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