Laser basics

Keep in mind that make-up air requirements can be substantial. For example, if your ventilation system is rated at 500 cfm, the replacement air is the volume of air that is contained in a room roughly 8'x8'x8'. We are talking about evacuating that much air per minute. A temporary make-up air system can be as simple as opening a window in the work area, although this may not be very practical in a commercial shop. Users often provide replacement air by running a length of duct from the outdoors to (near) the laser. A number of commercial systems incorporate heat exchangers to temper the air temperature closer to room temperature as it enters the shop.

outlet near the fan to plug the fan into and run a wire to a wall switch near your laser, which allows you to conveniently turn the vent system. Another solution would be to use a remote controlled switch device, like you have for your television, with a receiver connected to the on/off switch on the fan. This remote device must be able to handle the current of the exhaust fan, so be sure to get an industrial, heavy-duty-rated remote switch that will be up to the task. Much like your tv remote, remote switches are battery operated, so they can be placed within easy reach of your laser system. Make-up Air, one important design criterion for any heating, cooling or ventilation system is called make-up air or replacement air. This is based on the idea that whenever you pump air into a sealed room (air pressure) without providing a means for the same volume of air to escape, the result is to pressurize the atmosphere in the room. Or, if your laser vent system sucks air out of the sealed room, that creates a vacuum, unless you provide a source of make-up air to replace the air you are evacuating. This serves the same purpose as having air returns in each room of a house with a forced-air heating or cooling system. A source of make-up air is important in any laser vent system. Without adequate replacement air, system efficiency can drop and, in extreme cases, laser users have reported symptoms of nausea or light headedness from breathing laser fumes due to the inability of the vent system to remove the fumes from a room operating under vacuum pressure.

If you have a longer duct run or more than three or four bends in the duct, you will need to go to a larger size fan and possibly a slightly larger duct. Consulting an hvac company may be cartridge a good idea, just to be sure. An important design criterion is the location of the exhaust fan along the duct. Keep in mind that the electrical motor and the fan blade combined can make a droning noise which might be in excess of 50 decibels. So, mounting the fan in an outside location is best whenever possible. If you cannot locate the exhaust fan outside, i would recommend moving it as far away from the laser system as possible. Locating the exhaust fan at the far end of the duct adds an important safety bonus in addition to a lower noise level. With the fan located at the far end of the duct, it is creating a vacuum pressure that essentially sucks the air out of the duct. One safety advantage is that if there is a break in the seal that creates a small air leak, room air will leak into the duct rather than dangerous fumes leaking out into the work environment. Configuring the fan at the far end of the duct will also reduce the odor and fumes that otherwise might drift into the work area, making it safer in the long term.

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What About the fan? Of course, the fan itself is an integral part of an exhaust system. The cfm and pressure specifications we mentioned earlier are important when we choose the size and type of fan for our exhaust system. The type of fan we should use is an impeller unit with, typically, a 1/2, 3/4 or 1 horsepower electric motor to power. Choose a fan that is rated for continuous duty and has bronze or ball bearings in the motor. An exhaust fan with these ratings will give you many years of dependable service. Since the length and size of the duct and the number of bends in the run will reduce the amount of airflow and pressure, it is important to choose the correct size exhaust fan. Calculating the exact airflow will require an hvac eucerin engineer or using the more common method of slightly oversizing the fan size to give you adequate flow and pressure. A simple rule of thumb is, if you have a 20"x24" table laser system and a duct run of 20' or less, a 1/2 horsepower fan that is rated for 660 cfm at 4" static pressure will be enough to meet the system manufacturers specifications.

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Coatings for high-power CO2 lasers operating.6 microns are not very reliable, so bare surface reflectors should be used if possible for such applications. Another technique is to use a plane parallel mirror which then acts as a fabry-perot etalon and has any degree of reflectivity from 0 to a value given by Equation k, depending on wavelength. The reflectivities and physical properties of presently available materials for use.6 microns are given in Table. Output spectrum t he output spectrum of the co2 laser is determined by the exact mirror separation and its relationship to the co2 amplifying medium. When the discharge is on and the tube is not lasing, the medium exhibits a single-pass gain given by Equation l, where alpha is a function of frequency (see fig. 3, and refer to "CO2 Laser Theory" ). This gain curve is a series of bumps or lines about 50 mhz wide where each line corresponds to one of the j rotational levels of the 001 vibrational state with an excess density over the j 1 level in the 100 state.

This leads to the normal full-angle beam divergence in the far field given by Equation. At.6 microns, and with handen a cavity length of 1 meter, the waist diameter is 2 w.6. If large-diameter tubes and mirrors are used, the radial or off-axis modes may be generated, thus contributing to the total output power. However, for single-frequency operation the size of the longitudinal mode must be increased by increasing the curvature of the mirrors. It is usually desirable to make the transmitting mirror optically flat. Thus, with R2 infinity, the spot sizes at the two mirrors are given by Equations g and.

If R 1 is now increased to 50 meters, the spot diameter increases to about 10 mm and a single mode begins to fill a reasonable volume. Ideally, the curvature could be further increased, depending on available optical components. Output from the cavity can be obtained in various ways. The best of these is to use an output transmitting mirror with a uniform transmission coefficient over the whole aperture. Other techniques include hole coupling, spot coupling and iris coupling; but these tend to generate irregular modes in the cavity and should be avoided. When coupling through a transmitting mirror, it is possible to see the Fresnel reflection (whose magnitude is given by Equation J) from the bare mirror material as the feedback for the oscillator. Otherwise, the mirror may have a multilayer dielectric coating on one face to achieve any desired reflectivity while the other face has an antireflection coating.

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The engineer should be prepared to perform experimental exploration of his own system. Laser optics, the most common electromagnetic laser cavity consists of two mirrors with circular apertures, as shown. The reflecting surfaces are segments of spheres with radii. R 1 and, r 2 and separated by a distance,. The best alignment occurs when the line joining the centers of curvature is coincident with the geometrical axis through the mirror centers and the laser medium. For gas lasers, these mirrors may be directly mounted on the discharge tube, or one or both of them may be externally mounted.

If externally mounted, the vacuum exists between optically polished flat transmitting windows mounted at the Brewster angle given by Equation A wher e n is the refractive index of the window material. The advantages of a brewster-window system are that the laser radiation is polarized and that mirror tuning is independent of the tube. Its disadvantage is that the windows must withstand the high-power laser radiation field inside the optical cavity. If system aperture is sufficiently large, the radiation pattern inside the laser cavity can be approximated by a summation of normal modes-each with a different frequency and spatial distribution determined by the mirror curvatures and spacing. The longitudinal modes have a radial gaussian power distribution approximated by Equation B where the gaussian width, w, or spot size varies as a function of axial position. One such mode is sketched in Fig. For convenience the confocal geometry is shown with R 1 R2 l, where the spot size at the beam waist is given by Equation c and at the mirrors by Equation. The spot size generally increases as a function of distance from the beam waist; the relation is described by Equation.

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Horrigan and, whitehouse ; more detail can be obtained by referring to their reports. Optimizing the waar co 2, laser, performance of CO2 lasers may be optimized in several ways: Maximize multimode power; maximize single-mode power; maximize efficiency; and/or minimize size and complexity. The parameters that affect such optimization for flowing gas sale systems are: Tube length, diameter and wall temperature. Gas mixture, pressure, and flow speed. Optical mode control, wavelength control, and output coupling. Electrical discharge control and current density. In addition, for sealed-off CO2 lasers, it appears that the gas purity and tube materials are also important. Optimization is by no means simple, because the various parameters are strongly interrelated. All results, therefore, should be viewed only as indicative of performance trends.

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Moeller and Rigden with the addition. Other molecular systems have been made to lase, but so far co 2 is the most important power producer. Many parameters affect the design and operation of the co2 laser. The gas discharge can be powered with dc, ac, rf, repetitive pulses, or any combination thereof. The mirrors can be fixed, rotated for nekwervel Q-switching, or vibrated for reactive q-switching. The discussion here will be limited to dc excitation with fixed mirrors. Most of the technical data were obtained from.

With a fully reflecting mirror on the left and a partially transmitting mirror on the right, the device becomes a laser which radiates in the far infrared.6 microns. The co2 laser is comparable in simplicity, if not in size, to the microwave klystron. Both have a high-q electromagnetic cavity, a highly efficient power-output coupling technique, and an active medium fed from a simple dc power supply. Also, with slight modifications either can be used as an oscillator or an amplifier. The possibility of using molecular vibrations for laser action ayurvedic was clearly pointed out. More recently, patel has described in some detail the laser action on the vibrational-rotational (V-R) transitions of co 2 in an electrical discharge. Shortly afterwards, progress towards high power and efficiency was achieved. Patel with the addition of N2 and.

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Understanding co2 lasers, a detailed treatment of a laser system that holds great promise for high-power applications includes design procedures and parameter tradeoff considerations. Whitehouse, manager, laser Advanced development Center, raytheon., waltham, mass. The vergelijken great interest in carbon dioxide lasers stems from their continuous power capability, high efficiency and ease of construction. Table 1 graphically illustrates their advantages over other gas lasers. The co2 laser system is shown. Three gases (CO2, N2 and He) are mixed and fed into one end of a discharge tube at a pressure of a few torr. The gas flows down the end of the tube in about one second and is pumped out the far end with a mechanical forepump. An electrical discharge is maintained between the metallic end flanges of the tube. The ballast resistance is required because of the negative dynamic resistance of the discharge.

Laser basics
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