Francisco Madruga. Olga Conde. Fiber optics in structural health monitoring. Rodriguez, A. Quintela, A. Cobo, F. Madruga, Olga M. Conde, M. Lomer, M. Quintela, J. SHM can be understood as the integration of sensing intelligence and possibly also actuation devices to allow the structure loading and damaging conditions to be recorded, analyzed, localized and predicted in such a way that non-destructive testing becomes an integral part of the structure.
SHM sensing requirements are very well suited for fiber optic sensing technology. So in this paper, after a very brief introduction of the basic SHM concepts, the main fiber optic technologies for this application will be reviewed, several examples and the main current technical challenges will be addressed and, finally, the conclusions summarized. On the other hand, it is known that sudden collapses of representative structures with all the troubles and costs even with losses of human lives such the crashed AA flight on November 12th of , the collapse of the I35W Minneapolis Bridge  or the Belfast Railway Line viaduct Collapse in Dublin on 21st August have unfortunately happened.
They could have been determined or evenavoided, if they had been equipped with Structural Health Monitoring systems SHM. It must be remarked that modern structures should be equipped with monitoring systems able to automatically detect the damage, characterize it recognize, localize, quantify or rate , and report it, providing important input for structure managers or for the system intelligence.
According to the functionality and degree of complexity, the SHM systems can be classified in five levels, following what can be named the staircase of the SHM systems : the higher the level, the higher the complexity and functionality. In fact, it is a logical consequence of the example described using the human body to depict the SHM concept.
Level I SHM systems the simpler ones only detect the presence of damage without locating it on the structure. A key item or subsystem of the SHM systems is its sensing part for what OFS technologies suit properly the main technical requirements. After being detected, processed and conditioned, the system will deliver an output signal Oe , usually in the electric domain, which will be a valid reproduction of the object variable.
The transmitted or reflected light can be modulated by the measurand or modulating signal in its amplitude, phase, frequency or polarization characteristics. In this paper, the more successful fiber optic technologies for SHM will be reviewed, several examples and the main current technical challenges will be addressed and, finally, the conclusions summarized. Many companies were created to exploit commercially the new OFS technologies.
However not all of them have followed the proper path and survived successfully . In the following lines a very brief review of some of the most successful techniques used will be addressed. They will be structured regarding the fiber structure employed in the transducer. Typically, these kind of transducers are useful to measure the structural integrity of structures in a wide set of application sectors including architectural heritage and civil engineering applications [3,4,5].
When long gauge transducers are required, the most successful technology in the recent years has been the SOFO system. The transducer consists of a pair of single-mode fibres installed in the structure to be monitored Figure 1. Setup of the SOFO system. It is attached to it at its two extremities and pre-stressed in-between. On the other hand, the other fibre, the reference fibre, is placed loose in the same pipe. To make an absolute measurement of this path unbalance, a low-coherence double Michelson interferometer is used.
The first interferometer is made of the measurement and reference fibres, while the second is contained in the portable reading unit. This second interferometer can introduce, by means of a scanning mirror, a well-defined path unbalance between its two arms . The precision and stability obtained by this set-up has been quantified in laboratory and field tests to be 2 micron, independently from the sensor length over more than one year. Even a change in the fibre transmission properties does not affect the precision, since the displacement information is encoded in the coherence of the light and not in its intensity.
Since the measurement of the length difference between the fibres is absolute, there is no need to maintain a permanent connection between the reading unit and the sensors . Five improved generations of the SOFO system for static measurements have been developed and commercialized being now also available the version for dynamic measurements . It is based on the same transducer approach, but the reading or optoelectronic unit is based on a Mach-Zehnder interferometer instead of a mobile mirror used on the static SOFO unit.
In sensing, both short period Bragg and long period period much longer than the wavelength of the light are used [5,11]. The first ones because of their capability to measure both the strain and temperature of the structure and an ample variety of indirect measurands. Besides, they are also widely used because of their ability to create tunable filters, and for their multiplexing capabilities. Long period gratings  are used because of their high sensitivity to the cladding modes among others. An Optical Fiber Grating, FBG, can be understood as an optical fiber with a periodic refractive index perturbation pattern inscribed in the core such that it diffracts the optical signal in the guided mode at specific wavelengths into other core-bounded modes, cladding modes, or radiation modes .
This technology can be used in aerospace, medical, biomedical, environmental, electric power energy, and in military and civil engineering applications sectors [3,5,16]. In table I a summary of grating applications can be observed. Table I Summary of potential sensing applications of various types of fibre gratings.
New studies looking for new effects and structures continue nowadays [18,19]. Linear backscattering and, overall, non-linear back-scattering and non-linear forward-scattering having their own special advantages and disadvantages can be used to match the specific requirements of length and resolution of the measurand.
Handbook of Optical Fibre Sensing Technology
Distributed systems over larger distances up to km  and special resolutions of 1cm have already  been demonstrated. Due to their relevance in sensing, works are in course to improve their main technical characteristics . Figure 3. Despite the importance of this technology to solve real problems and in spite of the important flux of ideas coming from the research centers, their commercialization has not reach the expected level yet. They can be used both as the basis for the transducer mechanism or as fixed or tunable devices in the optoelectronic unit. The cavity can be active, for instance integrating a fibre laser sensor, or passive.
One very well tested approach is the Extrinsic Fabry-Perot Interferometers EFPI's that is constituted by a capillary silica tube containing two cleaved optical fibres facing each other, but leaving an air gap of a few microns or tens of microns between them. Optical fiber is also widely exploited as a nonlinear medium. The glass medium supports a host of nonlinear optical interactions, and the long interaction lengths possible in fiber facilitate a variety of phenomena, which are harnessed for applications and fundamental investigation.
Optical fibers doped with a wavelength shifter collect scintillation light in physics experiments. Fiber-optic sights for handguns, rifles, and shotguns use pieces of optical fiber to improve visibility of markings on the sight. An optical fiber is a cylindrical dielectric waveguide nonconducting waveguide that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials.
The boundary between the core and cladding may either be abrupt, in step-index fiber , or gradual, in graded-index fiber. The index of refraction or refractive index is a way of measuring the speed of light in a material. Light travels fastest in a vacuum , such as in outer space. The speed of light in a vacuum is about , kilometers , miles per second. The refractive index of a medium is calculated by dividing the speed of light in a vacuum by the speed of light in that medium. The refractive index of a vacuum is therefore 1, by definition. A typical singlemode fiber used for telecommunications has a cladding made of pure silica, with an index of 1.
From this information, a simple rule of thumb is that a signal using optical fiber for communication will travel at around , kilometers per second. To put it another way, the signal will take 5 milliseconds to travel 1, kilometers in fiber. The fiber in this case will probably travel a longer route, and there will be additional delays due to communication equipment switching and the process of encoding and decoding the voice onto the fiber.
When light traveling in an optically dense medium hits a boundary at a steep angle larger than the critical angle for the boundary , the light is completely reflected. This is called total internal reflection. This effect is used in optical fibers to confine light in the core. Light travels through the fiber core, bouncing back and forth off the boundary between the core and cladding.
Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding.
In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine of this maximum angle is the numerical aperture NA of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. Single-mode fiber has a small NA. Such fiber is called multi-mode fiber , from the electromagnetic analysis see below. In a step-index multi-mode fiber, rays of light are guided along the fiber core by total internal reflection.
Rays that meet the core-cladding boundary at a high angle measured relative to a line normal to the boundary , greater than the critical angle for this boundary, are completely reflected. The critical angle minimum angle for total internal reflection is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber.
The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber.
In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber.
This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis. Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics.
Handbook of optical fibre sensing technology | UNIVERSITY OF NAIROBI LIBRARY
Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or mono-mode fiber. The behavior of larger-core multi-mode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation hence the name.
The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes. The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave. The most common type of single-mode fiber has a core diameter of 8—10 micrometers and is designed for use in the near infrared. The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths.
Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometers and as large as hundreds of micrometers. The normalized frequency V for this fiber should be less than the first zero of the Bessel function J 0 approximately 2. These include polarization-maintaining fiber and fiber designed to suppress whispering gallery mode propagation. Polarization-maintaining fiber is a unique type of fiber that is commonly used in fiber optic sensors due to its ability to maintain the polarization of the light inserted into it.
Photonic-crystal fiber is made with a regular pattern of index variation often in the form of cylindrical holes that run along the length of the fiber. Such fiber uses diffraction effects instead of or in addition to total internal reflection, to confine light to the fiber's core. The properties of the fiber can be tailored to a wide variety of applications. Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam or signal as it travels through the transmission medium.
The medium is usually a fiber of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption.
Single-mode optical fibers can be made with extremely low loss. Corning's SMF fiber, a standard single-mode fiber for telecommunications wavelengths, has a loss of 0. It has been noted that if ocean water was as clear as fiber, one could see all the way to the bottom even of the Marianas Trench in the Pacific Ocean, a depth of 36, feet. The propagation of light through the core of an optical fiber is based on total internal reflection of the lightwave.
Rough and irregular surfaces, even at the molecular level, can cause light rays to be reflected in random directions. This is called diffuse reflection or scattering , and it is typically characterized by wide variety of reflection angles. Light scattering depends on the wavelength of the light being scattered. Thus, limits to spatial scales of visibility arise, depending on the frequency of the incident light-wave and the physical dimension or spatial scale of the scattering center, which is typically in the form of some specific micro-structural feature.
Since visible light has a wavelength of the order of one micrometer one millionth of a meter scattering centers will have dimensions on a similar spatial scale. Thus, attenuation results from the incoherent scattering of light at internal surfaces and interfaces. In poly crystalline materials such as metals and ceramics, in addition to pores, most of the internal surfaces or interfaces are in the form of grain boundaries that separate tiny regions of crystalline order.
It has recently been shown that when the size of the scattering center or grain boundary is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent. This phenomenon has given rise to the production of transparent ceramic materials. Similarly, the scattering of light in optical quality glass fiber is caused by molecular level irregularities compositional fluctuations in the glass structure. Indeed, one emerging school of thought is that a glass is simply the limiting case of a polycrystalline solid.
Within this framework, "domains" exhibiting various degrees of short-range order become the building blocks of both metals and alloys, as well as glasses and ceramics. Distributed both between and within these domains are micro-structural defects that provide the most ideal locations for light scattering. This same phenomenon is seen as one of the limiting factors in the transparency of IR missile domes.
At high optical powers, scattering can also be caused by nonlinear optical processes in the fiber. In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths, in a manner similar to that responsible for the appearance of color. Primary material considerations include both electrons and molecules as follows:.
The design of any optically transparent device requires the selection of materials based upon knowledge of its properties and limitations. The Lattice absorption characteristics observed at the lower frequency regions mid IR to far-infrared wavelength range define the long-wavelength transparency limit of the material. They are the result of the interactive coupling between the motions of thermally induced vibrations of the constituent atoms and molecules of the solid lattice and the incident light wave radiation.
Thus, multi-phonon absorption occurs when two or more phonons simultaneously interact to produce electric dipole moments with which the incident radiation may couple. These dipoles can absorb energy from the incident radiation, reaching a maximum coupling with the radiation when the frequency is equal to the fundamental vibrational mode of the molecular dipole e. Si-O bond in the far-infrared, or one of its harmonics. The selective absorption of infrared IR light by a particular material occurs because the selected frequency of the light wave matches the frequency or an integer multiple of the frequency at which the particles of that material vibrate.
Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies or portions of the spectrum of infrared IR light. Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects. When IR light of these frequencies strikes an object, the energy is either reflected or transmitted. Attenuation over a cable run is significantly increased by the inclusion of connectors and splices.
When computing the acceptable attenuation loss budget between a transmitter and a receiver one includes:. Connectors typically introduce 0. Splices typically introduce less than 0. The calculated loss budget is used when testing to confirm that the measured loss is within the normal operating parameters. Glass optical fibers are almost always made from silica , but some other materials, such as fluorozirconate , fluoroaluminate , and chalcogenide glasses as well as crystalline materials like sapphire , are used for longer-wavelength infrared or other specialized applications.
Silica and fluoride glasses usually have refractive indices of about 1. Typically the index difference between core and cladding is less than one percent. Plastic optical fibers POF are commonly step-index multi-mode fibers with a core diameter of 0. Silica exhibits fairly good optical transmission over a wide range of wavelengths. In the near-infrared near IR portion of the spectrum, particularly around 1. Such remarkably low losses are possible only because ultra-pure silicon is available, it being essential for manufacturing integrated circuits and discrete transistors.
A high transparency in the 1. Alternatively, a high OH concentration is better for transmission in the ultraviolet UV region. Silica can be drawn into fibers at reasonably high temperatures, and has a fairly broad glass transformation range. One other advantage is that fusion splicing and cleaving of silica fibers is relatively effective. Silica fiber also has high mechanical strength against both pulling and even bending, provided that the fiber is not too thick and that the surfaces have been well prepared during processing.
Even simple cleaving breaking of the ends of the fiber can provide nicely flat surfaces with acceptable optical quality. Silica is also relatively chemically inert. In particular, it is not hygroscopic does not absorb water. Silica glass can be doped with various materials. One purpose of doping is to raise the refractive index e. Doping is also possible with laser-active ions for example, rare-earth-doped fibers in order to obtain active fibers to be used, for example, in fiber amplifiers or laser applications.
Both the fiber core and cladding are typically doped, so that the entire assembly core and cladding is effectively the same compound e. Particularly for active fibers, pure silica is usually not a very suitable host glass, because it exhibits a low solubility for rare-earth ions. This can lead to quenching effects due to clustering of dopant ions. Aluminosilicates are much more effective in this respect. Silica fiber also exhibits a high threshold for optical damage. This property ensures a low tendency for laser-induced breakdown. This is important for fiber amplifiers when utilized for the amplification of short pulses.
Because of these properties silica fibers are the material of choice in many optical applications, such as communications except for very short distances with plastic optical fiber , fiber lasers, fiber amplifiers, and fiber-optic sensors. Large efforts put forth in the development of various types of silica fibers have further increased the performance of such fibers over other materials.
Fluoride glass is a class of non-oxide optical quality glasses composed of fluorides of various metals. Because of their low viscosity , it is very difficult to completely avoid crystallization while processing it through the glass transition or drawing the fiber from the melt. Thus, although heavy metal fluoride glasses HMFG exhibit very low optical attenuation, they are not only difficult to manufacture, but are quite fragile, and have poor resistance to moisture and other environmental attacks.
An example of a heavy metal fluoride glass is the ZBLAN glass group, composed of zirconium , barium , lanthanum , aluminium , and sodium fluorides. Their main technological application is as optical waveguides in both planar and fiber form. However, such low losses were never realized in practice, and the fragility and high cost of fluoride fibers made them less than ideal as primary candidates.
Later, the utility of fluoride fibers for various other applications was discovered. These include mid- IR spectroscopy , fiber optic sensors , thermometry , and imaging. Also, fluoride fibers can be used for guided lightwave transmission in media such as YAG yttrium aluminium garnet lasers at 2. Phosphate glass constitutes a class of optical glasses composed of metaphosphates of various metals.
Instead of the SiO 4 tetrahedra observed in silicate glasses, the building block for this glass former is phosphorus pentoxide P 2 O 5 , which crystallizes in at least four different forms. The most familiar polymorph see figure comprises molecules of P 4 O Phosphate glasses can be advantageous over silica glasses for optical fibers with a high concentration of doping rare-earth ions. A mix of fluoride glass and phosphate glass is fluorophosphate glass.
The chalcogens —the elements in group 16 of the periodic table —particularly sulfur S , selenium Se and tellurium Te —react with more electropositive elements, such as silver , to form chalcogenides. These are extremely versatile compounds, in that they can be crystalline or amorphous, metallic or semiconducting, and conductors of ions or electrons. Glass containing chalcogenides can be used to make fibers for far infrared transmission. Standard optical fibers are made by first constructing a large-diameter "preform" with a carefully controlled refractive index profile, and then "pulling" the preform to form the long, thin optical fiber.
The preform is commonly made by three chemical vapor deposition methods: inside vapor deposition , outside vapor deposition , and vapor axial deposition. Gases such as silicon tetrachloride SiCl 4 or germanium tetrachloride GeCl 4 are injected with oxygen in the end of the tube. When the reaction conditions are chosen to allow this reaction to occur in the gas phase throughout the tube volume, in contrast to earlier techniques where the reaction occurred only on the glass surface, this technique is called modified chemical vapor deposition MCVD. The oxide particles then agglomerate to form large particle chains, which subsequently deposit on the walls of the tube as soot.
The deposition is due to the large difference in temperature between the gas core and the wall causing the gas to push the particles outwards this is known as thermophoresis. The torch is then traversed up and down the length of the tube to deposit the material evenly. After the torch has reached the end of the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to form a solid layer.
This process is repeated until a sufficient amount of material has been deposited. For each layer the composition can be modified by varying the gas composition, resulting in precise control of the finished fiber's optical properties. In outside vapor deposition or vapor axial deposition, the glass is formed by flame hydrolysis , a reaction in which silicon tetrachloride and germanium tetrachloride are oxidized by reaction with water H 2 O in an oxyhydrogen flame. In outside vapor deposition the glass is deposited onto a solid rod, which is removed before further processing.
In vapor axial deposition, a short seed rod is used, and a porous preform, whose length is not limited by the size of the source rod, is built up on its end. Typical communications fiber uses a circular preform. For some applications such as double-clad fibers another form is preferred. Because of the surface tension, the shape is smoothed during the drawing process, and the shape of the resulting fiber does not reproduce the sharp edges of the preform.
Nevertheless, careful polishing of the preform is important, since any defects of the preform surface affect the optical and mechanical properties of the resulting fiber. In particular, the preform for the test-fiber shown in the figure was not polished well, and cracks are seen with the confocal optical microscope. The preform, however constructed, is placed in a device known as a drawing tower , where the preform tip is heated and the optical fiber is pulled out as a string.
By measuring the resultant fiber width, the tension on the fiber can be controlled to maintain the fiber thickness. The light is guided down the core of the fiber by an optical cladding with a lower refractive index that traps light in the core through total internal reflection. The cladding is coated by a buffer that protects it from moisture and physical damage. These coatings are UV-cured urethane acrylate composite or polyimide materials applied to the outside of the fiber during the drawing process.
The coatings protect the very delicate strands of glass fiber—about the size of a human hair—and allow it to survive the rigors of manufacturing, proof testing, cabling and installation. An inner primary coating is designed to act as a shock absorber to minimize attenuation caused by microbending. An outer secondary coating protects the primary coating against mechanical damage and acts as a barrier to lateral forces, and may be colored to differentiate strands in bundled cable constructions.
Fiber optic coatings are applied using one of two methods: wet-on-dry and wet-on-wet. In wet-on-dry, the fiber passes through a primary coating application, which is then UV cured—then through the secondary coating application, which is subsequently cured. In wet-on-wet, the fiber passes through both the primary and secondary coating applications, then goes to UV curing. Fiber optic coatings are applied in concentric layers to prevent damage to the fiber during the drawing application and to maximize fiber strength and microbend resistance.
Unevenly coated fiber will experience non-uniform forces when the coating expands or contracts, and is susceptible to greater signal attenuation. Under proper drawing and coating processes, the coatings are concentric around the fiber, continuous over the length of the application and have constant thickness. The thickness of the coating is taken into account when calculating the stress that the fiber experiences under different bend configurations. In a two-point bend configuration, a coated fiber is bent in a U-shape and placed between the grooves of two faceplates, which are brought together until the fiber breaks.
The stress in the fiber in this configuration is given by. The coefficient 1. Fiber optic coatings protect the glass fibers from scratches that could lead to strength degradation. The combination of moisture and scratches accelerates the aging and deterioration of fiber strength. When fiber is subjected to low stresses over a long period, fiber fatigue can occur.
Over time or in extreme conditions, these factors combine to cause microscopic flaws in the glass fiber to propagate, which can ultimately result in fiber failure. Three key characteristics of fiber optic waveguides can be affected by environmental conditions: strength, attenuation and resistance to losses caused by microbending. On the inside, coatings ensure the reliability of the signal being carried and help minimize attenuation due to microbending.
In practical fibers, the cladding is usually coated with a tough resin coating and an additional buffer layer, which may be further surrounded by a jacket layer, usually plastic. These layers add strength to the fiber but do not contribute to its optical wave guide properties. Rigid fiber assemblies sometimes put light-absorbing "dark" glass between the fibers, to prevent light that leaks out of one fiber from entering another. This reduces cross-talk between the fibers, or reduces flare in fiber bundle imaging applications. Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, high voltage isolation, dual use as power lines,  [ failed verification ] installation in conduit, lashing to aerial telephone poles, submarine installation, and insertion in paved streets.
The cost of small fiber-count pole-mounted cables has greatly decreased due to the high demand for fiber to the home FTTH installations in Japan and South Korea. This creates a problem when the cable is bent around corners or wound around a spool, making FTTX installations more complicated. This type of fiber can be bent with a radius as low as 7. Even more bendable fibers have been developed. Another important feature of cable is cable's ability to withstand horizontally applied force. It is technically called max tensile strength defining how much force can be applied to the cable during the installation period.
Some fiber optic cable versions are reinforced with aramid yarns or glass yarns as intermediary strength member. In commercial terms, usage of the glass yarns are more cost effective while no loss in mechanical durability of the cable. Glass yarns also protect the cable core against rodents and termites. Optical fibers are connected to terminal equipment by optical fiber connectors.
Optical fibers may be connected to each other by connectors or by splicing , that is, joining two fibers together to form a continuous optical waveguide. The generally accepted splicing method is arc fusion splicing , which melts the fiber ends together with an electric arc.
Fusion splicing is done with a specialized instrument. The fiber ends are first stripped of their protective polymer coating as well as the more sturdy outer jacket, if present. The ends are cleaved cut with a precision cleaver to make them perpendicular, and are placed into special holders in the fusion splicer. The splice is usually inspected via a magnified viewing screen to check the cleaves before and after the splice. The splicer uses small motors to align the end faces together, and emits a small spark between electrodes at the gap to burn off dust and moisture.
Then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends together permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding do not mix, and this minimizes optical loss. A splice loss estimate is measured by the splicer, by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side.
A splice loss under 0. The complexity of this process makes fiber splicing much more difficult than splicing copper wire. Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning and precision cleaving. The fiber ends are aligned and held together by a precision-made sleeve, often using a clear index-matching gel that enhances the transmission of light across the joint. Such joints typically have higher optical loss and are less robust than fusion splices, especially if the gel is used.
All splicing techniques involve installing an enclosure that protects the splice. Fibers are terminated in connectors that hold the fiber end precisely and securely. A fiber-optic connector is basically a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. The mating mechanism can be push and click , turn and latch bayonet mount , or screw-in threaded. The barrel is typically free to move within the sleeve, and may have a key that prevents the barrel and fiber from rotating as the connectors are mated.
A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick-set adhesive is usually used to hold the fiber securely, and a strain relief is secured to the rear. Once the adhesive sets, the fiber's end is polished to a mirror finish. Various polish profiles are used, depending on the type of fiber and the application. For single-mode fiber, fiber ends are typically polished with a slight curvature that makes the mated connectors touch only at their cores.
This is called a physical contact PC polish. The curved surface may be polished at an angle, to make an angled physical contact APC connection. Such connections have higher loss than PC connections, but greatly reduced back reflection, because light that reflects from the angled surface leaks out of the fiber core.
The resulting signal strength loss is called gap loss. APC fiber ends have low back reflection even when disconnected. In the s, terminating fiber optic cables was labor-intensive. The number of parts per connector, polishing of the fibers, and the need to oven-bake the epoxy in each connector made terminating fiber optic cables difficult.
Today, many connectors types are on the market that offer easier, less labor-intensive ways of terminating cables. Some of the most popular connectors are pre-polished at the factory, and include a gel inside the connector. Those two steps help save money on labor, especially on large projects. A cleave is made at a required length, to get as close to the polished piece already inside the connector.
The gel surrounds the point where the two pieces meet inside the connector for very little light loss. It is often necessary to align an optical fiber with another optical fiber, or with an optoelectronic device such as a light-emitting diode , a laser diode , or a modulator. This can involve either carefully aligning the fiber and placing it in contact with the device, or can use a lens to allow coupling over an air gap. Typically the size of the fiber mode is much larger than the size of the mode in a laser diode or a silicon optical chip.
In this case, a tapered or lensed fiber is used to match the fiber mode field distribution to that of the other element. The lens on the end of the fiber can be formed using polishing, laser cutting  or fusion splicing. In a laboratory environment, a bare fiber end is coupled using a fiber launch system, which uses a microscope objective lens to focus the light down to a fine point. A precision translation stage micro-positioning table is used to move the lens, fiber, or device to allow the coupling efficiency to be optimized.
Fibers with a connector on the end make this process much simpler: the connector is simply plugged into a pre-aligned fiberoptic collimator, which contains a lens that is either accurately positioned with respect to the fiber, or is adjustable. To achieve the best injection efficiency into single-mode fiber, the direction, position, size and divergence of the beam must all be optimized.