Optical Fundamentals and Testing. Tony Lowe Technical Sales Specialist May 2010

Transcription

1 Optical Fundamentals and Testing Tony Lowe Technical Sales Specialist May 2010

2 Optical Fundamentals and Testing Introduction to fiber optics Fiber Characteristics Optical Loss Testing Attenuation Insertion Loss Optical Return Loss Testing Rayleigh Backscatter Fresnel Backreflections Connectors Visual Fault Locators Live Fiber Detectors Introduction to OTDRs Set Up Parameters Live Demo Conclusion

3 Optical Challenges Due to the network growth and demand for bandwidth, network engineers, technicians, and contractors face new testing challenges. The ability to gather data and make decisions based on that data is becoming critical. Some basic optical terms covered in this session are: Optical Return Loss (ORL) Optical Loss Budget Reflectance Index of Refraction (IOR) Pulse Width Insertion Loss (IL) Attenuation Macrobends Gainers Distance Scale Launch Conditions Backscatter 3

4 Optical Testing Testing requirements are changing. Technicians are now expected to get more information from their test gear. Just knowing the fiber length is not enough. Manufacturers, Engineers and Managers need to be able to interpret the measurements made by the OTDR. The data obtained from the test gear must be: Credible Capable of Documentation Technicians need to be able to look at the data obtained by the equipment and make decisions based on the fact that: 1. Optical loss min/max windows at the receiver are tighter making even the smallest macrobend unacceptable 2. Lasers and networks becoming less tolerable with optical return loss. 3. Dirty connectors and poor reflectance events can take a network down. 4. Attenuation levels in older cables must be realized for WDM and high speed networks. 5. Launch conditions at the OTDR can jeopardize the credibility of the trace data 4

5 Advantages of Fiber Fiber optic cables have had specific advantages over copper cables. Major of advantages of fiber over copper include: Noise Immunity Low Loss Attenuation Optical fibers Tube Strain relief (e.g., Kevlar) High Bandwidth Light Weight Small Size Transmission Security Inner jacket Shield Outer jacket 5

6 Singlemode and Multimode Propagation Singlemode fiber NA Pulse X distance Loss db/km Multimode fiber NA Pulse X distance Pulse spreading = Modal Dispersion 6

7 Telecom Fiber Types Singlemode Multimode Core Cladding Core Cladding Core Cladding 9/125 (µm) 62.5/125 (µm) 50/125 (µm) 7

8 Index of Refraction (IOR) The velocity at which light travels through in a material is determined by the refractive index of that material. The refractive index (n) represents the ratio of the velocity of light in a vacuum to the velocity of light in a material. n= c v _ Speed of light in a vacuum (299,792,458 meters/sec) Speed of light in the material Water nm Diamond nm

9 Fiber Construction Fibers are made of glass consisting of a core and a cladding that will allow propagation of light by total internal refraction. Total internal refraction is achieved in the fiber by having two different refractive indexes the core IOR is higher than the cladding IOR Fiber Cladding The cladding IOR is slightly lower than the core IOR. This will «bend» the light to keep it in the core area. Fiber Core Will act as a «mirror tunnel» for the light propagation. Core IOR > Cladding IOR 9

10 Fiber Basics 101 Every fiber optic system has three basic components: A Source (Transmitter) A Receiver A Fiber for connection (Single Mode or Multimode) Transmitter The source of the light used in the optical communications. Their purpose is to convert electrical signals into an optical signal which can be carried over the fiber. Their purpose is called modulating the source of the light. This modulation can be digital or analog. Receiver Performs the opposite function of the transmitter, it converts optical signals into electrical. The signal can be corrupted through the fiber making it hard for the receiver to understand what signal was sent. Typical Fiber Link Tx Fusion Splice Bend Connector Pair Mechanical Splice Crack Fiber End Rx 10

11 db and dbm When should we use them? Decibel - db The db is a logarithmic scale that is used to tell us what percentage of the original power will be present after transmission along the fiber link Standard logarithmic unit used to express the ratio of two quantities. Is used to express GAIN or LOSS; usually used to compare power in to power out. dbm Decibel referenced to one milliwatt (mw). i.e. The ratio uses a constant of 1 mw as output power. Power in dbm Power in mw

12 Optical Loss Testing Once a fiber span is built, an optical loss test is done to ensure that adequate signal strength is available at the receiver. But before a loss test is done, at the engineering stage, a loss budget will be calculated based on the minimum output of the transmitter and the minimum sensitivity of the receiver. Typical Fiber Link Tx Fusion Splice Bend Connector Pair Mechanical Splice Crack Fiber End Rx Maximum output: -3 dbm Minimum output: -9 dbm Maximum input: -3 dbm Minimum input: -20 dbm There are two sources of loss that make up the total loss number. Attenuation Insertion Loss Optical Loss 12

13 Rayleigh Scattering - Attenuation One of the two contributing factors to optical loss is Rayleigh scattering. When a pulse of light propagates through a fiber, some of the photons of light are scattered in random directions from microscopic particles. This is a decrease in average optical power (attenuation) as the pulse travels to the receiver. Rayleigh scattering provides amplitude information along the length of the cable to the OTDR. Without this scattering, the OTDR could not give us a trace. Higher density of dopants in a fiber will also create more scattering and thus higher levels of attenuation. Example: flashlight in a fog at night 13

14 Insertion Loss Events Insertion loss is the loss of signal power resulting from the insertion of a device in an optical fiber. Splices, macrobends, connectors, cracks, and components cause insertion loss that add to the total loss of fiber links. OTDRs can be used to measure loss from these events. Differences between Rayleigh backscatter coefficients before and after the event affect insertion loss accuracy. 14

15 Optical Loss Test Sets Light source feature Can generate a modulated signal for fiber identification Power meter options and functions Capable of reading a modulated signal Options for detector thresholds Calibrated Wavelengths 15

16 Optical Return Loss (ORL) The measurement of ORL is becoming more important in the characterization of optical networks as the use of WDM and high speed data increases. These systems use lasers that have a lower tolerance for reflectance. ORL is a measure taken from one end of the total energy reflected back to the source by all the interfaces due to a variation of the index of refraction (IOR), breaks, voids, backscatter, etc., created inside a component or along a link. It is expressed as a positive value. Power in (dbm) Power Reflected (dbm) Optical Return Loss (db) 16

17 Optical Return Loss (ORL) What contributes to ORL? Rayleigh Backscatter Fresnel Backreflection Optical Return Loss (ORL) Rayleigh backscattering: intrinsic to the fiber and cannot be completely eliminated. Fresnel backreflections: caused by different network elements (mainly connectors and components) with air/glass or glass/glass interfaces and can always be improved by special care or better design. 17

18 Rayleigh Backscattering It comes from the natural reflection of the fiber. Backscatter is the amount of light from the outgoing pulse that is scattered back toward the OTDR, which looks at the returning signal and calculates loss based on the declining amount of light it sees coming back. It is a function of the attenuation of the fiber and the diameter of the core of the fiber. 18

19 Fresnel Reflections Fresnel reflection is due to the light reflecting off a boundary of two optical mediums, each having a different index of refractions (IOR). Common sources of reflections are: Open fiber ends Mechanical Splices Cracks Connectors 19

20 ORL Summary Poor ORL can cause: 1. Strong fluctuations in laser output power 2. Instability in the laser due to temperature increase 3. Receiver interference 4. Lower signal to noise ratio 5. Higher BER OC Gig OC Gig OC Gig FTTX Video 24 db 27 db 30 db 32 db The higher the ORL value, the better the network will perform 20

21 Measuring Optical Return Loss/Optical Loss Conclusions ORL testing remains the best insurance for preventing backreflection and promoting a high quality of signal transmission. OTDRs are less accurate measuring ORL since they are affected by noise, distance and pulse width. The last connector in the fiber path can be a major influence in the measuring of ORL and optical loss. Testing for ORL and optical loss is done in both directions. To get the credible data needed for current and future upgrades, testing should be done with equipment designed to test loss and ORL. 21

22 Connector Cleaning and Inspection Inspection techniques: A microscope or fiber probe can be used to inspect connectors A microscope will act 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 22

23 Connector Cleaning and Inspection Physical damages to a connector s endface are permanent and will, in most cases, require a connector replacement. Scratches can generate high loss, but more importantly, cause reflectance that can lead to transmitter issues. a) b) 23

24 Connector Cleaning and Inspection Clean Dust Liquid contamination WHOA! Dry residue Oil from hand Permanently damaged 24

25 Visual Fault Locators Visual fault locators (red lights) can be used for many purposes in testing fiber. A few of those purposes are: Identifies end of cables or jumpers Shows cracks at splice locations or connectors Finds breaks in the cable jumpers Illuminates macrobends in the fiber 25

26 Live Fiber Identifiers All fibers are live, but only one needs to be worked on or disconnected. Which one do I pull? Needs upgrade Rx Side Which one is it??? Tx Side 26

27 Live Fiber Identifiers Technicians will have the need to identify an optical fiber by detecting the optical signal being transmitted through the fiber. They might not have access to the end or cannot disconnect in fear of dropping an important customer s service. He might be looking for: Traffic detection and direction Power estimation Minimal loss <1 db Modulation Recognition 27

28 Live Fiber Identifiers Signals sent from the transmitter on fiber 1 might not be detected on fiber 1 by the receiver. Poor documentation can create a potential problem when the technician needs to do maintenance. 28

29 Live Fiber Identification Testing Solution 1. Verify the tone and traffic direction 2. Clip the generator onto the live fiber on the transmit side of the network. 3. Clip the live fiber detector device to the far end of the system 29

30 Live Fiber Detector Technology The unit clipped on to the transmit side creates a recognizable signal on the fiber. The signal characteristics are: - <1dB loss - 11Hz modulation - Non-disruptive signature on a live fiber 30

31 OTDR Definition and Overview OTDR Optical Time Domain Reflectometer How does the OTDR acquire data and create a trace? OTDRs launch short duration light pulses into a fiber and then measures, as a function of time after the launch, the optical signal returned to the instrument. As the optical pulses propagate along the fiber, they encounter reflecting and scattering sites resulting in a fraction of the signal being reflected back in the opposite direction. Raleigh scattering and Fresnel reflections are physical causes for this behavior. By measuring the arrival time and amplitude of the returning light, the locations and magnitudes of faults can be determined and the fiber link can be characterized. The OTDR has been used, and is today by many, to test the fiber length and determine if there are any broken fibers in the span. 31

32 Index of Refraction - IOR Mike Andrews How does an OTDR calculate optical distance? How does an OTDR calculate optical distance? 2008 EXFO Electro-Optical Engineering Inc. All rights reserved. It needs to know the values of two variables elapsed time and the speed of light in the glass (IOR) It measures the time launch and the reflection s return It calculates the speed of light in the glass from the Index of Refraction (IOR) Since different fiber types have different IOR values it is important to use the correct value. However, it isn t always known. So, EXFO provided default average values that will be reasonably close. IOR = C/V C equals the speed of light in a vacuum So, we program in the IOR from the fiber type and it does the rest IOR values for different transparent mediums Water: 1.33 Diamond: : :

33 OTDR Average Time Connector End-Face By the time the primary pulse reaches the end of a relatively long optical fiber, most of its energy has been dissipated. The OTDR records the results of the first pulse then launches another and then another. It averages the results of multiple pulse launches to give the operator a clean trace. The parameter that determines how long this happens is simply labeled Average Time in the OTDR setup and is adjustable by the operator. Typical average times range from 5 seconds to 3 minutes. Not much improvement will be seen by averaging longer than 3 minutes (diminishing returns ). In my experience, anything over a minute should lead us to investigate why. Mike Andrews 2008 EXFO Electro-Optical Engineering Inc. All rights reserved. 33

34 OTDR Distance Range Connector End-Face Before the OTDR can launch a 2 nd or 3 rd pulse it must wait long enough to allow all the reflections to have returned from the end of the optical fiber. If it launched the 2 nd (or subsequent) pulse too soon then the reflections from multiple pulses would arrive together leading to a meaningless trace. The OTDR can be set to display a specific length of fiber and this setting (Distance Range) also tells the algorithm how long to wait before launching the next pulse. If the distance range is set shorter than the actual length of the fiber then reflections from multiple pulses may overlap and show apparent events that aren t actually there. It is important to set the distance range to a value slightly greater than the length of the fiber. Good OTDRs have an Automatic feature that lets the algorithm determine the correct setting based on the last reflection that returns from a series of trial pulses. Mike Andrews 2008 EXFO Electro-Optical Engineering Inc. All rights reserved. 34

35 OTDR Pulse Width Another important parameter is the Pulse Width. Put simply, the longer the LASER stays on the more energy is injected into the fiber and the greater the effective range. It would seem that one would always select a long pulse-width, then. However, a long pulse also generates an equally long reflection. If the LASER stays on for 20 microseconds then the pulse itself will be approximately 1.3 miles long. This means that any events that are closer together than 1.3 miles will be masked as one large reflection. So, the operator would choose a shorter pulse width in order to resolve (separate) closely spaced events. This would be sacrificing range. Stated another way it s a balancing act. Choose a shorter pulse width to separate closely spaced events or a longer pulse width for range. Fortunately, good OTDRs have an automatic feature that lets the software algorithm choose the most appropriate pulse width for a given length of fiber. Mike Andrews 2008 EXFO Electro-Optical Engineering Inc. All rights reserved. 35

36 OTDR Pulse Width Pulse width on an OTDR can be represented as time or distance. Thinking about the pulse as distance can help the technician select the proper pulse depending on the information needed. 5 ns pulse = 1.6 feet 100 ns pulse = 33 feet 20 microsecond pulse = 1.3 miles Two events close together can be measured as a single event if the pulse traveling in the fiber goes thorough both events simultaneously. Another place this problem shows up is in splice closures. An OTDR may show a bad splice, but it can actually be a crack or stress point somewhere else in the splice closure. 36

37 Macrobends Macrobendings: A Visual Fault Locator (VFL) can be used to find macrobendings: Bad splices will also shine using a VFL: 37

38 Macrobends What causes a macrobend? What is the effect of a macrobend on a signal? Why is a macrobend more detectable at higher wavelengths (1310 versus 1550)? How do you determine what wavelength you are looking at based on this graph? To understand the cause and effect of a macrobend, you must first discuss how wavelengths propagate down a fiber and the power distribution of that wavelength. The mode field diameter of the wavelength holds the key. MFD is an expression of distribution of the optical power per unit area across the end face of a singlemode fiber EXFO Electro-Optical Engineering Inc. All rights reserved. 38

39 Trace Analysis Attenuation Changes Because of the way in which OTDRs generate their measurements, obtaining accurate splice loss can be difficult due to small variations in fiber characteristics. At a splice point, it is possible for the amount of backscattered light before the splice to be greater than after the splice or vice versa. Can create a mismatch in MFD of the two fibers. An OTDR trace will show these differences either as a gainer or as an exaggerated loss, depending on the direction of the measurement. While the differences in attenuation levels result in gainers and exaggerated losses, the effect on actual splice loss is relatively low (<0.04 db). 39

40 Bidirectional Traces The TIA fiber optical test procedure (TIA-FOTP-61) indicates that splice loss measurements with an OTDR must be conducted from both directions and averaged for accurate results because of gainers and exaggerated losses. Bi-directional traces address the directionality effect. 40

41 OTDR Trace - Events Launch Macrobend Connector Panel Attenuation Bad Fusion Splice 41

46 Questions? Thank you Tony Lowe Technical Sales Specialist 46