Gas Lasers: Principles, Types and Properties

• Gas lasers are a type of laser that uses a gas as the active gain medium.
• The term "laser" is an acronym for "light amplification by stimulated emission of radiation."
• Lasers emit light through optical amplification based on stimulated emission of electromagnetic radiation.

Principle of Laser

• Absorption occurs when energy is absorbed by an atom, exciting electrons into vacant energy shells.
• Spontaneous emission occurs when an atom decays from a higher energy level to a lower one, emitting a photon with energy hv in a random process.
• Stimulated emission occurs when atoms in an upper energy level are triggered by an incoming photon of a specific energy, causing them to emit a photon in phase with the incoming one.
• Stimulated photons have the same phase, wavelength, and direction as the incident photon.

Types of Lasers

• Lasers are classified into four types: semiconductor lasers, gas lasers, liquid lasers and solid-state lasers.

Properties of Laser

• Monochromatic: Lasers concentrate light in a narrow range of wavelengths (one specific color).
• Coherent: Emitted photons have a constant phase relationship with each other in both time and phase.
• Directional: Lasers produce a very tight, strong, and concentrated beam.

Population Inversion

• Population inversion is a state where a substance has more atoms or molecules in a higher excited state.
• Pumping, using lamps or electrical discharge, produces the population inversion.

Gas Laser

• Gas lasers work by discharging an electric current through a gas to produce coherent light.
• Gas lasers convert electrical energy into laser light output.
• Types of gas lasers: He-Ne laser, CO2 gas laser, N2 gas laser, and Argon gas laser.

He-Ne Laser

• The Helium-Neon laser was the first continuous laser.
• It was invented by Javan et al. in 1961.
• A He-Ne laser is a type of small gas laser with industrial and scientific uses, often used in laboratory demonstrations of optics.
• He-Ne lasers operate as four-level lasers.
• The usual operation wavelength is 632.8 nm, which is in the red portion of the visible spectrum.
• It operates in Continuous Wave (CW) mode.

Construction of He-Ne Laser

• The setup includes a discharge tube of 80 cm length and 1.5 cm bore diameter.
• The gain medium is a 5:1 to 20:1 mixture of helium and neon gases at low pressure within a glass envelope.
• The energy source is an electrical discharge of about 1000 volts through an anode and cathode at each end of the glass tube, with a current of 5 to 100 mA for CW operation.
• The optical cavity consists of a plane, high-reflecting mirror at one end and a concave output coupler mirror (1% transmission) at the other end.

He-Ne Laser Energy Level Diagram & Working

• When power is on, an electron collisionally excites a He atom to a higher energy state, He*(2 ¹S⁰).
• The excited He* atom collides with an unexcited Ne atom, transferring energy and resulting in an unexcited He atom and an excited Ne atom, Ne*(3s2).
• When the excited Ne atom transitions from the metastable state (3s) to a lower level (2p), it emits a photon of 632 nm.
• Reflected back and forth by the mirrors this photon stimulates additional excited Ne atoms to emit photons of 632nm.
• The stimulated transition from the 3s to 2p level is the laser transition.
• This process continues until the beam of coherent radiation becomes strong enough to escape through the partially silvered end.
• The Ne atom then spontaneously emits to reach the 1s level.

Application of He-Ne Laser

• The narrow red beam of He-Ne lasers is used in supermarkets to read bar codes.
• He-Ne lasers are used in holography for producing 3D images.

He-Ne Laser Pros

• High coherence length.
• Stable central wavelength.
• High spectral purity.
• Good beam quality and alignment.
• Low cost.

He-Ne Laser Cons

• Short life-time.
• Long warm-up time.
• Low output power.
• Very bulky.
• High voltage.

CO2 Laser Introduction

• The Carbon Dioxide (CO2) laser was one of the earliest gas lasers, invented by Kumar Patel of Bell Labs in 1964.
• CO2 gas lasers are the highest-power continuous wave lasers currently available.
• The raio of output power to pump power can be as large as 20%, making them quite efficient
• CO2 lasers produce a beam of infrared light with principal wavelength bands centering around 9.6 and 10.6 micrometers.

CO2 Laser Construction

• The discharge tube is about 2.5 cm in diameter and 5 cm in length, with two optically plane and parallel mirrors.
• The tube is filled with a mixture of CO2, nitrogen, and helium in a 15%:15%:70% ratio, respectively, at a pressure of a few mm of Mercury.
• A high DC voltage is used for electrical discharge.
• A small amount of water is added to regenerate the CO2 molecule as it breaks into CO and O, maintaining equilibrium.

CO2 Laser Energy Transition

• When a voltage is placed across the gas, electrons collide with the N2 molecules and excite them to their lowest vibrational levels.
• Excited N2 molecules populate the asymmetric vibrational states in the CO2 molecule through collisions, as the energy levels are very close.

CO2 Laser Application

• Because of their high power, CO2 lasers are frequently used in industrial applications for cutting and welding.
• They are also useful in surgical procedures because water absorbs this frequency of light very well.
• Medical uses include laser surgery and skin resurfacing (laser facelifts).

CO2 Laser Advantage

• Offers the lowest cost per watt with good beam quality, with high power levels ranging from a few watts to 15000 watts.
• The efficiency of CO2 gas lasers (i.e., 10% or higher) is better than He-Ne and argon lasers.
• Long sealed-off lifetime of greater than 20,000 hours.
• Small size per watt of output power.

CO2 Laser Disadvantage

• Divergence is greater than He-Ne and Argon lasers, typically ranging from 1 to 10 milli radians.
• Beam width varies from 3mm to 100mm.
• Some CO2 lasers have the disadvantage of a short and think optical cavity.
• cost is relatively high.

Argon Lasers

• used to treat glaucoma and diabetic eye diseases.
• Used in Raman spectroscopy.
• Used in holography and forensic science.
• Used as sources for optical pumping, and in laser shows for entertainment.