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