1. Introduction to Optoelectronics Optical communication (3) Optical components
Prof. Katsuaki Sato

2. 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)

3. 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

4. Photodiode Optical signal Incident photon
Structure and band diagram of photo-diode p n
Depletion layer
Reverse bias
Incident
photon
Electric signal
Bias voltage
Load resistance
Photocurrent
Optical signal
佐藤勝昭編著「応用物性」p.152

5. 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.

6. Fundamentals of photodiode
ｐ-type
ｎ-type
Depletion layer + -
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

7. 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.
EDF
Optical isolator
Pumping laser
Composer
Band pass filter
Light input
Asahi Glass Company
HPhttp://

8. 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://

9. 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

10. Polarization-dependent isolator
analyzer
Magnetic field
Returned beam
polarizer
Faraday rotator
Forward direction
Incident light
Reversed direction

11. Polarization-independent isolator
Fiber 2
Fiber 1
Forward direction
Reverse direction
½ waveplate C
Birefringent plate B2 B2 B1 F C
Birefringent plate B1
Faraday rotator F

12. 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.

13. 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

14. 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.

15. Magneto-optical circulator
Prism polarizer A
Faraday rotator
Prism polarizer B
Half wave plate
Port 1
Port 3
Port 2
Port 4
Reflection prism