by: Mrs. Aboli N. Moharil Assistant Professor, EXTC dept. Optical Amplifiers by: Mrs. Aboli N. Moharil Assistant Professor, EXTC dept.

Contents Basics of optical amplifier Semiconductor Optical Amplifier (SOA) Doped Fiber Amplifier (DFA) Raman Amplifier

Optical amplifier An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. Reasons to use the optical amplifiers: Reliability Wide spectral range Wavelength division multiplexing Low cost Fundamental types of optical amplifiers are: Semiconductor optical amplifier (SOA) Doped fiber amplifier (DFA) Raman Amplifier

Traditional Optical Communication System Loss compensation: Repeaters at every 20-50 km

Applications of Optical Amplifiers In line Amplifiers: An in line amplifier is used to compensate for the loss and increase the distance between regenerative repeaters. Preamplifier: A weak optical signal is amplified first before photo detection so the signal to noise ratio degradation caused by thermal noise can be supressed. Power amplifier: power or booster amplifier include placing the device immediately after an optical transmitter to boost the transmitted power.

Amplification process All optical amplifiers increase the power level of incident light through a stimulated emission or an optical power transfer process. Spontaneous Emission Stimulated Emission

Semiconductor Optical Amplifier SOA is a InGaAsP laser. Construction is analogous to the laser diode. The gain peak of SOA can selected in any narrow wavelength band extending from 1280nm to 1650nm in U band by varying composition of InGaAsP material. Most SOA’s are Travelling wave amplifier category.

Semiconductor Optical Amplifier In SOA the mechanism for creating population inversion that is needed for stimulated emission to occur is same as is used in Laser diodes. An external injection current is passed through the device that excites the electrons in the active region. When photons (light) travels through the active region it can cause these electrons to loose some of their energy in the form of more photons that match the wavelength of initial ones.

Semiconductor Optical Amplifier Advantages: They can be made to work in O band (around 1310nm) as well as in the C band. Consume less electrical power. Have fewer components and more compact. Rapid gain response which is of the order of 1 to 100ps.

Doped Fiber Amplifier Erbium doped fiber amplifier (EDFA) which are widely used in C band (1530nm to 1565nm) for optical communication networks. The active medium for operation in the S ,C, and L bands is created by lightly doping a silica (silicon dioxide) or tellurium oxide) fiber core with rare earth elements such as thulium (Tm), erbium (Er) or Ytterium(Yb)

EDFA

Physics of an EDFA

Erbium Properties Erbium: rare element with phosphorescent properties Photons at 1480 or 980 nm activate electrons into a metastable state Electrons falling back emit light in the 1550 nm range Spontaneous emission Occurs randomly (time constant ~1 ms) Stimulated emission By electromagnetic wave Emitted wavelength & phase are identical to incident one Erbium is a rare earth element which can absorb and release light energy in the communications band around 1550 nm. When light at 980 or 1480 nm is applied to fiber doped with Er, the fiber absorbs this energy, i.e., electrons are excited to a higher energy level where they remain in a metastable state for some time. If left undisturbed, the Er doped fiber will eventually release this energy in the band of frequencies from about 1530 to 1565 nm. If stimulated by an input signal in this band, the Er doped fiber will emit the stored energy at the stimulated wavelength. The 980 nm absorption band is narrower than the 1480 nm band. In addition it is more difficult to make reliable 980 nm lasers. However, pumping the amplifier with 980 nm can result into a better noise figure of the amplifier.

DFA Advantages of DFA includes the ability to pump the devices at several different wavelengths. Low coupling loss to compatible sized fiber transmission medium. They exhibit slow gain dynamics. Immune to interference effects such as cross talk and intermodulation distortion.

Raman amplifier A Raman optical amplifier is based on nonlinear effect called Stimulated Raman Scattering (SRS), which occurs in fibers at high power.

Raman amplifier Topologically simple to design-No special doping is required. Uses intrinsic optical non-linearity of the fiber. Amplification takes place throughout the length of the optical fiber. Hence also known as Distributed Amplifier.

Stimulated Raman Scattering Stimulated Raman Scattering is an interaction between lightwaves and the vibrational modes of silica. If photon with energy hv1 is incident on a molecule having vibrational frequency vm, the molecule can absorb some energy from photon. In this interaction, the photon is scattered, thereby attaining a lower frequency v2 and a corresponding lower energy hv2. λ5<λ4 < λ3 < λ2 <λ1 SRS transfers optical power from shorter wavelength to longer wavelength

Raman amplifier An atom absorbs a photon at a particular wavelength and releases another photon at lower energy, that is at a longer wavelength than that of the absorbed photon. The energy difference between the absorbed and released photons is transformed into photon, which is a vibrational mode of the material. The power transfer to higher wavelength occurred over a broad spectral range of 80 to 100nm. The shift to particular longer wavelength is referred to as strokes shift for that wavelength.

Raman Amplifier

Advantages and Disadvantages Variable wavelength amplification possible Compatible with installed SM fiber Can be used to extend EDFAs Can result in lower average power over a span, good for lower crosstalk Very broadband operation is possible Disadvantages: High pump power requirements. Sophisticated gain control needed. Noise is also an issue.

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