Gas Dynamic Laser is laser based on differences in relaxation velocities of molecular vibrational states.
The laser medium gas has such properties that an energetically lower vibrational state

Molecular vibration

A molecular vibration occurs when atoms in a molecule are in periodic motion while the molecule as a whole has constant translational and rotational motion...

 relaxes faster than a higher vibrational state,
thus a population inversion

Population inversion

In physics, specifically statistical mechanics, a population inversion occurs when a system exists in state with more members in an excited state than in lower energy states...

 is achieved in a particular time.

Pure Gas dynamic lasers usually use a combustion chamber, supersonic expansion nozzle

Nozzle

A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....

 and CO₂

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 as an active laser medium

Active laser medium

The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state...

 in mixture with nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 or helium

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

.
Gas dynamic laser could be however pumped

Laser pumping

Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...

 not only by combustion

Combustion

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

, but by any adiabatic expansion of gas.
Any hot and compressed gas with appropriate vibrational structure could be utilized.

Explosively pumped gas dynamic laser is a version of gas dynamic laser pumped by expansion of explosion products. Hexanitrobenzene

Hexanitrobenzene

Hexanitrobenzene, also known as HNB, is a high-density explosive compound with chemical formula C6N6O12, obtained by oxidizing the amine group of pentanitroaniline with hydrogen peroxide in sulfuric acid.-Properties:...

 and/or tetranitromethane

Tetranitromethane

Tetranitromethane or TNM is an organic oxidizer with chemical formula C4. Its chemical structure consists of four nitro groups attached to one carbon atom...

 with metal powder is preferred explosive. This device could have very high pulse peak power output applicable in laser weapons.

Function

1. Hot compressed gas is generated.
2. Gas expands through subsonic or supersonic expansion nozzle, the temperature of the gas becomes lower and according to maxwell–Boltzmann distribution the gas isn't in thermodynamic equilibrium
   Thermodynamic equilibrium
   In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. The word equilibrium means a state of balance...
   
    until the vibrational states relax.
3. The gas flows through the tube of a particular length for a particular time. In this time lower vibrational state does relax but higher vibrational state doesn't. Thus population inversion is achieved.
4. Gas flows through mirror area where stimulated emission
   Stimulated emission
   In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...
   
    takes place.
5. Gas returns to equilibrium and becomes warm. It must be removed from the laser cavity
   Optical cavity
   An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
   
    or it will interfere with the thermodynamics and vibrational state relaxation of the freshly expanded gas.

Application

Almost any chemical laser

Chemical laser

A chemical laser is a laser that obtains its energy from a chemical reaction. Chemical lasers can achieve continuous wave output with power reaching to megawatt levels...

 uses gas-dynamic processes to increase its efficiency.

High energy efficiency

Energy conversion efficiency

Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...

(as high as 30%) and very high power output makes GDL perspective for some (especially military) applications.