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Photonics

Photonics. Photonics is the branch of science that studies or uses the manipulation of photons (the basic unit of light) to manipulate, encode, transmit, decode and/or store information. It involves:

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Photonics

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  1. Photonics

  2. Photonics is the branch of science that studies or uses the manipulation of photons (the basic unit of light) to manipulate, encode, transmit, decode and/or store information. It involves: • using electronic systems to convert information into light (or other electromagnetic radiation) energy signals • transmitting the light — for example, through optical fibres • converting the light signal back into useful information — for example, sound • storing information using light — for example, by burning CDs (compact discs) or DVDs (digital video discs).

  3. Transducers The infra-red LED is an example of a TRANSDUCER that transforms electrical energy to electromagnetic radiation. The infra-red radiation is the carrier of the signal. The photodetector is an example of a TRANSDUCER that transforms electromagnetic radiation to electrical energy.

  4. A Brief History In 1870 Alexander Graham Bell patented the PHOTOPHONE. It used unguided light to carry speech. Sunlight was directed onto a flat mirror. Speech caused the mirror to vibrate. The reflected light was collected by a concave mirror and directed onto a selenium detector. The resistance of the selenium changes with the light intensity. The VOICE-MODULATED sunlight varied the current through the receiver and this reproduced the voice. Bell was able to use this system over a distance of 200 m.

  5. Photonic TRANSDUCERS are devices that are used in photonic systems to transform light energy to electrical energy or electrical energy to light energy. These can be further divided into DETECTORS and SOURCES. Examples of detectors are LDRs, Photodiodes and Phototransistors. Examples of sources are LEDs and Laser Diodes.

  6. LDRs A light-dependent resistor (LDR) is a semiconductor device that has a resistance which decreases with the amount of light falling on it.

  7. Photodiodes

  8. Phototransistors A phototransistor is similar to an ordinary transistor except that incident light on the p–n junction controls the response of the device. Phototransistors have a built-in gain and are more sensitive than photodiodes.

  9. Modulation Modulation is the changing of one wave train caused by another wave. An example of this is amplitude modulation (AM) used in radio transmission systems. In this case, the amplitude of the carrier signal is changed by superimposing the waveform of a sound such as music or voice.

  10. (a) Optical intensity modulation system block diagram and (b) adding the signals

  11. Demodulation Demodulation is the process whereby the transmitted signal is converted back into an electrical signal. This can then be amplified and used to drive a loudspeaker (for example, in an AM radio) or used by a controlling system to control another device (for example, a television set). In photonic systems, the light signal falls on a transducer called a detector. The most common devices used as detectors are the photodiode or phototransistor. The detector produces an electrical current whose magnitude is proportional to the intensity of the light. This produces an identical electrical signal to that which modulated the light carrier waveform.

  12. Optical intensity demodulation system block diagram and • restoring the electrical signal

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