chapter one
[SIZE="4"]i1.1 Introduction[/SIZE]
Carbon Dioxide (CO 2) laser is the gas discharge device .It operates by electric excitation of an active medium that is a mixture of helium, nitrogen, and carbon dioxide .The excited CO2 has many energy levels it can drops to, allowing emission on groups of lines around (9.6) and (10.6) µm..
All CO2 lasers are rated in Watts or kilowatts. The distinguishing characteristic of the CO2 lasing process that make these sustained power levels possible is its relatively high efficiency at least compared to most other common gas lasers. The typical electrical power into optical power out efficiency of a CO2 laser may be any, where from 10 to 30 percent [1].
Unlike the other lasers producing visible or near IR light, the output of a CO2 laser is medium IR radiation at 10.6µm .At this wavelength, normal glass and plastic are opaque, and water completely absorbs the energy in the beam .The 10.6 µm energy is ideal for cutting, engraving, welding, heat treating and other industrial processing of many types of materials including:
****ls, ceramics, plastics ,wood ,paper ,cardboard fabric ,composites.
The CO2 laser represents the classic heat ray of science fiction .The basic construction of CO2 laser is a gas filled tube between a pair of mirrors excited by an electrical discharge. ****l coated mirrors (e.g. solid molybdenum, gold or copper coated glass) may be used for the high reflector (totally reflecting mirror). Germanium or zinc selenide which have very low losses at 10.6µm are often used, but must be cooled to minimize losses for high power.
There are many different types of design for CO2 laser divided into different groups but the basic principles are the same.
We can summarized the properties of these lasers as fallow:
1
i1. High output power. Commercial CO2 Lasers produce more than 10kW continuously.
i2.Output spectrum is in the infrared (IR) spectrum.
i3. Very high efficiency (up to 30%).
i4.Can operate both continuously or pulsed.
i5.Average output power is 50 [W/m] for slow flow of gas, and up to few hundreds [W/m] for fast gas flow.
i6.Very simple to operate, and the gasses are non-toxic.
i1.2 Thesis Contents
In this thesis, the basic theory of CO2 laser and the parameters that effects on its reviewed in this chapter. Chapter two represents the basic design aspects for slow flow CO2 laser. The details of the laser system design and construction are described in chapter three. Results, discussion, conclusion and suggestions for further works will be present in chapter four.
i1.3 Types of CO2 Lasers:
There are many types of CO2 lasers, all based on the same physical principles. The difference between them is in their structure, excitation mechanism, and the output power radiation. CO2 laser can be separated into six categories[2] :
i1. Flowing CO2 gas lasers
a.The Slow Flow CO2 Laser.
b.Fast Flow CW CO2 Laser.
i1.Fast Axial Flow.
i2. Fast Transverse Flow.
i2. Sealed-off CO2 Laser.
i3. The Gas Dynamic Laser.
In these lasers a fresh gas mixture is flowing continuously through the laser tube while lasing lasts. Flowing gas is used when the maximum power is needed out of the CO2 laser. The gas flows along the tube and is released out into the atmosphere (since it is non-poisonous)
These lasers are very simple, and the requirements from the gas purity are small. Hundreds of watts can be achieved at the output of these lasers. There are two different types of flowing gas CO 2 laser slow flow and fast flow [3].
The slow flow of CO2 laser reliable, robust, easy to operate, and are produced by the industry in many versions with powers ranging from tens of watts to a kilowatt . Figure .1.1 shows a typical setup of a conventional diffusion cooled (slow flow type) CW CO2 laser.
Figure (1.1) Schematic Drawing of Slow Flow CW CO2 Laser [3].i
In these systems, the heat generated by the electrical discharge is removed by heat conduction through a water jacket surrounding the laser discharge tube .In order to continuously refresh the mixture, the gas is slowly pumped through the tube. Laser out put power can be done only by increasing the tube length [4]. i
For a fast flow system, mass transfer or the flowing gas removes the heat produced by the electric discharge. The output power is largely dependent on the mass flow rate (amount of the laser gas medium flow rate per second )of the gas, which can be as high as 120 to 150 W of output power for each gram per second of mass flow .The fast flow system can be divided into two major types as the fast-axial –flow and transverse –flow system .
Summary of CO2 lasers according to groups:
•Gas laser.
•Emit in the infrared (IR) spectrum (λ = 9-11 [μm]).
•Electrical excitation.
•Four level laser
•Continuous wave, although pulsed operation is possible.
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