Question

Summarize the conditions of Laser operation and explain shortly the essentials of He-Ne laser operation.

Answer 1

Laser means Light Amplification by Stimulated Emission of Radiation

Stimulated Emission happens by the method of Population Inversion

Population Inversion

Population inversion is the process of achieving greater population of higher energy state as compared to the lower energy state. Population inversion technique is mainly used for light amplification. The population inversion is required for laser operation.

To achieve population inversion the following steps must be included while designing a Laser

Pump Source

The pump source or energy source is the part of a laser system that provides energy to the laser medium. To get laser emission, first we need to produce population inversion. Population inversion is the process of achieving greater number of electrons in higher energy state as compared to the lower energy state.

The source of energy supplies sufficient amount of energy to the laser medium by which the electrons in the lower energy state are excited to the higher energy state.

Laser Medium

The laser medium is a medium where spontaneous and stimulated emission of radiation takes place. Generally, the population of lower energy state is greater than the higher energy state. However, after achieving population inversion, the population of higher energy state becomes greater than the lower energy state.

Optical resonator

The laser medium is surrounded by two parallel mirrors which provides feedback of the light. One mirror is fully reflective (100 % reflective) whereas another one is partially reflective (<100 % reflective). These two mirrors as a whole is called optical resonator. Optical resonator is also known as optical cavity or resonating cavity.

He Ne laser

Pump Source

In He Ne laser the pump source is a High voltage power supply

Laer medium

The gain medium of a helium-neon laser is made up of the mixture of helium and neon gas contained in a glass tube at low pressure. The partial pressure of helium is 1 mbar whereas that of neon is 0.1 mbar.

The gas mixture is mostly comprised of helium gas. Therefore, in order to achieve population inversion, we need to excite primarily the lower energy state electrons of the helium atoms.

Optical resonator

The glass tube (containing a mixture of helium and neon gas) is placed between two parallel mirrors. These two mirrors are silvered or optically coated.

Each mirror is silvered differently. One side mirror is partially silvered and is known as output coupler whereas the other side mirror is fully silvered and is known as the high reflector or fully reflecting mirror.

The fully silvered mirror will completely reflect the light whereas the partially silvered mirror will reflect most part of the light but allows some part of the light to produce the laser beam.