Optical Communication

1. Engr. Abbas Abbasi
Fiber Optic Communications
An Introduction

2. Electromagnetic Spectrum
Used for optical communications

3. Fiber Optic Communications
 Use high carrier frequencies (∼100 THz)
 Also known as Lightwave systems
 Guided and unguided lightwave systems
 Guided systems use fiber optics as the guiding medium

4. Historical perspective of communication technologies
•Advent in 1830
•Used Morse Code of dots
and dashes
•First transatlantic telegraph
cable in 1866
•Bit rate ~ 10 b/s
•Invented in 1876
•Analog transmission
•Used two copper wires as
the transmitting media
•First coaxial system in 1940
•3 MHz Bandwidth
•300 voice channels or a
single TV channel
•Losses increase when used
for frequencies higher than
10 MHz
•More advanced coaxial
systems operate at 274 Mb/s
•Repeater spacing ~ 1Km
•Frequency dependent
losses of coaxial cable led to
Microwave communications
•1-10 GHz
•Used AM, FM or PM
•First microwave system in
1948
•Larger repeater spacing
•Low bit rate
•The demand for High bit
rate and bandwidth led to
optical communication
•Laser invented in 1960
•Optical fibers suggested in
1966
•1960s fiber loss ~ 1000
dB/Km
•1970 fiber loss ~ 20 dB/Km
•First system in 1980 (USA)

5. Fiber optic advancements

6. Evolution of fiber-optic communication systems
•Operated near 0.8 µm
•GaAs Lasers
•Commercial
availability in 1980
•Bit rate 45 Mb/s
•L ~ 10Km
•Operated near 1.3 µm
•InGaAsP Lasers
•Commercial
availability in 1987
•Bit rate 1.7 Gb/s
•L ~ 50Km
•Loss ~ 0.5 dB/Km
•Operated near 1.55 µm
•InGaAsP Lasers
•Commercial availability
in 1990
•Bit rate 10 Gb/s
•L ~ 70Km
•Loss ~ 0.2 dB/Km
•Optical Amplifiers
•Wavelength Division
Multiplexing (WDM)
•Transatlantic and
transpacific systems
•Bit rates reached to
10Tb/s in 2001

7. Global fiber-optic undersea networks

8. Analog Vs Digital Signals
Bit: Binary Digit
TB : Bit Slot
Bit Rate = 1/TB
Example: ASCII code
Δf = Bandwidth of the signal
determined by Fourier
transform

9. Analog to digital conversion
Sampling theorem: a bandwidth-limited signal
can be fully represented by discrete samples,
without any loss of information, provided that
the sampling frequency fs satisfies the Nyquist
criterion fs ≥ 2Δf
M Discrete Intervals (M >Amax/AN)
Quantization noise
Dynamic range (Amax/AN)
SNR: Signal to noise ratio

10. Quantized to Digital conversion techniques
 Pulse-position modulation, pulse position within the bit slot is a measure of the sampled
value (rarely used)
 Pulse-duration modulation, the pulse width is varied from bit to bit in accordance with
the sampled value (rarely used)
 Pulse-code modulation (PCM), A binary code is used to convert each sampled value into
a string of 1 and 0 bits

11. Pulse Code Modulation (PCM)
 The number of bits m needed to code each sample is related to the number of quantized
signal levels M by the relation
M = 2m or m = log2M
 The bit rate associated with the PCM digital signal is thus given by
B = mfs ≥ (2Δf )log2M
where fs ≥ 2Δf
 M > Amax/AN
 log2M = (log210)(log10M)=3.33(log10 (Amax/AN))=(3.33/20)SNR=(1/6)SNR
 B> (2Δf )log2M  B> (Δf/3 )SNR

12. Example
 The analog audio signal contains frequencies in the range 0.3–3.4 kHz and has a SNR of
about 30 dB. Sampling rate is 8 kHz and each sample is represented by 8 bits.
 What is the minimum bandwidth requirement to transmit this signal?
 What is the bit rate for this transmission?

13. End of first module
 Thank you