Introduction to Optoelectronics Optical communication (3) Optical components

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Introduction to OptoelectronicsOptical
communication (3)Optical components

• Prof. Katsuaki Sato

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Optical components

• Previous lectures
• (1) Optical fibers
• Transmission of light by total reflection
• (2) Laser diodes
• The pn-junction is forward biased
• Above threshold current lasing occurs
• High density of carriers and photons are confined
  in thin active layer (DH structure)

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Component (3)Optical detectors

• Using photodiode
• Very fast response required
• pin photodiode or Schottky junction photodiode
  are used
• As material for photodiode InGaAs semiconductors
  are used

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Photodiode
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Photodetection

• Pin-PD
• Schottky PD
• Response is determined by capacitance of
  depletion layer where photocarrier flows
• Thinning of depletion layer and reduction of
  junction area is necessary

pin photodiode
Schottky diode
Andrew Davidson, Focused Research Inc. and Kathy
Li Dessau, New Focus Inc.
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Fundamentals of photodiode

• Illuminate the pn junction
• Electrons and holes are generated by an
  excitation across the gap
• Generated electrons and holes are separated and
  drift to electrodes by diffusion potential

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Component 4Fiber amplifier

• Light signal traveling in optical fiber for 100
  km suffers 20 dB(1/100)attenuation. Therefore the
  light intensity should be recovered. Optical
  fiber amplifier is used for this purpose.
• Optical amplifier consists of an erbium doped
  fiber (EDF) and a pumping laser. By introducing
  the strong pumping light to EDF the signal light
  can be amplified by stimulated emission from Er
  ion.

Asahi Glass Company HPhttp//www.agc.co.jp/news/20
00/0620.html??
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Amplification by Er ion

• EDF absorbs light with wavelength of 980nm
  or1480nm and emits infrared light with wavelength
  of1530nm. Optical amplification is possible
  utilizing stimulated emission of the 1530-nm
  luminescence.
• Inputting pumping laser light into EDF, Er ion
  become excited by absorbing the laser light and
  the signal light stimulate to make a transition
  to the ground level emitting the light with
  wavelength around 1530 nm, which is close to the
  signal light wavelength. Thus the incident light
  is amplified utilizing the emitted light.
• Luminescent intensity and spectrum width differ
  from sample to sample according to the
  concentration of doped Er-ion. The broader the
  bandwidth of the emission band the broader the
  bandwidth of communication.

From the Web-site of Asahi Glass Inc.
HPhttp//www.agc.co.jp/news/2000/0620.html
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Component 5Optical isolator

• Optical isolator is an optical component that
  makes the light direction oneway.
• Operation of laser diodes (LD) and optical
  amplifiers (EDFA) become unstable and generate
  noise when returned light enters.
• Optical isolator utilize Faraday effect to cut
  off the returned beam and stabilize the operation
  of lasers and amplifiers.

Shinkosha http//www.shinkosha.com/products/optica
l/
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Polarization-dependent isolator
analyzer
Magnetic field
Returned beam
polarizer
Faraday rotator
Forward direction
Incident light
Reversed direction
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Polarization-independent isolator
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Component 6WDMwavelength division multiplexing

• WDM technique can increase communication capacity
  by transmitting many different light signal of
  different wavelength simultaneously.
• Fiber cables can utilize wavelength region from
  1450 to1650nm since the transmission loss is
  very low (less than 0.3dB/km) in this region.

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Optical add-drop

• Optical add-drop can separately drop desired
  wavelength from multiplexed-signal network or can
  add a particular wavelength to the network

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Optoelectronic integrated circuits (OEIC)

• Integration of optical and electronic
  semiconductor devices
• Two types of OEIC exist
• One is integration of light emitting devices
  (example LD) and driving FET circuits
• The other is integration of optical detection
  device like PD and electronic circuits for
  amplification and signal processing
• Compound semiconductors such as GaAs-based and
  InP-based alloy semiconductors are used.

http//www2.nsknet.or.jp/azuma/o/o0028.htm
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Magneto-optical circulator