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Lasers & Fiber Optics

Engr. Hyder Bux MangrioEngr. Fayaz Hassan Mangrio

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Introduction

L&FO labs • Lab #01: Introduction to Fiber optics Communication System• Lab #02: Optical Sources• Lab #03: Optical Detectors• Lab #04: Optical fiber attenuation losses• Lab #05: Analog voice transmission• Lab #06: Understanding basic function of S122A splicer• Lab #07: Perform Fusion/Mechanical Splicing

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Introduction

• Lab #08: Understanding the basic function of OTDR• Lab #09: Perform fiber measurement on OTDR• Lab #10: Fiber attenuation measurement using Cut-Back method

• Lab #11: Optical Field Spectrum Analyzer• Lab #12: Overview of Power meter & Light Source

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Information

• Email: hyder.bux@faculty.muet.edu.pk

• Webpage: https://sites.google.com/a/faculty.muet.edu.pk/hydermangrio/

• Optical Communication Laboratory• Consultation Timings:

Monday(8am to 3pm) & Friday (8am to 1pm)

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Laboratory

• There will be at least 13 labs covering in 13 weeks course. Each lab will be approximately 2 hours long. The lab report / Handout is due to the lab assistant before next lab.

What is lightwave technology?

• Lightwave technology uses light as the primary medium to carry information.

• The light often is guided through optical fibers (fiberoptic technology).

• Most applications use invisible (infrared) light. (HP)

Why lightwave technology?

• Most cost-effective way to move huge amounts of information (voice, data) quickly and reliably.

• Light is insensitive to electrical interference.

• Fiber optic cables have less weight and consume less space than equivalent electrical links.

(HP)

Use Of Lightwave Technology

• Majority applications:– Telephone networks–Data communication systems–Cable TV distribution

• Niche applications:–Optical sensors–Medical equipment

LW Transmission Bands

Near InfraredFrequency

Wavelength1.6

229

1.0 0.8 µm0.6 0.41.8 1.4

UV

(vacuum) 1.2

THz193 461

0.2

353

Longhaul Telecom

Regional Telecom

Local Area Networks850 nm

1550 nm

1310 nmCD Players780 nm

HeNe Lasers633 nm

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Introduction to Fiber Optics

• Fiber optics is a medium for carrying information from one point to another in the form of light. Unlike the copper form of transmission, fiber optics is not electrical in nature.

• A basic fiber optic system consists of a transmitting device that converts an electrical signal into a light signal, an optical fiber cable that carries the light, and a receiver that accepts the light signal and converts it back into an electrical signal.

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Introduction to fiber Optics

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Optical Sources

• Two main types of optical sources– Light emitting diode (LED)• Large wavelength content• Incoherent• Limited directionality

– Laser diode (LD)• Small wavelength content• Highly coherent• Directional

Light Emitting Diodes (LED)

• Spontaneous emission dominates –Random photon emission

• Spatial implications of random emission–Broad far field emission pattern–Dome used to extract more of the

light• Spectral implications of random

emission–Broad spectrum

Laser Diode• Stimulated emission dominates

– Narrower spectrum– More directional

• Requires high optical power density in the gain region– Optical Feedback: Part of the optical power is reflected back into the cavity– End mirrors

• Lasing requires net positive gain– Cavity gain• Depends on external pumping• Applying current to a semiconductor pn junction

– Cavity loss• Material absorption• Scatter• End face reflectivity

Optical Detectors• Inverse device with semiconductor lasers– Source: convert electric current to optical power– Detector: convert optical power to electrical current

• Use pin structures similar to lasers• Electrical power is proportional to i2– Electrical power is proportional to optical power squared– Called square law device

• Important characteristics– Modulation bandwidth (response speed)– Optical conversion efficiency– Noise– Area

HOW DOES FIBRE OPTIC WORK ?

Carries Signals as Light Pulses

signals converted from electrical to light (and visa-versa) by special equipment

e.g. fibre-optic “transceiver” (transmitter / receiver)

FIBRE CONSTRUCTION

125 Cladding Glass 8, 50, 62.5Core Glass

PRIMARY BUFFER

Core (62.5)

Cladding 125

Primary Buffer 250

SECONDARY BUFFERSecondary Buffer 900

Primary Buffer 250

Cladding 125Core (62.5)

FIBRE MATERIAL Silica Glass

used for high-speed data applications

Plasticsused for low-speed data / voice applications

Composite Constructionsused for low-speed and specialized applications

FIBRE TRANSMISSION Multi-Mode

graded-index used for short / medium distance applications

step-indexearly fibre type - no longer used

Single-Mode a.k.a Mono-Mode

used for long-distance / very high-speed applicationse.g. cross-country and transatlantic communications

LIGHT TRANSMISSION

MultiModeStep Index

MultiModeGraded Index

SingleMode

COMMON FIBRE SIZES

125 µm

50 µm

125 µm

62.5 µm

140 µm

100 µm

125 µm

8 µm

MultiMode Graded Index SingleMode

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Advantages/Disadvantages of Fiber Optics

• Advantagesa) Enormous potential bandwidthb) Small size and weightc) Electrical Isolationd) Signal securitye) Low transmission lossf) Potential low cost

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Advantages/Disadvantages of Fiber Optics

• Disadvantagesa) High cost for connector and interfacingb) Requires specialized and sophisticated tools for

maintenance and repairingc) Higher initial cost in installation

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Light-Source

• What is light?• Properties of Light.• Refractive Index• Law of Refraction • Law of Reflection• Total Internal Reflection

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Refractive Index

• The guidance of the light beam which acts as a transmission channel for information (through the optical fiber) takes place because of the phenomenon of total internal reflection (TIR),which is dependent on the refractive index of the medium.

The refractive index (n) of a medium can be writtenas:

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Total Internal Reflection• A ray of light incident on a denser medium i.e. n1<n2According to Snell’s Law and the law of reflection we have

n1 sin θ1 =n2 sin θ2 and θ1=θ3

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Total Internal Reflection

• The angle of incidence, for which the angle of refraction is 90º, is known as the critical angle and is denoted by θc .Thus, when

θ1=θc =sin-1(n2/n1)

θ2=90. When the angle of incidence exceeds the

angle of critical (i.e.,θ1>θc), there is no refracted ray

and we have total internal reflection.

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Total Internal Reflection