May 19, 2011

Bill Reynolds Technical Support Engineer

Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

Fiber Optic Theory

Spectrum Optical fiber domain 850 nm  353 000 GHz λ (nm)=c (m/s) / f (Hz) Units Micrometers (mm) - 10-6 m Nanometers (nm) - 10-9 m

Index of Refraction n = c vac / c mat The speed of light = c = 299793 km/s under vacuum Any material that can transmit light has it’s own index of refraction represented by n Example n = c vac / c mat In a given index of refraction, the speed of light gets slower Speed of light in water ≈ 225 000 km/s Speed of light in fiber optic ≈ 215 300 km/s Index (n) 1.000 1.400 1.333 Material Air Fiber Optic Water

Fiber Optic Coating Cladding Core Acrylate, Teflon, polyimide Glass index n2 Core Glass index n1

Fiber Optic Coating diameter = 250 µm Cladding diameter = 125 µm Core diameter = 9, 50, 62.5 µm

Reflection θi = θR Reflection When a light beam I hits a material with a different index of refraction, a portion of the beam is reflected R The angle of this reflected beam is the same as the incident beam n1 θi θR Core I R n2 Cladding θi = θR

Refraction n1 sin(θi) = n2 sin(θr) Refraction For the same light beam hitting a different material, another portion is refracted This occurs when a the light goes through a material with a different index of refraction The angle of this beam changes because the speed of propagation changes n1 θi θR Core I R n2 θr T Cladding n1 sin(θi) = n2 sin(θr)

Total Internal Reflection Critical angle (Total internal reflection) There is a certain angle where 100% of the light is reflected and no light is refracted, we call this angle, the critical angle Fiber optics use this concept to propagate light Cladding Core I R1 θC θR R2

Fiber Types Singlemode Multimode ITU-T G.652D There are 2 fiber optics types in telecommunications Singlemode Multimode Core Core Core Cladding Cladding Cladding 62.5/125 (µm) 50/125 (µm) 9/125 (µm) ITU-T G.652D For telecommunications applications

Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

Inspecting & Cleaning

Inspection Inspection techniques: A microscope or fiber probe can be used to inspect connectors A microscope acts as a magnifying glass. If you inspect a connector on a live fiber, permanent damage can be done to your eyes! Using a fiber probe is the safest was to inspect a connector:

Cleaning Cleaning Techniques: The best way to clean connectors can be done by following these easy steps: Clean the outside of the ferrule with a wet pad Clean the ferrule using a dry pad Inspect the connector using a fiber probe If the connector is still dirty, repeat the 2 previous steps with a wet pad Dirty ferrule Clean ferrule

80% of network problems are related to dirty connectors! Why Clean? 80% of network problems are related to dirty connectors! Permanently burnt – combined high power and dirt Clean Permanent damage can occur on dirty connectors on high power systems RF video may reach +23 dBm @ 1550nm MDUs now includes MPO connectors MPO are tricky to clean View of an multi-fiber angle-polished connector 18

Cleaning Bad cleaning results Broken surface

ICIC: Inspect – Clean – Inspect - Connect Damaged Dirty Clean ICIC: Inspect – Clean – Inspect - Connect

Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

OTDR Testing

Attenuation in fiber is wavelength-dependent Water peak C: 1310nm = 0.34 dB/km D: 1383nm = 0.50 dB/km E: 1550nm = 0.19 dB/km Optical fiber is normally tested at the same wavelength that the fiber system will be operated. Water peak Corning SMF-28 SM Fiber

OTDR Principle The OTDR can be compared to a submarine radar. Instead of sending RF or audio (submarine) signal to detect distant objects, it sends short pulses of light to detect events in fiber. The OTDR locates and identifies events along the fiber.

Setting Up the OTDR

Rayleigh Backscattering Comes from the “natural” reflection of the fiber The OTDR will use the Rayleigh back reflections to measure the fiber’s attenuation (dB/Km). Back reflection level is around -75 dB (depends on pulse length) Higher wavelength will be less attenuated by the Rayleigh Backscatter Source Ray of light Dopant particules

Fresnel Reflections Fresnel back reflections Will come from abrupt changes in the IOR, ex: (glass/air) Fiber break, mechanical splice, bulkheads, connectors Will show as a “spike” on the OTDR trace Reflection are typically UPC –45 dB and APC –65 dB (Typical OTDR results) Fresnel reflections will be approximately 20,000 times higher than the fiber’s backscattering level Will create a « Dead Zone » after the reflection

Pulse versus resolution and dynamic range Short Pulse : more resolution but less energy Long Pulse width: more energy but less resolution

Pulse Width Short pulses will give a better resolution but less dynamic range: Two connectors 3 meters apart End of link (patch panel) Connectors are measured for distance and marked as separate events 5ns pulse End of fiber is not reached due to low power of short pulses Long pulses will give a better dynamic range but less resolution Connectors are « merged » and identified as one event 30ns pulse End of fiber is reached and located when using a larger pulse

10 us pulse

20 us pulse

OTDR Trace Single ended measurement UPC APC OTDR Connector Fusion splice Connector (P.P.) Connector (P.P.) End of link UPC Reflection Power (dB) Loss APC Slope shows fiber attenuation Distance (km)

Macrobend Loss in fiber is wavelength-dependent Acquisition at 1310 nm Acquisition at 1550 nm Shorter wavelengths are more attenuated by fiber’s scattering Longer wavelengths tend to leak out of the fiber more easily due to bending

Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

OLTS Testing Insertion Loss

Insertion testing is done in pairs Dual ended measurement

Measurement Units The laser output is -3.50 dBm dBm The dBm is use to measure the output power of a light source Instrument reading - 3.50 dBm Fiber optic Detector Laser source The laser output is -3.50 dBm

Measurement Unit 1 mW 0.00 dBm mW How to convert the dBm in mW dBm = 10*log(mW) The laser seen on previous page was emitting -3.50dB -3.50 dBm so 0.45 mW Optical Power mW vs dBm 10000 mW +40.00 dBm 1000 mW +30.00 dBm 10 mW +10.00 dBm 1 mW 0.00 dBm 500 µW -3.00 dBm 100 µW -10 dBm 10 µW -20 dBm 1 µW -30 dBm 100 nW -40 dBm 10 nW -50 dBm 1 nW -60 dBm 100 pW -70 dBm

Measurement Units dB (relative power) dB is the difference between 2 power measurements Take the -3.50 dBm laser shown previously Laser output = -3.50 dBm There is an event on the fiber and the detector reads -4.25 dBm -3.50 dBm To calculate this difference: (-3.50 dBm) – (-4.25 dBm) = 0.75 dB We have lost 0.75 dB So the insertion loss is -0.75dB -3.50 dBm -4.25 dBm

Reflectance [dB] = Preflected [dBm] - Pincident [dBm] Will come from abrupt changes in the IOR: Fiber break, mechanical splice, bulkheads, connectors, etc. We use the term « reflectance » when speaking of the amount of energy returned by specific points within the network Expressed as a negative value Connector 1 = -40dB Connector 2 = -50dB Q: Which one has the best reflection value? Reflectance [dB] = Preflected [dBm] - Pincident [dBm] Patch Panel Fiber section Connector Fiber section Fiber section Mec. Splice Patch Panel Connector reflectance: -55dB Connector reflectance: -45dB Mechanical splice reflectance: -45dB

Optical Return Loss (ORL) Comes from the amount of energy lost within components and fiber due to back reflections We use the term « ORL » when speaking of the amount of energy returned by a section or an entire link Expressed as a positive value Link ORL = 35dB Section 2 & 3 ORL = 45dB Patch Panel Fiber section Connector Fiber section Fiber section Mec. Splice Patch Panel

Rayleigh Backscatter + Reflectance = ORL Optical Return Loss (ORL) represents the sum of all of the light returned to the source from the fiber link under test.

Consequences of ORL • Less light is transmitted • Causes interference with light source signals • Creates higher bit error rate (BER) in digital systems • Reduces signal-to-noise ratio (SNR) in analog systems • Causes fluctuations in the light source’s central wavelength • Causes fluctuations in its output power • Damages the light source permanently

UPC Connectors SC/UPC FC/UPC Ultra Physical Contact LC/UPC

APC Connector SC/APC FC/APC Angle Physical Contact LC/APC

Mitigating ORL Backscatter Reflectance Is an inherent property of fiber and therefore there is nothing you can do about it. However backscatter will have minimal affect on ORL Reflectance Dirty connectors!!! Damaged endfaces Improper mating APC-UPC etc. Clean connectors, clean connectors, clean connectors!!

Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

Dispersion

Data rate on long fibers! Limits length on high data rate fibers! Dispersion is the Fundamental limiting factor in transmission links and determines the:

Dispersions Multimode Fiber Optical Paths Optical 1 Frequencies 2 Difference in Arrival Times Chromatic Dispersion Polarization Modes Polarization Mode Dispersion Input Pulse Output Pulse

A light pulse will spread and lose peak power as it travels down the Fiber.

The Pulse Broadening Causes intersymbol interference and bit errors Each light pulse representing a data bit is smeared out and runs into the time slot allotted to the next bit, giving bit errors

Dispersion was never an issue because: Low Transmission Rates (DS-0 to OC-48) Switching equipment (OEO) Regens were used, No EDFAs Single transmission wavelength was near zero dispersion point. (SMF-28 1300nm = Lambda 0)

PMD Testing

PMD is Stochastic (random) System Tolerance Average PMD

PMD for 99.9954% probability that the tolerable broadening will correspond to a mean power penalty of 1 dB SONET-SDH Bit rate (Gbit/s) Average DGD* (ps) 2.5 (OC-48) 10 (OC-192) 40 (OC-768) 40 10 2.5

Birefringence Speed varies with Index of Refraction (density) Fiber cross-section can have variable Index, which is called Birefringence

Birefringence Fiber manufacturing Fiber Geometry Internal Stress Core Concentricity Lateral Pressure Environmental constraint Heat Bend Wind (aerial fibers)

Birefringence Medium PMD High PMD The causes Asymmetries in fiber core geometry and/or stress distribution create fiber local birefringence. A "real" fiber is a randomly distributed addition of these local birefringent portions. Medium PMD High PMD

Visualizing PMD Let’s visualize a light pulse traveling into a fiber

Polarization Mode Dispersion PMD z,t

Polarization Mode Dispersion PMD h Coupling length β Bi-refringence

t z,t

T0 T t Dt fast axis z, t slow axis Dt

Dt t fast axis z, t slow axis Dt

PMD Impact When the detector receives this: It sees: Clear 1 and clear 0 0 1 0 1 0 When the detector receives this: It cannot differentiate 1 and 0, this is when we get BER!!! ? 1 ? 1 0

Typical Thresholds Average PMD Bit rate (ps) (Gbit/s) 2.5 40 10 10 40

PMD in a cable Cable 1 (18.6km): Fiber 1: 7.82ps Fiber 2: 23.15ps

Dual ended or Single Ended measurement

Cumulative PMD PMD adds quadratically: PMDTOT =N(PMDN)2 Example: 15ps + 2ps + 1ps + 6ps = 16.31ps

Mitigation of PMD

CD Testing

Dispersions Multimode Fiber Optical Paths Optical 1 Frequencies 2 Difference in Arrival Times Chromatic Dispersion Polarization Modes Polarization Mode Dispersion Input Pulse Output Pulse

Material Dispersion Each wavelength as it’s own speed in the glass material therefore it refracts at a different angle, separating the colors

Material Dispersion DFB Laser source Zoom Central Wavelength dBm DFB Laser source Zoom Wavelength Spectral width The pulse is made out of all laser frequencies Amplitude Amplitude Time Time

Waveguide Dispersion N1 N2 With increasing wavelength, more power is traveling in the cladding section of the fiber Mode Field Diameter N1 Wavelength 1 N2 Wavelength 2 Wavelength 3

Waveguide Dispersion Index Index profile of step index singlemode fiber G.652 Diameter Index Index profile of dispersion shifted fiber G.653 Diameter

1. Light will propagate both in core and cladding 2. Core and Cladding have different refractive indexes 3. Spectral width of source consists of several wavelengths 4. Different wavelengths will propagate at different velocities this is because the refractive index is wavelength dependant.

Pulse Spreading occurs because different wavelengths of light travel at different velocities Sources are not truly mono-chromatic (one λ) so each wavelength component will travel at a different speed 1 2 Single Mode Fiber

The Laser has finite line width Spectral width is measured at FWHM (full width at half maximum) really consists of several wavelengths. nm Wavelength (nm)

Therefore: Within the spectral width of the laser the different wavelengths will propagate at different velocities within the core and at different velocities between the light in the core and the cladding. This is the effect of both material dispersion and waveguide dispersion, which combines together into chromatic dispersion

Chromatic Dispersion (CD) specified in ps/nm*km Total Chromatic dispersion of a link is specified in ps/nm Lambda Zero or zero dispersion is the wavelength in which there is no Chromatic Dispersion SMF-28 has about 17ps/nm*Km at 1550nm CD Limits:

Chromatic Dispersion CD is an addition of 2 chromatic dispersion types

Fiber Types +17 dispersion unshifted G.652 Chromatic Dispersion + + Chromatic Dispersion (ps/nm-km)  dispersion shifted G.653  non-zero dispersion non-zero dispersion shifted G.655  (nm)      

Chromatic Dispersion (ps/nm -km) Corning LEAF Lucent TrueWave Balanced + Lucent TrueWave +4 Reduced Slope +2 G.653 Corning LS Chromatic Dispersion (ps/nm -km) Lucent TrueWave Balanced - 1530 1540 1550 1560 1570 - 2 Corning MetroCor - 4 S-band EDFA C-band EDFA L-band

Threshold

Threshold 2.5 Gb/s Time slot 125 us 10 Gb/s Time slot 125 us 40 Gb/s Cause #1 2.5 Gb/s Increasing the speed of transmission by 4 will decrease the threshold by 16. This is due to 2 reasons Time slot 125 us 10 Gb/s Time slot 125 us 40 Gb/s Faster bit rate means less space between pulses Time slot 125 us

Threshold The chirp effect @ 2.5 Gb/s modulation @ 10 Gb/s Cause #2 The chirp effect P @ 2.5 Gb/s P  modulation P  @ 10 Gb/s Pulse before modulation  Faster bit rate means broader pulses P @ 40 Gb/s 

Threshold Mask Threshold Increases BER

Dual ended or single ended measurement FLS-5800 & FTB-5800 Dual ended or single ended measurement

Results Table

Mitigation of CD: Compensation could be preformed: Per span: Negative dispersion fiber Per channel: DCM Per path: Tunable Dispersion Compensator

Summary: CD & PMD PMD and CD can increase BER and distortion. PMD and CD are expressed in ps PMD depends on the cable type and age and may change with environmental conditions, so it is important to test PMD on a regular basis. To correct PMD, faulty fiber sections must be identified and changed CD is a stable phenomenon as it is relative to refractive index of the fiber used To correct CD, compensators may be used

Overview

Agenda Fiber Optic Theory Connector Cleaning OTDR Testing OLTS Testing CD Testing PMD Testing CWDM/DWDM

CWDM/DWDM Difference testing

WDM Wavelength Division Multiplexing WDM technology supports simultaneous carriers/wavelengths/Protocols traveling along one fiber

WDM: Wavelength Division multiplexing In order to increase the number of bits transmitted per fiber, multiple different wavelengths could be transmitted in the same fiber or system. Mux/ demux and filters will combine and select the wavelength

WDM: Wavelengh Division Multiplexing Class of WDM : Wide WDM Devices: Access channel spacing > 50nm 1310 , 1490 , 1550, 1625... Coarse WDM : Metro channel spacing > 200 GHz (1.5nm @1550nm) but < 50 nm (typical less than 18 lambdas) Common spacing 20nm Dense WDM : Long haul channel spacing  200 GHz

WDM bands

CWDM Application CWDM is a WDM technology using 20nm between channels for a total count of 18 Channel in the O, E, S, C,L bands. This technology is deployed in Access and metro networks where the cost per bit is critical.

CWDM Specifications Chromatic dispersion transmission rates are a LOT lower than for DWDM CWDM is for now limited to 2.5Gbps Reason: Direct modulation causes CHIRP CD is important to test, even for 2.5Gbps CHIRP is induced by the modulation, the higher the bit rate the wider the signal mw Ghz

DWDM technology O-Band E-Band S-Band C-Band L-Band 1270 1290 1310 1330 1350 1370 1390 1410 1430 1450 1470 1490 1510 1530 1550 1570 1590 1610 1549.72 1550.12 1550.52 1550.92 1551.32 1551.72 1552.12 1552.52 1552.93 1553.33 1553.73 1554.13 1554.54 1554.94 1555.34 1555.75 1556.15 1556.55 1556.96 1557.36 1557.77 1558.17 These are only a small number of the ITU-T DWDM channel plan. There are hundreds more!

DWDM technology DWDM is usually deployed where high bit rate and long reach are required

CWDM OTDR Channel Verifier Optical Spectrum Analyzer Testing CWDM OTDR Channel Verifier Optical Spectrum Analyzer

CWDM Channel Verifier

OSA Optical Spectrum Analyzer

System Critical Parameters Channel Spacing Power Level Center Wavelength OSNR Peak Power Laser transmitter output power level, which should be as high and as stable as possible to increase the transmission span length. Puisque plusieurs éléments optiques varient en fonction de la puissance il est primordial que la puissance de chaque canaux soit connu et monitoré. Central Wavelength The central wavelength of each channel in the signal is probably the most important single characteristic, given that its accuracy ultimately determines the ability of the source to communicate with the receiver. These values must also be monitored during maintenance programs to detect unacceptable drift. The accuracy of central- wavelength measurement increases in importance as channel bandwidths and spacing are reduced. Évidemment toutes les composantes WDM sont fonction de la longueur d’onde, alors il est important que celle-ci soit précise et constante. Channel spacing The channel spacing capability in GHz (ITU-T has defined a standard channel spacing of 100 GHz—about 0.8 nm—in the ITU grid; 50 GHz is considered in the grid—about 0.4 nm—and eventually lower values will be added.) SNR Although the BER is the best single parameter to characterize the performance of a link, it is determined principally by the optical signal-to-noise ratio (OSNR). It is an indication of the readability of the received signal; a parameter of increasing interest as the limits for long distance applications are pushed farther and farther. Plusieurs system vendors demandent de mesurer seulement les SNR pendant le commissioning de leur system, pas de BER. SNR est finalement la somme de la puissance, du bruit, de l’amplification, du center wavelength et drift. De plus, le SNR pourra donner un bonne idée du problème selon la valeur obtenues.

A power meter will mesure the POWER versus wavelenght Power meter versus OSA Sensitivity Sensitivity Wavelength Wavelength A power meter will mesure the TOTAL POWER +5dBm An OSA will mesure the POWER versus wavelenght

CWDM Solution for Multiple Users Head-end Each customer has a dedicated wavelength Each customer has different distance from head-end Conventional OTDR cannot test through CWDM system Need CWDM OTDR for system installation, maintenance and trouble shooting

Troubleshooting CWDM

Testing CWDM Specialized test equipment with wavelengths that match the CWDM filters are capable of testing end-to-end loss and optical return loss thus assuring the user that all requirements are met.

Thank You