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IntroductionIn 1880, only four years after his invention of the telephone, Alexander Graham Bell used light for the transmission of speech. He called his device a photophone. It was a tube with a flexible mirror at its end. He spoke down the tube and the sound vibrated the mirror. The modulated light was detected by a photocell placed at a distance of 200 metres or so. The result was certainly not hi-fi but the speech could at least be understood.

Following the invention of the Ruby LASER (an acronym for Light Amplification by Stimulated Emission of Radiation) in 1960, the direct use of light for communication was re-investigated. However the data links still suffered from the need for an unobstructed path between the sender and the receiver.Nevertheless, it was an interesting idea and in 1983 it was used to send a message, by Morse code, over a distance of 240km (150 miles) between two mountaintops.

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IntroductionEnormous resources were poured into the search for a material with sufficient clarity to allow the development of an optic fibre to carry light over long distances.

The glass used for optic fibre is unbelievable clear. We are used to normal 'window' glass looking clear but it is not even on thesame planet when compared to the new silica glass. We could construct a pane of glass several kilometres thick and still match the clarity of a normal window. If water were this clear we would be able to see the bottom of the deepest parts of the ocean.

The early results were disappointing. The losses were such that the light power was halved every three metres along the route. This would reduce the power by a factor of a million over only 60 metres (200 feet). Obviously this would rule out long distance communications even when using a powerful laser. Within ten years, however, we were using a silica glass with losses comparable with the best copper cables.

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IntroductionPlastic optic fibre is finding increasing applications in hi-fi systems and in automobile control circuitry. Plastic optic fibre has impossibly high losses for long distance communications, but for short links of a few tens of metres it is satisfactory and simple to use.

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Sending information by light has proved so successful that over 90% of all long distance telephone calls are now carried on optical fibres as well as an increasing proportion of television signals.

By converting an input signal into short flashes of light, the optic fibre is able to carry complex information over distances of more than a hundred kilometres without additional amplification. This is at least five times better than the distances attainable using the best copper coaxial cables.

The system is basically very simple: a signal is used to vary, or modulate, the light output from a suitable source - usually a laser or an LED. The flashes of light travel along the fibre, and, at the far end, are converted to an electrical signal by means of a photo-electric cell or photodiode. Thus the original signal is recovered.

Introduction

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Some Basic PhysicsElectromagnetic waves:Radio waves and light are electromagnetic waves. The rate at which they alternate in polarity is called the frequency (f), and the speed of the electromagnetic wave in free space is approximately 3 x 108 m/s.The distance travelled during each cycle, the wavelength (λ) can be calculated by the relationship:Wavelength λ = speed of light / frequency = 3x108/f

Speed of light [material] = (speed of light [free space]) / (refractive index)The lower the refractive index the higher the speed, or vice versa, the higher the refractive index the lower the speed of the light travelling through the material.

The Speed of Light and Refractive Index:The speed of light is not, however, a constant, but depends upon the medium through which it is travelling. When it passes through a clear material, it slows down by an amount dependent upon a property of the material called its Refractive Index. For most materials that are used in optic fibres, the refractive index is in the region of 1.5.

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Some Basic Physics

We can see that as light travels through a material that the wavefronts are evenly spaced, denoting a constant speed.Consider the diagram above that shows what happens when a beam of light passes from a material with a higher refractive index to a material/medium with a lower refractive index.We can see that, in the material with the lower refractive index, that the wavefronts are further apart because of the increased speed (the wave travels further in a given time).

We can represent our light wave as a series of wavefronts, that represent the crests of the electromagnetic wave, see the diagram opposite:

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The direction that the light approaches the boundary between the two materials is very significant.

Refraction

In this section we will consider a ray of light travelling through two materials, but approaching the boundary at an angle other than 90°.

As the ray crosses the boundary between the two materials, one side of the ray will find itself travelling in the new, higher velocity, material while the other side is still in the original material. The result of this is that the wavefront progresses further on one side than the other, which causes the wavefront to swerve. The ray of light, now wholly in the new material, is again travelling in a straight line, but at a different angle and speed.

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The amount by which the ray swerves, and hence the new direction, is determined by the relative refractive indices of the materials and the angle at which the ray approaches the boundary.

Snell’s Law

Snell's Law:The angles of the rays are measured with respect to the normal. This is a line drawn at right angles to the boundary line between the two materials (refractive indices). The angles of the incoming and outgoing rays are called the angles of incidence and refraction respectively.

Notice how the angle increases as it crosses from the higher refractive index to the one with the lower refractive index.Snell's Law states: n1sinφ1 = n2sinφ2

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Critical Angle

As the angle of incidence in the first material is increased, there will come a time when, eventually, the angle of refraction reaches 90° and the light will be refracted along the boundary between the two materials. The angle of incidence that results in this effect is called the critical angle.

The critical angle is well-named as its value is indeed critical to the operation of optic fibres. At angles of incidence less than the critical angle, the ray is refracted.

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Total Internal ReflectionIf the light approaches the boundary at an angle greater than the critical angle, the light is actually reflected back from the boundary region into the first material. The boundary region acts as a mirror - this effect is called Total Internal Reflection (TIR).

The effect holds the solution to the puzzle of trapping the light in the fibre.If the fibre has parallel sides, and is surrounded by a material with a lower refractive index, the light will be reflected along at a constant angle - shown as Ø in the diagram below:

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Propagation of Lightand Practical ConstructionIt would seem, on initial inspection, that all that is required is an optic fibre, and light can be transmitted from one place to another. However there is a problem - the light wave does not travel only in the fibre. At the boundary, where total internal reflection occurs, part of the wavefront passes through the second material - in the case of a plain optic fibre this is air.

If the fibre is touched or contaminated, or even supported at a point, the boundary at that point will no longer be glass/air but glass/'other-material'.

The problem is that the 'other-material' will have a refractive index greater than 1, or may not even conduct light at all. This leads to losses as some light will be lost:(a) Because the critical angle has changed at the point of contamination (refractive index >1), some rays will then be approaching the surface at an angle less than the new critical angle, causing light to escape from the fibre.(b) The contamination does not conduct light, but rather absorbs it,causing little or no reflection at the point of contamination.

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Propagation of Lightand Practical Construction

The diagram below illustrates the loss of light at a point of contamination on a bare optical fibre without any cladding material:

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Propagation of Lightand Practical ConstructionThe solution is to cover the fibre (core) with another layer of glass that has a lower refractive index than the core. This new layer is called the cladding, which is added around the outside of the core during manufacture. The core and cladding form a single solid fibre of glass.

Immediately after manufacture a third layer, the primary buffer, is added prior to winding the optic fibre onto a reel. The primary buffer is purely a protective covering to stop the fibre from being damaged, and is normally a plastic coating.

When the optic fibre is incorporated into the final cable more layers of protection will be added depending upon the construction and use of the cable.

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Losses in Optic Fibres - AbsorptionAny impurities that remain in the fibre after manufacture will block some of the light energy. The worst culprits are traces of metal and water (hydroxyl) ions.Great care is taken during the manufacture of optic fibre to reduce these impurities as far as possible.

Losses in Optic Fibres - Rayleigh Scatter:This is the scattering of light due to small localised changes in the refractive index of the core and the cladding material. The changes are indeed very localised, typically less than the wavelength of the light.

When a ray of light hits one of these discontinuities, it is scattered in all directions -all light that then finds itself with an angle of incidence less than the critical angle escapes from the fibre. Some light is scattered back in the direction from which it came - 'back scatter'.However much of the light misses these discontinuities because they are so small.

There are two causes:(1) The inevitable slight fluctuations in the 'mix' of the ingredients.

These are random and impossible to eliminate.(2) Slight changes in the density as the silica cools and solidifies.

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When a ray of light strikes a change of refractive index and is approaching at an angle close to the normal (at right angles to the boundary), as is the case at the ends of the fibre where light enters or exits, most of the light passes straight through.

Most of the light exits, but not all. A very small proportion is reflected back off the boundary. If the refractive index of the glass is approximately 1.5 and it is passing into air (refractive index =1.0) the power that is reflected is approximately 4% - therefore only 96% of the light passes out of the fibre.

Losses in Optic Fibres - Fresnel Reflection

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A sharp bend in a fibre can cause significant losses as well as the possibility of mechanical failure.It is easy to bend a short length of optic fibre to produce higher losses than a whole kilometre of fibre in normal use.

In the diagram above the ray is safely outside the critical angle and therefore is propagated correctly.

Losses in Optic Fibres - Bending Losses

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For a bare fibre, consisting of core/cladding and primary buffer, a typical figure would be 50mm. For a cable, which is bare fibre plus other protective layers the minimum bend radius is typically 10 x cable outside diameter or 50mm (whichever is greater).

Remember that the normal is always at right angles to the surface of the core.If the core bends, as shown in the diagram opposite, the normal will follow it and the ray will now find itself on the wrong side of the critical angle and will escape.

Tight bends are therefore to be avoided, but how tight is tight?The manufacturers specification of the optic cable will normallygive the minimum allowable bend radius.

Losses in Optic Fibres - Bending Losses

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Effect of Attenuation onAnalogue and Digital SignalsObviously the losses in an optic fibre cause a loss of amplitude of the transmitted signal at the receiving end. In common with standard copper based cables, repeaters are placed at suitable intervals to boost the signal.

Analogue signals are susceptible to noise and have a great amplitude range. This means that lower amplitude signals can easily be lost by attenuation in the fibre or be badly degraded by any noise in the transmission line. Higher amplitude signals will be less affected by noise but will suffer attenuation.

The repeater will amplify the signals, but will also amplify any noise in the system, still leaving the signal degraded.

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The circuit/repeater that carries out this function is called a regenerator, since it re-builds/restores the original signal.Since the information is not carried in the digital signals amplitude, but rather pulse timing/pattern, it is not degraded by the attenuation/losses in the fibre.

Digital Signals, although still subject to noise, contain only two amplitude levels; high (1) or low (0). All the transmitted information is carried in the timing or pattern of pulses. These signals, if degraded by noise, can be reconstructed by passing the signal through a Schmitt Trigger, or other thresholding circuit.

Effect of Attenuation onAnalogue and Digital Signals

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Introduction - Dispersion:We have met the effect of dispersion in public address systems. As soon as the announcement ends, people turn to one another and say 'What did he/she say?' 'What was that all about?'.

Dispersion in Optic Fibres

The problem is that the sound is arriving by many different paths of different lengths, and parts of the sound are arriving at different times.Optic fibres suffer from a similar effect, called dispersion.

Two types of dispersion occur:(1) Intermodal - 'Inter' (between) modes/paths.(2) Intramodal, - 'Intra' (within) a single mode/path.

These are covered in the following sections:

We cannot make out what is being said. Hearing the sound is no problem, there is plenty of volume and increasing it further would not help and would probably make the situation worse.

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Imagine that two separate rays are launched into a fibre. Now, since both rays are travelling in material of the same refractive index they must be moving at the same speed.

Intermodal Dispersion

If we follow the two rays of light which have entered the core at the same time, as shown in the diagram above, we can see that one of them, Ray A, will travel a longer distance than the other, Ray B.

The effect of this is to cause the pulse of light to spread out as it moves along the fibre - as the ray taking the shorter route overtakes the other.This spreading effect, called dispersion, is shown in the diagram opposite:

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Intermodal Dispersion and its effect on the DataThe data can be corrupted by dispersion. If we send a sequence of ON OFF pulses, it would start its life as an electronic signal with nice sharp edges:

The pulses are used to switch a light source, usually an LED or a LASER and the resultant pulses of light are launched into the fibre.

We could make this degree of dispersion acceptable by simply decreasing the transmission frequency and thus allowing larger gaps between the pulses.This type of dispersion is called intermodal dispersion.

Dispersion causes the pulses to spread out and eventually they will blend together and the information will be lost.

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Overcoming Intermodal Dispersion

The problem of intermodal dispersion can be tackled in two ways:

(2) Eliminate all the modes/paths except for one.

The two methods are outlined in the pages that follow:

(1) Encourage all the modes/paths to travel at the same speed along the fibre.

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Overcoming Intermodal Dispersion

Method 1 - Graded Index Fibre:This design of fibre eliminates about 99% of intermodal dispersion.We can compensate for the ray that takes the longer route by making it move faster. If the speed/distance of each route is carefully balanced all the rays can be made to arrive at the same time.

Review: relationship between speed of light in a material and its Refractive Index is as follows:

Speed of light [material] = (speed of light [free space]) / (refractive index)

The solution to our problem is to change the refractive index progressively from the centre of the core to the outside.If the core centre has the highest refractive index, and the outer edge the least, the ray will increase in speed as it moves away from the centre.

The rate at which the refractive index changes is critical and is the result of intensive research. A parabolic profile is often employed but there are many others available in specialised fibres.

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Method 1 - Graded Index Fibre (Continued)

Overcoming Intermodal Dispersion

We can see that, in the GI fibre, the rays follow a curved path.This is one of the results of the change in refractive index as the ray moves away from the centre of the core. We can consider the core to be made up of a whole series of discreet changes in refractive index. At each refractive index change the light ray is refracted slightly until the ray approaches a layer at an angle greater than its critical angle and reflection occurs.

The standard SI (Step Index) and the GI (Graded Index) profiles are shown in the diagram opposite:

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Method 2 - Single Mode (SM) Fibre:This type of fibre solves the problem by making the core of a size that only one mode/path can be supported - the direct path.

Overcoming Intermodal Dispersion

This form of dispersion occurs because light of different frequencies/colours travel at different speeds. Refractive index and speed of travel are therefore dependent upon the wavelength of the ray of light.This form of dispersion has the greatest influence on transmission in Singlemode (SM) Fibres.

Intramodal (Chromatic) Dispersion

Multi mode fibre Single mode fibre

A comparison between Multi and Single mode fibres

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Intramodal (Chromatic) Dispersion

Although chromatic dispersion is generally discussed in terms of a single mode fibre, it does still occur in multimode fibres, but the effect is generally swamped by the Intermodal Dispersion.

This form of dispersion, where different light wavelengths travel at different speeds, and cause the pulse of light to spread, is called chromatic dispersion.

As can be observed from the diagram, the LASER produces a narrower range of light frequencies than the LED.

The light sources used in optoelectronics, as transmitters, do not emit light at a single wavelength. The typical spectral widths (bandwidth) of two sources, the LED and the LASER are shown in the diagram opposite:

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Immunity from Electrical Interference:Optic fibres can run comfortably through areas of high level electrical noise such as near machinery and discharge lighting.

Advantages / Applications of Optic Fibres

No Crosstalk:When copper cables are placed side by side for a long distance, electromagnetic radiation from each cable can be picked up by the others so the signals can be detected on surrounding conductors. This effect is called crosstalk. In a telephone circuit it results in being able to hear another conversation in the background. Crosstalk can be easily avoided in optic fibres even if they are closely packed.

Application: Sending information by light has proved so successful that over 90% of all long distance telephone calls are now carried on optical fibres as well as an increasing proportion of television signals.

Application: This property is used in military applications, such as aircraft/avionics, where short range communication is required that will not be affected by high pulse electromagneticradiation (as from a nuclear explosion).

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Advantages / Applications of Optic FibresGlass fibres are insulators:Being an insulator, optic fibres are safe for use in high voltage areas.

Low losses:Fibres are now available with losses as low as 0.2dB/km, and hence very wide spacing is possible between repeaters. This has significant cost benefits in long distance telecommunication systems, particularly for undersea operations.

Security:As the optic fibres do not radiate electromagnetic signals they offer a high degree of security.

Improved Bandwidths:Using light allows for very high bandwidths. Bandwidths of several Gigahertz are available on fibres, whereas the best copper cables are restricted to about 500MHz.

Application: Optical fibre communications/links can be used to isolate circuits at different potentials, or have different ground/0V references.

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Advantages / Applications of Optic FibresSize and Weight:The primary coated fibre is extremely small and light, making many applications like *Endoscopes possible. (*A medical instrument for looking inside the human body)

Even when optic fibres are used as part of a cable with strength members and armouring, the result is still much lighter and smaller than the copper equivalents.This provides many knock-on benefits like reduced transport costs, more cables can be fitted within existing ducts and they are easier to install.

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Wavelength considerations in an optic fibre cableEarly experiments with optic fibre were conducted in the visible light part of the spectrum between 400nm (blue) and 740nm (red).

It has also been found that water, in the form of hydroxyl ions, can be absorbed by optic fibres, especially those used in seabed cables. The hydroxyl ions cause very high absorption of infrared light with a wavelength of 1380nm.

As a result, most fibre optic systems now operate in the infrared region of the spectrum (wavelength >740nm). This light is of a longer wavelength (lower frequency) than visible light and therefore has lower losses and hence longer ranges. An infrared laser of less than 1 watt can communicate over 40km (25miles) of optic fibre.

It was soon discovered that the power loss in the fibre increases as the wavelength decreases. Indeed, the effect is very marked with the losses being inversely proportional to the fourth power of the wavelength.This means that if the wavelength halved (frequency doubled), the losses would increase (24) = 16 times.

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Including the effects of possible water absorption, the diagram of power loss in an optic fibre cable as a function of wavelength is shown opposite:

Wavelength considerations in an optic fibre cable

Taking into account the properties of lasers, LEDs and photo-detectors, it was decided that communication equipment should use just three areas of the available spectrum so equipment from various sources will work together. These three areas are called 'windows', as shown in the diagram.

The 2nd and 3rd windows are used by laser drivers in long distance communications.The 1st window is suitable for infrared LEDs.

Other LED devices are available working at 770nm (infrared) and 660nm (visible red) wavelengths. These other wavelengths are used for short-range communication where absorption loss is minimal.

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Light Sources and Detectors

The light output from an LED is expressed in terms of power. Typical output levels are in the 10μW to 50μW range, although high output devices are available in the 600μW to 2500μW range.

Light Emitting Diodes (LEDs):A light Emitting Diode is a PN-junction semiconductor device that emits light when forward biased.

These frequencies have been chosen primarily because most fibre-optic cables have the lowest losses in these frequency ranges.The most commonly used wavelength is 1300nm.

LEDs can be designed to emit virtually any colour light desired. The LEDs used for fibre-optic transmission are usually in the red and near-infrared ranges.

Typical wavelengths of LED light used are 820nm, 940nm, 1300nm and 1550nm. These wavelengths/frequencies are all in the near-infrared range just below red light, which is not visible to the naked eye.

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Light Sources and Detectors

Light Emitting Diodes (LEDs) (continued):

The typical LED is relatively slow to switch on and off. A typical switch-on/switch-off time is about 150ns. This is too slow for most data communications applications by fibre-optics.

Faster LEDs are made just for fibre-optic applications. These units are made of gallium arsenide or indium phosphide(GaAs or InP) and emit light at 1300nm. Other LEDs with as many as six multiple layers of semiconductor material are used to optimise the device for a particular frequency and light output.

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The most commonly user light source in fibre-optic systems is an Injection Laser Diode (ILD). Like an LED, it is a PN-junction diode usually made with Gallium Arsenide (GaAs). With a low-level forward-bias current, the ILD operates as a standard LED, producing a low-level light over a relatively broad frequency range. At a higher forward current level known as the threshold, the ILD operates as a laser and emits a brilliant light over a much narrower frequency range.

Laser Diodes:The other commonly used light transmitter is a laser, which is a light source that emits coherent monochromatic light. Monochromatic light is a pure single-frequency light, while an LED emits red light which covers a narrow spectrum around the red frequencies. Coherent refers to the fact that all the light waves emitted are in phase with one another. Coherence produces a focusing effect on a beam so that it is narrow and extremely intense.

Lasers can be made in a variety of ways. Some are made from a solid rod of special material that emits light when properly stimulated. Certain types of gases are also used. Lasers used in fibre-optic systems are usually specially made LEDs that can operate as lasers.

Light Sources and Detectors

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Injection Laser Diodes (ILDs) are capable of developing light power up to several watts. They are far more powerful than LEDs and therefore capable of transmitting over much longer distances.

Another advantage of Laser Diodes, over LEDs, is their ability to switch off and on at a faster rate. High-speed laser diodes are capable of gigabit-per-second digital data rates.

Injection lasers dissipate a tremendous amount of heat and, therefore, must be connected to a heat sink for proper operation. Because their operation is heat sensitive, most injection lasers are used in a circuit that provides some feedback for temperature control. This not only protects the laser but also ensures proper light intensity and frequency.

Light Sources and DetectorsLaser Diodes (continued):

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When light strikes the diode, this leakage current increases significantly. This increased current flows through a resistor and develops a voltage across it - the result is an output voltage pulse.

Photodiode:The most widely used light sensor is a photodiode. This is a silicon PN-junction diode that is sensitive to light. This diode is normally reverse biased, as shown in the diagram opposite:

Light Sources and Detectors

The only current that flows through is an extremely small reverse leakage current.

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PIN Diode:The sensitivity of a standard PN-junction photodiode can be increased and the response time decreased by creating a new device that adds an undoped or intrinsic (I) layer between the P and N semiconductors. The result is a PIN diode.

The thin P-layer is exposed to light which penetrates the junction, causing electron flow proportional to the amount of light. The diode is reverse biased, and used in a similar manner to the standard photodiode, in series with a resistor. The current is very low until light strikes the diode, which significantly increases the current.

PIN Diodes are significantly faster in response to rapid light pulses of high frequency, and their light sensitivity is far greater than that of an ordinary photodiode.

Light Sources and Detectors

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The avalanche photodiode (APD) is a widely used photosensor. It is the fastest and most sensitive photodiode available, and its circuitry is complex.Like the standard photodiode, the APD is reverse biased.

The APD uses the reverse breakdown mode of operation that is commonly found in Zener diodes.When a sufficient amount of reverse voltage is applied, an extremely high current flows because of the avalanche effect. The voltage applied is just below the avalanche threshold.When light strikes the junction, breakdown occurs, and a large current flows. This high reverse current requires less amplification than the small current in a standard photodiode.

Germanium APDs are also significantly faster than other photodiodes and are capable of handling the gigabit-per-second data rates possible in some systems.

Light Sources and Detectors