The Basics of Fiber Optics

The Basics of Fiber Optics Presented by: AshutoshRastogi Assistant Professor BBDNITM,Lucknow BBDNITM,Lucknow

Introduction • Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances. BBDNITM,Lucknow

Fiber v. Copper • Optical fiber transmits light pulses • Can be used for analog or digital transmission • Voice, computer data, video, etc. • Copper wires (or other metals) can carry the same types of signals with electrical pulses BBDNITM,Lucknow

Advantages of Fiber • Fiber has these advantages compared with metal wires • Bandwidth – more data per second • Longer distance • Faster • Special applications like medical imaging and quantum key distribution are only possible with fiber because they use light directly BBDNITM,Lucknow

Elements of a Fiber Data Link • Transmitter emits light pulses (LED or Laser) • Connectors and Cables passively carry the pulses • Receiver detects the light pulses Cable Transmitter Receiver BBDNITM,Lucknow

Fiber Fiber Fiber Fiber Repeater Repeater Repeater Repeaters • For long links, repeaters are needed to compensate for signal loss BBDNITM,Lucknow

Optical Fiber • Core • Glass or plastic with a higher index of refraction than the cladding • Carries the signal • Cladding • Glass or plastic with a lower index of refraction than the core • Buffer • Protects the fiber from damage and moisture • Jacket • Holds one or more fibers in a cable BBDNITM,Lucknow

Total Internal Reflection • There is a critical angle at which no light can be refracted at all, so 100% of the light is reflected • Light is trapped in the water and cannot escape into the air • This works with any dense medium, such as plastic or glass, the same way it works with water • Image from glenbrook.k12.il.us BBDNITM,Lucknow

How Light Travels in Fiber • Image from ece.umd.edu/~davis BBDNITM,Lucknow

Optical Fiber Transmission Modes BBDNITM,Lucknow

Multimode Step Index Step index has a large core, so the light rays tend to bounce around inside the core, reflecting off the cladding. This causes some rays to take a longer or shorter path through the core. Some take the direct path with hardly any reflections while others bounce back and forth taking a longer path. The result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. Typical Core: 62.5 microns. Index of refraction BBDNITM,Lucknow

Index of refraction Multimode Graded Index Graded index has a gradual change in the core's refractive index. This causes the light rays to be gradually bent back into the core path. This is represented by a curved reflective path in the attached drawing. The result is a better receive signal than with step index. LED light sources are used. Typical Core: 62.5 microns. BBDNITM,Lucknow

Index of refraction Single Mode • Single mode has separate distinct refractive indexes for the cladding and core. The light ray passes through the core with relatively few reflections off the cladding. Single mode is used for a single source of light (one color) operation. It requires a laser and the core is very small: 9 microns. • Best for high speeds and long distances • Used by telephone companies and CATV BBDNITM,Lucknow

Comparison of Optical Fibers BBDNITM,Lucknow

Sources and Wavelengths • Multimode fiber is used with • LED sources at wavelengths of 850 and 1300 nm for slower local area networks • Lasers at 850 and 1310 nm for networks running at gigabits per second or more BBDNITM,Lucknow

Sources and Wavelengths • Singlemode fiber is used with • Laser sources at 1300 and 1550 nm • Bandwidth is extremely high, around 100 THz-km BBDNITM,Lucknow

Fiber Optic Specifications • Attenuation • Loss of signal, measured in dB • Dispersion • Blurring of a signal, affects bandwidth • Bandwidth • The number of bits per second that can be sent through a data link • Numerical Aperture • Measures the largest angle of light that can be accepted into the core BBDNITM,Lucknow

Attenuation and Dispersion BBDNITM,Lucknow

Measuring Bandwidth • The bandwidth-distance product in units of MHz×km shows how fast data can be sent through a cable • A common multimode fiber with bandwidth-distance product of 500 MHz×km could carry • A 500 MHz signal for 1 km, or • A 1000 MHz signal for 0.5 km • From Wikipedia BBDNITM,Lucknow

Numerical Aperture • If the core and cladding have almost the same index of refraction, the numerical aperture will be small • This means that light must be shooting right down the center of the fiber to stay in the core BBDNITM,Lucknow

Fiber Types and Specifications BBDNITM,Lucknow

Optical Fiber - Benefits • Greater capacity • Data rates of hundreds of Gbps • Smaller size & weight • Lower attenuation • Electromagnetic isolation • Greater repeater spacing • 10s of km at least BBDNITM,Lucknow

Fiber Performance BBDNITM,Lucknow

Attenuation • Modern fiber material is very pure, but there is still some attenuation • The wavelengths used are chosen to avoid absorption bands • 850 nm, 1300 nm, and 1550 nm • Plastic fiber uses 660 nm LEDs • Image from iec.org (Link Ch 2n) BBDNITM,Lucknow

Three Types of Dispersion • Dispersion is the spreading out of a light pulse as it travels through the fiber • Three types: • Modal Dispersion • Chromatic Dispersion • Polarization Mode Dispersion (PMD) BBDNITM,Lucknow

Modal Dispersion • Modal Dispersion • Spreading of a pulse because different modes (paths) through the fiber take different times • Only happens in multimode fiber • Reduced, but not eliminated, with graded-index fiber BBDNITM,Lucknow

Chromatic Dispersion • Different wavelengths travel at different speeds through the fiber • This spreads a pulse in an effect named chromatic dispersion • Chromatic dispersion occurs in both singlemode and multimode fiber • Larger effect with LEDs than with lasers • A far smaller effect than modal dispersion BBDNITM,Lucknow

Polarization Mode Dispersion • Light with different polarization can travel at different speeds, if the fiber is not perfectly symmetric at the atomic level • This could come from imperfect circular geometry or stress on the cable, and there is no easy way to correct it • It can affect both singlemode and multimode fiber. BBDNITM,Lucknow

Modal Distribution • In graded-index fiber, the off-axis modes go a longer distance than the axial mode, but they travel faster, compensating for dispersion • But because the off-axis modes travel further, they suffer more attenuation BBDNITM,Lucknow

Equilibrium Modal Distribution • A long fiber that has lost the high-order modes is said to have an equilibrium modal distribution • For testing fibers, devices can be used to condition the modal distribution so measurements will be accurate BBDNITM,Lucknow

Mode Stripper • An index-matching substance is put on the outside of the fiber to remove light travelling through the cladding • Figure from fiber-optics.info (Link Ch 2o) BBDNITM,Lucknow

Mode Scrambler • Mode scramblers mix light to excite every possible mode of transmission within the fiber • Used for accurate measurements of attenuation • Figure from fiber-optics.info (Link Ch 2o) BBDNITM,Lucknow

Mode Filter • Wrapping the fiber around a 12.5 mm mandrel • Exceeds the critical angle for total internal reflection for very oblique modes • The high-order modes leak into the cladding and are lost • That creates an equilibrium modal distribution • Allows an accurate test with a short test cable • Figure from fiber-optics.info (Link Ch 2o) BBDNITM,Lucknow

Decibel Units BBDNITM,Lucknow

Power Out Power In Data Link Optical Loss in dB (decibels) • If the data link is perfect, and loses no power • The loss is 0 dB • If the data link loses 50% of the power • The loss is 3 dB, or a change of – 3 dB • If the data link loses 90% of the power • The loss is 10 dB, or a change of – 10 dB • If the data link loses 99% of the power • The loss is 20 dB, or a change of – 20 dB dB = 10 log (Power Out / Power In) BBDNITM,Lucknow

Absolute Power in dBm • The power of a light is measured in milliwatts • For convenience, we use the dBm units, where -20 dBm = 0.01 milliwatt -10 dBm = 0.1 milliwatt 0 dBm = 1 milliwatt 10 dBm = 10 milliwatts 20 dBm = 100 milliwatts BBDNITM,Lucknow

Bare Fiber • During 1920-1950, thin, flexible rods of glass or plastic were used to guide light • Such “bare” fibers require air outside each fiber • Image from Wikipedia BBDNITM,Lucknow

Fiber With Cladding • Developed in 1954 by van Heel, Hopkins & Kapany • Cladding is a glass or plastic cover around the core • Protects the total-reflection surface contamination • Reduces cross-talk from fibers in bundles BBDNITM,Lucknow

Optical Fiber BBDNITM,Lucknow

Attenuation BBDNITM,Lucknow

Optical Fiber - Applications • Long-haul trunks • Metropolitan trunks • Rural exchange trunks • Subscriber loops • LANs BBDNITM,Lucknow

Optical Fiber - Transmission Characteristics • Act as wave guide for 1014 to 1015 Hz • Portions of infrared and visible spectrum • Light Emitting Diode (LED) • Cheaper • Wider operating temp range • Last longer • Injection Laser Diode (ILD) • More efficient • Greater data rate • Wavelength Division Multiplexing BBDNITM,Lucknow

Fiber Optic Types • Multimode step-index fiber • the reflective walls of the fiber move the light pulses to the receiver • Multimode graded-index fiber • acts to refract the light toward the center of the fiber by variations in the density • Single mode fiber • the light is guided down the center of an extremely narrow core BBDNITM,Lucknow

Optical Fiber Optical fiber Optical fiber consists of thin glass fibers that can carry information at frequencies in the visible light spectrum and beyond. The typical optical fiber consists of a very narrow strand of glass called the core. Around the core is a concentric layer of glass called the cladding. A typical core diameter is 62.5 microns (1 micron = 10-6 meters). Typically Cladding has a diameter of 125 microns. Coating the cladding is a protective coating consisting of plastic, it is called the Jacket. BBDNITM,Lucknow

Refraction in Fiber An important characteristic of fiber optics is refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it "bends" as it passes from one medium to the other. An example of this is when we look into a pond of water. BBDNITM,Lucknow

Angle of Incidence If the angle of incidence is small, the light rays are reflected and do not pass into the water. If the angle of incident is great, light passes through the media but is bent or refracted. Optical fibers work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops light from escaping through it - it bounds the medium! BBDNITM,Lucknow

Thanks BBDNITM,Lucknow

The Basics of Fiber Optics