FIBER OPTIC SYSTEM DESIGN CHAPTER 5 FIBER OPTIC SYSTEM DESIGN

System Design Considerations Design is based on Application Type of signal Distance from transmitter to detector Performance standards Resource constraints (time, money, etc.) Implementation Components Format, power, bandwidth, dynamic range Amplification, amplitude, and spacing Multiplexing Security requirements Acceptable noise levels

System development schematic

Design of optical networks Design proceeds at several levels (rough classification) Physical: fiber, amplifiers, ADMs (hardware) Data link: Ethernet, SONET (encoding, access control) Network: ATM, IP (addressing, routing) There is interaction among these layers SONET may require particular physical layer configuration, e.g., rings Ethernet, especially GigE or 10GigE will require switches

Steps for physical layer design Determine topology needed Point-point Star Ring Determine key functional requirements Data rates Error rates Make initial design Use manufacturer data to complete/modify design Satisfy budgets Meet performance goals

System factors for designing from scratch Available choices Type of fiber Single mode, multimode, plastic Dispersion Repeaters, compensation Fiber nonlinearities Fiber characteristics, wavelengths used, transmitter power Operating wavelength (band) 780, 850, 1310, 1550, 1625 nm typical Transmitter power ~0.1 to 20 mw typical; usually expressed in dBm Light source LED, laser Receiver characteristics Sensitivity, overload Multiplexing scheme None, CWDM, DWDM

System factors (continued) Available choices Detector type PIN diode, APD, IDP Modulation scheme OOK, multilevel, coherent End-end bit error rate <10-9 typical; may be much lower Signal-to-noise ratio Specified in dB for major stages Max number of connectors Loss increases with number of connectors Max number of splices Loss increases with number of splices Environmental Humidity, temperature, sunlight exposure Mechanical Flammability, strength, indoor/outdoor/submarine

System factors (continued) Available choices Amplifiers Type, spacing Switches OEO, all optical Add/drop multiplexers Number, location

System Design Considerations System Power Budget Most important parameter is throughput or transfer function. Output power must be greater than the input sensitivity of the receiver. System budget Amount of power lost or gained in each component 1. Optical link loss (attenuation) 2. Dispersion 3. Signal-to-noise ratio System power margin Allows for component tolerances, system degradation, repairs and splices

System Design Considerations Power at the Source Transmitter must be appropriate for the application Number of signals Wavelength of signal Type of transmitter device (LED, Laser) Modulation Mode structure Tunability WDM and amplification capability Coupling efficiency

System Design Considerations Power in the Fiber Matching Source output pattern, core-size, and NA of fiber Coupling is critical Power at the Detector Sensitivity is the primary purpose of the detector Must support the dynamic range of the power levels

System Design Considerations Fiber Amplification For those fibers that require amplification Two types: Repeaters are rarely used. Optical amplifiers are the preferred amplification. Use manufacturers specifications to ensure optimization of the input signal.

Optical link loss budget Key calculations in designing a simple fiber optic link Objective is to determine launch power and receiver sensitivity Variables Environmental and aging Connector losses Cable losses Splices Amplifier Other components

Optical link loss budget Other allowances (repair, splice, safety margin Temperature Insertion loss Fiber coupling Insertion loss Attenuation Aging Temperature -5 Launch -10 +2 db +2 db +2 db 0-3 db 0-2 db -4 db +1 db 0-2 db 0-3 db -15 Receiver Sensitivity Range -20 -30 -35 -40 Source variables Fiber Receiver -45 Connector Connector

Optical link loss budget (continued) Connector -0.5 dB Connector -0.5 dB Transmitter -10 dBm Receiver -10 to –25 dBm Splice -0.1 dB Splice -0.1 dB 15.5 km @ 0.35 dB/km

Optical link loss budget (continued)

Optical link loss budget—example Point-to-point fiber optic link between 2 computers Path length measured as 1.2 km Multimode fiber to be used Patch panel at each end to facilitate connections 3 fusion splices required Transmitter power: -10 dBm Receiver sensitivity: -20 dBm Problem: choose type of fiber to be used

Example Transmitter Receiver -10 dBm -10 to –25 dBm 1.2 km Patch panel Splice -0.1 dB Splice -0.1 dB Splice -0.1 dB 1.2 km

Available fiber

Example (continued) Using 62.5/125 with 3.0 db/km loss:

Example (continued) Using 100/140 with 4.0 db/km loss:

Example (continued) Power at receiver: -10 dBm – 9.1 dBm = -19.1 dBm OK, since receiver sensitivity –25 dBm

Amplifier Placement Depends on Can be inserted Type of amplifier Transmitter Receiver Rise time Noise and error analysis Can be inserted Before regeneration Between regenerators

System Rise Time Budget Determines the bandwidth carrying capability Total rises time is the sum of the individual component rise times. Bandwidth is limited by the component with the slowest rise time.

Rise Time and Bit Time Rise time is defined as the time it takes for the response to rise from the 10% to 90% of maximum amplitude. Fall time is the time the response needs to fall from 90% to 10% of the maximum. Pulse width is the time between the 50% marks on the rising and falling edges.

Transmitters, Receivers, and Rise Time Rise time of transmitter is based on the response time of the LED or laser diode. Rise time of the receiver is primarily based on the semiconductor device used as the detector.

Fiber Rise Time Total Rise Time Comes directly from the total dispersion of the fiber as a result of modal, material, wave guide, and polarization mode dispersion Total Rise Time Sum of all the rise times in the system

Optical Power Most basic fiber optic measurement The basis for loss measurements as well as the power from a source or presented at a receiver Typically both transmitters and receivers have receptacles for fiber optic connectors, so measuring the power of a transmitter is done by attaching a test cable to the source and measuring the power at the other end For receivers, one disconnects the cable attached to the receiver receptacle and measures the output with the meter

Optical Power Optical power is based on the heating power of the light, and some optical lab instruments actually measure the heat when light is absorbed in a detector Optical power meters typically use semiconductor detectors since they are sensitive to light in the wavelengths and power levels common to fiber optics Most fiber optic power meters are available with a choice of 3 different detectors, silicon (Si), Germanium (Ge), or Indium-Gallium-Arsenide (InGaAs).

Optical Power Table 1. Optical power levels typical of fiber optic communication systems Network Type Wavelength, nm Power Range, dBm Power Range, W Telecom 1310, 1550 +3 to -45 dBm 50 nW to 2mW Datacom 650, 850, 1300  0 to -30 dBm 1 to 100uW CATV, DWDM 1310,1550 +20 to -6 dBm 250 uW to 10mW

Optical Power Measurement

Optical Power Measurement Measuring power requires only a power meter (most come with a screw-on adapter that matches the connector being tested), a known good fiber optic cable (of the right fiber size, as coupled power is a function of the size of the core of the fiber) and a little help from the network electronics to turn on the transmitter when measure power, the meter must be set to the proper range (usually dBm, sometimes microwatts, but never "dB" ­ - that's a relative power range used only for testing loss and the proper wavelength , ­ matching the source being used in the system (850, 1300, 1550 nm for glass fiber, 650 or 850 nm for POF).

Optical Power Measurement To measure power, attach the meter to the cable attached to the source that has the output you want to measure (see diagram to the right). That can be at the receiver to measure receiver power, or using a reference test cable (tested and known to be good) that is attached to the transmitter to measure output power Turn on the transmitter/source and give it a few minutes to stabilize. Set the power meter for the matching wavelength and note the power the meter measures. Compare it to the specified power for the system and make sure it's enough power but not too much.

Optical wavelength The wavelengths we use for transmission must be the wavelengths we test for losses in our cable plants. Our power meters are calibrated at those wavelengths so we can test the networking equipment we install The three prime wavelengths for fiber optics, 850, 1300 and 1550 nm drive everything we design or test. NIST (the US National Institute of Standards and Technology) provides power meter calibration at these three wavelengths for fiber optics

Optical wavelength Multimode fiber is designed to operate at 850 and 1300 nm, while singlemode fiber is optimized for 1310 and 1550 nm. The difference between 1300 nm and 1310 nm is simply a matter of convention, harking back to the days when AT&T dictated most fiber optic jargon Lasers at 1310 nm and LEDs at 1300 nm were used in singlemode and multimode fiber respectively

Fiber Optic Testing Testing is used to evaluate the performance of fiber optic components, cable plants and systems. As the components like fiber, connectors, splices, LED or laser sources, detectors and receivers are being developed, testing confirms their performance specifications and helps understand how they will work together. Designers of fiber optic cable plants and networks depend on these specifications to determine if networks will work for the planned applications

The following test: Continuity testing to determine that the fiber routing and/or polarization is correct and documentation is proper. End-to-end insertion loss using an OLTS power meter and source. Test multimode cables using TIA/EIA 526-14, and singlemode cables using TIA/EIA 526-7 (singlemode). Total loss shall be less than the calculated maximum loss for the cable based on Loss Budget calculations using appropriate standards or customer specifications. Optional OTDR testing may be used to verify cable installation and splice performance. However, OTDR testing should not be used to determine cable loss, especially on longer cables. Use of an OTDR in premises applications may be inappropriate if cables are too short.

The following test: If the design documentation does not include cable plant length, and this is not recorded during installation, read the length from the distance marking on the cable jacket or test the length of the fiber using the length feature available on an OTDR, or some OLTSs. If testing shows variances from expected losses, troubleshoot the problems and correct them.

Component Testing Fiber optic inspection microscopes are used to inspect connectors to confirm proper polishing and find faults like scratches, polishing defects and dirt. They can be used both to check the quality of the termination procedure and diagnose problems. A well made connector will have a smooth , polished, scratch free finish and the fiber will not show any signs of cracks, chips or areas where the fiber is either protruding from the end of the ferrule or pulling back into it.

Continuity Testing Perform continuity testing of optical fibers using a visual fiber tracer, visual fault locator, or OLTS power meter and source. Trace the fiber from end to end through any interconnections to ensure that the path is properly installed, and that polarization and routing are correct and documented.

Visual Tracing Continuity checking with a visual fiber tracer makes certain the fibers are not broken and to trace a path of a fiber from one end to another through many connections, verifying duplex connector polarity for example. It looks like a flashlight or a pen-like instrument with a light bulb or LED source that mates to a fiber optic connector Attach the fiber to test to the visual tracer and look at the other end of the fiber to see the light transmitted through the core of the fiber. If there is no light at the end, go back to intermediate connections to find the bad section of the cable

Visual Fault Location A higher power version of the fiber tracer called a visual fault locator (VFL) uses a visible laser that can also find faults. The red laser light is powerful enough for continuity checking or to trace fibers for several kilometers, identify splices in splice trays and show breaks in fibers or high loss connectors. You can actually see the loss of light at a fiber break by the bright red light from the VFL through the jacket of many yellow or orange simplex cables (excepting black or gray jackets, of course.)   It's most important use is finding faults in short cables or near the connector where OTDRs cannot find them

Insertion Loss Insertion loss refers to the optical loss of the installed fibers when measured with a test source and power meter (OLTS). Test multimode cables using TIA/EIA 526-14, and singlemode cables using TIA/EIA 526-7 (single mode). The insertion loss measurement is made by mating the cable being tested to known good reference cables with a calibrated launch power that becomes the "0 dB" loss reference.

Insertion Loss a) Test multimode fiber at 850 and 1300 nm, and singlemode fiber at 1310 and 1550 nm, unless otherwise required by other standards or customer requirements. b) Test reference test cables to verify quality and clean them often. c) Cabling intended for use with high speed systems using laser sources may be tested with appropriate laser sources to ensure that tests verify performance with that type of source

Insertion Loss There are two methods that are used to measure loss, a "patchcord test" which we call "single-ended loss" (TIA FOTP-171) and an "installed cable plant test" we call  "double-ended loss" (TIA OFSTP-14 (MM) and OFSTP-7 (SM).) Single-ended loss uses only the launch cable, while double-ended loss uses a receive cable attached to the meter also.

OTDR Testing OTDRs are powerful test instruments for fiber optic cable plants. When used by a skillful operator, OTDRs can locate faults, measure cable length and verify splice loss. Within limits, they can also measure the loss of a cable plant OTDR) uses optical radar-like techniques to create a picture of a fiber in an installed fiber optic cable. The picture, called a signature or trace, contains data on the length of the fiber, loss in fiber segments, connectors, splices and loss caused by stress during installation OTDRs are used to verify the quality of the installation or for troubleshooting. However, OTDR testing shall not be used to determine cable loss. OTDRs have limited distance resolution and may show confusing artifacts when testing short cables typical of premises applications. If OTDR testing of premises cables is desired, experienced personnel should evalute the appropriateness of the tests.

OTDR Testing

Bit Error Rate (BER) Testing Bit error rate(BER) is a fundamental measure of digital transmission quality. BER is essentially an error probability of digital bits in the received signal; it is also known as bit e.rror probability