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  • Mode Conditioning Patch Cable FAQ

    Mode Conditioning Patch Cable, or Mode Conditioning Patchcord (MCP), is duplex multimode patch cable that has a small length of single mode fiber at the start of the transmission length. The basic principle behind the MCP is to launch laser into the small section of single mode fiber. The other end of the single mode fiber is coupled to the multimode section of the MCP with the offset from the center of the multimode fiber.

    The MCP patch cord is required with -LX or  longwave Gigabit Ethernet Transceivers that use both single mode and multimode fibers. When launching into multimode fiber, the transceiver can generate multiple signals that causes Differential Mode Delay (DMD) which can severly limit transmission distances. A mode conditioning cable removes these multiple signals, eliminating problems at the receiver end.

    Using mode conditioning patch cable is not difficult, but there are also some notes that we should keep in mind. Here are some frequently-asked questions about mode conditioning patch cable for you.

    1. When & Where is Mode Conditioning Patch Cable Needed?

    • When using Gigabit 1000BASE-LX (or 10-Gigabit Ethernet 10GBASE-LRM and 10GBASE-LX4) equipment with existing multimode fiber cable plant. 1000BASE-LX is specified to work over a distance of up to 5 km over 9µm single-mode fiber. But 1000BASE-LX can also run over multi-mode fiber with a maximum segment length of 550 m.
    • For any link distance greater than 300 m, the use of a special launch mode conditioning patch cable may be required.


    2. How does Mode Conditioning Patch Cable Work?

    Answer: Mode conditioning patch cable launches the laser at a precise offset from the center of the multimode fiber. This causes the laser to spread across the diameter of the fiber core, reducing the effect known as Differential Mode Delay (DMD) which occurs when the laser couples onto only a small number of available modes in multimode fiber.


    3. Why should Mode Conditioning Patch Cable be ordered in Pairs?

    Answer: Mode conditioning patch cables are normally used in pairs. That means that one at each end to connect the equipment to the cable plant. So then these cables are usually ordered in even numbers. The usual reason why someone may order one cable is so they may keep it as a spare.


    4. How should Mode Conditioning Patch Cable be Connected?

    Answer: If Gigabit 1000BASE-LX switch is equipped with SC or LC connectors, the yellow leg (single mode) of the cable should be connected to the transmit side, and the orange leg (multimode) to the receive side of the equipment.


    5. Do all Multimode Fiber Types Require Mode Conditioning?

    Answer: Some manufacturers of the newer "high end" multimode fibers claim that that their premium line cables will not require mode conditioning.


    6. When holding the yellow Single Mode Cable up to a Light, the Light does not Come Through on the Other Side. Does This Indicate that it is a Defective Cable?

    Answer: The core of the single mode cable is so small than it does not gather enough light for it to be visible without a microscope on the other side. This is a normal condition for any single mode cable.


    7. Can Mode Conditioning Patch Cable be used to Convert Single Mode to Multimode or Vice Versa?

    Answer: No. Conversions of multimode and single mode require Fiber to Fiber Media Converters.

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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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  • Polarity and MPO Technology in 40/100GbE Transmission

    It have been proved that reducing cable diameters and increasing connection densities offered by fiber links would be extremely valuable during installation in constrained space, like data center, large enterprise equipment rooms, central office, etc. With the market turning to 40/100G transmission, to reduce congestion during cabling and make it easier to organize equipment cable runs, the network designers turns to MPO/MTP technology and components for today's duplex fiber transmission. Then, network designers face another challenge which is how to assure the proper polarity of these array connections using MPO/MTP components from end-to-end.

    Traditionally, a fiber optic link requires two fibers for full duplex communications. It is very important to ensure that the equipment on the link are connected properly at each end. However, when the link contains two or more fibers, maintain the correct polarity across a fiber network become more complex, especially when using multi-fiber MPO components for high data rate transmission. Luckily, pre-terminated MPO components adopt humanized design for polarity maintenance and the TIA 568 standard provides three methods for configuring systems to ensure that proper connections are made. This article will introduce polarity in MPO system and 40/100GbE polarization connectivity solutions in details.

    Polarity in MPO Components

    To maintain the correct polarity in MPO systems, the property of the components of MPO systems should be understood firstly. This part will introduce the basic components that are used in MPO system.

    MPO Connector: To understand the polarity in 40/100 GbE transmission, the key of MPO technology—MPO connector should be first introduced. MPO connector usually has 12 fibers. 24 fibers, 36 fibers and 72 fibers are also available. Each MTP connector has a key on one of the flat side added by the body. When the key sits on the bottom, this is called key down. When the key sits on the top, this is referred to as the key up position. In this orientation, each of the fiber holes in the connector is numbered in sequence from left to right and is referred as fiber position, or P1, P2, etc. A white dot is additionally marked on one side of the connector to denote where the position 1 is. (shown in the following picture) The orientation of this key also determines the MTP cable's polarity.

    MPO connector


    MPO Adapter: MPO (male) connectors are mated to MPO(female) connectors using a MPO adapter. As each MPO connector has a key, there are 2 types of MPO adapters:

    Type A—key-up to key-down. Here the key is up on one side and down on the other. The two connectors are connected turned 180° in relation to each other.Type B—key-up to key-up. Here both keys are up. The two connectors are connected while in the same position in relation to each other.


    MPO adapter


    MPO Cables: MPO trunk cable with two MPO connectors (male/female) on both side of the cable serves as a permanent link connecting the MPO modules to each other, which is available with 12, 24, 48, 72 fibers.

    MPO harness cable, which is terminated with a male/female connector on the MPO side and several duplex LC/SC connectors on the other side, provides a transition from multi-fiber cables to individual fibers or duplex connectors.

    MPO Cassette: Modular MPO cassette is enclosed unit that usually contains 12 or 24-fiber factory terminated fan-outs inside. It enables the user to take the fibers brought by a trunk cable and distribute them to a duplex cable with a MPO connector (at the rear) to the more common LC or SC interface (on the front side). The following is a MTP cassette with 6 duplex LC interface and a MTP connector.

    MTP cassette

    Three Cables for Three Polarization Methods

    The three methods for proper polarity defined by TIA 568 standard are named as Method A, Method B and Method C. To match these standards, three type of MPO truck cables with different structures named Type A, Type B and Type C are being used for the three different connectivity methods respectively. In this part, the three different cables will be introduced firstly and then the three connectivity methods.

    MPO Trunk Cable Type A: Type A cable also known as straight cable, is a straight through cable with a key up MPO connector on one end and a key down MPO connector on the opposite end. This makes the fibers at each end of the cable have the same fiber position. For example, the fiber located at position 1 (P1) of the connector on one side will arrive at P1 at the other connector. The fiber sequence of a 12 fiber MPO Type A cable is showed as the following:

    Type A cable

    MPO Trunk Cable Type B: Type B cable (reversed cable) uses key up connector on both ends of the cable. This type of array mating results in an inversion, which means the fiber positions are reversed at each end. The fiber at P1 at one end is mated with fiber at P12 at the opposing end. The following picture shows the fiber sequences of a 12 fiber Type B cable.

    Type B cable

    MPO Trunk Cable Type C: Type C cable (pairs flipped cable) looks like Type A cable with one key up connector and one key down connector on each side. However, in Type C each adjacent pair of fibers at one end are flipped at the other end. For example, the fiber at position 1 on one end is shifted to position 2 at the other end of the cable. The fiber at position 2 at one end is shifted to position 1 at the opposite end etc. The fiber sequence of Type C cable is demonstrated in the following picture.

    Type C cable

    Three Connectivity Methods

    Different polarity methods use different types of MTP trunk cables. However, all the methods should use duplex patch cable to achieve the fiber circuit. The TIA standard also defines two types of duplex fiber patch cables terminated with LC or SC connectors to complete an end-to-end fiber duplex connection: A-to-A type patch cable—a cross version and A-to-B type patch cable—a straight-through version.

    duplex patch cable

    The following part illustrates how the components in MPO system are used together to maintain the proper polarization connectivity, which are defined by TIA standards.

    Method A: the connectivity Method A is shown in the following picture. A type-A trunk cable connects a MPO module on each side of the link. In Method A, two types of patch cords are used to correct the polarity. The patch cable on the left is standard duplex A-to-B type, while on the right a duplex A-to-A type patch cable is employed.

    Method A

    Method B: in Connectivity Method B, a Type B truck cable is used to connect the two modules on each side of the link. As mentioned, the fiber positions of Type B cable are reversed at each end. Therefore standard A-to-B type duplex patch cables are used on both sided.

    Method B

    Method C: the pair-reversed trunk cable is used in Method C connectivity to connect the MPO modules one each side of the link. Patch cords at both ends are the standard duplex A-to-B type.

    Method C

    Upgrade to 40/100GbE With Correct Polarity

    The using of MPO/MTP connectors for 40/100G transmission is achieved with multimode fiber by transmitting multiple parallel 10G transmissions that will then be recombined when received. This method has been standardized. The following is to offer 40G transmission solution and 100G respectively.

    40G Transmission Connectivity

    The 40G transmission usually uses 12-fiber MPO/MTP connectors. There are eight lanes within twelve total positions being employed for transmitting and receiving signals. Looking at the end face of the MPO/MTP connector with the key on top, the four leftmost positions are used to transmit, the four rightmost positions are used to receive, the four in the center are unused. The following picture shows the optical lane assignments. (Tx stands for Transmit, Rx stands for Receive) This approach would transmit 40G using for parallel 10G lanes in each direction according to 40GBase-SR4.

    40G transmission

    100G Transmission Connectivity

    The 100G transmission over multimode requires a total of 20 fibers, 10 for transmitting and 10 for receiving. There are three options which is introduced as following:

    The first method is to use a 24-fiber MPO/MTP connector with the top center 10 positions allocated for receiving and the bottom 10 position allocated for transmitting,as shown in the following figure. This method is recommended by IEEE.

    100G transmission

    The second option is to use two 12-fiber MPO/MTP connectors side by side. The 10 positions in the center of the connector on the left are used for transmitting and the center 10 positions of the left are used for receiving.

    100G transmission

    The third way of 100G transmission also uses two 12-fiber MPO/MTP connectors, but it uses the stacked layout as showed in the following figure. The ten center positions of the top connector are used for receiving and the ten center position of the bottom are used for transmitting.

    100G transmission

    Understand Polarity in 40/100G

    Any transmit position should be connected to its own receive position. Here's an analogy to illustrate: Think of ball players. You have pitchers & catchers. For 10G transmission, Pitcher 1 needs to throw to Catcher 1, Pitcher 2 to Catcher 2 and so on. (showed on the left side of the following picture) For 40/100G, any pitcher can throw to any catcher.(showed on the right side of the following picture)

    10/40/100G polarity understanding

    But if you've got two catchers looking at each other as showed in the following picture, there isn't a whole lot happening.

    wrong polarity


    Network designer using MPO/MTP components to satisfy the increasing requirement for higher transmission speed, during which one of the big problems—polarity, can be solved by selecting the right types of MPO cables, MPO connectors, MPO cassette and patch cables. Consider the polarity method to be used and selecting the correct MPO/MTP components to support that methods, the proper solution for 40/100G transmission would be achieved with high density and flexibility and reliability.

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