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  • Polarization-Maintaining Fiber Tutorial

    Introduction to Polarization

    As light passes through a point in space, the direction and amplitude of the vibrating electric field traces out a path in time. A polarized lightwave signal is represented by electric and magnetic field vectors that lie at right angles to one another in a transverse plane (a plane perpendicular to the direction of travel). Polarization is defined in terms of the pattern traced out in the transverse plane by the electric field vector as a function of time.

    Polarization can be classified as linear, elliptical or circular, in them the linear polarization is the simplest. Whichever polarization can be a problem in the fiber optic transmission.

    FiberStore Polarization Coordinate System

    More and more telecommunication and fiber optic measuring systems refer to devices that analyse the interference of two optical waves. The information given by the interferences cannot be used unless the combined amplitude is stable in time, which means, that the waves are in the same state of polarization. In those cases it is necessary to use fibers that transmit a stable state of polarization. And polarization-maintaining fiber was developed to this problem. (The polarization-maintaining fiber will be called PM fiber for short in the following contents.)

     

    What Is PM Fiber?

    The polarization of light propagating in the fiber gradually changes in an uncontrolled (and wavelength-dependent) way, which also depends on any bending of the fiber and on its temperature. Specialised fibers are required to achieve optical performances, which are affected by the polarization of the light travelling through the fiber. Many systems such as fiber interferometers and sensors, fiber laser and electro-optic modulators, also suffer from Polarization-Dependent Loss (PDL) that can affect system performance. This problem can be fixed by using a specialty fiber so called PM Fiber.

     

    Principle of PM Fiber

    Provided that the polarization of light launched into the fiber is aligned with one of the birefringent axes, this polarization state will be preserved even if the fiber is bent. The physical principle behind this can be understood in terms of coherent mode coupling. The propagation constants of the two polarization modes are different due to the strong birefringence, so that the relative phase of such copropagating modes rapidly drifts away. Therefore, any disturbance along the fiber can effectively couple both modes only if it has a significant spatial Fourier component with a wavenumber which matches the difference of the propagation constants of the two polarization modes. If this difference is large enough, the usual disturbances in the fiber are too slowly varying to do effective mode coupling. Therefore, the principle of PM fiber is to make the difference large enough.

    In the most common optical fiber telecommunications applications, PM fiber is used to guide light in a linearly polarised state from one place to another. To achieve this result, several conditions must be met. Input light must be highly polarised to avoid launching both slow and fast axis modes, a condition in which the output polarization state is unpredictable.

    The electric field of the input light must be accurately aligned with a principal axis (the slow axis by industry convention) of the fiber for the same reason. If the PM fiber path cable consists of segments of fiber joined by fiber optic connectors or splices, rotational alignment of the mating fibers is critical. In addition, connectors must have been installed on the PM fibers in such a way that internal stresses do not cause the electric field to be projected onto the unintended axis of the fiber.

     

    Types of PM Fibers

    Circular PM Fibers

    It is possible to introduce circular-birefringence in a fiber so that the two orthogonally polarized modes of the fiber—the so called Circular PM fiber—are clockwise and counter-clockwise circularly polarized. The most common way to achieve circular-birefringence in a round (axially symmetrical) fiber is to twist it to produce a difference between the propagation constants of the clockwise and counterclockwise circularly polarized fundamental modes. Thus, these two circular polarization modes are decoupled. Also, it is possible to conceive externally applied stress whose direction varies azimuthally along the fiber length causing circular-birefringence in the fiber. If a fiber is twisted, a torsional stress is introduced and leads to optical-activity in proportion to the twist.

    Circular-birefringence can also be obtained by making the core of a fiber follows a helical path inside the cladding. This makes the propagating light, constrained to move along a helical path, experience an optical rotation. The birefringence achieved is only due to geometrical effects. Such fibers can operate as a single mode, and suffer high losses at high order modes.

    Circular PM fiber with Helical-core finds applications in sensing electric current through Faraday effect. The fibers have been fabricated from composite rod and tube preforms, where the helix is formed by spinning the preform during the fiber drawing process.

     

    Linear PM Fibers

    There are manily two types of linear PM fibers which are single-polarization type and birefringent fiber type. The single-polarization type is characterized by a large transmission loss difference between the two polarizations of the fundamental mode. And the birefringent fiber type is such that the propagation constants between the two polarizations of the fundamental mode are significantly different. Linear polarization may be maintained using various fiber designs which are reviewed next.

    Linear PM Fibers With Side Pits and Side Tunnels

    Side-pit fibers incorporate two pits of refractive index less than the cladding index, on each side of the central core. This type of fiber has a W-type index profile along the x-axis and a step-index profile along the y-axis. A side-tunnel fiber is a special case of side-pit structure. In these linear PM fibers, a geometrical anisotropy is introduced in the core to obtain a birefringent fibers.

     

    Linear PM Fibers With Stress Applied Parts

    An effective method of introducing high birefringence in optical fibers is through introducing an asymmetric stress with two-fold geometrical symmetry in the core of the fiber. The stress changes the refractive index of the core due to photoelastic effect, seen by the modes polarized along the principal axes of the fiber, and results in birefringence. The required stress is obtained by introducing two identical and isolated Stress Applied Parts (SAPs), positioned in the cladding region on opposite sides of the core. Therefore, no spurious mode is propagated through the SAPs, as long as the refractive index of the SAPs is less than or equal to that of the cladding.

    The most common shapes used for the SAPs are: bow-tie shape and circular shape. These fibers are respectively referred to as Bow-tie Fiber and PANDA Fiber. The cross sections of these two types of fibers are shown in the figure below. The modal birefringence introduced by these fibers represents both geometrical and stress-induced birefringences. In the case of a circular-core fiber, the geometrical birefringence is negligibly small. It has been shown that placing the SAPs close to the core improves the birefringence of these fibers, but they must be placed sufficiently close to the core so that the fiber loss is not increased especially that SAPs are doped with materials other than silica. The PANDA fiber has been improved further to achieve high modal birefringence, very low-loss and low cross-talk.

    PANDA Fiber and Bow-tie Fiber

    PANDA Fiber (left) and Bow-tie Fiber (right). The built-in stress elements made from a different type of glass are shown with a darker gray tone.

    Tips: At present the most popular PM fiber in the industry is the circular PANDA fiber. One advantage of PANDA fiber over most other PM fibers is that the fiber core size and numerical aperture is compatible with regular single mode fiber. This ensures minimum losses in devices using both types of fibers.

     

    Linear PM Fibers With Elliptical Structures

    The first proposal on practical low-loss single-polarization fiber was experimentally studied for three fiber structures: elliptical core, elliptical clad, and elliptical jacket fibers. Early research on elliptical-core fibers dealt with the computation of the polarization birefringence. In the first stage, propagation characteristics of rectangular dielectric waveguides were used to estimate birefringence of elliptical-core fibers. In the first experiment with PM fiber, a fiber having a dumbbell-shaped core was fabricated. The beat length can be reduced by increasing the core-cladding refractive index difference. However, the index difference cannot be increased too much due to practical limitations. Increasing the index difference increases the transmission loss, and splicing would become difficult because the core radius must be reduced. Typical values of birefringence for the elliptical core fiber are higher than elliptical clad fiber. However, losses were higher in the elliptical core than losses in the elliptical clad fibers.

     

    Linear PM Fibers With Refractive Index Modulation

    One way to increase the bandwidth of single-polarization fiber, which separates the cutoff wavelength of the two orthogonal fundamental modes, is by selecting a refractive-index profile which allows only one polarization state to be in cutoff. High birefringence was achieved by introducing an azimuthal modulation of the refractive index of the inner cladding in a three-layer elliptical fiber. A perturbation approach was employed to analyze the three-layer elliptical fiber, assuming a rectangular-core waveguide as the reference structure. Examination of birefringence in three-layer elliptical fibers demonstrated that a proper azimuthal modulation of the inner cladding index can increase the birefringence and extend the wavelength range for single-polarization operation.

    A refractive index profile is called Butterfly profile. It is an asymmetric W profile, consisting of a uniform core, surrounded by a cladding in which the profile has a maximum value of ncl and varies both radially and azimuthally, with maximum depression along the x-axis. This profile has two attributes to realize a single-mode single-polarization operation. First, the profile is not symmetric, which makes the propagation constants of the two orthogonal fundamental modes dissimilar, and secondly, the depression within the cladding ensures that each mode has a cutoff wavelength. The butterfly fiber is weakly guiding, thus modal fields and propagation constants can be determined from solutions of the scalar wave equation. The solutions involve trigonometric and Mathieu functions describing the transverse coordinates dependence in the core and cladding of the fiber. These functions are not orthogonal to one another which requires an infinite set of each to describe the modal fields in the different regions and satisfy the boundary conditions. The geometrical birefringence plots generated vs. the normalized frequency V showed that increasing the asymmetry through the depth of the refractive index depression along the x-axis increases the maximum value of the birefringence and the value of V at which this occurs. The peak value of birefringence is a characteristic of noncircular fibers. The modal birefringence can be increased by introducing anisotropy in the fiber which can be described by attributing different refractive-index profiles to the two polarizations of a mode. The geometric birefringence is smaller than the anisptropic birefringence. However, the depression in the cladding of the butterfly profile gives the two polarizations of fundamental mode cutoff wavelengths, which are separated by a wavelength window in which single-polarization single-mode operation is possible.

     

    Applications of PM Fibers

    PM fibers are applied in devices where the polarization state cannot be allowed to drift, e.g. as a result of temperature changes. Examples are fiber interferometers and certain fiber lasers. A disadvantage of using such fibers is that usually an exact alignment of the polarization direction is required, which makes production more cumbersome. Also, propagation losses are higher than for standard fiber, and not all kinds of fibers are easily obtained in polarization-preserving form.

    PM fibers are used in special applications, such as in fiber optic sensing, interferometry and quantum key distribution. They are also commonly used in telecommunications for the connection between a source laser and a modulator, since the modulator requires polarized light as input. They are rarely used for long-distance transmission, because PM fiber is expensive and has higher attenuation than single mode fiber.

     

    Requirments for Using PM Fibers

    Termination: When PM fibers are terminated with fiber connectors, it is very important that the stress rods line up with the connector, usually in line with the connector key.

    Splicing: PM fiber also requires a great deal of care when it is spliced. Not only the X, Y and Z alignment have to be perfect when the fiber is melted together, the rotational alignment must also be perfect, so that the stress rods align exactly.

    Another requirement is that the launch conditions at the optical fiber end face must be consistent with the direction of the transverse major axis of the fiber cross section.

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  • How to Install or Remove SFP Transceiver Modules on Cisco Device

    The SFP (small form Factor pluggables) transceiver modules are hot-pluggable I/O devices that plug into module sockets. The transceiver connects the electrical circuitry of the module with the optical or copper network. SFP transceiver modules are the key components in today's transmission network. Thus, it is necessary to master the skill of installing or removing a transceiver modules to avoid unnecessary loss. This tutorial are going to guide you how to install or remove SFP transceiver module in a right way.

     

    Things you should Know Before Installing or Removing SFP

    Before removing or installing a Transceiver Module you must disconnect all cables, because of leaving these attached will damage the cables, connectors, and the optical interfaces. At the same time please be aware that do not often remove and install an SFP transceiver and it can shorten its useful life. For this reason transceivers should not be removed or inserted more often than is required. Furthermore, transceiver modules are sensitive to static, so always ensure that you use an ESD wrist strap or comparable grounding device during both installation and removal.

     

    Required Tools

    You will need these tools to install the SFP transceiver module:
    Wrist strap or other personal grounding device to prevent ESD occurrences.Antistatic mat or antistatic foam to set the transceiver on.Fiber-optic end-face cleaning tools and inspection equipment

     

    Installing SFP Transceiver Modules

    SFP transceiver modules can have three types of latching devices to secure an SFP transceiver in a port socket:
    SFP transceiver with a Mylar tab latch.SFP transceiver with an actuator button latch.SFP transceiver that has a bale-clasp latch.
    Types of SFP Latching

    Determine which type of latch your SFP transceiver uses before following the installation and removal procedures.

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  • Introduction to Small Form-factor Pluggable (SFP) Transceiver Modules

    Introduction to Small Form-factor Pluggable (SFP) Transceiver Modules

    What Is SFP?
    SFP, short for small form-factor pluggable is a compact, hot-pluggable transceiver used for both telecommunication and data communications applications. SFP transceiver can be regarded as the upgrade version of GBIC module. Unlike GBIC with SC fiber optic interface, SFP is with LC interface and the main body size of SFP is only about half of GBIC so that it can save more space. SFP interfaces a network device mother board (for a router, switch, media converter or similar devices) to a fiber optic or copper networking cable. Meanwhile, SFP is a popular industry format supported by many network component vendors. It is designed to support SONET, Gigabit Ethernet, Fibre Channel, and other communications standards.

    Standardization
    The SFP transceiver is not standardized by any official standards body, but rather is specified by a Multi-source Agreement (MSA) among competing manufacturers. The SFP was designed after the GBIC interface, and allows greater port density (number of transceivers per cm along the edge of a mother board) than the GBIC, which is why SFP is also known as mini-GBIC. The related Small Form Factor transceiver is similar in size to the SFP, but is soldered to the host board as a through-hole device, rather than plugged into an edge-card socket.

    However, as a practical matter, some networking equipment manufacturers engage in vendor lock-in practices whereby they deliberately break compatibility with "generic" SFPs by adding a check in the device's firmware that will enable only the vendor's own modules. For example, in 2003 during a routine Internet Operating System (IOS) update on their Catalyst line of switches, Cisco added a feature that would cause the switch to reject optical modules that were not deemed "Cisco brand".

    Types of SFP Transceiver Modules
    SFP Transceivers are available with a variety of transmitter and receiver types, allowing users to select the appropriate transceiver for each link to provide the required optical reach over the available optical fiber type (e.g. multi-mode fiber or single-mode fiber).

    In the market, SFP transceiver modules are commonly available in several different categories:

    For multi-mode fiber, with black or beige extraction lever
    SX - 850 nm, for a maximum of 550 m at 1.25 Gbit/s (Gigabit Ethernet) or 150m at 4.25 Gbit/s (Fibre Channel)

    For single-mode fiber, with blue extraction lever
    LX - 1310 nm, for distances up to 10 km
    EX - 1310 nm,for distances up to 40 km
    ZX - 1550 nm, for distances up to 80 km
    BX - 1490 nm 1310nm, for distances up to 10 km
    1550 nm 40 km (XD), 80 km (ZX), 120 km (EX or EZX)

    For copper twisted pair cabling
    1000BASE-T - these modules incorporate significant interface circuitry and can only be used for Gigabit Ethernet, as that is the interface they implement. They are not compatible with (or rather: do not have equivalents for) Fibre channel or SONET.

    For WDM (Wavelength Division Multiplex) system
    BiDi SFP (Bidirectional SFP) for bi-directional traffic on a single fiber. Coupled with CWDM (Coarse Wavelength Division Multiplexing), these double the traffic density of fiber links
    CWDM and DWDM (Dense Wavelength Division Multiplexing) transceivers at various wavelengths achieving various maximum distances

     Applications of SFP Transceiver Module
    SFP is expected to perform at data speed of up to five gigabits per second (5Gbps), and possibly higher. Because SFP module can be easily interchanged, so electro-optical or fiber optic networks can be upgraded and maintained more conveniently than that with traditional soldered-in modules. Owing to its low cost, low profile and the ability to provide a connection to different types of optical fibers, SFP transceiver can result in a substantial cost savings, both in maintenance and in upgrading efforts. SFP transceiver is available with multi-mode single-mode fiber optics, allowing users to select the appropriate transceiver for each link in order to provide the required optical reach over the available optical fiber type. It is also available with copper cable interfaces, which allows a host device designed primarily for optical fiber communications to communicate over unshielded twisted pair networking cables. Modern optical SFP transceiver supports DDM (Digital Diagnostics Monitoring) functions, also known as DOM (Digital Optical Monitoring). This feature gives users the ability to monitor the real-time parameters of SFP transceiver, such as optical output power, optical input power, temperature, laser-bias current and transceiver supply voltage.

    Click on Link to buy Compufox SFP Transceivers

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  • Feds get huge response to request for IoT input

    By Sean Kinney   www.industrialiot5G.com

     

     

    More than 100 companies suggest ways U.S. government can help advance the IoT

    Many industry watchers feel the U.S. is slipping behind other countries, particularly Germany and China, in creating a unified national strategy for development of the Internet of Things or IoT. But federal leaders, in the early stages of involvement, reached out to the telecom industry for guidance.

    Back in April the National Telecommunications and Information Administration, a part of the U.S. Department of Commerce, issued a “request for comments on the benefits, challenges and potential roles for the government in fostering the advancement of the Internet of Things.”

    Two months later and the call for comment has been met in spades with more than 130 filings coming from a broad swath of telecom interests including carriers like AT&T, T-Mobile, Verizon and Vodafone; vendors including Nokia, Ericsson, Huawei and Samsung; and industry trade groups like the Wi-Fi Alliance, Wireless Infrastructure Association, the Open Connectivity Foundation and the GSMA.

    Here’s a full list of the respondents and their filings with NTIA. A review of some of the filings indicates a strong industry expectation that the rapid uptake of IoT will require global coordination and will likely create new markets while disrupting existing ones.

    Verizon representatives told NTIA: “To support this explosion of IoT devices, a robust and secure underlying communications network must serve as a foundation. That network requires both increased commercial spectrum and development of the underlying core infrastructure. We encourage all stakeholders to work together to ensure that these necessary building blocks for IoT development are available and accessible. To enable sufficient spectrum to power this new wave of connected innovation, private and public sectors must continue to cooperate, not only to develop more ways to effectively share spectrum, but also to provide federal users incentives to free up spectrum for commercial licensed and unlicensed use. As potentially billions of new IoT devices are deployed, they will drive data growth that – combined with the parallel growth in overall data usage by consumer devices – will require new commercial spectrum allocations to accommodate the unprecedented demands for more bandwidth. This includes spectrum necessary to support 5G, since 5G’s super-fast speeds and low latency will help facilitate new IoT use cases.”

    Ericsson commented: “In Ericsson’s view, 5G is the technology that will unleash the true potential of the Internet of Things. To support the IoT’s development, the government should unleash the resources that will ensure U.S. leadership in 5G by releasing more spectrum for commercial use. Through network slicing, 5G technology will allow a single infrastructure to meet the very different needs of Massive and Critical IoT devices – it will enable networks to handle the incredible increase in data from the billions of low energy, low data devices, while also providing very high reliability, availability and security for critical uses. We also encourage the government to support global standards and best practices and to allow industry to continue to innovate and coalesce around the most favorable IoT solutions.”

    And from the GSMA’s point of view: “The United States should forbear from regulating IoT and avoid reflexively extending legacy regulations designed for outdated technologies to the IoT…The U.S. government should support and promote industry alignment around interoperable, industry-led specifications and standards across the global IoT ecosystem…The U.S. government should promote the allocation of globally harmonized spectrum that can support IoT…The U.S. government should encourage industry to build trust into IoT devices. Existing laws and regulations, operating in tandem with self-regulatory regimes and best practices, will provide sufficient protection to consumers as the IoT develops…Finally, the U.S. government should engage on a bilateral and multilateral basis, as appropriate, to ensure that international IoT activities similarly encourage competition, investment, and innovation. Regulatory interference at this stage—from any source—could lead to fragmentation and impede innovation, inhibiting the IoT’s ability to reach its full potential to deliver benefits to consumers.”

     

     

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  • LSZH Fiber Optic Cables Tutorial

    Since the 1970s, the wire and cable industry has been using low-smoke, low-halogen materials in a number of applications. The objective was to create a wire and cable jacketing that was not only flame retardant but also did not generate dense, obscuring smoke and toxic or corrosive gases. Several notable fires over the years (such as the King's Cross Fire that killed 32 people in London's underground subway in 1987) increased the awareness of the role that wire and cable jacketing plays in a fire and contributed to a greater adoption of Low-Smoke Zero-Halogen (LSZH) cables.

    With an increase in the amount of cable found in residential, commercial and industrial applications in recent years, there is a greater fuel load in the event of a fire. Wire and cable manufacturers responded by developing materials that had a high resistance to fire while maintaining performance. Low-smoke, zero-halogen compounds proved to be a key materials group that delivered enhanced fire protection performance. Today, LSZH cables are being used in applications beyond the traditional transit, shipboard, military and other confined-space applications. This tutorial is provided to help you learn more about the LSZH fiber optic cables.

    What is LSZH Fiber Optic Cable?

    LSZH Fiber Optic Cable is a kind of fiber optic cable of which the jacket and insulation material are made of special LSZH materials. When these cables come in contact with a flame very little smoke is produced making this product ideal for applications where many people are confined in a certain place (office buildings, train stations, airports, etc.). While a fire may be very harmful in a building, the smoke can cause more damage to people trying to locate exits and inhalation of smoke or gases.

     

     



    Fiber optic cable insulation and jacket made from LSZH materials are free of halogenated materials like Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At), which are reported to be capable of being transformed into toxic and corrosive matter during combustion or decompositions in landfills.

    The most prominent characteristic of LSZH fiber optic cable is safety. LSZH fiber optic cables are used in public spaces like train and subway stations, airports, hospitals, boats and commercial buildings, where toxic fumes would present a danger in the event of a fire. Similarly, low-smoke property is also helpful. More people in fires die from smoke inhalation than any other cause. Using LSZH fiber optic cables which release low smoke and zero halogenated materials in these places would be really important to the safty of people.

    Applications of LSZH Fiber Optic Cables

    There is no doubt that the amount of fiber optic cables installed in buildings has been increasing as data communication proliferated. Central office telecommunication facilities were some of the first places that LSZH cables became common due to the large relative fuel load represented by wire and cable.

    Public Spaces like train stations, hospitals, school, high buidings and commercial centers where the pretection of people and equipment from toxic and corrosive gases is critical should apply LSZH fiber optic cable for the safty of people.

    Data Centers contain large amounts of cables, and are usually enclosed spaces with cooling systems that can potentially disperse combustion byproducts through a large area. In industrial facilities, the relative fuel load of cables will not be at the same level. Other materials burning may also contribute greater amounts of dangerous gases that outweigh the effect of the cables. There have been notable fires where cables burning contributed to corrosion (the Hinsdale Central Office fire is a famous example), but in some instances, better fire response techniques could have prevented this damage.

    Nuclear Industry is another area where LSZH cables have been and will be used in the future. Major cable manufacturers have been producing LSZH cables for nuclear facilities since the early 1990s. The expected construction of new nuclear plants in the U.S. in coming years will almost certainly involve some LSZH cable.

    One of the most important things to understand about LSZH fiber optic cable is that no two products are the same and that there are many factors that will define the suitability of the final product to its application. In fact, research done by a major pulling lubricant supplier tested 27 LSZH compounds and found a huge variation in physical properties. So even using material that meets the base requirements of one of the many specifications available may not result in the best material for the application. Understanding the goals, results and limits of these tests are key to finding the right product. In any case, the trend to consider environmental concerns with a greater weight relative to performance has increased and it can be generally stated that there is an enlarging market for fiber optic cables that can be demonstrated to be environmentally friendly.

    Conclusion

    When selecting or designing a fiber optic cable for any application, the operating enviroments where the fiber optic cable will be used, whether extreme or not, must be considered along with availability, performance, and price, among other things. And when the safety of humans and the enviroment is a consideration, along with high-performance and capability, then LSZH fiber optic cables are what you must specify.

    Warm Tips: When choosing LSZH fiber optic cables, factors such as the environment and price should be considered. An environmental factor such as the temperature of the installation could reduce the flexibility of the cable. Will the application be in an open area or confined? Will other flammable material be present? LSZH fiber optic cables also tend to be higher in cost. 

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