8 m sec, the penetration depth is usually limited to the size of spot diameter which is of the order of 0.5—1 mm. Energy level diagram: ! Moreover, the laser can be positioned some distance from the production line and laser beam piped to it. Major components of a resonator. 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. The figure below shows the 4 state energy level diagram of Nd:YAG laser: Here, E1 is the lowest energy state while E4 is the highest energy level. Figure 1: Energy level structure and common pump and laser transitions of the trivalent neodymium ion in Nd 3+:YAG. Describe the laser action occurred in Nd-YAG lasers. 14.29. 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. 4. Privacy Policy 8. So, in this way electrons by gaining single photon of energy releases the energy of 2 photons. These energy levels are those of Neodymium (Nd3+) ions. Following this is a more detailed description of the The four energy levels designated E 0 to E 3 and a laser transition of Nd 3+ ions are shown in Fig. 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. The common shielding gases used for laser welding are argon, CO2, helium and OFN (oxygen-free nitrogen). 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. Even if the concentration of Nd3+ ions in crystal increases, the spectrum of the oscillating light docs not become broad, because the valency and ion radius of Nd3+ are not much different from those of Y3+. 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. The optical fibre assemblies used to transmit laser welding powers are purpose-made and quite different from those used in electronics. 8 Fig. Hence, lasing efficiency depends mainly on … Therefore, electrons that were excited to this state by the application external pumping will not stay at this state for much longer duration and comes to lower energy state E3 very fastly but without radiating any photon. 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. Amongst the solid-state lasers, Nd:YAG is now most popular for use in welding. Plagiarism Prevention 4. 5.3. However, care should be taken when using short pulses (< 1 m see) and high power (say 1 kW) as weld undercutting can occur through excessive vaporisation and material ejection. 2. 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. Required fields are marked *. Process B indicates the 879 nm direct pumping which is to pump laser active ions from the Z 1 … Also, as the system is equipped with optical resonators so, more number of photons will get generated as the pumped energy will get reflected inside the active medium. [enter image description here] The energy levels E 1, E 2 and E 3 of Nd along with many other levels of YAG. 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. Use the above diagram to explain power losses in the laser. 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. 14.32 gives reliable weld protection. The necessary gas flow rate is mainly dependent on the laser power. Nd:YAG, liquid dyes, gases like CO2 or Helium/Neon, or semiconductors such as GaAs. Draw and label a diagram showing power flow in an Nd:YAG laser. The four energy levels designated E0 to E3 and a laser transition of Nd3+ ions are shown in Fig. The YAG rod and excitation lamp are installed in the cavity of a reflecting mirror. The most common Nd:YAG emission wavelength is 1064 nm. Active mediums contain atoms whose electrons may be excited to a metastable energy level by an energy source. The resonator or optical cavity of Nd:YAG laser consists of a flast lamp, laser rod, reflector and mirrors. Then the electrons present in the energy state E1 gains energy and moves to energy state E4. This is due to the fact that short wave length of 1.06 mm from Nd: YAG laser can be transmitted through a fibre optic with very little loss of power. Nd: YAG laser is a four-level laser system, which means that the four energy levels are involved in laser action. 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. When light is focused, the rays converge to a very small waist diameter, d, and length, L, Fig. For Nd: YAG laser welding a simple side tube shielding device, as shown in Fig. It is a 4 level systems i.e. Beam transmission efficiency is also impaired if the fibre is bent too tightly. Fig. 5. ENERGY LEVEL DIAGRAM FOR ND: YAG LASER 5. 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. Although continuous wave Nd: YAG lasers have also been developed but al present are not widely used for welding. 3. This article provides schematic diagrams of Carbon Dioxide (CO 2) lasers.. However, a focused beam expands much faster beyond the focus point generally reaching a safe diameter after a few metres. 14.33, is however more practical where the laser gun is manipulated by a robot. 5:50. 14.34. The Nd3+ ions form the oscillating medium to give four level laser action typical in solid-state lasers. Definition: Nd:YAG laser is the short form used for Neodymium-doped Yttrium Aluminium Garnet. ; it contains 4 energy levels. For example a gas (low rate of 10 to 20 lit/min. 12 Fig. Here, only those are shown which are significant for optical pumping with laser diodes and which are important for the laser process. Due to doping, yttrium ions get replaced by the Nd3+ ions. Figure 10. It is a solid state and 4 level system as it consists of 4 energy levels. 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. The fibre optic beam delivery systems are by far the most simple and versatile. 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 Nd:YAG laser is a four-level system as depicted by a simplified energy level diagram in and Figure 2. The shape of the cavity is an elliptical cylinder or a double ellipsoid; some typical cavities used in practice are shown, in Fig. Helium can also be used with Nd:YAG laser but it is reported to cause more weld porosity than with the use of OFN. Content Guidelines 2. Image Guidelines 5. 14.30, am be established for Nd:YAG lasers by taking a photographic print. Optical resonator: The two ends of the Nd:YAG rod is coated with silver. APPLICATIONS • Used widely in medical field. 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. Then the Nd ions get raised to higher energy levels and their transition produces a laser beam. 5 6. Unfortunately retina does not register pain caused by such blind spots thus the damage caused to the eye may not be realised immediately. Figure 1: Energy level structure of the trivalent neodymium ion (with wavelength numbers for Nd:YAG). The Nd:YAG laser is the most common laser used in laser designators and laser rangefinders.. Consider a ruby laser medium consisting of three energy levels E 1, E 2, E 3 with N number of electrons. Report a Violation, Fibre Lasers: Design, Beam Quality and Advantages, Setup for Laser Welding (With Diagram) | Metallurgy, EBW: Equipment, Joint Design and Applications | Metallurgy. Focused Spot Size, Depth of Focus and Focus Position: To achieve the required power density for keyhole welding (103 to 105 W/mm2) selection and maintenance of the focused spot size is most important. 14.27. Basically when the external energy source is provided then the electrons from lower energy state moves to higher energy state thereby causing lasing action to take place. Four-level energy diagram. ; it contains 4 energy levels. Nd:YAG laser is basically categorized into 3 domains that are the active medium, pumping source and the optical resonator. So, in this section, we will discuss the working of Nd:YAG laser with the help of the energy level diagram. Metallurgy, Welding Processes, Laser Welding, Nd:YAG Laser. 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.. are suggested. At powers from 3 to 5 kW, rates of 15-30 lit/min, and for those between 5 and 10 kW rates of 25 to 40 lit/min. So, for lasing action to take place an external pump source is required. Common TEM modes. Thereby attaining population inversion. 1.4. Fig. TOS 7. Moreover the focus position of the incoming beam has to be precise in relation to the end of the fibre. Laser output beam diameters increase with the power rating of the laser, for example 1,5, 10 and 25 kw lasers have beam diameters in the order of 10, 25, 40, and 70 mm respectively. Table 1 illustrates various types of material currently used for lasing and the wavelengths that are emitted by that type of laser. 9. While down ward transition shows the emission of energy / light or lasing present in the Neon atom only. 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. 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. The energy level diagram of Nd:YVO 4 is shown in Fig. The average power density across the diameters is in the order of 6 to 13 W/mm2; the actual concentration of power being distributed according to the beam mode (see Fig. The λ=1.06 μm laser transition is the strongest of the 4F 3 /2 →4I 11 /2 transitions. 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. The ability to control to the current pulse parameters allow the control of weld penetration depth, profile and appearance. Figure 1 shows the Nd:YAG energy level diagram. 14.28 shows schematically the essential features of a Nd: YAG laser welding unit. Figure 6.15: Energy Level Diagram of a Nd-YAG Laser As can be seen from the energy level diagram, Nd lasers are four level lasers. Energy level diagram for Nd:YAG. This is because the side tube provides good visual access to the target area for the spot weld. at a pulse repetition rate of 500 Hz when using an average power of 1 KW. 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. This type of laser also finds its application in medical field for the surgical purpose. 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. On the other hand, several very different stations can time share one laser. Continuous oscillation is possible with YAG. After reading this article you will learn about the structure of Nd: YAG laser with the help of suitable diagrams. The incident beam diameter, Fig. * How does Nd:YAG laser work? 14.39, without a significant loss of power. Also, the doping concentration is around 0.725% by weight. Maximum light is pumped into the laser rod with the help of reflector assembly thus exciting the Nd ions to produce laser beam by spontaneous and stimulated emission. Also included is a discussion of the second, third and fourth harmonic laser output gener-ated by the system. 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. A simplified energy-level scheme for Nd:YAG is shown in Fig. The figure below shows the road like the structure of Yd:YAG laser: Active medium: This is also known as the laser medium and is the middle portion of the structure i.e., Nd:YAG crystal. will suffice for a laser up to 3 kW capacity. Fabry-Perot resonator. When the krypton flash lamp is switched on, by the absorption of … Explain the construction and working of He-Ne lasers with a suitable energy level diagram. a telescope in reverse). 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