FIBER OPTIC BASICS The information contained in this document is subject to change without notice. This manual was produced by GN Nettest and should not be copied or reproduced in any form without permission. GN Nettest makes no warranties with respect to this document and disclaims any implied warranties of merchantibility or fitness for a particular purpose. GN Nettest shall not be liable for errors contained herein or for incidental or consequential damages connected to the furnishing, performance, or use of this material. For telephone assistance call 1-800-443-6154 in the U.S., or 315-797-4449. 109 N. Genesee Street Utica, New York 13502 Revision A Part # 32704 Table of Contents Chapter 1 The Nature of Light Classifying Light .................................................................... 1-3 Power .................................................................................... 1-4 Wavelength .......................................................................... 1-5 Reflection & Refraction ........................................................ 1-6 Rayleigh Scattering ............................................................ 1-11 Chapter 2 Optical Fiber Characteristics Typical Optical Fiber Parameters ......................................... 2-3 Optical Fiber Types ............................................................... 2-4 Singlemode vs. Multimode Fiber ......................................... 2-5 Fiber Geometry Problems..................................................... 2-6 Index of Refraction (n) ......................................................... 2-7 Attenuation .......................................................................... 2-9 Causes of Attenuation ....................................................... 2-10 Dispersion ........................................................................... 2-11 Splice Loss Due to Core Mismatch ..................................... 2-12 Causes of Connector Loss ................................................... 2-13 Chapter 3 Optical Measurements Testing Fiber - Why? ............................................................. 3-3 Testing Fiber - When? .......................................................... 3-4 Testing Fiber - What? ........................................................... 3-5 Attenuation Measurement .................................................. 3-6 OTDR Block Diagram ............................................................ 3-7 How an OTDR Works ............................................................ 3-8 FIBER OPTIC BASICS TOC-1 Reflected Light ..................................................................... 3-9 Returned Light ................................................................... 3-10 OTDR Distance Measurements ........................................... 3-11 OTDR Loss Measurements .................................................. 3-13 OTDR Trace Basics ............................................................... 3-15 Measuring Optical Return Loss .......................................... 3-16 Wavelength - Scattering Loss............................................. 3-17 Wavelength - Bending Loss ............................................... 3-18 Deadzones .......................................................................... 3-19 Event Deadzone ................................................................. 3-21 Attenuation Deadzone ...................................................... 3-22 Fresnel Characteristics ........................................................ 3-23 Fusion Splices ...................................................................... 3-24 Why Losers and Gainers? ................................................... 3-25 Dynamic Range ................................................................... 3-26 Backscatter vs. Dynamic Range .......................................... 3-27 Resolution ........................................................................... 3-28 Data Sampling Resolution ................................................. 3-29 Spatial Resolution - Long Pulse ......................................... 3-30 Spatial Resolution - Short Pulse ......................................... 3-31 Spatial Resolution - Long vs. Short Pulse Widths ............. 3-32 TOC-2 FIBER OPTIC BASICS Chapter 1 The Nature of Light FIBER OPTIC BASICS Page 1-1 FIBER OPTIC BASICS Page 1-3 . or 32 wavelength channels or more over a single fiber. • Increased demand for bandwidth is driving Dense Wave Division Multiplexing (DWDM) systems which transmit from 1520 to 1580nm using 4.16. Fiber optic systems typically transmit at 850. • Color (Wavelength) 380nm (blue) to 750nm (red) is visible to humans.Classifying Light • Optical Power is measured in dBm. 8. 1300. 1310 and 1550nm. Power • Like a light bulb: more wattage = brighter light • FO transmitters: about 1mw (0 dBm) • Power ranges: +20 dBm to -70 dBm Page 1-4 FIBER OPTIC BASICS . Wavelength • Measure of Color of light • Units in nanometers (nm) or microns (µm) • Different colors (wavelengths) exhibit different characteristics. yellow fog lights FIBER OPTIC BASICS Page 1-5 . ex: red & orange sunsets. Reflection & Refraction • Reflection is a light ray BOUNCING off of the interface of two materials • Refraction is the BENDING of the light ray as it changes speed going from one material to another Page 1-6 FIBER OPTIC BASICS . Some light is reflected off the glass-air surface back into the glass. FIBER OPTIC BASICS Page 1-7 . The amount of this refraction angle is constant for any single wavelength.Refraction & Reflection A ray of light in glass will bend (refract) away from the direction of travel as it escapes to the surrounding air. the reflected ray does not leave the glass at all. This angle is called the Critical Angle.Refraction & Reflection When the angle becomes shallow enough. but travels along the air-glass interface. Page 1-8 FIBER OPTIC BASICS . This condition is known as Total Internal Reflectance FIBER OPTIC BASICS Page 1-9 . all light is Reflected back into the fiber at the same angle it strikes the air-glass interface.Refraction & Reflection At angles beyond the Critical Angle. it will remain in the fiber until it reaches the other end.Refraction & Reflection As long as the light ray stays at the Critical Angle or less when it hits the air-glass interface. In a glass fiber the light remains in the core because light reflects at core/cladding boundary. Page 1-10 FIBER OPTIC BASICS . FIBER OPTIC BASICS Page 1-11 . The part that returns to the source is called BACKSCATTER. part of it is scattered in all directions.Rayleigh Scattering As light passes through a particle. Chapter 2 Optical Fiber Characteristics FIBER OPTIC BASICS Page 2-1 . Light travels in the core only. The Buffer protects the glass fiber.Typical Optical Fiber Parameters Most Transmission Glass Fiber has the Following Properties: The denser Core is centered within the Cladding. FIBER OPTIC BASICS Page 2-3 . Optical Fiber Types Multimode fiber has a large core relative to the cladding diameter. Page 2-4 FIBER OPTIC BASICS . Singlemode fiber has a smaller core relative to the cladding diameter. Multimode vs. Singlemode Fiber Multimode allows many paths (“modes”) for light to travel Singlemode allows only one single path for light to travel FIBER OPTIC BASICS Page 2-5 . Fiber Geometry Problems All fibers are allowed a certain tolerance in the core/cladding geometry. Page 2-6 FIBER OPTIC BASICS . This can cause light loss at joints between fibers. More dense glass causes light to go slower (smaller “v” =>larger “n”). FIBER OPTIC BASICS Page 2-7 .Index of Refraction (n) “C” is a constant. “V” depends on the density of the glass. These two values determine the acceptance angle and critical angle Θ of the fiber.Index of Refraction (n) How IOR Affects Fiber Characteristics The core of the fiber has an Index of Refraction (n1) which is different than the Index of Refraction of the cladding (n2). Page 2-8 FIBER OPTIC BASICS . four properties can cause attenuation: 1. ABSORPTION occurs when light strikes impurities in the core glass and is absorbed. 4. SCATTERING occurs when light strikes an area where the material density changes. 2. MACROBENDING is large-scale bending of the fiber which exceeds the fiber bend radius and causes light to leave the core and travel in the cladding (usually an installation problem). FIBER OPTIC BASICS Page 2-9 .Attenuation As light is guided through the core. MICROBENDING is microscopic distortion of the fiber which causes light to leave the core and travel in the cladding (created during manufacturing). 3. Causes of Attenuation Page 2-10 FIBER OPTIC BASICS . FIBER OPTIC BASICS Page 2-11 . MATERIAL (or CHROMATIC) dispersion occurs because different wavelengths (colors) of light travel at different velocities through the fiber. MODAL dispersion occurs when various modes of light follows different paths through the fiber and arrive at the far end at different times. This is similar to MODAL dispersion except that it can be of significance in singlemode fibers. 2. WAVEGUIDE dispersion occurs because light travels in both the core and cladding at slightly different speeds. There are 3 types of dispersion: 1. POLARIZATION MODE DISPERSION occurs when the X and Y polarization states of a light signal travel at different speeds through a fiber. It occurs only in multimode fibers.Dispersion DISPERSION is the spread of a pulse of light as it is guided through the fiber. 4. It is most significant in singlemode fibers. 3. Page 2-12 FIBER OPTIC BASICS .Splice Loss Due to Core Mismatch Off-center core in the second fiber does not receive all the light from the first fiber. The amount of light lost is the Splice Loss. Causes of Connector Loss End-Face Separation Angular Separation (end-face not cleaved to perpendicular) Core Misalignment FIBER OPTIC BASICS Page 2-13 . Chapter 3 Optical Measurements FIBER OPTIC BASICS Page 3-1 . Why? FIBER OPTIC BASICS • Verify specs • Check handling • Record best condition • Detect defects • Locate faults • Troubleshoot problems Page 3-3 .Testing Fiber . Testing Fiber .When? Page 3-4 • At Factory • When Received • After Placed • After/During Splicing • System Acceptance • Periodic (Annual) • Troubleshooting FIBER OPTIC BASICS . Testing Fiber .What? FIBER OPTIC BASICS • Continuity • Average Loss (dB/Km) • Splice Loss & Location • Reflectance / ORL • End-to-End Attenuation • Overall Length Page 3-5 . Page 3-6 FIBER OPTIC BASICS . Multimode optical measurements require a mandrel wrap at the source end. Shown here is the proper method for storing a reference and then measuring loss. Bi-directional testing further improves loss accuracy.Attenuation Measurement The simplest and most accurate method of measuring the end to end loss of an optical span is with a light source and a power meter. OTDR Block Diagram The OTDR sends out a pulse of light and measures the level of light that is reflected back. An optical coupler allows both optical source and optical receiver to be connected to the same fiber. FIBER OPTIC BASICS Page 3-7 . The OTDR knows how far it needs to measure because of the fiber length setting. SLOW) the unit will send out either 2048. or 261. depending on the current pulsewidth.How an OTDR Works The modern OTDR knows how fast light will travel through the core of the fiber under test because of the index of refraction (IOR) setting. The sample points collected are in this way averaged to present screen trace. Page 3-8 FIBER OPTIC BASICS . GN Nettest’s CMA4000 takes up to 16.288 pulses of light. MED. In Average mode (FAST. sampling would occur every 8 meters (128 km/16384). With this information. This means that if the fiber length setting was 128 km. 32768. the OTDR will repeatedly sample the level of reflected light. In REAL TIME mode the CMA4000 sends out between 64 to 256 pulses of light per trace.384 samples of reflected light per pulse of transmitted light. Reflected Light Rayleigh Backscatter = 5 log (Po WS)-10αx(log e) Where: Po is the launched optical power in watts W is the transmitted pulsewidth in seconds S is the scattering factor expressed in watts/Joule α is the fiber attenuation coefficient in Nepers/m x is the distance along the fiber FIBER OPTIC BASICS Page 3-9 . Page 3-10 FIBER OPTIC BASICS . As light passes from one density to another. there is some scattering of the light. When light passes from one index of refraction to another some light is always reflected back.Returned Light An OTDR relies on returned light for making measurements. Reflected light is called FRESNEL REFLECTION. and a small amount returns to the OTDR. Glass density is not uniform. This scattering is the result of variations in the density of the core glass. As light passes from the index of the core to the lower index of air. The backscattering of light is called RAYLEIGH SCATTERING. There is an air gap between fiber ends joined with mechanical connectors. Initial and End Fresnel Reflections are good examples of events resulting from the glass-to-air transition. There are two forms of returned light: reflected and scattered. a high amount of reflection can take place. if “n” is incorrect.OTDR Distance Measurement Where: d is distance c is the speed of light t is the time n is the Index of Refraction As shown in the formula above. then the distance measured will also be wrong! FIBER OPTIC BASICS Page 3-11 . OTDR Distance Measurements • Index of Refraction is set for a wavelength to measure fiber distance • More fiber than cable (approx. Need to compensate for loops & slack in fiber and cable • Use landmarks to correlate OTDR to ground distances Page 3-12 FIBER OPTIC BASICS . 2 to 6%) • Ground location is most important. and detects REFLECTIONS • Compares BACKSCATTER levels to determine loss between points in fiber • Splice losses determined by amount of shift in backscatter FIBER OPTIC BASICS Page 3-13 .OTDR Loss Measurements • OTDR measures BACKSCATTER. Page 3-14 FIBER OPTIC BASICS . The difference in strength between two points of backscatter is the same as the difference in strength between the test pulse at the same two points. so does the backscatter.OTDR Loss Measurements Backscatter is directly related to the signal in the test pulse. As the signal decreases. OTDR Trace Basics FIBER OPTIC BASICS Page 3-15 . Measuring “ORL” Optical Return Loss ORL is calculated as the total amount of light returning from the area between the cursors below the trace line to the noise level. Page 3-16 FIBER OPTIC BASICS . It includes total Backscatter and all Reflections. Wavelength Scattering Loss Difference FIBER OPTIC BASICS Page 3-17 . Wavelength Bending Loss Difference Higher wavelengths are more sensitive to bend losses. Page 3-18 FIBER OPTIC BASICS . Deadzones • Specified as a DISTANCE • Determines how CLOSE to the OTDR you can detect and measure a splice loss • Determines how CLOSE TOGETHER two events (splices) can be measured • Directly related to PULSE WIDTH: larger pulse widths produce larger dead zones FIBER OPTIC BASICS Page 3-19 . Page 3-20 FIBER OPTIC BASICS .Deadzones A dead zone is the portion of a trace where an OTDR cannot take accurate measurements because it is in a recovery or transitional state. where another pulse could be distinguished if present. FIBER OPTIC BASICS Page 3-21 .5 dB from the top of an unsaturated reflection.5 dB down after pulse recovery.Event Deadzone An Event Deadzone is the area between two points 1. An Event Deadzone ends 1. 5 dB above the extrapolated backscatter line.Attenuation Deadzone An Attenuation Deadzone is measured from the beginning of a pulse to a point 0. Page 3-22 FIBER OPTIC BASICS . Fresnel Characteristics FIBER OPTIC BASICS Page 3-23 . Fusion Splices A fusion splice exhibits a deadzone approximately equal to the pulsewidth. Page 3-24 FIBER OPTIC BASICS . add the results. The loss or gain at the splice can appear much larger than the actual transmission loss.Why Losers and Gainers? TBs equals the total backscatter W1 equals the field radii of transmitting fiber W2 equals the field radii of receiving fiber The loss or gain in backscatter power across a splice due to different mode field radii is calculated using the above formula. Therefore. This happens when the fiber is only measured in one direction. FIBER OPTIC BASICS Page 3-25 . then divide by two. for greater accuracy. Refer to the diagrams on page 3-24. take loss measurements in both directions. • Describes how much loss an OTDR can measure in a fiber.Dynamic Range • Measured in dB. which in turn describes how long of a fiber can be measured • Directly related to Pulse Width: larger pulse widths provide larger dynamic range • Increase by using longer PW and by decreasing noise through averaging Page 3-26 FIBER OPTIC BASICS . Typical range is 20-40dB or more. FIBER OPTIC BASICS Page 3-27 . This is the usable portion of the trace.Backscatter vs. Dynamic Range (SNR=1 method) is measured down from the highest point of normal backscatter to approximately 70.7% of the peak noise floor. This does not include initial Fresnel reflection and recovery. Dynamic Range Backscatter range is from the bottom of the screen at zero dB to the highest point of normal trace backscatter. Resolution • Described as a DISTANCE • Two Types: Data Sampling Spatial Resolution (from Dead Zones) • Determines: Accuracy of event location. Page 3-28 FIBER OPTIC BASICS . If you can measure two closely spaced splices in the fiber. FIBER OPTIC BASICS Page 3-29 . Pulsewidth is the same in both cases. illustrating the effective accuracy in splice location. and is not affected by sampling.Data Sampling Resolution The above graph charts resolution at both 8 meters and 16 meters. The OTDR reports this as a Grouped Event as it can not determine where one splice ends and the next begins. It is impossible to tell by looking at the trace which splice is causing the high loss. Page 3-30 FIBER OPTIC BASICS .Spatial Resolution Dead Zone Effects From Using Long Pulse Width Long Pulse Width produces a longer Dead Zone preventing the detection and measurement of individual splices. The Pulse Width strikes the second connection before clearing the first connection. The Pulse Width clears the first connection before striking the second. This produces Rayleigh Scattering between the splices allowing individual measurement.Spatial Resolution Dead Zone Effects From Using Short Pulse Width Short Pulse Width produces a shorter Dead Zone allowing each splice to be measured individually. It is now quite easy to determine which splice is causing the greater loss. FIBER OPTIC BASICS Page 3-31 . Short Pulse Widths By using a Long Pulse Width takes longer to make the transition from backscatter of the first fiber to backscatter of the second fiber.Spatial Resolution Long vs. Page 3-32 FIBER OPTIC BASICS . Short Pulse Width makes a sharper transition.