Fiber Optics

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Fiber optics is the technology based on optical fibers, i.e., on mostly flexible waveguides for light. The article on fibers describes the core technology, including various types of glass fibers (e.g. silica fibers and fluoride fibers) but also plastic optical fibers. Apart from the basic materials used, there can be differences in many other respects, particularly concerning the propagation characteristics of light in the core. For example, there are

• single-mode and multimode fibers, supporting a single guided mode or multiple modes
• large mode area fibers with particularly large effective mode area
• polarization-maintaining fibers
• low-loss versions for long-haul data transmission
• dispersion-decreasing fibers and dispersion-shifted fibers, exhibiting modified chromatic dispersion properties

• rare-earth-doped fibers for use in amplifiers and lasers, sometimes in the form of double-clad fibers for high-power operation
• highly nonlinear fibers, e.g. for supercontinuum generation
• hollow-core fibers, where light is partly guided in air
• multi-core fibers, containing multiple fiber cores

and various kinds of specialty fibers. Some belong to the important group of photonic crystal fibers (or microstructure fibers), which contain tiny air holes running along the fiber core.

One can also combine multiple fiber-optic elements. In all-fiber setups, the light may entirely stay within fiber waveguides.

Besides, there are fiber bundles and fiber-optic plates containing many thousand or even millions of fibers.

Handling of Fiber Ends

Fiber ends need to prepared with sufficiently high quality, such that the optical wavefronts are well preserved and possibly disturbing protusions are avoided. Cleaving of fiber ends is often sufficient and may be done manually with simple means or with a precision fiber cleaver. In many cases, some polishing is also required.

Fiber ends are often equipped with fiber connectors.

Fiber Cables

In an optical fiber cable, the actual fiber is embedded into a supporting structure, which protects it mostly against mechanical stress and moisture. Such cables are often terminated with fiber connectors, so that they can be plugged in a similar way as electrical cables, although fiber-optic connections are tentatively more delicate.

Fiber cables can differ in many respects:

• They can contain different types of fibers, for example single-mode or multimode glass fibers or plastic fibers with different specifications.
• A cable can contain different numbers of fibers – between one and several hundred.
• They can have different levels of protection, e.g. against mechanical damage and moisture.
• In addition, some cables are fire-retardant.

More details can be found in the article on fiber cables.

Fiber-optic Components

There are various types of fiber-optic elements, which may be connected with each other using fibers. Some of these are essentially made of fibers, whereas others consist of utterly different materials but are coupled to fibers, i.e., they offer fibers for input and output purposes. Some examples of fiber-optical components:

• Fiber-coupled laser diodes can be used as light sources. One may also use bulk solid-state lasers or other lasers in combination with a fiber launch system.
• Fiber couplers can be used e.g. for combining light from different sources into one fiber, or as fiber splitters e.g. for distributing television (cable-TV) signals to different users.
• Mode field converters can efficiently couple light between fibers with different effective mode areas.
• Fiber Bragg gratings can be used as strongly wavelength-selective reflectors, e.g. for add–drop multiplexers in telecom applications with wavelength division multiplexing. Another application is the introduction of tailored wavelength-dependent losses, e.g. for gain flattening of amplifier systems.
• Fiber connectors allow one to have removable and reconfigurable connections between devices – similar to electrical connections, although often more sensitive.
• Fiber collimators provide a connection between fiber optics and free-space optics: they can collimate the output from a fiber, or launch a collimated beam into a fiber.
• Fiber-coupled Faraday isolators, rotators and circulators can be used for manipulations based on beam polarization.
• There are various others fiber-coupled components for beam manipulation, such as fiber-optic modulators and saturable absorbers.
• There are fiber-coupled power meters and spectrometers for monitoring optical powers and spectra. Other devices can monitor the polarization state.

Fiber-optic Setups

One may combine multiple fiber-optical elements to obtain fiber-based optical setups with complex functionality.

In optical fiber communications, one transmits optical signals through fibers. Signals can be amplified in fiber amplifiers, and various types of fiber-coupled components can be used for filtering, regenerating and routing signals.

In the area of laser technology, one assembles diode-pumped (fiber lasers, see below) from fiber-coupled laser diodes, rare-earth-doped fibers and fiber couplers. Additional elements such as fiber-coupled saturable absorbers and fibers for dispersion compensation allow one to obtain mode-locked operation, where the laser emits a train of ultrashort pulses. One can also use elements for Q switching, power stabilization, wavelength tuning and various other purposes.

Fiber Management

In applications with a large fiber count, for example in data centers, special solutions are required for managing fibers:

• Cable identification ties and labeling systems help identify individual fiber, cables or bundles.
• Fiber optic storage reels organize and protect fiber cables on optical tables or breadboards.
• Products like cable trays, ladder racks, cable lacing shelves and mounting brackets help organize fiber cables.
• Fiber patch panels serve as organized distribution hubs for fiber optic cables. They are typically metal enclosures that house an array of ports or connectors. They can contain adapter panels, connector adapters, and sometimes splice trays with space for fiber storage. Rack-mount, wall-mount, outdoor, and DIN-mount panels are available. There are even robotic patch panels, allowing software-controlled reconfigurations.
• There are fiber shuffles, the essential purpose of which is to provide suitable routing of signals going through many fibers. Basic shuffles are relatively simple and provided fixed routing; they basically allow one to place the fibers in a compact and orderly manner. More complex reconfigurable shuffles, containing some type of fiber-optic switches, can be used for adaptive network management.

There are also software solutions for managing fibers. Essentially, such software can provide a virtual model of an existing system and keep track of all its relevant properties – for example, which signal channels (often with different wavelengths for wavelength division multiplexing systems) go through which fibers. It can be used for evaluating performance bottlenecks and the efficiency of resource use. This can be vital for planning further extensions, for example in response to a growth of performance demand.

Fiber Amplifiers and Lasers

In laser-active fibers, which are in most cases rare-earth-doped fibers, one can perform laser amplification processes based on stimulated emission. The laser-active ions, e.g. Yb³⁺, Er³⁺ or Tm³⁺, are pumped with some typically shorter-wavelength pump light, and can then amplify some signal light. Fiber amplifiers based on that technology can easily provide a power gain of several tens of decibels. High-power versions based on double-clad fibers can generate average output powers of hundreds or even thousands of watts. By incorporation of reflectors such as fiber Bragg gratings, or by building ring resonators, one can also realize fiber lasers.

Due to high laser gain, effects of amplified spontaneous emission, the quasi-three-level behavior of typical laser-active ions in fibers, strong gain saturation effects etc., the operation details of fiber amplifiers and lasers are often more complicated than those of bulk lasers. Therefore, detailed laser modeling and simulation is particularly important in this area to obtain a clear understanding, based on which device designs can be optimized.

Imaging with Fiber Optics

Fiber optics can also be used for imaging applications. For example, there are imaging fiber bundles which provide accurate image transfer by guiding light from each input point the corresponding output point with a typically rather small fiber. They are used in endoscopes, for example. Also, there are fiber-optic plates (faceplates), which are rigid parts containing many fibers, sometimes many millions, and are used in night vision devices, for example. Besides the basic function of the image transfer, one can obtain (de)magnification with fiber-optic tapers and also image inversion with twisted devices.

Comparison of Bulk Optics and Fiber Optics

Traditional bulk-optical setups comprise discrete optical elements such as mirrors, lenses, polarizers, filters, etc., whereas fiber optics may be use to make all-fiber setups.

The different technological approaches can differ in many respects:

• An important practical advantage of all-fiber setups is their robustness. All components are connected with each other, so that they cannot become misaligned after fabrication. Often, but not always, the contained fibers can be bent or twisted during operation without any detrimental effects. Different parts of a setup can be mounted on parts which are not rigidly connected with each other. As the light is entirely kept within the cores and closed optical components, there is no risk that dirt and dust particles can effect it.

• On the other hand, a bulk-optical setup is often more convenient during development, testing and maintenance, as one can more easily remove or replace components and access beams e.g. to measure their powers or beam profiles. One can thus more easily identify and cure the reason of faults or optimize single components. Also, one may easily change e.g. the beam sizes within a whole bulk laser setup by exchanging a single mirror or changing its position, whereas such an operation in a fiber-optic setup would require one to replace all or most components.
• Bulk-optical elements are often easier to procure. A problem with fiber-optic elements is that various additional parameters such as mode sizes, polarization-maintaining guidance or not, type and thickness of protective coating, etc. make it more difficult for suppliers to fabricate all combinations of interest and keep these on stock.

• Bulk-optical setups often need to contain a lot of expensive positioning equipment (opto-mechanics), and each fabricated device must undergo an alignment procedure which is not always easy to automate. Fiber-optical setups also need fine alignments, but usually only during fabrication, so that there can be large savings on opto-mechanical parts. On the other hand, the required lab equipment for working with fiber optics comprises expensive things such as fusion splicers. Therefore, cost savings with fiber optics are more likely for large quantities, but not for small quantities, as they often occur in optical technology.
• The article on fiber lasers versus bulk lasers discusses various specific aspects in the context of lasers – among others, influences on the technology on the possible performance of laser devices.

Of course, bulk and fiber technologies are also used in mixed forms, where the light partly travels through air and bulk-optical elements and partly through fibers. One may then obtain advantages of both technologies, but also disadvantages of both. For example, the robustness of a fiber-optical solution may be lost entirely if a setup contains only a single free-space beam path. (Note that re-launching light into a single-mode fiber requires a more sensitive alignment than that in many bulk-optical setups.)

Important Applications of Fiber Optics

In the following, we briefly discuss some particularly important areas of application in photonics technology:

• Optical fiber communications have become a core element of information technology, allowing the extremely fast and low-cost transmission of mostly digital data for telephony, video and television (cable-TV) signals, regional data networks, computing, etc. It is getting even more important with the widespread deployment of fiber to the home technology for providing fast Internet access to many companies and households, surpassing the performance of copper cable technology. The development of the Internet profits enormously from modern fiber optics. This holds not only for passive telecom fibers, which are used for the actual data transmission, but also for additional technology such as fiber amplifiers for compensating fiber losses, fiber couplers for combining or splitting of signals, fiber Bragg gratings for filtering purposes, specialty fibers for nonlinear data processing and various others fiber-optic devices. Glass fibers now totally dominate long-haul data transmission with data rates often reaching multiple terabits per second even in a single fiber; a cable can contain multiple such fibers. Even for short-distance transmission of information in buildings or even within apparatuses, fiber optics gains more and more ground – partly in the form of plastic optical fibers.
• Various types of fiber lasers have become important light sources not only for low-power applications, but even for very high output powers in the domain of multiple kilowatts of average power and megawatts to gigawatts of peak power (at least in conjunction with bulk-optical pulse compressors). They compete with various types of bulk lasers, and depending on many circumstances, one of these technologies may be more appropriate. For more details, see the article on fiber lasers versus bulk lasers.
• Fiber-optic sensors for quantities like temperature, stress and strain, rotation, chemical compositions etc. have pervaded various fields, including aircraft & space technology, oil exploration, and the monitoring of buildings (e.g. large bridges) and pipelines. Both localized and distributed fiber-optic sensors, based on a wide range of physical principles, are nowadays applied in many fields.
• Many fibers simply transport light from a source to an application – for example, from a high-power laser diode setup to a bulk laser, from a laser diode to a light-powered sensor system on a high-voltage transmission line (→ power over fiber), or from a high-power fiber laser to a welding robot in a car factory.

Modeling of Fiber Devices

Physical modeling is often crucial for analyzing and optimizing the operation details of fiber-optic devices. Many different aspects can be the subject of such modeling:

• The properties of the guided modes depend in non-trivial ways on the fiber designs – not just the glass composition, but also the waveguide properties. Optimized mode structures are often crucial for the performance of glass fibers.
• Although many aspects of light propagation can be described on the basis of modes, numerical beam propagation is often required, e.g. for studying effects of imperfections, bending and other external influences. Also, a mode-based analysis may not be practical in situations with a very large number of modes.
• The behavior of rare-earth ions in active devices (amplifiers and lasers) is essential for the power conversion in such devices. As extreme conditions in terms of intensities and gains often occur in fiber-optic devices, such modeling is tentatively more sophisticated than in bulk lasers.
• The propagation of ultrashort pulses in fibers introduces additional aspects such as influences of chromatic dispersion and nonlinearities. Note that such effects are particularly strong in fibers due to the typically long device length and small effective mode area.

For many such aspects, fiber simulation software is used – particularly for various kinds of numerical simulations.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 227 suppliers for fiber optics. Among them:

Diamond SA

Diamond is a Swiss company with a long tradition in the design, manufacture and assembly of high precision fiber optic components. As you explore our offerings, here are the key attributes that set our fiber optic components apart:

• Ultra-low loss interconnects: Our interconnects are meticulously engineered to minimize insertion loss to unprecedented levels.
• Custom solutions crafted for your needs: Customize your interconnect solutions to your exact specifications.
• Quantum-ready fiber-optic components: Our components are designed to meet the demanding requirements of quantum technology and position your network for what lies ahead.

For detailed insights and product specifications, navigate through our offerings or contact us to discuss your application.

Le Verre Fluore

LVF offers the largest range of fluoride fibers in the world, including passive fibers and active fibers for applications ranging from visible to mid-infrared.

• ZrF₄ (fluorozirconate) fibers transmit light from 0.3 µm up to 4.5 µm.
• InF₃ (fluoroindate) fibers transmit light from 0.3 µm up to 5.5 µm.
• GeO₂ (germanate) fibers are qualified for high power handling around 2.7–3.0 µm (Er:YAG and Er:YSSG medical lasers).

LVF fluoride fibers are the most transparent fibers on the market in the mid-infrared 2–5 µm band.

O-E Land

O/E Land offers various fiber-optic items:

• We make customized fiber couplers for various wavelengths; please contact us to discuss your requirements. We offer both single-window and dual-window broadband couplers, which meet the Bellcore GR-1209-CORE requirements.
• The low cost OEPBC-2000 polarization beam combiner is a micro-optics based, UV to mid infrared wavelength product for many applications like instrumentation, sensor, biomedical et al. It combines two light inputs of different linear polarization status coming from PM fiber into one single mode fiber. It can stand high optical powers with low insertion loss due to the epoxy-free light path.
• We also offer fiber patch cables and fiber-optic tapers.

Exail

Exail (formerly iXblue) offers a wide range of specialty optical fibers, either for lasers and amplifiers or sensing applications. Hundreds of fiber designs are available from stock on dedicated e-store. Custom versions are also available. Most of the fibers are also available in radiation-resistant versions, either for nuclear environments or for space missions.

NYFORS

NYFORS has gathered more than three decades of knowledge and experience in specialty splicing and fiber preparation applications. In particular, we offer

• high quality automatic fiber cleaving equipment
• devices for fiber end face inspection
• equipment for fiber recoating and proof testing
• different kinds of fiber stripping and preparation devices
• laser fusion splicers

Further, we offer consulting services on specialty splicing and fiber preparation.

GLOphotonics

We offer a large range of fiber products from award winning fiber technology. Our HCPCF stands out by guiding light in a hollow channel surrounded by a microstructured cladding.

GLO is a pioneering industrial player in this field by offering its partners varied and bespoke HCPCF. A Photonic Micro-Cell (PMC) is a length of HCPCF filled with a gas in a controllable fashion and hermetically sealed. PMC offers strong gas–light interaction.

Edmund Optics

Edmund Optics offers a variety of fiber optics, including jacketed or unjacketed optical grade or communications grade optical fibers. Optical grade fiber is ideal for general industrial lighting or short distance data transmission. Communications grade fiber is designed for optimal visible light transmission for digital or analog links. Jacketed fiber has increased durability while decreasing stray light. Edmund Optics also offers optical fiber components, including patchcords, collimators, faceplates and image conduits, fiber connectors, and the tools needed for cutting or stripping fibers.

Guiding Photonics

Guiding Photonics produces fiber-optic beam delivery solutions for mid-infrared, high-power and UV sources, including standard products, custom cables, and fiber bundles.

Sylex

SYLEX specializes in high-quality optical interconnect solutions and fiber optic sensing/monitoring solutions. Our Fiber Optic Interconnection Division serves telecom, datacom & LAN, on-board optics, general industry, defense, aerospace, and harsh environments.

SYLEX offers automated monitoring solutions based on advanced FBG technology, essential for monitoring structural health and operational conditions in industries like civil engineering, geotechnical fields, energy, transportation, chemicals, oil & gas, and process control. We also produce OEM and custom FBG sensors, partnering with research and development entities as a small and medium enterprise (SME).

Schäfter + Kirchhoff

We offer fiber optic components including the laser beam coupler of series 60SMS/60SMF for coupling into a polarization-maintaining fiber cables, fiber collimators of series 60FC. The polarization analyzers series SK010PA are universal measurement and test systems for coupling laser beam sources into polarization-maintaining fiber cables.

CSRayzer Optical Technology

CSRayzer provides various kinds of fiber-optic components, including polarization-maintaining and single-mode fiber couplers, WDM couplers, isolators, circulators, filters, phase shifters, collimators and hybrid components. These components could work in full temperature conditions, and suitable for special applications such as aerospace and military.

NKT Photonics

Optical fibers are at the heart of everything we do. We embed as many functions as possible directly into the fibers to make systems based on our fibers simpler, cheaper, and more reliable. Our Crystal Fibre portfolio spans from nonlinear fibers for octave-spanning supercontinuum generation, over the World’s largest single-mode ytterbium gain fibers for high-power lasers and amplifiers to advanced hollow-core fibers guiding the light in air.

Shalom EO

Hangzhou Shalom EO offers various fiber optics, including:

• TGG (Terbium Gallium Garnet) crystals and TSAG (Terbium Scandium Aluminum Garnet) crystals both in laser-grade polished versions and as ingots for applications such as Faraday rotators and Faraday isolators
• Fiber end caps for QBH (Quartz Block Head), fiber end caps can be made of quartz or fused silica, and various custom glass materials with custom AR coatings
• Beam combiners
• Fiber laser optics (C-lens, fast axis collimator, slow axis collimator, etc.)

art photonics

art photonics’ offers special items in fiber optics:

• copper alloy coated and aluminum coated silica fibers with high performance under harsh environmental conditions concerning temperature and humidity.
• chalcogenide mid-IR fibers for 1.1–6.5 μm with low transmission losses
• polycrystalline infrared fibers for 3–17 μm, non-hygroscopic, non-toxic, minimal ageing
• hollow glass waveguides for infrared transmission with low propagation losses
• indium and zirconium fluoride fibers: many single-mode and multimode versions
• various types of fiber cables and fiber bundles
• accessories like combiners and splitters, collimating and refocusing objectives, fiber couplers for CO and CO₂ lasers, mating sleeves

Fibercore

Fibercore has delivered over 40 years of innovation and excellence in developing and manufacturing speciality optical fiber. All of our fiber products have been developed with our customers in mind with market leading capabilities to produce an extensive range of fibers including;

• single-mode (SM) fiber
• doped fiber
• polarization-maintaining (PM) fiber
• multimode (MM) fiber
• multicore fiber
• spun HiBi fiber

We are continuously expanding our product ranges to cover wider and more demanding customer applications. So if you have a specific development project or require a custom fiber, we would like to discuss it further with you. We will work together with you to find the best solution.

AMS Technologies

AMS Techno­logies provides an exceptionally large portfolio of fibers and fiber optics, ranging from optical fibers, patch cables, fiber bundles and assemblies to a broad variety of fiber components:

• optical fibers – SM, MM, PM, plastic optical and doped fibers, including specialty fibers
• SM, MM, PM patch cables
• multi-fiber bundles and high-temperature assemblies
• V-grooves and two-dimensional fiber arrays
• fiber connector kits and parts, mating sleeves and adapters, fiber feedthroughs
• fiber optomechanics, receptacle collimators and focusers
• fiber WDMs, combiners, splitters, couplers, delay lines, isolators, amplifiers, circulators
• fiber optic switches, polarization controllers, attenuators

Bibliography

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