doped optical fibers

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  • 8/11/2019 Doped Optical Fibers


    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



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


    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


    Avoids thermo-optical problems even with significant heating.

    Based on solid-state laser technologies.

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    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


    Optical amplification occurs in the transmission fiber.

    Distributed along the transmission path.

    Optical signals are amplified up to 10 dB in the optical


    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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    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


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

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    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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    High-Power Fiber Lasers & Amplifiers Broad Wavelength Tunability

    Transitions of rare-earth-doped


    Transitions with very low oscillator


    Long radiative upper-state lifetimes

    Good energy storage qualities

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    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


    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


    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


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    Technological Future

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


    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


    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


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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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  • 8/11/2019 Doped Optical Fibers