optical fiber

Topics • Major milestones in electrical communication

• Communication Systems – 20th Century • Problems of Electrical Communication Systems • History of Optical Communication • Figure of merit for Communication Systems –

the B.L product • Optical Communication systems – free-space

and optical fiber • Optical Fiber Communication (OFC) –

advantages • Major developments in OFC since 1980

1.Major milestones in Electrical Communication

• 1838 – Samuel F.B. Morse invented Telegraphy • 1866 – first transatlantic telegraph cable • 1876 – Alexander Graham Bell invented

Telephone • 1905 – Triode based Electronic amplifier • 1940 – first coaxial-cable system (3 MHz –

3,000 voice channels or ONE television channel)

• 1948 – first microwave system (4 GHz) • 1975 – the most advanced coaxial system with a

bit rate of 274 Mb/s

2.Communication Systems of the 20th

Century • Wire – Telegraphy (2 wires for telegraph

transmission – simplex & duplex)

• Wire – Telephony (2 wires for telephone transmission of 1 channel)

• Carrier telephony (long-distance telephony for multiple channels – 4,8,16)

• Coaxial cable systems (for 32 channel PCM systems – 32x64kb/s = 2.048 Mb/s)

3.Problems of Electrical Communication systems • Affected by EMI • Low bandwidth (4 kHz – telephone,

100-500 MHz per km – coaxial cable ) • High attenuation (20 dB/km – typically) • High system cost (due to too many

repeaters for a given Bandwidth/ data rate) • Prone to tapping • Bulky

4.History of Optical Communication Systems

• Optical communication is older than electrical communication !

• 8th century B.C. - Greeks used fire signals for sending alarms, calls for help, etc

• 1792 – Claude Chappe’s Optical telegraph • 1880 – Alexander Graham Bell used light

beam for transmission of speech (Photophone)

• 1960 – invention of Ruby LASER

History of Opt.Commn…..contd

• 1966 – light confinement using sequence of gas lenses

• 1966 – suggestion to use optical fiber (Kao & Hockham)

• 1970 – Corning Glass optical fiber with 20 dB/km near 1 µm

• 1970 - Semiconductor Laser with CW operation at room temp.

• 1980 onwards – wide spread use of Optical Fiber Communication

5. Figure of merit for Communication Systems – the B.L product • A commonly used figure of merit for communication

systems is the bit rate-distance product, BL where B is the bit rate, and L is the repeater spacing.

• 1970 – Communication systems had a maximum value

of BL product = 100 Mb/s-km only, due to fundamental limitations.

• It was realized that BL product could only be increased through the use of optical waves as carrier.

6.What is Optical Fiber Communication (Fiber Optics) all about? • Optical transmission of electrical signals

using an electrical-to-optical converter (E/O converter), an optical fiber, and optical-to-electrical converter (O/E converter).

• E/O converters: LEDs, Laser Diodes

• O/E converters: Photodetectors

Increase in bit rate-distance product (BL) during the period 1850-2000.

(source: Chapter 1 - GP Agrawal, Fiber-Optic Communication Systems, 3rd

edition, John Wiley & Sons., Inc., New York, 2002)

Advantages of Optical Fiber Communication (Fiber Optics)

• Very high bandwidth (10 - 100 GHz, typ.)

• Very low attenuation (lowest 0.16 db/km)

• Immune to EMI

• Data security (almost impossible to tap information)

• Lower system cost (fewer repeaters due to low attenuation of fibers)

• Very low Bit Error Rate ( 10^-10 typically)

Basics of Optical Fiber Communication

An Optical Fiber Communication System consists of

• Transmitter (Optical source + driver circuit) • Optical Fiber • Receiver (Photodetector + receiver circuit) • Based on the communication system

requirements, the appropriate source, fiber, photodetector combination is chosen.

7. Optical Fiber Optical Fiber Types

Optical Fiber Dimensions

Cross section and refractive-index profile for step-index and graded- index fibers



Attenuation Characteristics – Single Mode Fiber

Optical Fiber Communication Transmission windows Improvements in Optical fiber attenuation and popular

transmission windows for Optical fiber communication (Source: Chapter 1, Gerd Keiser, Optical Fiber Communications, 3rd

edition, McGraw-Hill International Editions, Singapore, 2000)

Optical Fiber Cables • For Outdoor applications optical fibers

need to be armored. • Unlike copper cables optical fibers do not

have high tensile strength. • Optical fibers are put inside loose tubes or

V-grooves. • For Indoor applications tight buffered

cables with strengthening materials such as Kevlar are often employed.

Loose Tube Fiber Cable

Optical Fiber Cable for Outdoor Applications

Tight Buffered Cable

Optical Fiber Cable for Indoor Applications

8. Transmitter

• Optical transmitter is an electrical-to-optical

converter. • Sources – LED or Laser Diode (LD) • Principle – Varying the optical power of the

source by varying the current • LED – for short range and low data rate

applications • LD – for long range and high data rate

applications • Analog or Digital modulation of source current

Transmitter……contd.

• LEDs - used for low to medium bit rate applications (less

than 100 Mbits/sec) and lower optical link lengths. - are cheap and rugged - can be switched on and off (for digital modulation)

using simple logic drivers. • Laser diodes are used for high bit rate and longer optical

link applications. - are very sensitive to temperature changes - require sophisticated circuits for their field use. - Most commonly used circuits monitor the average

optical power and adjust the drive current automatically to maintain the required optical power.

Transmitter……contd. • Generally laser diodes come with fiber pigtails,

which are aligned in factory for optimum power coupling.

• An optical transmitter consists of an optical source (LED or LD) and a drive circuit which drives the required amount of current through the LED or LD.

• LED transmitters typically have output powers of 10 – 50 µW at the end of a fiber MMF pigtail.

• LDs typically give anywhere from 1mW – 20mW on a SMF pigtail.

Optical Spectrum of LEDs & Laser Diodes

9. Receiver

• An optical receiver is an optical-to-electrical converter +

amplifier and decision circuits. • Photodetectors are used for O/E conversion. • Two types - PIN and Avalanche Photodetector (APD) • Principle – generation of photo current using the light

from the fiber falling on the depletion region of a photo detector

• PIN – used for modest applications, no internal gain mechanism, cheap and rugged

• APD – used for applications requiring high sensitivity; provide internal optical gain of several tens. They require high bias voltages (>200V). Quite expensive.

Receiver………..contd.

• The photodetector (PIN or APD) followed by a

low noise amplifier. • The optical power detected is typically 1µW or

less. • front end amplifier must be a low noise

amplifier. • The bandwidth required at the receiver is

generally very high (several hundreds of MHz).

• Design of a fiber optic receiver circuit is quite a challenge.

Receiver………..contd.

– Most of the noise in the low noise amplifier is introduced by the first device.

– For high frequency applications a matching MESFET device is chosen as the front end amplifier device.

– For simple, low bit rate applications a simple current-to- voltage converter (using an opamp) is good enough.

– Low noise preamplifer circuit will be followed by a Post amplifier (to raise the electrical signal to the required levels)

– For digital applications a high-speed comparator employed to finally convert the signal to the required logic levels.

10. Optical Fiber Connectors 11. Permanent Joints - Splicing

• Fusion Splicing is the most common method used for joining fibers.

• Fibers for indoor use with primary and secondary buffer coatings generally come in lengths of about 2km.

• Outdoor fiber cables are quite bulky and come in much smaller lengths (100m to 500m).

• With modern day fusion splicing machines splice losses are typically of the order of 0.01 dB per splice.

• These machines automatically align the two pieces of fibers for maximum power before they are joined.

• Sophisticated splicing machines match the refractive- index profiles of the fibers as well.

12.Major developments in OFC since 1980

Increase in the capacity of optical fiber systems realized after 1980. The change in the slope after 1992 is due to the advent of WDM technology



Major developments since 1980…..contd.

Increase in the BL product since 1975 through several generations of

optical fiber systems (source: Chapter 1 - GP Agrawal, Fiber-Optic Communication Systems, 3rd


Major developments since 1980…..contd.

• First generation systems – 1975 to 1980 – 850nm systems, and multimode fibers, data rates below 100 Mb/s,

• Second generation systems – early 1980s – 1300nm systems, single mode fibers with 0.5 dB/km loss, data rates up to 1.7 Gb/s, repeater spacing of 50km

• Third generation systems – mid 80s - 1550nm, 0.2 dB/km loss, dispersion-shifted fibers with minimum dispersion at 1550nm, data rates 4Gb/s, repeater spacing of 100km

Fourth Generation systems • Drawback of the 3rd generation systems – signal

regenerated using electronic repeaters, spaced typically 60-70km.

• Demonstration of Fiber amplifiers - 1989

• 4th generation systems – 1990 - make use of Optical amplification (for increased repeater spacing) and Wavelength-division multiplexing (WDM) for increased data rate.

• Resulted in a data rate of 10 Tb/s by 2001.

4th Generation systems…contd.

• In most systems fiber losses are

compensated periodically using erbium- doped fiber amplifiers spaced at 60-70km.

• 1991 – demonstration of a data transmission using re-circulating-loop configuration

- 21,000 km at 2.5Gb/s

- 11,300 km at a bit rate of 5 Gb/s

Under sea Cable (submarine cable) Communication • One of the most challenging means of

communication - used since 1858 • The cable of 1858 worked only for a few weeks • 1866 – the first transatlantic telegraph cable

(North America to Europe) • Telegraph operator could send about 17 words

per minute, at a cost of $5 per word. • 1956 – the first transatlantic telephone cable

(TAT-1) – 48 telephone circuits between Newfoundland and Scotland.

• Was based on analog systems

Undersea cables….contd.

• By 1983 – TAT cable capacity increased

to 4200 voice circuits using Frequency Division Multiplexing (FDM)

• From 1956 to 1983 the capacity of the TAT increased at an annual rate of 20%

• 1988-89 – the first undersea fiber optic communication system with a capacity of 280 Mb/s on each of the three fiber pairs.

Optical Fiber Undersea cable communication…contd.

• First system used hybrid optical systems – repeaters converted the incoming signals from optical to electrical, regenerated the data with high-speed ICs, and retransmitted the data with a local semiconductor laser.

Power on repeatered cables

• repeaters need to be powered. • The standard approach is to send a constant current

of about 1A from one end of the cable to the other, along a copper sheath which lies outside the fibres and inside the armour (if present).

• Each km of cable offers a resistance of some 0.7 ohm. Voltage drop across each repeater is typically 40V (on four fibre-pair cable)

• a requirement of close to 10 KV across a typical 7500 km transatlantic crossing with 100 repeaters.

Wavelength division multiplexing (WDM)

• Transmitting signals at more than one

wavelength on each fiber pair, thus increasing bandwidth.

• STM-16 (2.5 Gbps) is the transmission speed in the SDH hierarchy which is being most widely used today

• Modern submarine cable systems can transmit STM-16 signals at four or eight different wavelengths, to give a total capacity of 10 or 20 Gbps per fiber pair.

4th Generation Optical Fiber

Submarine Systems • 1996 - the first cable (TAT-12/13) using fully

optical amplification via erbium-doped fiber amplifiers (EDFAs) came into service.

• Because of the optical amplifiers the need for the two signal conversions is avoided.

• This change from regeneration to optical amplification considerably reduced the number of active components which had to be qualified for 25 years of undersea service

• Significantly improved the intrinsic reliability of the cable systems (though that is so high that it is difficult to measure).

WDM Cable Network between Germany and Singapore (SEA-ME-WE-3)

Documents

optical fiber