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Five Basics About Fiber Optic Cable

A fibre optic cable is a network cable that contains strands of glass fibres inside an insulated casing. They’re designed for high performance data networking and telecommunications. Fibre optic cable carry communication signals using pulses of light, faster than copper cabling which uses electricity. They are becoming the most significant communication media in data centre. Then how much do you know about them? This post serves as a guide for beginners.

Fibre Components

The three basic elements of a fibre optic cable are the core, cladding and coating. Core is the light transmission area of the fibre, either glass or plastic. The larger the core, the more light that will be transmitted into the fibre. The function of the cladding is to provide a lower refractive index at the core interface, causing reflection within the core. Therefore the light waves can be transmitted through the fibre. Coatings are usually multi-layers of plastics applied to preserve fibre strength, absorb shock and provide extra fibre protection.

Fiber Components

Fibre Type

Generally, there are two basic types of fibre optic cables: single mode fibre (SMF) and multimode fibre (MMF). Furthermore, multimode fibre cores may be either step index or graded index.

Single mode and multi-mode fiber-optic cables

Single mode optical fibre is a single strand of glass fibre with a diametre of 8.3 to 10 microns that has one mode of transmission. The index of refraction between the core and the cladding changes less than it does for multimode fibres. Light thus travels parallel to the axis, creating little pulse dispersion. It’s often used for long-distance signal transmission.

Step index multimode fibre has a large core, up to 100 microns in diametre. As a result, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternative pathways cause the different groupings of light rays to arrive separately at a receiving point. Consequently, this type of fibre is best suited for transmission over short distances.

Graded index fibres are commercially available with core diametres of 50, 62.5 and 100 microns. It contains a core in which the refractive index diminishes gradually from the centre axis out toward the cladding. The higher refractive index at the centre makes the light rays moving down the axis advance more slowly than those near the cladding.

Fibre Size

Single mode fibres usually has a 9 micron core and a 125 micron cladding (9/125µm). Multimode fibres originally came in several sizes, optimised for various networks and sources, but the data industry standardized on 62.5 core fibre in the mid-80s (62.5/125 fibre has a 62.5 micron core and a 125 micron cladding. It’s now called OM1). Recently, as gigabit and 10 gigabit networks have become widely used, an old fibre design has been upgraded. 50/125 fibre was used from the late 70s with lasers for telecom applications. 50/125 fibre (OM2) offers higher bandwidth with the laser sources used in the gigabit LANs and can allow gigabit links to go longer distances. Laser-optimised 50/125 fibre (OM3 or OM4) today is considered by most to be the best choice for multimode applications.

Basic Cable Design

The two basic cable designs are loose-tube cable, used in the majority of outside plant installations, and tight-buffered cable, primarily used inside buildings.

loose-tube-or-tight-buffered-cable

The modular design of loose-tube cables typically holds up to 12 fibres per buffer tube with a maximum per cable fibre count of more than 200 fibres. Loose-tube cables can be all dielectric or optionally armored. The modular buffer-tube design permits easy drop-off of groups of fibreers at intermediate points, without interfering with other protected buffer tubes being routed to other locations.

Tight-buffered cables can be divided into single fibre tight-buffered cables and multi-fibre tight-buffered cables. single fibre tight-buffered cables are used as pigtails, patch cords and jumpers to terminate loose-tube cables directly into opto-electronic transmitters, receivers and other active and passive components. While multi-fibre tight-buffered cables also are available and are used primarily for alternative routing and handling flexibility and ease within buildings.

Connector Type

While there are many different types of fibre connectors, they share similar design characteristics. Simplex vs. duplex: Simplex means 1 connector per end while duplex means 2 connectors per end. The following picture shows various connector styles as well as characteristics.

fiber cable connectors

Summary

Ultimately, what we’ve discussed is only the tip of the iceberg. If you are eager to know more about the fibre optic cable, either basics, applications or purchasing, please visit www.fs.com for more information.

OTDR Selection Guide

As the use of fibre in premise networks continues to grow, so do the requirements for testing and certifying it. An optical time-domain reflectometer (OTDR) is an electronic-optical instrument used to characterize optical fibres. It locates defects and faults, and determines the amount of signal loss at any point in an optical fibre by using the effects of Rayleigh scattering and Fresnel reflection. By sending a pulse of light into a fibre and measuring the travel time (“time domain”) and strength of its reflections (“reflectometer”) from points inside the fibre, it produces a characteristic trace, or profile, of the length vs. returned signal level on a display screen (as shown in the following picture). This article will describe the key specifications that should be considered when choosing an OTDR.

OTDR

When choosing an OTDR, it is important to select the specific OTDR performance and features according to the required specifications listed below.

Dynamic Range

The dynamic range of an OTDR determines how long of a fibre can be measured. The total optical loss that an OTDR can analyze is mainly determined by the dynamic range. The dynamic range affects the accuracy of the link loss, attenuation and far-end connector losses. Thus, having sufficient dynamic range is really important. The manufacturers specify dynamic range in different way. The higher the dynamic range, the longer the distance an OTDR can analyze.

Dead Zones

Dead zone refers to the space on a fibre trace following a Fresnel reflection in which the high return level of the reflection covers up the lower level of backscatter. To specify an OTDR’s performance, it is important to analyze the dead zone and ensure the whole link is measured. Dead zones are characterized as an event dead zone and an attenuation dead zone. Event dead zone refers to the minimum distance required for consecutive reflective events to be “resolved” (for example, to be differentiated from each other). Attenuation dead zone refers to the minimum distance required, after a reflective event, for the OTDR to measure a reflective or non-reflective event loss.

Resolution

There are two resolution specifications: loss (level), and spatial (distance). Loss resolution is the ability of the sensor to distinguish between levels of power it receives. When the laser pulse gets farther out in the fibre, the corresponding backscatter signal gets weaker and the difference between backscatter levels from two adjacent measurement points becomes larger. Spatial resolution is how close the individual data points that make up a trace are spaced in time (and corresponding distance). The OTDR controller samples the sensor at regular time intervals to get the data points. If it takes readings from the sensor very frequently, then the data points will be spaced close together and the OTDR can detect events in the fibre that are closely spaced.

Pass/Fail Thresholds

This is an important feature because a great deal of time can be saved in the analysis of OTDR traces if the user is able to set pass/fail thresholds for parameters of interest (such as splice loss or connector reflection). These thresholds highlight parameters that have exceeded a warning or fail limit set by the user and, when used in conjunction with reporting software, it can rapidly provide re-work sheets for installation/commissioning engineers.

Post-Processing and Reporting

Report generation could be another major time saver. For example, some OTDRs with specialized post-processing software allow fast and easy report generation, which might reduce the post-processing time up to 90 percent. These reports also include bidirectional analyses of OTDR traces and summary reports for high-fibre-count cables.

Your Applications and Users

Some OTDRs are designed to test long distance optical fibres and some others to test short distance optical fibres. For example, if you are to test premises fiber networks where short distance optical fibres are installed, OTDRs designed for testing long distance optical fibres are not suitable. Besides, knowing your users and the time it will cost is also necessary. Because some types of OTDRs are easy to use and some others are complicated to set up.

When selecting an OTDR, you’re supposed to take all the above factors into consideration. Fiberstore supplies a wide range of OTDRs available with various fibre types and wavelengths (including single-mode fibre, multi-mode fibre, 1310nm, 1550 nm, 1625 nm, etc). They also supply OTDRs of famous brands, such as JDSU MTS series, EXFO FTB series, YOKOGAWA AQ series and so on. OEM portable and handheld OTDRs are available as well.

Understanding Wavelengths in Fiber Optics

The light we are most familiar with is surely the light we can see. Our eyes are sensitive to light whose wavelength is in the range of about 400 nm to 700 nm, from the violet to the red. But for fiber optics with glass fibers, we use light in the infrared region which has wavelengths longer than visible light. Because the attenuation of the fiber is less at longer wavelengths. This text may mainly tell you what the common wavelengths used in fiber optics are and why they are used.

wavelength-nm

Wavelengths Definition

In fact, light is defined by its wavelength. It is a member of the frequency spectrum, and each frequency (sometimes also called color) of light has a wavelength associated with it. Wavelength and frequency are related. Generally, the radiation of shorter wavelengths are identified by their wavelengths, while the longer wavelengths are identified by their frequency.

Common Wavelengths in Fiber Optics

Wavelengths typically range from 800 nm to 1600 nm, but by far the most common wavelengths actually used in fiber optics are 850 nm, 1300 nm, and 1550 nm. Multimode fiber is designed to operate at 850 nm and 1300 nm, while single-mode fiber is optimized for 1310 nm and 1550 nm. The difference between 1300 nm and 1310 nm is simply a matter of convention. Both lasers and LEDs are used to transmit light through optical fiber. Lasers are usually used for 1310nm or 1550nm single-mode applications. LEDs are used for 850nm or 1300nm multimode applications.

wavelength-nm

Why Those Common Wavelengths?

As mentioned above, the most common wavelengths used in fiber optics are 850 nm, 1300 nm and 1550 nm. But why do we use these three wavelengths? Because the attenuation of the fiber is much less at those wavelengths. Therefore, they best match the transmission properties of available light sources with the transmission qualities of optical fiber. The attenuation of glass optical fiber is caused by two factors: absorption and scattering. Absorption occurs in several specific wavelengths called water bands due to the absorption by minute amounts of water vapor in the glass. Scattering is caused by light bouncing off atoms or molecules in the glass.

It is strongly a function of wavelength, with longer wavelengths having much lower scattering. From the chart below, we can obviously see that there are three low-lying areas of absorption, and an ever-decreasing amount of scattering as wavelengths increase. As you can see, all three popular wavelengths have almost zero absorption.

wavelength-nm

Conclusion

After reading this passage, you may know some basic knowledge of wavelengths in fiber optics. Since the attenuation of the wavelengths at 850 nm, 1300 nm, and 1550 nm are relatively less, they are the most three common wavelengths used in fiber optic communication. Fiberstore offer all kinds multimode and single-mode fiber optic transceivers which operate on 850 nm and 1310 nm respectively very well. For more information, please visit fs.com.

Do You Know About Mode Conditioning Patch Cord?

The great demand for increased bandwidth has prompted the release of the 802.3z standard (IEEE) for Gigabit Ethernet over optical fiber. As we all know, 1000BASE-LX transceiver modules can only operate on single-mode fibers. However, this may pose a problem if an existing fiber network utilizes multimode fibers. When a single-mode fiber is launched into a multimode fiber, a phenomenon known as Differential Mode Delay (DMD) will appear. This effect can cause multiple signals to be generated which may confuse the receiver and produce errors. To solve this problem, a mode conditioning patch cord is needed. In this article, some knowledge of mode conditioning patch cords will be introduced.

What Is a Mode Conditioning Patch Cord?

A mode conditioning patch cord is a duplex multimode cord that has a small length of single-mode fiber at the start of the transmission length. The basic principle behind the cord is that you launch your laser into the small section of single-mode fiber, then the other end of the single-mode fiber is coupled to multimode section of the cable with the core offset from the center of the multimode fiber (see diagram below).

mode conditioning patch cord

This offset point creates a launch that is similar to typical multimode LED launches. By using an offset between the single-mode fiber and the multimode fiber, mode conditioning patch cords eliminate DMD and the resulting multiple signals allowing use of 1000BASE-LX over existing multimode fiber cable systems. Therefore, these mode conditioning patch cords allow customers an upgrade of their hardware technology without the costly upgrade of their fiber plant.

Some Tips When Using Mode Conditioning Patch Cord

After learning about some knowledge of mode conditioning patch cords, but do you know how to use it? Then some tips when using mode conditioning cables will be presented.

    • Mode conditioning patch cords are usually used in pairs. Which means that you will need a mode conditioning patch cord at each end to connect the equipment to the cable plant. So these patch cords are usually ordered in numbers. You may see someone only order one patch cord, then it is usually because they keep it as a spare.
    • If your 1000BASE-LX transceiver module is equipped with SC or LC connectors, please be sure to connect the yellow leg (single-mode) of the cable to the transmit side, and the orange leg (multimode) to the receive side of the equipment. The swap of transmit and receive can only be done at the cable plant side. See diagram below.

mode conditioning patch cord

  • Mode conditioning patch cords can only convert single-mode to multimode. If you want to convert multimode to single-mode, then a media converter will be required.
  • Besides, mode conditioning patch cables are used in the 1300nm or 1310nm optical wavelength window, and should not be used for 850nm short wavelength window such as 1000Base-SX.

Conclusion

From the text, we know that mode conditioning patch cords really significantly improve the data signal quality and increase the transmission distance. But when using it, there are also some tips must be kept in mind. Fiberstore offer mode conditioning patch cords in all varieties and combinations of SC, ST, MT-RJ and LC fiber optic connectors. All of the Fiberstore’s mode conditioning patch cords are at high quality and low price. For more information, please visit fs.com.