Augmented reality

Augmented reality (AR) is a live, direct or indirect, view of a physical, real-world environment whose elements are augmented by computer-generated sensory input such as sound, video, graphics or GPS data. 

It is related to a more general concept called mediated reality, in which a view of reality is modified (possibly even diminished rather than augmented) by a computer. As a result, the technology functions by enhancing one’s current perception of reality. 

By contrast, virtual reality replaces the real world with a simulated one. Augmentation is conventionally in real-time and in semantic context with environmental elements, such as sports scores on TV during a match. 

With the help of advanced AR technology (e.g. adding computer vision and object recognition) the information about the surrounding real world of the user becomes interactive and digitally manipulable. 

Artificial information about the environment and its objects can be overlaid on the real world.

Fiber Network : Test Equipment and Techniques

Test Equipment and Techniques

Experience tells us just what happens when you install a new communications system of any kind. Nothing. You turn it all on and wait and nothing happens! (This is actually quite a good outcome - sometimes you get smoke from one or other piece of equipment!) Then you have the task of tracking down just what is and is not happening and where the problem is. Experience also tells us that the most likely problem is that someone plugged a cable into the wrong socket! Perhaps the biggest irony of optical communications is that you can't see anything! So if we want to find out what's going on in the system in order to make it work then we appropriate test equipment on hand. In many situations a simple optical power meter is quite sufficient but in others very sophisticated equipment is essential.

Optical Power Meters (Optical Multimeter)

Optical Power Meter - Logical Structure

The simplest and most basic piece of equipment used in the field is the optical power meter. This is shown in Figure and requires very little explanation. Different models have different connector types and are specialised to either multimode or single-mode fibre. There is always a wavelength switch to adjust the power readings for the particular wavelength being received. Also there is usually a range switch which determines the range of signal power expected - although this last function can be automatically determined by the meter itself.

Optical Time-Domain Reflectometers (OTDRs)

OTDR Display – Schematic

The Optical Time-Domain Reflectometer enables us to look at a fibre link from inside the fibre. In reality it is just a radar system for looking at fibre. High intensity pulses are sent into the fibre from a specialised laser and when the pulse returns its strength is displayed on an oscilloscope screen in the form of a trace. A schematic of such a display is shown in Figure. Important considerations are as follows:

Reflections from within the Fibre

In the trace you see reflections coming from all along the fibre itself. This is the result of Rayleigh scattering. Rayleigh scattering was mentioned in the section on optical fibre as the major limiting factor in fibre attenuation. This scattering occurs backwards towards the transmitter and we can receive it and display the result.

Faults and Joints etc.

Every time there is a discontinuity or imperfection in the fibre the effect can be seen in the trace. Such events can be the presence of a connector or a splice or some more serious imperfection such as a crimp in the cable due to poor installation. In the schematic above we can see the reflections from the beginning and end of the fibre as well some imperfections in between.

Noise Floor

At the end of the fibre you see a characteristic large (4%) reflection followed by the signal dropping to the noise floor.

Measurable Parameters

From an OTDR you can quickly determine the following characteristics of the fibre link under test:

·         The length of the fibre:This is not as precise as it sounds. What you can calculate is the length of the fibre itself. Most long distance cables employ “loose tube” construction and the fibre length is between 5% and 10% longer than the cable itself.The attenuation in dB of the whole fibre link and the attenuation of separate sections of fibre (if any).

·         The attenuation characteristics of the basic fibre itself.

·         The locations of connectors, joints and faults in the cable. These locations are measured from the beginning of the fibre and can be as accurate as a few metres.

Usually you can't see the fibre close to the instrument. A “dead zone” extends from the connector at the instrument itself for about 20 metres into the fibre. So if the trace in the figure above was real we would need to have a piece of fibre about 20 metres long connecting from the OTDR to the end of the fibre under test. Often you don't have this luxury in the field. In which case many OTDRs come equipped with a red laser source which you can use to illuminate the fibre. When you look at a fibre illuminated in this way (from the side) bad joints and fibre faults will scatter the red light and you can see it. But beware YOU MUST NOT LOOK AT THE SOURCE OR AT THE END OF THE FIBRE DIRECTLY. This can damage your eyes. Before using a feature like this read the manufacturer's instructions carefully.

Advantages

The major advantage of the OTDR is that tests can be done from one end of the link and you don't need access to the other end. This means you don't need two people to do the test and you save the problem of coordinating between people. Also the testing is much quicker. So even simple tests which could be performed with a basic optical source at one end of the link and a power meter at the other are often performed with an OTDR.

Characteristics

OTDRs today are extremely sophisticated devices and come with many options. They can be large fixed laboratory instruments or small portable ones about the size of a laptop computer. Different models are available for multimode and single-mode fibres. Of course different models have different levels of sensitivity (and price). There is always a range of options for the user to control such as wavelength used for the test, timescales, pulse duration etc.

Many modern OTDRs come with additional functions such as optical power meter or laser source so that a good OTDR often has all of the function needed by a technician in the field. In addition many OTDRs offer computer output so that you can collect OTDR data in the form of digital  readings and analyse it later on a computer.

 OTDR Operational Logic

The principle of operation of a typical OTDR is shown in Figure. In the figure a circulator has been used to enable transmission and reception of the pulse from the single strand of fibre under test. Other means of signal splitting/combining are used but circulators offer the least attenuation. As might be expected the big problem with an OTDR is that the returning signal is very low level especially on long distance fibre sections. We can't use signal pulses of too high a power for many reasons and so pulses of 10-20 mW are typically sent. The problem of low return power is addressed in two ways:

1. A very sensitive APD detector is used.  Thus an APD becomes very sensitive indeed at the very low pulse rates used. The penalty for using APDs is additional noise but this is mitigated by the averaging

process.

2. A “boxcar averager” circuit is used to average many thousands of returning pulses. The averaging process removes a large amount of noise. (Most of the noise comes from the APD and its associated circuitry.) In some (very sensitive, long distance) OTDRs the averaging time can be of the order of several minutes! The averager provides logarithmic scaling of its output so that the vertical scale on the display can be displayed in dBm. The pulse rates used are quite slow! Since the optical signal propagates at approximately 5 microseconds per kilometer we have to allow 10 microseconds per kilometer of fibre length. So for 20 km of fibre we need to wait at least 200 microseconds between pulses and so a pulse rate of 5000 pulses per second would be the maximum possible.

Spectrum Analysers

Spectrum Analyser - Display Schematic

There are many occasions where we want to look at the wavelength spectrum of the signal(s) on a fibre. One such occasion would be to examine the wavelength spectrum of a WDM system to help understand system operation and to diagnose faults. A spectrum analyser scans across a range of wavelengths and provides a display showing the signal power at each wavelength.

From this display we can calculate:

1. The power levels of each channel.

2. The spectral width of each channel.

3. Any interference between channels such as crosstalk possibilities.

4. By connecting it in different places through the system we can track many potential problems such as laser drift etc.

Spectrum Analyser - Logical Structure

The logical structure of the device is shown in Figure. . Light input from the fibre is passed through a tunable Fabry-Perot filter. . The filter is scanned at quite a slow rate (perhaps 10 times per second) through the range of wavelengths that we want to examine.

·         Optical output of the FP filter is fed to an APD to convert it to electronic form.

·         The output of the APD will contain rapid variations due to modulation of the signal in each channel etc. These modulations are averaged out electronically so that the electrical signal level now represents the average power level of the optical signal (average over a few milliseconds). The electronic signal now needs to be scaled logarithmically as we need the y-axis scale to be in dBm.

·         The electronic signal is now fed to the y-axis control of an oscilloscope.

·         The x-axis is swept across in synchronism with the wavelength setting of the FP filter.

·         This results in a display similar to that in the figure.

Like OTDRs, spectrum analysers vary widely in their capabilities and prices. They range from large, very accurate and expensive laboratory instruments to small, much less expensive devices about the size of a laptop computer. You can even buy one that does not have a display and instead connects to your laptop

computer. In using one you need to be aware of the resolution (minimum width) of each wavelength measured and also of the accuracy of the instrument.

Eye Diagrams

When you think about it there is a paradox involved in much of the testing we would like to do. You have a signal that is varying extremely quickly - so quickly that we need sophisticated receiver circuitry to detect its changes of state. Yet we expect to be able to measure and display the signal very accurately - much more accurately than we could ever possibly receive it.

The secret is that we receive the signal many times (indeed millions of times) and display the aggregate. Signals when they carry information vary and therefore we can never get a good solid picture of a particular state or change of state. However we can get an excellent idea of the aggregate.

The eye-diagram has over the years become the recognised way of looking at an electronic signal and determining its “goodness” as a carrier of information. It consists of many (from hundreds to millions) of instances of the signal displayed over the top of one another. In extremely fast equipment you might get only one or two points on a trace at a single sweep. But displaying them together allows us to assess the quality of the received signal very well indeed.

Eye Diagram - Schematic

The diagram is produced by feeding the result of the analogue section of the receiver circuit to the y-axis control of an oscilloscope. The sweep is set to display one full cycle (2-bit times) and is usually triggered from the receiver's PLL circuit (the receiver's derived clock).

The following aspects of the eye are important:

1. The vertical eye opening indicates the amount of difference in signal level that is present to indicate the difference between one-bits and zero-bits. The bigger the difference the easier it is to discriminate between one and zero. Of course this is affected significantly by noise in the system.

2. The horizontal eye opening indicates the amount of jitter present in the signal. The wider the eye opening is on this axis the less problem we are likely to have with jitter.

3. The thickness of the band of signals at the zero-crossing point is also a good measure of jitter in the signal. However, you need to be careful here as the sweep is usually triggered from the receiver PLL and variations here are as much an indicator of the quality of the PLL as they are of the signal itself!

4. The best indication of signal “goodness” is just the size of the eye opening itself. The larger it is the easier it will be to detect the signal and the lower will be the error rate. When the eye is nearly closed it will be very difficult or impossible to derive meaningful data from the signal.

Real Eye Diagrams

Figure  shows two real eye diagrams of a modulated signal (actually 100 Mbps FDDI code). On the left is a very good situation with a wide open eye and very little jitter. On the right we have an example of a signal which will be relatively easy to detect but which contains a significant amount of jitter.

Fiber Network - System Power Budgeting

Fiber Network - System Power Budgeting

Attenuation of both multimode and single-mode fibre is generally linear with distance. The amount of signal loss due to cable attenuation is just the attenuation per kilometer (at the signal wavelength) multiplied by the distance. To determine the maximum distance you can send a signal (leaving out the effects of dispersion), all you need to do is to add up all the sources of attenuation along the way and then compare it with the “link budget”. The link budget is the difference between the transmitter power and the sensitivity of the receiver.

Thus, if you have a transmitter of power -10 dBm and a receiver that requires a signal of power -20 dBm (minimum) then you have 10 dB of link budget. So you might allow:

·         10 connectors at .3 dB per connector = 3 dB

·         2 km of cable at 2 dB per km (MM GI fibre at 1300 nm) = 4 dB

·         Contingency of (say) 2 dB for deterioration due to ageing over the life of the system.

This leaves us with a total of 9 dB system loss. This is within our link budget and so we would expect such a system to have sufficient power. Dispersion is a different matter and may (or may not) provide a more restrictive limitation than the link budget.

The amount of power that we have to use up on the link and in connectors is determined by the characteristics of the components we select as transmitters and receivers.

Figure  shows the characteristics of some typical devices versus the transmission speed (in bits per second). A number of points are interesting here:

1. The power output of a laser doesn't vary much with modulation speed. Every laser has a limit to the maximum speed at which it can be modulated but up to that limit power output is relatively constant.

2. LEDs on the other hand produce less and less output as the modulation rate is increased. In the figure, the difference in fibre types only relates to the amount of power you can couple from an LED into the different types of fibre.

3. All receivers require higher power as the speed is increased. This is more a rule of physics than anything else. To reliably detect a bit a receiver needs a certain number of photons. This depends on the receiver itself but there is a theoretical limit of 21 photons per bit needed. Real receivers require around ten times this but it is a relatively fixed amount of optical power needed per bit. Therefore every time we double the modulation speed we need to also double the required power for a constant signal-to-noise ratio.

4. In addition to the point above there is another important problem when we get to seriously high speeds (above 10 Gbps). Within a pin detector at speeds above 10 Gbps the time taken for electrons to diffuse/drift across the i-layer (in the p-i-n structure) becomes a significant limitation. So if you want the device to respond faster you have to reduce the thickness of the i-layer. But reducing the thickness of this layer increases the capacitance between the p and n layers. So you have to reduce the detector surface area to compensate. Both of these actions reduce the volume (size) of the i-layer and hence they reduce

the probability that an incident photon will be absorbed and create an electron/hole pair. Thus the quantum efficiency of the detector is significantly reduced. Up to 10 Gbps we expect a (best case) quantum efficiency in pin detectors of around .8. At 20 Gbps this is reduced to .65, at 40 Gbps it reduces again to .33 and at 60 Gbps it becomes .25 or so. This reduction in quantum efficiency effect operates over and above the doubling of power you need when you double the line speed (as discussed in the previous point).

Current research is under way on the use of travelling wave principles in detectors to increase the quantum efficiency at these extreme speeds.

If you look in the figure for a given bit rate (vertical line) there will be a difference between the required receiver power and the available transmitter power. This difference is the amount we have available for losses in the fibre and connectors (and other optical devices such as splitters and circulators). It is also very important to allow some margin in the design for ageing of components (lasers produce less power as they age, detectors become less sensitive etc...).

Connector and Splice Loss Budgeting

The signal loss experienced at a connector or splice is not a fixed or predictable amount! We know roughly how much loss to expect from a particular connector type or from a particular type of splice in a fibre. The problem is the measured losses in actual splices and actual connectors vary considerably from each other. The good news is that actual measurements form (roughly) a “normal” statistical distribution about the mean (average).

Previous section shows “typical” losses that may be expected from different connector types. This table was complied from specifications obtained from connector manufacturers. However, in the practical world things are a bit more complex than this:

 1. For a connection using almost any modern single-mode connector where both connectors (halves of the connection) are from the same supplier you can expect a mean loss of .2 dB with a standard deviation of .15 dB.

2. If the manufacturers of the two connectors (halves of the connection) are different (the any-to-any case) then you can expect a loss of .35 dB (average) with a standard deviation of .25 dB. One type of single-mode connector may have an “average loss” of .2 dB but in practical situations this loss might vary from perhaps .1 dB to .8 dB (for the any-to-any case). In budgeting power for a link including multiple connectors we have a real problem deciding how much loss to allow for them.

If a hypothetical link has 10 connectors there is a statistical probability (albeit minuscule) that all will be high in loss (in this example .8 dB each) and so perhaps to be safe we need to allocate 8 dB for connector losses. But there is also a probability that each will be .1 dB and therefore we might need to allocate only 1 dB for the loss budget. In fact the probability of each of the above events is smaller than “minuscule” - it is somewhere between about 1 in 1010 and 1 in 10 depending on the exact way in which the extreme best and worst case (.1 dB and .8 dB) figures were arrived at in the first place. The same principle applies to fibre splices.

It is possible to get very sophisticated with statistics in predicting the amount of loss but things can be simplified significantly: If you know the average loss for a single connector and the standard deviation (.) of the connector loss for a particular situation then you can calculate these figures for any given combination.

1. The average (mean) of the total is just the average loss of a single connector multiplied by the number of connectors. Thus if we have 5 connectors in a link with an average loss of .35 dB per connector then the average loss of the total link will be 5 x .35 or 1.75 dB.

2. It is very important that the term “average loss” in this context be understood. If we fit 5 connectors (pairs) into a single link the total doesn't have an average loss - it has an actual loss. This actual loss will be quite a bit different from the average quoted above. If (hypothetically) we were to make a large number of links (say 100) each with five connectors then we could compute the mean (average) loss of a 5-connector link just by averaging over the 100 links. This mean would be very close indeed to 5 times the mean loss of a single connector. But we need to take care statistically of the fact that any real 5-connector link will be different from the mean. This is done by quoting not only the mean (for the combination of 5 connectors) but also a standard deviation from the mean.

3. The standard deviation (.)93 of the total is just the standard deviation of a single connector multiplied by the square root of the number of connectors involved. If we have 5 connectors each with a . of .25 then the . of the total is Õ5 times .25. That is, 2.235 x .25 which equals .559. For the above example (5 connectors) then we have a mean of 1.75 dB and a . of .559. Using a knowledge of the basic characteristics of a statistical “normal distribution” we can now calculate amount of loss to allow for the combination based on the probability we are prepared to accept of our being correct (or wrong!). . We know that 84.13% of the time the total will fall below one standard deviation (.) above the mean. So, If we allow a loss for the 5 connectors of the mean plus one standard deviation (1.75+.559 = 2.31 dB) then we will be safe 84.13% of the time. That is to say the real value will be less than our allowance 84.13% of the time.

. The total will fall below two standard deviations above the mean 97.72% of the time. So if we allow 2.868 dB for the connectors we will be safe 97.72% of the time.

. If we allow 2.32 times the standard deviation then we will be safe 99% of the time.

. In practice, many people like to use the “3-.” value where we can be confident of being safe 99.87% of the time.

 For this example the 3-. value would be 1.75+(3 x .559) which equals 3.427 dB.

We have taken a few shortcuts here. For example we have assumed that the distribution of connector losses is a statistically “normal” distribution. Also we have assumed that all connections are between connectors made by different manufacturers (the any-to-any case). Statistically some of them will really be like-to-like. But while we have taken shortcuts, the result is close enough.

1. If the number of cascaded splices is large (say more than 30) you can safely use the average loss and multiply it by the number of splices involved and ignore the variations. There is a statistical law here sometimes referred to as the “law of large numbers”. When you add up a large number of variable “things” (with the same characteristics) the variation in the sum gets smaller and smaller (in relation to the total) as the number gets larger.

2. With a very small number of splices (say two) you can allocate the worst case for each of them. The formula will arrive very close to this anyway.

3. For numbers in between use the calculation method described above. In long distance links it is common to regard splices as part of the fibre loss. So you might get raw SM fibre with a loss (at 1550 nm) of .21 dB/km. After cabling this will increase to perhaps .23 dB/km. For loss budget purposes you might

allocate .26 dB/km for installed cable. Cable is typically supplied in 2 km lengths so in a 100 km link there will be a minimum of 50 splices. Similarly, in the 1310 nm band, a typical cable attenuation might be .36 dB/km but it is typical to allocate .4 dB/km for fibre losses in new fibre used in this wavelength band.

The same piece of installed fibre cable would then be budgeted at .4 dB/km when used in the 1310 nm band and at .26 dB when used in the 1550 nm band.

Power Penalties

There are a number of phenomena that occur within an optical transmission system that can be compensated for by increasing the power budget. In each case the amount of additional power required to overcome the problem is termed the “power penalty”.

In all commodity communications products and in most pre-planned systems the effects of power penalties are already included by means of adjustment of the receiver sensitivity. The user systems engineer can usually ignore them quite safely. Nevertheless it is important to understand what they are and get some idea of the magnitude of the penalty. The three most important issues here for digital systems are:

1. System noise

2. Effect of dispersion and

3. Extinction ratio

Signal-to-Noise Ratio (SNR)

The quality of any received signal in any communication system is largely determined by the ratio of the signal power to the noise power - the SNR. Obviously, the SNR is a function of both the amount of noise and the signal power. You can always improve the SNR by increasing the signal power (if you can do it without also increasing the noise).

When noise is present the amount of increase in signal power necessary to compensate for the noise and produce the same SNR at the output can be expressed as an amount of power increase in decibels. This is the power penalty due to noise. In simple systems most of the noise comes from within the receiver itself and so is usually compensated for by an adjustment of the receiver sensitivity specification. In complex systems with EDFAs, ASE noise becomes important and to compensate we indulge in power level planning throughout the system.

Inter-Symbol Interference (ISI)

Dispersion causes bits (really line states or bauds) to merge into one another on the link. When this becomes severe it will prevent successful link operation but at lower levels of severity, dispersion adds noise to the signal.

We can compensate for this by increasing the signal power level and thus for certain levels of dispersion we can nominate a system power budget (allowance) to compensate.

Extinction Ratio

If a zero bit is represented by a finite power level rather than a true complete absence of power then the difference between the power level of a 1-bit and that of a 0-bit is narrowed. The power level of the 0-bit becomes the noise floor of every 1-bit. The receiver decision point has to be higher and therefore there is an increased probability of error.

This can be compensated for by an increase in available power level at the receiver. An extinction ratio of 10 dB incurs a power penalty (in either a pin-diode receiver or an APD) of about 1 dB over what it would have been with a truly zero value for a 0-bit. An extinction ratio of 3 dB causes a power penalty of 5 dB in a pin-diode receiver and 7 dB in an APD.

Bit Error Rates (BER)

In a digital communication system the measure of system “goodness” is the bit error rate or BER. This is the number of error bits received as a proportion of the number of good bits. It is usually expressed just as a single number such as 10-6 which means one in a million. It must be realised that errors are normal events in communications systems - there is always the probability of an error (however small).

When an optical communications system is planned the BER is a key objective of the system design and measure of success. It is determined by the link speed, its power, the distance, the amount of noise etc.

The question of what is an “adequate” BER in a particular situation or what is a good one is purely a judgement call on the part of the people who set the system objectives. However when considering BERs some points should be borne in mind. . When modern networking systems (such as ATM and Sonet/SDH) were designed it was assumed that they would operate over very low error rate optical links. Errors have a disruptive effect on both of these protocols. . In the early days, computer networking error rates of 10-6 and 10-5 on slow speed copper connections were normal and higher level systems were designed to  recover and give acceptable throughput. Many modern networking systems will fail entirely if operated over links this bad.

. In current networking technologies an error at the lowest layer has its effects multiply as you proceed up the protocol stack. A single bit error at the physical layer could (in the extreme) cause loss of frame synchronisation in the SDH layer which might cause the loss of perhaps 30 frames. The loss of 30 SDH

frames might mean the loss of 100 ATM cells and the loss of these might cause the re-transmission of up to 50 cells for every one lost. So the network could well end up re-transmitting 3000 cells to recover from a single bit error! (This is an extreme and highly unlikely example but the principle is sound.)

. On many public network optical networks today error rates of 10-14 are consistently achieved and so user expectation is that errors will be very rare events indeed.

. Public network operators seem to consider the minimum acceptable error rate to be around 10-12.

. In many research reports you find optical network error rates of 10-9 quoted. Many people feel that in the context of their use as lowest-layer network within a stack of networks that this figure is just not good enough. This is a judgement call - but...

. The faster the link the lower we need the error rate to be! But the harder that low error rate becomes to deliver. . In many standards (such as the ATM recommendations) the expected error rate performance of links over which the system will be run are specified in the standard.

Fiber Maintainence

Fiber Maintainence

The diameter of the core in an optical fiber is very small and any irregularity (such as a join) can result in significant loss of power. To get the maximum light transfer from the cut end of one fiber into another, both ends must be cut precisely square and polished flat. They must then be butted together so that there is minimal air (or water) between the butted ends and these ends must match up nearly exactly.

In a practical situation outside the laboratory this is very difficult to do. In the early days of optical data communication (1983), one industry standard specified an optical cable for use in the office environment which was step-index 100/140 μm in diameter. The 100 micron core is very wide and is certainly not the best size for communication. However, at the time it was the best specification for making joins in the field. (This specification is still supported by some systems - including FDDI.)

Joining fibres together is not a trivial task. Light travelling in a fibre is not “like” electricity travelling in a wire except in the most superficial way. Light travelling in a fibre is a guided wave and the fibre is a waveguide. Any imperfection or irregularity (such as a join) is a potential source of loss and of noise. The problem (obvious as it is) is that the dimensions of a fibre are tiny and accuracy of alignment is critical.

  

There are three general ways of joining fibres:

1.     By fusion splicing (a type of weld)

2. Use of index matching epoxy glues

3. With mechanical connectors of different types

The common requirement of all three methods is that the cores must be aligned. However, we can't always line the cores up - we line the fibres themselves up. This is not the same. The core is not always in the centre of the fibre. The manufacturers try very hard but there is always a variation. Difference between the axis of the core and the axis of the cladding is expressed as the “concentricity” of the fibre. This means that unless you do something to align the cores whilst making the join, there is a random misalignment (and hence loss) imposed by the concentricity error. The major improvement in connector losses observed between the middle 1980's and the 1990's was due generally to better fibre manufacture as much as to better connectors and connection techniques.

Fusion Splicing

A fibre join is a type of weld. The fibre ends are cut, polished, butted up to one another and fused by heat. (Incidentally, with silica fibers you need quite a high temperature - much higher than the melting point of ordinary soda glass.) In practice, a light loss of only .1 dB is the current budget for power loss in a single-mode fiber join. But it should be realised that .1 dB is quite a lot in that it represents the total loss of one half of a kilometer of cable.

Fusion Splicing Schematic

A device setup for fusion splicing is illustrated in above.

1. Each fibre is stripped of its primary coating and the end cleaved such that it is square.

2. The fibre ends are positioned a few mm from one another and clamped to positioning blocks. There is often a groove provided in the mounting block to aid in correct alignment.

3. The fibre ends are then aligned with one another and brought closer together.

4. When alignment is satisfactory an electric arc is started between the two electrodes and the fibres brought into contact. Heat from the arc melts the glass and the join is made. There are two major issues here - alignment of the fibre and precise control of the heating arc.

Fiber Alignment using optical feedback

Fibre Alignment

When fibre ends are melted and touched together there are significant effects caused by the surface tension that tend to align the outside of the cladding. This is very convenient when joining multimode fibres but in the case of SM it can cause a number of problems. A common method of fibre alignment is illustrated in Figure. The primary coating is stripped from the fibre for several cm from the end.

·         When it is mounted in the fusion splicer the fibre is bent tightly around two mandrels (one at each end).

·         Light from a laser (or LED) is focused onto a spot on the fibre bend such that some of it enters the core in a guided mode.

·         At the other side of the splicer there is an optical detector positioned to capture light radiated from the tight bend.

·         One end of the fibre is moved (by moving the mounting block with a piezo-electric actuator) until the output of the detector is at a maximum.

This method works reasonably well but with SM fibres the surface tension effect can change the position as the join is made! Another problem is the fact that the primary coating has to be removed for a long distance either side of the join. This makes the join hard to protect from later damage. Many current automatic fusion splicers use a visual method of positioning. The fibre ends are examined (magnified) through a digital imaging process. Initially, this is displayed on a screen on the splicer so the operator can

check easily for faults in the fibre endfaces. You can clearly see the fibre endface and even the cores this way. Once the operator is satisfied that the ends look okay, a microcomputer in the splicer examines the images of the fibre ends and aligns them automatically. This method works remarkably well and can produce very low-loss splices. A big advantage is that you don't need to strip the fibre back more than a

few cm from the splice.

Control of Heating

Precisely how the fibre is melted and joined is a very important issue. To make a good mechanical join you should melt each fibre end completely and allow for intimate mixing of the glass from the two fibres. However, a join that does this is likely to have a strong perturbation of the refractive index and hence high loss. If you heat only a very thin layer on the endface of each fibre you get the best optical properties but the mechanical characteristics of the join are not good. Once the splice is made it must be protected. This is usually done by covering it with a sleeve of heat-shrink material and then applying gentle heat. The material contracts around the fibre and protects the splice. A metal strengthening pin is often integrated into the side of the wrapping to provide additional mechanical strength.

Cleaving the Fibre

Before a splice of any kind is made or a connector fitted it is of critical importance that the fibre ends be cut square. (Except for the unusual case where we are making a diagonal splice.) In the case of connectors the ends are often polished to a desired shape later but we need to start off with square cut fibre ends. The established technique for ensuring a good square fibre end is called “cleaving”.

This is just the same technique we use when cutting glass to replace a broken window.

1. A scratch or nick is made in the side of the fibre. This destroys the local surface tension and gives the glass a point from which to crack.

2. A stress is applied to the fibre so that it will crack across its diameter. You can apply this stress in many ways. An older technique called for the fibre to be bent around a rod. Unfortunately this had a habit of creating a “lip” on the side of the fibre opposite to the nick. In todays world you use a little machine which delivers a short sharp blow to the fibre in exactly the right place. Usually a good square cleave is obtained.

Mechanical Splicing

In this technique the fibre ends are cleaved and polished, aligned with one another and the gap between filled with an epoxy resin which has the same RI as the fibre core. There are various ways of aligning the fibre ends but we consider the splice to be “mechanical” if the outside of the fibre cladding is aligned without reference to alignment of the cores. There are many ways of aligning the outside of the fibres:

·         One common method here is to use a glass tube into which each end of the fibre is pushed. A small amount of the epoxy resin is placed on the end of one of the fibres before insertion. Usually there is a small hole in the tube at the point of the join so that excess epoxy can escape.

·         There are many other methods of obtaining mechanical alignment of the outside of the fibres. V-groves, slots and alignment rods are all used. In addition heat-shrink elastomer tubes are also used sometimes.

This splicing technique is the lowest cost but it is also not very good. The quality of the join depends on:

1. The concentricity of the fibre

2. The accuracy of the outside diameter of the fibre

3. The circularity of the outside of the fibre

4. The tolerances and precision of the alignment device used

However, this makes a solid, permanent connection and is used for fibre-to-fibre joins in many situations.

In similar techniques epoxy glues are often used for pigtailing microoptic devices (like circulators). There is a wide range of epoxies available which will meet most requirements. In a recent experimental situation a suitable special purpose epoxy could not be found. However, a dental (uv cured) epoxy was available (made for filling teeth) which had almost exactly the needed RI and a very low coefficient of expansion (you don't want your fillings falling out!). However, there is significant doubt about the long term stability of epoxy resins. Resins (might) break down and cause scattering over time. We don't know. Recently there has been a major shift in the industry away from the use of epoxy resins for just this reason.

Mechanical Splicing with Alignment and Bonding

This process is very similar to straight mechanical splicing but the fibres are actively positioned in the same way as with fusion splicing. The cleaved fibres are inserted into silica sleeves and bonded in place. The sleeve ends with the fibres exposed are then polished to get a very accurate surface. After this the sleeves are actively aligned so that the maximum optical power is transferred. They are then bonded with epoxy and covered with another protective sleeve. In reality, the role of the inner sleeves is simply to provide rigidity and bulk to the fibre to make handling and positioning easier and gluing of the endfaces

mechanically strong. This technique provides very high-quality splices but it is very time consuming (and

hence costly) to perform.

Losses in Fibre Joins

Losses in fibre joins are commonly classified into two kinds:

1. Extrinsic losses are those caused by factors concerned in joining the fibre but are unrelated to the properties of the fibre itself.

2. Intrinsic losses are losses caused by some property inherent in the construction of the fibre.

Extrinsic Losses

Longitudinal Misalignment

Lateral Misalignment

Angular Misalignment

Fibre end not cut square

Fibre end irregular or rough

Sources of Loss due to Misalignment in Fibre Joins. Because of the fact that these losses are caused by factors external to the fibre itself they are called “Extrinsic” losses. As shown in Figure  above there are many ways to make a bad fibre join. This applies whether a fused join is to be made or a connector is to be used. Losses and reflections for the different types of mismatches vary but all are to be avoided.

Longitudinal Misalignment

Longitudinal misalignment (or endface separation) has two loss effects. The first is just loss of signal power caused by the fact that light exiting one fibre endface diffuses outwards and (depending on the amount of separation) some of it will not be within the NA of the other fibre and hence cannot enter it in a guided mode. The second effect is that the separated endfaces themselves constitute a Fabry-Perot interferometer. Depending on the wavelength and the exact distance between the endfaces the attenuation can vary between zero and 100%.

Lateral Misalignment

Lateral misalignment is a major potential source of signal loss in all fibres but especially in single-mode fibres. A lateral displacement of one micron in an otherwise perfect join will result in a loss of .2 dB of signal. A displacement of 2.5 microns results in a loss of just more than 1 dB! Fibre End Not Cut Square

If the fibre end is not cut square then you can't mate the two surfaces closely together.

Angular Misalignment

This problem is worst in single-mode fibres due to the very small mode field and the low RI contrast (low NA). A misalignment of only 1 degree produces a loss of .2 dB. A misalignment of 2 degrees causes a loss of around 1 dB!

Fibre End Irregular or Rough

Rough ends on the fibre scatter the light and prevent close contact between the fibre ends. Most of the above comments apply to losses when connectors are used rather than when a fused join is made. In the case of a fused join, most of the above faults create a constriction in the fibre itself and a random perturbation of the RI. Losses in this context are hard to predict quantitatively but can be very large.

Intrinsic Losses

Core Concentricity Core Shape (Ellipticity)

Core Diameter Cladding Diameter

Sources of Intrinsic Loss in Fibre Joins Losses that are caused by factors involving the fibre itself are called “intrinsic losses”. The major ones are summarised below:

Concentricity Error

As mentioned earlier, one of the major causes of loss in fibre joins is concentricity error in the fibre. Concentricity error comes about when the axis of the core and that of the total fibre itself are not exactly aligned. That is, the core is not exactly centred in the fibre. Even assuming that the fibres are lined up exactly on the outside, concentricity error will cause the cores to be misaligned. Concentricity error is a problem for both SM and MM fibre but it is a significantly greater problem in SM fibres. However, vast improvements in fibre manufacture have been made and major fibre manufacturers have recently (1997) announced big improvements in this area.

Core Shape (Ellipticity)

No matter how precise the manufacture the core will always exhibit a (hopefully very slight) ellipticity. When a fibre is cut and re-joined the orientation of the core will usually not be the same and some light will be lost. This is not a big problem with MM fibre. With SM fibre, any ellipticity causes the fibre to be birefringent. That is, the fibre will exhibit different RIs to orthogonal polarisations of light travelling through it. A join in this case can be a source of birefringent noise.

Core Diameter

In MM fibre light is obviously lost (some modes escape into the cladding) when a core of a larger diameter is joined to one of a smaller diameter. This happens in the natural situation of every join where the diameter of the fibre core cannot ever be exact. There is always a difference however slight. Note that if the fibres are aligned correctly the loss will occur only when light passes from the larger diameter fibre to the smaller diameter one. Light travelling in the other direction (from smaller to larger) is not lost. In the situation where two fibres of different specifications (with different diameters) are being joined with a connector, then a lot of light is usually lost. This is a common situation, where fibres with a 62.5 μm core can be connected to fibres with a 50 μm core. This happens often because most available data communications equipment is pigtailed using 62.5 micron MM fibre. Some users have installed 50 micron MM cabling and so a mismatch is inevitable. Loss of light in this situation (about 3 dB) is unavoidable. Again, this happens only in the direction where light travels from the larger diameter core to the smaller one.

Mode Field Diameter

In SM fibres the actual core diameter is not very relevant in considering joins. The diameter of the “Mode Field” (generally larger than the core diameter) is the important parameter.

Cladding Diameter

When fibres are joined we line the fibres up with each other using the outside of the fibre (you can't see the core). This means that at some point on the outside of the cladding both fibres must align with each other. If the outside diameters of these claddings are different from one another then the cores cannot be aligned.

Numerical Aperture

When MM fibres of different NAs are joined some modes that were possible in the fibre of higher NA cannot travel in the fibre of lower NA. These will enter the cladding and ultimately be lost. Thus some optical power will be lost. Loss from this source will occur in only one direction (from the higher NA fibre to the lower NA one). Light travelling in the opposite direction will be retained. There is another source of loss and noise here. Fibres with different NAs usually have different RIs in the core or the cladding or both. When you join fibres of different RIs the RI changes at the join. The join then becomes a partial mirror and some light will be reflected back down the fibre. This can cause noise (as well as loss) due to the phenomenon of “Return Loss Variation”.

Refractive Index Profile

Differences in RI profile in the joined fibres can cause the same effects as described above for numerical aperture.