8/11/2019 Doped Optical Fibers

1/20

Doped Optical FibersFocused on Erbium-Doped Fibers as Examples

Used as Laser Gain Mediums and Optical Amplifiers

by Ryan McKinney

8/11/2019 Doped Optical Fibers

2/20

Concept

Erbium-Doped Fiber Amplifiers Most important fiber amplifiers in long-range optical fiber

communications.

Efficiently amplify light in the 1.5-m wavelength region,

where telecom fibers have their loss minimum.

Fiber Lasers used with Amplifiers

High-power fiber lasers and particularly amplifiers with

output powers of tens or hundreds of watts; even several

kilowatts from a single fiber.

Very high surface-to-volume ratio and the waveguide

effect,

Avoids thermo-optical problems even with significant heating.

Based on solid-state laser technologies.

8/11/2019 Doped Optical Fibers

3/20

Concept

Raman Amplifiers Optical amplifier based on Raman gain.

Optical gain from stimulated Raman scattering.

Occur in transparent solid media, liquids and gases under the

influence of intense pump light.

Magnitude depends mainly on the optical frequency offset

between pump and signal wave.

Can be fairly strong in optical fibers where substantial optical

intensities can be maintained over long lengths.

8/11/2019 Doped Optical Fibers

4/20

Material Composition & Operation

Typical fiber amplifier works in the 1550 nm band. Consists of a length of fiber doped with Erbium.

Pumped with a laser at 980 nm.

Stimulated emission stimulates more emission

Rapid exponential growth of photons in the doped fiber. Gains of >40 dB (10,000X) possible with power outputs >+20

dBm (100 mW)

8/11/2019 Doped Optical Fibers

5/20

Material Composition & Operation

The most efficient fiber amplifiers have been Erbium-Doped Fiber Amplifiers (EDFAs) operating in the

1550 nm range.

Amplifier emits light energy in a signal wavelength.

Energy supplied to it by photons in a pump wavelength. When stimulated by incoming photons in the signal the

emitted photons stimulate other emissions, so there is an

exponential growth of photons.

8/11/2019 Doped Optical Fibers

6/20

Material Composition & Operation

Why Erbium ions? (Er3+) Erbium ions have quantum levels that allow them to be

stimulated to emit in the 1540nm band,

Band with the least power loss in most silica-based fiber.

Gives them the ability to amplify signals in a band where high-

quality amplifiers are most needed.

Can also be excited by a signal at 800nm or 980nm.

Silica-based fiber can carry these without great losses but

are not in the middle of the signal wavelengths.

Bands are also far enough away from the signal bandsthat it is easy to keep the pump beam and the signal

beam separated.

8/11/2019 Doped Optical Fibers

7/20

Material Composition & Operation

Raman Fiber Amplifiers A high-power pump laser and a WDM or directional

coupler.

Optical amplification occurs in the transmission fiber.

Distributed along the transmission path.

Optical signals are amplified up to 10 dB in the optical

fiber.

Can be combined with EDFAs but fibers used for Raman

amplifiers are not exclusively doped with rare earth ions.

Pump laser has a wavelength of 1535 nm.

Circulator injects light backwards

into the transmission path.

Preferred as transfer of noise from

the pump to the signal is reduced.

8/11/2019 Doped Optical Fibers

8/20

Applications

Erbium-Doped Fiber Amplifiers Light Distance Amplifier.

Overcomes traditional electronic fiber optic repeater limitations.

Digital Signal & Analog Signal Conversion

Repeater Changes and Replacements

Complex Structure & Expenses

EDFA does not have to be replaced if input signal is changed.

Equipment is expansive for optical wavelength division

multiplexing.

Transmitter Amplifier & Optical Receiver Preamplifier Signal transmission distance can be increased to 100-200km

8/11/2019 Doped Optical Fibers

9/20

Applications

High-Power Fiber Lasers & Amplifiers Special General-Purpose Solid-State Lasers

High Potential for High Average Output Power

High Power Efficiency & Beam Quality

8/11/2019 Doped Optical Fibers

10/20

Applications

High-Power Fiber Lasers & Amplifiers Broad Wavelength Tunability

Transitions of rare-earth-doped

crystals/glasses

Transitions with very low oscillator

strength

Long radiative upper-state lifetimes

Good energy storage qualities

8/11/2019 Doped Optical Fibers

11/20

Applications

Raman Fiber Amplifiers Optical Telecommunications

All-Band Wavelength Coverage

In-Line Distributed Signal Amplification

8/11/2019 Doped Optical Fibers

12/20

Issues & Problems

Fiber Amplifiers Most operate on quasi-three-level transitions.

In the unpumped state amplifiers exhibit some

losses caused by the active ions.

Only when a certain excitation level is

exceeded does actual amplification occur.

Quasi-three-level nature also has an increased noise figure.

Can be minimized by certain design optimizations.

Most are not made with polarization-maintaining fibers.

Polarization state of the input is not preserved. Amplification process is normally not polarization-dependent

in telecommunications.

In some cases polarization hole burning can cause problems.

8/11/2019 Doped Optical Fibers

13/20

Issues & Problems

Fiber Lasers Subject to problems with uncontrolled birefringence.

Refractive index depends on polarization or double refraction.

This often changes the polarization state from linear to

elliptical depending on temperature and bending.

May require readjustment of polarization controllers with

temperature changes.

May be acceptable for laboratory use but not for commercial

devices.

Special fibers and fiber devices are often not available withpolarization-maintaining fibers.

Mode locking with nonlinear polarization rotation would not work

with a polarization-maintaining fiber.

8/11/2019 Doped Optical Fibers

14/20

Issues & Problems

Fiber Lasers Devices require pump sources with higher beam

quality and brightness.

Generally increases the price per watt of the pump source.

Very sensitive to optical feedback.

Especially when used as master oscillator power amplifiers.

Backreflected light is even amplified on its way back to the seed

laser.

Fibers consist of glass material where the composition is

often poorly defined. Even the manufacturer may not know the exact composition and

it is not revealed to customers.

Fiber parameters such as the core diameter or refractive index

profile can be uncertain and vary between different samples.

8/11/2019 Doped Optical Fibers

15/20

Issues & Problems

Raman Amplifiers Relatively poor pumping efficiency by comparison.

Require longer gain fibers.

Fast response time creates new sources of noise.

Reacts to minor unwanted inputs other amplifiers may not. More directly couple pump noise to the signal than laser

amplifiers.

8/11/2019 Doped Optical Fibers

16/20

Technological Future

Dense Wavelength Division Multiplexing Pools data from different sources together on an optical

fiber.

80+ separate wavelengths or channels of data can be

multiplexed into a lightstream transmitted on a single

optical fiber.

Each channel carries a time division multiplexed signal.

In a system with each channel

carrying 2.5 Gbps (billion bits per

second), up to 200 billion bitscan be delivered a second by the

optical fiber.

8/11/2019 Doped Optical Fibers

17/20

Technological Future

Developments Within the 12601610 nm Bands Typically fell within ranges absorbed by water.

Tiny amounts of water vapor within fiber glass affected

transmissions.

Fiber amplifiers and diode lasers in development within

this band to expand it as useable bandwidth.

Will allow for even greater ranged of wavelength-division

multiplexing, especially within dense wavelength-division

systems.

8/11/2019 Doped Optical Fibers

18/20

Technological Future

Semiconductor Optical Amplifiers Direct competition with erbium-doped fiber amplifiers.

Compact setup containing only a small semiconductorchip with electrical and fiber connections.

The output powers are significantly smaller.

The gain bandwidth is smaller. Faster reaction to pump or signal input changes.

The noise figure is typically higher.

Often used currently in telecom systems in the form offiber-pigtailed components.

A single, short optical fiber with exposed fiber at one end usedto separate bundles into individual fibers spliced to otherequipment.

8/11/2019 Doped Optical Fibers

19/20

References

RP PHOTONICSENCYCLOPEDIA

HTTP://WWW.RP-PHOTONICS.COM/ENCYCLOPEDIA.HTML

THEFIBEROPTICSASSOCIATIONAMPLIFIERS

HTTP://WWW.THEFOA.ORG/TECH/FIBERAMP.HTM

FIBER-OPTICS.INFOOPTICALAMPLIFIERS

HTTP://WWW.FIBER-

OPTICS.INFO/ARTICLES/OPTICAL_AMPLIFIERS

UNIVERSITYOFSOUTHAMPTONFIBERLASERS

HTTP://WWW.ORC.SOTON.AC.UK/61.HTML

8/11/2019 Doped Optical Fibers

20/20