Tutorial On Optical Fiber-01

OPTICAL DRIVERS AND DETECTORS IDC

Light sources ●

Characteristics required » Effective coupling to fibres down to 8.5 microns diameter » Easily modulated with linear characteristics » High output power » High reliability » Small size and weight » Low cost

IDC

●

Light emitting diodes (LEDs) » Made from p- and n-type semiconductor substrates » Most common materials – Gallium aluminium arsenide (GaAlAs) for 800 to 900 nm wavelengths – Gallium arsenide (GaAs) for 930 nm – Gallium arsenide phosphorous (GaAsP) for 660 nm (red for plastic fibres) – Indium gallium arsenide phosphide (InGaAsP) for 1300 and 1550 nm

IDC

» P - type substrate – More protons than electrons - holes – Apply +ve voltage

» N - type substrate – More electrons than protons – Apply -ve voltage

» Electrons and holes flow toward the junction and combine, emitting photons » Forbidden bandgap determines the energy of the emitted photon IDC

Basic LED operation

Light Emission

+

+ -

+

-+

Bias Voltage

p - region

-+ -

Junction

n - region

Recombination

IDC

Energy (eV)

Bandgap determines photon energy emission

IDC

-

Conduction Band (Free Electrons)

hf

Bandgap (w)

+

-

Valence Band + (Bound Electrons - Free Holes)

Amount of Energy released when electron drops to valence band and recombination occurs

●

LED geometry » Standard LEDs emit light in all directions » Fibre LEDs must emit light in a narrower confined beam » Types – Burrus ●

A hole is etched into the substrate for fibre

– Edge emitting diodes ●

IDC

Uses a very thin active region to confine the light

Edge emitting LED 300 µm

Metal

Light Emission

P - Region 50 µm

Active Layer n - Region (substrate) SiO2 Insulating Layer Metal 100

IDC

µm

Metal Stripe Confines Charge Carriers Laterally

●

Operating characteristics » Output power – 1 mW max. Down to several µW

» Power consumption – 20 to 100 mA, 1.2 to 1.8 V, up to 180 mW

» Spectral widths – 3 dB optical power bandwidth – 40 nm at 850 nm, 80 nm at 1300 nm – Increased spectral width causes increased dispersion

» Operating lifetime IDC

– When output power has dropped by 3 dB – ~ 11 years » Cont.

DIODE AND LASER CHARACTERISTICS

IDC

» Modulation – Mostly digital - (LED turned on and off) – Analogue possible (requires forward DC bias)

» Temperature effects – Operating range -65 to 125 degrees C – Output power decreases with temperature 0.012 dB / °C

IDC

Practical LED packages Fibre a)

LENS coupling (for large fibres ~1000 microns)

b)

Microlensed LED (50 micron fibres)

LENS LED

Metal Cap

TO18 Header

Fibre Transparent Window Microlens LED

Metal Cap

IDC

TO18 Header

Diode Packaging

IDC

Integrated Connectors

IDC

ANALOG AND DIGITAL DIODE CONTROLS

IDC

Laser diodes Laser “light amplification by stimulated emission of radiation” ● Operate on similar principles to edge LEDs. ● A resonant optical cavity ●

» Reflective ends » Dc biased » Photons strike electrons and stimulate further photon emission » Light amplified and escapes as laser beam IDC

Laser diode operation

IDC

●

Operating characteristics » Output power – 1mW to several hundred mW

» Power consumption – 30 to 250 mA, 1.2 to 2 V, 30 to 500 mW

» Spectral width – 1 nm at 850nm, 3 nm at 1300nm

» Lifetimes – 11 years at 22 degrees C – 1 year at 70 degrees C IDC

» Modulation – Mostly digital ●

Switched on/off close to threshold

– Analogue possible ●

Above threshold current

» Temperature effects – Threshold current increases with temperature 1.5 % per degree C – Output power decreases as temperature increases

IDC

Laser diode power - current temperature curves

IDC

VCSEL Vertical Cavity Surface Emitting Laser ● Cheap laser diode ● Less energy required for laser operation ● Emits narrow, more circular beam ● Operate at 850 and 1300 nm ● Speeds up to 10 Gbps ●

IDC

VCSEL Chip

IDC

ANALOG AND DIGITAL LASER CONTROL CIRCUITS

IDC

Laser diode package Insulation Contact Wire Laser Diode Circuit Connection

Fibre Pigtail

Grooved Block IDC

Threaded Ground Terminal

Cap

Laser diode transmitter module Photodetector

Diode Terminal

Laser Diode

Grooved Block

Fibre IDC

Laser Module

IDC

Laser safety ! All laser devices should be considered hazardous to the human eye. Under no circumstances look directly into the laser path! ● The 1300 and 1550 nm wavelengths are invisible to the human eye but will burn the retina. ●

IDC

Optical detectors ●

Converts photon light energy to electrical energy

●

Must be very low inherent noise device

●

Provide amplification of low level

IDC

signals

●

Pin photodiodes » Use reverse process of the LED » Common response times of 0.5 to 10 ns (2 GHz to 100 MHz). » Special devices down to 10 ps (100 GHz)

IDC

Pin photodiode Photon

-

-

+

+

I V

IDC

P+

π Intrinsic Region

n+

RL

VOUT

●

Avalanche photodiodes » High reverse bias voltages cause an increasing avalanche of charges in the semiconductor areas. » Advantages – Increased sensitivity to incoming light – Internal amplification – Twice the response time for standard devices

» Disadvantages – More complex – Increase costs – Less reliable IDC

» Used for low signal to noise requirements and long distance telecommunications.

Avalanche photodiodes

IDC

IDC

APD Receiver

IDC

Amplifiers » Required because the very low levels of light produce tiny currents in the detectors – A light signal of 1 µW will produce 600 nA in a pin diode – need to amplify..

» Can be FET – Greater sensitivity at lower data rates & lowest noise

» Or bipolar – For high data rates

IDC

Optical amplifiers » Amplification without conversion to electrical energy » Use stimulated emission principle for single photon pass » Doped fibres – Erbium for 1550 nm – Praseodymium for 1300 nm

» Semiconductor lasers

IDC

– Problems matching wide diameter fibre cores to small laser areas – Active layer narrow - 9 µm to 1 µm

Doped Fibre Amplifier Pump laser 980nm Erbium doped fibre Raises energy of Erbium atoms

Incident Light 1540nm IDC

Stimulates Emission at 1540nm

Amplified Output 1540nm

IDC

EDFA

IDC

Semiconductor Laser Amplifier Semiconductor Laser Amplifier Core

Core

INPUT FIBRE

OUTPUT FIBRE Incident light beam

Output light beam Active Layer

IDC