Engr. Hyder Bux Mangrio Engr. Fayaz Hassan Mangrio Lasers & Fiber Optics Engr. Hyder Bux Mangrio Engr. Fayaz Hassan Mangrio

Introduction L&FO labs Lab #01: Introduction to Fiber optics Communication System Lab #02: Optical Sources Lab #03: Optical Detectors Lab #04: Optical fiber attenuation losses Lab #05: Analog voice transmission Lab #06: Understanding basic function of S122A splicer Lab #07: Perform Fusion/Mechanical Splicing

Introduction Lab #08: Understanding the basic function of OTDR Lab #09: Perform fiber measurement on OTDR Lab #10: Fiber attenuation measurement using Cut-Back method Lab #11: Optical Field Spectrum Analyzer Lab #12: Overview of Power meter & Light Source

Information Email: hyder.bux@faculty.muet.edu.pk Webpage: https://sites.google.com/a/faculty.muet.edu.pk/hydermangrio/ Optical Communication Laboratory Consultation Timings: Monday(8am to 3pm) & Friday (8am to 1pm)

Laboratory There will be at least 13 labs covering in 13 weeks course. Each lab will be approximately 2 hours long. The lab report / Handout is due to the lab assistant before next lab.

What is lightwave technology? Lightwave technology uses light as the primary medium to carry information. The light often is guided through optical fibers (fiberoptic technology). Most applications use invisible (infrared) light. Optical fiber is the latest evolution in the area of high frequency transmission line media. Coaxial cable, metal waveguides, and copper wires are examples of transmission lines which can carry high frequency signals with varying degrees of success. The currrent popularity of fiber stems from the frequencies which can be transmitted on fiber. Whereas a good coax cable (depending on physical dimensions) might have an upper frequency limit of 20 GHz, typical long haul fiber operates at 193,000 GHz. Since the bandwidth of a transmission medium defines its maximum information carrying ability, a single coax cable could carry two 10 GHz channels, but a single optical fiber could theoretically carry thousands of 10 GHz channels. In addition to its enormous information bandwidth, fiber is physically smaller and lighter than coax (more fibers/cable), and can have hundred times longer spans between amplifiers due to significantly lower attenuation. It is relatively inexpensive to manufacture and very resistant to external interference or unauthorized signal tapping. (HP)

Why lightwave technology? Most cost-effective way to move huge amounts of information (voice, data) quickly and reliably. Light is insensitive to electrical interference. Fiber optic cables have less weight and consume less space than equivalent electrical links. For point-to-point telecom applications, lightwave technology can support much more bandwidth than other technologies with comparable cost. Optical cables therefore have become the medium of choice for most high volume, long distance traffic routes. Even base stations for mobile communications often are connected via lightwave technology. Because optical fibers can be safely brought into areas where electrical interference prohibits most electrical solutions, you can find them along high voltage power lines (as part of the ground wire) or along railways. Within buildings, duct space is often very limited. Lightwave based backbones are therefore often the only economically viable solution for upgrading local area networks (LANs) that then can connect thousands of users. (HP)

Use Of Lightwave Technology Majority applications: Telephone networks Data communication systems Cable TV distribution Niche applications: Optical sensors Medical equipment Fiber optic cable was originally used for long haul transmission of voice. As the demand for data transmission continues to grow, fiber is finding its way into every possible information transmission application: computers, satellites, electronic devices, LANs, etc. Because the telecommunication market is huge, many designs in lightwave have been optimized for that application. Nevertheless optical fibers have been penetrating many other areas, especially where electrical signals or metal conducts can be a concern.

LW Transmission Bands Frequency Wavelength Near Infrared UV (vacuum) 193 229 353 461 THz Frequency Near Infrared UV Wavelength (vacuum) 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 µm HeNe Lasers 633 nm Longhaul Telecom Regional Telecom Local Area Networks 1550 nm Lightwave transmission systems operate just below the frequency of visible light. Using the f*lambda=c equation, you can calculate that the frequency of a 1550 nm wavelength signal is ~193 THz (~193,000,000,000,000 cycles/s). This is also why light is the carrier of choice for high speed data signals: even if you can use only 2 to 3% of the spectrum around that center frequency there is more than 5,000 GHz of bandwidth available! Most lightwave frequencies are reported as wavelengths rather than frequencies (the opposite convention prevails are lower frequencies), a hold- over from physicists reporting emissions bands as spectral lines of a specific wavelength. Please note that the term “microns” is often used for micrometers (µm). 1.55 µm = 1550 nm = 1550,000 pm. CD Players 780 nm 1310 nm 850 nm

Introduction to Fiber Optics Fiber optics is a medium for carrying information from one point to another in the form of light. Unlike the copper form of transmission, fiber optics is not electrical in nature. A basic fiber optic system consists of a transmitting device that converts an electrical signal into a light signal, an optical fiber cable that carries the light, and a receiver that accepts the light signal and converts it back into an electrical signal.

Introduction to fiber Optics

Optical Sources Two main types of optical sources Light emitting diode (LED) Large wavelength content Incoherent Limited directionality Laser diode (LD) Small wavelength content Highly coherent Directional

Light Emitting Diodes (LED) Spontaneous emission dominates Random photon emission Spatial implications of random emission Broad far field emission pattern Dome used to extract more of the light Spectral implications of random emission Broad spectrum

Laser Diode End mirrors Lasing requires net positive gain Cavity gain Stimulated emission dominates Narrower spectrum More directional Requires high optical power density in the gain region Optical Feedback: Part of the optical power is reflected back into the cavity End mirrors Lasing requires net positive gain Cavity gain Depends on external pumping Applying current to a semiconductor pn junction Cavity loss Material absorption Scatter End face reflectivity

Optical Detectors Inverse device with semiconductor lasers Source: convert electric current to optical power Detector: convert optical power to electrical current Use pin structures similar to lasers Electrical power is proportional to i2 Electrical power is proportional to optical power squared Called square law device Important characteristics Modulation bandwidth (response speed) Optical conversion efficiency Noise Area

HOW DOES FIBRE OPTIC WORK ? Carries Signals as Light Pulses signals converted from electrical to light (and visa-versa) by special equipment e.g. fibre-optic “transceiver” (transmitter / receiver)

FIBRE CONSTRUCTION 8, 50, 62.5 125 Cladding Glass Core Glass The slide above shows the construction at the very heart of optical fibre, namely the “core” and the “cladding”. These two elements are both made of glass, in a manufacturing process which creates them as an inseparable unit. The core and cladding layers have slightly different optical properties, which combine to enable to the core to transmit the light pulses produced by the transmitter. The property used in this case is the “refractive index” of the glass, which is a measure of how much it can “bend” light - the same property which makes a straw appear to be bent where it enters a glass of water. As we said earlier, the fibre itself is very small, typically only a few thousandths of an inch in diameter. The most commonly-used fibre actually has a core of 62.5 microns diameter, and a cladding of 125 microns (about 0.005”) diameter. How big is a micron? One micron - or micrometre - is 1,000 of a millimetre, or approximately 0.00004”. By comparison, an average human hair is a huge 0.004” diameter - 100 times as large !

PRIMARY BUFFER Primary Buffer 250 Cladding 125 Core (62.5) As the optical fibre is so small, it needs some measure of protection so that it can be easily handled and installed. To this end, the manufacturer puts a coating called the “Primary Buffer” onto the fibre during the process of drawing the glass from a rod down into a fibre. ALL optical fibre has a Primary Buffer coating, regardless of what type of construction is ultimately used to make the cable. The Primary Buffer is usually a polymer material, and gives some degree of physical protection to the fibre, while making it easier to handle (and easier to see !). The Primary Buffer is always 250 microns (0.25 mm, or approx. 0.010”) in diameter. If the fibre is to be used in a cable construction known as “loose-tube”, this is the only coating applied directly to the fibre, and is normally coloured so that individual fibres in a multi-fibre bundle can be easily identified.

SECONDARY BUFFER Secondary Buffer 900 Primary Buffer 250 Cladding 125 Core (62.5) For fibre which is to be used in a cable type known as “tight-jacketed”, the fibre is given a further protective coating, known as the “Secondary Buffer”. This coating is normally applied by the cable manufacturer, rather than the fibre manufacturer, and is always 900 microns (0.9 mm, or approx. 0.036”) in diameter. The Secondary Buffer is usually a nylon type of material, which gives considerable extra strength and resilience to the fibre. As with Primary Buffer, the Secondary Buffer coating is normally colour-coded when the fibre is to be used in a multi-fibre cable.

FIBRE MATERIAL Silica Glass used for high-speed data applications Plastics used for low-speed data / voice applications Composite Constructions used for low-speed and specialized applications Although we have so far only mentioned glass as the material for the fibre itself, other materials are sometimes used, such as various types of plastics. Such materials are only used for low-speed applications, as their performance in terms of speed and distance is nowhere near that of glass. They do however have the advantage of being very cheap to produce compared with glass, and are most robust in the “bare” state. Typical applications for plastic fibres include process-control sensors in factories, especially in industries such as petro-chemicals, where the absence of electrical connections means that the risks of hazards such as sparks are reduced to nil. Another increasing use of plastic fibre is for the interconnections between hi-fi units, especially from CD players, which are all-digital in operation.

FIBRE TRANSMISSION Multi-Mode graded-index used for short / medium distance applications step-index early fibre type - no longer used Single-Mode a.k.a Mono-Mode used for long-distance / very high-speed applications e.g. cross-country and transatlantic communications Although a detailed discussion of the transmission through optical fibre is beyond the scope of this course, we do need to mention that there are several different methods used. The most common method uses what is known as “Multi-Mode” transmission, in which rays of light are propagated along the length of the fibre by being “bounced” between the sides of the core. This method is further sub-divided into two variations on the theme, namely “Step-Index”, and “Graded-Index”. The Step-Index method was the first to be employed, but has been superseded by a more sophisticated technique called Graded-Index, in which light takes a more curved path through the core, rather than being bounced between the walls of the core in straight lines. This results in a much-improved quality of signal at the receiving end of the fibre, which in turn has allowed higher-speed signals to be transmitted over greater distances. Step-Index fibre is no longer used in the commercial environment. Applications using the highest data rates, over the longest distances, require even higher signal quality to be maintained over the length of the fibre, and use the technique known as “Single-Mode” (or “MonoMode”) propagation. In this method, Lasers are used as the transmitters instead of LEDs, with the light beam travelling straight down the middle of the core, rather than being bounced between the core walls. As might be expected, the fibre and transceivers needed for Single-Mode operation are considerably more expensive than their Multi-Mode counterparts.

LIGHT TRANSMISSION MultiMode Step Index MultiMode Graded Index The slide above illustrates the three different types of propagation within optical fibres. SingleMode

COMMON FIBRE SIZES 125 µm 50 µm 62.5 µm 140 µm 100 µm 8 µm MultiMode Graded Index SingleMode Unlike copper cables, which need to be produced in a very wide range of conductor sizes, fibre only needs to be made in a relatively small range of size options, and in fact just two of the four variants shown above are commonly used. Fibre sizes are expressed by the diameters of the core and cladding, e.g. 50/125, 100/140, and so on. The most common by far is the type designated “62.5/125”, in which the core is 62.5 microns, and cladding is 125 microns in diameter. Earlier installations used the “50/125” standard, whereas the 100/140 size was used by IBM in the early optical systems. Because of the propagation method used in Single-Mode fibre, the core is a very much smaller diameter than for Multi-Mode. Typically, Single-Mode fibre is designated “8/125”, i.e. with core size of just 8 microns. The cladding is left as 125 microns to provide some degree of stability, and make it possible to use normal termination techniques.

Advantages/Disadvantages of Fiber Optics Enormous potential bandwidth Small size and weight Electrical Isolation Signal security Low transmission loss Potential low cost

Advantages/Disadvantages of Fiber Optics High cost for connector and interfacing Requires specialized and sophisticated tools for maintenance and repairing Higher initial cost in installation

Light-Source What is light? Properties of Light. Refractive Index Law of Refraction Law of Reflection Total Internal Reflection

Refractive Index The guidance of the light beam which acts as a transmission channel for information (through the optical fiber) takes place because of the phenomenon of total internal reflection (TIR), which is dependent on the refractive index of the medium. The refractive index (n) of a medium can be written as:

Total Internal Reflection A ray of light incident on a denser medium i.e. n1<n2 According to Snell’s Law and the law of reflection we have n1 sin θ1 =n2 sin θ2 and θ1=θ3

Total Internal Reflection The angle of incidence, for which the angle of refraction is 90º, is known as the critical angle and is denoted by θc .Thus, when θ1=θc =sin-1(n2/n1) θ2=90. When the angle of incidence exceeds the angle of critical (i.e.,θ1>θc), there is no refracted ray and we have total internal reflection.

Total Internal Reflection