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<< cont. from part 1
Graded index
A graded index multimode has a core that has a gradual changing cross-sectional refractive index. The center of the core has the highest refractive index that gradually reduces moving away to the edges of the core. Because of this smooth changing refractive index, the light ray refracts (rather than reflects as in step index fibers) as it moves through the core, and sets up a set of sinusoidal light wave patterns in the fiber. This is illustrated in FIG. 19.
FIG. 19 Multimode graded index fiber transmission
The practical effect of this curved refractive index profile is that the light rays that are traveling further from the center axis of the core will have the furthest distance to travel but will be traveling through the lowest refractive index glass and will therefore be traveling the fastest (as suggested by Snell's law). The light rays that are traveling closer to the center of the core will be traveling the shortest distance but will be traveling through the highest refractive index glass and will therefore be traveling the slowest. The end result of all this is that the rays will all tend to arrive at the same point on the center axis of the core at the same time, which means that the modal dispersion has been greatly reduced.
The center of the core has a refractive index of approximately 1.48, which gradually reduces to 1.46 as it approaches the cladding. The acceptance angle or θ1 is typically around 12° (an NA of approximately 0.2). In practice, graded index fibers have a modal dispersion which is well below 5 nanoseconds and transmission bandwidths on good quality cables that go as high as 2 GHz/km (standard readily available fibers have bandwidths of 600 MHz-km) for both 850 µm and 1300 µm operating wavelengths. This allows data transmission rates up to approximately 2-300 Mbps for a cable with approximately 600 MHz/km and up to 1 Gbps for fiber with better than 2 GHz/km (the latter using VCSEL laser transmission devices). This type of fiber is suitable for virtually all industrial applications and for a number of commercial telecommunication applications. Although modal dispersion is low in graded index fibers compared to step index fibers, it is still significant and does have a limiting effect on the bandwidth.
Some graded index fibers are constructed in such a way that there is no distinction between the core and cladding refractive indices, just a gradual reduction from the center axis to the outside of the cladding.
Standards
The international standard for customer premises cabling ISO/IEC11801, which contains recommendations for both copper and optical fiber cable, has classified different multimode fiber for customer premises' use. At the time of writing this guide, the standard was under review and being updated. The standard provides details on three different classes of multimode fiber (OM1 to OM3) and one class of single-mode fiber (OS1). It details the required minimum bandwidth-kilometer product that is expected from each of the cables. It also distinguishes between when the fibers are driven from LEDs and when they are driven from lasers (VCSELs are normally used - refer to Section 6). The following table shows the bandwidth-distance product for the three multimode fiber types.
Table 1 -- The bandwidth-distance product for the three multimode fiber types
Although LEDs may be able to reach 1 Gbps in multimode fiber, it is not in
normal practice to manufacture links at this speed using LEDs. The preferred
practice is to use the cheaper laser products (VCSELs), which will provide
data speed up to 10 Gbps on OM-3 quality singlemode fibers. 1 Gbps Ethernet
applications on multimode fiber are now running up to 500 meters at the 850
nm wavelength. 1300 nm VCSELs have just been released and 1550 nm VCSELs are
still under development.
4.7 Singlemode fibers
With reference to the formula given in section 4.3, it can be seen that in order to reduce the number of modes traveling down a fiber (and therefore reduce the modal dispersion) the core diameter and/or the NA must be reduced, and/or the wavelength of transmission should be increased. These are the fundamental principles behind the operation of singlemode cables.
A singlemode fiber (or sometimes referred to as a single mode cable) is basically a step index fiber with a very small core diameter. In theory, because the cores are so small, only a few modes of light can travel down the fiber. To further reduce the number of modes, the fiber is constructed with very little difference between the refractive indices of the core and the cladding. From the formula given in section 2.4, as the difference between the refractive indices of the core and cladding decrease, the critical angle increases. Therefore, only light approaching at a very large angle of incidence will be internally reflected and all other rays will dissipate into the cladding. Because of this construction, only a single mode of light is able to transverse down the fiber (i.e. the fundamental mode). This is illustrated in FIG. 20.
FIG. 20 Singlemode optic fiber transmission.
For the transmission of light down a singlemode fiber to operate as described above, the pulse of light that is injected into the core must be very precisely aimed down the center of the core or the majority of the light will be lost in the cladding. If the system is implemented correctly, the input signal pulse into the fiber will appear at the output of the fiber as a signal pulse with almost exactly the same shape. With only the fundamental mode traveling down the fiber, there can theoretically be no modal dispersion in singlemode fiber. The core diameter of a singlemode fiber is generally in the region of 8 to 9 µm. A typical measurement specification for a singlemode fiber is 8.5/125/250 µm.
Attenuation
A typical singlemode fiber will exhibit between 0.35 and 1.0 dB attenuation per kilometer at an operating wavelength of 1310 nm, and between 0.25 and 1.0 dB attenuation per kilometer at an operating wavelength of 1550 nm. Recent research has achieved losses for the 1550 nm wavelength down to approximately 0.15 dB/km.
Bandwidth
A modern singlemode fiber will generally exhibit very high bandwidths, often in excess of 100 GHz/km. Presently, this represents commercial data transmission rates of between approximately 10 & 40 Gbps for systems operating with single wavelength. Laboratory work is currently being carried out on lasers that transmit up to 100 Gbps. Data speeds beyond this for one wavelength are becoming more difficult to obtain because the response time of the receiving devices are unable to detect the difference between the bit length and the wavelength of light. As has been previously mentioned, the use of wavelength division multiplexing will allow singlemode transmission speeds up to several Tera bps on a single fiber.
The NA of singlemode fibers is extremely small (generally in the region of 0.1 to 0.15), which greatly reduces the number of modes of light that can pass down the fiber. To overcome the problem of a very small acceptance angle, lasers are used to provide a coherent and powerful beam of light that is very precisely aligned on to the end of the fiber to ensure maximum amount of energy is radiated into the fiber. Misaligned light rays will dissipate into the cladding and be lost, therefore, correct alignment is very important.
The singlemode fiber has a cladding diameter of 125 µm and it is therefore physically possible to connect this fiber to multimode source and detector equipment. Due to the small NA and core diameter, very little light energy will radiate down the fiber and therefore the system will not operate satisfactorily. For the reverse of this scenario where singlemode source and detector equipment are connected to multimode cables, the systems will work very successfully for speeds of up to 1 Gbps over relatively long distances.
Distance between repeaters
Modern singlemode fiber optic link systems that are of a very high quality are capable of distances between repeaters of up to 300 km for speeds up to 2.5 Gbps (using non zero - dispersion shifted fiber). Research programs currently taking place are quoting future distances between repeaters of up to two orders of magnitude greater than this.
Wavelength of operation
Most singlemode fiber systems operate with the 1300 nm and 1550 nm wavelengths, although there is a preference to operate in the 1550 nm region because of the lower fiber attenuation at this wavelength. The lasers operating at the latter wavelength are not as efficient as the 1300 nm lasers but a significant amount of research and development is currently being carried out to improve them.
One of the problems with the construction of singlemode cables that was discussed is that since there is little difference between the core and the cladding refractive indices a small amount of light tends to travel down the cladding. This will add further distortion to the output signal. This is generally referred to as 'waveguide dispersion'. This is discussed further in section 6.1. The manufacturer will generally include this dispersion figure in the published chromatic dispersion figure in the optical fiber specification.
4.8 Comparison of data rate, distance and fiber type
The following graph shows the expected data rates and transmission distances for the various mode and indexed fibers at different wavelengths.
10,000 1,000 100 10
FIG. 21 Comparison of data rate, distance and fiber type
4.9 Cost
Contrary to what is often expected, the price of singlemode fiber is cheaper than multimode fiber. It is the authors' opinion that the reason for this aberration is simply that the quantity of multimode fiber produced is enormous when compared to the quantity of singlemode fiber produced. Therefore, economies of scale prevail. The cost of fabrication of both multimode and singlemode fibers are very similar.
Unfortunately, the cost of light source (lasers) and detection equipment for singlemode equipment is anything from three to ten times the cost of the equipment used with multimode fiber systems (LEDs). Therefore, the overall cost of installing singlemode systems is prohibitive unless there is a requirement for data systems that operate at greater than 1 Gbps or for distances of greater than 5 km.
The introduction of VCSEL lasers (discussed in Section 6) is beginning to change this scenario. VCSELs are cheaper to manufacture, easier to test and more efficient than traditional lasers. In the near future, more VCSELs will begin to replace LEDs in short distance singlemode and multimode applications.
5. Bandwidth
The concept of bandwidth in the frequency spectrum was discussed in Section 2. With respect to optical fibers, the operational bandwidth does not correspond to changes in frequency to the extent that it does with copper cable, but is more directly related to distance. All the factors that affect the bandwidth will increase as the length of the cable increases. For example, as the length of the cable increases, the modal dispersion increases (increasing the pulse widths at the end of the cable and therefore increasing the degree of inter-symbol interference), which effectively decreases the maximum data transmission rate. Other factors that affect bandwidth will be discussed in section 6.
The bandwidth for a fiber is given in the manufacturer's data sheets. It will be specified in the form, 'frequency bandwidth by kilometers' (i.e. MHz/km). The bandwidth of a fiber is where the operating baseband has dropped by 3 dB in optical power (6 dB in electrical power from an optical power detector). As an example, if a fiber system is specified to operate at a mean wavelength of 1310 nm, which is equivalent to a laser or LED operating at a frequency of 2.3 × 10^14 Hz, and the optical fiber has a specified bandwidth of 500 MHz, then the source will be able to modulate at a rate that produces frequency components up to 250 MHz either side of that operating frequency. This is illustrated in FIG. 22.
FIG. 22 Bandwidth of operation of a fiber Therefore a cable with an operating
bandwidth of 500 MHz-km will provide a 3 dB frequency response of 500 MHz over
1 km or a 3 dB response of 250 MHz over 2 km and so on. Over 5 km, this fiber
would have a net bandwidth of 100 MHz.
6. Wave division multiplexing
A method that is commonly used in long line bearer systems to significantly increase bandwidth is the techniques of wave division multiplexing (WDM) or sometimes referred to as dense wave division multiplexing (DWDM). This technique works by multiplexing highly precise temperature stabilized lasers operating on slightly different wavelengths onto a single fiber. Optical filters are used at the transmitting ends of the link to precisely define the wavelengths and to ensure that there is no overlap from each transmitting laser.
Filters are also used at the receiving end to enable each channel (wavelength) to be captured and decoded.
For example, a system may consist of 32 lasers transmitting 32 different wavelengths into an optical multiplexing unit. The wavelengths will be centered around either the 1310 or 1550 nm wavelengths but be spaced very close together. A system of this size may have wavelengths that are 1.6 nm apart. Commercial systems are commonly available that combine 80 wavelengths at 0.4 nm apart onto a single fiber. If each laser is transmitting at a speed of 10 Gbps, then the single fiber system will have a capacity of 800 Gbps.
Systems are now appearing in the market that will support up to 160 wavelengths onto a single fiber.
There is also a range of lower capacity cheaper systems available in the market, which are sometimes referred to as course wave division multiplexing (CWDM) systems. These are fundamentally WDM systems that do not use temperature-stabilized lasers and have distances of 20 nm between wavelengths. These provide cost effective high capacity solutions. A system like this may operate with 8 wavelengths at speeds of 1.2 Gbps each, providing a total of 9.6 Gbps throughput.
7. Effects on optical signal transmission
There are a number of physical characteristics that are inherent in optical fibers. These characteristics affect the bandwidth, attenuation, and signal quality of the transmission. In multimode fibers, the main factor that affects signal transmission quality is modal dispersion. This was discussed in detail in section 6.1. The following section examines in detail the other factors that affect the transmission characteristics of an optical fiber. The losses that are incurred in optical fibers behave in a similar manner to losses that are incurred in most other dielectric physical environments by electromagnetic energy.
The phenomenon that is noted is an exponential loss of energy that is directly proportional to the linear length of the fiber. This is illustrated in FIG. 23.
FIG. 23 Attenuation curve for an optical fiber
This diagram shows that the output power will drop by half for every incremental length of fiber i.e. it absorbs energy at an exponential rate. It is worth noting here that radio waves experience a similar attenuation as they travel through air or free space.
7.1 Chromatic dispersion
The major dispersion effect in multimode fibers is modal dispersion. In singlemode fibers, there are no modal dispersion effects. More complex dispersion problems occur with singlemode fibers and also with graded index multimode fibers to a significant degree. . There are two further types of dispersion to be discussed:
• Material dispersion
• Waveguide dispersion
These are combined together and referred to as chromatic dispersion. The reason that other types of dispersion occur is because the refractive index of glass is a function of wavelength. Therefore, with reference to Snell's law discussed earlier in this section, the speed of light in years will also be a function of wavelength. Therefore the overall modal transmission effects of the fiber refractive index also depend upon wavelength.
Material dispersion is a phenomenon that occurs because light sources put out a signal, which contains a number of different wavelengths. No light source can produce just one frequency (wavelength). It will produce a spectral spread around a central frequency. As the different wavelengths travel through the same material, they will effectively encounter different refractive indexes. Relating this to Snell's law, this means different rays of light will be traveling at different speeds. The result of this is similar in nature (but lesser in degree) to modal dispersion in which, the light rays will arrive at the end of the fiber at different times. This phenomenon is particularly noticeable with LED light sources because they emit a very broad spectrum of light. However, material dispersion is significantly less in magnitude than modal dispersion and generally is not a significant problem with LEDs unless the system is operating at relatively high speeds at a wavelength of 850 mm. The use of lasers as a source of light significantly reduces material dispersion because the laser provides a coherent beam of light with a very narrow spectral spread (i.e. range of wavelengths). When manufacturing singlemode fibers, not only is the diameter reduced, but the difference between the core and the cladding refractive indices is also reduced. Here the effect of modal dispersion disappears, but then material dispersion becomes the significant problem. The effects of material dispersion become more noticeable in single mode fibers because of the higher bandwidths (data rates) that are expected of them. For example, a few picoseconds of dispersion at a data rate of 10 Gbps can cause severe data corruption. It is possible to partially compensate for this problem by allowing a certain amount of modal dispersion in the singlemode fiber so that the faster rays travel the longer distances and therefore arrive at roughly the same time as the slower rays. In this case, the faster wavelengths would be traveling the higher order modes and the slower wavelengths, the lower order modes.
The second form of dispersion that makes up chromatic dispersion is waveguide dispersion. Waveguide dispersion occurs in singlemode fibers (which are of step index construction) where a certain amount of the light travels in the cladding. The dispersion occurs because the light moves faster in the low refractive index cladding than in the higher refractive index core. The degree of waveguide dispersion depends on the proportion of light that travels in the cladding.
Chromatic dispersion in real terms is a measure of the change in the refractive index with wavelength (ps/nm/km). For this reason dispersion measurement can read as going positive or negative. That is, the change in refractive index with wavelength can be an increase or a decrease. Material dispersion has a positive slope of change and the waveguide has a negative slope of change. At roughly 1300 nm, the two dispersion types tend to cancel each other out. This is referred to as the zero dispersion wavelength. This phenomenon is illustrated in FIG. 24.
FIG. 24 Chromatic dispersion showing the zero dispersion wavelength
In a physical sense, it can be perceived that material dispersion is causing the pulse to travel faster (relative to other wavelengths) and waveguide dispersion is causing the pulse to travel slower, and therefore the overall effect is the cancellation of some of this movement.
Therefore, at present, most moderately priced high-speed fiber optic data link systems tend to operate at the 1300 nm wavelength. If very high-speed operation over longer links is required, then the 1550 nm wavelength is used. These link systems cost more because special fiber has to be installed (discussed later). It is important to remember that chromatic dispersion is primarily a function of wavelength and has nothing to do with whether the cable is multimode or singlemode.
Cable suppliers generally provide a chromatic dispersion figure in the cable specification. The unit of measure will be given as picoseconds of pulse spreading per kilometer of fiber per nanometer of the light source spectral spread (bandwidth of source). Refer to section 8.3.3 for details of how chromatic dispersion is calculated.
It is generally preferred to operate at a wavelength of 1550 nm because of the lower signal attenuation of this wavelength compared to the 1300 nm operation. But as indicated in FIG. 24, operation at this wavelength introduces dispersion. This can be overcome to some degree by using a laser source that emits only a very narrow spectral spread, i.e. it has very narrow bandwidth. This type of laser is commonly used for longer distance requirements. Note that this will reduce the pulse spreading due to the number of frequencies traveling, but not that caused by the inherent chromatic dispersion of the fiber itself.
Another method is to use what is referred to as 'dispersion shifted fibers'. This technique uses fibers that have lower chromatic dispersion at 1550 nm. It is not possible to change the overall effect of material dispersion, as this is a function of glass material itself. But as waveguide dispersion is caused by a certain amount of the light traveling in the cladding, it is possible to change the construction of the core and cladding (and also to add further layers of cladding) so that the waveguide dispersion is shifted downward and therefore makes the zero dispersion wavelength move toward 1550 nm. This technique works well with only an extremely small increase in attenuation, but the cost of producing the fibers increases significantly.
One common dispersion shifted fiber used today is the non-zero dispersion shifted fiber (NZ DSF). This is used extensively for DWDM systems operating at distances of over 70 kms in the 1440 to 1625 nm region. It also helps to compensate for other non linearities that occur in singlemode systems such as wave mixing and phase modulation.
A typical NZ DSF fiber will have a dispersion figure of better than 8 ps/nm/km. Most of the major fiber manufacturers have a version of the NZ DSF commercially available. The NZ DSF fibers are designed for use at high data rates and are used extensively with the 40 Gbps lasers. Commonly, single non-repeated links are established up to 300 kms at 2.5 Gbps.
7.2 Absorption losses
During the manufacturing process of optical fiber, every effort is made to fabricate the glass as pure as possible. The requirements for cleanliness, purity and quality control in the manufacturing process are as stringent as those applied to the semiconductor industry.
Unfortunately, it is impossible to produce 100% pure glass. The impurities that are left in the glass will absorb light. These impurities are in the form of ionized molecules. Metal ions such iron, copper and nickel are the main offenders. They absorb the light particles (photons) and in the energy exchange process the fiber will heat up. The absorption losses caused by metal ion impurities are substantial in poor quality glass.
Glass will also contain significant amounts of water ion ( OH-) impurities that resonate at certain frequencies. Most signal attenuation due to water ions occurs in the 850 nm wavelength region. This was the major source of signal attenuation when fiber optic cables were first manufactured on a commercial basis. Significant advances in the manufacturing technology have greatly reduced this problem.
7.3 Scatter losses
There are two types of scatter losses that occur in fibers. The first type occurs because all manufactured or naturally occurring material is never perfect in its molecular structure throughout the entire volume of the material. If a piece of optical fiber is placed under an electron microscope it can be seen that there are irregularities in the molecular structure of the glass. These irregularities (or inhomogenetics as they are referred) are unavoidable because molecules and atoms are naturally random in nature and will set in a random pattern when the material is formed. The irregularities will scatter some of the light waves as they transverse through the length of fiber, which will then be dispersed in the cladding and lost. This type of scatter loss is referred to as Rayleigh scattering. The degree of scattering very rapidly decreases with increasing wavelength and acceptably low scatter losses are achieved using the infrared wavelengths (1300 and 1550 nm). The Rayleigh scattering loss in fused silica glass is approximately 0.8 dB per kilometer at a wavelength of 1000 nm.
The second type of scatter loss that occurs is due to irregularities in the core/cladding interface. These appear as physical imperfections and are introduced during the fabrication process. When a light ray strikes one of these imperfections, it may change to a higher order mode and be dissipated through the cladding. This results in higher signal attenuation. The various scattering that takes place causes light to often change modes. For example, a lower order mode may scatter and become a higher order mode. This is referred to as mode coupling or mode mixing. Modal mixing can be advantageous by averaging the traveled distances of the light rays and therefore helping to cancel some modal dispersion.
7.4 Bending losses
It is sometimes assumed intuitively that if a fiber is bent, then losses will be introduced into the transmission path. This is not true as the inside of a fiber is normally seen as a mirror to light rays, and slight bends in the fiber do not introduce losses. Losses occur only when the radius of the bend causes the light ray to be incident at an angle less than the critical angle. This could be from a ray that is directly incident into the bend at less than the critical angle or from a ray that reflects off a bend and then into the cladding at an angle less than the critical angle. The manufacturer of the cable will specify the minimum installed bending radius requirement for that particular fiber optic cable. This specified figure indicates the minimum inside radius a bend in the cable is allowed to have when the cable has been finally laid to rest.
There are two types of bend that cause losses. The first is referred to as a 'Macrobend'. This is where the cable is installed with a bend in it that has a radius less than the minimum bending radius. Light will strike the core/cladding interface at an angle less than the critical angle and will be lost into the cladding. This is illustrated in FIG. 24.
FIG. 25 Losses due to macrobending
The second type of bending loss is referred to as 'Microbending'. The microbend takes the form of a very small sharp bend (a kink) in the cable. Microbends can be caused by imperfections in the cladding, ripples in the core/cladding interface, tiny cracks in the fiber and external forces. The external forces may be from a heavy sharp object being laid across the cable or from the cable being pinched, as it is pulled through a tight conduit.
As for the occurrence of macrobends, the light ray will hit the bend at an angle less than the critical angle and will be refracted into the cladding. This is illustrated in FIG. 26.
FIG. 26 Losses due to microbending
7.5 Radiation losses
A close analysis of the energy field of the light pulse that is carried down the fiber shows that a certain amount of the total light energy is carried in the cladding of the fiber. This is particularly noticeable in graded index multimode and in step index singlemode fibers where the core/cladding refractive index change is minimal. The total cross-section of the moving energy field in the fiber will try to move as a constant field. There appears to be a natural cohesion between the light rays that keep the energy field constant, as it moves through the fiber. When there is a bend in the fiber, the light energy traveling through the larger outer curve will be required to travel faster than the energy traveling in the center of the core. The light will naturally resist this and will tend to radiate away.
The amount of energy that is normally lost through large radius bends in the fiber is almost negligible. But if the outside light rays are required to travel faster than the speed of light because there is a very sharp kink in the fiber (i.e. there is a very small radius bend in the fiber) then the radiation loss becomes quite significant and can be disastrous to the transmission link. The radius at which this occurs is very small, generally around 60 µm depending on the type of optical fiber that is being used. This is a further reason to avoid microbends.
7.6 Fresnel connection loss
It was previously discussed in section 1.8 that where the core and the cladding meet and light strikes this boundary at an angle less than the critical angle, about 4% of the light is reflected back into the core. The same phenomenon, referred to as Fresnel reflection, is also noted where two fibers are joined together. Even though the two fibers may have been joined with perfectly flat and smooth ends, there is still an unavoidable change in the refractive indices due to a small amount of air between the two fibers. This effectively represents a 4% loss in signal level at each interface (glass to air and air to glass). Therefore, the total amount of energy lost is 8%. When assessing power loss over a link, this represents approximately 0.17 dB loss per fiber to air interface and 0.34 dB loss per fiber-to-fiber joint. This is illustrated in FIG. 27. Section 5.1.6 discusses this phenomenon further and describes methods of overcoming this problem.
FIG. 27 Fresnel loss due to fiber-to-fiber connections
7.7 Fiber size and NA mismatch
Although it is not desirable, the occasion does arise when it is required to connect fibers of different sizes and of different NAs. If the fiber from which the light is emanating is larger than the fiber, which is receiving the light, then light rays will escape out of the fringes of the larger fiber. If the two fibers have the same diameters but have different NAs and the fiber from which light is emanating has the larger NA, then this fiber will lose a small amount of its energy through refraction into the cladding of the second fiber.
If the fiber from which the light is emanating has a smaller diameter or smaller NA than the receiving fiber, then there is no signal loss incurred. The mismatch is illustrated in FIG. 28. The following formula approximates the losses incurred.
Loss (dBs) = -20log (NA1/NA2) for NA1 > NA2
Loss (dBs) = -20log (D1/D2) for D1 > D2
FIG. 28 Losses due to diameter and NA mismatch Note that it is implicit
that there is a NA mismatch if there is a diameter mismatch because NA is dependent
upon diameter.
8. Other losses
There are two other main types of losses in fiber systems:
• Connector to fiber losses is covered in Section 5.
• Source/detector to fiber mismatches are covered in Section 6.
9. Other types of fibers
Glass (fused silica) is by far the most commonly used material for manufacturing optical fibers.
It is worth noting though that there are many other types of optical fibers available. The following section provides a short description of a number of such other types of optical fibers.
9.1 Plastic fibers
Optical fiber transmission principles also work very successfully with optical fibers made from plastic. They are however limited to a step index multimode construction. Because of the nature of plastics, the fabrication process produces a core and cladding with significantly different refractive indices. Typical figures obtained are a core with a refractive index of approximately 1.5 and a cladding of approximately 1.4. A typical NA is 0.5, which represents an acceptance angle of 30 degrees (an acceptance cone of 60 degrees). There are three main advantages associated with using plastic fibers. Firstly, they are a lot more robust than glass fibers. They can sustain significantly more shock, pressure and stress without damage than glass fibers can. For this reason, they are often used in harsh environments such as in automobiles. Secondly, they are far more flexible, easier to handle and therefore easier to terminate than glass fibers. Finally, if they are bought in bulk, they are generally cheaper than glass fibers.
On the other hand, there are a number of significant disadvantages. They have a significantly higher attenuation than glass fibers and are generally used over very short distances (100 m maximum) only. Plastic has an optimum operating wavelength of 650 nm (red LEDs are used), which has an attenuation of approximately 300 dB per kilometer. They also have very limited operating bandwidths with maximum operating data speeds up to approximately 10 Mbps over a maximum of 50 m. Therefore they are rarely used for telecommunications purposes. They are sometimes used with RS-232 to optical converters up to 20 kbps and very short distance Ethernet data links to 10 Mbps. One further problem of note is that they have much lower maximum operating temperature than glass fibers (generally around 85°C).
9.2 Ultraviolet fibers
Although most fiber transmission is in the near infrared region because of the correspondingly low attenuation through glass at these frequencies, some fibers are fabricated for special applications in the ultraviolet region. Typical attenuation figures are around 200 dB per kilometer at 350 nm and 2000 dB per kilometer at 200 nm. Therefore they are only used for specific applications over very short distances. The most common application is for scientific measurements.
9.3 Mid-infrared fibers
The lower infrared frequencies provide the lowest losses in glass fibers. Also, the longer the wavelength the less scattering that occurs, which is a significant cause of signal attenuation in fiber transmission. Therefore, it would be desirable to operate with longer wavelength systems, moving into the mid-infrared region. Unfortunately, glass almost absorbs these frequencies totally.
However, there are certain materials that provide very low resistance to the flow of energy at these wavelengths. It has been suggested by scientists that attenuation as low as 0.001 dB per kilometer should be possible using these special materials. Zirconium fluoride and barium fluoride are two compounds currently being researched. The problem encountered though is the difficulty and the cost associated with fabricating these substances in a very pure form. The best attenuation figures that have been achieved to date are approximately 25 dB per kilometer at 2600 nm and approximately 700 dB per kilometer at 5500 nm. Despite this, scientists are enthusiastic about the future of this form of fiber technology, which will mean significantly greater distances between repeaters.
9.4 Polarized fibers
In section 2.5, it was discussed how light has an electric field and a magnetic field. The electric field is classified as traveling either vertically or horizontally to the ground. In the former case, the light ray is said to be vertically polarized and in the latter case, the light ray is said to be horizontally polarized. The light rays can also be of some degree of polarization in between these. When polarized light is passed along an optical fiber, the plane of the polarization generally changes with distance along the fiber.
Optical fibers can be constructed to maintain its polarization as the light passes down the fiber. At present, these types of fibers are only used for special sensing applications but it is planned to use them in the future with very high data speed singlemode fiber systems.
10. Fabrication of fibers
Optical fibers are manufactured from very pure raw materials and in spotlessly clean conditions. As an indication of the purity of glass, it is said that a kilometer thick block of optical fiber glass has about the same translucence as a normal windowpane.
There are four main methods of manufacturing optical fibers, which are briefly discussed below.
10.1 Inside chemical vapor deposition
This fabrication process commences with a hollow Quartz tube about 15 mm in diameter and about 4 mm thick. The tube is rotated on a burner to heat it up. Various chemical components of glass are then passed through the tube in a gaseous form. Included in this gas are various other chemicals that act as reactants, metal impurities, and catalysts to form glass of differing refractive indices. Once the quartz is heated, the gases will react with it to form a layer of glass on the inside of the tube. Depending on how long and to what temperature the quartz is heated, the impurities will impregnate the quartz to different depths and concentrations. Different concentrations of gasses are passed through the tube to form different layers of the core and cladding refractive index. Once this is complete, the tube is passed through a furnace and collapsed into a solid tube of glass. This tube is approximately 10 mm thick and is referred to as a preform.
The tip of the preform is then fed into an extrusion unit, which is encased in a furnace.
A fiber is drawn from the tip down from a high drawing tower. Lasers are used to monitor the fiber diameter, and then the diameter size is fed back to control the speed at which the fiber is drawn (which directly correlates to the drawn diameter). The fiber is then passed through a plastic fluid to provide the fiber sheath. This process is illustrated in FIG. 29.
FIG. 29 Inside chemical vapor deposition
This method of fiber fabrication is the most commonly used.
10.2 Outside chemical vapor deposition
This is a similar fabrication process to that described above except that the glass is layered on the outside of a rotating metal rod. The glass gaseous compounds are fed into the burner and are formed into layered glass onto the outside of the rod, as the burner moves along the rod. Once the glass formation is completed, the metal rod is removed and the glass tube is fed into a furnace and collapsed into a preform. Once the preform is complete, the fiber is drawn in the manner described above. This process is illustrated in FIG. 30.
Moving burner; Rotating metal rod; Fuel; Gaseous glass compounds; Vapor Deposition
FIG. 30 Outside chemical vapor deposition
10.3 Vapor axial deposition
This fabrication process uses a glass seed rod on which the glass 'preforms'. The seed rod is held vertically and rotated while the gaseous glass chemicals are blown onto the end of the seed rod through a burner. As the preform is forming, the glass seed rod is raised at a controlled speed so that the preform is constantly at the same distance from the burners (and therefore the same diameter). Different concentrations of gases are blown through different burners to form the required different refractive indices of the core and the cladding. The preform that is formed here is very porous, so it is fed through a heater and collapsed into the required preform density. Once the preform is complete, the fiber is drawn in the conventional manner. This process is illustrated in FIG. 30.
FIG. 30 Vapor axial deposition
This technique is becoming the most popular method of fabrication. The advantage of this method is that very long preforms can be made, which in turn means that long continuous fibers can be drawn. Continuous fibers up to 100 km are regularly drawn using preforms from the process. With the first two fabrication methods discussed the preform length is limited to the length of the quartz tube or metal rod used in the lathe.
10.4 Double crucible drawing
This fabrication process is the cheapest and easiest to implement, but produces the poorest quality cable. It is generally used to produce large quantities of step index multimode fiber. If extreme care is taken, graded index multimode fibers can be manufactured.
FIG. 31 Double crucible drawing
Separate glass rods are produced, one with the refractive index of the core and the second with the refractive index of the cladding. These are fed into a furnace, which has two concentric crucibles. The core glass is fed into the central crucible and the cladding glass is fed into the outer crucible. As the rods are fed into the crucibles, the furnace melts them and turns them into molten glass. At the bottom of the double crucible is a small hole through which the fiber is drawn. This process is illustrated in FIG. 31.