Essential Factors for Laser Generation

Essential Factors for Laser Generation

Laser working material (or medium), resonator cavity (or laser cavity), and pump source (or excitation source) are the three essential elements for generating a laser.

Laser Working Material

Different types of lasers have different working materials, typically including the following:

(1) Solid-state lasers

The working materials are chromium-doped ruby, neodymium-doped yttrium aluminum garnet (YAG), and neodymium-doped glass rod.

(2) Gas lasers

The working materials are CO2, He-Ne, N2, Ar, etc.

(3) Semiconductor lasers

The working material is a compound formed by covalent bonds (such as GaAs).

(4) Dye lasers

The working material is an organic dye solution (Rhodamine 6G).

Resonator Cavity

The resonator cavity is an important component of a laser, not only being a necessary condition for laser oscillation but also significantly affecting the output mode, power, and beam divergence angle. By selecting an appropriately structured optical resonator cavity, the direction and frequency of the generated stimulated radiation beam can be controlled.

The resonator cavity consists of a totally reflecting mirror and a partially reflecting mirror (output mirror), from which the laser is emitted. Depending on the specific circumstances, a stable cavity, unstable cavity, or critical stable cavity can be chosen. The principles of the resonator cavity’s reflecting mirrors are shown in Figure 1-8.

(1) Mode selection of the light wave (not all light waves of any frequency and propagation direction can exist).

(2) Feedback amplification (the light repeatedly passes through the working material between the two mirrors and is repeatedly amplified).

(3) Threshold condition (the necessary condition for generating a laser is that the laser oscillator must oscillate, i.e., the threshold condition).

1) Gain inside the resonator cavity = losses of the resonator cavity. At this point, the threshold condition is satisfied, and the laser oscillator starts oscillating.

2) Gain > losses: the light intensity gradually increases, the oscillation becomes stronger, and when saturation is reached, the gain gradually decreases; this is the gain saturation effect.

3) Gain < losses: the light intensity quickly decays to zero, unable to oscillate.

Pump Source (Excitation Source)

The pump source provides energy to the laser working material, enabling the working material to be in a stable excited state (the driving force of particle migration).

Optical excitation: The working material is irradiated with light, and after absorbing light, it undergoes population inversion. This is usually achieved using high-efficiency, high-intensity light sources such as lamps or solar energy. The construction and principles of the pump source are shown in Figure 1-9.

 Figure 1-8 Principle of Resonant Cavity Mirror
Figure 1-8 Principle of Resonant Cavity Mirror
Figure 1-9 Construction and Principle of Pump Source

Electrical excitation: Under high voltage, gas molecules undergo ionization and become conductive, known as gas discharge. During the discharge process, gas molecules (or atoms, ions) collide with electrons accelerated by the electric field, absorb electron energy, and undergo transitions to high energy levels, resulting in population inversion.

Thermal excitation: High-energy level gas particles are increased through high-temperature heating, followed by a sudden decrease in gas temperature. Due to the different thermal relaxation times of the high and low energy levels, population inversion can be achieved.

Nuclear excitation: High-energy particles, radiation, or fission fragments released from nuclear fission reactions are used to excite the working material, also achieving population inversion.

(Note: Relaxation time is a characteristic time of a dynamic system. It represents the time needed for a variable of the system to transition from a transient state to a steady state. In statistical mechanics and thermodynamics, relaxation time indicates the time required for a system to transition from an unstable state to a stable state.)

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