Optical Fibre Devices (Series in Optics and Optoelectronics)
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In Charles K. After a period of research starting from , the first commercial fiber-optic communications system was developed which operated at a wavelength around 0. The second generation of fiber-optic communication was developed for commercial use in the early s, operated at 1.
These early systems were initially limited by multi mode fiber dispersion, and in the single-mode fiber was revealed to greatly improve system performance, however practical connectors capable of working with single mode fiber proved difficult to develop. The first transatlantic telephone cable to use optical fiber was TAT-8 , based on Desurvire optimised laser amplification technology.
It went into operation in Third-generation fiber-optic systems operated at 1. This development was spurred by the discovery of Indium gallium arsenide and the development of the Indium Gallium Arsenide photodiode by Pearsall. Engineers overcame earlier difficulties with pulse-spreading at that wavelength using conventional InGaAsP semiconductor lasers.
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Scientists overcame this difficulty by using dispersion-shifted fibers designed to have minimal dispersion at 1. These developments eventually allowed third-generation systems to operate commercially at 2. The fourth generation of fiber-optic communication systems used optical amplification to reduce the need for repeaters and wavelength-division multiplexing to increase data capacity. The focus of development for the fifth generation of fiber-optic communications is on extending the wavelength range over which a WDM system can operate.
The conventional wavelength window, known as the C band, covers the wavelength range 1. Other developments include the concept of " optical solitons ", pulses that preserve their shape by counteracting the effects of dispersion with the nonlinear effects of the fiber by using pulses of a specific shape. In the late s through , industry promoters, and research companies such as KMI, and RHK predicted massive increases in demand for communications bandwidth due to increased use of the Internet , and commercialization of various bandwidth-intensive consumer services, such as video on demand.
Internet protocol data traffic was increasing exponentially, at a faster rate than integrated circuit complexity had increased under Moore's Law. From the bust of the dot-com bubble through , however, the main trend in the industry has been consolidation of firms and offshoring of manufacturing to reduce costs. Modern fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send through the optical fiber, a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal.
The information transmitted is typically digital information generated by computers, telephone systems and cable television companies. The most commonly used optical transmitters are semiconductor devices such as light-emitting diodes LEDs and laser diodes. The difference between LEDs and laser diodes is that LEDs produce incoherent light , while laser diodes produce coherent light.
For use in optical communications, semiconductor optical transmitters must be designed to be compact, efficient and reliable, while operating in an optimal wavelength range and directly modulated at high frequencies. In its simplest form, an LED is a forward-biased p-n junction , emitting light through spontaneous emission , a phenomenon referred to as electroluminescence. However, due to their relatively simple design, LEDs are very useful for low-cost applications.
The large spectrum width of LEDs is subject to higher fiber dispersion, considerably limiting their bit rate-distance product a common measure of usefulness. LEDs have also been developed that use several quantum wells to emit light at different wavelengths over a broad spectrum and are currently in use for local-area WDM Wavelength-Division Multiplexing networks. The narrow spectral width also allows for high bit rates since it reduces the effect of chromatic dispersion.
Furthermore, semiconductor lasers can be modulated directly at high frequencies because of short recombination time. Laser diodes are often directly modulated , that is the light output is controlled by a current applied directly to the device. For very high data rates or very long distance links , a laser source may be operated continuous wave , and the light modulated by an external device, an optical modulator , such as an electro-absorption modulator or Mach—Zehnder interferometer.
External modulation increases the achievable link distance by eliminating laser chirp , which broadens the linewidth of directly modulated lasers, increasing the chromatic dispersion in the fiber. A transceiver is a device combining a transmitter and a receiver in a single housing see picture on right.
Fiber optics have seen recent advances in technology. The main component of an optical receiver is a photodetector which converts light into electricity using the photoelectric effect. The primary photodetectors for telecommunications are made from Indium gallium arsenide. The photodetector is typically a semiconductor-based photodiode. Several types of photodiodes include p-n photodiodes, p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal MSM photodetectors are also used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.
Optical-electrical converters are typically coupled with a transimpedance amplifier and a limiting amplifier to produce a digital signal in the electrical domain from the incoming optical signal, which may be attenuated and distorted while passing through the channel. Further signal processing such as clock recovery from data CDR performed by a phase-locked loop may also be applied before the data is passed on. An optical communication system transmitter consists of a digital-to-analog converter DAC , a driver amplifier and a Mach—Zehnder-Modulator.
Digital predistortion counteracts the degrading effects and enables Baud rates up to 56 GBaud and modulation formats like 64 QAM and QAM with the commercially available components. The transmitter digital signal processor performs digital predistortion on the input signals using the inverse transmitter model before uploading the samples to the DAC. Older digital predistortion methods only addressed linear effects. Recent publications also compensated for non-linear distortions. Berenguer et al models the Mach—Zehnder modulator as an independent Wiener system and the DAC and the driver amplifier are modelled by a truncated, time-invariant Volterra series.
Duthel et al records for each branch of the Mach-Zehnder modulator several signals at different polarity and phases. The signals are used to calculate the optical field.
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Cross-correlating in-phase and quadrature fields identifies the timing skew. The frequency response and the non-linear effects are determined by the indirect-learning architecture. An optical fiber cable consists of a core, cladding , and a buffer a protective outer coating , in which the cladding guides the light along the core by using the method of total internal reflection. The core and the cladding which has a lower- refractive-index are usually made of high-quality silica glass, although they can both be made of plastic as well. Connecting two optical fibers is done by fusion splicing or mechanical splicing and requires special skills and interconnection technology due to the microscopic precision required to align the fiber cores.
Two main types of optical fiber used in optic communications include multi-mode optical fibers and single-mode optical fibers. However, a multi-mode fiber introduces multimode distortion , which often limits the bandwidth and length of the link. Furthermore, because of its higher dopant content, multi-mode fibers are usually expensive and exhibit higher attenuation.
Hybrid Optical Fibers – An Innovative Platform for In‐Fiber Photonic Devices
Both single- and multi-mode fiber is offered in different grades. In order to package fiber into a commercially viable product, it typically is protectively coated by using ultraviolet UV , light-cured acrylate polymers , then terminated with optical fiber connectors , and finally assembled into a cable. After that, it can be laid in the ground and then run through the walls of a building and deployed aerially in a manner similar to copper cables. These fibers require less maintenance than common twisted pair wires once they are deployed.
Specialized cables are used for long distance subsea data transmission, e. New — cables operated by commercial enterprises Emerald Atlantis , Hibernia Atlantic typically have four strands of fiber and cross the Atlantic NYC-London in 60—70ms. Another common practice is to bundle many fiber optic strands within long-distance power transmission cable.
This exploits power transmission rights of way effectively, ensures a power company can own and control the fiber required to monitor its own devices and lines, is effectively immune to tampering, and simplifies the deployment of smart grid technology. The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than was received, thus counteracting the loss incurred in the previous segment.
An alternative approach is to use optical amplifiers which amplify the optical signal directly without having to convert the signal to the electrical domain. Optical amplifiers have several significant advantages over electrical repeaters. First, an optical amplifier can amplify a very wide band at once which can include hundreds of individual channels, eliminating the need to demultiplex DWDM signals at each amplifier.
Second, optical amplifiers operate independently of the data rate and modulation format, enabling multiple data rates and modulation formats to co-exist and enabling upgrading of the data rate of a system without having to replace all of the repeaters. Third, optical amplifiers are much simpler than a repeater with the same capabilities and are therefore significantly more reliable.
Optical amplifiers have largely replaced repeaters in new installations, although electronic repeaters are still widely used as transponders for wavelength conversion. Wavelength-division multiplexing WDM is the practice of multiplying the available capacity of optical fibers through use of parallel channels, each channel on a dedicated wavelength of light.
This requires a wavelength division multiplexer in the transmitting equipment and a demultiplexer essentially a spectrometer in the receiving equipment. Arrayed waveguide gratings are commonly used for multiplexing and demultiplexing in WDM. Using WDM technology now commercially available, the bandwidth of a fiber can be divided into as many as channels  to support a combined bit rate in the range of 1. This value is a product of bandwidth and distance because there is a trade-off between the bandwidth of the signal and the distance over which it can be carried.
Engineers are always looking at current limitations in order to improve fiber-optic communication, and several of these restrictions are currently being researched. Each fiber can carry many independent channels, each using a different wavelength of light wavelength-division multiplexing. The net data rate data rate without overhead bytes per fiber is the per-channel data rate reduced by the FEC overhead, multiplied by the number of channels usually up to eighty in commercial dense WDM systems as of [update].
The following summarizes the current state-of-the-art research using standard telecoms-grade single-mode, single-solid-core fibre cables. Ethical principles and practice. Corporate Communications: Theory and Practice. Neurosurgery: principles and practice. Recommend Documents. Read Petrology of the igneous rocks, 13th Wireless communications principles and practice www.
It also enables a viscous boundary between elements that would otherwise react or mix with an interfacing liquid metal during the drawing process. An alternative approach recently proposed for electrical conductivity inside optical fibers relies on ionic conductivity through the doping of a phosphate glass with silver. The emergence of multiple LSPRs is rather exceptional and can be explained by a cavity model: here, the LSPR is formed by a fundamental SPP propagating to and from between the apex and the base of the ellipsoid.
Optical Fibre Devices
As a result of the nanoscale confinement, the LSPRs can be used to probe near fields with a resolution substantially below the operating wavelength one example where the probe traces an evanescent field is shown in Figure 7 a. Light scattered via the plasmon resonances partly scatters into the tapered capillary and is subsequently coupled into the multimode fiber, which ultimately guides the light to a desired location. The LSPRs strongly depend on the actual nanoscale shape of the tip, which makes it difficult to predict the exact spectral position of the resonance the scattering spectra of three different tips and an example of the polarization dependence is shown in Figure 7 b,c.
From the application point of view, beams with a cylindrical polarization state are interesting and are used in a large variety of fields such as stimulated emission depletion microscopy 81 or superfocusing. A particularly important example of a HOF with unique polarization properties is a solid core PCF containing a gold NW in the center of the structure Figure 8 a and inset of b.
It is important to note that an entirely azimuthal polarization state can be converted straightforwardly to its radial counterpart simply via two wave plates. These unique polarization properties of this HOF device arise from the different fraction of magnetic field of the three supported modes inside the NW. All three supported modes have annular shapes and rather equal excitation probabilities when being excited with an external Gaussian beam, giving rise to an overall azimuthal polarization efficiency of 10 dB, which is sufficient for many kinds of applications.
In addition to single wires, arrays of metallic wires also allow the integration of novel functionalities into fiber. If the metallic features are substantially smaller than the operation wavelength, such an array becomes an effective medium, i. Noncircular wires—a slotted cylinder resonator or split ring resonator—were also integrated into fiber by the direct drawing method to demonstrate a basic magnetic MM Figure 9 b. The simplest application of these basic MM structures can be filters, particularly for the THz spectrum where components are still in relatively early development.
This allows the propagation of high spatial frequency modes that would otherwise be evanescent, i. In addition, a magnifying hyperlens consisting of a tapered wire array was demonstrated. The example in Figure 11 a,b has an 8 mm diameter that tapers to a 1 mm diameter. Finally, metallic domains may not only be used as high conductivity materials, but their optical properties can also be exploited to confine electromagnetic waves and hence provide a guiding mechanism for optical transport.
Gold and copper wires surrounding a silica solid core have been used to make metallic optical fibers that rely on metallic confinement to guide light. Theoretical studies on these types of fibers also reveal the strong potential of metallic microwires and metamaterials to propose novel guiding mechanism and mode engineering for optical fibers.
Semiconductors are key elements of any practically relevant optoelectronic system and are used in particular for the fast and efficient detection and modulation of electromagnetic radiation. One of the main motivations of the field of HOF is to integrate semiconductor functionalities directly into optical fibers, with the perspective of making a new generation of large area, flexible and even wearable photonic devices with efficiencies on par with or beyond their rigid planar counterparts. Various devices such as detectors or modulators have been implemented up to now and a selection of those will be discussed in this section.
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One important application of semiconductors in optoelectronics is the fast and efficient detection of light, in particular NIR wavelengths that form the basis of telecommunications. The optimal solution to this problem is to include the detector functionality directly into the fibers. One particular interesting approach was presented by the Badding group, 65 where they used HPCVD to deposit multilayers of differently doped crystalline semiconductor materials inside silica capillaries and in selected holes of PCFs Figure 12 a.
Using a sophisticated contacting scheme this device showed rise and fall times of the order of several tens of picoseconds when excited by picosecond optical pulses Figure 12 c , which is a sufficient figure for many optoelectronic detection applications. In a next step a junction with reduced complexity was created inside a selected hole of a PCF, where strong coupling between the fundamental PCF mode and the modes in the junction was observed. While semiconductor devices with excellent performance can be achieved using HPCVD techniques, the main limitation of such an approach is its scalability, as only short fiber lengths can be functionalized.
Integrating semiconducting functionalities, traditionally reserved for smaller scale and rigid wafer substrates, inside flexible 1D systems with the length and surface associated with optical fibers, can herald a novel path towards large area, flexible, and even wearable optoelectronic devices. Several applications can be envisioned and have been proposed for these one dimensional fiber systems as opposed to point devices in imaging, 41 , , industrial monitoring, remote and distributed sensing, 34 , 35 energy harvesting 75 , and functional fabrics.
Chalcogenide glasses, a class of materials based on the Chalcogen elements S, Se, Te , have the double advantage of being compatible with this process, and exhibiting relevant optoelectronic properties that are tunable depending on their composition with As, Ge or Sn being typical additive materials. They have been extensively used in photonic crystal fibers owing to their large refractive index 29 , 30 and strongly nonlinear properties.
For example, a thermal sensing layer could be added around Bragg mirror of a photonic bandgap fibers. The resulting fiber had the ability to monitor itself against the presence of defects that could locally heat and destroy the polymer cladding. For example, by cascading several of such layers in a single fiber, each independently electrically contacted by metallic electrodes Figure 13 f , fibers that could sense not only impinging optical wave fronts but also extract their angle of incidence, wavelength and power were fabricated.
This first generation of optoelectronic fibers has given rise to new 1D devices that can be integrated in myriad of applications and configurations. The examples presented so far still suffer, however, from three main limitations: 1 the axial symmetry that prevents the extraction of information on the stimuli distribution along the fiber axis; 2 the amorphous nature of the semiconducting material that result in rather poor optoelectronic properties, and; 3 the limited available semiconducting materials.
To address the first point, a configuration was proposed to localize axial position of one or more incident beams impinging onto a photodetecting fiber Figure 13 d.
Optical Fibre Devices: 1st Edition (Paperback) - Routledge
The low conductivity of the CPC material compared to its metallic electrode counterpart enabled the tailoring of the electric field distribution along the fiber axis. By measuring the current generated while varying the potential drop applied at both fiber ends, one can measure independent currents and hence reconstruct the location of the beam.
Indeed, thermally drawn fibers with amorphous materials reveal the relatively poor electronic and optoelectronic properties due to the glassy state of the semiconductors. This configuration alleviates the optical losses experienced by the CL signal in purely optical systems. Indeed, most of the emitted photons are directly absorbed and converted into an electrical signal by the surrounding semiconducting layer at the spot where the hazardous chemical triggered the emission.
The sensitivity of this fiber sensor is hence no longer dictated by a signal collection deficiency, but is governed by the performance of the distributed photodetector. This highlights the strong potential for distal and distributed sensing of a variety of stimuli enabled by the multimaterial fiber approach schematically depicted in Figure 14 d.
This approach has the advantage of accessing a wider class of materials, as long as a compatible glassy cladding can be found to encapsulate the liquid as it flows from preform to fiber. In particular, materials more commonly used in electronic and optoelectronic components such as silicon and germanium can be integrated with this technique, which can bring a higher level of performance, novel optoelectronic applications and superior optical properties.
So far, it is the particular optical properties of crystalline semiconductors, rather than their optoelectronic attributes, that has been exploited in crystalline core optical fibers. Si and Ge for example are highly nonlinear materials with Raman gain coefficients much higher than silica and are transparent in the IR examples of Si core fiber shown in Figure 2 g. Note that the approach of introducing Silicon powder into a tube under vacuum has also been proposed.
This was recently highlighted in work where an aluminium rod inside a silica cladding at the preform level resulted in a Silicon core and alumina domains in the fiber. Another intriguing phenomenon recently shown in multimaterial HOFs is the compound formation induced by the drawing process. Depending on the materials in contact, the interdiffusion coefficients and the enthalpy of formation of various compounds, it can be anticipated that synthesis may occur and different materials end up in the fiber compared to those found in the preform. The first work that presented what could be named a multimaterial fiber draw synthesis approach was realized after the volatization of some elements initially in the core of the preform, resulting in a different composition in the fiber.
The only functional optoelectronic device using fiber draw synthesis was recently presented where a Se 97 S 3 layer was brought in contact with a Sn 85 Zn 15 eutectic electrode. This layer with a large bandgap in between the Se layer and the metal Figure 15 a , would explain the formation of a heterojunction that induced a barrier to the transport of holes.
While the short circuit current was very small given the unoptimized parallel configuration in the fiber, the high open circuit voltage opens the way towards creating truly distributed and scalable photovoltaic fibers that could combine the flexibility of polymers with the efficiency of inorganic compounds. It should be noted that this approach contrasts with other commonly used ways to integrate photovoltaic functionalities into fiber using HPCVD or solution process layers onto existing fibers.
By choosing the dielectric properties of the material to be integrated into the fibers, the characteristics of the propagating waveguide modes can be engineered almost as desired. Dispersion engineering is extremely relevant, for example, broadband supercontinuum generation, as shown by the examples discussed below. The efficient and broadband generation of light is clearly an important prerequisite for any practically relevant photonic system. HOFs hold a great potential in this field, as they naturally address all these mentioned issues on an entirely integrated waveguide platform. Exciting the fundamental mode in a 2 mm long sample using an intense femtosecond laser pulse center wavelength: 1.
Besides PAMF, highly nonlinear materials can also be incorporated into microstructured fibers via direct fiber drawing, examples of which include SCG inside chalcogenide wires embedded in polymer matrices. The Abouraddy group at Creol showed that dual layer chalcogenide nanotapers can be integrated into a thermoplastic polymer jacket Figure 17 a. Another example of a hybrid nonlinear fiber is a chalcogenide glass included into a microstructured tellurite fiber by direct fiber drawing an As 2 S 3 rod embedded in a structured tellurite perform Figure 18 a.
The dispersion properties of the final hybrid fiber were mainly adjusted via the microstructured cladding and the core diameter core diameter: 1. Very large spanning supercontinuum from about nm to 2. However, compared to the chalcogenide glass systems, one major drawback of silicon is its comparably large two photon cross section when being pump below 2.
One particular interesting feature of liquid core fibers is the emergence of a retarded optical nonlinearity, which is a result of a torque applied to the individual molecules of the liquid when being exposed to strong femtosecond pulses. It was theoretically predicted that this kind of nonlinearity can lead to noninstantaneous solitonic excitations, which does not suffer from a distinct Raman shift towards longer wavelength while propagating through the waveguide.
Another advanced fiber device recently developed relied on the modulation of a surface emitting fiber laser by liquid crystal LC channels, electrically addressed, where the channels and sandwiching electrodes run along the entire fiber length Figure 20 b. The polarization of this emitted laser light can be changed as it goes through the LC channel. This modulation is a function of the alignment of LC molecules that is directly related to the potential applied across the electrodes.
This transverse functionality of direction modulation of an emitted coherent light over large surfaces can pave the way towards a variety of novel devices. The example presented here Figure 20 c represents an application where such a fiber could be introduced with minimal invasion inside a vessel on which a tumor has grown on one side. Emission could be controlled to illuminate and treat only the tumor and not the surrounding tissue. Another example is a MO fiber which has been realized via PAMF and consists of diamagnetic tellurite glass core and a silica cladding Figure 21 a,b.