Design And Construction Of Axial Slow Flow Cw Co2 Laser

[ماجستير هندسة ليزر]

chapter three

3.1i Introduction:

In this chapter, the results for the design and construction have been analyzed and discussed where different parameters are varied.i
3.2 Effect of The Pressure on The Discharge Voltage:i

Gas mixture pressure plays a dominant role in determining the gas laser output. This parameter may affect drastically on the lasing action. The gas pressure is a function of the applied discharge voltage. So, the increase of the pressure means increase the discharge voltage and that lead to increase the output laser power to a certain limit.i

Figure 3.1 presents the practical results obtained in the laboratory for the present designed at an active length 0.75 m. The required discharge voltage for normal glow discharge increases as the pressure increases. The normal glow discharge obtained at high vacuum (pressure in the tube less than 0.1mbar) for 0.75m tube length was 3.5kV and at steady state was 2kV. For both one and two fold and because the input voltage to the folded system are parallel but the current is series , So the input current for each fold was 10mA.The discharge in the two fold system configuration was successful obtained in both tubes (see figure 3.2)i

Fig. 3.1: Practical Result for One Fold and Two Folds Discharge Voltage Variation with Pressure at Active Laser Media Length 0.75m.)i

Figure 3.2:Picture of the Two Folds at Discharge.)i

Fig.3.3:The Behavior of The Discharge Current as Function of Discharge Voltage For Tube Length 0.75 m at Different Pressure and Flow Rate 50liter/min.)i


As we can see from the chart that increasing the pressure for a certain value will decrease the knee of discharge point, which obey the Pachen curve. )i


3.3)i Effect of the Pressure and Gas Flow Rate on The Output Poweri

The variation of the output power of our conductive cooled CO2 laser as function of pressure and gas flow rate is illustrated by figure 3.4.)i

To optimize the output power ,we fixed the gas flow rate and change the pressure. The maximum output power obtained was at 16 mbar with 50 liter /min flow rate as shown in figure 3.4 This means that by increasing the flow rate , the power increases
The above result represent the practical result obtained in the lap where, the maximum output power obtained for only one fold =23.5W,discharge voltage =8 kV, discharge current =28mA at flow rate =50 liter /min and at pressure = 16 mbar.)i

Fig.3.4: Relative Output Power of CO2 Laser as Function of Pressure for Several Gas Flow Rates.)i

Fig. 3.5: Output Power Behavior as Function of Pressure at Different Discharge Current for Only One Fold.
As we can see from the fig.3.5, increasing the pressure of the inner gas will decrease the knee point at the pachen curve which will reduce the input power , the thing that affect the output power positively.)i

3.4)i Effect of The Discharge Current on The Power Outputi

All CO2 laser mixtures show current saturation of the output power, as the current is increased the laser output increases steadily to maximum and the decreases slowly as the current rises above the optimum value.)i

In general the small signal gain of the convectively cooled CO2 laser increases linearly as a function of discharge current for given gas flow rate .A point is finally reached where the small signal gain saturates, and as the discharge current increased beyond this point , the small signal gain begins to decrease .)i


[The decrease of the small signal gain with increasing discharge current an indication of excessive gas heating due to the inability of the gas flow to sufficiently cool the discharge .As the gas flow rate increased , the saturation of the small signal gain peaks successively reaches higher values and at higher discharge currents. )i

Fig. 3.6: Output Power Behavior as A function of Discharge Current at Different Gas Pressure.)i

The small signal gain is relatively insensitive to gas flow rate variation at low –discharge current. Fig.3.6 illustrates experimental results of output power as a function of discharge current at various gas pressure. we can notice that as soon as the ignition start, the voltage will drop and the current will build up and therefor the output power will increase till a saturation point where there is no longer increasing in the power, and after a while the power will start to fall reaching the sustain current condition.)i

The maximum output power obtained at discharge current of 28 mA, flow rate of 50 liter /min and pressure=16mbar, for only on fold is 23.5W and for the two fold (V-configuration see fig.3.2) at current 40 mA and pressure of 16mA at flow rate of 50liter/min the maximum output power is 37W.