Transmitting Video Via Fiber Optics
May 1, 2002 12:00 PM,
Alvin G. Sydnor
Communicating with light has been a dream at least since AlexanderGraham Bell conceived the idea in the 1870s. A century later, in June1977, Bell Telephone Company began the commercial evaluation of fiberoptic communications, at that time called light-wavecommunications.
In the company’s system, messages were coded into pulses of lightand transmitted through hair-thin glass fibers. The new system carriedvoice and data as well as video signals over two miles of undergroundcable that connected two switching offices of the Illinois BellTelephone Company and a commercial office building in downtownChicago.
In the 1990s, long-distance telecommunications had made thetransition to glass as fiber optics surpassed the carrying capacity ofcoaxial cable. As the demand for bandwidth increases, there is agrowing demand for multiple-channel transmission, that is, bundlingmany channels into a single cable. We can also expect increasing demandfor carrying capacity within buildings; historically, bandwidthrequirements have doubled every two years, and installed fiber-opticsystems are poised to meet that trend. That means sound and videocontractors installing and servicing high-quality video systems arelikely to see the applicability of fiber-optic technology rise.Therefore it is useful to look at the basic concepts and components offiber-optic systems.
to illustrate the concept of fiber-optic cable, imagine a pipe whoseinterior is lined with mirrors. If you shine a flashlight down thepipe, most of the light will go through the pipe and come out the otherend — even if the pipe is bent into right angles, because themirrored walls will bounce the light around the bends. But some lightwill be lost along its path because of the small size of the light beamcompared with the diameter of the pipe. The light has a lot of room tobounce around in the mirrored walls of the pipe, and as light wavesbounce, they interfere with each other, thus dissipating their energiesduring the collisions. In order to be functional, fiber optic cablemust be small enough to prevent the light from bouncing too much. Theword small means microns. (One micron equals one millionth of ameter or 0.00004 inches.) Figure 1 shows a cross section of a six-fibercable.
Each optical fiber consists of a core plus cladding layers, eachwith a different index of refraction. Those two layers determine thatcable’s numerical aperture and are generally designed to optimize it. Alarge aperture allows light rays to enter the fiber at greater anglesto the fiber’s center.
Fiber-optic cables use either single-mode fiber or multimode fiber.The core in the single-mode fiber promotes direct travel of one lightray. The core in the multimode fiber, constructed of differentmaterials, allows multiple rays to bounce inside the core. Single-modefiber provides the ultimate in bandwidth, and it must be used withlaser light sources.
- FIBER TYPESA light ray propagates through a fiber in a limited number of ways.Light guided by a fiber may be separated into the modes of propagationthat are permitted in that particular fiber. Smaller diameter fibersallow fewer modes. Fibers can be classified into three categories bythe way light is propagated through them: single-mode step index,multimode step index and multimode graded index (see Figure 2).A single-mode fiber allows only one mode of light to propagate. Thefiber has a very small core with a diameter on the order of onewavelength of the light frequency the fiber can carry. Single-modepropagation will allow a large light signal bandwidth and a highcarrying capacity, but practical difficulties exist in coupling lightenergy into such small fibers.Multimode step index fiber will accommodate many propagation modes.Coupling light into multimode fiber is easier than coupling light intosingle-mode fiber because the fiber core can be wider, but thepropagation modes have different velocities within the fiber. Thesediffering velocities cause dispersions or pulse spreading, an effectanalogous to a high-frequency signal being passed through a low-passfilter. Sharp waveforms become rounded and spread out, so dispersionlimits the bandwidth of the signals that can be guided.In multimode graded index fiber, the refraction index falls offgradually from the core’s center to the outside of the cladding with nodistinct core-cladding interface. The gradually changing refractionindex allows all propagation modes to have nearly the same velocity,which eliminates pulse spreading. The core diameter in multimode gradedindex is also relatively large, which allows more light to enter thefiber.
- ATTENUATIONAttenuation is the loss of signal over a length of cable. Whenrunning coaxial cable in a video system, attenuation is a function ofits length due to distributed capacitance. At times the practical wayto deal with coax attenuation is to equalize the video signal at thesource, which means distorting it to compensate for the cableattenuation characteristics. In very long runs, amplifiers or repeatersare put on the line that have equalization circuits that have to betrimmed for the length of the cable. The core of optical fiber, on theother hand, consists of relatively low-loss glass fiber (silica) orplastic fiber. General-purpose fiber exhibits about 60 to 75 percentattenuation per kilometer at a wavelength of about 850nm.
- BANDWIDTHPerhaps the most important disparity between coax and fiber isbandwidth, or how much data can be carried on a single cable.Fiber-optic cable can handle more than 10 times the data that coaxialcable can. As an example, RG-59 coaxial cable can be used to carrybandwidths up to about 400 MHz; twisted-pair can handle about 10 MB persecond. Fiber optic cable can handle between 10 and 320 GB per second— that’s gigabytes, not megabytes. It’s no wonder the telephonecompanies are using fiber-optics.
- DISADVANTAGESFiber optics are known for lower durability than wire-based cable;however, the fragility of fiber cable is mostly due to cablemanufacturing techniques, and improvements are being engineeredcontinually.
FIBER OPTIC SYSTEM COMPONENTS
a point-to-point fiber optic system consists of three basicelements: the optical transmitter, the fiber-optic cable and theoptical receiver. The optical transmitter converts an electrical analogor digital signal into a corresponding optical signal. The source ofthe optical signal can be either a light-emitting diode or a solidstate laser diode (LD). The most common wavelengths of operation foroptical transmitters are 850, 1500 or 1550 nanometers. The cableconsists of one or more glass fibers that act as waveguides for theoptical signal. At the end of the signal chain, the optical receiverconverts the optical signal back into a replica of the originalelectrical signal. The detector of the optical signal can be either aPIN-type photo-diode or the avalanche-photo diode.
- OPTICAL TRANSMITTERSThe basic fiber-optical transmitter consists of a driver and asource. The driver’s output delivers the current required to operatethe source. Video signals use a 1V peak-to-peak level. Digital systemsuse different standards that depend upon the type of logic being usedwithin the system. The logic circuits determine the levels for thehighs and the lows that represent the 1s and 0s of digital data.When specifying a system’s speed it’s important to recognize thedifference between data rate and signal rate. Data rate is the numberof data bits being transmitted in bits per second. Signal speed isexpressed in baud. The signal speed and the data rate may or may not bethe same depending on the modulation code being used.The transmitter’s output power is important because it is specifiedfor the required power being coupled into a given fiber. The poweroutput being coupled into the fiber cable requires an increase in thecore diameter and numerical aperture. The NA indicates fibers that havelower attenuation and higher bandwidths. It is important for systemdesigners to bear in mind that a NA-mismatch occurs when the NA of thetransmitting fiber is larger than that of the receiving fiber. Corediameter mismatch can also occur when the core or the diameter of thetransmitting fiber is larger than that of the receiving fiber.
- OPTICAL RECEIVERSThe receiver consists of a detector, an amplifier and an outputsection. The output performs several functions, including separation ofthe clock rate and data, pulse reshaping and gain control. Thereceiver’s sensitivity is specified as the weakest optical signal thatthe receiver can detect, and that depends on the receiver’s front-endnoise level, which must be included in its sensitivity rating. Thesensitivity ratings are expressed in microwatts or dBm. Fiber opticreceivers employ two different designs: low-impedance amplification andtransimpedance amplification. The bandwidth of the low-impedanceamplifier is determined by the RC time constant of the circuit(resistance in ohms times capacitance in farads); and thetransimpedance amplifier’s bandwidth is affected by the gain of theamplifier.When the proximity method is used in coupling the light into thefiber, the amount of light that enters the fiber is a function of theLED or LD intensity, the light surface area, the acceptance angle ofthe fiber and losses due to scattering and reflections. The intensityof an LED or LD is a function of its design and is specified as poweroutput at a particular drive current. Some manufacturers express thisfigure in actual power that is delivered into a specific type of fiber.The actual amount of light entering into a fiber is a function of thearea of the light-emitting surface as compared to area of the fibercore accepting the light. The smaller the ratio, the more light will belaunched into the fiber core.
- OPTICAL AMPLIFIERSAn important light-handling device that contributed to the use offiber optics in the long-haul communications backbone is the opticalamplifier. Optical amplifiers boost the light signal so the signal cantravel longer distances (similar to electronic repeaters). Today’sfiber-optical amplifiers can maintain a cleaner signal with highersignal-to-noise ratio, increasing the distance the signal can travelbefore additional amplification is needed. In the 1990s, the distancehad doubled from 25 miles between electronic repeaters to 50 milesbetween optical amplifiers. Optical amplifiers are simpler, morereliable and less expensive than electronic repeater systems andprovide more optical fiber bandwidth.
- THE SYSTEMIn designing a fiber optic system, it is paramount to ensure thatenough light reaches the receiver. A lack of the proper amount of lightwill jeopardize the entire system. In putting a fiber optic systemtogether, these steps must be carefully considered:
- Select the proper optical transmitter and receiver based on thesignal to be used (analog, digital, audio, video, RS-232, RS-422,RS-485 and so on).
- Make sure the fiber bandwidth is sufficient to handle the systemspecifications. If the fiber bandwidth is not adequate for the systemit may be necessary to select a different transmitter/receivercombination or consider the use of a lower-loss fiber.
- Calculate the total optical loss in dB from the technical datasupplied with each individual component by adding cable loss, splicelosses and connector losses. Compare the total system loss to thereceiver’s input requirements and allow at least a 3dB safetymargin.
DIGITAL FIBER OPTICS
analog fiber systems that interface with digital systems can take inanalog or digital video, audio and data signals and output thesesignals in their original formats. In interfacing a digital system, theincoming base-band signal is fed into an analog-to-digital converterwithin the optical transmitter. The A/D converter converts the incomingsignals to a series of 1s and 0s (a digital stream). When more than onesignal is being processed, the transmitter combines all the digitalstreams into a single digital stream. This stream is used to turn onand off the light source (LED or LD) focused into the fiber at a veryhigh speed that corresponds to the 1s and 0s being transmitted. Theprocess performed by the transmitter is reversed at the receiver. Atthe receiver, the combined digital stream is separated into multiplebit streams that represent each of the original transmitted signals.The multiple bit streams are fed into digital-to-analog converters, andthe receiver output is the same as the original format.
Digital fiber optic processing systems offers flexibility overtraditional analog AM and FM systems. In digital systems that transmitone or more signals over short or long distances, the signal fidelityof the video, audio and data signals remain constant throughout thesystem. The only signal distortion occurring in a digital fiber systemis a function of the A/D and D/A conversion. Today’s A/D and D/Aconverters can provide video and audio quality that far exceedsconventional systems.
A great advantage of digital video over fiber is the ability toplace repeaters on the fiber line at points where the light diminishesbelow detectable level. The repeater regenerates and restores thedigital signal to its original form without any degradation of theoriginal signal because the repeater deals only with the data streamthat represents the original signal.
To round out your reading, peruse the accompanying glossary offiber-optics terms as they apply to video signal transmission. Thistechnology is certainly an important one to be familiar with, as it’slikely to be a preferred option for quite a few years to come.
A former electronics engineer, Alvin G. Sydnor is now retiredfrom the audio products division of GE.
9 Advantages OF FIBER OPTICS:
- Extremely wide bandwidth
- Complete electrical isolation
- Immunity from electromagnetic interference
- No short circuits
- No crosstalk
- Wide temperature range
- Long service life
Fiber optic terms applicable to video signal transmission:
ABSORPTION: That portion of attenuation that results from theconversion of optical power to heat.
ADD/DROP MULTIPLEXER: A multiplexer that can selectively addor drop lower data rate signals from a higher data rate signal.Typically an intelligent network element allowinginstruction-controlled inputs through embedded data communicationschannels.
AMPLIFIER NOISE FIGURE: The ratio of actual output noise tothat which would remain if the amplifier noise contributed only gainwithout adding internal noise.
ANALOG: Represents one continuously variable physicalquantity with another continuously varying indicator.
ANALOG CARRIER: The transmission of multiple voice frequencysignals over the same medium by placing the individual signals atsuccessively higher frequencies, then combining these signals fortransmission over a single path.
ANALOG TRANSMISSION: The transmission of a continuouslyvariable signal as opposed to a digital signal.
ASYNCHRONOUS TRANSFER MODE (ATM): A connection-typetransmission mode carrying information organized into blocks (headerplus information fields); it is asynchronous in the sense thatrecurrence of blocks depends on the required or instantaneous bitrate.
ATTENUATION: The decrease in signal strength along a fiberoptic cable caused by absorption and scattering. Usually expressed indecibels per kilometer (Db/km).
ATTENUATOR: A device used to reduce the optical signalstrength.
BANDPASS: The width of the spectrum (wavelength range) of alight source focused into a slit. Bandpass usually decreases with theslit width.
BANDWIDTH: For an optical fiber, defined as the lowestfrequency at which the magnitude of the base band frequency response inoptical power has decreased by 3 dB compared to zero frequency.
BENDING LOSS: Attenuation caused by high-order modesradiating from the outside of a fiber optic waveguide. Bending lossesoccur when the fiber is bent around a restrictive radius.
BEND RADIUS: The radius of curvature that a fiber can be bentwithout breaking.
BIT: Contraction of binary digit, the smallest unit ofinformation in the binary number system, either a 0 or a 1.
BIT-ERROR RATE (BER): The fraction of bits transmitted thatare received incorrectly.
BIT RATE: The speed at which bits are transmitted. Usuallyexpressed in bits per second.
BRIDGING AMPLIFIER: An amplifier designed for connectionacross a trunk with minimum reduction of trunk signal levels.
BROADBAND: 1) A service or system requiring transmissionchannels that are capable of supporting rates greater than the primaryrate. 2) A frequency-division multiplexing technique for LANs that usecable TV technology to provide multiple signals (digital and or analog)on the same distribution medium.
BROADCAST TRANSMISSION: Unidirectional distribution to allsubscribers where the message transmission may be read by a largenumber of destinations.
BUFFER: A protective coating over the fiber.
BUNDLE: A group of individual glass fibers contained within asingle jacket acting as one transmission channel.
BURIED (SINGLE) HETEROSTRUCTURE: A subsurface layer ofmaterial placed above a laser’s active, or emitting, region. Having adifferent index of refraction, the layer keeps light confined to thehorizontal plane. Some light emitted downward is absorbed by thedevice’s substrate.
BUS: A shared medium for transmitting signals or power, forexample, a LAN bus operating in broadcast transmission mode.
BYTE: A binary string, usually 8 bits operating as a unit.The byte is typically shorter than a computer word and often representsa character.
CABLE: One or more optical fibers enclosed within aprotective jacketed cable that can transmit optical signals.
CABLE COMPATIBLE: A term used to describe a TV, VCR or otherTV tuning device with 75-ohm input impedance capable of tuning all orpart of the cable mid-band, super-band or hyper-band channels inaddition to standard VHF and UHF channels.
CARRIER: A continuous frequency capable of being modulated byan information carrying signal.
CARRIER-TO-NOISE RATIO: The ratio of a carrier’s power to thenoise in the bandwidth of a specific system. It is usually determinedbefore any nonlinear process such as amplitude limiting and detectiontakes place.
CHROMINANCE SIGNAL: That portion of the NTSC color-televisionsignal that contains the color information.
CHROMATIC (MATERIAL) DISPERSION: The spreading of a lightpulse caused by the different propagation delays imposed on the pulse’svarious wavelengths as it travels along a fiber or through air.
CLADDING: A low refractive index material that surrounds thecore of a fiber, causing the transmitted light to travel down the coreand providing protection to the core.
COHERENT: A light source in which the amplitude of all wavesare exactly equivalent and convergent. Most systems operate by varyingthe intensity of the light source, and direct intensity detection isused for reception.
COHERENT SYSTEM: A fiber optic system that can synchronouslydetect or heterodyne (mix) two or more light-wave sources.
CONNECTOR: Hardware installed on cable ends to providephysical and optical cable attachment to a transmitter, receiver oranother cable.
CORE: The central light transmission part of the fiber with arefractive index higher than that of the cladding.
CROSSTALK: The phenomenon of light leakage or informationtransfer from a waveguide to one adjacent. Also called opticalcoupling.
CUTBACK METHOD: A technique for measuring optical fiberattenuation by measuring the optical power at two points at differentdistances from the test source.
CUTOFF WAVELENGTH: In single-mode fiber, the wavelength belowwhich the fiber ceases to be single-mode.
DECIBEL (dB): A unit of measurement that indicates relativeoptical power on a logarithmic scale. Usually expressed in reference toa fixed value, such as dBm (1 MW).
DECODING: Changing a digital signal into an analog form orinto another type of digital signal. The reverse of decoding isencoding.
DENSE-WAVELENGTH MULTIPLEXING (DWDM): Combines multipleoptical signals into a single fiber by transmitting each signal on adifferent wavelength. DWDM is used to increase the capacity of existingfibers without adding more fibers to the system. It is primarily usedin the C band (1530 to 1570 nm).
DETECTOR: The receiving photo-diode.
DIFFERENTIAL GAIN: An amplitude change of the 3.58 MHz colorsubcarrier, caused by the overall circuit as the luminance is variedfrom blanking to white level. It can be expressed as a percentage or indecibels.
DIGITAL SIGNAL LEVEL (DSL): The digital transmission pulserate within the time-division multiplexing hierarchy. A DSL-0 level is64 KB per second, which is equivalent to one voice channel. A DSL-1 is1.544 MB/s.
DISPERSION: The cause of bandwidth limitations in a fiber.Dispersion causes a broadening of input pulses along the length of thefiber. Two major types are mode dispersion caused by differentialoptical path lengths in a multimode fiber and material dispersioncaused by a differential delay of various wavelengths of light in awaveguide material.
DUPLEX TRANSMISSION: Data transmission over a system that iscapable of transmitting in both directions at the same time.
DOUBLE HETEROSTRUCTURE: A structure that places a barrierlayer above and below a laser’s thin active region, thereby confininglight totally to the horizontal plane for greater efficiency.
ELECTRO-OPTICAL SWITCH: A device that allows the routing ofoptical signals without an intermediary conversion to electronicsignals.
ENCODING: Transforming an analog signal into digitalformat.
FIBER: A single, separate optical transmission element,characterized by a core and a cladding.
FIBER CABLE: One or more optical fibers enclosed within aprotective covering.
FIBER HUBBING: A network that uses fiber transmission tiedinto a facility hub. At the hub, such functions as automatic digitalcross connection, testing and network configuration take place.
FIBER OPTICS: Light-wave transmission through optical fibersfor communication or signaling.
FOCAL LENGTH: The distance from the exit-imaging mirror tothe flat focal plane. Increasing the focal length usually increasesresolution and reduces the bandpass.
ƒ NUMBER: The input aperture of the monochromator. Itmeasures the ability of a device to collect radiation in the entranceslit. The light gathering power of an optical device increases as theinverse square of the ƒnumber.
FREQUENCY: The number of cycles per unit of time, denoted byHertz (Hz).
FREQUENCY DIVISION MULTIPLEXING (FDM): A method that allowsthe transmission of more than one signal over a common path byassigning each signal a different frequency band.
FRESNEL REFLECTION: The reflection losses at ends of fibercaused by differences in refractive index between glass and air.
FUSION SPLICER: An instrument that permanently bonds twofibers together by heating and fusing them.
GIGABIT (GB): One billion bits.
GIGAHERTZ (GHz): A unit of fiber frequency equal to onebillion hertz.
GRADED-INDEX FIBER: A type of fiber where the refractiveindex of the core varies smoothly with the radius. This type of fiberprovides high bandwidth capabilities.
GRADED SECTION: A layer of material, or region of acomponent, where the index of refraction decreases from the centeroutward in order to decrease modal dispersion and minimize band-gapenergy and gradients, and thus increase the bandwidth of a device.
HYBRID OPTICAL CIRCUIT: A device in which the various circuitelements are fabricated in different substrate materials and joinedtogether in such a way that the materials can be chosen to optimize theperformance of each type of device within the optical integratedcircuit.
INCOHERENT FIBER BUNDLE: A bundle of optical glass or plasticmaterials that transmit light only. The incoherent fiber bundle doesnot transmit optical images.
INDEX MATCHING MATERIAL: A material whose refractive index isequal to the core index. Used to reduce Fresnel reflections from theoptical fiber’s end face.
INFRARED (IR): The band of electromagnetic wavelength between0.75 μm and 1000 μm. This region is sometimes subdivided intothree categories: near infrared, 0.75 μm to 3.0 μm; middleinfrared, 3 μm to 30 μm; and far infrared, 30 μm to 1000μm.
INTRINSIC FIBER LOSS: Optical power loss in a fiber, itssplice, connection or coupling.
INJECTION LASER DIODE (ILD): A solid-state component thatconsists of one p-n junction that is capable of emitting coherentstimulated radiation under specific conditions.
KILOBIT (KB): A thousand binary digits or bits.
KILOMETER: The standard length measurement for optical fibers(1000 meters or 3281 feet or 0.621 statue miles).
LASER (LIGHT AMPLIFICATION BY SIMULATED EMISSION OFRADIATION): A device that produces a high-intensity narrow band oflight that is monochromatic, of a single wavelength and coherent of thesame phase. Most lasers used in fiber optic systems are solid-statesemiconductor devices.
LASER CHIRP: A phenomenon in lasers in which the wavelengthof the light being emitted changes during modulation.
LASER DIODE: A semiconductor that emits coherent light whenforward biased.
LASER MODULATION: Turning a laser on and off.
LIGHT EMITTING DIODE (LED): A semiconductor device that emitsincoherent light formed by the p-n junction of the semiconductormaterial. The light intensity is roughly proportional to electricalcurrent flow.
MICROBEND LOSS: An increased attenuation caused by curvedfiber around sharp bends. Excessive bend loss can be traced tomanufacturing techniques.
MICRON (μm): Micrometer, one millionth of a meter.
MONOCHROMATIC: Light radiated from a source that isconcentrated in only a very narrow wavelength range.
MULTIMODE FIBER CABLE: An optical cable having more than onefiber.
NANOMETER (nm): One billionth of a meter.
NUMBERICAL APERTURE (NA): As a number, the NA expresses thelight gathering power of a fiber. It is mathematically equal to thesine of the acceptance angle.
OPTICAL POWER: The distribution of the available light powerthat is required to complete a circuit, usually expressed indecibels.
REFLECTION: A change in the direction of a light beams at aninterface between two dissimilar materials.
REFRACTIVE INDEX: The ratio of the velocity of light in avacuum to the velocity of a given frequency of light in thetransmission medium whose refractive index is desired.
SINGLE-INDEX FIBER: An optical fiber that supports thepropagation of only one wavelength. It is usually classified as alow-loss fiber that has a core diameter ranging between 2 to 12microns. Sometimes called single-mode fiber.
SOURCE AGING LOSS: The loss of optical power resulting fromaging of the light source (emitter), typically 3 dB.
SPECTROMETER: An instrument that measures the distribution ofradiation from a broadband source. The instrument’s principlecomponents are a monochromator, a radiant power detector such as asilicon detector and a photo-multiplier tube.
SPECTRAL WIDTH: A measure of a spectrum width. For a source,it’s the width of wavelengths contained in the output at one half ofthe wavelength of peak power. A typical spectral width for an LED is 20to 60 nanometers and between 2 and 5 nanometers for a laser diode.
STABILIZED LIGHT SOURCE: An LED or laser diode that emitslight with a controlled and constant spectral width, central wavelengthand a peak power with respect to time and temperature.
STEP-INDEX FIBER: An optical fiber that exhibits an abruptchange in refractive index between the core and cladding along itsdiameter, the core refractive index is higher than the claddingrefractive index.
SYNCHRONOUS TRANSMISSION: A method employing thesynchronizing of characters at the sending and receiving ends whereeach operate continuously at the same frequency and are kept in phasewith each other. Also often referred to as bi-sync or binarysynchronous.
THRESHOLD CURRENT: The minimum current required for photomultiplication to start in laser diodes.
TIME DIVISION MULTIPLEX (TDM): A device or process in whichmore than one signal can be sent over a single channel by usingdifferent time intervals for the different signals. That isaccomplished by varying the pulse duration, pulse amplitude and pulseposition.
TEMPERATURE VARIATION LOSS: Loss of optical power due totemperature changes.
TRANSMISSION LOSS: The total optical power loss occurringduring transmission through a system.
WAVEGUIDE: A conducting or a dielectric structure able tosupport and propagate one or more electromagnetic field modes. Fibercable is often referred to as a waveguide.
WAVELENGTH-DIVISION MULTIPLEXING (WDM): A means of combiningand separating a number of optical signals of different wavelengthssent along an optical fiber. WDM greatly increases the transmissioncapacity of a single fiber.