EE 594, Spring 2001 Instructor: Dr. Alexander Cartwright

Optical Communication

Chunhai Ji

2/26/01

Optical Communication System

Reference: http://www.corningfiber.com/

• Transmitter – laser source, amplifier, modulator and channel coupler, etc.

• Communication channel –fiber and optical amplifier, etc.

• Receiver – channel coupler,Photodetector and demodulator, etc.

Optical Fiber — History

Reference: Jeff Hecht, City of Light, 1999 Oxford University Press.

• 1880 Alexander Graham Bell patented an optical telephone system• 1930 Heinrich Lamm, a medical student demo image transmission through a bundle of optical fiber• 1966 Kao and Hockham proposed intrinsic loss of silica-base glass could be low enough to enable use as a light guide• 1970 Maurer, Keck and Schultz announced a single-mode fiber with attenuation below 20 dB/km; At the same year, Bell lab made first CW semiconductor diode laser able to work at room temperature• Three generations of optical fibers: first, 2dB/km; Second, using with InGaAsP lasers at 1300nm, 0.5dB/km; Third, with 1500nm lasers, 0.2dB/km.• 1987 Mears, et. Al. invented erbium-doped optical fiber amplifiers; in early 1990s became commercially available and make DWDM practical.

How Fiber Works

Reference: http://jhsu.www3.50megs.com/tech-dwdm.html

vc

n ?

Total Internal Reflection

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1

21sinnn

c?

When n1>n2

Types of Fibers ? multimode fiber

Reference: http://www.corningfiber.com/

Multimode fiber is the type of fiber to be commercialized and is commonly used in data communication. In multimode fibers many rays, or modes, propagate down the fiber simultaneously. Over long distance, these

modes spread and may overlap, cause bit errors. So multimode fibers only can be used within 2km or less.

Types of Fibers ? single mode fiber

Reference: http://www.corningfiber.com/

In single mode fiber, only one ray, or mode,of lights propagates down a fiber at a time. It has the ability to transmit very high data rate and over very long distance. Normally it works at wavelength longer than 1300nm where attenuation and dispersion are minimum.

Fabrication of Optical Fiber

Reference: http://www.corningfiber.com/

HClSiOHOSiClHClGeOHOGeCl

4242

2224

2224

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Outer Vapor Deposition (OVD):

Other method: modified CVD (MCVD), plasma CVD (PCVD), plasma enhanced CVD (PMCVD) and vapor axial deposition (VAD)

Technical Characteristics of Fibers

Reference:Thomas, Ackerman, Prucnal, and Cooper, Phy. Today, vol. 53, Sep. 2000

Attenuation is the measure of the signal loss over distance, unit is dB/km.

Transmission loss has three key components:

• OH absorption (producing both the big peak at approximately half the wavelength of the fundamental OH mode ? 0 and the little peak at slightly shorter wavelength)• Rayleigh scattering (the red dotted line, proportional to 1/? 4)• Urbach tail (the green dotted line, an exponential band tail of SiO vibrations).

The Rayleigh and Urbach contributions sum to the blue solid line, which may be the clarity limit in SiO2 glass.

Technical Characteristics of Fibers

Reference: http://www.corningfiber.com/

Dispersion refers to the spreading of a pulse of a light that occurs over distance as the pulse travel through the fiber.

Dispersion includes:• Modal dispersion – the spreading of optical signals of different modes.• Chromatic Dispersion – the dispersion of light waves of different wavelength. Two components included: materials and waveguide. • Polarization mode dispersion – due to light can vibrate in one or two principal polarization modes. X-axis slow mode and y-axis fast mode causePMD

Advances in Optical Amplifiers

Reference: Y. Sun et. al., BLTJ, Jan-Mar. 1999, p187-206

• Before the invention of EDFA, the optical losses were compensated every tens of km by an electronic repeater – Drawbacks of the electronic repeater are high cost, fairly unreliable and most importantly, working only for the designed bit rate at only one carrier wavelength.• 1987 Mears, et. Al. invented erbium-doped optical fiber amplifiers• 1989, the first EDFA pumped by efficient semiconductor laser had been demonstrated• 1994 the first undersea optical amplifiers were deployed by AT&T Submarine System.• 1995 the first transatlantic communication system using opticalamplifiers was deployed and soon followed by transpacific system

Typical EDFAs structure

Reference: http://www.newport.com/Photonics/

• An EDFA consists of a short length of optical fiber whose core has been doped with less than 0.1 percent erbium , an optically active rare earth element.

• An laser diode pump normally with 980nm or 1480nm

How EDFAs work

Reference: Y. Sun et. al., BLTJ, Jan-Mar. 1999, p187-206

• The erbium ions are pumped to an upper energy level by the absorption of light from the pump source at, for example, 980 nm.• The transition to the ground state emits a photon and may be either spontaneous (i.e. the natural decay of the excited ion in the absence of any interactions), or stimulated (i.e. in the presence of photons possessing the transition energy, stimulated emission produces additional photons identical to the stimulating photons at a rate proportional to their flux). • Signal photons in the EDFA stimulate depopulation of the excited state, which amplifies the signal. The long lifetime of the excited state, approximately 10 milliseconds (ms), assures that, instead of emitting noise by spontaneous emission, most erbium ions will wait to amplify signals by stimulated emission.

Raman Amplifier

Reference: Thomas, Ackerman, Prucnal, and Cooper, Phy. Today, vol. 53, Sep. 2000

• Coherent process --stimulated Raman scattering• Boost a signal in an untreated SiO2 fiber

phspump ??? ??

Other fiber functional products

• UV-induced fiber grating technology

• Microstructured optical fibers

Lasers for Optical Communication

• Laser for light source

• Laser for pumping energy of EDFA and Raman amplifier

Laser Transmitter

Reference: W.F. Brinkman et. al., BJTL, Jan-Mar 2000, p150-167

Lasers for Optical Communication

Reference: Thomas, Ackerman, Prucnal, and Cooper, Phy. Today, vol. 53, Sep. 2000

Gain Tuning ModulatorAmplifier

• MQW confines carriers and tends to to direct the emmitted light along the layer

• DBR, distributed Bragg reflection -- distributed mirror with periodically spaced elements and tune the central wavelength by injection carrier.

•Amplifier, structurally similar to the gain section, but without mirror

•EA, electroabsorption, Modulator, turns light on and off to add digital info to light

Materials for Laser Source

Reference: CORD Projects and Activities: Optics and Photonics Resources

Comparison of Laser Source

Reference: CORD Projects and Activities: Optics and Photonics Resources

Type Wavelength(nm)

Power(mW)

Current(mA)

SpectralWidth(nm)

BeamDivergence(degrees)

AlGaAsDoubleheterojunctionlaser

850 10 300 2.5 5 × 20

AlGaAs TJSlaser 830 15 65 0.1 13 × 40

InGaAsP laser 1300, 1550 7 250 4 10 × 30Multiple-stripe AlGaAslaser

850 to 500 1600 2 10 × 35

LD Pump for Optical Amplifier

Reference: http://www.lasertroninc.com/

• In fiber optic amplifiers, the wavelength of laser is chosen in order to maximize amplification efficiency and minimize the noise generated in the amplification process.

• Efficient pumping is possible by using semiconductor lasers operating near 980 nm and 1480 nm wavelengths. It is possible to obtain high amplifier gains in the range 30-40 dB with only a few milliwatts of pump powers. Efficiencies as high as 11 dB/mW have been achieved with 980nm pumping.

Corning Lasertron

Properties of Photodetectors

Reference: G.P. Agrawal, Fiber-optic Communication Systems 2nd ed., Wiley, 1997

• Responsivity: ? ?24.1

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• Bandwidth – determined by the speed with which it responds to variations in the incident optical power

? ? 1)(2 ???? RCtrf ???

• Dark current Id

Materials for Photodiode

Reference: G.P. Agrawal, Fiber-optic Communication Systems 2nd ed., Wiley, 1997

Working Principle of Photodiode

Reference: G.P. Agrawal, Fiber-optic Communication Systems 2nd ed., Wiley, 1997

p-i-n Photodiode

Reference: Brock Koren and Marek Szawlowski, Laser Focus World, Nov. 1998

InP

InGaAs

InP

• Decreasing the P- and n-regions will make the diffusion contribution small

• Increasing the i-region will make moreincident optical power absorbed in it

• InGaAs as middle layer and InP for P- and n- layer to make more light with 1300-1500nm wavelength absorbed at i-region

Characteristics of p-i-n photodiode

Reference: G.P. Agrawal, Fiber-optic Communication Systems 2nd ed., Wiley, 1997

Avalanche Photodiodes (APDs)

Reference: G.P. Agrawal, Fiber-optic Communication Systems 2nd ed., Wiley, 1997

• Impact ionization

• Impact ionization coefficient determined by materials and applied electric field to accelerates electrons and holes

•

Characteristics of APDs

Reference: G.P. Agrawal, Fiber-optic Communication Systems 2nd ed., Wiley, 1997

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