m Chapter 14
`
`FIGURE 14.6
`Important coupler
`types.
`
`Input
`
`O ut
`
`O ut
`
`T Coupler
`
`O n e Input
`
`M a n y O u tp u ts
`
`1 x n ,o r
`Tree, Coupler
`
`W .
`
`Wavelength-
`Selective
`Coupler
`
`Wavelength -
`selective couplers
`distribute signals
`according to their
`wavelength.
`
`Wavelength-selective couplers distribute signals according to their wavelengths. Their main
`uses are to route W DM signals to their proper destinations and to separate wavelengths trans-
`mitted for different purposes through the same fiber, such as separating the light pumping an
`optical amplifier from the amplified signal. Wavelength-selective couplers are supposed to block
`other wavelengths from reaching the wrong destination. Chapter 15 covers them in detail.
`Bulk and Micro Optics
`In the world of fiber optics, bulk optics are conventional lenses, mirrors, and diffraction
`gratings, the sort of things you can hold in your hands. Bulk optics do not have to be large;
`they may be made quite small to match the dimensions of optical fibers and light sources.
`Such micro optics may be tiny, but they are still based on the same optical principles as
`larger bulk optics, so I will cover them together.
`
`MASIMO 2014
`PART 6
`Apple v. Masimo
`IPR2020-01526
`
`

`

`Beamsplitter transmits half
`the input light, reflects the other half.
`
`i O u tp u t 2
`I
`
`Couplers and Other Passive Components
`
`FIGURE
`0p tic 'ai
`
`14.7
`
`coupler: A
`beamsplitter
`divides a signal
`in half.
`
`Bulk optics were the basis of many early types of couplers, and they still work. Figure 14.7
`shows a simple example, the use of a device called a beamsplitter to split one input signal into
`two outputs. Like a one-way mirror, the beamsplitter transmits some light that hits it and
`reflects the rest. Collect the light from the two outputs in fibers, and you have a T coupler.
`Bulk optical couplers often include lenses that expand, collimate, or focus light. The
`simple coupler of Figure 14.7 generally works better if a lens expands the light emerging
`from a fiber and focuses it onto a large area of the beamsplitter. This function is collimation,
`and such optics are called collimators. Then additional lenses focus the output beams into
`output fibers. Standard lenses with curved surfaces may be used; generally they are tiny, to
`match the sizes of fibers.
`Alternatively, gradient-index (GRIN) lenses (or rods) may be used. These are rods or
`fibers in which the refractive index of the glass changes either with distance along the rod
`or with distance from the axis. The refractive-index gradient makes GRIN lenses focus light
`in a way functionally equivalent to ordinary lenses, but GRIN lenses are smaller and easier
`to adapt to fiber systems.
`Another application of bulk optics is the use of a diffraction grating to separate wave-
`lengths. A diffraction grating is an array of closely spaced parallel grooves, which act
`together to scatter light at an angle that depends on its wavelength, generating a rainbow
`of colors.
`Fused-Fiber Couplers
`Normally you can’t transfer light between fibers just by touching them together. The light-
`guiding cores are covered by claddings that keep light from leaking out. If you want to cou-
`ple light between fibers, you have to transfer it between the cores. That means you have to
`remove the claddings from one side of each fiber so the cores can touch. That is the basis
`of fused-fiber couplers, made by melting together fibers, usually with claddings removed
`partly or totally from one side, as shown in Figure 14.8. Often the fibers are twisted to-
`gether to improve light transfer. Fused-fiber couplers sometimes are called biconic couplers,
`which should not be confused with biconic connectors, an early type in very limited use
`today. They are the most common technology used to make couplers.
`Although Figure 14.8 shows the cores merged, they don’t have to merge completely in the
`middle zone as long as they come close. A phenomenon called evanescent-wave coupling can
`transfer light energy through a thin cladding or material with lower refractive index than the
`light-guiding zone. As you learned in Chapter 4, a small amount of the light energy guided
`
`Micro optics are
`tiny versions of
`conventional
`lenses and other
`optical
`components,
`shrunk in size to
`work with fibers.
`
`GRIN lenses are
`rods or fibers with
`refractive index
`graded so they
`act like ordinary
`
`Fused-fiber
`couplers are the
`most widely used
`type.
`
`

`

`m Chapter 14
`
`FIGURE 14.8
`A 2 X 2 fused-
`fib er coupler.
`
`Input L ig h t
`
`in the core of an optical fiber actually penetrates a short distance into the cladding. This light
`is called the evanescent wave, and you can see its effect in single-mode fibers, where it makes
`the mode-field diameter larger than the core of a step-index single-mode fiber. Evanescent-
`wave coupling is important in both fused-fiber and waveguide couplers.
`Fusing two fibers produces a 2 X 2 coupler with two inputs and two outputs. In prac-
`tice, these are often turned into 1 X 2 couplers by cutting one fiber end inside the case.
`This design is functionally directional, although it is bidirectional in the sense that light can
`go through it in either direction. If light enters the fiber end at upper left in Figure 14.8, the
`only way light can reach the fiber end at lower left is by reflection or scattering. Directivity
`is measured by comparing the input power, Pj, to the power reflected back through the
`other fiber end on the input side, P4:
`
`Directivity (dB) = — lO log f —VP \
`For a typical fused-fiber coupler, the directivity is 40 to 45 dB.
`The details of fused-fiber coupler operation depend on whether the fibers are multimode
`or single-mode. In multimode couplers, the higher-order modes leak into the cladding and
`into the core of the other fiber; the degree of coupling depends on the length of the
`coupling zone, and does not depend on wavelength. In single-mode fibers, light transfers
`between the two cores in a resonant interaction that varies with length. If all the light
`enters in one fiber, it gradually transfers completely to the other, then transfers back as it
`travels farther, shifting back and forth cyclically. The distance over which the cycling takes
`place depends on the coupler design and the wavelength, as Chapter 15 will describe in
`more detail.
`The fused-fiber coupler design can be extended to multiple fibers using the same basic
`principles. The important change is adding more fibers, so signals from all input fibers mix
`in the coupling zone and emerge out of all the output fibers. This approach can be used to
`make star couplers with many distinct inputs and outputs. Such multifiber fused couplers
`are bidirectional.
`
`

`

`Couplers and Other Passive Components
`
`Planar Waveguide Couplers
`As you learned earlier, optical fibers are not the only type of optical waveguides. Like fibers,
`planar waveguides confine light in a region of high refractive index surrounded by material
`with a lower refractive index. The planar waveguide may be a thin strip embedded in the
`surface of a flat substrate, as you saw in Figure 6.12, or it could be a strip deposited on
`the top of a flat substrate. Air and the substrate combine to serve the function of the
`cladding in a fiber.
`Waveguide patterns are written using the same techniques that write integrated elec-
`tronic circuits onto semiconductor wafers. They can branch or merge, making them the
`equivalent of fused-fiber couplers. A simple example is a Y-shaped structure that divides
`one input waveguide to form two outputs, as shown in Figure 14.9. (An actual split would
`be much smaller than shown.) If the outputs split at equal angles, as shown, the light
`divides equally between them. This approach can be extended to more outputs by adding
`Y couplers to divide the output signals. Alternatively, the input waveguide could be divided
`to give more outputs, but the power would not be evenly divided.
`Two closely spaced waveguides on the same substrate can also transfer light by
`evanescent-wave coupling through a thin layer of lower-index material, as in fused-fiber
`
`Two output waveguides
`each carry half the input light.
`
`Simple waveguide
`couplers are
`branched planar
`waveguides.
`
`FIGURE 14.9
`Planar waveguide
`splits in two, so
`light divides
`equally between
`arms o f the
`Y coupler.
`
`

`

`Chapter 14
`
`FIGURE 14.10
`Light transfer
`between two
`evanescently
`coupled
`wavev
`
`couplers. This type of waveguide device is called an evanescent-wave coupler and shown in
`Figure 14.10.
`An evanescent-wave coupler gradually transfers light between the two waveguides, along
`the region where the waveguides are closely spaced. In Figure 14.10, light enters in the top
`waveguide, and gradually transfers to the lower waveguides. This continues until all light
`shifts from the upper to the lower waveguide at a point called the transfer length, which
`depends on the optical characteristics of the waveguide. At this point, all the light is in
`the lower waveguide, and the process reverses, with the light starting to shift back from the
`lower waveguide to the upper one. Thus the distribution of light energy between the two
`waveguides oscillates back and forth between them with distance, as shown in the lower
`part of Figure 14.10. This oscillation stops at the end of the coupling region, determining
`the final distribution of light. The same process occurs in single-mode fused-fiber couplers.
`Designers select lengths and optical properties of the two parallel waveguides to give the
`desired distribution of light (e.g., 50/50 or 75/25). In practice some light is lost within
`the waveguide and in transferring between the two waveguides.
`Surface waveguides can be fabricated in complex patterns on a variety of materials.
`When they are made on the same substrate with many other devices, they are often called
`integrated optics, but then they usually contain active devices such as lasers, switches, or
`modulators. Chapter 16 covers such devices.
`Active Couplers
`Devices called active couplers also look to the user as if they split signals from fiber-optic
`transmission lines, but if you look closely, they work quite differently. An active coupler is
`essentially a dedicated repeater in which the signal from a receiver drives two (or more)
`
`Evanescent-wave
`couplers depend
`on light leakage
`between two
`closely spaced
`waveguides.
`
`An active coupler
`is a repeater with
`two or more
`outputs.
`
`

`

`Couplers and Other Passive Components
`
`Attenuators
`reduce light
`intensity uniformly
`across the
`spectrum.
`
`transmitters, which can generate optical and/or electronic output signals. This means that
`active couplers are not passive devices. However, they do function as couplers, so they are
`mentioned here.
`Active couplers are mostly used in local-area networks. For example, a fiber that runs to
`a network node may drive a receiver that generates two electronic outputs. One goes to an
`optical transmitter, which generates an optical signal to send through the next length of
`fiber in the network. The other is transmitted in electronic form to the terminal attached
`to that network node. Other configurations also are possible.
`
`Attenuators
`
`As you learned in Chapter 11, too much light can overload a receiver. Attenuators reduce
`light intensity, by transmitting only a fraction of the input light. They are needed when a
`transmitter could deliver too much light, such as when it is too close to the receiver.
`An attenuator is a type of optical filter, which should affect light of all wavelengths trans-
`mitted by the system equally. Attenuators are like sunglasses, which protect your eyes from
`being dazzled by bright lights. Fiber-optic attenuators generally absorb the extra light energy,
`which is too little to heat the attenuator noticeably. They should not reflect the unwanted
`light, because it could return through the input fiber to cause noise in a laser transmitter.
`Most attenuators have fixed values that are specified in decibels. For example, a 5-dB
`attenuator should reduce intensity of the output by 5 dB. Attenuators designed for general
`optics use may have attenuation specified as the percent of light transmitted (T) or as
`optical density. Optical density is defined as:
`
`Optical Density = logj,
`
`T,
`
`This should look familiar, because it’s close to the formula for attenuation in decibels, with-
`out the factor of 10. You can think of optical density as 0.1 times attenuation in dB, so a
`filter with optical density of 2 has a 20-dB loss.
`Variable attenuators also are available, but they usually are used in precision measurement
`instruments.
`If you’re familiar with electronics, it may be tempting to think of an attenuator as an
`optical counterpart of a resistor. This is not a good general analogy. An attenuator does
`limit the flow of light like a resistor limits current flow— but resistors also serve other
`circuit functions, such as providing voltage drops, and controlling circuit loads. The only
`job of attenuators in a fiber-optic system is to get rid of excess light.
`It’s important to distinguish between attenuators and other types of optical filters.
`Attenuators should have the same effect on all wavelengths used in the fiber system. That
`is, if the attenuator reduces intensity at one wavelength by 3 dB, it should do the same at
`all wavelengths. Other types of filters typically do not affect all wavelengths in the same
`way. For example, a filter might transmit light in the 1530 to 1565 erbium-amplifier band,
`but have 50 dB attenuation in the 980-nm pump band. In fiber-optic systems, the term
`filter is used for filters in which light transmission varies significantly with wavelength; they
`are used in wavelength-division multiplexing, and covered in Chapter 15.
`
`

`

`Chapter 14
`
`Optical isolators
`transmit light only
`in one direction.
`
`Optical Isolators
`
`Optical isolators are devices that transmit light only in one direction. They play an impor-
`tant role in fiber-optic systems by stopping back-reflection and scattered light from reach-
`ing sensitive components, particularly lasers. You can think of them as optical one-way
`streets with their own traffic cops or as the optical equivalent of an electronic rectifier
`(which conducts current only in one direction).
`The operation of optical isolators usually depends on materials called Faraday rotators,
`which rotate the plane of polarization of light. The rotation is always by the same angle
`when seen from the viewpoint of the light source. But for light transmitted from sources
`on opposite sides of the Faraday rotator, the angles are in different directions. With a bit of
`smart design, this feature can separate light going in different directions, so light going in
`the desired direction gets through, but light going the wrong way is stopped.
`Figure 14.11 shows a simple example. First consider light going from left to right, the
`desired direction. The input light is unpolarized, but it passes through a linear polarizer,
`which transmits only light polarized vertically. Then the Faraday rotator twists the plane of
`polarization 45° to the right. The light then encounters a second polarizer, which is
`oriented so it transmits only light with its plane of polarization oriented 45° to the right of
`
`FIGURE 14.11
`An optical isolator
`transmits light in
`only one direction.
`
`U n p o la rize d
`
`Light
`
`P o larizatio n D irection
`
`©
`
`0
`
`Light
`Transmitted
`
`P o la riz e r 2
`O rie n te d
`/
`
`0 U n p o ia rize d
`
`Light
`
`P o la riz e r 1
`O rie n te d
`
`F arada y
`R o ta to r
`
`
`^
`
`Light Blocked
`by Crossed
`Polarizer
`
`P o la riz e r 1
`O rie n te d
`
`t
`
`Faraday
`R o ta to r
`
`r ©
`
`P o la riz e r 2
`O rie n te d
`
`/
`
`

`

`Couplers and Other Passive Components
`
`vertical. That’s all of the light going from left to right, so the signal goes through unim-
`peded except for a 3-dB loss because the input polarizer blocked half of the unpolarized
`input signal.
`Now consider light going in the opposite direction, from right to left. The polarizer at
`right transmits only light polarized at 45° to the vertical, and the Faraday rotator turns the
`plane of polarization another 45° to the right. That makes the plane of polarization hori-
`zontal, so the light is blocked by the vertical polarizer at the left. A little stray light does
`leak through, but light headed in the wrong direction can be attenuated by 40 dB or more,
`protecting lasers from stray light that could induce noise.
`One drawback of this simple design is that it’s polarization sensitive. The input polar-
`izing fdter blocks half the input light that is not vertically polarized, causing 3-dB loss. More
`refined polarization-insensitive designs instead separate the input signal into two beams:
`one vertically polarized and the other horizontally polarized. One approach uses transpar-
`ent crystals in which light travels at different speeds depending on its polarization. Prisms
`of such strongly birefringent materials separate vertically and horizontally polarized light so
`they follow different paths; these prisms are sometimes called beam displacers. They can be
`combined with focusing elements and Faraday rotators so that light traveling in one direc-
`tion in focused from the input fiber into the output fiber, while light traveling in the op-
`posite direction is defocused to prevent it from going into the input fiber. Although this
`design is a bit more complicated, it avoids 3 dB of loss.
`
`Optical Circulators
`
`The optical circulator is a cousin of the optical isolator in both its function and design. Its
`function is to serve as a one-way street for light passing through a series of ports, so light
`that enters in port 1 must go to port 2, and any light entering at port 2 goes to port 3, and
`so on. Like the optical isolator, it uses polarization to do its job.
`One way to make an optical circulator is with a pair of optical isolators. One can be
`inserted between port 1 and port 2, blocking light going backwards from port 2. A second
`can be inserted between ports 2 and 3, blocking light trying to go back from port 3 to
`port 2. However, these designs lose the blocked light.
`Figure 14.12 shows a more elegant and efficient optical circulator, which is assembled
`from three types of components. Faraday rotators and birefringent beam displacers also are
`used in the optical isolators described above. Recall that the displacers separate light of dif-
`ferent polarization, while the Faraday rotators always rotate the polarization by 45° from
`the viewpoint of a photon passing through them. If light goes back and forth through the
`same Faraday rotator, its polarization changes a total of 90°.
`Optical circulators also include devices called waveplates, which also rotate the polariza-
`tion by 45°, but in a different way. If light passes through a waveplate one way, it’s rotated
`45° to the right; if it goes through the other way, the light rotates 45° to the left. That
`means the net change on a round trip is 0°, so light that makes a round trip through a
`waveplate emerges with the same polarization it started with.
`Now go back to Figure 14.12 and trace the paths of the two polarizations from port 1 to
`port 2. The vertically polarized input is deflected up, then rotated +45° by the waveplate
`
`Polarization-
`insensitive optical
`isolators do not
`have 3-dB internal
`loss.
`
`An optical
`circulator sends
`light in one
`direction through
`a series of ports.
`
`

`

`go backwards.
`
`to
`
`it
`
`Optical circulator directs light from part 1—2, 2—3, and 3~4, without allowing
`FIGURE 14.12
`
`when
`Faraday rotators shift polarization +45
`
`light goes
`
`either direction.
`
`in
`
`

`

`Couplers and Other Passive Components
`
`and another +45° by the Faraday rotator, a total of 90°, making it horizontally polarized
`so it passes straight through the second beam displacer. Then it is rotated —45° by the
`waveplate and + 45° by the Faraday rotator, a net change of zero, so it remains horizontally
`polarized through the third displacer and out port 2. The horizontally polarized input, in
`contrast, goes straight through the first displacer, and is rotated —45° by the waveplate and
`+45° by the Faraday rotator, a net of zero. It then goes straight through the second
`displacer. At the second rotator, it is rotated + 45° by the waveplate and + 45° by the
`Faraday rotator, a total of 90°, which makes it vertically polarized so the displacer bends it
`upwards— and aims it out port 2, where it is supposed to go.
`The tricky part is following the path from port 2 to port 3 (right to left in Figure 14.12).
`The beam displacer splits the two polarizations so they pass through the second rotator. This
`time the top (horizontally polarized) beam is rotated + 45° by the waveplate and + 45° by
`the Faraday rotator, changing its polarization to vertical. The middle beam displacer bends
`the beam upward, and it emerges from the upper side of the middle beam displacer on its
`way to port 3. In this case, it is rotated —45° by the waveplate and +45° by the Faraday
`rotator, so it remains vertically polarized, and is deflected downward to port 3. The bottom
`(vertically polarized) light from port 2 is deflected downward, where the waveplate rotates
`it —45° and the Faraday rotator rotates it +45°, leaving it vertically polarized as it enters
`the middle beam displacer. It’s bent upward, and arrives at the lower position on its way to
`port 3. Here the polarizer rotates it +45° and the Faraday rotator rotates it +45°, changing
`it to horizontally polarized light that goes straight through the beam displacer to port 3.
`Each level of the optical circulator is identical, so the steps can be repeated as long as you
`want. The crucial tricks are separating the polarizations, routing them through different
`components, and taking advantage of the different ways Faraday rotators and waveplates
`rotate polarization.
`
`What Have You Learned?
`
`1. Couplers connect three or more fibers or ports. Dividing an optical signal
`among two or more ports reduces its strength because it divides the photons
`in the signal.
`2. Several different types of couplers are used; their design depends on the
`application.
`3. Direction is important in couplers. Most couplers are directional in the sense
`they transmit signals from one or more inputs to one or more outputs, with
`little light going from one input to another. Most designs also are bidirectional,
`in the sense that the input and output ports could be reversed to change light
`coupling.
`4. T or Y couplers, or taps, are three-port devices. Tree, or l-to-«, couplers divide
`one input among n output ports. Star couplers have multiple inputs and
`outputs. Outputs are usually distinct from inputs.
`5. Wavelength sensitivity is important in couplers. It is desirable for wavelength-
`division multiplexing, but not for most other applications.
`
`

`

`6. Many couplers are made from bulk or micro optics, such as beamsplitters.
`7. GRIN lenses are rods or fibers with refractive index graded so they refract light
`like ordinary lenses.
`8. Fused-fiber couplers transfer light between the cores of two fibers melted
`together. Single-mode fused-fiber couplers work differently than multimode
`versions. Multifiber fused-fiber couplers are possible.
`9. There are two types of planar waveguide couplers. Some simply divide light
`between two waveguides branching in a Y from a single-input guide. Others rely
`on evanescent-wave coupling to transfer light between two parallel waveguides.
`Evanescent-wave couplers are sensitive to wavelength.
`10. Active couplers are repeaters with two or more outputs.
`11. Attenuators block light uniformly across a range of wavelengths to reduce signal
`strength at the receiver.
`12. Optical isolators transmit light in only one direction. They rely on polarizing
`optics and Faraday rotators.
`13. Optical circulators route light through a series of ports, feeding output from
`one port to the next, and taking input from the second port and routing it
`to a third. They rely on birefringent crystals, Faraday rotators, and other
`polarization rotators.
`
`What's Next?
`
`Chapter 15 covers wavelength-selective optics used for wavelength-division multiplexing.
`
`Further Reading
`
`Morris Hoover, “New coupler applications in today’s telephony networks,” Lightwave,
`Vol. 17, March 2000.
`Luc B. Jeunhomme, Single-Mode Fiber Optics: Principles and Applications (Marcel Dekker,
`1990). See Chapter 6, “Passive Components.”
`
`Questions to Think About
`
`1. Suppose your input signal is —10 dBm and your receivers require a signal of
`at least 0 dBp.. You want to distribute signals to as many terminals as possible.
`If there is 3 dB of fiber loss between you and each receiver, how many terminals
`can you deliver signals to? How much coupler loss does this correspond to on
`each channel? Assume you can buy a star coupler with as many ports as you
`want, which has no excess loss.
`
`

`

`Couplers and Other Passive Components
`
`2. Suppose that all star couplers available for the system described in Question 1
`have excess loss of 3 dB. How many terminals can you reach with these
`couplers, and what is the total loss per channel?
`3. An alternative design is to cascade a series of 3-dB T couplers. The first splits
`the signal in half, then each output has its own 3-dB coupler, dividing that
`output in half, yielding one-quarter of the original output. Adding more layers
`further divides the signal. Suppose you can get as many 3-dB couplers as you
`want and each one has no excess loss. How many terminals can you divide
`signals among in Question 1?
`4. A local-area network includes 90/10 couplers, which split 10% of the input
`signal and deliver it to a local terminal. Suppose you have 10 of them in series
`and the input power is —10 dBm. What is the power delivered by the last
`coupler out each of its ports?
`5. If your receiver requires 1 pW of power, how many more 90/10 couplers could
`you have in series before the 10% side does not deliver enough power for
`reliable operation? Assume the same —10 dBm input as in Question 4.
`6. An optical amplifier delivers 0.5 mW/channel on each of 32 channels. You want
`to monitor its performance by diverting a small portion of its output to an
`optical performance monitor that requires 1 dBp input per optical channel.
`What fraction of the output power do you need to divert to the performance
`monitor?
`7. Neglecting excess internal losses, what is the difference in attenuation between
`the following two optical circulators? The first uses the simple optical isolators of
`Figure 14.11— one oriented from port 1 to 2 and the second from port 2 to 3,
`with a 50/50 T coupler splitting input signals from port 2 between two routes.
`The other is the more complex optical circulator of Figure 14.12.
`
`Chapter Quiz
`
`1 . You have a coupler that divides an input signal equally among 16 outputs. It
`has no excess loss. If the input signal is —10 dBm, what is the output at any
`one port?
`a. —12 dBm
`b. —20 dBm
`c. —22 dBm
`d. —26 dBm
`e. —30 dBm
`2. A 1 X 20 coupler has output signals of —30 dBm at every port if the input
`signal is —10 dBm. What is its excess loss?
`a. 0 dB
`b. 1 dB
`
`

`

`m Chapter 14
`
`c. 2 dB
`d. 4.2 dB
`e. 7 dB
`3. A coupler splits an input signal between two ports with a 90/10 ratio. If the
`input signal is —20 dBm and the coupler has no excess loss, what is the output
`at the port receiving the smaller signal?
`a. —21 dBm
`b. —29 dBm
`c. —30 dBm
`d. —31 dBm
`e. —110 dBm
`4. What type of coupler could distribute identical signals to 20 different terminals?
`a. T coupler
`b. tree coupler
`c. star coupler
`d. M X N coupler
`e. wavelength-selective coupler
`5 . What type of coupler divides one input signal between two output channels?
`a. T coupler
`b. tree coupler
`c. star coupler
`d. M X N coupler
`e. wavelength-selective coupler
`6. You find a coupler with four ports and no label on it. You measure attenuation
`from port 1 to the other three ports. The values are: —40 dB
`to port 2, 3 dB
`to port 3, 3 dB to port 4. What type of coupler do you have?
`a. star coupler with three unequal outputs
`b. tree coupler with three unequal outputs
`c. a directional 2-by-2 coupler with two inputs and two outputs
`d. a nondirectional T coupler
`e. a broken coupler
`7 . A Y coupler that equally divides light between two outputs has a 3-dB loss on
`each channel. What is the right explanation?
`a. The 3-dB figure is excess loss.
`b. Half the photons that enter the coupler go out each output, corresponding
`to a 3-dB loss on each channel.
`c. The coupler polarizes the light going out each port, causing 3-dB loss.
`
`

`

`Couplers and Other Passive Components
`
`d. Every coupler has at least 3-dB loss no matter how it divides the input
`light.
`e. The optics are dirty, causing loss of half the input light.
`8. Evanescent waves cause light energy to transfer between channels in what type
`of coupler?
`a. planar-waveguide coupler
`b. single-mode fused-fiber coupler
`c. bulk optical coupler
`d. multimode fused-fiber coupler
`e. a and b
`f. b and d
`9. An attenuator
`a. is a filter that blocks one wavelength but transmits others.
`b. polarizes input light, causing loss of the other polarization.
`c. reduces light intensity evenly across a range of wavelengths.
`d. selectively blocks photons produced by spontaneous emission.
`
`10. How many polarizers does a simple polarization-sensitive optical isolator use to
`
`block transmission of light in the wrong direction?
`a. none
`b. 1
`c. 2
`d. 3
`e. 4
`
`

`

`

`

`Wavelength-Division
`Multiplexing Optics
`
`15
`
`About This Chapter
`
`W avelength-division m ultiplexing (W D M ) multiplies transmission capacity by
`allowing a single optical fiber to carry separate signals at multiple wavelengths, but that
`benefit comes at a cost in complexity. Additional optical components are needed to
`combine and separate optical channels at closely spaced wavelengths, and to process
`the signals as they are transmitted. Specific requirements vary with system design, and
`several technologies are in use.
`This chapter first outlines the basic requirements, then explains how these require-
`ments relate to the operation of W DM systems. Then it covers specific technologies,
`their operation, and their capabilities. The chapter introduces the important concepts
`of wavelength-selective optical filtering, and describes the types of devices that can
`perform it. You will recognize a little overlap with the couplers and attenuators
`described in Chapter 14, which can be adapted for wavelength selectivity. The current
`chapter concentrates on passive technologies like those in Chapter 14, but does men-
`tion some active technologies that serve similar purposes in “dynamic” versions of pas-
`sive components. Chapter 16 will cover switching, modulation, and other “active”
`technologies.
`
`WDM Requirements
`
`At first glance, wavelength-division multiplexing looks simple; it’s easy to shine light
`of many different colors into an optical fiber. The hard part seems to be the dem ulti-
`plexing, when you have to separate the output signals by their wavelength. Unfortunately,
`reality is not quite that simple. Demultiplexing is a more difficult task, but care does
`
`Demultiplexing is
`harder than
`multiplexing.
`
`

`

`Chapter 15
`
`Many advanced
`concepts were
`never
`implemented
`outside the lab.
`
`need to be taken in multiplexing, to control background noise that might otherwise be
`introduced into the fiber. Other types of processing also require some type of wavelength
`selection.
`There are a number of basic requirements for W DM , which often differ in degree:
`
`9 Wavelength multiplexing or combination, which must transmit the desired signal
`while blocking noise at other wavelengths, which might interfere with other optical
`channels.
`9 Wavelength demultiplexing, to isolate individual optical channels for detection or
`further processing.
`• Add-drop multiplexing, often called “optical add-drop multiplexing,” which separates
`and combines only a few wavelengths from many wavelengths transmitted by a system.
`9 Wavelength separation, typically to isolate a signal wavelength from a pump
`wavelength, as in an optical amplifier.
`• Wavelength-selective processing, such as attenuating signals at some wavelengths to
`balance the strength of all optical channels in a system. This may be static or dynamic.
`9 Wavelength conversion, to change a signal from one wavelength to another. (See
`Chapter 12.)
`9 Wavelength switching, to redirect signals at one or more wavelengths while
`transmitting others unchanged. (See Chapter 16.)
`
`The specific requirements depend on system properties such as the number of channels,
`the spacing between th