Skip to main content
Physics LibreTexts

8.6: Lasers

  • Page ID
    4539
  • [ "article:topic", "authorname:openstax", "monochromatic", "coherent light", "laser", "metastable state", "population inversion", "stimulated emission", "license:ccby" ]

    laser is device that emits coherent and monochromatic light. The light is coherent if photons that compose the light are in-phase, and monochromatic if the photons have a single frequency (color). When a gas in the laser absorbs radiation, electrons are elevated to different energy levels. Most electrons return immediately to the ground state, but others linger in what is called a metastable state. It is possible to place a majority of these atoms in a metastable state, a condition called a population inversion.

    An illustration of the amplification of light in a laser. Two energy levels are shown as dotted lines, one above the other at three different times. Electrons are in the higher state which is a metastable state, and transition to the lower state. A light wave with energy h f arrives, causing the electron to drop to the lower state. Two identical, in phase photons of energy h f are emitted and absorbed by more electrons in the metastable state. These electrons drop to the lower state and emit four identical, in phase photos of energy h f, which are then absorbed by the third set of electrons. The electrons transition to the lower state and emit eight identical, in phase photons of energy h f.

    Figure \(\PageIndex{1}\): The physics of a laser. An incident photon of frequency f causes a cascade of photons of the same frequency.

    When a photon of energy disturbs an electron in a metastable state (Figure \(\PageIndex{1}\)), the electron drops to the lower-energy level and emits an addition photon, and the two photons proceed off together. This process is called stimulated emission. It occurs with relatively high probability when the energy of the incoming photon is equal to the energy difference between the excited and “de-excited” energy levels of the electron (\(\Delta E = hf\)). Hence, the incoming photon and the photon produced by de-excitation have the same energy, hf. These photons encounter more electrons in the metastable state, and the process repeats. The result is a cascade or chain reaction of similar de-excitations. Laser light is coherent because all light waves in laser light share the same frequency (color) and the same phase (any two points of along a line perpendicular to the direction of motion are on the “same part” of the wave”). A schematic diagram of coherent and incoherent light wave pattern is given in Figure \(\PageIndex{2}\).

    An illustration of coherent light wave pattern and incoherent light wave pattern. The coherent light consists of waves of the same wavelength, phase and amplitude, so that all the crests are aligned and all the troughs are aligned. The incoherent light consists of waves of different wavelengths, phases and amplitudes, resulting in overlapping crests and troughs of different waves.

    Figure \(\PageIndex{2}\): A coherent light wave pattern contains light waves of the same frequency and phase. An incoherent light wave pattern contains light waves of different frequencies and phases.

    Lasers are used in a wide range of applications, such as in communication (optical fiber phone lines), entertainment (laser light shows), medicine (removing tumors and cauterizing vessels in the retina), and in retail sales (bar code readers). Lasers can also be produced by a large range of materials, including solids (for example, the ruby crystal), gases (helium-gas mixture), and liquids (organic dyes). Recently, a laser was even created using gelatin—an edible laser! Below we discuss two practical applications in detail: CD players and Blu-Ray Players.

    CD Player

    A CD player reads digital information stored on a compact disc (CD). A CD is 6-inch diameter disc made of plastic that contains small “bumps” and “pits” nears its surface to encode digital or binary data (Figure \(\PageIndex{3}\)). The bumps and pits appear along a very thin track that spirals outwards from the center of the disc. The width of the track is smaller than 1/20th the width of a human hair, and the heights of the bumps are even smaller yet.

    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.

    Figure \(\PageIndex{3}\): 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 Figure \(\PageIndex{4}\).

    A photograph of the inner working of a CD player

    Figure \(\PageIndex{4}\): 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 Figure \(\PageIndex{5}\). 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.

    Figure \(\PageIndex{5}\): Comparison of laser resolution in a CD, DVD, and Blu-Ray Player.

    Contributors

    • Samuel J. Ling (Truman State University), Jeff Sanny (Loyola Marymount University), and Bill Moebs with many contributing authors. This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).