Optical Recording and Optical Recording and That audio or video is - PDF document

Optical Recording and Communications 1 Optical Recording and Communications 2 Observations about Optical Observations about Optical Recording and Communications Recording and Communications � Optical disks can store lots of audio or video Optical disks can store lots of audio or video Optical Recording and Optical Recording and � That audio or video is of the highest quality That audio or video is of the highest quality Communications Communications � Optical disks continue to play perfectly for years Optical disks continue to play perfectly for years � Playback of optical disks involves lasers Playback of optical disks involves lasers � Lasers and fibers are used in communication Lasers and fibers are used in communication Optical Recording and Communications 3 Optical Recording and Communications 4 5 Questions about Optical Recording 5 Questions about Optical Recording Question 1 Question 1 and Communication and Communication � How is information represented digitally? How is information represented digitally? � How is information represented digitally? How is information represented digitally? � How is information recorded on an optical disk? How is information recorded on an optical disk? � How is information read from an optical disk? How is information read from an optical disk? � How can light carry information long distances? How can light carry information long distances? � Why does light follow an optical fiber’s bends? Why does light follow an optical fiber’s bends? Optical Recording and Communications 5 Optical Recording and Communications 6 Review of Digital Representation Review of Digital Representation Digital Audio Digital Audio � Audio or video info is a sequence of numbers Audio or video info is a sequence of numbers � The air pressure in sound The air pressure in sound � � Each number can be represented digitally Each number can be represented digitally is measured thousands of is measured thousands of � times per second times per second � by putting specific symbols in a set of digits. by putting specific symbols in a set of digits. � Digital representations often involve binary digits, � Digital representations often involve binary digits Digital representations often involve binary digits Digital representations often involve binary digits, � Each meas rement is � Each measurement is Each meas rement is Each measurement is � which can each hold only two symbols: which can each hold only two symbols: 0 0 and and 1 1. . represented digitally using represented digitally using � Each symbol is a discrete value of a physical quantity Each symbol is a discrete value of a physical quantity about 16 binary digits, about 16 binary digits, � Digital representations have Digital representations have each a each a 0 0 or a or a 1 1. . � good noise good noise- -immunity immunity � and permit error correction (elimination of noise). and permit error correction (elimination of noise). 1

Optical Recording and Communications 7 Optical Recording and Communications 8 Question 2 Question 2 Structure of CDs and DVDs Structure of CDs and DVDs � How is information recorded on an optical disk? How is information recorded on an optical disk? � These disks have spiral tracks in reflective layers These disks have spiral tracks in reflective layers � Each track contains pits and flats Each track contains pits and flats � Digital symbols consist of the Digital symbols consist of the lengths of the pits and flats l l lengths of the pits and flats h h f h f h i i d fl d fl � The track structure is made as The track structure is made as small as can be detected by the small as can be detected by the playback system, so as to maximize playback system, so as to maximize the density of information. the density of information. Optical Recording and Communications 9 Optical Recording and Communications 10 Question 3 Question 3 Playback Techniques Playback Techniques � How is information read from an optical disk? How is information read from an optical disk? � Laser light is focused on the shiny layer Laser light is focused on the shiny layer � Reflection is weaker from a pit than from a flat Reflection is weaker from a pit than from a flat � Reflected light is directed to photodiodes Reflected light is directed to photodiodes � Reflected light intensity Reflected light intensity indicates pits or flats indicates pits or flats Optical Recording and Communications 11 Optical Recording and Communications 12 Playback Issues Playback Issues Advantages of Digital Recording Advantages of Digital Recording � Light must hit pits perfectly Light must hit pits perfectly � Freedom from noise and media damage issues Freedom from noise and media damage issues � Feedback optimizes the position of the light spot Feedback optimizes the position of the light spot � Digital representation avoids information loss Digital representation avoids information loss � Error correction ensures clean information Error correction ensures clean information � Light must hit only one pit Light must hit only one pit � Surface contamination doesn’t matter (much) � S rf Surface contamination doesn’t matter (much) S rf nt min ti n d nt min ti n d n’t m tt r (m n’t m tt r (m h) h) � Use laser light Use laser light—a single wave U U l l li h li h a single wave i i l l � High information density High information density � Use a converging lens to focus that wave to a spot Use a converging lens to focus that wave to a spot � The wave forms a “waist” The wave forms a “waist”— —its minimum width its minimum width � Data compression is possible Data compression is possible � Wave limits on focusing are known as diffraction Wave limits on focusing are known as diffraction � Perfect, loss Perfect, loss- -less copies are possible less copies are possible � Waist can’t be much smaller than a wavelength Waist can’t be much smaller than a wavelength � so pit size can’t be much smaller than a wavelength. so pit size can’t be much smaller than a wavelength. 2

Optical Recording and Communications 13 Optical Recording and Communications 14 Question 4 Question 4 Optical Communication Optical Communication � How can light carry information long distances? How can light carry information long distances? � Both analog and digital representations possible Both analog and digital representations possible � Analog representation Analog representation � often involves AM modulation of the light often involves AM modulation of the light � and is often used for remote process monitoring. and is often used for remote process monitoring. d i d i f f d f d f i i i i � Digital representation Digital representation � often uses pulses with discrete amplitudes as symbols often uses pulses with discrete amplitudes as symbols � and provides noise and provides noise- -immunity, error correction, immunity, error correction, compression, and channel- compression, and channel -sharing. sharing. Optical Recording and Communications 15 Optical Recording and Communications 16 The Components of Optical The Components of Optical Transmission Techniques Transmission Techniques Communications Communications � Light in air: direct line Light in air: direct line- -of of- -sight sight � Transmitters Transmitters � Incandescent lamps (poor performance) Incandescent lamps (poor performance) � Infrared remote controls Infrared remote controls � Light Emitting Diodes (adequate performance) Light Emitting Diodes (adequate performance) � Infrared computer links Infrared computer links � Laser Diodes (high performance) Laser Diodes (high performance) � Light in optical fibers: arbitrary paths Light in optical fibers: arbitrary paths Li h i Li h i i i l fib l fib bi bi h h � Receivers Receivers � Optical cables and networks Optical cables and networks � Photoresistive cells (poor performance) Photoresistive cells (poor performance) � Photodiodes (high performance) Photodiodes (high performance) � Conduits Conduits � Optical Fibers (ranging from poor to high performance) Optical Fibers (ranging from poor to high performance) Optical Recording and Communications 17 Optical Recording and Communications 18 Question 5 Question 5 Total Internal Reflection Total Internal Reflection � Why does light follow an optical fiber’s bends? Why does light follow an optical fiber’s bends? � As light enters a material with a lower index of As light enters a material with a lower index of refraction, it bends away from the perpendicular refraction, it bends away from the perpendicular � If that bend exceeds 90 If that bend exceeds 90° °, , the light reflects instead the light reflects instead the light reflects instead the light reflects instead � That reflection is perfect: That reflection is perfect: total internal reflection total internal reflection 3