What is a three-level laser system?

A three-level laser system consists of three energy levels where population inversion is achieved between a metastable state and the ground state. Ruby laser is a classic example.

What is a four-level laser system?

A four-level laser system uses four energy levels, making population inversion easier because the lower laser level is above the ground state. Helium-Neon laser is a common example.

Why is four-level laser more efficient than three-level laser?

Four-level lasers are more efficient because population inversion is achieved more easily and requires less pumping power compared to three-level lasers.

What is population inversion in lasers?

Population inversion occurs when more atoms are present in a higher energy state than in a lower energy state, which is essential for laser action.

Give examples of three-level and four-level lasers.

Ruby laser is an example of a three-level laser system, while Helium-Neon and Nd:YAG lasers are examples of four-level laser systems.

Three and Four Level Laser Systmes

A three-level system consisting of energy levels E₁, E₂, and E₃ (Fig.1). With the help of pumping (optical) atoms are excited from level E1 to level E3. From which they decay rapidly to level E₂ through some nonradiative process. Level E₂ is generally a meta stable state and when there will be a transition from E2 level to E₁ level, coherent laser radiation are resulted. Ruby fall under this three level laser scheme.

Figure 1.Three level laser scheme

2) Four level laser scheme

The four-level laser system is shown in Fig 2. Level E₁ is the ground level and levels E₂, E₃, and E₄ are excited levels of the system. Atoms from level E₁ are pumped to level E₄ from where they make a fast nonradiative relaxation to level E₃. Level E₃ which corresponds to the upper laser level is usually a metastable level having a long lifetime. The transition from level E₃ to level E₂ forms the laser transition. In order that atoms do not accumulate in level E₂ and hence destroy the population inversion between levels E₃ and E₂, level E₂ must have a very small lifetime so that atoms from level E₂ are quickly removed to level E₁ ready for pumping to level E₄.

Figure 2. Four level laser scheme

Dielectrc Properties of Materials

Three and Four Level Laser Systmes

2) Four level laser scheme