Design And Construction Of Axial Slow Flow Cw Co2 Laser

[ماجستير هندسة ليزر]

chapter one

1.4i Historical Background
It was some time after discovery of gas laser that special properties of the mixture of gases used in the CO2 laser were recognized .Patel et al [5].(Patel 1964 )observed laser action in pure CO2 at 10.6μm .using a 5-m laser he obtained outputs of 1 mW CW with DC excitation and 10 mW peak with 1 μse excitation pulses [6].This report was rapidly followed by the observation of laser action in a CO2-N2 mixture (Patel 1964)[7].In this case a stream of N2 was passed through high frequency discharge and the active nitrogen so formed was mixed with CO2 in an interaction region between the mirrors of a Fabry-Perot resonator; an output of over 1mW was observed with a 20-cm long interaction region .i

Further work by Patel (1965)[8] led to the direct excitation of the gas within the laser resonator and the attainment of 12 watts from a 2-m tube containing a mixture of CO2 and air. He observed oscillation on R branch lines as well as P branch lines and noted that the flow rate of the mixture effected the gain .Moeller and Rigden (1965) [9]reported 10Wm-1 from a sealed tube containing a CO2-He mixture and also mentioned,with no details ,that a flowing mixture of CO2,N2 and He gave good results ,but the simultaneous report by Patel (1965) gave a detailed account of a water–cooled tube using a flowing CO2-N2-He mixture which produced over 50 Wm-1 [10].i


Pulse energies of 1.1mj were reported in the first study of a Q-switched CO2 laser (Kovacs et al 1966)[11].i

The year1969 marked attuning point in the development of high –power CW and pulsed CO2 laser .By 1969 a 750 –f t long CW CO2 laser based on the design of Patel et al (1965) had been built and operated at a power output of 8.8 kW (Horrigan et al., 1969)[12]. Subsequently, the development of electric discharge convection and gas dynamic laser has been resulted in the generation of CW laser powers in excess of 100 kW.i

The same year also saw the development of lasers based on convective cooling (Lavaini et al., 1969; Deutsch et al., 1969;Cool and Shirley ,1969)[13] and a report was published describing the operation of a compact closed system CW laser using rapid transverse gas flow capable of generating output powers of 1kw (Tiffany et al., 1969)[14]. i

Late in 1969 ,Bealieu (1969)[15] reported that CO2 laser emission could be obtained at atmospheric pressure and above by exciting the gas transversely so that the discharge passed perpendicular to the optical axis .i

The extraordinary in powers obtained from CO2 lasers in the 10 years, 10-15 kW CW CO2 can be purchased (Locke and Hella, 1974)[16].i

Al-aobaidi A.R M., in 1981[17] discussed the design and construction of CO2 CW laser and its application in the detection of the absorption line in some gases using opt.-acoustic technique. The designed laser has been operated with TEM00 mode with an output power of 27 W, and gas mixture ratio of CO2: N2: He /12:14.3:73.7% respectively .He used a grating to tune the laser emission for the required lines. i

Noory Z.T.,(1989)[18]described the design and construction of continuous waves fast flow CO2 laser system ,the total electric discharge consist of two equal parts 112cm with DC of 15kVand 600mA power supply output power was 30 W at pressure 60mbar and gas flow rate of 240l/min.
Al-Yaseen U. A.,(2001)[19] constructed a CW CO2 laser system .The laser cavity of different lengths 50,60,70and 80cm.The maximum output power was 60 W at flow rate 150 l/min .i

Kazim Kh. N.,(2002)[20]constructed a CW CO2 laser system of different lengths60, 70 cm with internal diameter of 2cm.The maximum output power and efficiency obtained of 83 Wand 8.5% respectively at resonator length 204 cm..i


1.5i Lasing Transition in CO2 Laser:

Lasing transitions in CO2 laser occur when the molecule is going from higher energy level of the asymmetric mode into one of the following modes see figure 1.2.i

1. The transitions to the symmetric stretching mode correspond to the wavelength of 10.6 [µm].i2. The transitions to the bending mode correspond to the wavelength of 9.6 [µm].i

Fig. 1.2 : Energy Level Diagram of CO2 Laser [22 ].i
Each of the vibrational energy level is subdivided into many rotational levels. Transitions canoccur between vibrational energy levels with different rotational levels, so there are many lasing lines around the main vibration transitions [20].i

The carbon dioxide molecule is liner asymmetric triatomic molecule. The molecule has three normal modes of vibration and the three atoms are situated on a straight line with the Carbon atom in the middle. i

In figure 1.3 the three vibration modes of CO2 molecule are illustrated: i

i1.Symmetric stretch mode (υ1). i
i2.Bending mode (υ2). i
i3.Asymmetric stretch mode (υ3). i

Vibration levels are designated by three numbers representing the number of vibration quanta in each mode associated with the particular level and written in the form (υ1,υ2,υ3 ). The molecule can simultaneously vibrate in more than one mode and can have more than one quantum of vibration energy in each mode [21].i


Electric discharge is created in the laser tube. The energy of the accelerated electrons is transferred by collisions to the Nitrogen molecules and to the CO2 molecules. .i

Nitrogen molecules help in the process of the excitation of the CO2 molecules. The first vibrations energy level of the Nitrogen molecule is very similar to the asymmetric stretching mode of the CO2 molecule (see figure 1.4), so energy can be easily transferred from the excited Nitrogen molecules to the CO2 molecules [22]..i

Figure 1.3: Oscillation Modes of CO2 Molecule [22]..i

The (00˚1) upper laser level of CO2 at 2349.3cm-1 lines 18.6 cm-1 above the υ =1 level of N2 .The (10º0) and (02º0) lower laser levels for the 10.4µm and 9.4µm bands of CO2 lie at 1388.3 and 1285.5 cm-1 , respectively .These two levels are in Fermi resonance and vibration levels belonging to different vibrations have nearly the same energy .The mixed states denoted as (100,020) lying at 1388.3 and 1285.5 cm-1 respectively .The (01º0) bending mod of CO2 lies at 667.3cm-1 For exhibit laser action ,(00º1)(100,02º0) near 9.4µm,consist of an R-J is the rotational quantum number .iThe IR transitions between various levels are governed by the selection rules [24]. .i

ΔV= ±1
ΔJ= ±1
Where, J is the rotational quantum number.i
V is the vibration quantum number.i