Light sources and detectors

Most light sources and detectors are electronic devices built from the same semiconductor materials as are used in transistors and integrated circuits. The design of these devices is a separate study and will not be considered here. Instead, our view will be restricted to the characteristics which are of interest to the user.

Lasers

The most common form of laser diode is called an injection laser diode (ILD) or just injection diode (ID). The word injection is not of interest — it merely refers to part of the process occurring inside the semiconductor material. A laser provides a light of fixed wavelength which can be in the visible region around 635 nm or in any one of the three infrared windows. The light has a very narrow bandwidth, typically only a few nanometers wide. This ensures that chromatic dispersion is kept to a low value and this, together with fast switching, allows high data transmission rates.

As the laser device itself is barely visible to the unaided eye, it must be contained in some form of package. Two typical examples are shown in Figure 14.1.

Lasers for visible light

The light is launched via a lens system to allow it to be concentrated into a beam. Visible laser light finds applications in bar code readers, CD players, medical and communication systems. They are usually fitted with a built-in light detector so that they can receive reflected information as in the case of the bar code reader.

Lasers for 850 nm use

These can be packaged in either of the ways illustrated in Figure 14.1 depending on their application. The fact that their output is not visible allows for use in security, ranging, automotive and industrial and military applications. They also provide the light source for short and medium range fiber communications.

Lasers for singlemode communications

Successful launching into singlemode fibers requires very high precision and this is achieved by optimizing the position of an attached pigtail which can then be connected to the main fiber run by any desired method.
A photoelectric cell is also included as a monitoring device to measure the output power. This provides feedback to allow for automatic control of the laser output power.
The output power of a laser is affected by any change in its temperature, generally decreasing in power as the temperature increases. Some laser modules include a temperature sensor to combat this problem. It provides internal temperature information which is used to control a thermo-electric cooler like a small refrigerator, to maintain the temperature. The temperature stability is also improved by bolting the laser package to some form of heat sink such as the instrument casing.

Laser safety

Both visible and infrared light can cause immediate and permanent damage to the eyes. The shorter wavelengths cause damage to the retina and the longer wavelengths attack the cornea, in neither case can medical science offer remedy once the damage is done. Permanent loss of eyesight in less than a second by exposure to light we can’t even see — it doesn’t seem fair somehow.
It is extremely important that we take sensible precautions.
Never look into:

• A live laser source
• An unknown light source
• Any fiber until you have ascertained that it is safe. Check it yourself even if trusted colleagues say ‘its OK we’ve just checked it out’. They may be talking about a different fiber or they may have made a mistake.

If an instrument such as a live fiber detector is used, make sure it is working.

Beware of concentrating the light by instruments such as will happen when checking a cleave or the end condition of a connector with a microscope.
Laser classifications are based on an international standard titled Radiation Safety of Laser Products, Equipment Classification, Requirements and User’s Guide, referred to as IEC standard 825. Additional national standards apply in each country.

The IEC 825 classification has used four classes of laser based on the accessible emission limit (AEL). Every laser must carry a warning label stating the class of laser as shown in Figure 14.2. It is the responsibility of the manufacturer to

determine the classification of the laser and they do so by measuring the wavelength, output power and the pulsing characteristics.

IEC classifications

Class 1: Safe under reasonably foreseeable conditions of operations. Note that it doesn’t say ‘safe under any conditions’.

Class 2: Visible lasers with light output within the visible spectrum of 400–700 nm. There is an assumption here that the blink reflex will close the eyes within a fraction of a second and hence provide protection. Prolonged exposure will cause damage.

Class 3a: Safe for viewing by the unaided eye either visible or infrared light but possibly unsafe when viewed with instruments.

Class 3b: Direct viewing is hazardous but reflected light is normally OK. Note the normally. Not to be viewed with instruments.

Class 4: Horribly dangerous. Even reflections are hazardous and the direct beam can cause fires and skin injury. Not normally used for communications.

Control measures

For classes 2, 3 and 4, control measures are employed such as interlocks, keys, laser ‘on’ warning lights, remote switching, prevention of reflections across walkways. The precautions depend on the situation, use and power of the laser. The appropriate national standards as well as IEC 825 should be consulted for guidance.

Laser specifications

Wavelength

The wavelength quoted is only a typical value. So if we want to buy a laser for the 1300 nm window, the one offered may well be quoted as 1285–1320 nm and the actual frequency will fall somewhere between these limits. Sometimes it would just be sold as 1300 nm (nominal).

Rise and fall time — Figure 14.3
This is a measure of how quickly the laser can be switched on or off measured between the output levels of 10% to 90% of the maximum. A typical value is 0.3 ns.

Threshold current — Figure 14.4

This is the lowest current at which the laser operates. A typical value is 50 mA and the normal operating current would be around 70 mA.

Spectral width — Figure 14.5

This is the bandwidth of the emitted light. Typical spectral widths lie between 1 nm and 5 nm. A laser with an output of 1310 nm with a spectral width of 4 nm, would emit infrared light between 1308 nm and 1312 nm.

Operating temperature

No surprises here. Typical values are –10°C to +65°C and therefore match the temperature ranges of fibers quite well.

Voltages and currents

The specifications also list the operating voltages and currents of the monitor detector, the cooler current and the thermistor resistance. These are generally only of interest to the equipment designer or the repair technician.

Output power
The output power may be quoted in watts or in dBm.

LEDs — light emitting diodes

LEDs can provide light output in the visible spectrum as well as in the 850 nm, 1350 nm and the 1500 nm windows. Compared with the laser, the LED has a lower output power, slower switching speed and greater spectral width, hence more dispersion. These deficiencies make it inferior for use with high speed data links and telecommunications. However it is widely used for short and medium range systems using both glass and plastic fibers because it is simple, cheap, reliable and is less temperature dependent. It is also unaffected by incoming light energy from Fresnel reflections etc. Although the lower power makes it safer to use, it can still be dangerous when the light is concentrated through a viewing instrument. Typical packages are shown in Figure 14.6.

PIN diodes

A PIN diode is the most popular method of converting the received light into an electronic signal. Their appearance is almost identical to LEDs and lasers. Indeed the diagrams in Figure 14.6 would serve equally well for PIN diodes if the labels were changed. They can be terminated with SMA, ST, SC, biconic and a variety of other connectors or a pigtail.
It may be of interest to have a brief look at its name. It uses a semiconductor material, either germanium or silicon. The pure semiconductor material is called an intrinsic semiconductor — this is the I in the name. To make it work, we have to add a controlled amount of impurity into the semiconductor to change its characteristics. The semiconductor is converted into two types, one called P-type semiconductor and the other called N-type. These are arranged either side of the I material to make an I sandwich. Hence P-I-N or PIN diode. The theory of its operation will not be considered further.
While we can still buy straightforward PIN diodes, it is more usual for it to have an amplifier built into the module to provide a higher output signal level.
Avalanche diode also called an avalanche photo diode or APD
Higher output signals can be achieved by an avalanche diode. It uses a small internal current to generate a larger one in the same way that a snow-ball rolling down a mountainside can dislodge some more snow which, in turn, dislodges even more snow and eventually gives rise to an avalanche.
They have the advantages of a good output at low light levels and a wide dynamic range — it can handle high and low light levels. However there are a number of disadvantages which tend to outweigh the benefits. It has higher noise levels, costs more, generally requires higher operating voltages and its gain decreases with an increase in temperature.

Light receiver specifictions

Wavelength
This is quoted as a range e.g. 1000 nm to 1600 nm, or by stating the frequency that provides the highest output e.g. peak wavelength = 850 nm.

Dynamic range or optical input power

Dynamic range is the ratio of the maximum input power to the lowest. It is quoted in decibels e.g. 21 dB. The optical input power is the same information expressed in watts. e.g. 1 μW to 125 μW.

Responsivity
A measure of how much output current is obtained for each watt of input light. e.g. 0.8AW–1. This means that the current will increase by 0.8 amps for every watt of increased light power.

Response time
This is the rise and fall time that we saw in Figure 14.3. It determines the fastest switching speed of the detector and hence limits the maximum transmission rate e.g. tr or tf = 3.5 ns.

Bit rate or data rate or bandwidth

These are both measures of the maximum speed of response to incoming signals and is therefore determined by the response time above.

Kevin M Contreras H
CI 18.255.631
CRF
http://www.kiet.edu/ensite/downloads/Introduction%20to%20Fiber%20Optics%20-%20John%20Crisp.pdf

Fusion splicing

Fusion splicing is the most permanent and lowest loss method of connecting optic fibers. In essence, the two fibers are simply aligned then joined by electric-arc welding. The resulting connection has a loss of less than 0.05 dB, about 1% power loss. Most fusion splicers can handle both single mode and multimode fibers in a variety of sizes but, due to the losses involved, we only splice multimode to multimode or singlemode to singlemode. There are also splicers that can automatically splice multicore and ribbon cable up to 12 fibers at a time.

Preparation of the fiber

The fibers must first be stripped, cleaned and cleaved as we have seen in Chapter 9. To allow spare fiber for easy access and to allow for several attempts, a length of at least five meters of jacket should be removed. The primary buffer is only stripped to about 25 mm. The exact length is determined by the fusion splicer in use.
The quality of the cleave is of paramount importance. However much money we spend on buying the most sophisticated splicing apparatus, it will all be wasted if we cannot cleave the fiber accurately. Both cleavers and splicers come in a range of prices with splicers being the more expensive by a factor of at least ten and sometimes a hundred. It is never a good idea to save money by buying an inadequate cleaver — it is always better to buy the cleaver you have confidence in, then, if necessary, recover the money by buying a slightly cheaper version of the splicer. Most splicers nowadays measure the accuracy of the cleave and if found wanting, the fiber is rejected until you have redone it to a satisfactory standard. Most splicers consider an end angle of better than about 3° as satisfactory.

Protecting the fiber

Splice protector

In the preparation phase, we have stripped the fiber of all its mechanical and waterproof protection. Once the fiber has been spliced, some protection must be restored since the splicing process will have reduced the fiber strength to less than 30% of its former value. This is achieved by a device called a splice protector. It consists of a short length (about 60 mm) of heatshrink sleeving enclosing some hot-melt glue and a stainless steel wire rod as seen in Figure 10.1.

Prior to joining the fiber, the splice protector is slid onto the fiber. After thesplice is completed, the protector is centered over the splice and heated, usually in a purpose-built oven although a hot-air gun can be used. The oven is a simple tray with a lid, a heater and a timer which are normally built-in features of the splicers. The hot-melt glue keeps the protector in position whilst the stainless steel rod provides proof against any bending that may occur. The outer sleeve offers general mechanical and water protection to replace the buffer that has been removed. To ensure that the fiber is fully protected along its length, at least 10 mm of the protector must overlap the primary buffer at each end of the splice.

Enclosures (termination enclosures)

After the splice is completed, we are left with a length of fiber deprived of its outer jacket. The fiber must be protected from mechanical damage, and from water. This is achieved by an enclosure (Figure 10.2).

The design, and cost, of the enclosures depend on the environment in which the fiber is going to live. Obviously something to protect fiber under water has to be superior to a plastic box in an air-conditioned office.
The significant feature is a means of ensuring that the fiber is well supported within the container in such a way that bending loss is avoided. This is done by having something to wind the fiber around, like a reel, referred to as a cassette, or at least a few clips to support the fiber and the splices. They are readily available in different sizes to hold everything from 4 to 240 fibers. Each fiber must be identified, otherwise a simple job could become a real nightmare. This is achieved by attaching labels to the fibers or splice protectors and by using colored splice protectors.
There are some other factors to consider which may not immediately spring to mind such as:

• Security of the data. With the outer jacket removed a simple live fiber detector can be clipped onto the fiber and all the data being passed can be copied.
• Access to an enclosure is the easy way to sabotage a communication system.
• There is also a problem with light, again with no jacket and as bends are inevitable, there is a risk of light entering the system so the container should be light proof.
• Unpleasant environments. Salt spray, acids, high temperatures, crushing and all sorts of other nasties.
• Access for repairs or for testing purposes.
  

We stripped off five meters of outer jacket to enable the fiber to be lifted out of the enclosure with enough spare fiber to be easily connected to test equipment or a fusion splicer.

Holding and moving the fibers in the splicer

The fibers are held in vee-grooves cut into steel or ceramic blocks (Figure 10.3). As usual, cleanliness is all-important. The fiber is cleaned and cleaved then the vee-groove is cleaned by a lint free cloth, tissue or a ‘cotton bud’ moistened with isopropyl alcohol. Do not use compressed air cleaners as any contamination will turn it into a grit blaster and damage the critical dimensions of the veegroove. The fiber is gently pressed into the vee-groove by a magnetic or gravity clamp.

Once the fibers are safely clamped into their vee-grooves, they are moved, veegrooves and all, until the fibers are aligned with each other and positioned directly under the electrodes from which the electric arc will be produced. We are aiming to achieve positioning with an accuracy of better than 1 μm. In the least accurate splicers, suitable only for multimode fibers, this can be achieved by simple microgears operated manually. More precision is required for single mode fiber since the core is so much smaller. A 1 μm error in positioning an 8 μm core causes a lateral misalignment of 12.5% whereas the same error on the larger 62.5 μm core in a multimode fiber would represent a lateral misalignment of less than 2%. The extra precision is provided by using stepper motors. A stepper motor is an electric motor that behaves in a different manner to a ‘normal’ electric motor. We usually picture electric motors spinning round as power is applied. Stepper motors, instead, turn a set number of degrees and then lock in that position. The amount it turns can be precisely controlled by digital input signals and, in conjunction with a gear train, is able to provide extremely accurate alignment of the fibers.

Observing the alignment

All fusion splicers are fitted with some means to observe the fiber positioning and the condition of the electrodes. This is achieved by either a microscope or by a CCD camera (CCD = charge coupled device — a semiconductor light sensor) and a liquid crystal display (LCD ). The trend is towards CCD cameras since they are more pleasant to use and have the safety advantage of keeping our eyes separated from the infrared light which can, of course, cause irreparable damage to the eyes if we accidentally observe an active fiber through the microscope. The optic system always allows viewing from two angles as fibers can otherwise hide one behind the another and appear to be aligned. One way of achieving this is shown in Figure 10.4.

Automatic positioning

There are two methods.

PAS — the profile alignment system — Figure 10.5

This is the standard method of aligning the fibers in modern fusion splicers. The idea is very simple. A light is shone through the fiber and is detected by a CCD camera. The change of light intensity at the edge of the cladding and at the core due to the changes in refractive indices allows the system to detect their positions. Several readings are taken from each fiber and averaged out to reduce any slight errors.
Once the positions are detected, small stepper motors are activated to bring thetwo fibers into alignment. The viewing angle is switched through 90° to allow

the system to check in both planes and further small adjustments are made until the splicer is quite satisfied. The whole operation is usually automatic but we can follow the process on the liquid crystal display. As the system is able to detect the core position as well as the cladding, any eccentricity error in the core can be compensated for.

LID — light injection and detection system — Figure 10.6

This system makes use of bend loss, the light leakage that occurs when moderately tight bends are introduced into a fiber. Remember that the light can go into the fiber at a bend as well as being able to escape from it.
A bend is introduced at the input side of the splicer and light is injected into the fiber through the primary buffer. The light travels down the fiber and jumps the gap into the other fiber. A similar bend in the other length of fiber allows light to escape.
A stepper motor is used to move one of the fibers horizontally and the output light is monitored to detect the point of maximum light transfer. This means that the cores are aligned, at least in one plane. The fiber is then moved in the vertical plane until, once again, the point of maximum light transfer is discovered. The whole process is repeated once or twice making finer and finer adjustments until it homes in on the point of best light transfer. Once this has been achieved, the fibers are spliced.

There are one or two slightly worrying aspects with this design. The first is the severity of the bends introduced. The radius of these bends is generally tighter than the fiber specification allows. This means that if the fiber fails, immediately or at a later date, the fiber manufacturer will not be interested since you exceeded the fiber limits. It does not usually break, of course, but there is still a slight feeling of unease. There is another thought, too. If the primary buffer happens to be opaque, the light cannot penetrate. LID systems are sometimes offered as a bolt-on goody to the standard PAS splicer.

Fusion splicing of the fibers

The arc that occurs between the two electrodes is about 7000 volts with an adjustable current up to 25 mA — a very unpleasant and dangerous combination. Safety precautions are built in to splicers to ensure that we cannot accidentally come into contact with the arc. There are two approaches to this problem, the first is to be sure we know where our fingers are before the arc starts. This is done by involving two buttons both of which must be held down during the operation and are spaced so that both hands must be used. The alternative is to enclose the dangerous area with a cover that must be closed before the arc can be energized.

Prefuse

The first stage of the fusing process is to align the fibers with an end gap equal to one fiber diameter and apply a short, relatively low power arc. This is called a prefuse. Its purpose is to clean and dry the end surfaces of the cleaves so that nothing untoward gets trapped inside the splice. It can remove very slight tangs from the cleave but don’t expect miracles, it won’t repair a poorly prepared fiber.

Main fuse

The fibers are then brought together and some additional end pressure is applied. The additional pressure allows the fibers to move towards each other slightly as they melt. How far they move, called the overfeed, autofeed or stuffing distance is critical. Too much or too little and the splice will not be satisfactory. The reason for introducing the overfeed is illustrated in Figure 10.7. In the figure, the cleaved end of the fiber is shown greatly magnified and, even with a good cleaver the surface is never completely smooth. When two fibers are brought together there are some small air gaps present which means that within

the area marked A there is actually less glass than in a similar length, shown as B. When the electric arc melts the end of the fiber, the glass tends to collapse inwards, filling the air gaps. This produces some waisting as shown on the page of disasters (see Figure 10.9). The main fusing arc is more powerful and lasts for a longer period of time, between 10 and 20 seconds. If the splicer is fitted with a microscope, it is important to check in the instruction book to see if the fiber can be viewed during splicing. The arc emits ultraviolet light and as the microscope concentrates the light there is a danger of suffering from arc-eye, a temporary or permanent damage to the eyes. This is the result of electric arc welding without eye protection. Most viewing microscopes have a UV filter (and an infrared filter) to prevent eye damage, but check the manual first. This is another advantage of choosing one with a CCD camera and monitor.

Once fusing is completed, have a good look at the splice. If it is difficult to see where the splice is, then it’s probably a good one (Figure 10.8). We are looking for the outer edges of the cladding to be parallel, just like a new continuous length of fiber. Sometimes a small white line appears across the core but this is not important and can be ignored. See Figure 10.9 for all the
disasters.

Setting up the splicer

Different types of fibers require particular values of arc current and lengths of time since this determines the temperature to which the fiber rises. There are several automatic programs installed as well as user-defined settings that we can employ in the light of experience.
The equipment is powered by nicad batteries giving up to 300 splices per charge, somewhat less (50 or so) if the oven is used to shrink the splice protector. Automatic splice testing
When the splice is completed, the fiber positions are rechecked and an estimated splice loss is calculated and displayed. Although the estimate is reasonably accurate, it is only an estimate and not a measurement. To be really certain, the loss must be measured and this must be done after the splice protector has been fitted just in case the splice is damaged during this operation.

Kevin M Contreras H
CI 18.255.631
CRF

http://www.kiet.edu/ensite/downloads/Introduction%20to%20Fiber%20Optics%20-%20John%20Crisp.pdf

Mechanical splices

The mechanical splice performs a similar function to the fusion splice except that the fibers are held together by mechanical means rather than by a welding technique. Physically, they often look very similar to splice protectors.

Advantages and disadvantages

There are several advantages. They do not require any power supplies. Indeed, many designs require no tools at all beyond a stripper and cleaver, so the mechanical splice can be used in situations that may be considered hostile to many fusion splicers. Mechanical splices are often re-usable and can be fitted in less than a couple of minutes, which makes them ideal for temporary connections. The disadvantage is that they cause a loss, called the insertion loss, of about 0.1–0.3 dB per connection which is significantly higher than a good fusion splice. This would suggest the use of a fusion splice as the first choice in situations where losses are critical. Mechanical splices can be used to connect either singlemode or multimode fibers.

Cost

On cost grounds alone, the choice between a fusion splicer and mechanical splices depends on the number of splices to be undertaken. If we already have a fusion splicer, the cost of each fusion splice is negligible but for the cost of a reasonably good fusion splicer we could purchase a thousand or more mechanical splices. It is also possible to hire fusion splicers and other fiber optic equipment.

How they work

In essence, it is very easy. The fiber must be stripped, cleaned and cleaved. They must then be aligned and then held in position either by epoxy resin or by mechanical clips. There are only three basic designs.

Vee-groove

This was the obvious choice since it worked so well in positioning the fibers in the fusion splicer. See Figure 11.1. Most mechanical splices are designed around the vee-groove. They consist of a base plate into which the vee-groove has been cut, ground or molded.

The prepared fibers are placed in the groove and their ends are brought into contact. Some index matching gel is used to bridge the gap between the two ends to prevent gap loss and to reduce Fresnel reflection. A gripping mechanism then holds the fibers in position and provides mechanical protection for the fiber. As an alternative to the index matching gel, an index matching epoxy can be used. This performs the same index matching task as the gel but also holds the fiber in position. It is usually cured by UV light.

Bent tube — Figure 11.2

If a length of fiber is pushed into a tube which is curved, the springiness of the fiber will force it to follow the outside of the curve. Now, if the tube is of square cross-section, the fiber will follow the far corner. This is very similar to a vee groove since the fiber is now positioned by a vee-shaped wall of the tube. This is called a bent tube design. A small spot of index matching gel is added before the fibers are inserted. In some designs, a bent tube with a circular cross section is used but the principle is just the same.

Precision tube

This type is very simple. A hole, very slightly larger than the fiber diameter isformed through a piece of ceramic or other material. When a piece of bare fiber is inserted from each end, the two fibers are inevitably aligned when they meet. The insertion losses are higher than the other types due to tolerances in the holediameter.

Specifications

The specifications will give information about several things.

Cladding and buffer diameter

The cladding diameter will usually be 125 μm for most fibers. The buffer diameter is likely to be either 250 μm or 900 μm.

Insertion loss

This is the loss caused by the device when it is installed in the system. Typical values are around 0.2 dB.

Return loss

This is the proportion of the incoming light that is reflected back along the fiber. Usually between –40 dB and –60 dB. The low loss is the result of adding the index matching gel to reduce Fresnel reflection.

Fiber retention

How much tension can be applied to the completed splice before the splice fails? The failure mode may be obvious and catastrophic as in the fiber actually becoming disconnected or the ends of the fiber being pulled apart very slightly causing enormous gap loss but no visible damage. Typical values are around 4 N but some are much more rugged with figures up to 180 N when the correct external protection is applied.

A practical guide to fitting a typical mechanical splice

All mechanical splices come with an instruction sheet. It is essential that thetime is taken to read it as each splice has a slightly different fixing method. As mentioned earlier, some are permanent fixings using epoxy resin and some are designed to be re-usable. It is not a good idea to pump in epoxy resin first, then settle down to read the instructions!

• Strip, clean and cleave the fiber leaving about 12 mm of primary buffer removed.
• If re-using the splice (if this is possible) clean with isopropyl alcohol and a piece of lint free cloth and use a syringe to inject a small bead of index matching gel into the center of the splice.
• Release the small clip at one end of the splice and insert the fiber until it comes to a stop. Operate the clip to lock the fiber in position.
• Release the clip at the other end and insert fiber until it comes up against the fiber already loaded. Operate the clip to lock that fiber also.
• Test the operation and if not satisfactory, the clips can be released to allow the fiber positions to be optimized.

Kevin M Contreras H
CI 18.255.631
CRF

http://www.kiet.edu/ensite/downloads/Introduction%20to%20Fiber%20Optics%20-%20John%20Crisp.pdf

Pulsos y solitones

En las comunicaciones ópticas los principales problemas son la atenuación, la dispersión, los efectos no lineales y el ruido. La dispersion y los efectos no lineales provocan la distorsión de la señal. Las pérdidas de la fibra óptica se compensan empleando amplificadores pero estos incrementan el ruido (ruido ASE, amplifier spontaneuos emission ) en la señal, por lo empeora la SNR. Para mejorar la SNR se debe incrementar el nivel de la señal pero esto supone agravar los efectos no lineales. Las consecuencia de estos problemas se agravan con la distancia de propagación de la señal.

Los solitones, a diferencia de otro tipo de pulsos, tienen la propiedad de ser transmitidos largas distancias sin cambiar de forma debido a la dispersión producida por la velocidad de grupo, GVD, se compensa con los efectos de la SPM , self-phase modulation .

Los solitones son pulsos estrechos con un pico de potencia elevado y una forma especial. El tipo de solitones más empleado son los llamados solitones fundamentales, cuya forma se muestra en la siguiente figura.

La ventaja de los solitones para las comunicaciones ópticas es que con ellos se compensan los efectos perjudiciales de la dispersión cromática. El empleo en las comunicaciones ópticas de solitones y amplificadores ópticos, para compensar los efectos de la dispersión y la atenuación respectivamente, permite alcanzar altas velocidades.

Propagación de solitones

Para obtener la expresión matemática de un solitón se resuelve la ecuación no lineal de Schrödinger (NLSE, Nonlinear Schrödinger Equation ) que rige la propagación de señales ópticas por la fibra, en presencia de SPM y GVD,

donde a represetan las pérdidas de la fibra; los parámetros β₂ y β₃ representan los efectos de la dispersión de segundo y tercer orden respectivamente; y γ representa los efectos no lineales y se define como el cociente entre el coeficiente del indice no lineal (n₂), la longitud de onda (λ) y el area efectiva del nucleo de la fibra (A_eff ),

En el régimen de dispersión anómala (transmisión en 3ª ventana en fibra monomodo estándar y fibras de dispersión desplazada) el parámetro β₂ de la GVD es negativo. A parte de estas consideraciones para simplificar resolución de la NLSE se considera que α=0, β₃=0. En este caso, y considerando los siguientes cambios de variables,

donde T₀ el ancho del pulso, P₀ es el pico de potencia del pulso y L_D es la longitud de dispersión, esto es,

Entonces, tenemos la NLSE normalizada,

Donde el parámetro N se define como,

Cuando N es un entero, la ecuación anterior puede resolverse analíticamente, y la envolvente del pulso resultante tiene una amplitud independiente de ξ (para N=1) o periódica con ξ (para N=2). Esto supone que estos pulsos se propagan sin cambiar su anchura o con cambios periódicos, respectivamente. Estas soluciones son los solitones y N es su orden.

Se puede demostrar que la solución de la NLSE normalizada para el caso descrito y N=1, es

El pulso correspondiente a esta envolvente es el solitón fundamental. En la siguiente figura se representa el pulso y su envolvente. Como muestra la ecuación anterior la forma del pulso no cambia con z. Sin embargo, el pulso adquiere un desplazamiento de fase que es lineal con z al propagarse.

Como la longintud de onda esta fijada, β₂ y γ vienen determinados por la fibra; para un solitón de un orden N, el pico de potencia P₀ del pulso se incrementa cuando la anchura de éste, T₀ , disminuye (ecuación 8 ). Como trabajar con velocidades de transmisión muy altas requiere pulsos muy estrechos, esto supone que los picos de potencia de los solinotes en los sistemas de comunicaciones ópticos sean muy altos, por lo que los problemas debidos a los efetos no lineales se acentuan.

Para el caso de la envolvente de un solitón de orden dos, la solución es,

En la siguiente figura se muestra la envolvente normalizada del pulso solitón de orden 2 en función de ξ y τ. La periodicidad de la envolvente del pulso con respecto a ξ se muestra en esta figura. En cada periodo, la envolvente primero se comprime debido al signo positivo del chirp provocado por la SPM y a continuación se ensancha para finalmente recuperar su forma orignal.

El pulso recupera su forma cuando se verifica que ξ=mπ/2, donde m es un valor entero. Luego el periodo del soliton, z₀ , es decir, la distancia a la que recorre un soliton de orden superior hasta que recupera su forma orignal, se define a partir de la ecuación 6 como,

Transmisores de solitones

Tras el analisis de los solitones en los anteriores apartados se deduce las fuentes ópticas empeladas para generear solitones han de poder generar alto número de pulsos por unidad de tiempo, siendo éstos muy estrechos (del orden de picosegundos), sin chirp y con una forma lo más aproximada posible a una secante hiperbólica. Además, estas fuentes han de operar en la tercera ventana, donde las pérdidas son menores y donde operan los EDFA.

El efecto de las pérdidas en los solitones

Los solitones mantiene su anchura gracias a la SPM ( self-phase modulation ) que contrarestra los efectos de la GVD ( Group Velocity Dispersion ). Cuando las pérdidas reducen lo sufienciente el maximo de potencia del pulso el efecto de la SPM no es lo suficentemente fuerte como para contrarrestar la GVD.

Incluyendo las pérdidas en la NLSE ( ecuación 7) para el caso de un soliton fundamental (N=1) se tiene,

donde Γ=αL_D, son las perdidas introducidas en la longitud de dispersión. La solución obtenida cuando Γ<<1>

Esta expresión muestra como la anchura del pulso se incrementa de forma exponencial según aumentan las pérdidas,

La solución a este problema es amplificar periódicamente la señal para asegurar que el efecto de la SPM contrarrestra a los de la GVD.

Kevin M Contreras H
CI 18.255.631
CRF

http://nemesis.tel.uva.es/images/tCO/contenidos/tema1/tema1_7_3.htm

Fibra óptica en aplicaciones de energía solar

La energía solar se va popularizando progresivamente para cumplir diferentes tipos de demandas.

En los parques solares, muchos de varios megavatios de potencia se generan a partir del sol unas enormes intensidades de corriente eléctrica. Una gran cantidad de corriente eléctrica producida en corriente continua y cuyo movimiento por el parque produce enormes pérdidas. El aislamiento galvánico es especialmente importante para asegurar la calidad y fiabilidad del sistema de generación. La fibra óptica ofrece protección de aislamiento de fallos de corriente/alto voltaje y señales no deseadas en los controles y comunicaciones del equipo de potencia y comunicación.
Pero la fibra óptica tiene también utilidad para reducir las pérdidas en la captación de los paneles solares. Las comunicaciones por fibra óptica pueden cubrir distancias de conexión mayores comparadas con el conductor de cobre.

Las aplicaciones clave de los componentes de la fibra incluyen:

• Drivers electrónicos de los inversores.
• Control del seguimiento del sol y panel de comunicaciones.
• Automatización de la subestación del parque solar y relés de protección.

Los paneles solares colectan energía solar y la convierten en energía eléctrica a través de los módulos fotovoltaicos o colectores térmicos solares. Para integrar la energía generada desde los paneles solares a las líneas de transmisión de energía, necesitamos convertirla a en corriente alterna y a una tensión soportada por la línea de distribución. Es uno de los problemas de los parques solares, ya que con tanta conversión lo único que se generan son pérdidas.
Para producir la corriente alterna requerida, el dispositivo semiconductor actúa a la frecuencia correcta para asegurar que la corriente alterna es fiable, y para ello se suele usar un controlador embebido DSP vía una conexión de fibra óptica, lo cual permite una alta capacidad de aislamiento galvánico.

Ejemplos de dispositivos semiconductores disponibles en el mercado son:

• Insulated Gate Bipolar Transistor (IGBT).
• Gate Turn Off Thyristor (GTO).
• Integrated Gate Commutated Thyristor (IGCT).
• Symmetrical Gate Commutated Thyristor (SGCT).
• Emitter Turn Off Thyristor (ETO).

Los componentes de fibra óptica comúnmente se usan para controlar dispositivos de alto voltaje y dispositivos de conmutación de corriente, con control y señales de retroalimentación fiables.

Control de paneles solares

Hay dos formas principales de maximizar la conversión de energía eléctrica en las instalaciones fotovoltaicas; una de ellas es usar el panel solar más eficiente. La segunda es mediante el seguimiento del movimiento del sol a lo largo del día, mediante los denominados seguidores solares. Experimentalmente se demuestra que el uso de seguidores solares tiene un efecto altamente positivo en la energía obtenida por los paneles. Dicho de otra forma, para conseguir una producción de energía dada puede reducirse el número de paneles sensiblemente si se usan seguidores solares. No entramos en detalles en este artículo, pero hemos comprobado que la utilización de seguidores es realmente efectiva. El sol produce muy poca energía en el panel cuando sus rayos son oblicuos al panel, por lo que los soportes fijos están perdiendo mucha capacidad de generación tanto en la mañana como en la tarde.
Cuando los parques solares empiezan a ser cada vez más grandes, están equipados con características inteligentes que controlan el rendimiento de cada panel solar. La producción eléctrica y la temperatura del panel
Cuando los parques solares son más y más grandes, controlar el ángulo y dirección de los paneles solares es una cuestión muy importante. En estos parques comerciales que generan varios megavatios, los paneles se instalan en enormes áreas, y el control de estas redes sólo es posible con redes de fibra óptica.

Automatización de subestaciones

Las subestaciones se conectan a las redes de distribución a través de una subestación, donde vuelven a producirse nuevas pérdidas. Las subestaciones modernas, que se basan en la norma CEI 61850, se diseñan para mejorar la fiabilidad total del sistema y reducir significativamente el número de conductores de cobre utilizados.

Ya que la mayoría de los equipos (ej. Cuadros eléctricos, transformadores, interruptores, etc.) en subestaciones operan a medio/alto voltaje, es necesario tener aislamiento galvánico que proporcione protección para los dispositivos de bajo voltaje conectados. Estos equipos también generan un gran campo electromagnético debido al alto voltaje y corriente de conmutación. Para asegurar un control fiable, la norma requiere que las líneas de comunicaciones sean inmunes a los campos electromagnéticos. En este caso, la fibra óptica es la mejor solución para tales requerimientos en controles de automatización de subestaciones y líneas de comunicaciones.

Kevin M Contreras H
CI 18.255.631
CRF

http://www.dforcesolar.com/energia-solar/fibra-optica-en-aplicaciones-de-energia-solar/

DETECTORES FOTODIODOS

CLASIFICACIÓN

Existe una gran variedad de detectores de luz.
-Detectores térmicos: donde la radiación es absorbida y transformada en calor (responde al cambio de temperatura).
-Detectores cuánticos: los que responden directamente a la densidad de fotones incidentes.

Es natural que para los sistemas que nos ocupa usemos detectores cuánticos. Pudiéndose realizar una subclasificación:

Detectores fotoemisivos: se aprovecha la emisión de electrones desde un material cuando inciden fotones según el efecto fotoeléctrico. Responden a fototubos de vacío con multiplicación interna.
Detectores fotoconductivos: la densidad de fotones hace variar la conductividad del fotosensor. Existen los tipos: fotorresistivos (elemento resistivo o un semiconductor intrínseco) y fotodiodos (semiconductores dopados).
Detectores fotovoltaicos: donde al incidir fotones se genera una tensión; tal el caso de las celdas solares.

La elección recae sobre los detectores cuánticos fotoconductivos con semiconductores dopados. Las razones son: tienen el menor peso y tamaño, son más robustos que los tubos de vacío. Tienen estabilidad térmica y temporal. Su costo es reducido.Tienen gran respuesta espectral, más linealidad y velocidad de conmutación

DETECTORES FOTODIODOS

La elección del semiconductor adecuado para la longitud de onda se realiza mediante la respuesta espectral. En la Fig 01 se representa el coeficiente de absorción α en unidades de cm-1 para distintos semiconductores. La inversa del coeficiente es la longitud de penetración de la radiación en el material. Para lograr altas velocidades el valor de 1/α debe ser menor que 1 μm. El Si tiene en la primer ventana 0,85 μm un valor de 1/α=10 μm. Sin embargo, por el alto grado de conocimiento de este semiconductor se lo ha adoptado como detector en esta ventana.

RESPONSIVIDAD. Se denomina responsividad o factor de respuesta R a la relación que existe entre la corriente I que entrega el fotodiodo al circuito eléctrico y la potencia óptica P que recibe de aquél. En un amplio margen de dependencia la misma es lineal y se expresa como:

R = I / P = (e.re)/(E.rf) = e.λ.η/h.c

donde e es la carga del electrón, E la energía del fotón, re y rf son el número promedio de electrones y fotones, η la eficiencia cuántica (re/rf). El valor de R se aproxima por:

R = 0,8.λ.η

si la longitud de onda λ se mide en μm. Se observa que R aumenta con λ en forma lineal. Sin embargo, existe un valor de la longitud de onda donde los fotones tienen una energía E inferior al salto entre las bandas BC-BV. A partir de este punto los pares electrón-laguna generados por los fotones disminuyen rápidamente. En el Si la R se torna insignificante más allá de 1,1 μm. Por ello debemos adoptar otros materiales para las longitudes de onda más largas. Entre los materiales posibles se encuentra el Ge y InP.
En el mejor de los casos (con η=1) se tiene que un fotón genera un par electrón-laguna. En condiciones normales este par se vuelve a recombinar. Para separarlo y extraer una corriente del material es necesario disponer de una juntura PN con una polarización inversa de forma que sólo hay corriente cuando incide luz. En la juntura existe una diferencia de potencial que separa al par.

En la Fig 02 se muestra las distintas formas de junturas. En una juntura simple PN los pares creados fuera de la zona de juntura se vuelven a recombinar. Para incrementar la zona de detección se coloca una capa intermedia levemente dopada o intrínseca constituyendo un diodo PIN. Si ahora se agrega una capa adicional altamente dopada se crea una elevada diferencia de potencial que produce una ganancia interna debido a una avalancha de electrones, dando lugar al fotodiodo por avalancha APD.

FOTODIODO AVALANCHA. La elevada diferencia de potencial del APD permite que algunos electrones cedan parte de la energía para formar otros pares adicionales, lo cual se logra cuando el campo eléctrico de la juntura supera los 3.105 V/cm. El proceso de multiplicación por avalancha crea una ganancia interna M que es aleatoria. Desde el punto de vista eléctrico del APD se comporta como un diodo ideal en paralelo con un generador de corriente proporcional a la luz incidente. Cuando llega a un valor de tensión de polarización inversa que crea un campo interno suficiente para iniciar la avalancha, se produce un aumento de la fotocorriente. La tensión necesaria es de 150 a 400 V en el Si y de 10 a 50 V en el Ge.

Sin incidencia de luz se tiene una corriente residual de oscuridad, que resulta ser la suma producida por la difusión de portadores minoritarios generados térmicamente fuera de la zona de deplexión y la tunelización de electrones entre la BV y BC. En resumen se usarán el Si en 0,85 μm; y el Ge o InP en 1,3 ó 1,55 μm y con dos estructuras posibles el PIN o APD. En general se usa la estructura APD para el Si y Ge y la estructura PIN para los derivados del InP. En el diodo PIN la velocidad de los portadores en un campo típico de 2 V/μm es de 84 μm/ns para el electrón y de 44 μm/ns para las lagunas. Para un ancho de carga espacial de 20 μm, con 40 V de tensión externa el electrón lo recorre en 250 ps y la laguna en 500 ps; la luz es absorbida cerca de la juntura PI y las lagunas recorren por ello un camino menor. La eficiencia cuántica del diodo PIN es del 80%.


Kevin M Contreras H
CI 18.255.631
CRF
https://uvirtual.unet.edu.ve/file.php?file=%2F539%2Fmaterial_para_foro_3%2F07_RECEPTOR_DE_ENLACE_OPTICO.pdf

Network Simulation Platforms for Fiber Optic Equipment Testing

As the use of fiber optic equipment in communications systems continues to grow worldwide, network simulation testing has become vitally important for vendors seeking to integrate their equipment in the field.

In the past, many engineers and technicians were satisifed by using spools of optical fiber that were exposed and sitting out on the test bench. This practice, although usually working for a short period of time, generally leads to a number of issues that cost users both time and money.

Potential Issues of Using Exposed Fiber Spools

• Bare optical fiber is easily damaged when exposed in the work environment
• Extra care must be used when moving spools or risk damage to the fiber
• Spools take up valuable workspace, especially in large facilities at vendors performing network simulation testing frequently

With the production/use of specialty optical fibers (carrying a higher cost) on the rise and more budget restrictions, engineers must be wary of these issues. Fortunately, by purchasing an affordable network simulation platform, users can eliminate all the above issues, while making their fiber optic test routines much easier.

Types of Fiber Optic Network Simulation Platforms

Today, there are a variety of affordable network simulation platforms available in the marketplace. Both portable enclosures for single spools, and rack-mount enclosures for multiple-spools (and longer distances) are available. In addition, most are custom built exactly to customer specification, offering a vareity of optical fiber types, lengths, and connectors. Lastly, these plaforms offer testing stability and better organize the entire workspace.

Multi-spool w/ 100km Optical Fiber

Single Spool w/ 25km Optical Fiber

With optical fiber being extremely valuable for network simulation tesing, it only makes sense to protect the fiber and fortunately that is easily accomplished.

Kevin M Contreras H
CI 18.255.631
CRF
http://www.m2optics.com/blog/