Introduction to Optoelectronics
Optical communication (3)
Optical components
Prof. Katsuaki Sato
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)
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
Photodiode
Optical signal
Photocurrent
Incident
photon
Reverse
bias
Electric signal
Load resistance
Bias voltage
p
Deplet
ion
layer
Structure and band diagram of photo-diode
佐藤勝昭編著「応用物性」p.152
n
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.
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
ｐ-type
-
+
+
+
+
ｎ-type
Depletion layer
Component ４
Fiber 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.
Optical isolator
Asahi Glass Company
HPhttp://www.agc.co.jp/news/2
000/0620.htmlより
EDF
Light
input
Composer
Pumping laser
Band pass filter
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
Component ５
Optical 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/optical/
Polarization-dependent isolator
Magnetic
field
analyzer
Returned
beam
polarizer
Faraday
rotator
Incident
light
Forward direction
Reversed direction
Polarization-independent
isolator
Faraday rotator F
½ waveplate C
Birefringent plate
B2
Birefringent plate B1
Fiber 1
Fiber 2
Forward direction
B1
F
C
B2
Fiber 1
Fiber 2
Reverse direction
Component ６
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
1450 to1650nm since the transmission loss is very
low (less than 0.3dB/km) in this region.
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
Optical circulator
Optical circulator
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
Magneto-optical circulator
Prism polarizer A
Faraday rotator
Reflection prism
Port 1
Port 3
Half wave plate
Port 2
Port 4
Prism polarizer B