levels complicates processing, so manufacturers prefer to make as much as possible of the
`fiber from pure silica.) Both designs are used for single-mode fiber. An alternative used for
`multimode step-index fiber is a pure silica core clad with a lower index plastic.
`As you learned in Chapter 4, the refractive-index profiles of dispersion-shifted fibers are
`considerably more complex, to provide the extra waveguide dispersion needed to shift the zero-
`dispersion point to longer wavelengths. So are the profiles of graded-index multimode fibers.
`The same dopants are used in these more complex fibers as in simple step-index fibers.
`
`Silica Fiber Manufacture
`The trickiest stage in the manufacture of fused-silica optical fibers is making the preform
`from which the fibers are drawn. Several processes have been developed; they share some
`common features but have important differences.
`The crucial common feature is the formation of fluffy fused-silica soot by reacting SiCl4
`(and GeCl4, when it is used as a dopant) with oxygen to generate S i02 (and G e 02 if the
`silica is doped). The crucial variations are in how the soot is deposited and melted into the
`final preform.
`One approach is to deposit the soot on the inside wall of a fused-silica tube, as shown
`in Figure 6.2. Typically, the tube serves as the outer cladding, onto which an inner cladding
`layer and the core material are deposited. Variations on the approach are called inside va-
`por deposition, modified chemical vapor deposition, plasma chemical vapor deposition,
`and plasma-enhanced chemical vapor deposition. The major differences center on how the
`reaction zone is heated.
`The chemicals react to deposit a fine glass soot, and the waste gas is pumped out to an
`exhaust. To spread soot along the length of the tube, the reaction zone is moved along the
`tube. Heating melts the soot, and it condenses into a glass.
`
`Chapter 6
`
`Fused-silica
`preforms can be
`made by
`depositing glass
`soot inside a tube
`of fused silica,
`which becomes the
`cladding.
`
`FIGURE 6.2
`Soot deposition
`inside a fused-silica
`tube.
`
`Reactant
`gases
`enter
`
`MASIMO 2014
`PART 3
`Apple v. Masimo
`IPR2020-01526
`
`

`

`Fiber Materials, Structure, and Manufacture
`
`The process can be repeated over and over to deposit many fine layers of slightly differ-
`ent composition, which are needed to grade the refractive index from core to cladding in
`graded-index fibers. The doping of input gases is changed slightly for each deposition step,
`producing a series of layers with small steps in the refractive index. Step-index profiles are
`easier to fabricate, because the whole core has the same doping. A final heating step collapses
`the tube into a preform.
`Another important approach is the outside vapor-deposition process, which deposits
`soot on the outside of a rotating ceramic rod, as shown in Figure 6.3. The ceramic rod
`does not become part of the fiber; it is merely a substrate. The glass soot that will be-
`come the fiber core is deposited first, then the cladding layers are deposited on top of
`it. The ceramic core has a different thermal expansion coefficient than the glass layers
`deposited on top of it, so it slips out easily when the finished assembly is cooled before
`the glass is sintered to form a preform. The central hole is closed either in making the
`preform or drawing the fiber.
`The third main approach is vapor axial deposition, shown in Figure 6.4. In this case, a
`rod of pure silica serves as a “seed” for deposition of glass soot on its end rather than on its
`surface. The initial soot deposited becomes the core. Then more soot is deposited radially
`outward to become the cladding, and new core material is grown on the end of the pre-
`form. Vapor axial deposition does not involve a central hole.
`All three processes yield long, glass cylinders or rods called preforms. They are essentially
`fat versions of fibers, composed of a high-index fiber covered with a lower-index cladding.
`They have the same refractive-index profile as the final fiber.
`
`Preforms also can
`be made by
`depositing soot on
`the outside or on
`the end of a rod.
`
`C la d d in g
`M ate rial
`
`S o o t D e p o sits
`B u ilt U p on R od
`
`FIGURE 6.3
`Outside vapor
`deposition to
`a preform.
`
`

`

`Chapter 6
`
`FIGURE 6.4
`Vapor axial
`deposition to make
`a preform.
`
`•
`Fibers are drawn
`from the bottoms
`"
`'
`
`C la d d in g
`
`Drawing Fibers
`Optical fibers are drawn from preforms by heating the glass until it softens, then pulling
`hot g[ass away from the preform. This is done in a machine called a drawing tower.
`Drawing towers typically are a couple of stories high and loom above everything else on
`the floor of a fiber factory. The preform is mounted vertically at the top, with its bottom
`end in a furnace that heats the glass to its softening point. Initially a blob of hot glass is
`pulled from the bottom, stretching out to become the start of the fiber. (This starting
`segment of the fiber normally is discarded.)
`The hot glass thread emerging from the furnace solidifies almost instantaneously as it
`cools in open air. As shown in Figure 6.5, the bare glass fiber passes through a device that
`monitors its diameter, then is covered with a protective plastic coating. The end is attached
`to a rotating drum or spool, which turns steadily, pulling hot glass fiber from the bottom
`of the preform and winding plastic-coated fiber onto the drum or spool. The actual length
`of the draw zone is longer than shown in the figure, to allow the fiber to cool and the
`plastic coating to cure properly.
`
`FIGURE 6.5
`Drawing glass
`fibers from
`preforms. (Courtesy
`o f Corning Inc.)
`
`P reform
`
`F urn a ce
`
`D ia m e te r M o n ito r
`
`C o a te r
`
`

`

`Fiber Materials, Structure, and Manufacture
`
`Typically the fiber is drawn at speeds well over a meter per second. A single, large pre-
`form can yield over 20 kilometers of fiber; smaller preforms yield a few kilometers. After
`the fiber is drawn, it is proof tested and wound onto final reels for shipping.
`
`Types of Silica Fibers
`Silica is the standard material used for most communication fibers. Except for a few
`special cases, both core and cladding are made of silica, differentiated by different doping
`levels. Typically the cores contain dopants that increase refractive index above that of pure
`silica, while the cladding is either pure silica or doped with index-depressing materials such
`as fluorine, as shown in Figure 6.1 and discussed earlier.
`This basic design is used for the single-mode and graded-index multimode fibers used
`for communications. Figure 6.6 shows typical attenuation curves for a high-quality
`nonzero dispersion-shifted (ITU G.655) single-mode fiber and a graded-index multimode
`
`Nonzero
`Dispersion-Shifted
`Single-Mode
`Fiber
`
`All-silica fibers
`are used for
`communications.
`
`FIGURE 6.6
`Attenuation o f
`non-zero
`dispersion-shifted
`ITU G .655 fiber
`(left) and graded-
`index multimode
`fiber (below).
`(Courtesy of
`Corning Inc.)
`
`Wavelength (nm)
`
`e C
`
`D
`
`

`

`fiber. Attenuation for step-index single-mode fiber is slightly lower than for the ITU G.655
`fiber, but the difference is not significant and would not show on this scale. Low-water
`single-mode fibers lack the absorption peak near 1.38 |xm. A quick comparison shows
`higher loss for the graded-index fiber, but this is not significant for the short-distance
`applications in which they are used.
`Different designs are used for step-index multimode silica fibers. Typically these fibers have
`a pure silica core, which is clad either with silica doped to reduce its refractive index or with
`a plastic having lower refractive index than silica. This approach simplifies the manufacturing
`process and avoids the need for dopants in cores that are 100 (Jtm or larger. Typically
`the claddings are thin— 20 |xm on a fiber with a 100-p.m core and a 140-p.m cladding
`diameter— with a protective plastic coating 50 to 100 |xm thick applied over the cladding.
`Large-core step-index silica fibers come in a variety of configurations, and typically are
`used for data transmission, laser beam delivery, or illumination. The oldest type is plastic-
`clad silica (PCS), in which the cladding is a silicone plastic that is fairly easy to strip from
`the silica core. Easily removed cladding is good for some applications, but bad for others.
`H ard-clad silica (HCS) fibers have a tougher plastic cladding, which makes the fibers more
`durable. Silica-clad fibers can handle higher powers than either type of plastic-clad fiber, an
`important consideration for fibers delivering high laser powers.
`Figure 6.7 shows attenuation for a selection of large-core silica fibers. The values vary
`depending on the type of cladding and the amount of moisture in the silica core. Fibers
`made in a low-water environment contain little OH and are more transparent in the
`near-infrared, while fibers that contain more OH are more transparent in the ultravio-
`let. (The fibers in Figure 6.7 all have low OH levels, and the plot does not show ultra-
`violet attenuation.)
`Typical core diameters of large-core step-index fibers range from 100 to 1000 |xm. The
`smaller fibers may be used for short-distance communication, but the larger fibers are used
`mostly for illumination. The largest-core fibers can carry considerable power, making them use-
`ful for laser-beam delivery, but they are significandy stiffer. For example, the rated continuous
`
`Chapter 6
`
`Large-core silica
`fibers are used
`for data
`transmission, laser
`beam delivery, or
`illumination.
`
`FIGURE 6.7
`Spectral
`attenuation o f
`various large-core
`silica fibers.
`(Courtesy o f 3M
`Specialty Optical
`Fibers.)
`
`Wavelength (nm)
`
`

`

`Fiber Materials, Structure, and Manufacture
`
`Table 6.1 Characteristics of large-core step-index silica fibers. Bandwidths of fibers with cores over
`200 pm generally are unrated because they are very rarely used in communications
`
`Fiber Type
`
`Silica clad
`Hard clad
`Plastic-clad, low OH
`Plastic-clad, high OH
`Silica clad
`Hard clad
`Silica clad
`Plastic-clad, low OH
`
`Core/Clad
`Diameter (pm)
`100/120
`125/140
`200/380
`200/380
`400/500
`550/600
`1000/1250
`1000/1400
`
`Attenuation at
`0.82 pm
`
`5 dB/km
`20 dB/km
`6 dB/km
`12 dB/km
`12 dB/km
`12 dB/km
`14 dB/km
`8 dB/km
`
`Bandwidth at
`0.82 pun
`
`N A
`0.22
`20 MHz-km
`0.48
`20 MHz-km
`20 MHz-km 0.40
`20 MHz-km
`0.40
`0.16
`—
`0.22
`—
`0.16
`—
`0.40
`
`—
`
`power capacity of one family of silica-clad fibers increases from 0.2 kW for 200-pm core
`fibers to 1.5 kW for 550-pm core fibers, and the rated minimum bend radius increases by
`a factor of 2.5. Table 6.1 summarizes important optical characteristics of selected fibers.
`Although most large-core silica fibers have step-index profiles, some are made with a
`graded-index core, surrounded by a thin silica cladding and typically a plastic coating and
`outer buffer layer. Their main application is in delivering high-power laser beams.
`
`Plastic Fibers
`
`Plastic optical fibers have long been a poor relation of glass. Traditionally regarded as inex-
`pensive, flexible, lightweight, and easy to handle, plastic seems to offer some important
`attractions. These potential advantages can be hard to realize in practice, since silica fibers
`are reasonably priced and flexible in the small diameters used for telecommunications
`applications. However, the biggest problem of plastic fibers has been attenuation levels many
`times that of glass, making commercial types impractical for distances beyond 100 meters.
`Years of research have reduced plastic loss considerably, but it still remains far higher
`than that of glass. The best laboratory plastic fibers have minimum loss around 50 dB/km.
`At the 650-nm wavelength preferred for communications using red LEDs, commercial
`plastic fibers have minimum attenuation as low as 150 dB/km. Unlike glass fibers, the loss
`of plastic fibers is somewhat lower at shorter wavelengths and much higher in the infrared,
`as shown in Figure 6.8.
`For this reason, plastic optical fibers have found only limited applications. One is in flex-
`ible bundles for image transmission and illumination, where the light doesn’t have to travel
`far and the flexibility and lower cost of plastic are important. Another application is in
`short data links, particularly within automobiles, where the ease of handling plastics is a
`major advantage and the required distances and data rates are small.
`
`•
`Multimode fibers
`made entirely of
`|
`^
`sj|ica fibers
`
`

`

`A Chapter 6
`
`FIGURE 6.8
`Attenuation versus
`wavelength fo r one
`commercial
`PMMA step-index
`fiber. (Courtesy o f
`Toray Industries
`Ltd.)
`
`4 0 0
`
`5 0 0
`
`6 0 0
`
`7 0 0
`
`8 0 0
`
`9 00
`
`W a v e le n g th (nm )
`
`Another important concern with plastic optical fibers is long-term degradation at high
`operating temperature. Typically plastic fibers cannot be used above 85°C (185°F). This
`may sound safely above normal room temperature, but it leaves little margin in many
`environments. The engine compartments of cars, for example, can get considerably hotter.
`Newer plastics can withstand temperatures to 125°C (257°F), but their optical properties
`are not as good.
`Plastic fibers are made using the same principles as glass fibers. A low-index core surrounds
`a higher index cladding. The refractive-index difference can be large, so many plastic fibers
`have large numerical apertures. Commercial plastic fibers are multimode types with large
`cores. Most are step-index but a few are graded-index. There is little interest in single-mode
`plastic fibers because the material’s high loss makes long-distance transmission impossible.
`Step- and Graded-index Plastic Fibers
`Standard step-index plastic fibers have a core of polymethyl methacrylate (PMMA) and a
`cladding of a lower index polymer, which usually contains fluorine. The differences in
`refractive index typically are larger than in silica or glass fibers, leading to a large numerical
`aperture. For example, one commercial plastic fiber designed for short-distance communi-
`cation has a PMMA core with refractive index of 1.492 and a cladding with index of 1.402,
`giving an NA of 0.47.
`Plastic fibers typically have core diameters from about 85 |xm to more than 3 mm
`(3000 pm). You can find larger light-guiding rods of flexible plastic, which sometimes are
`called fibers, but it’s hard to think of something as thick as a pencil as a “fiber.” The small-
`est fibers typically are used only in bundles, but larger fibers are used individually. Typically
`the claddings are thin, only a small fraction of overall fiber diameter. Large-core plastic
`fibers cannot carry optical powers as high as those carried by large-core silica fibers, but
`
`Traditional plastic
`fibers are made
`of PMMA, with
`large step-index
`cores. They are
`used in bundles
`and for short
`data links.
`
`

`

`Fiber Materials, Structure, and Manufacture
`
`they are more flexible and less expensive. Plastic fibers with diameters up to around a
`millimeter are used for some short-distance communication because they are much easier
`to handle than glass fibers. For example, technicians can splice and connect plastic fibers
`on site with minimal equipment, instead of the expensive precision equipment required for
`glass fibers. Figure 6.8 plots attenuation of one PMMA fiber against wavelength, on a scale
`of decibels per meter. The minimum loss, near 500 nm, is equivalent to 70 dB/km, but for
`communications transmission normally is at the 650-nm wavelength of inexpensive red
`LEDs. The step-index profile also limits bandwidth, so signals normally are limited to trav-
`eling within a building or between adjacent structures.
`Graded-index plastic fibers are a recent development because it had been difficult to pro-
`duce good graded-index profiles in plastic. Typically a preform is heat-treated to make
`high-index materials diffuse from a fluorinated plastic core and raise the index of lower
`index plastics in the cladding. This plastic preform is then drawn into fiber, much like glass
`fibers but at much lower temperatures.
`As in silica fibers, the advantage of a graded-index profile is broader transmission band-
`width than step-index fibers. Graded-index plastic fibers with core diameters of 50 to 200 fxm
`can transmit 2.5 Gbit/s over distances of 200 to 500 meters, making them attractive for
`high-speed local area networks. The fluorinated plastic fibers have attenuation around
`60 dB/km over a broad range from about 800 to 1340 nm, allowing operation at the 850
`and 1300 nm windows. However, attenuation through the entire range is tens of decibels
`per kilometer, limiting transmission to much shorter distances than with silica fibers, and
`the fibers are relatively expensive.
`
`Issues in Developing Plastic Fibers
`High attenuation has been a stubborn problem in plastic optical fibers. Bonds between
`atoms found in plastics— notably carbon-hydrogen and carbon-oxygen bonds— absorb
`light at visible and near-infrared wavelengths, even in plastics that look transparent to the
`eye. Fused silica is much more transparent because these bonds are not present in it.
`Efforts to reduce loss have concentrated on changing the chemical composition of the
`plastics. One step is to replace normal hydrogen with the heavier (stable) isotope deu-
`terium, which shifts the absorption peaks of carbon-hydrogen bonds to longer wave-
`lengths. Another step is to use fluorinated plastics instead of standard hydrocarbon plastics,
`because carbon-fluorine bonds have lower attenuation. Figure 6.9 compares attenuation
`curves for fibers made of standard hydrogen-based PMMA, deuterated PMMA, and one
`type of fluorinated plastic between 550 and 850 nm. Loss of the fluorinated plastic remains
`relatively low at wavelengths to 1.3 |xm. However, changing composition raises other issues,
`including the need for more expensive materials.
`Another important issue, mentioned earlier, is the durability of plastics, both over time
`and under extreme conditions. Plastic fibers generally are more flexible than glass, and are
`easier to cut and install. They generally work fine in a controlled environment such as an
`office. However, plastics are not as resistant to heat and sunlight as glass. Temperature lim-
`itations have proved a particular problem in areas such as the automotive industry, where
`equipment installed in the engine compartment must withstand frequent temperature
`cycling and extremes.
`
`Graded-index
`profiles can be
`made in plaslic
`fibers, increasing
`bandwidth.
`
`Attenuation is a
`key issue in
`plastic fibers.
`
`

`

`Chapter 6
`
`FIGURE 6.9
`Attenuation spectra
`o f graded-index
`plastic fibers made
`with regular
`PMMA, a
`fluorinated plastic,
`and deuterated
`PMMA. (Courtesy
`o f Takaaki
`Ishigure.)
`
`W a v e le n g th (nm )
`
`Exotic Fibers and Light Guides
`
`From time to time, you may encounter some unusual optical fibers, light guides, or optical
`waveguides based on novel materials. They presently play little role in communications, but
`have other applications.
`Liquid-Core Fibers (or Light Guides)
`In the very early days of fiber-optic communications, developers desperately seeking low-loss
`materials turned to liquids. They filled thin silica tubes with tetrachloroethylene, a dry-
`cleaning fluid that is extremely clear and has a refractive index higher than fused silica. The
`index difference was adequate to guide light, and developers eventually reduced loss to several
`decibels per kilometer, very good for the time, and better than current plastics.
`Liquid-core fibers were far from a practical communications technology. Filling the tiny
`capillary tubes took a very long time, but the real problem was thermal expansion. The liq-
`uid expanded at a different rate than the tube that held it, so the liquid-core fiber acted like
`a thermometer, with liquid rising and falling with temperature. If you weren’t careful, the
`liquid could squirt out the ends.
`Now larger diameter liquid-core light guides are finding a new life transmitting visible
`s^ ort distances for illumination. Single liquid-core light guides 2 to 10 mm thick are
`an aIternafive t0 standard illuminating bundles. Using suitable fluids, they have lower
`attenuation than standard bundle fibers, particularly at green and blue wavelengths. The
`liquid is housed in a plastic tube rather than glass, so the liquid waveguide is more flexible
`than a large solid fiber. Because lengths are modest— at most 20 m and typically only a few
`meters— thermal expansion poses little problem.
`Midinfrared Fibers
`The extremely low scattering losses expected at wavelengths longer than 1.55 |Jtm
`prompted interest in those wavelengths for long-distance communications in the 1980s. The
`
`. j .
`^ |.
`plastic or glass
`tube can act like
`a fiber core if its
`refractive index is
`higher than the
`tube.
`
`

`

`Fiber Materials, Structure, and Manufacture
`
`Fibers made of
`nonsilica glasses
`transmit infrared
`wavelengths,
`which silica
`absorbs.
`
`absorption of silica rises rapidly at longer wavelengths, so developers looked to other mate-
`rials that are transparent in that region. Theorists hoped that extremely low-loss glass fibers
`could be made from some of those materials. (Recall that glass is a disordered material, not
`necessarily made from silica.) If other losses could be avoided, the floor set by scattering
`loss suggested attenuation might be as low as 0.001 dB/km. Such incredibly low loss would
`allow extremely long transmission distances without amplifiers or repeaters.
`Unfortunately, very low-loss infrared fibers have proven exceedingly difficult to make.
`Purification of the materials is difficult. The raw materials are far more expensive than those
`for silica fibers. (Despite occasional jokes about desert nations concerning the market on raw
`materials, silica fibers can’t be made from raw sand, as can many glass products.) Infrared
`materials are harder to pull into fibers because they are much less viscous than silicate glass
`when molten. The fibers that can be produced are weaker mechanically than silica and
`suffer other environmental limitations. In short, infrared fibers have been a bust for ultra-
`long-distance communications.
`On the other hand, fibers made from nonsilicate glasses can transmit infrared wave-
`lengths that do not pass through silica fibers. This makes them useful in specialized appli-
`cations such as infrared instrumentation, although their losses are much larger than the
`minimum loss of silica fibers at shorter wavelengths.
`Fluorozirconate fibers transmit light between 0.4 and 5 |xm. Often simply called fluo-
`ride fibers, they are made primarily of zirconium fluoride (ZrF,^ and barium fluoride
`(BaF2), with some other components added to form a glass compound. The lowest losses
`for commercial fluorozirconate fibers are about 25 dB/km at 2.6 pm, but loss as low as
`about 1 dB/km has been reported in the laboratory. A typical transmission curve is shown
`in Figure 6.10, along with other infrared fibers. Fluoride fibers are vulnerable to excess
`humidity, so they should be stored and used in low-humidity environments. Fluoride
`
`0
`
`2
`
`4
`
`8
`6
`W a v e le n g th (m ic ro n s )
`
`10
`
`12
`
`14
`
`16
`
`

`

`Chapter 6
`
`Hollow
`waveguides can
`transmit longer
`infrared
`wavelengths.
`
`Microstructured
`photonic materials
`confine light.
`
`fibers are used in some fiber amplifiers because of desirable optical characteristics.
`However, they have a refractive index higher than 2, so they have high reflection from
`their ends.
`Fibers made from silver halide compounds (AgBrCl in Figure 6.10) have useful trans-
`mission between about 3 and 16 |xm in the infrared. They are not a true glass, but a solid
`made of many small crystals.
`Synthetic crystalline sapphire (AI2O3) can be drawn into single-crystal fibers that trans-
`mit between 0.5 and 3.1 |xm. As Figure 6.10 shows, their loss is higher than fluoride fibers,
`but the material is much more durable.
`
`Hollow Optical Waveguides
`Hollow optical waveguides were first developed in the 1960s, after the laser stimulated in-
`terest in optical communications. Work on hollow waveguides for the visible and near-
`infrared stopped shortly after the first low-loss glass fibers were made in the 1970s. However,
`new types of hollow optical waveguides are being developed for infrared wavelengths longer
`than a few micrometers. I mention them here because they serve the same purpose as infrared
`fibers, and compete successfully for some infrared applications. There are two basic types
`of hollow infrared waveguides, metal and hollow glass.
`Hollow metal waveguides are coated inside with a nonconductive dielectric material to
`make them more reflective. The infrared light bounces along the shiny walls, with high
`reflectivity limiting loss to about 500 dB/km. That isn’t bad considering how many reflec-
`tions the light undergoes. Many hollow glass waveguides work on the same principle; they
`have the advantage of very smooth surfaces that give loss as low as 0.5 dB/m with suitable
`coatings, as shown in Figure 6.10.
`Other hollow glass waveguides work on a different principle. At certain wavelengths,
`some materials have an effective refractive index less than 1. Functionally, that means they
`absorb those wavelengths strongly, but it also means they can serve as a low-index cladding
`surrounding a hollow core of air, which at that wavelength has a higher refractive index.
`Silica glass meets those conditions at wavelengths of 7 to 9.4 |xm, and sapphire at 10 to 17 |Jim.
`These waveguides are called attenuating total internal reflection guides because the fraction
`of the wave in the cladding is absorbed, so loss is over 1 dB/m. However, hollow sapphire
`guides can be used at the important 10.6-p.m wavelength of carbon dioxide lasers.
`
`Photonic or Microstructured Fibers
`A new family of optical fibers, largely in the research stage, relies on internal microstruc-
`tures to control the propagation of light in ways impossible with conventional fibers. These
`are often called photonic fibers, but also have been called “microstructured” or “holey”
`fibers because their internal structures typically have holes running along their length.
`Figure 6.11 shows how microstructured fibers are made. Hollow glass tubes and solid
`rods are stacked together with the desired proportions and enclosed in an outer tube. In
`the design shown, a single solid rod is at the center of the array. Then the glass is fused
`together and drawn into a fiber. Careful processing produces finished fiber with fine holes
`running along its length.
`
`

`

`~1 mm
`
`\
`
`Stack
`
`Fiber Materials, Structure, and Manufacture
`
`-15 mm
`
`FIGURE 6.11
`Structure and
`fabrication o f
`photonic crystal
`fib er with solid
`core. (Courtesy
`o f Tim Birks,
`University o f Bath)
`
`Drawn
`Fiber
`
`Hot-Zone
`of Furnace
`
`Preform
`
`Microstructured fibers developed from the phenomenon of the photonic bandgap,
`which arises in materials with internal structures that make it impossible for light to prop-
`agate at certain wavelengths. In that sense, the photonic bandgap is analogous to the elec-
`tronic bandgap, a range of energy levels that electrons can’t occupy in semiconductors.
`Semiconductor bandgaps are used to confine electrons; photonic bandgaps are used to
`confine the path of light.
`The two basic components of a microstructured fiber are a glass matrix and holes con-
`taining air (or some other gas or liquid). The relative packing and size of the holes and glass
`can range from nearly solid glass with a few tiny holes that act as flaws to a thin lattice of
`glass spread through a volume that is largely air. Specialists divide microstructured fibers
`into two broad classes, depending on whether the light is confined in a central solid area
`or within a central hole.
`
`9 Photonic crystal fibers, like the one shown in Figure 6.11, have a solid core
`surrounded by a layer containing holes running the length of the fiber. The central
`solid region is a defect in a sense, because it lacks the holes present in the surrounding
`microstructure. The microstructured zone is a photonic bandgap material with an
`average refractive index lower than that of the solid core. That makes photonic crystal
`
`

`

`Chapter 6
`
`Photonic bandgap
`fibers confine light
`in a hollow core.
`
`Planar waveguides
`are thin strips on
`flat substrates that
`guide light by
`total internal
`reflection.
`
`Planar waveguides
`are used in
`couplers, lasers,
`modulators, and
`switches.
`
`fibers act somewhat like a conventional solid fiber, with a high-index core surrounded
`by a lower index cladding. However, the light guiding of the structure depends on
`the size and spacing of the holes, which determine the effective refractive index of
`the holey cladding layer.
`9 Photonic bandgap fibers have a hollow core surrounded by a photonic bandgap
`cladding, which reflects all light at certain wavelengths. In this way, it guides light
`through the air-filled core, which has a lower refractive index than the surrounding
`material. Such light guiding is impossible in conventional fibers because the refractive
`index of air is lower than that of any conventional solid, so these fibers can only be
`described in terms of photonic bandgaps.
`
`These types of fibers can be designed to have properties impossible in standard fibers.
`Photonic crystal fibers with a large fraction of the cladding filled with air and small
`features can confine light in effective mode areas as small as one square micrometer,
`which is useful for producing nonlinear effects. Large-holed microstructures also can
`produce high waveguide dispersion for use in dispersion compensation or shifting.
`Other photonic crystal structures can confine light in a larger core than otherwise pos-
`sible, reducing nonlinear effects.
`Photonic bandgap fibers have received less attention because they are a more recent
`development, but they offer other possibilities. Guiding light in air should allow very low
`attenuation and reduce nonlinear effects. It also could allow light transmission at wave-
`lengths where no usable transparent solids are available. Because the photonic bandgap effect
`is wavelength-dependent, such fibers would guide some wavelengths but not others.
`
`Planar Waveguides
`Planar waveguides work on the same principle of total internal reflection as optical fibers,
`but they come in a different form. A planar waveguide is a thin layer on the surface of a
`flat material, which has higher refractive index than the bulk material. Typically the high
`refractive index is produced by doping the substrate material with something that increases
`its refractive index. Figure 6.12 shows the basic idea. The boundaries of the doped area
`form an interface that guides light, like the core-cladding interface in optical fibers. In
`Figure 6.12, the substrate provides the low-index materials on the sides and bottom, while
`air is the low-index medium on the top.
`An alternative approach is to deposit a layer of high-index material on a lower index sub-
`strate. In this case, the waveguide is a raised stripe on the substrate, surrounded by air on
`top and on the sides, and contacting the substrate only on the bottom. As with the doped
`waveguide, total internal reflection confines light in the waveguide layer.
`From a theoretical standpoint, both types of planar waveguides are dielectric slab wave-
`guides. That means they are made of nonconducting (dielectric) materials, and are rectan-
`gular in cross section, rather than round like a fiber. The theory of such waveguides is quite
`well developed.
`From a practical standpoint, planar waveguides also have some attractions. The tech-
`nology for making thin stripes of material on flat substrates has been well developed by
`the semiconductor electronics industry. The technology can be used with a wide variety
`
`

`

`Fiber Materials, Structure, and Manufacture
`
`A r
`
`FIGURE
`Planar waveguide.
`
`6.12
`
`of materials, including silica glass and other compounds as well as semiconductors. Active
`optical components such as lasers and photodetectors can be made on the semiconductor
`materials. So can a wide variety of passive optical components, such as demultiplexers and
`couplers that divide and combine optical signals. That opens the possibility of integrating
`optical components on a chip.
`On the other hand, planar waveguides also suffer serious practical drawbacks. Their
`attenuation is much higher than optical fibers, so they can’t send signals very far. Their flat,
`wide geometry differs greatly from the round cores of optical fibers, so light is inevitably lost in
`transferring from a fiber to a waveguide. Such problems limit the uses of planar waveguides.
`Nonetheless, planar waveguide device