m Chapter 12
`
`FIGURE 12.2
`Electro-optic
`repeater and
`optical amplifier.
`
`3-R regeneration
`(re)amplifies,
`reshapes, and
`retimes pulses.
`
`E lectrical
`O u tp u t
`
`Input Fiber
`
`D e te c to r
`
`O p tic a l
`
`| | ____
`
`S ig n a ls
`
`R e ceiver
`C irc u it
`
`S ignal
`R e g e n e ra tio n
`
`T ra n sm itte r
`D rive
`C irc u it
`
`Lig h t
`S o u rc e
`
`O u tp u t F iber
`
`■ E lectrical S ig n a ls -
`
`O p tic a l S ig n a ls
`
`a. Electro-Optic Repeater
`
`Input F iber
`
`O p tic a l
`A m p lifie r
`
`O u tp u t Fiber
`
`b. Optical Amplifier
`
`telegraphs. Today, however, regenerator is a distinct term with its own meaning— a device
`that generates a fresh version of a digital input signal by removing noise and distortion.
`The standard regenerators used in today’s fiber-optic systems are electro-optic devices.
`Like digital receivers, electro-optic regenerators convert input optical signals into electronic
`form, then run them through discrimination circuits that examine the time-varying input
`signal and decide which changes in signal strength are data bits and which are noise. They
`also contain retiming circuits that put the pulses into their proper time slots. Regeneration
`typically is performed at the ends of a system, with amplifiers used to boost signal strength
`along the route. Often regeneration is performed within the receiver stage of a large switch
`or other electronic device, so you won’t see a big box labeled “regenerator.”
`Optical regeneration is also possible, although still largely at the laboratory stage. There
`are two variations: 2-R regeneration and 3-R regeneration.
`Both types amplify the signal optically, but sometimes the term ream plification is used
`to justify the “R ” terminology. 2-R regenerators also reshape the pulses, using a dis-
`crimination circuit to generate fresh sharp pulses. 3-R regenerators follow that with a
`retiming stage. Retiming is difficult to implement optically, so 2-R regenerators are bet-
`ter developed.
`Wavelength conversion changes the wavelength of a transmitted signal. Such conversion
`may be required to provide the same wavelength throughout a system, or to meet other
`needs of optical networks. The wavelength-conversion function is separate from amplifica-
`tion and regeneration, but in practice these functions may be combined. One way to con-
`vert the signal wavelength is to pass it through an electro-optic repeater that generates a
`different output wavelength. These devices are often called OEO transponders, and are
`essentially special-purpose repeaters or regenerators. Like regenerators, they may be built into
`a terminal switch at a network node, so they are hard to identify as discrete devices. As you
`will learn later in this chapter, some all-optical devices also can convert signal wavelengths.
`
`MASIMO 2014
`PART 5
`Apple v. Masimo
`IPR2020-01526
`
`

`

`Repeaters, Regenerators, and Optical Amplifiers
`
`Long-Fiber
`
`Short-Fiber
`
`FIGURE 12.3
`Roles fo r optical
`amplifiers.
`
`System Requirements
`
`The need for repeaters, regenerators, optical amplifiers, and wavelength converters depends
`on system design.
`Electro-optic repeaters are rarely used today simply to amplify signals except in certain
`types of local-area networks, where one receiver generates an electronic signal that drives
`two or more transmitters. Such repeaters may be used where an electronic signal must be
`generated to drive a terminal device and an optical signal is needed for transmission to
`the next node.
`Electro-optic regenerators require expensive multiplexing and demultiplexing of signals
`transmitted through the same fiber at different wavelengths, so typically signals are regen-
`erated only at the end of a system by the optical receiver. Demultiplexing is required at this
`point anyway, and electronic outputs are often required. In practice, regeneration is usually
`invisible because it is built into the receiver.
`Optical amplifiers may be used at several different points in communication systems, as
`shown in Figure 12.3.
`
`9 Postamplifiers are placed immediately after a transmitter to increase strength of a
`signal being sent through a length of fiber. It might seem easier just to crank up the
`transmitter output, but that can degrade the quality of the output signal. External
`amplification of a lower-power transmitter output gives a cleaner signal. Postamplifiers
`also can generate powerful signals that can be split among many separate output fibers
`if a single transmitter is distributing signals to many points.
`• In-line am plifiers compensate for signal attenuation in long stretches of fiber.
`The goal is to amplify a weak signal sufficiently to send it through the next segment
`of fiber. These generally are required in long telecommunication systems but may be
`used in some networks where many branching points reduce transmitted power.
`Signals may require regeneration after a series of many amplifiers.
`
`An electro-optic
`repeater can
`convert
`wavelengths if the
`transmitter end
`emits a
`wavelength
`different from that
`of the input
`signal.
`
`Optical
`amplification is
`needed in-line,
`after transmitters,
`before receivers,
`and after lossy
`components.
`
`

`

`w Chapter 12
`
`Electro-optic
`repeaters
`essentially link
`two systems end
`to end.
`
`Regeneration is
`normally done in
`electronic switches
`at the end of a
`system.
`
`Electro-optic
`repeaters can
`convert signals to
`different
`wavelengths.
`
`9 Preamplifiers amplify a weak optical signal just before it enters a receiver, in effect
`increasing the sensitivity of the receiver and stretching transmission distances.
`• Offsetting component losses that otherwise would reduce signals to unacceptably low
`levels. Optical couplers must physically divide the signal among multiple terminals,
`which reduces the signal strength arriving at each one. For example, splitting a signal
`in half reduces each output to a level 3 dB below the input. Dividing a signal among
`20 terminals reduces signal strength by 13 dB— assuming every output gets exactly
`of the input. Placing an optical amplifier before or after the lossy component can raise
`the signal strength to compensate for the loss.
`
`Repeaters and Regenerators
`
`You saw earlier that an electro-optic repeater or regenerator is essentially a receiver and
`transmitter placed back to back in a single unit. The input end performs the usual receiver
`functions; the output end performs the standard transmitter functions. In the middle they
`amplify and typically clean up the signal. You can think of them as joining two separate
`fiber-optic systems together end to end.
`Electro-optic repeaters were widely used in long-distance fiber-optic systems installed
`before the mid-1990s, when optical amplifiers became available. Most of those systems
`operated at 1310 nanometers, the zero-dispersion wavelength in standard step-index single-
`mode fibers. Optical amplifiers have replaced electro-optic repeaters for boosting signal
`strength to extend transmission distance, and operation has shifted to the 1550-nm range
`where the best optical amplifiers operate.
`Electro-optical regeneration is largely performed within receivers or electronic switches
`at the end of a fiber-optic system. Operators would rather install the sensitive electronic
`equipment in climate-controlled buildings than in the field along the cable route.
`The performance of electro-optic repeaters and regenerators depends on the internal
`electronic circuits. These circuits are designed to operate at specific data rates, with clock
`circuits set to generate pulses at the same speed as the input signal. That means that
`repeaters, like transmitters and receivers, must be changed if the system is to operate at a
`speed different from that of the original design. This is not true for optical amplifiers.
`Another limitation is that electro-optic repeaters and regenerators can process only one
`signal at a time, so signals transmitted on separate wavelengths through the same fiber
`must be divided among separate repeaters or regenerators. This is not true for optical
`amplifiers.
`Electro-optic repeaters do offer a straightforward way to convert wavelengths, which has
`given them a new life as OEO (opto-electronic-optical) transponders. Like a standard
`EO repeater, the OEO transponder converts the input optical signal to electronic form for
`amplification and other processing. However, the transmitter module generates a wavelength
`different from the input wavelength. It may not be elegant, but it works. The performance
`can be enhanced by using a tunable laser to generate a user-specified wavelength. As with
`electro-optic regenerators, this wavelength-conversion function can be buried within a
`larger electronic switch that processes signals at a network node.
`
`

`

`Repeaters, Regenerators, and Optical Amplifiers
`
`PHOTON VIEW
`
`L a s e r M a te ria l
`^ — n
`
`/•-
`
`The stimulated
`emission can
`stimulate more
`emission.
`
`-
`S tim u la te d
`E m ission fro m
`E xcite d A tom
`
`E xcite d A to m s
`
`A m p lifie d O u tp u t
`
`FIGURE 12.4
`Optical
`am plification o f
`individual photons
`(top), and o f a
`signal (bottom).
`
`A m p lifie d O u tp u t
`S ignal
`
`Input P h oton -
`
`W e a k Input ■
`
`Optical Amplifiers
`
`You read earlier that optical amplifiers are based on the same principle as the laser. The dif-
`ference between lasers and optical amplifiers is that lasers generate a signal internally, while
`optical amplifiers amplify a signal from another source.
`Figure 12.4 sketches the idea of an optical amplifier. The amplifier material is excited so
`that some of the atoms store excess energy, as you saw for a laser in Chapter 9, but it is not
`placed between a pair of mirrors. Instead, a weak input signal enters the material, stimu-
`lating some of the excited atoms to release energy as light. The photons produced by this
`stimulated emission are duplicates of the photons in the input signal, at the same wave-
`length, in the same phase, and going in the same direction. This process multiplies the
`strength of the input signal as the light makes a single pass through the amplifier.
`The effectiveness of optical amplification depends on how well the input wavelength
`matches the stimulated-emission properties of the material. The probability of stimu-
`lated emission varies with wavelength, so the input wavelength must match the material’s
`emission wavelength.
`Gain and Power Levels
`The performance of an optical amplifier is measured by the total output power and by the
`amount of amplification, called the gain. These quantities depend on several factors. For
`simplicity, we’ll consider an input signal that consists of only a single wavelength.
`The input pow er is the starting point for the amplifier. Optical amplifiers are analog
`devices, so as in electronic systems the power of input signal should be well above the
`background noise.
`The gain is the amplification, usually measured in decibels, which depends on the
`input power and the amplifier design. Often gain is measured per unit length, usually as a
`fraction or percentage per centimeter. The gain measures how much emission the input
`signal can stimulate, which in turn depends on how many excited atoms are available and
`how easy it is to stimulate emission. The number of excited atoms, in turn, depends on
`
`Optical amplifiers
`are based on
`stimulated
`emission.
`
`Gain depends on
`input wavelength
`and amplifier
`design.
`
`

`

`Chapter 12
`
`FIGURE 12.5
`Saturation o f fib er
`am plifier gain.
`
`10 dB m
`
`1 dB m
`
`30 d B gain
`
`1 dBn.
`
`10 dBfj,
`
`100 dB(i
`
`10 0 0 dB^
`
`In p u t S ignal
`
`the structure of the material, how fast the atoms are being excited, and how fast the input
`signal is taking away their extra energy by stimulating emission. In the real world, fur-
`ther complications include how uniformly the excitation energy is distributed along the
`amplifier.
`For low input powers, the output power is the product of the input power, the gain per
`unit length, and the length:
`
`Output = ^input X gain x length
`This is called the small-signal approximation, and it assumes that enough excited atoms
`are always available along the entire length of the amplifier.
`At higher input signals, the output power may saturate because too few excited atoms are
`available to further amplify emission. Essentially, the amplifier runs out of energy. Figure 12.5
`gives an example of this effect for an erbium-fiber amplifier. The higher the input power,
`jower ^ gain. If you keep increasing the input power, eventually you extract all the
`available energy from the amplifier, and raising the input power further will not produce
`any additional output.
`Note that this saturation effect actually distorts the amplified signal, like trying to turn
`up an audio amplifier beyond its operating range. However, this doesn’t affect digital
`transmission.
`So far, we’ve considered only amplification of one optical signal at one wavelength.
`However, the saturation effect depends on total power at all input wavelengths in the
`amplification band. If you look carefully at Figure 12.5, you will notice that it takes a fair
`amount of input power to saturate this optical amplifier. Gain levels off when the input signal
`is 1 dBm, which you’re unlikely to encounter at the receiver end of a system carrying only
`a single optical channel. This figure was plotted for an amplifier that carries wavelength-
`division multiplexed signals at many different wavelengths in the amplification band, and
`the numbers are for total input and output power.
`
`•
`Amplifier gain
`saturates at high
`power eve s.
`
`

`

`Repeaters, Regenerators, and Optical Amplifiers
`
`Erbium fiber
`amplifiers work
`from 1530 to
`about 1605 nm.
`
`Erbium-doped
`fiber amplifiers
`are the most
`common optical
`amplifiers.
`
`Wavelength Range and Material
`Optical amplifiers work over the range of wavelengths that can stimulate emission from the
`excited atoms. That is, the probability of stimulated emission varies with wavelength: It
`peaks at one wavelength, then drops from that level, eventually reaching zero. The wave-
`length range depends on the amplifier material and structure.
`The best optical amplifiers, based on erbium-doped optical fibers, typically work at
`wavelengths from about 1530 to 1605 nm, where silica fibers have their lowest loss. These
`wavelengths have become standard for long-distance transmission that requires amplifica-
`tion. Other amplifiers are available at other wavelengths.
`Optical amplification works only as long as the atoms in the amplifier are being excited.
`Once the excitation stops, the atoms drop to their lower energy states. In many optical
`amplifiers, notably erbium, these lower energy states can absorb light at the same wave-
`lengths that are amplified when the atoms are excited. That means an erbium-doped fiber
`that is not being excited strongly absorbs the signal and can shut down transmission.
`
`Types of Optical Amplifiers
`Three types of optical amplifiers are now used in fiber-optic systems. Before describing
`them in more detail, we’ll make a quick comparison.
`D oped fib er am plifiers have cores doped with atoms of an element that light from an
`external laser can excite to a state in which stimulated emission can occur. The doped fibers
`are specialty products, described in Chapter 7. Pump light from the external laser steadily
`illuminates one or both ends of the fiber and is guided along the fiber length to excite the
`atoms in the core. The core guides the input signal and the amplified light. By far the most
`important of these amplifiers is the erbium -doped fib er am plifier (EDFA), which is widely
`used in long-distance fiber systems.
`Raman fib er am plifiers are based on a process called stim ulated Raman scattering, which
`also causes nonlinear effects in fibers. An atom absorbs a pump photon at one wave-
`length and, while it holds that extra energy, is stimulated to emit most of the energy by
`a second photon with longer wavelength. The effect converts light energy from the
`shorter wavelength to the longer one. For amplification, the fiber is pumped with strong
`light at one wavelength to amplify a weak signal at a longer wavelength. This is a non-
`linear process with gain per unit length much weaker than in doped fiber amplifiers; but
`it can be spread over many kilometers of fiber, so the total amplification can be signifi-
`cant. It requires no special doping of the fiber core and can be produced in ordinary
`telecommunications fiber.
`Semiconductor optical am plifiers are essentially semiconductor lasers without mirrors. A
`weak signal enters the junction layer and is amplified when the photons stimulate emis-
`sion from recombining electron-hole pairs. The energy comes from a current flowing
`through a semiconductor diode, as in semiconductor lasers. The gain per unit length is
`much higher than in doped fiber amplifiers; but the length is much shorter, so the overall
`amplification is comparable. The edges of the semiconductor chip can be coated to pre-
`vent reflections, or the amplifier may be integrated within a semiconductor waveguide to
`avoid reflections.
`
`

`

`Chapter 12
`
`Opfical signals
`are amplified by
`erbium atoms in
`the fiber core.
`
`Standard pump
`wavelengths are
`980 and
`1480 nm.
`
`We will start by looking at the erbium-doped fiber amplifier, the most widely used type.
`Then we’ll cover other fiber amplifiers and semiconductor amplifiers.
`
`Erbium-Doped Fiber Amplifiers
`
`The erbium-doped fiber amplifier tremendously expanded fiber-optic transmission
`capacity, which fed the telecommunications bubble. Its ability to amplify wavelength-division
`multiplexed signals broke the traditional bandwidth bottleneck that had limited the capacity
`of long-distance systems. By rare good fortune, erbium has especially attractive properties for
`an amplifier, with gain at wavelengths of 1530 to 1625 nm, closely matching the minimum-
`attenuation band of standard optical fibers.
`Function
`Erbium-fiber amplifiers simultaneously amplify weak light signals at wavelengths across
`their operating range. This range varies with amplifier design, as described later in this sec-
`tion, but this capability is crucial for wavelength-division multiplexing. Because fiber am-
`plifiers respond very rapidly to variations in input signal strength, they amplify signals
`across a wide range of modulation speeds, although the response is not unlimited.
`Fiber amplifiers can be used in various locations in a system, or cascaded so one ampli-
`fies a signal that has earlier been amplified by another amplifier, as shown in Figure 12.3.
`In practice, the accumulation of noise limits the number of amplifiers that can be cascaded.
`The accumulation of noise can be reduced by limiting the gain per amplifier and reducing
`the spacing between amplifiers. Thus, a series of erbium-fiber amplifiers spaced 50 km
`apart can transmit signals farther than a series of amplifiers 100 km apart.
`
`Structure and Operation
`Figure 12.6 shows amplification in an erbium-doped fiber amplifier. Small quantities of er-
`bium are present in the fiber core. When light excites the erbium atoms, a weak signal in
`the erbium amplification band guided along the fiber core stimulates emission, and the sig-
`nal grows in strength along the length of the fiber.
`Figure 12.7 shows the overall structure of an erbium-fiber amplifier, omitting the details
`inside the fiber. The input signal enters from the left (in this example, a single optical chan-
`nel at 1550 nm). It passes through an optical isolator, which blocks light from going back
`toward the light source, and a filter, which transmits the signal wavelength but blocks the
`wavelength of the pump laser. Then the signal enters the erbium-doped amplifying fiber.
`Light from the pump laser is coupled into the other end of the erbium-doped fiber to excite
`erbium atoms, which amplify the signal passing through the loop of fiber. Then the ampli-
`fied signal is separated from the pump at a wavelength-selective coupler on the right, and
`exits through another optical isolator into the next leg of the fiber-optic system.
`The pump light must be at specific wavelengths in order to stimulate emission from the
`erbium atoms. Standard pumps are semiconductor lasers that emit 980 or 1480 nm. Each
`wavelength has its own distinct advantages.
`
`

`

`Repeaters, Regenerators, and Optical Amplifiers
`
`E rbium A tom
`
`FIGURE 12.6
`Amplification in
`an erbium -doped
`fib er amplifier.
`
`Pump light (A,pump)
`excites Er atoms.
`
`E xcite d Er
`
`A m p lifie d Light
`at
`
`Signal at erbium wavelength (A.Er)
`stimulates emission.
`
`What the communication system sees.
`
`A m p lifie r
`
`s\s\s\s\s\/^-
`XE (w eak)
`
`/\ \z\z\r
`
`S tim u la te d E m ission
`
`mwvi,
`
`>.Er (s tro n g sig n a l)
`
`Operating Wavelengths
`Erbium-doped fibers can amplify light over a surprisingly wide range of wavelengths.
`Figure 12.8 gives an indication of this range by plotting the cross section for stimulated
`emission as a function of wavelengths. This cross section measures the likelihood that a
`photon of that wavelength can stimulate emission from an excited erbium atom. The cross
`section depends on the glass “host” as well as the erbium atom; it is highest for a special
`glass formulation containing tellurium and is somewhat lower for fluoride and silica-based
`
`

`

`Chapter 12
`
`FIGURE 12.7
`Erbium -doped
`fib er amplifier.
`
`E rb iu m -D o p e d
`A m p lify in g F ib e r
`
`P u m p
`Filter
`
`Pump light excites erbium
`atoms, which then amplify
`1550 nm.
`
`O ptical
`Isolator
`
`W a v e le n g th -
`S p littin g
`C o u p le r
`
`O ptical
`Iso la to r
`
`•
`Small-signal gain
`of erbium-doped
`fiber ampli iers is
`^ k s'a H S d o 'to
`1535 nm
`
`glass. (The silica glass shown has extra aluminum and phosphorus to enhance erbium
`emission.)
`You can’t actually realize amplification across this entire range. Erbium atoms absorb
`light at the shorter wavelengths, damping possible amplification. In addition, the amplifi-
`cation process concentrates gain at the wavelengths where the probability of stimulated
`emission is highest. For relatively short lengths of fiber— a few meters— the gain is highest
`3t
`t0
`nm’ aS s^own ‘n Figure 12.9. This figure shows gain at various wave-
`lengths for different amounts of input power. Recall that the gain is highest for small input
`signals. As Figure 12.9 shows, for small inputs, gain varies significantly with wavelength—
`by more than 10 dB from the peak between 1530 and 1535 nm to the plateau at 1540 to
`1560 nm. However, for high inputs, where gain saturates, gain is more uniform across that
`wavelength range.
`
`FIGURE 12.8
`Stimulated
`emission cross
`section fo r erbium-
`doped fibers o f
`various
`compositions.
`
`sHi
`-o
`Bjo
`=3£
`55
`
`Wavelength (nm)
`
`

`

`Repeaters, Regenerators, and Optical Amplifiers
`
`FIGURE 12.9
`Erbium -fiber
`am plifier gain
`versus wavelength
`at different input
`powers. (Courtesy
`o f Corning, Inc.)
`
`Erbium gain ai
`absorption line
`overlap in the
`1550-nm regie
`
`Most erbium
`amplifiers operate
`in the C-band.
`
`W ave le n g th (nm )
`
`Erbium atoms have both gain and absorption at a broad range of wavelengths near
`1550 nm, as you can see in Figure 12.10. Gain is high at short wavelengths, but it is offset
`by high absorption, so most erbium amplifiers operate at wavelengths longer than 1530 nm.
`Gain drops at wavelengths longer than about 1560 nm, but the absorption also drops, and
`the gain remains higher than the absorption for wavelengths out to about 1625 nm. This
`produces a net gain for light at those wavelengths as long as the fiber is excited with pump
`light. Although that gain is not large, it does accumulate, allowing amplification in a long
`fiber. “Long” in this case means 100 m or more, but the fiber can be packaged as a coil inside
`a case, which opens that range of wavelengths to erbium-fiber amplifiers.
`Design of erbium-fiber amplifiers differs for the high-gain and low-gain wavelengths.
`Two different types have emerged:
`
`• C-band amplifiers are designed for the high-gain band from 1530 to 1565 nm and
`use several meters of optical fiber. C-band erbium amplifiers are by far the most
`widely used optical amplifier. Their gain is highest at 1530 to 1535 nm, but this
`bandwidth is sometimes avoided in W DM systems to keep gain uniform across the
`operating range. Operation at shorter wavelengths is limited by absorption and noise
`from amplified spontaneous emission (described later).
`• L-band amplifiers are designed for the lower-gain wavelengths longer than
`1565 nm and use 100 m or more of erbium-doped fiber optimized for low-gain
`operation. Erbium-doped fiber has gain at wavelengths to 1625 nm, but in practice
`L-band erbium amplifiers are limited to wavelengths shorter than about 1605 nm.
`L-band amplifiers are not widely used with standard fibers, but they are used with
`zero dispersion-shifted fibers because they shift the operating range away from the
`zero-dispersion wavelength at 1550 nm. (WDM is impractical in the C-band in zero
`dispersion-shifted fibers because of four-wave mixing.)
`
`L-band amplifiers can supplement C-band amplifiers when no more channels can be
`accommodated in the C-band. In practice, a 5-nm gap is left between the C- and L-bands,
`so the signals can be split between a pair of parallel amplifiers and both bands can be used
`
`

`

`Chapter 12
`
`FIGURE 12.10
`Gain and
`absorption in a
`typical erbium-
`doped fib er (top)
`and calculated
`gain fo r C-band
`and L-band
`am plifiers
`(bottom).
`(Courtesy o f
`Nufern)
`
`(0
`CD
`
`W a v e le n g th (nm )
`
`Ec
`
`S t
`
`o
`CD
`
`W a v e le n g th (nm )
`
`simultaneously, as shown in Figure 12.11. The long-wavelength end of the L-band depends
`on the manufacturer and system requirements. A typical L-band operating range is 1570
`to 1605 nm, but can be extended to 1620 nm in some cases.
`You should remember one other thing about erbium-doped fibers: if the pump light is
`turned off, the gain goes away but the absorption remains, and the fiber strongly absorbs
`the light it is supposed to amplify.
`
`

`

`S e p a ra te A m p lific a tio n in P a rallel A m p lifie rs
`
`Repeaters, Regenerators, and Optical Amplifiers
`
`FIGURE 12.11
`High- and
`low -band optical
`am plifiers in
`parallel.
`
`WDM and Erbium-Fiber Amplifiers
`One advantage of erbium-fiber amplifiers is their ability to simultaneously amplify signals
`at several different wavelengths in the erbium band. Without this ability, wavelength-
`division multiplexing would be cumbersome and impractical. Nonetheless, multiwavelength
`operation does pose some complications.
`The same population of excited erbium atoms amplifies all the wavelengths of light in
`the signal, so all the atoms draw power from the same pump laser. Thus, if the signal con-
`tains only one wavelength, all erbium atoms are available to amplify that wavelength; but
`if it contains multiple wavelengths, the pump power has to be shared among them.
`As long as the amplifier is operating in the small-signal regime, where there is power to
`spare, that’s not a problem. However, adding more channels at the same input power can
`saturate the amplifier. The total output from the amplifier on all channels depends on the
`total input power on all channels. Figure 12.5 shows that the total output from the
`amplifier increases only 5 dB as the input signal increases from 10 dBp to 100 dBp, a sign
`of saturation. The effect is the same whether the total power is all on one channel or dis-
`tributed among 10 input channels. At higher power levels the total available power satu-
`rates completely. If saturation limits the output from an amplifier to 100 mW at one
`wavelength, dividing the signal among 40 wavelengths would leave each channel with
`only 2.5 mW.
`Another complication is that erbium-fiber amplifiers do not have uniform gain across
`their spectrum. As you can see in Figure 12.9, the gain peaks at 1535 nm for small signals
`when plenty of erbium atoms are available to amplify light. Saturation tends to reduce this
`differential gain, but it doesn’t go away completely. Differential gain also builds up in a
`series of amplifiers, with the strong wavelengths getting stronger and the weak wavelengths
`getting weaker. This same phenomenon concentrates stimulated emission at a narrow range
`of wavelengths in a laser, but is undesirable when you’re trying to amplify signals at multi-
`ple wavelengths.
`Gain can be equalized across the erbium spectrum either by adding optical filters to
`reduce the peaks or by adding different types of amplifiers to boost the strength of the
`
`All optical
`channels share
`power from the
`same pump laser.
`
`Adding more
`channels can
`saturate an
`erbium-fiber
`amplifier.
`
`

`

`m Chapter 12
`
`FIGURE 12.12
`A m plified
`spontaneous
`emission noise in
`a fib er amplifier.
`
`A m p lifie d
`
`Amplified
`spontaneous
`emission
`generates
`background noise
`in fiber amplifiers.
`
`Erbium can also
`be used in
`waveguide
`amplifiers.
`
`weaker wavelengths. You’ll learn more about how this works in Chapter 22, which covers
`optical network design.
`Noise and Amplified Spontaneous Emission
`As analog devices, optical amplifiers inevitably amplify any input noise that arrives with the
`signal. They also generate background noise by a process called am plified spontaneous emission.
`As you saw in Chapter 9, the light that starts simulated emission in a laser is emitted
`spontaneously when an excited atom releases its excess energy without outside stimulation.
`A laser resonator bounces this light back and forth through the laser cavity to amplify it by
`stimulated emission. Fiber amplifiers lack resonator mirrors, so they don’t build up a laser
`beam in the same way. However, spontaneous emission that occurs within the fiber can be
`amplified if it’s guided along the fiber, creating background noise.
`Amplified spontaneous emission is spread across the whole operating range of a fiber
`amplifier, as shown in Figure 12.12. The power is much lower than at the amplified wave-
`lengths, shown as peaks in Figure 12.12. However, it remains in the background and can
`be amplified in successive amplifiers. As a broadband noise, it’s analogous to static in the
`background of an AM-radio signal.
`Erbium-Doped Waveguide Amplifiers
`Erbium atoms can amplify light by stimulated emission in rods or waveguides as well as
`in fibers. Rods are used in erbium lasers, and erbium-doped waveguides are used as opti-
`cal amplifiers. The physics of erbium-doped waveguides are similar to those of erbium-
`doped fibers, although the erbium-doped waveguide confines the input signal and
`stimulated emission from the erbium atoms in a high-index region rather than in the core
`of a fiber.
`The details of erbium-doped waveguide amplifiers differ considerably from those of
`erbium-doped fiber amplifiers. Waveguide amplifiers are much shorter than fiber amplifiers—
`centimeters instead of meters. This makes them more compact, but the erbium must be in
`higher concentrations in the waveguide to get reasonable gain in the C-band. (L-band
`
`

`

`Repeaters, Regenerators, and Optical Amplifiers
`
`operation is more difficult for waveguide amplifiers.) Even with the higher erbium concen-
`tration, erbium waveguides have considerably less small-signal gain than typical erbium-
`fiber amplifiers, so they are used for different applications.
`Erbium-Amplifier Configurations
`In theory, erbium amplifiers can be made with a variety of optical characteristics. In
`practice, they generally fall into a few distinct configurations that meet specific com-
`mercial needs:
`
`9 Metro amplifiers, compact devices with moderate gain for use in metro
`networks, where only modest (10 to 20 dB) gain is needed.
`• Single-channel am plifiers with higher gain.
`9 WDM am plifiers with higher gain and higher total power, able to amplify
`many channels simultaneously. These may span up to 100 km for
`terrestrial systems.
`• Ultra-long-haul and submarine amplifiers, optimized to have very low noise
`and moderate gain. These are used with fiber spans shorter than those
`used in normal terrestrial long-distance systems.
`• Cable-television optim ized amplifiers, able to deliver higher total powers
`when signals are split among many outputs.
`
`Other variations have been demonstrated in the laboratory. One of these is an erbium-fiber
`amplifier that demonstrates net gain between 1480 and 1530 nm, which are wavelengths not
`normally produced by erbium amplifiers. This requires pumping at 980 nm and suppressing
`amplified spontaneous emission, which otherwise would overwhelm the signal.
`
`Other Doped Fiber Amplifiers
`
`The success of the erbium-doped fiber amplifier and its compatibility with wavelength-
`division mult