Gas Laser Lecture Notes PDF

# Gas Laser * Engineering Technical College * Department of. Laser and Electro Optical ENG * Laser design 4th stage * Lec-2 * By Msc--zahraa abbass ## 1- Gas Laser * A gas laser is a laser in which an electric current is discharged through a gas inside the laser medium to produce laser light. * I...

# Gas Laser * Engineering Technical College * Department of. Laser and Electro Optical ENG * Laser design 4th stage * Lec-2 * By Msc--zahraa abbass ## 1- Gas Laser * A gas laser is a laser in which an electric current is discharged through a gas inside the laser medium to produce laser light. * In gas lasers, the laser medium is in the gaseous state, used in applications that require laser light with very high beam quality and long coherence lengths. * Is the first laser that works on the principle of converting electrical energy into light energy. * It produces a laser light beam in the infrared region of the spectrum at 1.15 µm. * Different types: * Helium (He) Neon (Ne) lasers * Argon ion lasers * Carbon dioxide lasers (CO2 lasers) * Carbon monoxide lasers (CO lasers) * Excimer lasers * Nitrogen lasers, hydrogen lasers, etc. ## 2- Gas Laser Power Supplies Continuous-Wave * The power supplies for continuous-wave gas lasers are similar in design to those used in direct-current power supplies. * Gas laser power supplies tend to be current-limited regulated DC power supplies. * The designs are basically the same for all gas-discharge devices. * The details depend on the particular voltage-current characteristics of the gas and the configuration of the laser. * Three essential elements are used in the design of all gas laser power supplies: * The starter or ignition circuit * The operating supply * A current-limiting element ## 3- Electrical Characteristics of Gas Discharges * Most gas lasers are pumped by an electrical discharge that flows through the gas mixture between electrodes in the gas. * Collisions between electrons in the electric discharge and the molecules in the gas transfer energy from the electrons to the energy levels of the molecule. * Electrical discharges in gases are characterized by *current/voltage* characteristics shown in Figure 1. * The exact characteristics, of course, depend on the nature of the gas, its pressure, and the length and diameter of the discharge. * Figure 1: Current/voltage curve for a gas discharge ## 4- Power Supplies for Helium-Neon Lasers * The requirements for power supplies for gas lasers derive from the characteristics of the curve in Figure 1. * The exact design for a particular gas laser power supply will depend on the specific *current/voltage* curve for the gas mixture that is being excited, but three essential elements for any gas laser power supply are: * A *Starter Circuit.* This portion of the power supply provides an initial voltage pulse. The peak value of the voltage pulse must exceed the breakdown voltage of the gas. The pulse drives the gas past point B and into region C. * An *Operating Supply*. This part of the power supply provides a steady current flow through the gas mix. The gas has reached region C. It must operate at the appropriate voltage and current levels to sustain the current in the particular gas. * A *Current Limiter*. This limits the current through the gas to a desired value and prohibits the unbounded increase of current. It usually takes the form of a ballast resistor in series with the discharge. * In steady operation, the power supply must sustain the flow of electrical current through the gas mixture, accelerating free electrons to energies sufficient for excitation of the helium atoms and providing enough current flow to produce an adequate population inversion. * The basic blocks of the power supply for a typical small helium-neon laser are shown in Figure 2. * Figure 2: Block diagram of power supply for helium-neon laser * The characteristics of the power supply for the helium-neon laser derive from the properties of the *current/voltage* curve for the helium-neon gas mixture. * We already have discussed the general properties of *current/voltage* curves with respect to Figure 1. * We now discuss some details of circuits that have been used for driving helium-neon lasers. A number of different configurations have been used by different manufacturers. * We shall not attempt to describe them all, but shall choose some representative examples. ## 5- Switching Elements * Helium-neon lasers almost always are operated continuously. After the startup, there is no need for high-voltage pulsing of the power supply. * We now shall turn to a discussion of gas lasers that frequently are operated as pulsed lasers, and for which there is a need for short-duration high-voltage pulses. * As a preliminary to the discussion of these lasers, we first shall describe high-voltage switching elements that often are employed with pulsed gas lasers. * The two high-voltage switches that we will describe are *spark gaps* and *thyratrons*. * Figure 7: Power output of helium-neon laser as a function of tube current * There are many different power supplies for helium-neon lasers but most of them use a voltage doubler circuit. * Figure 4: Operating supply combining rectifier and voltage-doubler functions * Figure 5: Voltage doubler with internal starting circuit * Figure 7: Power supply used for helium-neon laser