Fiber-Optic Applications
Figure 3-1 Cross Section of a Fiber-Optic Cable
The index of refraction is calculated by dividing the speed of light in a vacuum by the speed of light in another medium, as shown in the following formula:
Figure 3-2 shows the propagation of light down the fiber-optic cable using the principle of total internal reflection. As illustrated, a light ray is injected into the fiber-optic cable on the left. If the light ray is injected and strikes the core-to-cladding interface at an angle greater than the critical angle with respect to the normal axis, it is reflected back into the core. Because the angle of incidence is always equal to the angle of reflection, the reflected light continues to be reflected. The light ray then continues bouncing down the length of the fiber-optic cable. If the angle of incidence at the core-to-cladding interface is less than the critical angle, both reflection and refraction take place. Because of refraction at each incidence on the interface, the light beam attenuates and dies off over a certain distance.
Figure 3-2 Total Internal Reflection
The critical angle is fixed by the indices of refraction of the core and cladding and is computed using the following formula:
Figure 3-2 shows a light ray entering the core from the outside air to the left of the cable. Light must enter the core from the air at an angle less than an entity known as the acceptance angle (
a):
The optical fiber also has a numerical aperture (NA). The NA is given by the following formula:
Figure 3-3 Acceptance Cone
The wide range of power values makes decibel a convenient unit to express the power levels that are associated with an optical system. The gain of an amplifier or attenuation in fiber is expressed in decibels. The decibel does not give a magnitude of power, but it is a ratio of the output power to the input power.
Signals weaker than 1 mW have negative dBm values, whereas signals stronger than 1 mW have positive dBm values.
Optical-Cable Construction
The core is the highly refractive central region of an optical fiber through which light is transmitted. The standard telecommunications core diameter in use with SMF is between 8
m and 10 m, whereas the standard core diameter in use with MMF is between 50 m and 62.5 m. Figure 3-4 shows the core diameter for SMF and MMF cable. The diameter of the cladding surrounding each of these cores is 125 m. Core sizes of 85 m and 100 m were used in early applications, but are not typically used today. The core and cladding are manufactured together as a single solid component of glass with slightly different compositions and refractive indices. The third section of an optical fiber is the outer protective coating known as the coating. The coating is typically an ultraviolet (UV) light-cured acrylate applied during the manufacturing process to provide physical and environmental protection for the fiber. The buffer coating could also be constructed out of one or more layers of polymer, nonporous hard elastomers or high-performance PVC materials. The coating does not have any optical properties that might affect the propagation of light within the fiber-optic cable. During the installation process, this coating is stripped away from the cladding to allow proper termination to an optical transmission system. The coating size can vary, but the standard sizes are 250 m and 900 m. The 250-m coating takes less space in larger outdoor cables. The 900-m coating is larger and more suitable for smaller indoor cables.
Figure 3-4 Optical-Cable Construction
Fiber-optic cable sizes are usually expressed by first giving the core size followed by the cladding size. Consequently, 50/125 indicates a core diameter of 50 microns and a cladding diameter of 125 microns, and 8/125 indicates a core diameter of 8 microns and a cladding diameter of 125 microns. The larger the core, the more light can be coupled into it from the external acceptance angle cone. However, larger-diameter cores can actually allow in too much light, which can cause receiver saturation problems. The 8/125 cable is often used when a fiber-optic data link operates with single-mode propagation, whereas the 62.5/125 cable is often used in a fiber-optic data link that operates with multimode propagation.
Three types of material make up fiber-optic cables:
Figure 3-5 Inside Plant Ribbon-Cable System
Figure 3-6 shows a typical six-fiber, inside-plant cable system. The central core is composed of a dielectric strength member with a dielectric jacket. The individual fibers are positioned around the dielectric strength member. The individual fibers have a strippable buffer coating. Typically, the strippable buffer is a 900-m tight buffer. Each individual coated fiber is surrounded with a subunit jacket. Aramid yarn strength members surround the individual subunits. Some cable systems have an outer strength member that provides protection to the entire enclosed fiber system. Kevlar is a typical material used for constructing the outer strength member for premise cable systems. The outer jacket is OFNP, OFNR, or LSZH.
Figure 3-6 Cross Section of Inside-Plant Cables
Figure 3-7 shows a typical armored outside-plant cable system. The central core is composed of a dielectric with a dielectric jacket or steel strength member. The individual gel-filled subunit buffer tubes are positioned around the central strength member. Within the subunit buffer tube, six fibers are positioned around an optional dielectric strength member. The individual fibers have a strippable buffer coating. All six subunit buffer tubes are enclosed within a binder that contains an interstitial filling or water-blocking compound. An outer strength member, typically constructed of aramid Kevlar strength members encloses the binder. The outer strength member is surrounded by an inner medium-density polyethylene (MDPE) jacket. The corrugated steel armor layer between the outer high-density polyethylene (HDPE) jacket, and the inner MDPE jacket acts as an external strength member and provides physical protection. Conventional deep-water submarine cables use dual armor and a special hermetically sealed copper tube to protect the fibers from the effects of deep-water environments. However, shallow-water applications use cables similar to those shown in Figure 3-7 with an asphalt compound interstitial filling.
Fiber-Optic Characteristics
Optical-fiber systems have many advantages over metallic-based communication systems. These advantages include interference, attenuation, and bandwidth characteristics. Furthermore, the relatively smaller cross section of fiber-optic cables allows room for substantial growth of the capacity in existing conduits. Fiber-optic characteristics can be classified as linear and nonlinear. Nonlinear characteristics are influenced by parameters, such as bit rates, channel spacing, and power levels.
Figure 3-12 Attenuation Versus Wavelength
The OH- symbols indicate that at the 950-nm, 1380-nm, and 2730-nm wavelengths, the presence of hydroxyl radicals in the cable material causes an increase in attenuation. These radicals result from the presence of water remnants that enter the fiber-optic cable material through either a chemical reaction in the manufacturing process or as humidity in the environment. The variation of attenuation with wavelength due to the water peak for standard, single-mode fiber-optic cable occurs mainly around 1380 nm. Recent advances in manufacturing have overcome the 1380-nm water peak and have resulted in zero-water-peak fiber (ZWPF). Examples of these fibers include SMF-28e from Corning and the Furukawa-Lucent OFS AllWave. Absorption accounts for three percent to five percent of fiber attenuation. This phenomenon causes a light signal to be absorbed by natural impurities in the glass and converted to vibration energy or some other form of energy such as heat. Unlike scattering, absorption can be limited by controlling the amount of impurities during the manufacturing process. Because most fiber is extremely pure, the fiber does not heat up because of absorption.
Rayleigh Scattering@As light travels in the core, it interacts with the silica molecules in the core. Rayleigh scattering is the result of these elastic collisions between the light wave and the silica molecules in the fiber. Rayleigh scattering accounts for about 96 percent of attenuation in optical fiber. If the scattered light maintains an angle that supports forward travel within the core, no attenuation occurs. If the light is scattered at an angle that does not support continued forward travel, however, the light is diverted out of the core and attenuation occurs. Depending on the incident angle, some portion of the light propagates forward and the other part deviates out of the propagation path and escapes from the fiber core. Some scattered light is reflected back toward the light source. This is a property that is used in an optical time domain reflectometer (OTDR) to test fibers. The same principle applies to analyzing loss associated with localized events in the fiber, such as splices.
Short wavelengths are scattered more than longer wavelengths. Any wavelength that is below 800 nm is unusable for optical communication because attenuation due to Rayleigh scattering is high. At the same time, propagation above 1700 nm is not possible due to high losses resulting from infrared absorption.
A macrobend is a large-scale bend that is visible, and the loss is generally reversible after bends are corrected. To prevent macrobends, all optical fiber has a minimum bend radius specification that should not be exceeded. This is a restriction on how much bend a fiber can withstand before experiencing problems in optical performance or mechanical reliability.
The second extrinsic cause of attenuation is a microbend. Microbending is caused by imperfections in the cylindrical geometry of fiber during the manufacturing process. Microbending might be related to temperature, tensile stress, or crushing force. Like macrobending, microbending causes a reduction of optical power in the glass. Microbending is very localized, and the bend might not be clearly visible on inspection. With bare fiber, microbending can be reversible.
As the pulses spread, or broaden, they tend to overlap and are no longer distinguishable by the receiver as 0s and 1s. Light pulses launched close together (high data rates) that spread too much (high dispersion) result in errors and loss of information. Chromatic dispersion occurs as a result of the range of wavelengths present in the light source. Light from lasers and LEDs consists of a range of wavelengths, each of which travels at a slightly different speed. Over distance, the varying wavelength speeds cause the light pulse to spread in time. This is of most importance in single-mode applications. Modal dispersion is significant in multimode applications, in which the various modes of light traveling down the fiber arrive at the receiver at different times, causing a spreading effect. Chromatic dispersion is common at all bit rates. Chromatic dispersion can be compensated for or mitigated through the use of dispersion-shifted fiber (DSF). DSF is fiber doped with impurities that have negative dispersion characteristics. Chromatic dispersion is measured in ps/nm-km. A 1-dB power margin is typically reserved to account for the effects of chromatic dispersion.
Single-mode optical fiber and components support one fundamental mode, which consists of two orthogonal polarization modes. This asymmetry introduces small refractive index differences for the two polarization states. This characteristic is known asbirefringence. Birefringence causes one polarization mode to travel faster than the other, resulting in a difference in the propagation time, which is called the differential group delay (DGD). DGD is the unit that is used to describe PMD. DGD is typically measured in picoseconds. A fiber that acquires birefringence causes a propagating pulse to lose the balance between the polarization components. This leads to a stage in which different polarization components travel at different velocities, creating a pulse spread as shown in Figure 3-13. PMD can be classified as first-order PMD, also known as DGD, and second-order PMD (SOPMD). The SOPMD results from dispersion that occurs because of the signal's wavelength dependence and spectral width.
PMD is not an issue at low bit rates but becomes an issue at bit rates in excess of 5 Gbps. PMD is noticeable at high bit rates and is a significant source of impairment for ultra-long-haul systems. PMD compensation can be achieved by using PMD compensators that contain dispersion-maintaining fibers with degrees of birefringence in them. The introduced birefringence negates the effects of PMD over a length of transmission. For error-free transmission, PMD compensation is a useful technique for long-haul and metropolitan-area networks running at bit rates greater than 10 Gbps. Note in Figure 3-13 that the DGD is the difference between Z1 and Z2. The PMD value of the fiber is the mean value over time or frequency of the DGD and is represented as ps/ km. A 0.5-dB power margin is typically reserved to account for the effects of PMD at high bit rates.
Figure 3-13 Polarization Mode Dispersion
OSNR is an important and fundamental system design consideration. Another parameter considered by designers is the Q-factor. The Q-factor, a function of the OSNR, provides a qualitative description of the receiver performance. The Q-factor suggests the minimum signal-to-noise ratio (SNR) required to obtain a specific BER for a given signal. OSNR is measured in decibels. The higher the bit rate, the higher the OSNR ratio required. For OC-192 transmissions, the OSNR should be at least 27 to 31 dB compared to 18 to 21 dB for OC-48.
XPM can be mitigated by carefully selecting unequal bit rates for adjacent WDM channels. XPM, in particular, is severe in long-haul WDM networks, and the acceptable norm in system design to counter this effect is to take into account a power penalty that can be assumed equal to the negative effect posed by XPM. A 0.5-dB power margin is typically reserved to account for the effects of XPM in WDM fiber systems.
The effects of FWM are pronounced with decreased channel spacing of wavelengths and at high signal power levels. High chromatic dispersion also increases FWM effects. FWM also causes interchannel cross-talk effects for equally spaced WDM channels. FWM can be mitigated by using uneven channel spacing in WDM systems or nonzero dispersion-shifted fiber (NZDSF). A 0.5-dB power margin is typically reserved to account for the effects of FWM in WDM systems.
SRS refers to lower wavelengths pumping up the amplitude of higher wavelengths, which results in the higher wavelengths suppressing signals from the lower wavelengths. One way to mitigate the effects of SRS is to lower the input power. In SRS, a low-wavelength wave called Stoke's wave is generated due to the scattering of energy. This wave amplifies the higher wavelengths. The gain obtained by using such a wave forms the basis of Raman amplification. The Raman gain can extend most of the operating band (C- and L-band) for WDM networks. SRS is pronounced at high bit rates and high power levels. The margin design requirement to account for SRS/SBS is 0.5 dB.
Connecting two fiber-optic cables requires precise alignment of the mated fiber cores or spots in a single-mode fiber-optic cable. This is required so that nearly all the light is coupled from one fiber-optic cable across a junction to the other fiber-optic cable. Actual contact between the fiber-optic cables is not even mandatory.
There are two principal types of splices: fusion and mechanical. Fusion splices use an electric arc to weld two fiber-optic cables together. The process of fusion splicing involves using localized heat to melt or fuse the ends of two optical fibers together. The splicing process begins by preparing each fiber end for fusion. Fusion splicing requires that all protective coatings be removed from the ends of each fiber. The fiber is then cleaved using the score-and-break method. The quality of each fiber end is inspected using a microscope. In fusion splicing, splice loss is a direct function of the angles and quality of the two fiber-end faces.
The basic fusion-splicing apparatus consists of two fixtures on which the fibers are mounted with two electrodes. An inspection microscope assists in the placement of the prepared fiber ends into a fusion-splicing apparatus. The fibers are placed into the apparatus, aligned, and then fused together. Initially, fusion splicing used nichrome wire as the heating element to melt or fuse fibers together. New fusion-splicing techniques have replaced the nichrome wire with carbon dioxide (CO2) lasers, electric arcs, or gas flames to heat the fiber ends, causing them to fuse together. Arc fusion splicers can splice single fibers or 12- and 24-fiber-count ribbon fibers at the same time. The small size of the fusion splice and the development of automated fusion-splicing machines have made electric arc fusion one of the most popular splicing techniques in commercial applications. The splices offer sophisticated, computer-controlled alignment of fiber-optic cables to achieve losses as low as 0.02 dB.
Splices can also be used as optical attenuators if there is a need to attenuate a high-powered signal. Splice losses of up to 10.0 dB can be programmed and inserted into the cable if desired. This way, the splice can act as an in-line attenuator with the characteristic nonreflectance of a fusion splice. Typical fusion-splice losses can be estimated at 0.02 dB for loss-budget calculation purposes. Mechanical splices are easily implemented in the field, require little or no tooling, and offer losses of about 0.5 to 0.75 dB.
Figure 3-16 Fiber-Optic Communication System
Figure 3-17 shows the optical power output, P, from each of these devices as a function of the electrical current input, I, from the modulation circuitry. As the figure indicates, the LED has a relatively linear P-I characteristic, whereas the LD has a strong nonlinearity or threshold effect. The LD can also be prone to kinks when the power actually decreases with increasing input current. LDs have advantages over LEDs in the sense that they can be modulated at very high speeds, produce greater optical power, and produce an output beam with much less spatial width than an LED. This gives LDs higher coupling efficiency to the fiber-optic cable. LED advantages include a higher reliability, better linearity, and lower cost.
Figure 3-17 LED and LD P-I Characteristics
A key difference between the optical output of an LED and a LD is the wavelength spread over which the optical power is distributed. The spectral width, σ, is the 3-dB optical power width (measured in nanometers or microns). The spectral width impacts the effective transmitted signal bandwidth. A larger spectral width takes up a larger portion of the fiber-optic cable link bandwidth. Figure 3-18 shows the spectral width of the two devices. The optical power generated by each device is the area under the curve. The spectral width is the half-power spread. An LD always has a smaller spectral width than an LED. The specific value of the spectral width depends on the details of the diode structure and the semiconductor material. However, typical values for an LED are around 40 nm for operation at 850 nm and 80 nm at 1310 nm. Typical values for an LD are 1 nm for operation at 850 nm and 3 nm at 1310 nm.
Figure 3-18 LED and LD Spectral Widths
Other transmitter parameters include packaging, environmental sensitivity of device characteristics, heat sinking, and reliability. With either an LED or LD, the transmitter package must have a transparent window to transmit light into the fiber-optic cable. It can be packaged with either a fiber-optic cable pigtail or with a transparent plastic or glass window. Some vendors supply the transmitter with a package having a small hemispherical lens to help focus the light into the fiber-optic cable. Packaging must also address the thermal coupling for the LED or LD. A complete transmitter module can consume more than 1 watt, which could result in significant heat generation. Plastic packages can be used for lower-speed and lower-reliability applications. However, high-speed and high-reliability transmitters need metal packaging with built-in fins for heat sinking.
There are several different schemes for carrying out the modulation function. These include intensity modulation (IM), frequency shift keying (FSK), phase shift keying (PSK), and polarization modulation (PM). Within the context of a premise fiber-optic data link, the only one really used is IM. IM is used universally for premise fiber-optic data links because it is well matched to the operation of both LEDs and LDs. The carrier that each of these sources produces is easy to modulate with this technique. Passing current through them operates both of these devices. The amount of power that they radiate (sometimes referred to as the radiance) is proportional to this current. In this way, the optical power takes the shape of the input current. If the input current is the waveform m(t) representing the binary information stream, the resulting optical signal looks like bursts of optical signal when m(t) represents a 1 and the absence of optical signal when m(t) represents a 0. This is also known as direct modulation of the LED or LD.
Figure 3-19 Schematic of an Optical Receiver
A receiver is generally designed with a transmitter. Both are modules within the same package. The light detection is carried out by a photodiode, which senses light and converts it into an electrical current. However, the optical signal from the fiber-optic cable and the resulting electrical current will have a small amplitude. Consequently, the photodiode circuitry must be followed by one or more amplification stages. There might even be filters and equalizers to shape and improve the information-bearing electrical signal.
The receiver schematic in Figure 3-19 shows a photodiode, bias resistor circuit, and a low-noise pre-amp. The output of the pre-amp is an electrical waveform version of the original information from the source. To the right of this pre-amp is an additional amplification, filters, and equalizers. All of these components can be on a single integrated circuit, a hybrid, or discretely mounted on a printed circuit board.
The receiver can incorporate a number of other functions, such as clock recovery for synchronous signaling, decoding circuitry, and error detection and recovery. The receiver must have high sensitivity so that it can detect low-level optical signals coming out of the fiber-optic cable. The higher the sensitivity, the more attenuated signals it can detect. It must have high bandwidth or a fast rise time so that it can respond fast enough and demodulate high-speed digital data. It must have low noise so that it does not significantly impact the BER of the link and counter the interference resistance of the fiber-optic cable transmission medium.
There are two types of photodiode structures: positive intrinsic negative (PIN) and the avalanche photodiode (APD). In most premise applications, the PIN is the preferred element in the receiver. This is mainly due to fact that it can be operated from a standard power supply, typically between 5 and 15V. APD devices have much better sensitivity. In fact, APD devices have 5 to 10 dB more sensitivity. They also have twice the bandwidth. However, they cannot be used on a 5V printed circuit board. They also require a stable power supply, which increases their cost. APD devices are usually found in long-haul communication links and can increasingly be found in metro-regional networks (because APDs have decreased in cost).
The demodulation performance of the receiver is characterized by the BER that it delivers to the user. The sensitivity curve indicates the minimum optical power that the receiver can detect compared to the data rate, to achieve a particular BER. The sensitivity curve varies from receiver to receiver. The sensitivity curve considers within it the SNR parameter that generally drives all communications-link performance. The sensitivity depends on the type of photodiode used and the wavelength of operation. Figure 3-20 shows sensitivity curve examples.
Figure 3-20 Receiver Sensitivity Curves
The quantum limit curve serves as a baseline reference. In a sense it represents optimum performance on the part of the photodiode in the receiver—that is, performance in which there is 100 percent efficiency in converting light from the fiber-optic cable into an electric current for demodulation. All other sensitivity curves are compared to the quantum limit.
Nonlinear effects occur at high bit rates and power levels. These effects must be mitigated using compensators, and a suitable budget allocation must be made during calculations.
The overall span loss, or link budget as it is sometimes called, can be determined by using an optical meter to measure true loss or by computing the loss of system components. The latter method considers the loss associated with span components, such as connectors, splices, patch panels, jumpers, and the optical safety margin. The safety margin sets aside 3 dB to compensate for component aging and repair work in event of fiber cut. Adding all of these factors to make sure their sum total is within the maximum attenuation figure ensures that the system will operate satisfactorily. Allowances must also be made for the type of splice, the age and condition of the fiber, equipment, and the environment (including temperature variations).
m multimode fiber-optic cable. Refer to Tables 3-1, 3-2, and 3-3 for appropriate attenuation, component, and nonlinear loss values.
Figure 3-21 MMF Span Analysis
The system operates at 155 Mbps or approximately 155 MHz. At such bit rates, there is no need to consider SPM, PMD, or SRS/SBS margin requirements. Because the link is a single-wavelength system, there is no need to include XPM or FWM margins. However, it is safe to consider the potential for a degree of chromatic dispersion, because chromatic dispersion occurs at all bit rates.
The use and demand for optical fiber has grown tremendously and optical-fiber applications are numerous. Telecommunication applications are widespread, ranging from global networks to desktop computers. These involve the transmission of voice, data, or video over distances of less than a meter to hundreds of kilometers, using one of a few standard fiber designs in one of several cable designs.
Carriers use optical fiber to carry plain old telephone service (POTS) across their nationwide networks. Local exchange carriers (LECs) use fiber to carry this same service between central office switches at local levels, and sometimes as far as the neighborhood or individual home (fiber to the home [FTTH]).
Optical fiber is also used extensively for transmission of data. Multinational firms need secure, reliable systems to transfer data and financial information between buildings to the desktop terminals or computers and to transfer data around the world. Cable television companies also use fiber for delivery of digital video and data services. The high bandwidth provided by fiber makes it the perfect choice for transmitting broadband signals, such as high-definition television (HDTV) telecasts.
Intelligent transportation systems, such as smart highways with intelligent traffic lights, automated tollbooths, and changeable message signs, also use fiber-optic-based telemetry systems.
Another important application for optical fiber is the biomedical industry. Fiber-optic systems are used in most modern telemedicine devices for transmission of digital diagnostic images. Other applications for optical fiber include space, military, automotive, and the industrial sector.
The Physics Behind Fiber Optics
A fiber-optic cable is composed of two concentric layers, called the core and the cladding, as illustrated in Figure 3-1. The core and cladding have different refractive indices, with the core having a refractive index of n1, and the cladding having a refractive index of n2. The index of refraction is a way of measuring the speed of light in a material. Light travels fastest in a vacuum. The actual speed of light in a vacuum is 300,000 kilometers per second, or 186,000 miles per second.Figure 3-1 Cross Section of a Fiber-Optic CableThe index of refraction is calculated by dividing the speed of light in a vacuum by the speed of light in another medium, as shown in the following formula:
Refractive index of the medium = [Speed of light in a vacuum/Speed of light
in the medium]The refractive index of the core, n1, is always greater than the index of the cladding, n2. Light is guided through the core, and the fiber acts as an optical waveguide.Figure 3-2 shows the propagation of light down the fiber-optic cable using the principle of total internal reflection. As illustrated, a light ray is injected into the fiber-optic cable on the left. If the light ray is injected and strikes the core-to-cladding interface at an angle greater than the critical angle with respect to the normal axis, it is reflected back into the core. Because the angle of incidence is always equal to the angle of reflection, the reflected light continues to be reflected. The light ray then continues bouncing down the length of the fiber-optic cable. If the angle of incidence at the core-to-cladding interface is less than the critical angle, both reflection and refraction take place. Because of refraction at each incidence on the interface, the light beam attenuates and dies off over a certain distance.
Figure 3-2 Total Internal ReflectionThe critical angle is fixed by the indices of refraction of the core and cladding and is computed using the following formula:
qc = cos–1 (n2/n1)The critical angle can be measured from the normal or cylindrical axis of the core. If n1 = 1.557 and n2 = 1.343, for example, the critical angle is 30.39 degrees.
Figure 3-2 shows a light ray entering the core from the outside air to the left of the cable. Light must enter the core from the air at an angle less than an entity known as the acceptance angle (
a):qa = sin–1 [(n1/n0) sin(qc)]In the formula, n0 is the refractive index of air and is equal to one. This angle is measured from the cylindrical axis of the core. In the preceding example, the acceptance angle is 51.96 degrees.
The optical fiber also has a numerical aperture (NA). The NA is given by the following formula:
NA = Sin qa = ?(n12 – n22)From a three-dimensional perspective, to ensure that the signals reflect and travel correctly through the core, the light must enter the core through an acceptance cone derived by rotating the acceptance angle about the cylindrical fiber axis. As illustrated in Figure 3-3, the size of the acceptance cone is a function of the refractive index difference between the core and the cladding. There is a maximum angle from the fiber axis at which light can enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine of this maximum angle is the NA of the fiber. The NA in the preceding example is 0.787. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. Single-mode fiber has a smaller NA than MMF.
Figure 3-3 Acceptance ConePerformance Considerations
The amount of light that can be coupled into the core through the external acceptance angle is directly proportional to the efficiency of the fiber-optic cable. The greater the amount of light that can be coupled into the core, the lower the bit error rate (BER), because more light reaches the receiver. The attenuation a light ray experiences in propagating down the core is inversely proportional to the efficiency of the optical cable because the lower the attenuation in propagating down the core, the lower the BER. This is because more light reaches the receiver. Also, the less chromatic dispersion realized in propagating down the core, the faster the signaling rate and the higher the end-to-end data rate from source to destination. The major factors that affect performance considerations described in this paragraph are the size of the fiber, the composition of the fiber, and the mode of propagation.Optical-Power Measurement
The power level in optical communications is of too wide a range to express on a linear scale. A logarithmic scale known as decibel (dB) is used to express power in optical communications.The wide range of power values makes decibel a convenient unit to express the power levels that are associated with an optical system. The gain of an amplifier or attenuation in fiber is expressed in decibels. The decibel does not give a magnitude of power, but it is a ratio of the output power to the input power.
Loss or gain = 10log10(POUTPUT/PINPUT)The decibel milliwatt (dBm) is the power level related to 1 milliwatt (mW). Transmitter power and receiver dynamic ranges are measured in dBm. A 1-mW signal has a level of 0 dBm.
Signals weaker than 1 mW have negative dBm values, whereas signals stronger than 1 mW have positive dBm values.
Optical-Cable Construction
The core is the highly refractive central region of an optical fiber through which light is transmitted. The standard telecommunications core diameter in use with SMF is between 8
m and 10 m, whereas the standard core diameter in use with MMF is between 50 m and 62.5 m. Figure 3-4 shows the core diameter for SMF and MMF cable. The diameter of the cladding surrounding each of these cores is 125 m. Core sizes of 85 m and 100 m were used in early applications, but are not typically used today. The core and cladding are manufactured together as a single solid component of glass with slightly different compositions and refractive indices. The third section of an optical fiber is the outer protective coating known as the coating. The coating is typically an ultraviolet (UV) light-cured acrylate applied during the manufacturing process to provide physical and environmental protection for the fiber. The buffer coating could also be constructed out of one or more layers of polymer, nonporous hard elastomers or high-performance PVC materials. The coating does not have any optical properties that might affect the propagation of light within the fiber-optic cable. During the installation process, this coating is stripped away from the cladding to allow proper termination to an optical transmission system. The coating size can vary, but the standard sizes are 250 m and 900 m. The 250-m coating takes less space in larger outdoor cables. The 900-m coating is larger and more suitable for smaller indoor cables.Figure 3-4 Optical-Cable ConstructionFiber-optic cable sizes are usually expressed by first giving the core size followed by the cladding size. Consequently, 50/125 indicates a core diameter of 50 microns and a cladding diameter of 125 microns, and 8/125 indicates a core diameter of 8 microns and a cladding diameter of 125 microns. The larger the core, the more light can be coupled into it from the external acceptance angle cone. However, larger-diameter cores can actually allow in too much light, which can cause receiver saturation problems. The 8/125 cable is often used when a fiber-optic data link operates with single-mode propagation, whereas the 62.5/125 cable is often used in a fiber-optic data link that operates with multimode propagation.
Three types of material make up fiber-optic cables:
- Glass
- Plastic
- Plastic-clad silica (PCS)
Glass Fiber-Optic Cable
Glass fiber-optic cable has the lowest attenuation. A pure-glass, fiber-optic cable has a glass core and a glass cladding. This cable type has, by far, the most widespread use. It has been the most popular with link installers, and it is the type of cable with which installers have the most experience. The glass used in a fiber-optic cable is ultra-pure, ultra-transparent, silicon dioxide, or fused quartz. During the glass fiber-optic cable fabrication process, impurities are purposely added to the pure glass to obtain the desired indices of refraction needed to guide light. Germanium, titanium, or phosphorous is added to increase the index of refraction. Boron or fluorine is added to decrease the index of refraction. Other impurities might somehow remain in the glass cable after fabrication. These residual impurities can increase the attenuation by either scattering or absorbing light.Plastic Fiber-Optic Cable
Plastic fiber-optic cable has the highest attenuation among the three types of cable. Plastic fiber-optic cable has a plastic core and cladding. This fiber-optic cable is quite thick. Typical dimensions are 480/500, 735/750, and 980/1000. The core generally consists of polymethylmethacrylate (PMMA) coated with a fluropolymer. Plastic fiber-optic cable was pioneered principally for use in the automotive industry. The higher attenuation relative to glass might not be a serious obstacle with the short cable runs often required in premise data networks. The cost advantage of plastic fiber-optic cable is of interest to network architects when they are faced with budget decisions. Plastic fiber-optic cable does have a problem with flammability. Because of this, it might not be appropriate for certain environments and care has to be taken when it is run through a plenum. Otherwise, plastic fiber is considered extremely rugged with a tight bend radius and the capability to withstand abuse.Plastic-Clad Silica (PCS) Fiber-Optic Cable
The attenuation of PCS fiber-optic cable falls between that of glass and plastic. PCS fiber-optic cable has a glass core, which is often vitreous silica, and the cladding is plastic, usually a silicone elastomer with a lower refractive index. PCS fabricated with a silicone elastomer cladding suffers from three major defects. First, it has considerable plasticity, which makes connector application difficult. Second, adhesive bonding is not possible. And third, it is practically insoluble in organic solvents. These three factors keep this type of fiber-optic cable from being particularly popular with link installers. However, some improvements have been made in recent years.NOTE
For data center premise cables, the jacket color depends on the fiber type in the cable. For cables containing SMFs, the jacket color is typically yellow, whereas for cables containing MMFs, the jacket color is typically orange. For outside plant cables, the standard jacket color is typically black.
Multifiber Cable Systems
Multifiber systems are constructed with strength members that resist crushing during cable pulling and bends. The outer cable jackets are OFNR (riser rated), OFNP (plenum rated), or LSZH (low-smoke, zero-halogen rated). The OFNR outer jackets are composed of flame-retardant PVC or fluoropolymers. The OFNP jackets are composed of plenum PVC, whereas the LSZH jackets are halogen-free and constructed out of polyolefin compounds. Figure 3-5 shows a multiribbon, 24-fiber, ribbon-cable system. Ribbon cables are extensively used for inside plant and datacenter applications. Individual ribbon subunit cables use the MTP/MPO connector assemblies. Ribbon cables have a flat ribbon-like structure that enables installers to save conduit space as they install more cables in a particular conduit.Figure 3-5 Inside Plant Ribbon-Cable SystemFigure 3-6 shows a typical six-fiber, inside-plant cable system. The central core is composed of a dielectric strength member with a dielectric jacket. The individual fibers are positioned around the dielectric strength member. The individual fibers have a strippable buffer coating. Typically, the strippable buffer is a 900-
m tight buffer. Each individual coated fiber is surrounded with a subunit jacket. Aramid yarn strength members surround the individual subunits. Some cable systems have an outer strength member that provides protection to the entire enclosed fiber system. Kevlar is a typical material used for constructing the outer strength member for premise cable systems. The outer jacket is OFNP, OFNR, or LSZH.Figure 3-6 Cross Section of Inside-Plant CablesFigure 3-7 shows a typical armored outside-plant cable system. The central core is composed of a dielectric with a dielectric jacket or steel strength member. The individual gel-filled subunit buffer tubes are positioned around the central strength member. Within the subunit buffer tube, six fibers are positioned around an optional dielectric strength member. The individual fibers have a strippable buffer coating. All six subunit buffer tubes are enclosed within a binder that contains an interstitial filling or water-blocking compound. An outer strength member, typically constructed of aramid Kevlar strength members encloses the binder. The outer strength member is surrounded by an inner medium-density polyethylene (MDPE) jacket. The corrugated steel armor layer between the outer high-density polyethylene (HDPE) jacket, and the inner MDPE jacket acts as an external strength member and provides physical protection. Conventional deep-water submarine cables use dual armor and a special hermetically sealed copper tube to protect the fibers from the effects of deep-water environments. However, shallow-water applications use cables similar to those shown in Figure 3-7 with an asphalt compound interstitial filling.
Fiber-Optic Characteristics
Optical-fiber systems have many advantages over metallic-based communication systems. These advantages include interference, attenuation, and bandwidth characteristics. Furthermore, the relatively smaller cross section of fiber-optic cables allows room for substantial growth of the capacity in existing conduits. Fiber-optic characteristics can be classified as linear and nonlinear. Nonlinear characteristics are influenced by parameters, such as bit rates, channel spacing, and power levels.
Interference
Light signals traveling via a fiber-optic cable are immune from electromagnetic interference (EMI) and radio-frequency interference (RFI). Lightning and high-voltage interference is also eliminated. A fiber network is best for conditions in which EMI or RFI interference is heavy or safe operation free from sparks and static is a must. This desirable property of fiber-optic cable makes it the medium of choice in industrial and biomedical networks. It is also possible to place fiber cable into natural-gas pipelines and use the pipelines as the conduit.Linear Characteristics
Linear characteristics include attenuation, chromatic dispersion (CD), polarization mode dispersion (PMD), and optical signal-to-noise ratio (OSNR).Attenuation
Several factors can cause attenuation, but it is generally categorized as either intrinsic or extrinsic. Intrinsic attenuation is caused by substances inherently present in the fiber, whereas extrinsic attenuation is caused by external forces such as bending. The attenuation coefficient α is expressed in decibels per kilometer and represents the loss in decibels per kilometer of fiber.Intrinsic Attenuation
Intrinsic attenuation results from materials inherent to the fiber. It is caused by impurities in the glass during the manufacturing process. As precise as manufacturing is, there is no way to eliminate all impurities. When a light signal hits an impurity in the fiber, one of two things occurs: It scatters or it is absorbed. Intrinsic loss can be further characterized by two components:- Material absorption
- Rayleigh scattering
Figure 3-12 Attenuation Versus WavelengthThe OH- symbols indicate that at the 950-nm, 1380-nm, and 2730-nm wavelengths, the presence of hydroxyl radicals in the cable material causes an increase in attenuation. These radicals result from the presence of water remnants that enter the fiber-optic cable material through either a chemical reaction in the manufacturing process or as humidity in the environment. The variation of attenuation with wavelength due to the water peak for standard, single-mode fiber-optic cable occurs mainly around 1380 nm. Recent advances in manufacturing have overcome the 1380-nm water peak and have resulted in zero-water-peak fiber (ZWPF). Examples of these fibers include SMF-28e from Corning and the Furukawa-Lucent OFS AllWave. Absorption accounts for three percent to five percent of fiber attenuation. This phenomenon causes a light signal to be absorbed by natural impurities in the glass and converted to vibration energy or some other form of energy such as heat. Unlike scattering, absorption can be limited by controlling the amount of impurities during the manufacturing process. Because most fiber is extremely pure, the fiber does not heat up because of absorption.
Rayleigh Scattering@As light travels in the core, it interacts with the silica molecules in the core. Rayleigh scattering is the result of these elastic collisions between the light wave and the silica molecules in the fiber. Rayleigh scattering accounts for about 96 percent of attenuation in optical fiber. If the scattered light maintains an angle that supports forward travel within the core, no attenuation occurs. If the light is scattered at an angle that does not support continued forward travel, however, the light is diverted out of the core and attenuation occurs. Depending on the incident angle, some portion of the light propagates forward and the other part deviates out of the propagation path and escapes from the fiber core. Some scattered light is reflected back toward the light source. This is a property that is used in an optical time domain reflectometer (OTDR) to test fibers. The same principle applies to analyzing loss associated with localized events in the fiber, such as splices.
Short wavelengths are scattered more than longer wavelengths. Any wavelength that is below 800 nm is unusable for optical communication because attenuation due to Rayleigh scattering is high. At the same time, propagation above 1700 nm is not possible due to high losses resulting from infrared absorption.
Extrinsic Attenuation
Extrinsic attenuation can be caused by two external mechanisms: macrobending or microbending. Both cause a reduction of optical power. If a bend is imposed on an optical fiber, strain is placed on the fiber along the region that is bent. The bending strain affects the refractive index and the critical angle of the light ray in that specific area. As a result, light traveling in the core can refract out, and loss occurs.A macrobend is a large-scale bend that is visible, and the loss is generally reversible after bends are corrected. To prevent macrobends, all optical fiber has a minimum bend radius specification that should not be exceeded. This is a restriction on how much bend a fiber can withstand before experiencing problems in optical performance or mechanical reliability.
The second extrinsic cause of attenuation is a microbend. Microbending is caused by imperfections in the cylindrical geometry of fiber during the manufacturing process. Microbending might be related to temperature, tensile stress, or crushing force. Like macrobending, microbending causes a reduction of optical power in the glass. Microbending is very localized, and the bend might not be clearly visible on inspection. With bare fiber, microbending can be reversible.
Chromatic Dispersion
Chromatic dispersion is the spreading of a light pulse as it travels down a fiber. Light has a dual nature and can be considered from an electromagnetic wave as well as quantum perspective. This enables us to quantify it as waves as well as quantum particles. During the propagation of light, all of its spectral components propagate accordingly. These spectral components travel at different group velocities that lead to dispersion called group velocity dispersion (GVD). Dispersion resulting from GVD is termed chromatic dispersion due to its wavelength dependence. The effect of chromatic dispersion is pulse spread.As the pulses spread, or broaden, they tend to overlap and are no longer distinguishable by the receiver as 0s and 1s. Light pulses launched close together (high data rates) that spread too much (high dispersion) result in errors and loss of information. Chromatic dispersion occurs as a result of the range of wavelengths present in the light source. Light from lasers and LEDs consists of a range of wavelengths, each of which travels at a slightly different speed. Over distance, the varying wavelength speeds cause the light pulse to spread in time. This is of most importance in single-mode applications. Modal dispersion is significant in multimode applications, in which the various modes of light traveling down the fiber arrive at the receiver at different times, causing a spreading effect. Chromatic dispersion is common at all bit rates. Chromatic dispersion can be compensated for or mitigated through the use of dispersion-shifted fiber (DSF). DSF is fiber doped with impurities that have negative dispersion characteristics. Chromatic dispersion is measured in ps/nm-km. A 1-dB power margin is typically reserved to account for the effects of chromatic dispersion.
Polarization Mode Dispersion
Polarization mode dispersion (PMD) is caused by asymmetric distortions to the fiber from a perfect cylindrical geometry. The fiber is not truly a cylindrical waveguide, but it can be best described as an imperfect cylinder with physical dimensions that are not perfectly constant. The mechanical stress exerted upon the fiber due to extrinsically induced bends and stresses caused during cabling, deployment, and splicing as well as the imperfections resulting from the manufacturing process are the reasons for the variations in the cylindrical geometry.Single-mode optical fiber and components support one fundamental mode, which consists of two orthogonal polarization modes. This asymmetry introduces small refractive index differences for the two polarization states. This characteristic is known asbirefringence. Birefringence causes one polarization mode to travel faster than the other, resulting in a difference in the propagation time, which is called the differential group delay (DGD). DGD is the unit that is used to describe PMD. DGD is typically measured in picoseconds. A fiber that acquires birefringence causes a propagating pulse to lose the balance between the polarization components. This leads to a stage in which different polarization components travel at different velocities, creating a pulse spread as shown in Figure 3-13. PMD can be classified as first-order PMD, also known as DGD, and second-order PMD (SOPMD). The SOPMD results from dispersion that occurs because of the signal's wavelength dependence and spectral width.
PMD is not an issue at low bit rates but becomes an issue at bit rates in excess of 5 Gbps. PMD is noticeable at high bit rates and is a significant source of impairment for ultra-long-haul systems. PMD compensation can be achieved by using PMD compensators that contain dispersion-maintaining fibers with degrees of birefringence in them. The introduced birefringence negates the effects of PMD over a length of transmission. For error-free transmission, PMD compensation is a useful technique for long-haul and metropolitan-area networks running at bit rates greater than 10 Gbps. Note in Figure 3-13 that the DGD is the difference between Z1 and Z2. The PMD value of the fiber is the mean value over time or frequency of the DGD and is represented as ps/ km. A 0.5-dB power margin is typically reserved to account for the effects of PMD at high bit rates.
Figure 3-13 Polarization Mode DispersionPolarization Dependent Loss
Polarization dependent loss (PDL) refers to the difference in the maximum and minimum variation in transmission or insertion loss of an optical device over all states of polarization (SOP) and is expressed in decibels. A typical PDL for a simple optical connector is less than .05 dB and varies from component to component. Typically, the PDL for an optical add/drop multiplexer (OADM) is around 0.3 dB. The complete polarization characterization of optical signals and components can be determined using an optical polarization analyzer.Optical Signal-to-Noise Ratio
The optical signal-to-noise ratio (OSNR) specifies the ratio of the net signal power to the net noise power and thus identifies the quality of the signal. Attenuation can be compensated for by amplifying the optical signal. However, optical amplifiers amplify the signal as well as the noise. Over time and distance, the receivers cannot distinguish the signal from the noise, and the signal is completely lost. Regeneration helps mitigate these undesirable effects before they can render the system unusable and ensures that the signal can be detected at the receiver. Optical amplifiers add a certain amount of noise to the channel. Active devices, such as lasers, also add noise. Passive devices, such as taps and the fiber, can also add noise components. In the calculation of system design, however, optical amplifier noise is considered the predominant source for OSNR penalty and degradation.OSNR is an important and fundamental system design consideration. Another parameter considered by designers is the Q-factor. The Q-factor, a function of the OSNR, provides a qualitative description of the receiver performance. The Q-factor suggests the minimum signal-to-noise ratio (SNR) required to obtain a specific BER for a given signal. OSNR is measured in decibels. The higher the bit rate, the higher the OSNR ratio required. For OC-192 transmissions, the OSNR should be at least 27 to 31 dB compared to 18 to 21 dB for OC-48.
Nonlinear Characteristics
Nonlinear characteristics include self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS).Self-Phase Modulation
Phase modulation of an optical signal by itself is known as self-phase modulation (SPM). SPM is primarily due to the self-modulation of the pulses. Generally, SPM occurs in single-wavelength systems. At high bit rates, however, SPM tends to cancel dispersion. SPM increases with high signal power levels. In fiber plant design, a strong input signal helps overcome linear attenuation and dispersion losses. However, consideration must be given to receiver saturation and to nonlinear effects such as SPM, which occurs with high signal levels. SPM results in phase shift and a nonlinear pulse spread. As the pulses spread, they tend to overlap and are no longer distinguishable by the receiver. The acceptable norm in system design to counter the SPM effect is to take into account a power penalty that can be assumed equal to the negative effect posed by XPM. A 0.5-dB power margin is typically reserved to account for the effects of SPM at high bit rates and power levels.Cross-Phase Modulation
Cross-phase modulation (XPM) is a nonlinear effect that limits system performance in wavelength-division multiplexed (WDM) systems. XPM is the phase modulation of a signal caused by an adjacent signal within the same fiber. XPM is related to the combination (dispersion/effective area). CPM results from the different carrier frequencies of independent channels, including the associated phase shifts on one another. The induced phase shift is due to the walkover effect, whereby two pulses at different bit rates or with different group velocities walk across each other. As a result, the slower pulse sees the walkover and induces a phase shift. The total phase shift depends on the net power of all the channels and on the bit output of the channels. Maximum phase shift is produced when bits belonging to high-powered adjacent channels walk across each other.XPM can be mitigated by carefully selecting unequal bit rates for adjacent WDM channels. XPM, in particular, is severe in long-haul WDM networks, and the acceptable norm in system design to counter this effect is to take into account a power penalty that can be assumed equal to the negative effect posed by XPM. A 0.5-dB power margin is typically reserved to account for the effects of XPM in WDM fiber systems.
Four-Wave Mixing
FWM can be compared to the intermodulation distortion in standard electrical systems. When three wavelengths (λ1, λ 2, and λ 3) interact in a nonlinear medium, they give rise to a fourth wavelength (λ 4), which is formed by the scattering of the three incident photons, producing the fourth photon. This effect is known as four-wave mixing (FWM)and is a fiber-optic characteristic that affects WDM systems.The effects of FWM are pronounced with decreased channel spacing of wavelengths and at high signal power levels. High chromatic dispersion also increases FWM effects. FWM also causes interchannel cross-talk effects for equally spaced WDM channels. FWM can be mitigated by using uneven channel spacing in WDM systems or nonzero dispersion-shifted fiber (NZDSF). A 0.5-dB power margin is typically reserved to account for the effects of FWM in WDM systems.
Stimulated Raman Scattering
When light propagates through a medium, the photons interact with silica molecules during propagation. The photons also interact with themselves and cause scattering effects, such as stimulated Raman scattering (SRS), in the forward and reverse directions of propagation along the fiber. This results in a sporadic distribution of energy in a random direction.SRS refers to lower wavelengths pumping up the amplitude of higher wavelengths, which results in the higher wavelengths suppressing signals from the lower wavelengths. One way to mitigate the effects of SRS is to lower the input power. In SRS, a low-wavelength wave called Stoke's wave is generated due to the scattering of energy. This wave amplifies the higher wavelengths. The gain obtained by using such a wave forms the basis of Raman amplification. The Raman gain can extend most of the operating band (C- and L-band) for WDM networks. SRS is pronounced at high bit rates and high power levels. The margin design requirement to account for SRS/SBS is 0.5 dB.
Stimulated Brillouin Scattering
Stimulated Brillouin scattering (SBS) is due to the acoustic properties of photon interaction with the medium. When light propagates through a medium, the photons interact with silica molecules during propagation. The photons also interact with themselves and cause scattering effects such as SBS in the reverse direction of propagation along the fiber. In SBS, a low-wavelength wave called Stoke's wave is generated due to the scattering of energy. This wave amplifies the higher wavelengths. The gain obtained by using such a wave forms the basis of Brillouin amplification. The Brillouin gain peaks in a narrow peak near the C-band. SBS is pronounced at high bit rates and high power levels. The margin design requirement to account for SRS/SBS is 0.5 dB.Splicing
Fiber-optic cables might have to be spliced together for a number of reasons—for example, to realize a link of a particular length. Another reason might involve backhoe fade, in which case a fiber-optic cable might have been ripped apart due to trenching work. The network installer might have in his inventory several fiber-optic cables, but none long enough to satisfy the required link length. Situations such as this often arise because cable manufacturers offer cables in limited lengths—usually 1 to 6 km. A link of 10 km can be installed by splicing several fiber-optic cables together. The installer can then satisfy the distance requirement and avoid buying a new fiber-optic cable. Splices might be required at building entrances, wiring closets, couplers, and literally any intermediate point between a transmitter and receiver.Connecting two fiber-optic cables requires precise alignment of the mated fiber cores or spots in a single-mode fiber-optic cable. This is required so that nearly all the light is coupled from one fiber-optic cable across a junction to the other fiber-optic cable. Actual contact between the fiber-optic cables is not even mandatory.
There are two principal types of splices: fusion and mechanical. Fusion splices use an electric arc to weld two fiber-optic cables together. The process of fusion splicing involves using localized heat to melt or fuse the ends of two optical fibers together. The splicing process begins by preparing each fiber end for fusion. Fusion splicing requires that all protective coatings be removed from the ends of each fiber. The fiber is then cleaved using the score-and-break method. The quality of each fiber end is inspected using a microscope. In fusion splicing, splice loss is a direct function of the angles and quality of the two fiber-end faces.
The basic fusion-splicing apparatus consists of two fixtures on which the fibers are mounted with two electrodes. An inspection microscope assists in the placement of the prepared fiber ends into a fusion-splicing apparatus. The fibers are placed into the apparatus, aligned, and then fused together. Initially, fusion splicing used nichrome wire as the heating element to melt or fuse fibers together. New fusion-splicing techniques have replaced the nichrome wire with carbon dioxide (CO2) lasers, electric arcs, or gas flames to heat the fiber ends, causing them to fuse together. Arc fusion splicers can splice single fibers or 12- and 24-fiber-count ribbon fibers at the same time. The small size of the fusion splice and the development of automated fusion-splicing machines have made electric arc fusion one of the most popular splicing techniques in commercial applications. The splices offer sophisticated, computer-controlled alignment of fiber-optic cables to achieve losses as low as 0.02 dB.
Splices can also be used as optical attenuators if there is a need to attenuate a high-powered signal. Splice losses of up to 10.0 dB can be programmed and inserted into the cable if desired. This way, the splice can act as an in-line attenuator with the characteristic nonreflectance of a fusion splice. Typical fusion-splice losses can be estimated at 0.02 dB for loss-budget calculation purposes. Mechanical splices are easily implemented in the field, require little or no tooling, and offer losses of about 0.5 to 0.75 dB.
Fiber-Optic Communications System
As depicted in Figure 3-16, information (voice, data, and video) from the source is encoded into electrical signals that can drive the transmitter. The fiber acts as an optical waveguide for the photons as they travel down the optical path toward the receiver. At the detector, the signals undergo an optical-to-electrical (OE) conversion, are decoded, and are sent to their destination.Figure 3-16 Fiber-Optic Communication SystemTransmitter
The transmitter component of Figure 3-16 serves two functions. First, it must be a source of the light launched into the fiber-optic cable. Second, it must modulate this light to represent the binary data that it receives from the source. A transmitter's physical dimensions must be compatible with the size of the fiber-optic cable being used. This means that the transmitter must emit light in a cone with a cross-sectional diameter of 8 to 100 microns; otherwise, it cannot be coupled into the fiber-optic cable. The optical source must be able to generate enough optical power so that the desired BER can be met over the optical path. There should be high efficiency in coupling the light generated by the optical source into the fiber-optic cable, and the optical source should have sufficient linearity to prevent the generation of harmonics and intermodulation distortion. If such interference is generated, it is extremely difficult to remove. This would cancel the interference resistance benefits of the fiber-optic cable. The optical source must be easily modulated with an electrical signal and must be capable of high-speed modulation; otherwise, the bandwidth benefits of the fiber-optic cable are lost. Finally, there are the usual requirements of small size, low weight, low cost, and high reliability. The transmitter is typically pulsed at the incoming frequency and performs a transducer electrical-to-optical (EO) conversion. Light-emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs) are used to drive MMF systems, whereas laser diodes are used to drive SMF systems. Two types of light-emitting junction diodes can be used as the optical source of the transmitter. These are the LED and the laser diode (LD). LEDs are simpler and generate incoherent, lower-power light. LEDs are used to drive MMF. LDs generate coherent, higher-power light and are used to drive SMF.Figure 3-17 shows the optical power output, P, from each of these devices as a function of the electrical current input, I, from the modulation circuitry. As the figure indicates, the LED has a relatively linear P-I characteristic, whereas the LD has a strong nonlinearity or threshold effect. The LD can also be prone to kinks when the power actually decreases with increasing input current. LDs have advantages over LEDs in the sense that they can be modulated at very high speeds, produce greater optical power, and produce an output beam with much less spatial width than an LED. This gives LDs higher coupling efficiency to the fiber-optic cable. LED advantages include a higher reliability, better linearity, and lower cost.
Figure 3-17 LED and LD P-I CharacteristicsA key difference between the optical output of an LED and a LD is the wavelength spread over which the optical power is distributed. The spectral width, σ, is the 3-dB optical power width (measured in nanometers or microns). The spectral width impacts the effective transmitted signal bandwidth. A larger spectral width takes up a larger portion of the fiber-optic cable link bandwidth. Figure 3-18 shows the spectral width of the two devices. The optical power generated by each device is the area under the curve. The spectral width is the half-power spread. An LD always has a smaller spectral width than an LED. The specific value of the spectral width depends on the details of the diode structure and the semiconductor material. However, typical values for an LED are around 40 nm for operation at 850 nm and 80 nm at 1310 nm. Typical values for an LD are 1 nm for operation at 850 nm and 3 nm at 1310 nm.
Figure 3-18 LED and LD Spectral WidthsOther transmitter parameters include packaging, environmental sensitivity of device characteristics, heat sinking, and reliability. With either an LED or LD, the transmitter package must have a transparent window to transmit light into the fiber-optic cable. It can be packaged with either a fiber-optic cable pigtail or with a transparent plastic or glass window. Some vendors supply the transmitter with a package having a small hemispherical lens to help focus the light into the fiber-optic cable. Packaging must also address the thermal coupling for the LED or LD. A complete transmitter module can consume more than 1 watt, which could result in significant heat generation. Plastic packages can be used for lower-speed and lower-reliability applications. However, high-speed and high-reliability transmitters need metal packaging with built-in fins for heat sinking.
There are several different schemes for carrying out the modulation function. These include intensity modulation (IM), frequency shift keying (FSK), phase shift keying (PSK), and polarization modulation (PM). Within the context of a premise fiber-optic data link, the only one really used is IM. IM is used universally for premise fiber-optic data links because it is well matched to the operation of both LEDs and LDs. The carrier that each of these sources produces is easy to modulate with this technique. Passing current through them operates both of these devices. The amount of power that they radiate (sometimes referred to as the radiance) is proportional to this current. In this way, the optical power takes the shape of the input current. If the input current is the waveform m(t) representing the binary information stream, the resulting optical signal looks like bursts of optical signal when m(t) represents a 1 and the absence of optical signal when m(t) represents a 0. This is also known as direct modulation of the LED or LD.
Receiver
Figure 3-19 shows a schematic of an optical receiver. The receiver serves two functions: It must sense or detect the light coupled out of the fiber-optic cable and convert the light into an electrical signal, and it must demodulate this light to determine the identity of the binary data that it represents. The receiver performs the OE transducer function.Figure 3-19 Schematic of an Optical ReceiverA receiver is generally designed with a transmitter. Both are modules within the same package. The light detection is carried out by a photodiode, which senses light and converts it into an electrical current. However, the optical signal from the fiber-optic cable and the resulting electrical current will have a small amplitude. Consequently, the photodiode circuitry must be followed by one or more amplification stages. There might even be filters and equalizers to shape and improve the information-bearing electrical signal.
The receiver schematic in Figure 3-19 shows a photodiode, bias resistor circuit, and a low-noise pre-amp. The output of the pre-amp is an electrical waveform version of the original information from the source. To the right of this pre-amp is an additional amplification, filters, and equalizers. All of these components can be on a single integrated circuit, a hybrid, or discretely mounted on a printed circuit board.
The receiver can incorporate a number of other functions, such as clock recovery for synchronous signaling, decoding circuitry, and error detection and recovery. The receiver must have high sensitivity so that it can detect low-level optical signals coming out of the fiber-optic cable. The higher the sensitivity, the more attenuated signals it can detect. It must have high bandwidth or a fast rise time so that it can respond fast enough and demodulate high-speed digital data. It must have low noise so that it does not significantly impact the BER of the link and counter the interference resistance of the fiber-optic cable transmission medium.
There are two types of photodiode structures: positive intrinsic negative (PIN) and the avalanche photodiode (APD). In most premise applications, the PIN is the preferred element in the receiver. This is mainly due to fact that it can be operated from a standard power supply, typically between 5 and 15V. APD devices have much better sensitivity. In fact, APD devices have 5 to 10 dB more sensitivity. They also have twice the bandwidth. However, they cannot be used on a 5V printed circuit board. They also require a stable power supply, which increases their cost. APD devices are usually found in long-haul communication links and can increasingly be found in metro-regional networks (because APDs have decreased in cost).
The demodulation performance of the receiver is characterized by the BER that it delivers to the user. The sensitivity curve indicates the minimum optical power that the receiver can detect compared to the data rate, to achieve a particular BER. The sensitivity curve varies from receiver to receiver. The sensitivity curve considers within it the SNR parameter that generally drives all communications-link performance. The sensitivity depends on the type of photodiode used and the wavelength of operation. Figure 3-20 shows sensitivity curve examples.
Figure 3-20 Receiver Sensitivity CurvesThe quantum limit curve serves as a baseline reference. In a sense it represents optimum performance on the part of the photodiode in the receiver—that is, performance in which there is 100 percent efficiency in converting light from the fiber-optic cable into an electric current for demodulation. All other sensitivity curves are compared to the quantum limit.
Fiber Span Analysis
Span analysis is the calculation and verification of a fiber-optic system's operating characteristics. This encompasses items such as fiber routing, electronics, wavelengths, fiber type, and circuit length. Attenuation and nonlinear considerations are the key parameters for loss-budget analysis. Before implementing or designing a fiber-optic circuit, a span analysis is recommended to make certain the system will work over the proposed link. Both the passive and active components of the circuit have to be included in the loss-budget calculation. Passive loss is made up of fiber loss, connector loss, splice loss, and losses involved with couplers or splitters in the link. Active components are system gain, wavelength, transmitter power, receiver sensitivity, and dynamic range.Nonlinear effects occur at high bit rates and power levels. These effects must be mitigated using compensators, and a suitable budget allocation must be made during calculations.
The overall span loss, or link budget as it is sometimes called, can be determined by using an optical meter to measure true loss or by computing the loss of system components. The latter method considers the loss associated with span components, such as connectors, splices, patch panels, jumpers, and the optical safety margin. The safety margin sets aside 3 dB to compensate for component aging and repair work in event of fiber cut. Adding all of these factors to make sure their sum total is within the maximum attenuation figure ensures that the system will operate satisfactorily. Allowances must also be made for the type of splice, the age and condition of the fiber, equipment, and the environment (including temperature variations).
NOTE
Considerations for temperature effects associated with most fibers usually yield ?1 dB that could be optionally included in optical loss-budget calculations.
Transmitter Launch Power
Power measured in dBm at a particular wavelength generated by the transmitter LED or LD used to launch the signal is known as the transmitter launch power. Generally speaking, the higher the transmitter launch power, the better. However, one must be wary of receiver saturation, which occurs when the received signal has a very high power content and is not within the receiver's dynamic range. If the signal strength is not within the receiver's dynamic range, the receiver cannot decipher the signal and perform an OE conversion. High launch powers can offset attenuation, but they can cause nonlinear effects in the fiber and degrade system performance, especially at high bit rates.Receiver Sensitivity and Dynamic Range
Receiver sensitivity and dynamic range are the minimum acceptable value of received power needed to achieve an acceptable BER or performance. Receiver sensitivity takes into account power penalties caused by use of a transmitter with worst-case values of extinction ratio, jitter, pulse rise times and fall times, optical return loss, receiver connector degradations, and measurement tolerances. The receiver sensitivity does not include power penalties associated with dispersion or with back reflections from the optical path. These effects are specified separately in the allocation of maximum optical path penalty. Sensitivity usually takes into account worst-case operating and end-of-life (EOL) conditions. Receivers have to cope with optical inputs as high as –5 dBm and as low as –30 dBm. Or stated differently, the receiver needs an optical dynamic range of 25 dB.Power Budget and Margin Calculations
To ensure that the fiber system has sufficient power for correct operation, you need to calculate the span's power budget, which is the maximum amount of power it can transmit. From a design perspective, worst-case analysis calls for assuming minimum transmitter power and minimum receiver sensitivity. This provides for a margin that compensates for variations of transmitter power and receiver sensitivity levels.Power budget (PB) = Minimum transmitter power (PTMIN) – Minimum receiver sensitivity (PRMIN)You can calculate the span losses by adding the various linear and nonlinear losses. Factors that can cause span or link loss include fiber attenuation, splice attenuation, connector attenuation, chromatic dispersion, and other linear and nonlinear losses. Table 3-1 provides typical attenuation characteristics of various kinds of fiber-optic cables. Table 3-2 provides typical insertion losses for various connectors and splices. Table 3-3 provides the margin requirement for nonlinear losses along with their usage criteria. For information about the actual amount of signal loss caused by equipment and other factors, refer to vendor documentation.
Span loss (PS) = (Fiber attenuation * km) + (Splice attenuation * Number of splices) + (Connector attenuation * Number of connectors) + (In-line device losses) + (Nonlinear losses) + (Safety margin)
Case 1: MMF Span Analysis
Consider the fiber-optic system shown in Figure 3-21 operating at OC-3 (155 Mbps). The minimum optical transmitter launch power is –12.5 dBm, and the maximum optical transmitter launch power is –2 dBm at 1310 nm. The minimum receiver sensitivity is –30 dBm, and the maximum receiver sensitivity is –3 dBm at 1310 nm. The example assumes inclusion of two patch panels in the path, two mechanical splices, with the system operating over 2 km of graded index 50/125-m multimode fiber-optic cable. Refer to Tables 3-1, 3-2, and 3-3 for appropriate attenuation, component, and nonlinear loss values.Figure 3-21 MMF Span AnalysisThe system operates at 155 Mbps or approximately 155 MHz. At such bit rates, there is no need to consider SPM, PMD, or SRS/SBS margin requirements. Because the link is a single-wavelength system, there is no need to include XPM or FWM margins. However, it is safe to consider the potential for a degree of chromatic dispersion, because chromatic dispersion occurs at all bit rates.
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