In nitrogen laser, active or gain or lasing medium is nitrogen molecules in the gaseous phase. It is a three-level laser. Population inversion in nitrogen laser is achieved by an electrical pumping source. Normally, pumping is provided by direct electron impact.
Construction and Working of Nitrogen Laser
The Active Medium ( or Nitrogen laser gain Medium )
The main component of a Nitrogen laser is a medium in the form of Nitrogen molecules called an active medium. The main characteristics of the active medium are as follows:
- It must have a pair of energy levels separated by a certain amount of energy. The energy level having energy is known as an upper energy level or higher excited energy level and the energy level having low energy is known as low energy or ground state.
- It must allow a population inversion between two energy levels.
it is an external source of energy that provides the necessary energy to the active medium to produce a state of population inversion essential for lasing action.
A resonant cavity
Population inversion is achieved to amplify the signal by ( or Photon ) stimulated emission. However, in practice, most of the atoms in the excited state emit spontaneously and do not contribute to the overall output.
Only a few atoms in the excited state emit via stimulated emission and hands overall gain of the output is small. Therefore, we require a positive feedback mechanism to make most of the atom in the excited state to emit via stimulated emission for contributing to the current output.
A device used to you have a positive feedback mechanism for Maximum coherent output is known as a resonator or resonant cavity.
Thus, a resonant cavity or resonator is a feedback device that makes the photon to move back and forth through the active medium. In this process, the number of photons emitted due to stimulated emission are multiplied.
A resonant cavity consists of a pair of plane or spherical mirrors placed parallel to each other at the end of the active medium. One of the mirrors is a fully reflecting mirror and the other is a partially transmitting mirror.
The laser output is taken out through the partially transmitting mirror which is also called the output coupler mirror.
Population inversion in nitrogen laser
Population inversion in nitrogen laser is achieved by electric discharge pumping. In this case, voltage is applied across the electrodes of the gas discharge tube which is filled with a low-pressure gas mixture known as the gain medium.
The applied voltage produces an electric field within the tube. This electric field accelerates electrons within the gas. These electron collides with the gas atom or gain medium and excite their atom to higher energy levels or excited energy levels.
If the atom in the lower-lying energy level makes the transition to the excited state faster than the atom in the higher-lying energy level makes the transition to the lower-lying energy levels, then the population of atom in the higher energy level is more than the population of atom in lower energy levels. Hence, population inversion in gases is achieved.
Construction of Nitrogen Laser
The arrangement to produce a nitrogen pulsed laser is shown in the figure.
It consists of a tube formed by two electrodes connected to the discharge voltage source (~ 10-40 keV). The active medium in the form of Nitrogen molecules in the gaseous phase is placed between these two electrodes.
The arrangement is enclosed in a resonant cavity formed by two planes and parallel mirrors M1 and M2. Mirror M1 is a perfectly reflecting mirror, while mirror M2 is a partially reflecting mirror through which pulse the laser comes out.
Working of Nitrogen laser
The energy level diagram of the Nitrogen molecule in the gaseous phase is shown in the figure. Each energy level consists of a series of vibrational energy levels depending on the internuclear separation.
Nitrogen molecule is excited from the ground state E1 to the excited energy level 3 ( called upper laser label ULL) by direct collision with electrons in the discharge tube. Nitrogen molecule falls from energy level E3 energy level E2 ( called lower laser level LLL ) by emitting a photon of ultraviolet light of wavelength 337.1 nm.
The lifetime of the upper energy level is less than the lifetime of the lower energy level. The output of the Nitrogen laser is in the form of pulses i.e., nitrogen laser is a pulsed laser.
In Nitrogen laser, the width of the laser pulse is narrow. It is because when a laser transition takes place, the population of upper laser level E3 decreases, and the population of lower laser level E2 increases.
The nitrogen molecule then Falls from energy level E2 to the ground state and Laser action stops. That is why nitrogen laser is known as self-terminating laser.
Disadvantages of a Nitrogen laser
- The Beam quality of the Nitrogen laser is poor.
- The divergence of the Nitrogen laser is large as compared to the divergence of other lasers.
- The output power is low.
- The efficiency of a Nitrogen laser is low.
Applications of a Nitrogen laser
- Pumping of dye lasers.
- Measuring air pollution.
- Scientific research.
- Spectroscopy and fluorescence studies.
- Fast-speed photography.