What is a Gas Laser?

1 Answer
Can you answer this question?

- GoPhotonics

Jul 13, 2023

A gas laser is a type of laser that uses a gas-filled medium to produce coherent and amplified light. The first gas laser, the helium-neon (He-Ne) laser, was developed in 1961 by Ali Javan, William R. Bennett Jr., and Donald R. Herriott. Gas lasers operate based on the principles of stimulated emission and optical amplification. They exhibit a broad power range, spanning from several milliwatts to kilowatts. These lasers can operate in either continuous mode or pulsed mode, offering output frequencies that extend across the ultraviolet to infrared spectrum.

Structure of Gas Laser

A gas laser utilizes one or more gases, such as Helium, Neon, Argon, Carbon Dioxide or a mixture of these gases as its lasing medium. The specific gas or gas mixture employed distinguishes different types of gas lasers and determines the characteristics of the laser output. The lasing medium is kept in a resonator cavity, a sealed discharge tube made of a suitable material such as glass or metal. The optical resonator consists of two mirrors at both ends. One of the mirrors is a fully reflective mirror (HR mirror) and a partially reflective mirror (output coupler). The reflective surfaces of the mirrors allow the laser light to bounce back and forth, amplifying and reinforcing the coherent light.

Gas lasers require an external energy source to pump energy into the gas medium and create a population inversion, which is necessary for stimulated emission and laser operation. The pump source can vary depending on the specific gas laser type and may include electrical discharges, flash lamps, radio frequency excitation, or optical pumping. The output wavelength of a gas laser depends on the specific gases present in the lasing medium. For example, the He-Ne gas laser has an output wavelength of 632.8 nm, while Ar-ion gas lasers can emit laser light at various wavelengths, including 488 nm, 514.5 nm, and 647.1 nm.

In some gas laser configurations, Brewster windows are employed. Brewster windows are specialized optics that allow the laser beam to pass through at a specific angle known as the Brewster angle. These windows reduce reflections and losses while also polarizing the laser beam.

Working Principle

Gas lasers operate based on the principles of stimulated emission and optical amplification. These lasers are excited by different pumping mechanisms such as electrical discharge, chemical reactions, or optical pumping. The gas atoms, ions, or molecules interact with energetic electrons, leading to their excitation. This method of electrical excitation is preferred over optical excitation because gases have narrow absorption lines, unlike solids. This excitation energizes the gas atoms or molecules, causing them to transition from lower energy states to higher energy states. As they return to lower energy states, they emit photons, which are then reflected and amplified within an optical resonator formed by two mirrors. The partially reflective mirror allows a portion of the light to escape as a coherent laser beam.

Types of Gas lasers

In gas lasers, the broadening of spectral lines is not as strong as in solid-state lasers. This is because gas lasers operate with a different type of material, a gas, where the energy levels and interactions between atoms or molecules are different. The main broadening mechanisms in gas lasers are collisional broadening and Doppler broadening.

Collisional broadening occurs when atoms or molecules in the gas collide with each other, causing the emission or absorption lines to become broader. This broadening depends on the density and temperature of the gas and the nature of the collisions.

Doppler broadening happens because of the random motion of atoms or molecules in the gas. As they move, the observed frequency of the emitted or absorbed light is shifted due to the Doppler effect. This leads to a broadening of the spectral lines.

However, in gas lasers, the linewidths, which are the width of the spectral lines, are relatively small compared to solid-state lasers. This is because gas lasers typically operate at low pressures, resulting in less collisional broadening. The energy levels in the gas medium are relatively narrow, and a sharp emission line is needed for efficient excitation. Optical pumping, which exhibits a broad emission spectrum, is unsuitable for efficiently pumping gas lasers. Hence, electrical pumping stands as the prevalent method for exciting the active medium in gas lasers. Additionally, pumping can be achieved through chemical means, with an electron beam, or by gas-dynamic expansion. These methods allow the gas lasers to have a narrow emission spectrum and produce focused laser beams. They are mainly classified based on the active medium such as:

  • Neutral atom lasers

Neutral atom lasers, also known as atom lasers, are devices that generate coherent beams of matter waves using neutral atoms rather than photons. A collection of atoms, cooled to ultra-low temperatures using laser and evaporative cooling, is trapped and manipulated using magnetic or optical fields, termed as coherent manipulation. Using techniques such as radiofrequency or magnetic field gradients neutral atoms are extracted out. The extracted atoms then form a coherent matter wave, analogous to the coherent beam of light in a conventional laser. Most common examples of neutral atom lasers include Sodium atom laser, Rb atom laser, Li atom laser, Sr atom laser, etc. 

  • Ion lasers

Ion gas lasers are generally four-level lasers, and the active medium is an ionized inert gas, with a typical operating pressure of about 1 Torr. There are a variety of ion lasers, including argon, krypton, xenon, and mercury ion lasers. These have essentially the same design. The operating wavelength range of ion lasers depends on the specific gas mixture and the transition levels of the ions involved. It includes Ar-Ion lasers, Kr-Ion lasers, He-Ion lasers, etc.

  • Metal vapor lasers

Metal vapor lasers are a type of gas laser that use vaporized metallic elements as the lasing medium. They produce laser light by exciting the vaporized atoms of the metal to higher energy levels and then releasing that energy as coherent light when the atoms return to their ground state. These lasers are known for their high output power and a wide range of operating wavelengths. Metal vapor lasers operate in the visible and ultraviolet (UV) regions of the electromagnetic spectrum. This laser includes Copper & Gold Vapor Laser, He-Cd laser, Ar-Ion laser, He-Ne lasers, etc.

  • Molecular lasers

Molecular lasers are a specific type of gas laser that utilize excited molecules as the lasing medium. These lasers operate by exciting the rotational or vibrational energy levels of the gas molecules, which results in the emission of coherent light. This type of gas laser includes CO, CO2, N2, Excimer, HF, DF lasers, etc.

Lifetime of Gas lasers

The lifetime of a gas laser is influenced by several factors, including the gas medium, the construction of the laser tube, and the operating conditions. Gas lasers typically have a finite operational lifetime due to factors such as gas depletion, contamination, or degradation of the optical components. The lifetime can vary from thousands of hours for some gas lasers to tens of thousands of hours for others. Proper maintenance, including gas replenishment and regular cleaning, can extend the operational lifespan of gas lasers.

Advantages of Gas lasers

Gas lasers offer several advantages over other types of lasers, some of them include:

  • They can contain a large volume of the active medium, which allows for a high concentration of excited atoms or molecules. This high volume contributes to increased power output and efficiency compared to other types of lasers.
  • The gases used as the active medium in gas lasers are typically inexpensive and readily available. This makes gas lasers cost-effective in terms of the materials required for their operation.
  • These lasers are known for their robustness and durability. The gas medium is contained within a sealed tube, which protects it from external elements and reduces the risk of damage.
  • Gas lasers have a relatively efficient heat dissipation mechanism. As the gas medium absorbs energy during excitation, the heat generated can be easily removed through cooling systems. This helps in maintaining stable laser performance and prolonging the lifetime of the laser system.

Applications of Gas lasers

Gas lasers have found numerous applications across various fields due to their unique properties and capabilities. Here are some common applications of gas lasers:

  • Industrial cutting and welding
  • Used in medical procedures such as in dermatology, ophthalmology, and gynecology.
  • Laser Marking and engraving
  • Popular in holography for creating three-dimensional images and in laser light shows for producing vibrant and colorful laser displays.
  • Range Finding and LiDAR