Sec 14.5 - Light Amplication and Lasers 475 requency Proper...

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Light Amplif cation and Lasers 475 Proper Frequency Required A laser resonator is like an exclusive nightclub—only certain frequencies are allowed. IN-DEPTH LOOK 14.1: LASER RESONATOR FREQUENCIES As explained above, in a resonator of length L , the only allowed wavelengths of light are those for which a whole number of half-wavelengths F t exactly between the two walls. If we denote the wavelength by λ , and the whole number by the symbol m ( m = 1,2,3,…), we can state this condition as an equation. m L = 2 We can use this equation to determine the allowed frequencies of the light in the reso- nator. Recall that wavelength equals the distance traveled by a wave during a time equal to one oscillation period T , which also equals one divided by frequency (1/ f ). Denoting the speed of light by c , as usual, we have = c / f . The condition above then gives: m c f L = 2 Multiplying both sides by f and dividing both sides by L gives allowed resonating frequencies: f m c L m =⋅ = 2 123 (, , , ) 14.5 HOW A LASER WORKS A laser is a device that emits a highly directional beam of pure-colored, coherent light. To make a laser, we combine the idea of light ampliF cation with the idea of a resonator. Figure 14.9 shows the layout of a laser, consisting of a gain medium with an energy pump to transfer energy to the electrons in the medium’s atoms and two re± ective mir- rors at the ends, making a resonator for light. A large circle shows an atom that contains stored energy and can contribute to light ampliF cation. A small circle shows an atom that has no stored energy and will absorb (not amplify) light. The resonator is made having two mirrors, one each placed at the ends of the gain medium. A mirror does not necessarily re± ect 100% of the light incident on it. A mirror that re± ects less than 100% of the incident light is called a partially re± ecting TAF-K10173-08-1107-014.indd 475 TAF-K10173-08-1107-014.indd 475 4/24/09 2:16:37 PM 4/24/09 2:16:37 PM
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476 The Silicon Web: Physics for the Internet Age mirror. An example of such a mirror can be found in one-way sunglasses, with reF ec- tivity equal to 0.90. This means that 90% of the light hitting either surface (inside or outside) is reF ected back, and 10% is transmitted through. The ref ectivity of a mirror equals the fraction of incident power that is reF ected when a light beam hits the mirror, as illustrated in Figure 14.10 . That is, R P reflected P incident P reflected R P == () ,( ) ( incident ) The value of the reF ectivity of each mirror making up the resonator is denoted by the symbol R . The value of R is between 0 (no reF ection) and 1 (total reF ection). A partially reF ecting mirror is typically made by coating a thin layer of aluminum or other material onto the surface of a clear piece of glass. This thin layer is shown in ±igure 14.9 as a bold line. Mirrors behave symmetrically; that is, the ref ectivity of light is the same regardless of which side of the mirror the light is incident on.
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Sec 14.5 - Light Amplication and Lasers 475 requency Proper...

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