Laser Generation 101: The Core Principles Explained

(1) Stimulated Emission

Stimulated absorption, spontaneous emission, and stimulated emission are the three interaction processes between light and matter. As illustrated in Figure 3-1, lasers are generated through the process of stimulated emission.

According to quantum theory, an energy level corresponds to an electron’s energy state, which is determined by the principal quantum number n (n=1,2,…).

Laser Generation 101 The Core Principles Explained

However, to accurately describe an electron’s motion within an atom, other factors such as orbital angular momentum L and spin angular momentum s, which are also quantized, must be considered, as described by corresponding quantum numbers. Bohr once proposed a quantization formula for orbital angular momentum, Ln=nh, but this formula is not strictly accurate, being derived from the assumption of electron motion as an orbital path.

Strict quantization of energy and angular momentum should be derived from quantum mechanical theory. An electron can only transition from a higher to a lower energy state between two states with angular momentum quantum numbers l differing by ±1, which is known as a selection rule.

If the selection rule is not satisfied, the probability of transition is very low, even approaching zero. In atoms, there can exist certain energy levels where electrons, once excited to these levels and unable to satisfy the selection rules for transitions, have abnormally long lifespans and are unlikely to transition spontaneously to lower energy levels; such energy levels are known as metastable states.

However, under the stimulation of external light, the electrons can be rapidly transitioned to lower energy levels while emitting photons, a process known as stimulated emission, as shown in Figure 3-1(c). The concept of stimulated emission was first introduced by Albert Einstein in 1917 while deriving Planck’s law of black body radiation, where he theoretically predicted the possibility of atoms undergoing stimulated emission.

Figure 3-1 illustrates the interaction rules between light and matter.

The process of stimulated emission is as follows: The atom initially resides at a high energy level Ex. When an incoming photon’s energy exactly matches the energy difference between two levels, Ex-E0, the atom can be induced by this photon to transition from the higher energy level Ex to the lower energy level E0.

The photons produced by stimulated emission have distinctive characteristics; they are identical to the inducing photons not only in frequency (energy) but also in direction of emission, polarization, and phase of the light wave.

Thus, one incident photon results in the emission of two identical photons, which means the original light signal is amplified. The light generated and amplified in this stimulated process is what we call laser light.

(2) Population Inversion

The inducing photons can cause not only stimulated emission but also stimulated absorption. Therefore, stimulated emission transitions can only dominate when the number of atoms at a higher energy level exceeds the number of atoms at a lower energy level.

It is crucial for a light source to emit laser light, rather than ordinary light, that there are more atoms at the higher energy level E2 than at the lower energy level E1, a condition known as population inversion. Under thermal equilibrium, atoms are almost exclusively in the lowest energy state (ground state), and thus light passing through the medium only diminishes in energy rather than being amplified.

To give stimulated emission the upper hand, the number of particles at the higher energy level Ex must exceed the number at the lower energy level E0. Therefore, achieving population inversion technically is a necessary condition for generating laser light.

To achieve population inversion, an external excitation energy source is first required. In theory, imparting a certain amount of input energy, such as light, heat, or electricity, to any substance can cause particles at a lower energy level to transition to a higher level, disrupting the normal distribution of particle numbers at thermal equilibrium and achieving population inversion.

This is commonly accomplished through gas discharge, wherein electrons with kinetic energy excite the laser material—this is known as electrical excitation. Alternatively, pulse light sources can be used to irradiate the medium atoms within the optical cavity, a method referred to as optical excitation. There are also thermal and chemical excitation methods. These various excitation methods are figuratively called “pumping.”

To maintain continuous laser output, it is necessary to constantly “pump” to replenish the high-energy particles that transition back down, compensating for their depletion.

Furthermore, achieving population inversion also requires a suitable active medium. For most substances, the particles at the excited state’s higher energy level have a very short lifetime (generally between 10-11 and 10-8 seconds), and they quickly transition back to the ground state. Thus, it is necessary to find a substance with a long-lived excited state, known as a metastable state.

The metastable state can last from 10-3 to 10-2 seconds, or even up to one second, providing sufficient time for laser generation. Such a substance is the working material for a laser.

In summary, two essential conditions are required to achieve population inversion: one is an external excitation energy source, and the other is a suitable active medium.

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