Department of Mechatronics, Gwangju Institute of Science and Technology, Republic of South Korea
Laser technologies are utilised in various fields like micromanufacturing, instrumentation, imaging, medicine, communications, etc. Although each of these fields is as important as others, we will limit our discussion to the application of laser technologies for manufacturing only.
Application of lasers to micromanufacturing has several advantages such as noncontact processing, capability of remote processing, automation, no tool wear, and the possibility of machining hard and brittle materials. In this chapter, the principle of laser light generation, laser beam properties, characteristics of practical laser systems, and examples of application technologies are discussed.
The word 'LASER' is originally an acronym for 'Light Amplification by Stimulated Emission of Radiation'. The generation of laser light is achieved by an amplification of light intensity through a physical process called 'stimulated emission'. Stimulated emission is a special form of light interaction with material in atomic scale.
Consider that a medium is irradiated with light, not necessarily a laser light but also sunlight or flash light. In a microscopic point of view, the interaction between the incident light and the medium can be described as an interaction between the light and the atoms consisting of the medium.
It is well known that atoms consist of a heavy, positively charged nucleus that takes most of the atomic mass and electrons of negligible mass orbiting the nucleus like the planets orbiting the sun. For the sake of simplicity, let us consider an isolated atom with only one electron orbiting the nucleus as shown in Fig.3.1(a), which is the Bohr model for a hydrogen atom (Wilson and Hawkes 1987; Milonni and Eberly 1988). The negatively charged electron will experience an attractive force Coulomb toward the positively charged nucleus, and a centrifugal force in the opposite direction due to its circling motion. Under thermal equilibrium, these two forces are balanced and the orbit along which the electron travels is determined. An atom at this condition is said to be at its ground state.
In the energy point of view, the atom possesses minimum available energy at the ground state. If external energy is supplied to a ground state atom, it will absorb the energy and reach a higher energy state, an excited state. Obviously, the radius of the electron orbit of an excited atom will be larger, (Fig. 3.1(b)), because the electron overcome attraction more easily. It is even possible for the electron to become free from the nucleus if the energy supply is sufficiently large.
However, there exists a rule that governs the excitation of an atom. This rule based on quantum mechanics, states that there exist discrete energy values permitted for an atom to occupy upon excitation such that E0<E1<E2<E3..., where E0 denotes the ground state energy.
Therefore, when external energy is supplied to a ground state atom, it may be excited to a higher energy state of E1 or E2 but it cannot be excited to an intermediate level between E0 and E1 or E1 and E2. Specifically, absorption of external energy by an atom occurs only if the amount of supplied energy matches the energy gap AEij=Ej-Ei between two energy states Ei and Ej, assuming Ei < Ej. A more thorough discussion about the discrete energy states of atoms, molecules, and solids can be found in Milonni and Eberly (Milonni and Eberly 1988).
Let us now consider the excitation of a ground state atom by incident light as shown in Fig. 3.2(a). The amount of energy carried by a photon is equal to hf, where f is frequency of the incident light and h is the Plank's constant (h=6.625*10-34 J-sec). Note that 'photon' is a term describing the particle characteristics of light interacting with a medium and used to express the energy transport by light. According to above statement, excitation of the atom from Ei to Ej may occur only if AEij= hf is satisfied.
Otherwise, absorption of the photon would not happen. Because the energy gap AEij between any two states of a specific material can be known, it is possible to induce an absorption by irradiating the atom with a light of frequency fij for which,
Where, the last term is from c=f describing the propagation speed of light in vacuum c as a function of frequency and wavelength X.
This type of induced absorption of incident light by an atom is called 'stimulated absorption'. Although excitation of an atom is possible using stimulated absorption, the atom is not stable at the excited state and thus tends to return to the ground state without any external influence.
(Ground state) (Excited state)
Fig. 3.1. Schematic diagram illustrating energy sates of an atom
To return to the ground state, the atom needs to release excess energy, which can be done by transferring its energy to surrounding atoms through collision or by emitting light (Not to mention we are more interested in the emission of light). The duration that an atom remains at the excited state before falling to the ground state is called the lifetime and typically it is on the order of 10 nanoseconds (Hecht 1998). When the falling happens naturally and is accompanied by an emission, it is called the 'spontaneous emission', Fig.3.2(b).
Stimulated emission also represents the falling of an excited atom to a lower energy state with emission. However, in the stimulated emission, the falling is induced by an incident light of matching frequency, where matching frequency means that the incident light satisfies the relation in Eq. 3.1. Note that for the stimulated emission the lower state (Ei) does not have to be the ground state but can be any energy level below the excited state (Ej) as long as Eq. 3.1 is satisfied. The uniqueness of stimulated emission can be better understood in reference to Fig. 2(c). When a photon of frequency fj is incident on the excited atom, the atom is then stimulated not only to release its excess energy via emission but also in a fashion that the emitted photon has the same phase, polarization, and propagation direction as the incident photon. Therefore, an amplification of light intensity is achieved through the stimulated emission process as was symbolized by the word LASER.
Light amplification by stimulated emission can be achieved by shining light with proper frequency on excited atoms. However, under thermal equilibrium, most atoms are at the ground state rather than at excited states. The ratio of number of atoms between two energy states Ei and Ej (E; < Ej) is expressed by the Boltzman equation,
Where, N is the number of atoms (or population) at E, kB is the Boltzman constant (1.3807x10"23 J/K), and T is the absolute temperature. Because the population at lower energy state is much greater than that at higher energy state, under equilibrium condition it is far more likely that stimulated absorption may take place upon light irradiation than stimulated emission. Hence, to obtain a laser light it is a prerequisite to produce a nonequilibrium condition at which more atoms exist at the excited (or upper) state rather than the lower state. This nonequilibrium distribution of atoms is known as population inversion. To see how to achieve a population inversion and thus laser light generation, let us consider the schematic diagram in Fig. 3.3. The core element of a laser is the active medium, the material undergoes stimulated absorption and emission processes for light generation, and it could be a solid, liquid or gas. Many lasers are named after the active medium; for example, ruby laser, Nd-YAG (Neodymium-doped Yttrium Aluminum Garnet) laser, CO2 (carbon dioxide) laser, etc. Supply of external energy to the active medium, pumping, can be done electrically or optically. The schematic diagram in Fig. 3.3 shows an optical pumping in which a flashlamp generates a very bright and intense white light due to high voltage discharge. A portion of this white light whose frequency matches the absorption band is absorbed by the active medium by stimulated absorption, leading to a sharp increase of the population of excited atoms within the active medium.
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