Fiber Optic Comms - nanoHUB

Fiber Optic Communications

Lecture 1

• Overview of optical fibers• Multi-mode fibers• Single-mode fibers• Graded Index (GRIN) fibers• Step-Index fibers• Numerical Aperture• Polarization-maintaining fibers• Fabrication

1/7/2019 ECE 695 - Peter Bermel - Purdue University

Fiber History

• 1854: John Tyndall guides light in a jet of water flowing from a tank

• 1930: Heinrich Lamm shows first image transmission through fiber bundle

• 1960: Lawrence Curtiss made first glass clad fiber (1000 db/km loss)

• 1970: Corning Glass Works demonstrates 20 dB/km @ 633 nm

• 1988: First transatlantic optical cable (TAT-8)

• 1996: First demonstration of photonic crystal fibers

1/7/2019 ECE 695 - Peter Bermel - Purdue University 2

Milestones of undersea cables

1866 First undersea telegraph cablesNorth America to Europe17 words/minute, $5/word

1956 First undersea telephone cables (coax) 48 phone circuits

1983 4200 voice circuitsBW: ~ 10+1 MHz Repeater spacing ~ 9.5 km

1988-89 First undersea fiber optics cableThree fiber pairs, 280 Mb/s @1.3mmUpgraded to 2.5 Gb/s @ 1.55 mmRepeater spacing: ~ 50 kmErbium doped fiber amplifiers

0.35 Gm, 350,000 km

Undersea Cables


TransAtlantic cables: 6000 to 7500 km(~ 2002)

G. MARRA ET AL/SCIENCE 2018

Global network of submarine fiber-optic cables

Existing cables are shown in purple; planned cables are in blue.



Basic Fiber Types

• Standard telecom fibers

• Core pumped active fibers

• Double clad active fibers

• Hollow core Photonic bandgap fibers

• Photonic Crystal Fibers (PCFs)

Materials for Fiber Lasers

• Erbium (telecom, 1550 nm)

• Ytterbium (most fiber lasers – 1 µm)

• Erbium-Ytterbium (high power 1550nm systems)

• Thulium

• Neodymium

Applications

• Data transport

• Signal amplification in telecom networks

• Fiber lasers

• Supercontinuum sources

• Lighting and decoration

• Power transport

• Sensors (gyros, heat, tension)


Standard Fiber


Typical Telecom Fiber

Materials• Synthetic silica (base of

99% of all fibers)• ZBLAN (fluorozirconate glass:

mixture of zirconium, barium, lanthanum, aluminum, and sodium fluorides, ZrF4-BaF2-LaF3-AlF3-NaF)

• Polymers

For index control:• Index raising

– Aluminum– Germanium

• Index lowering– Flour– Boron

ncl = 1.44 to 1.46

(ncore – ncl)/ncore = 0.0010.021/7/2019 ECE 695 - Peter Bermel - Purdue University 8

Rayleigh scattering: from random localized variation of the molecular positions in glass which creates random inhomogeneities in indexInfrared absorption: from vibrational transitions

Absolute minimum at 1.55 micron, at 0.16dB/km, about 3.6% per km

From: http://www.tpub.com/neets/tm/106-14.htm

Propagation Loss in Fibers

Att

enu

atio

n d

B/k

m


From: http://science.nasa.gov/newhome/headlines/msad05feb98_1.htm

Propagation Loss


Numerical Aperture

The acceptance angle of the fiber to achieve TIR is:

This acceptance angle determines the cone of external rays that are guided by the fiber. The numerical aperture therefore describes the light-gathering capacity of the fiber.

Central core (light is guided)Outer cladding (lower index)

Cf. Sec. 9.1

Low-loss materials


Step-Index Fibers

• The fractional index change between the core and cladding is quite small:

• Most fibers are made of high purity SiO2 glass. Slight changes in the refractive index are effected by adding low concentrations of doping materials such as titanium, germanium, or boron.


Graded-Index Fibers

Rays in meridional planes follow oscillatory planartrajectories, whereas skewed rays follow helicaltrajectories.


Graded-Index Fibers

Because different modes have different group velocities, one way to compensate is by using graded-index fibers.The refractive index of these fibers varies radially with the highest index in the middle of the fiber. Because rays in the center travel the least distance, speed of these modes are slowed by the higher index.

A grade profile parameter pdetermines the steepness of the

profile


The V Parameter

• a : core radius

• λo : wavelength in vacuum

• n1 : the maximum refractive index of the core

• n2 : the refractive index of the cladding

• NA: The numerical aperture

• For single-mode operation is required that V < 2.405, which is the first root of the Bessel function J0

The V-number determines the number of modes in the fiber

𝑉 =2𝜋𝑎

𝜆𝑜𝑛12 − 𝑛2

2 =2𝜋𝑎

𝜆𝑜NA


Single-Mode Fiber• Single mode operation is achieved via a small core diameter

and small Numerical Aperture (n2 is close to n1).

• The fundamental mode: Gaussian with the highestconfinement

• Only one mode - one group velocity - much higher data ratesare possible

• Single-mode fibers provide lower power attenuation(compared to multi-mode)

• Avoids unwanted random interference effects seen inmultimode fibers (known as modal noise or speckle)

Sec. 9.21/7/2019 ECE 695 - Peter Bermel - Purdue University 16

Single-Mode Fiber

As the V parameter increases (frequency or fiberdiameter increases), the effective refractive indexincrease from n1 to n2, meaning the mode is moreconfined to the core.


w

k

Light-line of claddingw=ck/n1

Radiation modes

Light-line of corew=ck/n2

Guided modes

Guiding modes: exist between the light line of the cladding and light line of the core

Band Diagram: Standard Fiber


w

k

Light-line of claddingw=ck/n1

Radiation modes

Light-line of corew=ck/n2

Guided modes

Fundamental mode

Higher order mode

The V-number determines the number of modes in the fiber

Lower and Higher Order Modes


Fundamental mode Higher order mode

TE mode – when Ez = 0 TEM – both Ez, Hz = 0TM mode – when Hz = 0Hybride modes – when Ez, Hz 0 (HE - E, H of the same signs; EH - opposite)L – azimuthal mode number (azimuthal variations in fields)m – radial mode number (radial variations in fields)LP modes – linearly-polarized modes

Lower and Higher Order Modes


Number of Modes


Field patterns of various modes



Intensity Distribution

Calculated intensity distribution for the modes guided by a step index fiber with V=7.1


Polarization-Maintaining Fibers• In principle, there should be no change in polarization components within

the fiber. In practice, however, slight random imperfections and uncontrollable strains in the fiber result in random power transfer between the two polarizations. Linearly polarized light at the fiber input is generally transformed into elliptically polarized light at the fiber output.

• To avoid this, we do not use circular-shaped fiber. Instead, elliptical cross section or stress-induced anisotropy of the refractive index is employed, making the propagation constants of the two polarizations different.


Preform Manufacturing

POCl4

GeCl4

SiCl4

Hydrogen burner

O2

Rotating glass tube

Fused silica Unfused silica

• The Modified Chemical Vapor Deposition (MCVD) process



Alternative Processes

• Plasma Modified Chemical Vapor Deposition (PMCVD)

• Plasma Chemical Vapor Deposition (PCVD)

• Outside Vapor Deposition (OVD)

• Vapor-phase Axial Deposition VAD


Fiber Drawing

Preform

Heating zone

Fiber Spool


Fiber Drawing Tower

1/7/2019ECE 695 - Peter Bermel - Purdue University 30

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