Lasers can be divided into gas lasers, solid state lasers and liquid lasers according to the active medium used.

Gas Lasers:

Figure 2.3: Scheme of energy levels of He-Ne laser

The active medium of CO₂ laser is a mixture of CO₂, helium and nitrogen gases, the approximate constitute is CO₂:N₂:He::0.8:1:7. Pumping is realized by AC or DC electrical discharge. First most of the electrical discharge energy is absorbed by nitrogen gas, only a small part of the energy is absorbed by CO₂ molecules directly which raise them from ground state (000) to upper state (001). Large amounts of CO₂ molecules collide with the nitrogen molecules and gain the excitation energy. Once excitation is achieved, the CO₂ molecules at (001) state will give out energy and jump to lower energy state (100) or (020), thus giving out laser light at frequency 10.6m m or 9.6 m m respectively. The remaining decay from state (100) to (010), (020) to (010) or (010) to ground state (000) will dissipate energy in the form heat instead of light.

Figure 2.4: CO₂ laser energy level

Helium gas has high thermal diffusivity, while the thermal diffusivity of CO₂ is low. This combination makes CO₂ gas to have high lasing efficiency compared with other lasing materials and in the same time, the system also has good thermal diffusivity. Thus the role of helium is evident. The cooling of the carbon dioxide gas mixture becomes very important for the stable operation of CO₂ lasers, there are three basic cooling designs.

Slow Flow Lasers:

At too high temperatures, the lower energy levels of CO₂ molecules inside the laser can not be emptied to ground level fast enough, population inversion condition might be disturbed, and lasing might stop. Thus the lasing action is limited by a upper temperature, T_lim. For slow flow lasers, cooling is through the walls of the cavity, typically gas flow is about 20 l/min, coolant flow is about 7 l/min at room temperature.

Figure 2.5: Cooling designs for CO₂ lasers

The highest power gain per unit cavity length can be computed as:

P=4p h kL(T_lim-T_c)

Where h is the energy conversion efficiency from pumping to optical, typical value 0.12; k is the heat conductivity of He, 0.14W/mK at 0 degree; L is the length of cavity, T_lim is the highest lasing temperature, 250° C, T_c is the coolant temperature, typical value 10° C.

For the above values, P is around 50W/m, not very high. To reach a higher power, the cavity has to be very long.

Fast Axial Flow Lasers:

Such lasers use gas convection for cooling, gas flows at the speed of 300~500 m/s through the discharge zone, the flow, discharge and optical oscillation are all along the optical axis, which favors the symmetrical power distribution of the laser beam. The typical power gain is about ten times more than the slow flow lasers. The output power is also decided by the cooling efficiency.

Transverse Flow Lasers:

The axial convection cooling is good for symmetrical beam energy distribution, but the transverse flow design is more effective, thus compact high power lasers are designed in this way. The disadvantage is the lack of symmetry.

There are many other flow patterns, for example, using spiral flow around the cavity. Since the flow pattern has so close relation with the quality of lasers, this area is a big concern in laser design.

Solid State Lasers

In solid state lasers, ions are suspended in crystalline matrix to generate laser light. The ions emit electrons when excited, the crystalline matrix spread the energy among the ions. The first solid state laser is ruby laser, but it is no longer used because of its low efficiency. Two common solid state lasers are Nd:YAG lasers and Nd:glass lasers, there structures are very similar. Both use krypton or xenon flash lamps for optical pumping.

For Nd:glass lasers, the glass rod has the advantage of growing into larger size than YAG crystals, but the low thermal conductivity of glass limits the pulse repetition rate of Nd:glass laser. So Nd:glass lasers are used in applications which require high pulse energies and low pulse repetition rates. It is suitable for hole piercing and deep keyhole welding operations.

YAG crystal has a higher thermal conductivity than glass, so the thermal dissipation in Nd:YAG laser cavity can be improved, operation power can be up to several hundred watts in continuous mode, and high pulse rates (50kHz) can be reached. YAG is a complex crystal of Yttrium-Aluminium-Garnet with chemical composition of Y₃Al₅O₁₂, it is transparent and colorless. About 1% Nd³⁺ ions are doped into the YAG crystal, the crystal color then changed to a light blue color. The wavelength of Nd:YAG laser is 1.06m m.

Solid state lasers are widely used in laser machining.

Figure 2.6: Construction for our Q-Switched UV Nd-YAG laser

G 2.4 Audio: NdYAGLaser.wav

Liquid Lasers