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    Doped Optical FibersFocused on Erbium-Doped Fibers as Examples

    Used as Laser Gain Mediums and Optical Amplifiers

    by Ryan McKinney

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    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.

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    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.

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    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)

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    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.

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    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.

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    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.

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    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

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    Applications

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

    High Potential for High Average Output Power

    High Power Efficiency & Beam Quality

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    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

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    Applications

    Raman Fiber Amplifiers Optical Telecommunications

    All-Band Wavelength Coverage

    In-Line Distributed Signal Amplification

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    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.

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    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.

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    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.

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    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.

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    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.

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    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.

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    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.

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    References

    RP PHOTONICSENCYCLOPEDIA

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

    THEFIBEROPTICSASSOCIATIONAMPLIFIERS

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

    FIBER-OPTICS.INFOOPTI

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