Optical communication lab manual

Sign Up. Create your account now. Signup with Email. Gender Male Female. Create Account. Already Have an Account? Preeti Singh Mr. Neeraj Sharma Lab Technician: Ms. Swaran Kaur List of Experiments: 1. Demonstration and study of different types of Optical Fibers and connectors. To establish and Study of a nm fiber optic analog link.

To establish and Study of a nm fiber optic digital link. Study of Intensity Modulation Technique using Analog input signal.

To obtain intensity modulation of the analog signal, transmit it over a fiber optic cable and demodulate the same at the receiver and to get back the original signal. Study of Intensity Modulation Technique using digital Input signal.

The objective of this experiment is to obtain intensity modulation of digital signal, transmit it over fiber optic cable and demodulate the same at the receiver end to get back the original signal.

To measure propagation or attenuation loss in optical fiber. To measure propagation loss in optical fiber using optical power meter. To measurement of the Numerical Aperture NA of the fiber. Experiment No. Theory: For much of modern telecommunication, the path over which the signals travel is optical fiber. Optical fiber for most purposes is made of a very special kind of glass that is drawn into a very thin, long fiber.

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Optical Instrument views. Optical active views. Optical Sources - Transmitters views. Integrated Optical Devices views. Some of the optical fibers in use are: 1. Multimode step index fibers. Multimode graded index fibers.

Single mode step index fibers. Plastic - clad fibers. All plastic fibers. Dimensions of fiber optic cables are written as a ratio e. Choice of operating frequency: Once we had the laser and the new optic fiber available, everything was in place for a significant upsurge in communications. This resulted in two driving forces: one towards the ability to send more data faster and secondly to send the data to greater distances without being re-amplified. More Data Faster: As the transmission rate of data is increased, the required bandwidth increases and this can best be accommodated by increasing the carrier frequency.

This premise has stood us in good stead over many years. The speech and poor quality music transmissions on the medium frequency, AM radio, give way to the higher frequency of FM radios which accommodate the increased bandwidth necessary for improved music quality. When television required even higher data rates, we responded by moving to even higher frequencies. These previous experience rather suggested that the light used for fiber optic communications should be of the highest frequency possible.

But there was a surprise in store. Lower frequencies mean lower losses: The first experiments used visible light of different colours frequencies. As the losses were measured, we found that the higher frequencies caused more losses. The losses actually increased by the 4th power of the frequency. This means that a trebling of the frequency would result in the losses increasing by 34 or 81 times.

Therefore, in most real installations, we tend to go for the relatively low frequencies of infrared light, which is just below the visible spectrum. Fiber windows: We now have an infrared range between nm — nm with one part of it around nm which is best avoided.

It seemed sensible to agree on standard wavelengths so that equipment from different manufacturer can be compatible. Figure 7 This has resulted in three standard wavelengths called windows. The windows were really the result of looking at the available light sources. Some wavelengths of LED and laser light are easier and less expensive than others to produce. The design design and manufacture of the optic fiber is then optimized for these frequencies.

Note: The infrared light is very dangerous to eyes it can cause irreversible damages and since it is invisible care should be taken to ensure that the optic fiber is not live.

Some photons of light are lost, causing attenuation of signal. Several mechanisms are involved, including absorption by materials within the fiber, scattering of light out of the core caused by environmental factors. The degree of attenuation depends on the wavelength of light transmitted. Attenuation measures the reduction in signal strength by comparing output power with input power. Measurements are made in decibels dB.

It is defined as dB loss 10 10 2. Material absorption losses It is a loss mechanism related to the material composition and fabrication process of the fiber which result in the dissipation of some of the transmitted optical power as heat in waveguide. The absorption of light may be intrinsic caused by one or more major components of glass or extrinsic caused by impurities within the glass.

Linear scattering losses Linear scattering mechanisms cause the transfer of some or all of the optical power contained within one propagating mode to be transferred linearly proportionally into a different mode. This process tends to result in attenuation of the transmitted light light as the transfer may be to a leaky or radiation mode which does not continue to propagate within the fiber core, but is radiated from the fiber.

It is mainly of two types. RayLeigh Scattering b. Mie Scattering Ray Leigh Scatter: When the infrared light strikes a very - very small place where the materials in the glass are imperfectly mixed. This gives rise to localized changes in the refractive index resulting in the light being scattered in all directions.

Some of the light escapes the optic fiber, fiber, some continues in the correct direction and some is returned towards the light source. This is called back scatter. Figure 8 E. Mie scattering These result from the non - perfect cylindrical structure of the waveguide. It may be the caused by the imperfections such as irregularities in the core cladding interface core, cladding refractive index difference along the fiber length, diameter fluctuations, fluctuat strains and bubbles.

The scattering created by such in homogeneities is mainly in the forward direction. Non linear scattering Optical waveguide does not behave linearly, several non-linear non linear effects occur, which in the case of scattering cause disproportionate attenuation usually at high optical power level.

This non- non linear scattering causes the optical power from one mode to be transferred transferred in either the forward or backward direction to the same, or other modes at different frequency. It depends critically upon the optical power density within the fiber and hence only becomes significant above threshold power levels.

Micro bending and macro bending A problem which often occurs in cabling of the optical fiber is the meandering of the fiber core axis on a microscopic scale within the cable form. This phenomenon, known as micro bending result from small lateral forces exerted on the fiber fiber during the cabling process and it causes losses due to radiation in both multimode and single mode fiber. Macro bends: The light propagates down the optic fiber solely because the incident angle exceeds the critical angle.

If a sharp bend occurs, the normal and the critical angle move round with the fiber. The incident ray continues in a straight line and it finds itself approaching the core - cladding boundary at a an angle less than the critical angle and much of light is able to escape.

Figure 9 E. The light spreads out. This effect is called dispersion. In figure 10 the light pulse shown before and after it has travelled through the cable.

Figure 10 It is going to limit how fast we can send data - how many bits per second we can transmit through a fiber optic link. In fact it is the main limit to the data transmissionrate for long distance communication system.

Over a giventransmission path, there are only two remedies. This is not a very exciting solution andwould clash with one of the main reasons for using optic fiber. There are two types of Dispersion. Inter Inter modal dispersion 2. Intra Intra modal dispersion Inter modal Dispersion: You will recall that, to be propagated down the core of the optic fiber, fiber, the light must enter at an angle greater than the critical angle. Let us consider just two such rays of light as they travel along a section of optic fiber.

Which ray would reach first? Figure 11 Ray A will reach the far end before Ray B since it is travelling a shorter distance. Assuming that rays A and B are part of the same pulse of light and start at the same time, we can now see how the spreading of the pulses can occur.

Each and every ray being propagated at its own angle will arrive at slightly different times at the far end. This spreading effect will occur all along the fiber so it is also the fiber so it is also important to appreciate that the longer the optic fiber, the greater the dispersion.

Transmission rates that are actually possible in in an optic fiber therefore depend on its length. In practice, there are only particular angles of propagation which are able to betransmitted down the optic fiber.

It is only really significant in single mode usage since, being very slight, it is completely swamped by the inter modal dispersion in the multimode case. The cause is simple enoughen - the refractive index of material is determined to some extent by the wavelength of the light source. Can you see how this causes dispersion? A change in refractive index will change the speed of that particular wavelength of light.

Now if your light source produces different wavelengths concurrently, we will havecomponents of the transmitted light pulse travelling simultaneously. If will have components of the transmitted light pulse travelling at difference speeds.

The total package of light will spread sp out - hence the dispersion. The cure for inter modal dispersion: A large core diameter means many modes and severe inter modal dispersion. The curefor this type of dispersion is quite simple. Reduce the core size, the number of modesdecreases, and the inter modal dispersion is reduced. We can do better than justreducing the inter modal dispersion, we can completely eliminate it. Simply make thecore so small that only one mode is propagated.

A single ray cannot possibly go attwo different speeds so inter inter modal dispersion cannot occur. In practice the core isreduced to about 9 m. The optic fiber which now carries only a single mode is nowreferred to as a 'single mode fiber'. The larger core optic fibers for short and medium distances carry many modes and arecalled ' Multimode'. The cure for intra modal dispersion: The cure is apparently so simple; use a light source which emits only one wavelength of light.

Unfortunately, it has not yet been invented. Our light sources in current use are the LED and the laser. Study figure 12 and decide which of the two two would cause the lesser amount of intra modal dispersion.

Figure 12 The laser would cause less intra modal dispersion because its light is more concentrated around the central wavelength. The spread of wavelength measured between the points where the power output falls to half of the peak power is called the spectral width. Some lasers have spectral widths as low as 0. The low spectral width together with its high power and fast switching makes the laser first choice for long distance communications using single mode optic fiber.

Also there are some losses due to coupling ing in between the fibers and at LED and photo detector ends. A Live Fiber Detector Here is the problem: long distance fiber optic systems employ powerful lasers operating in the infrared region of the spectrum. This infrared light has two properties which are very significant to the engineers and technicians working on the system. We have various pieces of test equipment that can be used to check the system. The 'live' fiber detector is able to find which fibers are carrying data in most day to day checks, but read the instruction manual first to ensure that the instrument is suitable for the type of optic fiber you are checking.

The fiber under test is slipped in between them and when the jaws close it will cause the fiber to be bent sufficiently to cause a macro bend. The escaping light can be detected by the photocell and used to activate the LED indicator. One flaw in the system is that it relies on the buffer beingtransparent to the infrared light. It is the most versatile piece of test equipment that we have for making measurements on fiber optic systems.

It provides us with two different measurements: 1. It It can measure the magnitude of any losses that occur along optic fiber. It It can measure distance along the optic fiber.

If the optic fiber has been cut as it has to be when fitting aconnector, the end face of the glass causes a reflection of energy. It is also usual forthis to occur at the extreme end of the optic fiber. This cause a localized increase inenergy returned to the OTDR. There is always aFresnel reflection at the start of the fiber due to the connector on the front panel of theOTDR.

Distance: We obtain timing information by starting the display and the pulse generator at the same instant. This is achieved by the synchronizing pulse which switches on both the laser and the receiver at the same instant. If we know how long it takes for the backscatter backscatter light to return to the OTDR then we only have to know how fast the infrared light is travelling along the optic fiber to be able to calculate how far the light hastravelled.

Some light returns after say, ns, it follows that it has travelled to a total t of meters. This represents 50 meters out along the optic fiber and 50 meters back. You will remember that the actual speed of propagation is determined by the refractive index of the core of the optic fiber. The value of the refractive index is quoted by the manufacturer. The synchronizing synchroniz pulse simply provides a "start" to the generator and to the display circuits to allow them to determine the travel-time time of the laser light and the backscatter.

Notice that both the macro bends and fusion splices are shown as a sudden loss of power at a particular point. Indeed, it is not possible to distinguish between a macro bend and a fusion splice just just by observing the OTDR display. It just shows a localizedloss. The loss may appear a vertical drop or as sloping line depending upon the speed at which the screen being scanned on the OTDR.

The connector has a similar loss but it also has afresnel reflection. Typical value of losses: Fusion splice 0. Two other Applications of Back scatter. Distributive temperature sensing DTS The amount of backscatter occurring in an optic fiber is dependent upon the manufacture of the optic fiber, the optic window used, and upon the temperature of the optic fiber.

Now, when we find a characteristic of the optic fiber, which depends on the temperature, tem it is but a small step away from using the effect to measure temperatures. This new technique is called distributive temperature sensing DTS.

It is an optic fiber connected to equipment operating just like an OTDR, which is then passed through through the areas to be measured. We know however, that a macro bend would allow the light to escape and hence the data to be copied. An OTDR monitoring the line would immediately detect the power loss of the macro bend and be able to measure asure its distance along the optic fiber to an accuracy of approximately 0.

The same immediate detection would occur as with the security matting shown in figure 17 a b c. Figure 17 Recommended Testing Instruments for experiments 1. Dual Dual trace oscilloscope 20 MHz 2. Oscilloscope Probes E.

Theory: Fiber optic links can be used for transmission of digital as well as analog signals. Basically a fiber optic link contains three main elements, a transmitter, an optical fiber and a receiver. The transmitter module takes the input signal in electrical form and then transforms it into optical light energy containing the same information. Theoptical fiber is the medium which takes the energy to the receiver. At the receiver light is converted back into electrical form with the same pattern as originally fed to the transmitter.

Transmitter: Fiber optic transmitters are typically composed of a buffer, driver and optical source. The driver provides electrical power to the optical source. Finally, the optical source converts the electrical current to the light energy with the same pattern. Commonly used optical sources are light emitting diodes iodes LED s and Laser beam.