Design And Construction Of Axial Slow Flow Cw Co2 Laser

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chapter one

1.7i Glow Discharge in a CO2Laser
Brown`s classification of discharge [27 ]is shown in fig.(1-4) .A glow discharge is characterized by a relatively high specific electric power (1-10 W/cm3 )and by “gab” between the electron temperature Te (Te 1-2 eV) and the gas temperature Tg (Tg=300 –600 K).These two feature are important for the use of glow discharge to excited nitrogen –molecule vibrations and to pump the vibrational (001) level of the CO2 molecule.i
The glow discharge in a tube, the from investigated detail before the invention of the CO2 laser, is divided in to three regions: the cathode drop re (its value in N2 is 340 V, and its length d=0.4 /p, where d is in cm and p in Torr), the anode drop region (the voltage drop is several times ten volts definite (for the given gas ) electric –field strength E proportional to the pressure and weakly dependent on the tube diameter and on the current flowing through the column .For nitrogen we have E/P= 5 V/cm. Torr..i

The current density in the column has a maximum at the tube center and drops to zero at the wall .In a round tube of radius a it varies with the column radius in proportion to the Basal function J0 (2.4 r/a)[26]..i

1.7.1.i Normal Density of Current at Cathode
The current distribution on the cathode surface is different: at low currents the cathode emission (observed at those where current flows) covers a cathode area fraction proportional to the total current ,i.e., the current density on the cathode is constant –this is called the normal density , proportional to the esquire of the gas pressure and equal to 0.240 mA/(cm.torr)2 for nitrogen and copper cathode .As the current is increased and the emission covers the entire cathode area the current density increases in proportion to the current , and the cathode drop increases slowly (it doubles approximately when the current density is increased by two orders above the normal value )[26]..i

1.7.2.iNormal Anode Current Density
Glowing spots were observed also on a solid anode at place s where current flowed into the anode . This phenomenon was observed in animmobile gas both at low pressure and at pressures of several times ten torr in air and in nitrogen. Measurements of the normal current density ja in a stainless steel anode in this pressure range yielded ja =4.2 10-4p2 ja in A/(cm.torr)2 and p in torr for nitrogen ,and ja =2.6*10-4 p2 for air .glowing layer were observed on an anode placed in a laminar produced by introducing gauze grids (at a velocity 30 m/sec and pressures 20-70 torr of the air ,N2, and CO2 )[27]..i

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chapter one

1.8i Self –Sustained Glow Discharge:
When along cylindrical glass tube with plane electrodes at its ends is filled with a gas at a pressure of ≈mmHg and the potential difference “V” between the electrodes is slowly raised, then a small current of about 10-12 A can be observed to flow through the gas.i
This current causes the ionization process in the gas .As V increases ,the ionization by collision in the gas beings to increase as well as the current rises . When the potential difference across the discharge tube reach the ionization by collision in the gas being to increase as well as the current rises. When the potential difference across the discharge tube reaches the breakdown value VB as determined by the gas, its pressure ,and the electrode spacing ,the current jumps to about 10-6 through it varies with the potential difference in a complicated way. .i

A resistor, of high resistance R, is connected in series with the discharge tube to limit the current at value of the supply voltage E. The current then takes such value that the voltage drop across the series resistor is equal to the difference between the supply voltage and the potential difference VB across the tube..i

The self-sustained glow discharge is, used to pump the longitudinal and transverse CO2 laser systems for different pressure [27]. Fig.(1-5)shows the various regions of the self –sustained glow discharge ,,which can be classified into very narrow dark space (Aston )close to the cathode. This is followed by a thin relatively weak luminous layer (the cathode glow) ,which in turne is followed by the cathode dark space ..i

Aston’s dark boundary separates the cathode glow are not always clearly visible .A sharp boundary separates the cathode dark space from the negative glow, which becomes progressively dimmer towards the Faraday Dark Space. The positive end of this is the positive column. It is either the region of uniform luminosity or regularly striated. At the positive end of this positive column there is sometimes visible an anode dark space followed by the anode glow close to it [27]..i

Fig.(1-5)istribution of the visible and dark regions, electrical field ,charges and current density in the normal glow discharge[27]..i

Consider an electron emitted from the cathode, which is accelerated in a strong field, however it performs few ionizing collisions due to its sufficient energy. Further from the cathode, the field has become weaker, the electron ionizes more efficiently. Near the boundary between the cathode space and the negative glow , the field has become very weak ,thus only the fast electron , which have not lost energy by inelastic collisions, will be able to ionize in that region .However ,a large number of electrons will cross the boundary and enter the negative glow [24]..i

In the negative glow occurs the recombination process between the electrons, which have low energy with positive ion produced from the collision between the electrons of high energy and this recombination produce the electron ion pairs. The negative glow is not affected by discharge tube dimension but effected by the cathode cross-section and the discharge voltage and current density .In the positive column the axial component of the electrical field is found to be constant at any point, it follows that the net space charge is zero (i.e. n+=n- )[20]..i

At the anode side of the positive column the anode attracts the electrons ,and the positive ions are repelled . A negative space charge is set up in front of the anode this lead to an increase of electric field as well as a rise in potential. The anode is therefore covered with a luminous sheath “the anode glow “[24]. .i

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chapter one

1.9i Output Stabilisation by the Opto –Voltaic Effect:
The phenomenon of static resistance fluctuation in a CW laser discharge due to the intra-cavity coherent radiation is well known as opt. -Voltaic or opt. galvanic effect .It can be used successfully for stabilising a single –line CO 2 laser with high accuracy, the alignment of a laser cavity, and the detection of radiation in CO2 laser.i
In general, the resistance fluctions of the discharge due to stimulated emission in a current stabilised or ballast resistor regime result in fluctuations of the power that is dissipated in the discharge, and hence in temperature variation of the plasma and the discharge tube. The relative changes of static resistance of the laser tube due to changes of the tube voltage V (or longitudinal electric field) and the discharge current I..i

where:.i
:
Changes of the static resistance of the laser tube..i

:
Static resistance of the laser tube..i

:
Changes of the tube voltage..i

:
Discharge tube voltage..i

:
Change of the tube current..i

:
Tube current..i

We shall consider the discharge laser tube as a nonlinear resistance with parametrical change of its voltage –current characteristic by the coherent radiation in the discharge tube .The operating current and voltage on the characteristic are determined by the voltage Ua of the power supply and the serial ballast resistor Rb ,the value of which must be larger than the absolute value of the negative dynamic resistance of the discharge tube ..i
The operating values of V and I0 change with the radiation power along the working –line determined by ballast resistor Rb and supply voltage Ua. .i

The impedance of the discharge tube, however, increases with radiation produced. This means that the variation of the discharge voltage ,called the opt-galvanic effect ,is in phase with variation of the radiation power and simultaneously the variation of the current ,called the opt-galvanic effect,has the opposite phase[27]. .i

.i 1.9.2 Gas Kinetic Temperature.

It is important that the gas kinetic temperature of the CO2 laser plasma be kept as low as possible. This follows both from Patel’s treatment of the gain of molecular laser (Patel 1964) which shows that the gain αT-3/2.The wall temperature, gas pressure and discharge current all affect the gas kinetic temperature, which varies radically, being maximum on the tube axis [15]. .i



1.9.3.i Helium Molecule.
The cooling of discharge gas is effectively obtained by the addition of He .The first excited state of the helium occurs at 159850 cm-1 , which is above the upper laser level (001) in only 2349 cm-1 above the ground level .The thermal conductivity of the He is about six times as large as that of CO 2 and N2.The difference between the temperature of the gas and the wall of the tubes cased by the discharge heat is inversely proportional to the conductivity of the gas .The considerable increase of heat transfer obtained by the addition of He means that the radiation production of the system saturate at higher discharge current [29]..i

As a result of the laser action, population of the (0110) level increases steadily. Since this level is close to the ground state, and since (k T) at room temperature is 210 cm –1 (where k is Boltzman constant (k=1.3805 *10-23 JK-1),and T Is absolute temperature in Kelvin ),it acts as bottleneck which prevents molecules transition down to the ground level .Such problems are overcome by the addition of He to the CO2 ,N2 gas mixture .Its addition effects the rat of dissipation of heat in the discharge tube that consequently effects the gas temperature ,and the rat of the thermal relaxation of each laser level .the effects of adding helium can be summarized by the following reactions :.i

CO2 (10˚0) + He → CO2 (00˚0) + He + K.E

He are added to the gas mixture in order to:.i

1.Empty the lower laser energy level so that population inversion is maintained. .i
2.Stabilize the electrical discharge by taking heat away from the lasing area. .i
((Advanced: (The specific heat (which determines the thermal conductivity) of He (1.24 [cal/gr* 0K] is five times that of Nitrogen (0.249 [cal/gr* 0K]).)) .i
Gas pressure inside the CO2 laser tube is 5-30 [Torr], of which 10% CO2 gas, 10% N2 and the rest is He [36][37]. .i

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chapter one

1.12i Optical Material for CO2 Laser

For optical materials are commonly used for CO2 laser. These are listed in table (1) Germanium (Ge) is the most common output coupler material for lower –power models (<100W) because of the cost advantage .It cannot be used on higher- power lasers as it absorbs significant amount of the laser beam and experiences thermal runaway at approximately 50&ordm;C.This means that ,as the temperature of the substrate increases ,its until the mirror is destroyed by fracture .i

Gallium arsenide (GaAs) and zinc selenide (ZnSe) are used as output couplers for higher power CO2 lasers. Gallium arsenide has a lower absorption coefficient than germanium and a higher thermal runaway point .It is also resistant to damage from high peak power and thus is popular for pulsed CO2 lasers. Zinc selenide has an even lower absorption coefficient, but its thermal conductivity is also low..i

Zinc selenide has the advantage of transmitting visible light. This makes optical alignment of the laser much easier. Both of these materials are widely used ,with zinc selenide being more popular in high-power kilowatt range for CW CO2 lasers.Because the index of refraction of these materials is high,antireflaction coatings are required for all transmitting optical components[8] ..i

1.13.i Optical Cavity Types.

The optical design of the cavity must taking into account the actual path of the light rays (geometric optics), mirror losses (such as absorption), diffraction (physical optics), and the density inhomogeneities of the flowing gas that cause phase distortion in the laser beam [29]..i

1.13.1.i Resonators

The most simple optical resonator consists of a pair of plan or spherical mirrors located opposite one another .They are center to a common optical axis and are perpendicular to this axis ..i

There are basically three types of optical resonators : plane parallel resonator (A),hemispherical resonator (B), spherical resonator (C)..i

For laser in the low to medium power range (1 mW-500W) ,the hemispherical resonator is mainly used .Its features include high output powers with relatively uncritical mechanical adjustment. The output power depend on how much of the laser- active material use ..i

For the plane parallel resonator (A),in which the light beam is only refected and not modified in shape ,it must be ensured that both plane parallel mirrors are adjusted exactly parallel to another .This type of resonator is the most difficult to adjust and to maintain in a correctly adjusted condition [2]..i

Figure 1.7: Types of resonator [2]..i

The spherical resonator (C) is the most simple to adjust; but has the advantage that undesired transverse modes can easily start to oscillate .This means that the laser power is split up over a number of modes which are separated spatially from one another and which cannot be focused to acommon point as with longitudinal modes ..i

The hemispherical resonator has become very popular since it exploits in special manner the desired mode characteristics of the plane parallel resonator and the advantages of adjustment associated with the spherical resonator [32]. .i

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chapter one

1.14i High Power System. (FOLDING)

The use of very log tube can introduce problems not encountered in smaller systems.practically log tube are usually divided into sections(with alternate anodes and cathodes )to keep the excitation potential to reasonably low value .However ,great care must be taken to eliminate all possibly reflections ,particularly wall reflections. It is necessary to break the system up into optically isolated sections each with smaller gain .Further problem encountered with very long systems is the negative lens effect produced by the refractive index gradients in the discharge plasma [30][31] .i

The folding methods which have been used are illustrated in figure 1.8 ,the most widely used is a pair of plan mirrors each set at 45˚ to the primary beam direction which reverse and laterally displace the beam .A variant of this idea is to use a corner cube in place of pair of mirrors .This configuration is exceptionally stable against mechanical vibrations ,through it dose involve an extra reflecting surface

(a)Two mirror or rooftop prism fold

(b)Corner cub fold

Figure (1.8):Schematic diagram of two-mirror and corner-cube folds[31].

1.15.i Cooling of CO2 Laser:
There are two basic ways of disposing of the waste heat .The first is to use the heat conductive properties of the gas and the structure containing it to carry off the heat Conductive –cooling is simple but ineffective because of the low thermal conductivity of the gas .Laser that use this cooling method generate an output of about 50 W/m of active length and therefor, must be relatively long to produce industrially interesting power level ..i

The second way to remove waste heat is to hot gas and replace it with cool gas by forced convection. Such flow –cooled laser are often called often called fast-flow lasers. Where as almost no heat leaves the active region of conduction –cooled lasers except through the tube walls with flow cooled lasers no significant heat flows to or through the tube walls compared with the amount of heat carried out of the active region of the gas flow .Power output for this type is not dependent on the length of the active region .It depend only on the mass flow rate of the gas and can be as high as 120 to 150 W of power output for each gram per second of mass flow [33][34]..i


A. Diffusion – Cooled Laser..i
In a diffusion cooled laser, waste energy is rejected in a characteristic time approximately that of the diffusion time (τD). If an electrical discharge column of diameter D is assumed, the number of mean free paths during which the energy diffuses is given by D/ where is the mean free path of the CO2 molecules in the gas mixture .The mean free time between collision is given by the ratio of D to the molecular speed. Since diffusion is a random- walk process, is not equal to but is equal to



Since the power achievable from a laser is approximately inversely proportional to such characteristic cooling time ,the power (PL ) of a diffusion- cooled laser is proportional to


Where


is the gas density.


B.Connective –Cooled Laser..i
If the gas is moved at speed is rejected in a characteristic time (τF ) which is given [35]..i




For the same active volume and gas density ,the ratio of the laser power achievable with a diffusion-cooled (PLd ) and with convection–cooled(PLc)laser is simply proportional to the ratio of the characteristic cooling times :.i



Since eq. (1-8) is much than unity for even relatively slow gas flow velocity, the advantages of convective cooling over diffusion cooling are readily apparent. .i

Assume that we have a rectangular volume of gas as schematically illustrated in figure(1-9),having a cross-sectional area A and a thickness χ .We dissipate PE watts of electrical power in the volume .We extract PL watts of laser power from the volume and PH watts of heat by convectively flowing the gas through the volume .If the laser extraction efficiency is η ,then



And



Where,.i



Where
[IMG][/IMG] is the gas density,
is the specific heat of the gas ,
is the velocity of the flow, and is the volume .Substituting (1-7) into (1-8 ) we obtain:





Figure (1.9): Schematic of Volume of Gas of Length x and Area A[36] .i

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chapter one

1.16i Output Power Design Equations:

The first aspect to be consider is the dependence of the output power ,P(w) on the cavity ,the output mirror transmission T ,and the saturation I, the output power is given by[37] :i

Where, :i

A is the cross sectional area of tube diameter (cm2):i

Iο the output intensity(w/cm2):i

K an empirically determined factor relating the effective uniform output intensity I to actual output intensity.:i

It is found experimentally that for TEMοο mode operation the proportionality constant K is 0.8.:i

It is convenient to write the expression for Iο in equation (1-13) as product of the ratio of the effective intensity Io /Is times the saturation intensity Is, the resulting output power is : :i

where,:i

dt is the tube inner diameter (m):i

a general expression for Io/Is as a function of the signal gain ,active laser cavity length La (length of discharge providing gain )in m, mirror transmission T ,and losses l is given by::i

So, the output power is ::i

Where
And

Where,:i

. ℓ is the active medium loss
is the absorbing confession.:

i


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chapter two

Co2 laser system

2.1i Introduction:

The first start to design CO2 laser system that we should know the relative optical output power that we wont to get it and the efficiency of the system that we should work with it .i

In this project the target optical power was 500W and the efficiency that we assume to work in 10%.So, the input electrical power should be of about 5000W and because we work in slow flow this mean that the available output power with respect to the length of the discharge tube of about 50 W/m.. So for 500W the overall active medium length of about 10m.i

The use of very long tube can introduce problem not encountered in smaller systems. So, long tube were divided into section with alternate anode and cathode to keep the excitation potential to reasonably low value .It divided into seven folds each folds consists from two discharge double jacket tubes with parallel input discharge voltage ( see figure 2.1) .By using stability condition of the optical resonator we found that the radius of curvature of the higher reflector mirror should be greater than or equal to 10m we chose hemispherical stable resonator to provide higher gain and easy in the alignment process..i

Using the above input parameters to the system and by using eq: (1-16) ,(1-17),(1-18),(1-22) and (1.23). at different variable parameter a compromise has been made between these parameters of the optical resonator using forward and backward calculation to specified the optimum values of diameter ,the best radius of curvature of higher reflector mirror ,the beam waist of the output window gain coefficient , Frenal number and the best output power that we can get from all these variable parameter (see table 2-1)..i

We found from the above calculation that the best discharge tube length equal to 0.75m, tube inner diameter equal to 0.17m. the best mirror reflectivity of 60%. Assume that we work at an output beam of TEM00 mode ..i

Fig.(2.1):Multi Folds Structure for Our Design System ..i

Table (2-1): Theoretical Calculation for Output Power at Different Variable..i

In the present work five different electrodes shapes and discharge tubes have been designed and constructed .The details for these terminals, discharge tubes and power supply unit, cooling system, vacuum circuit and gas supply have also been described. .i