Introduction to Optoelectronics
Optical communication (3)
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
Component (3)
Optical detectors
• Using photodiode
• Very fast response required
• pin photodiode or Schottky junction photodiod
e are used
Photodiode
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Structure and band diagram of photo-diode
p Depletion n layer
Reverse bias
Incident photon
Electric signal
Bias voltage Load resistance
Photocurrent
Photodetection
• Pin-PD
• Schottky PD
• Response is determin ed by capacitance of depletion layer where photocarrier flows
• Thinning of depletion l ayer and reduction of junction area is neces sary
Andrew Davidson, Focused Research Inc. and Kathy Li Dessau, New Focus Inc.
pin photodiode
Fundamentals of photodiode
• Illuminate the pn junctio
n
• Electrons and holes are
generated by an excitati
on across the gap
• Generated electrons an
d holes are separated a
nd drift to electrodes by
diffusion potential
Component
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Fiber amplifier
• Light signal traveling in optical fiber for 100 km suffers 20 dB
佐 1/100 佐 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) an d a pumping laser. By introducing the strong pumping light t o EDF the signal light can be amplified by stimulated emissi on from Er ion.
Asahi Glass Company
HPhttp://www.agc.co.jp/news/20 00/0620.html 佐佐
EDF Optical isolator
Pumping laser
Composer Band pass filter
Amplification by Er ion
• EDF absorbs light with wavelength of 980nm or1480nm and e mits infrared light with wavelength of1530nm. Optical amplificat ion is possible utilizing stimulated emission of the 1530-nm lum inescence.
• Inputting pumping laser light into EDF, Er ion become excited b y absorbing the laser light and the signal light stimulate to mak e a transition to the ground level emitting the light with wavelen gth around 1530 nm, which is close to the signal light waveleng th. 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 bro ader the bandwidth of the emission band the broader the band width of communication.
Component
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Optical isolator
• Optical isolator is an optical comp onent that makes the light directio n oneway.
• Operation of laser diodes (LD) an d optical amplifiers (EDFA) beco me unstable and generate noise when returned light enters.
• Optical isolator utilize Faraday eff ect to cut off the returned beam a nd stabilize the operation of laser s and amplifiers.
Polarization-dependent isolator
polarizer
analyzer
Faraday rotator
Magnetic field
Incident light
Returned beam
Forward direction
Polarization-independent
½ waveplate CBirefringent plate B2
B2 B1
F C
Birefringent plate B1
Fiber 2
Fiber 1
Component
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WDM=wavelength 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
Optical add-drop
• Optical add-drop can separately drop desired
wavelength from multiplexed-signal network or
can add a particular wavelength to the network
Grating optical fiber
Optoelectronic integrated circuits
(OEIC)
• Integration of optical and electronic semicond
uctor devices
• Two types of OEIC exist
– One is integration of light emitting devices (exampl e: 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 an d InP-based alloy semiconductors are used.
Magneto-optical circulator
Prism polarizer A Faraday rotator
Prism polarizer B
Half wave plate
Port 1
Port 3
Port 2
Port 4
