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8.6 Lasers  (Page 2/9)

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An illustration of the details of a compact disc. A laser beam hits the disc from below at right angles. The disc consists of three layers. The lower layer is a polycarbonate plastic layer with alternating pits and bumps. A thin layer of Aluminum is deposited on top of the plastic layer. A layer of laquer covers the disc, filling in the bumps and pits and forming a smooth upper surface. The entire disc, including all three layers, is 1.2 m m thick.
A compact disc is a plastic disc that uses bumps near its surface to encode digital information. The surface of the disc contains multiple layers, including a layer of aluminum and one of polycarbonate plastic.

A CD player uses a laser to read this digital information. Laser light is suited to this purpose, because coherent light can be focused onto an incredibly small spot and therefore distinguish between bumps and pits in the CD. After processing by player components (including a diffraction grating, polarizer, and collimator), laser light is focused by a lens onto the CD surface. Light that strikes a bump (“land”) is merely reflected, but light that strikes a “pit” destructively interferes, so no light returns (the details of this process are not important to this discussion). Reflected light is interpreted as a “1” and unreflected light is interpreted as a “0.” The resulting digital signal is converted into an analog signal, and the analog signal is fed into an amplifier that powers a device such as a pair of headphones. The laser system of a CD player is shown in [link] .

A photograph of the inner working of a CD player
A CD player and its laser component.

Blu-ray player

Like a CD player, a Blu-Ray player reads digital information (video or audio) stored on a disc, and a laser is used to record this information. The pits on a Blu-Ray disc are much smaller and more closely packed together than for a CD, so much more information can be stored. As a result, the resolving power of the laser must be greater. This is achieved using short wavelength ( λ = 405 nm ) blue laser light—hence, the name “Blu-” Ray. (CDs and DVDs use red laser light.) The different pit sizes and player-hardware configurations of a CD, DVD, and Blu-Ray player are shown in [link] . The pit sizes of a Blu-Ray disk are more than twice as small as the pits on a DVD or CD. Unlike a CD, a Blu-Ray disc store data on a polycarbonate layer, which places the data closer to the lens and avoids readability problems. A hard coating is used to protect the data since it is so close to the surface.

The different pit sizes and player-hardware configurations of a CD, DVD, and Blu-Ray player are illustrated. In each case, the pits are smaller than the size of the spot made by the laser beam on the surface of storage medium. On the left, the CD player, with 0.7 GB storage capacity, is shown. The CD laser has a wavelength of lambda equal to 780 nanometers, corresponding to a red color. It is focused by a lens, penetrating the CD material to a depth of 1.2 m m and forming a relatively large spot on the surface of the CD. In the middle, the DVD player, with 4.7 GB storage capacity, is shown. The DVD laser has a wavelength of lambda equal to 650 nanometers, corresponding to a reddish-orange color. It is focused by a lens, penetrating the DVD material to a depth of 0.6 m m and forming a smaller spot on the surface of the DVD than we saw on the CD. On the right, the Blue-Ray player, with 25 GB storage capacity, is shown. The blue-Ray laser has a wavelength of lambda equal to 405 nanometers, corresponding to a blue color. It is focused by a lens, penetrating the blue-ray disc material to a depth of 0.1 m m and forming a small spot on the surface of the disc.
Comparison of laser resolution in a CD, DVD, and Blu-Ray Player.

Summary

  • Laser light is coherent (monochromatic and “phase linked”) light.
  • Laser light is produced by population inversion and subsequent de-excitation of electrons in a material (solid, liquid, or gas).
  • CD and Blu-Ray players uses lasers to read digital information stored on discs.

Key equations

Orbital angular momentum L = l ( l + 1 ) ℏ
z -component of orbital angular momentum L z = m ℏ
Radial probability density function P ( r ) d r = | ψ n 00 | 2 4 π r 2 d r
Spin angular momentum S = s ( s + 1 ) ℏ
z -component of spin angular momentum S z = m s ℏ
Electron spin magnetic moment μ → s = ( e m e ) S →
Electron orbital magnetic dipole moment μ → = − ( e 2 m e ) L →
Potential energy associated with the magnetic
interaction between the orbital magnetic dipole
moment and an external magnetic field B → 		U ( θ ) = − μ z B = m μ B B
Maximum number of electrons in a subshell of
a hydrogen atom		 N = 4 l + 2
Selection rule for atomic transitions in a
hydrogen-like atom		 Δ l = ± 1
Moseley’s law for X-ray production ( Z − 1 ) = constant f

Conceptual questions

Distinguish between coherent and monochromatic light.

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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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