Introduction to Laser

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© Springer Nature Switzerland AG 2020

S. Stübinger et al. (eds.)Lasers in Oral and Maxillofacial Surgerydoi.org/10.1007/978-3-030-29604-9_2

2. An Introduction to Laser

Gholamreza Shayeganrad1  
(1)

Optoelectronic Research Centre, University of Southampton, Southampton, SO17 1BJ, UK
 
 
Gholamreza Shayeganrad

Abstract

For a better understanding of the special advantages of laser light in oral and maxillofacial surgery, we need to know the principle of generation of laser light and the properties that distinguishes it from conventional light or other energy sources, as well as, how a laser works and the different types of lasers that can be used in medical applications. Light theory branches into the physics of quantum mechanics, which was conceptualized in the twentieth century. Quantum mechanics deals with behavior of nature on the atomic scale or smaller.

This chapter briefly deals with an introduction to laser, properties of laser light, and laser-beam propagation. It begins with a short overview of the theory about the dual nature of light (particle or wave) and discusses the propagation of laser beam, special properties of laser light, and the different types of lasers that are used in medical applications.

Keywords

Laser principleCoherenceGaussian beam opticsLaser medicineLaser surgery

2.1 Introduction

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum that includes radio waves (AM, FM, and SW), microwaves, THz, IR, visible light, UV, X-rays, and gamma rays. The primary properties of light are intensity, brightness, wave vector, frequency or wavelength, phase, polarization, and its speed in a vacuum, c = 299, 792, 458 m/s. The speed of light in a medium depends on the refractive index of the medium, which is c/n. Intensity is the absolute measure of power density of light wave and defines the rate at which energy is delivered to a surface. Brightness is perceptive of intensity of light coming from a light source and depends on the quality of the light wave as well. The frequency of a light wave determines its energy. The wavelength of a light wave is inversely proportional to its frequency. The wave vector is inversely proportional to the wavelength and is defined as the propagation direction of the light wave. Phase cannot be measured directly; however, relative phase can be measured by interferometry. A light wave that vibrates in more than one plane like sunlight is referred to as unpolarized light. Such light waves are created by electric charges that vibrate in a variety of directions. Depending on how the electric field is oriented, we classify polarized light into: linear polarization, circular polarization, elliptical polarization, radial and azimuthal polarization. We say a light wave is linearly polarized if the electric field oscillates in a single plane. If electric field of the wave has a constant magnitude but its direction rotates with time at a steady rate in a plane perpendicular to the direction of the wave, it is called a circular polarized wave. In general case, electric field sweeps out an ellipse in which both magnitude and direction change with time, which is called elliptical polarized wave . Radially and azimuthally polarized beams have been increasingly studied in recent years because of their unique characteristic of axial polarization symmetry, and they can break the diffraction limit with a strong longitudinal electromagnetic field in focus [13]. The unpolarized light can be transformed into polarized light by wire grid, polaroid filter, molecular scattering, birefringent materials, retarder waveplates, reflection at Brewster’s angle, polarizing cubes, total internal reflection, optical activity, electro-optic effect, or liquid crystals.

The understanding of light refers to the late 1600s with raising important questions about the dual nature of light (particle or wave). Sir Isaac Newton held the idea that light travels as a stream of particles. In 1678, Dutch physicist and astronomer Christiaan Huygens believed that light travels in waves. Huygens’ principle was the successful theory to introduce the appearance of the spectrum, as well as the phenomena of reflection and refraction, which indicated that light was a wave. Huygens suggested that the light waves from point sources are spherical with wavefronts, which travel at the speed of light. This theory explains why light bends around corners or spreads out when shining through a pinhole or slit rather than going in a straight line. This phenomenon is called diffraction. Huygens stated that each point on the wavefront behaves as a new source of radiation of the same frequency and phase. Although Newton’s particle theory came first, the wave theory of Huygens better described early experiments. Huygens’ principle predicts that a given wavefront in the present will be in the future.

None of these theories could explain the complete blackbody spectrum, a body with absolute temperature T > 0 that absorbs all the radiation falling on it and emits radiation of all wavelengths. In 1900, Max Planck proposed the existence of a light quantum to explain the blackbody radiation spectrum. In 1905, Albert Einstein individually proposed a solution to the problem of observations made on photoelectric phenomena. Einstein suggested that light is composed of tiny particles called “photons,” and each photon has energy of , where h = 6.63 × 10−34 J/s is Planck’s constant and ν is the frequency of photons.

In 1924, de Broglie proposed his theory of wave–particle duality in which he said that not only photons of light but also particles of matter such as electrons and atoms possess a dual character, sometimes behaving like a particle and sometimes as a wave. He gave a formula, λ = h/p, to connect particle characteristics (momentum, p) and wave characteristics (wavelength, λ). Light as well as particle can exhibit both wave and particle properties at the same time. Light waves are also called electromagnetic waves because they are made up of both electric (E) and magnetic (H) fields. Electromagnetic radiation waves can transport energy from one location to another based on Maxwell’s equations. Maxwell’s equations describe the electromagnetic wave at the classical level. Light is a transverse wave and electromagnetic fields E and H are always perpendicular to each other and oscillate perpendicular to the direction of the traveling wave. The particle properties of light can also be described in terms of a stream of photons that are massless particles and traveling with wavelike properties at the speed of light. A photon is the smallest quantity (quantum) of energy that can be transported.

In 1803, Thomas Young studied the wave properties of light by interference of light through shining two narrow slits separated equally from the center axis. The light emerging from the double-slit spreads out according to Huygens’ principle, and the interference pattern appears after overlapping two wavefronts as shown in Fig. 2.1. The two beams exiting from the two slits are electromagnetic waves and can be described by A0sin(2πνt) and A0sin(2πνt + δ), respectively, where A0 is the field amplitude, ν is the frequency of light, t is the time, and δ = d∙sin(θ) is the path difference between the two beams at angle θ.

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Fig. 2.1

Thomas Young’s double-slit interference experiment

2.2 Physics of Laser

Although there are many different types of lasers, most lasers follow similar operation principle. The Light Amplification by Stimulated Emission of Radiation (LASER) was developed by Theodore Maiman first [4]. Since then, laser have found a wide range of different scientific and technical applications from the industrial to applied and fundamental research including information technology, consumer electronics, medicine, industry, military, law enforcement, and research. The invention of the laser in 1960 dates from the nineteenth century, when Albert Einstein explained the concept of “stimulated emission of radiation” in a paper delivered in 1916 and German physicist Max Planck proposed the quantum theory of light in 1900. As mentioned before, Planck assumed energy should be composed of discrete packets, or quanta, in the form of photons. According to Planck’s radiation law, when an oscillator changes from an energy state E2 to a state of lower energy E1, a photon with discrete amount of energy E2 – E1 =  emits.

The Danish physicist Niels Bohr expanded the quantum theory to help explain the structure of atoms called Bohr’s model . In Bohr’s model, the nucleus of an atom is surrounded by orbiting electrons that are confined to specific energy states depending on the chemical structure of the atom. In other words, electrons can only occupy certain energy states, which are fingerprints of each atom. An electron can absorb a photon and thereby can be pushed into a higher energy state, called absorption. When an electron is in an exited state, it is inherently unstable and will spontaneously drop to the lower energy states by releasing a photon, called spontaneous emission. The underlying principle of the laser phenomenon stimulated emission is purely a quantum effect. Einstein postulated that emission would be triggered by other photons, in which, an incoming photon of a specific frequency can interact with an excited electron causing it to drop to a lower energy state and release two photons with specific properties, including identical phase, frequency, polarization, and similar direction of propagation. The process of absorption, spontaneous emission, and stimulated emission is depicted in Fig. 2.2.

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Fig. 2.2

Schematic of absorption (a), spontaneous emission (b), and stimulated emission (c) of two photons with different frequencies

Notice that the root of the invention of laser lies in fundamental physics research, specifically, a 1917 paper by Albert Einstein on the quantum theory of radiation or stimulated emission, but it was a paper on laser theory published in 1958 by two physicists, Charles Townes and Arthur L. Schawlow, which spurred the race to make the first working laser. According to the Einstein principle, there is an equal probability that a photon will absorb or emit. Thereby, according to the Boltzmann distribution that when there are more atoms in the ground state than in the excited states and light is incident on the system of atoms, in thermal equilibrium, the probability of absorption of energy is much higher than emission. However, in the case that more atoms are in an excited state than in a ground state and strike with photons of energy similar to the excited atoms, many of atoms will induce the process of stimulated emission, whereby a single excited atom would emit a photon identical to the interacting photon. Under the proper conditions, a single input photon can result in a cascade of stimulated photons, and thereby amplification of photons will result. All of the photons generated in this way are in phase, traveling in the same direction, and have the same frequency as the input photon.

As shown in Fig. 2.3, a laser requires three major parts: (1) gain medium (e.g., gas, solid, liquid dye or semiconductor); (2) pump source (e.g., an electric discharge, flashlamp, or laser diode); and (3) the feedback system, e.g., optical resonator. For instance, in the case of the first invented laser, the gain medium was ruby, and the population inversion was produced by intense broadband illumination from a xenon flashlamp. However, in the case of diode-pumped lasers, the population inversion is produced by laser diode that benefits from higher total conversion efficiency. Laser wavelength emission is determined by the gain medium and the characteristics of the optical resonator (see, for example, [512]). It is noticeable that some high-gain lasers do not use an optical oscillator and work based on amplified spontaneous emission (ASE) without needing feedback of the light back into the gain medium. Such lasers emit light with low coherence but high bandwidth.

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Fig. 2.3

Schematic diagram of a typical laser (top) flashlamp pumped, (middle) laser-diode end-pumped, and (bottom) laser-diode side-pumped configurations, showing the three major parts: (1) laser gain medium, (2) pump source, and (3) optical resonator

For optical frequencies, population inversion cannot be achieved in a two-level system. In 1956, Bloembergen proposed a mechanism in which atoms are pumped into an excited state by an external source of energy. A lasing medium consists of at least three energy levels: a ground state E1, an intermediate (metastable) state; E2, with a relatively long lifetime, ts, and a high energy pump state; and E3, as shown in Fig. 2.4. To obtain population inversion, ts must be greater than t3, the lifetime of the pump state E3. Note that a characteristic of the three-level laser material is that the laser transition takes place between the excited laser level E2 and the final ground state E1, the lowest energy level of the system. The three-level system has low efficiency. The four-level system avoids this disadvantage.

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Fig. 2.4

Schematic diagram of three-level laser (a) and four-level laser (b)

Figure 2.4 compares schematically the three-level and four-level laser systems. In three-level lasers, initially, all atoms of the laser material are in the ground state level E1. The pump radiation rises the ground state atoms to a short-lived pump state E3. Atoms from this state undergo fast decay (radiationless transition) to a metastable state E2. In this process, the energy lost by the electron is transferred to the lattice. A population inversion takes place between ground state and the metastable state where the lasing transition occurs. In general, the “pumping” level 3 is actually made up of a number of bands, so that the optical pumping can be accomplished over a broad spectral range. If pumping intensity is below laser threshold, atoms in level 2 predominantly return to the ground state by spontaneous emission. While, when the pump intensity is above laser threshold, the stimulated emission is the dominated processes compared with spontaneous emission. The stimulated radiation produces the laser output beam.

In the case of four-level lasers , the pump excitation extends again by radiation from the ground state (now level E0) to a broad absorption band E3. As in the case of the three-level system, the atoms so excited will transfer fast radiationless transitions into the intermediate sharp level 2. The electrons return to the fourth level E1, which is situated above the ground state E0, by the emission of a photon to proceed the laser action. Finally, the electrons return to the ground level E0 by radiationless transition. In a true four-level system, the terminal laser level E1 will be empty. To qualify as a four-level system, a material must possess a relaxation time between the terminal laser level and the ground level, which is fast compared to the fluorescence lifetime, ts. In addition, the terminal laser level E1 would be far above the ground state E0 so that its thermal population can be considered as negligible. In a kind of situation, where the lower laser level is so close to the ground state that an appreciable population in that level occurs in thermal equilibrium at the operating temperature, the laser called quasi-three-level laser. As a consequence, the unpumped gain medium causes some reabsorption loss at the laser wavelength same as three-level lasers.

The purpose of the resonator is to provide the positive feedback necessary to cause oscillation. The resonator has mirror at the ends so that photons are reflected back and forth and are constantly renewing the process of stimulated emission as they strike more of the excited atoms in the laser medium. The mirrors also align the photons so that they force to travel in the same direction. Typically, one will be a high reflector (HR) and the other will be a partial reflector (PR). The latter is called output coupler that allows some of the light to transmit out of the resonator to produce the laser output beam. The buildup of oscillation is triggered by spontaneous emission. The produced photons by spontaneous emission are reflected by the mirrors back into the laser medium and amplified by stimulated emission. Other optical devices, such as prism, Q-switch modulators, filters, etalon, and lens, may be placed within the optical resonator to produce a tunable laser, pulsed laser, and narrow bandwidth laser or shape the laser beam.

If the gain medium has a homogeneous (Lorentzian) gain profile, as the oscillating intensity grows and the population of excited atoms depletes by causing sufficient stimulated emission, photons oscillating at ν0 can emit from all atoms in the medium, and oscillation at frequency ν0 can suppress oscillation at any other frequency under the gain profile. Generally, the oscillation will build up with frequency, which has maximum emission probability. However, in an inhomogeneously broadened gaseous medium, the additional oscillation at frequencies far away from ν0 is also possible.

2.3 Laser Light Properties

The laser happens when stimulated-emission process is dominant compared with absorption and spontaneous emission. It means, in laser, stimulated emission leads to the unique characteristics, e.g., (1) coherence, (2) divergence and directionality, (3) monochromatic, and (4) brightness. These properties differentiate laser light from ordinary light and make it very interesting for a range of applications.

2.3.1 Coherence

Coherence of electromagnetic radiation means maintaining a constant phase difference between two points of wavefront of the wave in space (spatial coherence) and in time (temporal coherence). Coherence is one of the most important concepts in optics and is strongly related to the ability of light to exhibit interference effects. Temporal coherence is related to the intrinsic spectrum bandwidth of the light source, while spatial coherence can be affected by size of the light source. Laser radiation has high spatial and temporal coherence compared with ordinary light sources. In ordinary light sources like bubble lamp, sodium lamp, and torchlight, the electron transition from higher energy level to lower energy level occurs in spontaneous process. In other words, electron transition in ordinary light sources is random in time. In these sources, no phase relation exists between the emitted photons, and the phase difference between different atoms changes in time. Thus, the photons emitted by an ordinary light source are out of phase as illustrated schematically in Fig. 2.5.

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Fig. 2.5

Schematic of incoherent (left) and (right) coherent light waves

In contrast to incoherent sources, in laser, a phase relation between electron transitions exists. In other words, in laser, electron transition occurs in specific time. Therefore, the emission of laser is in phase in space and time as illustrated schematically in Fig. 2.5. In laser, the stimulated emission process produces coherence light. Because of the coherence, a large amount of power can be concentrated in a narrow space. For a light source with a Gaussian emission spectrum, the coherence length can be obtained as follows:

$$ {l}_{\mathrm{c}}=c{\tau}_{\mathrm{c}}=\sqrt{\frac{2\ln 2}{\uppi n}}\frac{\lambda^2}{\Delta \lambda }=\sqrt{\frac{2\ln 2}{\uppi n}}\frac{c}{\Delta v} $$
(2.1)

where c is the speed of light, n is the refractive index of the medium, λ is the central wavelength, and Δλ is the full-width half-maximum (FWHM) of the emission peak in wavelength spectrum. The light sources with a small Δλ such as lasers are highly temporally coherent, while the light sources with a large Δλ such as white light lamps are temporally incoherent. The coherence length and coherence time of some medical optical sources and ordinary sources are compared in Table 2.1.

Table 2.1

Comparing the coherence length and coherence time of some medical laser systems and ordinary sources

Source

Δνc (THz)

τc

lc = c

Filtered sunlight (400–800 nm, λ0 = 550 nm)

374

1.8 fs

0.32 μm

InGaAs (Eg = 0.9 eV, λ0 = 1300 nm)

6.2

0.11 ps

17.7 μm

Low-pressure sodium lamp (λ0 = 589 nm)

0.5

1.33 ps

0.399 mm

Single-mode He-Ne laser (λ0 = 633 nm)

1 × 10−6

0.66 μs

198 m

CO2 laser (λ0 = 10.6 μm)

40 × 10−6

0.16 ns

4.8 m

Ruby laser (λ0 = 694 nm)

0.36

1.85 ps

0.555 mm

Nd:YAG (λ0 = 1064 nm)

0.18

3.7 ps

1.11 mm

Nd:Glass (λ0 = 1059 nm)

9

73.8 fs

22.2 μm

Dye laser (Typ. R6G λ0 = 570–610 nm)

100

6.6 fs

1.98 μm

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Jul 22, 2021 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Introduction to Laser
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