Welcome optics

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Optical Amplifier (OA)

• Due to attenuation, there are limits to how long
  a fiber segment can propagate a signal with
  integrity before it has to be regenerated.
• The OA has made it possible to amplify all the
  wavelengths at once and without
  optical-electrical-optical (OEO) conversion.
  Besides being used on optical links, optical
  amplifiers also can be used to boost signal power
  after multiplexing or before demultiplexing, both
  of which can introduce loss into the system.
• The explosion of dense wavelength-division
  multiplexing (DWDM) applications make these
  optical amplifiers an essential fiber optic
  system building block.
• OAs allow information to be transmitted over
  longer distances without the need for
  conventional repeaters.
• OA can be semiconductor optical amplifiers
  (SOAs), erbium doped fiber amplifiers (EDFAs), or
  Raman optical amplifiers.

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Regenerative repeater

• It is made of 3 electronic components
• Photodiode
• clock recovery
• laser
• 3 major disadvantages.
• photodiode is unable to differentiate one signal
  from another.
• it has a fixed capacity.
• it is expensive.

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Type of amplifiers

• Power (Booster) amplifier
• In line amplifier
• Preamplifier

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• Power Amplifier/Booster
• Power amplifiers (also referred to as booster
  amplifiers) are placed directly after the optical
  transmitter.
• This application requires the EDFA to take a
  large signal input and provide the maximum output
  level. Small signal response is not as important
  because the direct transmitter output is usually
  -10 dBm or higher.
• The noise added by the amplifier at this point is
  also not as critical because the incoming signal
  has a large signal-to-noise ratio (SNR).
• In line amplifier
• In-line amplifiers or in-line repeaters, modify a
  small input signal and boost it for
  retransmission down the fiber.
• Controlling the small signal performance and
  noise added by the EDFA reduces the risk of
  limiting a systems length due to the noise
  produced by the amplifying components.
• Preamplifier
• Past receiver sensitivity of -30 dBm at 622 Mb/s
  was acceptable however, presently, the demands
  require sensitivity of -40 dBm or -45 dBm. This
  performance can be achieved by placing an optical
  amplifier prior to the receiver.
• Boosting the signal at this point presents a much
  larger signal into the receiver, thus easing the
  demands of the receiver design.
• This application requires careful attention to
  the noise added by the EDFA the noise added by
  the amplifier must be minimal to maximize the
  received SNR.

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Amplifier Wavelength Bands

• C band 1530-1560 nm (Uses EDFAs)
• L band 1570-1610 nm (Uses gain-shifted EDFA,
  Raman Amplifiers)
• L band 1610-1650 nm

• S band 1450-1480 nm
• S band 1480-1530 nm
• (Uses Raman amplifier)

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Challenge for amplifier design

• The need for more optical channels along
  installed fibers, wider optical bandwidths, and
  increased channel count, pushed EDFA technology
  beyond its performance limit
• Operate over extended wavelength range, (beyond
  the 30 nm bandwidth of EDFAs)
• Have higher output powers to maintain sufficient
  power per WDM channel
• Provide equal gain to each channel to prevent the
  build-up of dominant channels
• Have low-noise characteristics to maximize
  amplifier spacing
• Prevent crosstalk between channels, even if some
  channels are lost, or added
• Have controllable gain

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Future technology

• Recent advances in amplifier design have improved
  performance by using dual-stage amplifiers with
  mixed Raman and doped-fiber technologies.
• Multiple-wavelength-band amplifiers have
  incorporated sophisticated technological
  innovation, including
• Raman amplification using multiple pump
  wavelengths
• Cascaded pumping of amplifiers (where pump
  wavelengths undergo conversion before finally
  pumping the amplification medium)
• Pump reuse by reflection, passing to another
  band, and so on
• Novel dopants and hosts for doped amplifiers
• Multistage amplifiers, bi-directional amplifiers
  and other topological innovations

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Erbium-Doped Fiber Amplifier

• By making it possible to carry the large loads
  that DWDM is capable of transmitting over long
  distances, the EDFA was a key enabling
  technology. At the same time, it has been a
  driving force in the development of other network
  elements and technologies.
• Erbium is a rare-earth element that, when
  excited, emits light around 1.54 micrometersthe
  low-loss wavelength for optical fibers used in
  DWDM.
• The next figure shows a simplified diagram of an
  EDFA. A weak signal enters the erbium-doped
  fiber, into which light at 980 nm or 1480 nm is
  injected using a pump laser. This injected light
  stimulates the erbium atoms to release their
  stored energy as additional 1550-nm light. As
  this process continues down the fiber, the signal
  grows stronger.
• The spontaneous emissions in the EDFA also add
  noise to the signal this determines the noise
  figure of an EDFA.
• The key performance parameters of optical
  amplifiers are gain, gain flatness, noise level,
  and output power. EDFAs are typically capable of
  gains of 30 dB or more and output power of 17 dB
  or more.
• The target parameters when selecting an EDFA,
  however, are low noise and flat gain. Gain should
  be flat because all signals must be amplified
  uniformly.

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EDFA A Key Enabler for WDM

• High power
• Low noise figure
• Bit-rate transparent
• No cross-talk
• Wide bandwidth
• Excellent mech. property
• and more

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Communication Window and Er
Absorption
Gain
Natures gift to optical communications Erbium
gain spectrum and transmission fiber minimum
loss wavelengths coincide.
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EDFA

• While the signal gain provided with EDFA
  technology is inherently wavelength-dependent, it
  can be corrected with gain flattening filters.
  Such filters are often built into modern EDFAs.
• Low noise is a requirement because noise, along
  with signal, is amplified. Because this effect is
  cumulative, and cannot be filtered out, the
  signal-to-noise ratio is an ultimate limiting
  factor in the number of amplifiers that can be
  concatenated and, therefore, the length of a
  single fiber link.
• In practice, signals can travel for up to 120 km
  (74 mi) between amplifiers. At longer distances
  of 600 to 1000 km (372 to 620 mi) the signal must
  be regenerated. That is because the optical
  amplifier merely amplifies the signals and does
  not perform the 3R functions (reshape, retime,
  retransmit).
• EDFAs are available for the C-band and the
  L-band. Recently the S-band also has been
  introduced using a depressed cladding EDF.

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EDFA Achitecture

• Single pumping ? typ. 17dB gain Dual pumping ?
  typ. 35dB gain
• Counterdirectional pumping ? allows higher gain
• Codirectional pumping ? gives better noise
  performance
• 980 nm pumping is preferred produces less noise
  and larger population inversion than 1480 nm
  pumping

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EDFA Gain

• The purpose of the EDFA is to provide gain, which
  is defined as the ratio of the output signal
  power to the input signal power. Gain is found
  from the following formula
• Gain 10 Log (G)
• where,
• G Linear gain
• ls Amplifier signal wavelength (nm)
• Pin (ls) Level of input signal (W)
• Pout (ls) Level of output signal (W)
• Pase ASE level (W)
• When these power levels are measured on a
  logarithmic scale, with units of dBm (decibels
  relative to 1 milliwatt), the gain is calculated
  as the difference between the two signals, as
  shown in next figure.

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Amplifier Gain

• The input and output power levels measured are
  actually the sum of the signal power and the
  small amount of spontaneous emission power within
  the optical spectrum analyzers resolution
  bandwidth at the signal wavelength.
• This additional measured power usually has a
  negligible impact on the gain calculation, but it
  can be a factor when high spontaneous emission
  levels are present.
• This is corrected for by subtracting, from each
  of the power measurements, the spontaneous
  emission power in the measured spectrum at the
  signal wavelength.

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PCE
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Amplified Spontaneous Emission

• Ideally, an EDFA would amplify the input signal
  by its gain and produce no additional output.
  However, the EDFA also produces amplified
  spontaneous emission, which adds to the
  spontaneous emission produced by the source.
• Because the output spectrum contains spontaneous
  emission from both the source and the EDFA under
  test, the EDFA ASE cannot be determined directly
  from the output spectrum measurement.
• The calculation of EDFA noise figure requires
  that the portion of the output ASE level that is
  generated by the EDFA is known. This is
  calculated as the difference between the output
  spontaneous emission power and the equivalent
  source spontaneous emission power at the
  amplifier output.

EDFA input and output spectra showing signal and
spontaneous emission levels
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EDFA Noise Figure

• The EDFA noise figure is defined as the ratio of
  the input signal-to-noise ratio (SNR) to the
  output SNR.
• The experimental determination of the noise
  figure is given by
• where PASE is the amplified spontaneous emission
  (ASE) power, G is the amplifier gain, h is the
  Planck constant, n is the signal frequency and Dn
  is the optical bandwidth of the photodetector.
  The first term on the right-hand side corresponds
  to the signal spontaneous beat noise and 1/G
  corresponds to the shot noise.
• The noise figure equation contains two terms that
  contribute to noise at the electrical output of a
  photodetector used to detect the optical signal.
• The first term is due to mixing, at the
  photodetector, of the signal and the amplified
  spontaneous emission at the same wavelength.
• The second term represents the level dependent
  shot noise produced at the photodetector.
• This calculation assumes that a third noise term,
  the mixing of spontaneous emission with itself,
  is negligible in the determination of noise
  figure. This tends to be the case when either the
  signal power level is large enough to drive the
  amplifier into compression, or the output of the
  amplifier is passed through a narrow bandpass
  filter prior to the photodetector, or both.

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EDFA Noise Figure

• In order to correctly determine the noise figure,
  the ASE level must be determined at the signal
  wavelength. Unfortunately, this cannot be
  measured directly because the signal power level
  masks the ASE level at the signal wavelength.
• The noise figure measurement made by the EDFA
  test personality is based on the interpolation
  technique. It is so called because the amplified
  spontaneous emission of the EDFA at the signal
  wavelength is determined by measuring the ASE
  level at a wavelength just above and just below
  the signal, and then interpolating to determine
  the level at the signal wavelength.

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EDFA in Saturated Region
Change in channel loading Moving of operation
point Dynamics in optical networks QoS and
operation
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EDFA- System Application
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EDFA- System Application
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Stimulated Raman Scattering (SRS)

• Raman scattering are inelastic processes in which
  part of the power is lost from an optical wave
  and absorbed by the transmission medium. The
  remaining energy is then re-emitted as a wave of
  lower frequency.
• Raman scattering process can become nonlinear in
  optical fibres due to the high optical intensity
  in the core and the long interaction lengths
  afforded by these waveguides.
• Stimulated Raman scattering (SRS) occur when the
  light launched into the fibre exceeds a threshold
  power level for each process. Under the
  conditions of stimulated scattering, optical
  power is more efficiently converted from the
  input pump wave to the scattered Stokes wave.
• The scattered wave is frequency-shifted from the
  pump and in the case of SRS the Stokes wave can
  be shifted from the pump wave by typically 10 to
  100-nm and continues to propagate forwards along
  the fibre with the pump wave.
• If the pump is actually one channel of a
  multi-wavelength WDM communication system, then
  its Stokes wave may overlap with other channels
  at longer wavelengths - leading to crosstalk and
  Raman amplification.
• Raman amplification causes shorter wavelength
  channels to experience power depletion and act as
  a pumps for the amplification of longer
  wavelength channels. This can skew the power
  distribution among the WDM channels - reducing
  the signal-to-noise ratio of the lowest frequency
  channels and introducing crosstalk on the high
  frequency channels. Both of these effects can
  lower the information-carrying capacity of the
  optical system.

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Raman scattering.

• Light incident on a medium is converted to a
  lower frequency during Raman scattering as shown
  the figure.
• A pump photon, ?p, excites a molecule up to a
  virtual level (nonresonant state). The molecule
  quickly decays to a lower energy level emitting a
  signal photon ?s in the process.
• The difference in energy between the pump and
  signal photons is dissipated by the molecular
  vibrations of the host material. These
  vibrational levels determine the frequency shift
  and shape of the Raman gain curve.
• The frequency (or wavelength) difference between
  the pump and the signal photon (?p-?s ) is called
  the Stokes shift, and in standard transmission
  fibers with a Ge-doped core, the peak of this
  frequency shift is about 13.2 THz.
• For high pump power, the scattered light can grow
  rapidly.

• Schematic of the quantum mechanical process
  taking place during Raman scattering.

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Raman amplifier

• Raman optical amplifiers differ in principle from
  EDFAs or conventional lasers in that they utilize
  stimulated Raman scattering (SRS) to create
  optical gain. Initially, SRS was considered too
  detrimental to high channel count DWDM systems.
• In optical systems, as light traveled down the
  fiber, energy would be "robbed" from the shorter
  wavelength channels, boosting the amplitude of
  the longer wavelength channels
• A Raman optical amplifier is little more that a
  high-power pump laser, and a WDM or directional
  coupler. The optical amplification occurs in the
  transmission fiber itself, distributed along the
  transmission path. Optical signals are amplified
  up to 10 dB in the network optical fiber.
• The Raman optical amplifiers have a wide gain
  bandwidth (up to 10 nm). They can use any
  installed transmission optical fiber.
  Consequently, they reduce the effective span loss
  to improve noise performance by boosting the
  optical signal in transit.
• They can be combined with EDFAs to expand optical
  gain flattened bandwidth.

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Properties of Raman Amplifiers

• The peak resonance in silica fibers occurs about
  13 THz from the pump wavelength. At 1550 nm this
  corresponds to a shift of about 100 nm.
• As indicated power is transferred from shorter
  wavelengths to longer wavelengths.
• Coupling with the pump wavelength can be
  accomplished either in the forward or counter
  propagating direction.
• Power is coupled from the pump only if the signal
  channel is sending a 1 bit.

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Raman Amplifier - Theory
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Raman Amplifier - Theory
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Raman gain
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Raman gain profiles for a 1510-nm pump in three
different fiber types.
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Architecture of RA
Type of RA

• Distributed RA - the fiber being pumped is the
  actual transmission span that links two points.
  (Typical fiber length gt 40km)
• Discrete RA - the amplifier is contained in a box
  at the transmitter or receiver end of the system.
  (Typical fiber length 5km)

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Advantages and disadvantages
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The noise of RA
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Pump Sources of RA

• A key enabler of Raman amplification is the
  relatively high power sources needed to
  accomplish Raman amplification.
• For telecommunication wavelengths in the
  15001600-nm region, the required pump
  wavelengths for first-order pumping are in the
  1400-nm region.
• Two competing pump sources have emerged
• (a) semiconductor diode lasers
• Diode lasers employed for pumping of RAs should
  provide fiber coupled power in excess of 100 mW,
  and power levels as high as 400 mW are often
  desirable. Moreover, they should operate in the
  wavelength range 14001500 nm if the Raman
  amplifier is designed to amplify signal in
  wavelength region from 1530 1620 nm.
• High-power InGaAsP diode lasers operating in this
  wavelength range were developed during the 1990s
  for the purpose of pumping Raman amplifiers.
• (b) Raman fiber lasers (RFL)
• Conceptually, a complete RFL consist of three
  parts. The first portion is a set of multimode
  diodes that are the optical pumps. The next
  section is a rare-earth-doped cladding-pumped
  fiber laser (CPFL), which converts the multimode
  diode light into single mode light at another
  wavelength. Finally, this single mode light is
  converted to the desired wavelength by a cascaded
  Raman resonator.
• RFL usually provide a higher power (gt1W) compared
  with LD.

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Pump Arrangement to Extend the Range for
Stimulated Raman Amplification

• An array of laser diodes can be used to provide
  the Raman pump. The beams are combined and then
  coupled to the transmission fiber. The pump beams
  can counter propagate to the direction of the
  signal beams.

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Comparison between DFA and RA
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Comparison between DFA and RA
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Semiconductor Optical Amplifiers (SOA)

• SOAs are essentially laser diodes, without end
  mirrors, which have fiber attached to both ends.
  They amplify any optical signal that comes from
  either fiber and transmit an amplified version of
  the signal out of the second fiber.
• SOAs are typically constructed in a small
  package, and they work for 1310 nm and 1550 nm
  systems. In addition, they transmit
  bidirectionally, making the reduced size of the
  device an advantage over regenerators of EDFAs.
• However, the drawbacks to SOAs include
  high-coupling loss, polarization dependence, and
  a higher noise figure.
• The figure illustrates the basics of a
  Semiconductor optical amplifier.

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Modern optical networks utilize SOAs in the
follow ways

• An SOA consists of an amplifying medium located
  inside a resonant (Fabry-Perot type) cavity.
• The amplification function is achieved by
  externally pumping the energy levels of the
  material. In order to get only the amplification
  function, it is necessary to protect the device
  against self-oscillations generating the laser
  effect.
• This is accomplished by blocking cavity
  reflections using both an antireflection (AR)
  coating and the technique of angle cleaving the
  chip facets.
• Unlike erbium-doped fiber amplifiers (EDFAs),
  which are optically pumped, SOAs are electrically
  pumped by injected current.

• Power Boosters Many tunable laser designs output
  low optical power levels and must be immediately
  followed by an optical amplifier. ( A power
  booster can use either an SOA or EDFA.)
• In-Line Amplifier Allows signals to be amplified
  within the signal path.
• Wavelength Conversion Involves changing the
  wavelength of an optical signal.
• Receiver Preamplifier SOAs can be placed in
  front of detectors to enhance sensitivity.

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Types of SOA

• Depending on the efficiency of the AR coating,
  SOAs can be classified as resonant devices or
  traveling-wave (TW) devices.
• Resonant SOAs are manufactured using an AR
  coating with a reflectivity around 10-2. They
  typically feature a gain ripple of 10 to 20 dB
  and a bandwidth of 2 to 10 GHz.
• TW devices incorporate a coating with a
  reflectivity less than 10-4 (see figure 2). They
  show a gain ripple of a few dB and a bandwidth
  better than 5 THz (e.g., 40 nm in the 1550 nm
  window).

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TWA
The amplifier gain versus signal wavelength for ?
SOA whose facets are coated to reduce reflectivity
to about 0.04 ? 3dB-BW70nm
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• The input optical power Pin injected into the
  SOA waveguide is amplified according to Pout
  Gsp Pin, where Gsp is the single pass gain over
  the length L of the TW SOA such that Gsp exp
  (gnet L). The net gain gnet is given by gnet Gg
  a where G, g, and a are the optical confinement
  factor, the material gain, and the optical loss,
  respectively.

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• Telecom applications require a TW design, which
  can be used for applications such as
  single-channel or WDM amplification in the metro
  space, optical switching in core network nodes,
  wavelength conversion in optical cross-connects,
  and optical reshaping and reamplification (2R)
  regenerators or optical reshaping,
  reamplification, and retiming (3R) regenerators
  for long-haul transport networks.
• Using titanium oxide/silicon oxide (TiO2/SiO2)
  layers for AR coating technology, it is possible
  to achieve reflectivities on the order of 10-5.
  By combining tilted facets (about a 7 angle)
  with an AR coating, a device with a highly
  reproducible and extremely low residual
  reflectivity can be achieved, leading to gain
  ripples as low as 0.5 dB.
• A large number of incoming channels can saturate
  an SOA. Gain saturation caused by one channel
  modifies the response of the other channels,
  inducing crosstalk between channels. WDM
  applications thus require a device with high
  output saturation power.
• In a GC-SOA, the design is modified to
  incorporate a Bragg grating in each of the two
  passive waveguides. This creates a resonant
  cavity and thus a lasing effect. By programming
  the SOA to generate the lasing effect at a
  wavelength ?laser located outside of the desired
  amplification bandwidth of the SOA, it is
  possible to stabilize the gain.

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SOAs in action (Amplifiers)

• Discrete stand-alone SOAs can be used as compact
  booster amplifiers (a standard device for
  single-channel operation, a gain-clamped version
  for WDM operation), or to achieve
  high-sensitivity optically preamplified receivers
  as an interesting alternative solution to replace
  avalanche photodiodes for data rates of 40 Gb/s
  or higher.
• Noise figure is a key consideration for
  amplification applications. Noise figure is
  defined as nsp/C1 where nsp is the inversion
  factor and C1 is the overall input loss (mainly
  input coupling loss of about 3 dB). Because nsp
  and C1 depend on the polarization state of the
  input light, noise figure is defined for each
  polarization state. Usually, for
  nonpolarization-dependent amplifiers such as
  EDFA, noise figure is defined as 2nsp/C1. Thus, a
  3 dB difference exists between SOAs and EDFAs.