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.

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.

Type of amplifiers Power (Booster) amplifier In line amplifier Preamplifier

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 system’s 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.

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)

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

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

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 micrometers—the 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.

EDFA: A Key Enabler for WDM High power Low noise figure Bit-rate transparent No cross-talk Wide bandwidth Excellent mech. property and more … Data Out WDM Point-to-Point XMTR O M U X Data In OA RCVR D • l1 l2 lN

Communication Window and Er Absorption Gain Nature’s gift to optical communications: Erbium gain spectrum and transmission fiber minimum loss wavelengths coincide.

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.

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

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.

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 analyzer’s 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.

PCE

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

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.

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.

EDFA in Saturated Region Output Power (dBm) Gain (dB) p=980 nm Pp=70 mW Linear Regime 3 dB Compression Saturated Regime Psat Psat3dB Change in channel loading Moving of operation point Dynamics in optical networks QoS and operation

EDFA- System Application

EDFA- System Application

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.

Raman scattering. Light incident on a medium is converted to a lower frequency during Raman scattering as shown the figure. A pump photon, νp, excites a molecule up to a virtual level (nonresonant state). The molecule quickly decays to a lower energy level emitting a signal photon νs 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 (νp-νs ) 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.

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.

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.

Raman Amplifier - Theory

Raman Amplifier - Theory

Raman gain

Raman gain profiles for a 1510-nm pump in three different fiber types.

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

Advantages and disadvantages

The noise of RA

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 1500–1600-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 1400–1500 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 (>1W) compared with LD.

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.

Comparison between DFA and RA

Comparison between DFA and RA

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.

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. Modern optical networks utilize SOAs in the follow ways: 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.

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).

TWA The amplifier gain versus signal wavelength for ⇐ SOA whose facets are coated to reduce reflectivity to about 0.04% ⇒ 3dB-BW~70nm

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 = Γg – α where Γ, g, and α are the optical confinement factor, the material gain, and the optical loss, respectively.

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 λlaser located outside of the desired amplification bandwidth of the SOA, it is possible to stabilize the gain.

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.