Introduction to Optoelectronics
Optical Communication(1)
Prof. Katsuaki Sato
Answer to the quiz
1. Can you tell a range of wavelength (in nm) of visible light? 
•	

1.5eV

3eV

2.5eV

2eV

800nm

300nm

3.5eV

4eV

The visible wavelength region is between 380 nm and 780 nm
•	Light whose wavelength is shorter than 380nm is called ultraviolet.
•	Light whose wavelength is longer than 780 nm is called infrared. 

2. Laser is an abbreviation of a word string. Spell it out.
•	Light Amplification by Stimulated Emission of Radiation
・	“Stimulated emission”
antonym of “spontaneous emission”
・	optical transition stimulated by the effect of electric field of light wave
on the contrary usually emission occur spontaneously without help of  electric field

Threshold current

Spontaneous 
emission

Laser action
With stimulated
emission

Forward bias current

Wavelength

Spontaneous 
emission

Laser action
With stimulated
emission

Light Intensity

(a)

(b)

3. What is the difference between LED and LD?
•	LED is light emitting diode
•	LD is laser diode
・	Diode is a semiconductor device which has an effect of rectification
・	Both LED and LD are semiconductor diode with a forward bias. Both emit light
・	LED emits light by spontaneous emission mechanism, while LD has an optical cavity which enables multiplication of photon by stimulated emission 
LED and LD
•	LD (laser diode) works as LED if the operating current does not exceed the threshold value.

4. What is the difference between CD-R and CD-RW? 
•	CD-R is a write-once memory.
It uses organic dye which is decomposed by laser irradiation
•	CD-RW is a rewritable memory
It uses phase change for recording and erasing

5. Explain how the light is transmitted through optical fiber.
•	Light is transmitted along the core by total reflection mechanism at the boundary with the cladding layer

N=1.46

N=1.48

cladding

core

Light Ray Entering
Core from Air

Light is propagated by Total internal reflection

CROSS SECTION

http://magazine.fujitsu.com/vol48-3/6.html

Light wave network

WDM

EDFA

ADSL

FTTH

Data carrying capacity (bps)

Development of Optical Communication

Development of Communication： Trend of increase in data carrying capacity
•	To meet with the growing need for large capacity information exchange, optical fiber communication system has been developed.

Broadband and Narrow band
•	Recently we hear the word “broadband”. What does the “broadband” mean?  
•	Broadband stands for broad bandwidth in communication path. The band width is defined in the frequency domain, the broader the frequency region for communication the larger is the capacity of information.
•	The broadness of communication path is expressed in terms of bit transfer rate, the unit being bps=bit per second.
•	Analog telephone line with metallic cable has a bit rate of 56 kbps, while digital telephone (ISDN=integrated service digital network) line has a rate of 128 kbps. These are called “narrowband” systems.
•	On the contrary, even metallic line can have higher rate by using  ADSL (asymmetric digital subscriber line) which has asymmetric data rate of 1.5Mbps to 50 Mbps for downward steam, and 12kbps to 3 Mbps for upward steam. Higher rate is  provided only for close area from station.
•	
down

up

FTTH (fiber to the home) supports 100Mbps for both up and down stream.

Optical fiber is becoming the leading actor of information network
•	FTTH can transfer as large as100Mbit=13MB information per second
•	It means you take only one minute to download information on one CD sheet（~640MB）

QUIZ１
1.	The distance between Tokyo and Paris is approximately 10,000km. Calculate the time necessary to transfer signal between two cities, assuming the refractive index of fiber as n=1.5. (Light speed in the fiber c’ can be calculated as c/n)
2.	The height from the earth of a communication satellite is about 35,000km. Estimate the delay of signal to go to and fro between the earth and the satellite.

Important components for optical communication
•	Light source：LD=laser diode
•	pn junction, DH structure, DFB structure
•	Transmission line: Optical fiber
•	Total reflection, attenuation by Rayleigh scattering and infrared absorption
•	Photo detector: PD=photodiode
•	Amplifier: EDFA=Erbium-doped fiber amplifier
•	Elements: isolator, attenuator, circulator

critical angle
c

medium 2

medium 1

ic

ic

evanescent field

total reflection and evanescent wave

Component（１）
Optical Fiber
•	Material： 
(fused silica SiO2)
•	Structure: 
concentric arrangement of core and cladding
•	Transmission:
total reflection of light at the boundary between core and cladding 

Total reflection

Transmission loss of optical fiber
•	Short wavelength: Rayleigh scattering
•	Log wavelength:
Infrared absorption
•	Loss peak at 1.4m comes from water vapor

Transmission loss of fiber

Physics　Today　Online
http://www.aip.org/pt/vol-53/iss-9/captions/p30cap1.html

Attenuation and dispersion in optical fiber
•	Attenuation: reduction of light amplitude
•	
http://www.tpub.com/neets/tm/106-13.htm

Dispersion: deterioration of waveform

ＱＵＩＺ２
1.	Obtain the critical angle for core index of 1.5 and cladding index of 1.3.
2.	Obtain critical angle for realistic system: core index of 1.4 and cladding index of 1.38
3.	Using a fiber with a loss of 0.2dB/km, calculate an attenuation of light intensity when transmitted between Tokyo and Mt. Fuji. Use 1dB=10log(I0/I) for light intensity

dB decibel; logarithmic expression of signal intensity change
For voltage  dB=20log10(output voltage/input voltage)
For power   dB=10 log10(output power/input power)