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Five Basics About Fiber Optic Cables

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

Fiber Components

The three basic elements of a fiber optic cable are the core, cladding and coating. Core is the light transmission area of the fiber, either glass or plastic. The larger the core, the more light that will be transmitted into the fiber. 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 fiber. Coatings are usually multi-layers of plastics applied to preserve fiber strength, absorb shock and provide extra fiber protection.

Fiber Components

Fiber Type

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

Single mode and multi-mode fiber-optic cables

Single mode optical fiber is a single strand of glass fiber with a diameter 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 fibers. Light thus travels parallel to the axis, creating little pulse dispersion. It’s often used for long-distance signal transmission.

Step index multimode fiber has a large core, up to 100 microns in diameter. 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 fiber is best suited for transmission over short distances.

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

Fiber Size

Single mode fibers usually has a 9 micron core and a 125 micron cladding (9/125µm). Multimode fibers originally came in several sizes, optimized for various networks and sources, but the data industry standardized on 62.5 core fiber in the mid-80s (62.5/125 fiber 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 fiber design has been upgraded. 50/125 fiber was used from the late 70s with lasers for telecom applications. 50/125 fiber (OM2) offers higher bandwidth with the laser sources used in the gigabit LANs and can allow gigabit links to go longer distances. Laser-optimized 50/125 fiber (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 fibers per buffer tube with a maximum per cable fiber count of more than 200 fibers. Loose-tube cables can be all dielectric or optionally armored. The modular buffer-tube design permits easy drop-off of groups of fibers at intermediate points, without interfering with other protected buffer tubes being routed to other locations.

Tight-buffered cables can be divided into single fiber tight-buffered cables and multi-fiber tight-buffered cables. single fiber 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-fiber 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 fiber 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 fiber optic cable, either basics, applications or purchasing, please visit www.fs.com for more information.

Optical Fiber Selection for Network Interconnection

The emergence of Data Centers, Storage Area Networks and other computing applications drives the needs for ultra-high speed data interconnections and structured cabling. The interconnect media choices include wireless technology, copper cable and optical fiber cable. Fiber cable offers the highest bandwidth and supports the highest data rates. There are single-mode and multimode fiber types. Different types of fiber connect with fiber optic transceivers resulting in different performances and costs. So it’s important for the network designers to understand the fiber types and select the right fiber and corresponding fiber optic transceivers for network interconnection.

Optical Fiber Types

There are three main types of optical fiber suitable for network interconnection use:
9/125μm Single-mode fiber
50/125μm multimode fiber
62.5/125μm multimode fiber

optical-fiber-types

The above numbers respectively mean the diameter of the glass core where the light travels and outside glass cladding diameter which is almost the same to most fiber types. So the difference of each fiber type is caused by the core diameter. It has great impact on system performance and system cost when balanced against network application needs. Two primary affected factors are attenuation and bandwidth.

Factors Affected by the Fiber Core Diameter

Attenuation is the reduction of signal power, or loss, as light travels through an optical fiber. Fiber attenuation is measured in decibels per kilometer (dB/km). The higher the attenuation, the higher rate of signal loss of a given fiber length. Single-mode fibers generally operate at 1310 nm (for short range) while multimode fibers operate at 850 nm or 1300 nm. Attenuation is not usually considered to be the main limiting factor in short rang transmissions. But it can cause big differences in high speed network such as 100Gb/s.

Bandwidth means the carrying capacity of fiber. For single-mode fiber, the modal dispersion can be ignored since its small core diameter. Bandwidth behavior of multimode fibers is caused by multi-modal dispersion during the light traveling along different paths in the core of the fiber. It has an influence on the system performance and data rate handling. Multimode fiber uses a graded index profile to minimize modal dispersion. This design maximizes bandwidth while maintaining larger core diameters for simplified assembly, connectivity and low cost. So manufacturers start to develop higher-performance multimode fiber systems with higher bandwidth.

System Costs: Single-mode and Multimode Fibers

A fiber optic transceiver usually consists the optical light sources, typically LED–light emitting diode and optical receivers. Since the core diameter size and primary operating wavelengths of single-mode fiber and multimode fiber are different, the associated transceiver technology and connectivity will also be different. So is the system cost.

To utilize the single-mode fibers generally for long distance applications (multi-kilometer reach), transceivers with lasers such as SFPP-10GE-LR (an SFP+ 1310nm 10 km transceiver supporting single-mode fibers) that operate at longer wavelengths with smaller spot-size and narrower spectral width. But these kinds of transceivers need higher precision alignment and tighter connector tolerance to smaller core diameters. Thus, it causes higher costs for single-mode fiber interconnections. To lower the cost, manufacturers produce transceivers based on VCSEL (vertical cavity surface emitting laser), for example, 10G-SFPP-SR (an SFP+ 850nm 300m transceiver supporting multimode fibers), which are optimized for use with multimode fibers. Transceivers applying low cost VCSEL technology to develop for 50/125μm multimode fibers, take advantage of the larger core diameter to gain high coupling efficiency and wider geometrical tolerances. OM3 and OM4 multimode fibers offer high bandwidth to support data rates from 10Mb/s to 100Gb/s.

Conclusion

Optical fiber is an easily-installed medium that is immune to electromagnetic interface and is also more efficient in terms of power consumption. What’s more, fiber optic cable can save space and cost with higher cabling density and port density over copper cabling. For single-mode fiber and multimode fiber, each one has its advantages and disadvantages. Network designers should better select the right fiber type and related fiber optic transceivers according to specific situations for higher system performance. Of course, cost is another important factor to be considered.

OTDR Selection Guide

As the use of fiber 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 fibers. It locates defects and faults, and determines the amount of signal loss at any point in an optical fiber by using the effects of Rayleigh scattering and Fresnel reflection. By sending a pulse of light into a fiber and measuring the travel time (“time domain”) and strength of its reflections (“reflectometer”) from points inside the fiber, 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 fiber 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 fiber 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 fiber, 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 fiber 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-fiber-count cables.

Your Applications and Users

Some OTDRs are designed to test long distance optical fibers and some others to test short distance optical fibers. For example, if you are to test premises fiber networks where short distance optical fibers are installed, OTDRs designed for testing long distance optical fibers 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 fiber types and wavelengths (including single-mode fiber, multi-mode fiber, 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.

Link Budget Evaluation Over SMF and MMF

Evaluating a link budget is equivalent to calculating the total loss suffered by a transmitted signal along fiber channels with the minimum receiver power to maintain normal operation. Calculating the link budget helps network architects to identify the feasibility of a physical-layer deployment.

Optical fibers come in several different configurations, each ideally suited to a different use or application. Early fiber designs that are still used today include single-mode fiber (SMF) and multimode fiber (MMF). And the most common optical communication data links include point-to-point transmission, WDM and amplified transmission. This article depicts the rules to be applied in order to evaluate link budget of these optical transmissions over SMF and MMF.

Link Budget for Point-to-Point Transmissions over Multimode Fibers

In this first case, the rule is fairly simple. A few parameters need to be taken into account:
● The minimum transmit power guaranteed (minTx), expressed in dBm
● The minimum receive power required (minRx), expressed in dBm
● The loss of optical connectors and adapters (L), expressed in dB
● The number of connectors and adapters (n)
● The normalized fiber loss (FL), expressed in dB/km
● The reach or distance to be achieved (d), expressed in km
With these parameters, the link budget (LB) expressed in dB is given as follows:
● (LB) = (minTx) – (minRx)
This value needs to be compared to the total loss (TL) suffered by the transmitted signal along the given link, and expressed in dB:
● (TL) = n*(L) + d*(FL)
If LB is greater than TL, then the physical deployment is theoretically possible.
In these calculations n is at least equal to 2 since there are a minimum of 2 connectors at each end, L is typically around 0.5 to 1 dB, and FL is of about 1 to 1.5 dB per km.

Link Budget for Point-to-Point Transmissions over Single-mode Fibers

At first, you need to know that the lasers deployed in optical communications typically operate at or around 850 nm (first window), 1310 nm (second window), and 1550 nm (third and fourth windows). In this second case, the calculations are exactly similar to the previous case. Only the numerical values will differ. For single-mode point-to-point transmissions, n is at least equal to 2, L is typically around 0.3 to 0.5 dB, and FL is of about 0.4 dB per km in the second window and about 0.25 dB per km in the third window.

The following drawings show the power budget of a 2km hybrid multimode/singlemode link with 5 connections (2 connectors at each end and 3 connections at patch panels in the link) and one splice in the middle.

power budget

Link Budget for WDM and Amplified Transmissions over Single-mode Fibers

In the case of WDM transmissions, passive modules are used to multiplex and demultiplex various wavelengths respectively before and after the signal propagates along the fiber channel. These passive modules introduce additional insertion losses suffered by the signal transmitted.

Additionally, the signal may be amplified and compensated for dispersion, and in this case, the amplifier gain and the dispersion compensation unit’s loss need to be taken into account. Dispersion and OSNR (optical signal noise ratio) penalties suffered by the receiver shall be considered as well.

Therefore all the parameters needed for a proper link budget evaluation are:
● The minimum transmit power guaranteed (minTx), expressed in dB/m
● The minimum receive power required (minRx), expressed in dB/m
● The loss of optical connectors and adapters (L), expressed in dB
● The number of connectors and adapters (n)
● The normalized fiber loss (FL), expressed in dB/km
● The reach or distance to be achieved (d), expressed in km
● The loss of passive add/drop modules (A and D), expressed in dB
● The gain of the amplifier (G), expressed in dB
● The penalty suffered by the receiver (P), expressed in dB
● The loss of a dispersion compensation unit (DCU), expressed in dB With these parameters, (LB) is given as for previous cases:
● (LB) = (minTx) – (minRx)
And the total loss is expressed as follows:
● (TL) = n*(L) + d*(FL) + (A) + (D) – (G) + (DCU) + (P)
Here again, if LB is greater than TL, then the physical deployment is feasible. Please note that for simplicity, only one amplifier, one dispersion compensation unit, and one set of add/drop modules are considered in this example. If more devices are planned to be deployed, their loss or gain should be added or subtracted accordingly in order to calculate TL.

Link budget is a way of quantifying the link performance. And the performance of any communication link depends on the quality of the equipment being used. Thus, when evaluating a link budget, you are supposed to consider the types of applications, the reach to be achieved, as well as the types of optical fibers deployed. For more information about fiber optical link products, please visit FS.COM.

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.

What Is Single Mode Fiber?

It is obvious that the fiber-optics are steadily replacing copper wire in telecommunications. The main difference between fiber optic systems and copper wire systems is how they transmit information. Fiber optic cable transmit signals by using light pulses instead of electronic pluses. The light pulses enable fiber optic cable to transmit more data with further distance and fewer signal loss. So fiber optical cables are now widely used for television services, university campuses, office buildings or other long-distance applications. Here will depict one kind of optical fiber: single mode fiber.

Introduction to Single Mode Fiber

Single mode fiber is one kind of fiber which is a single stand of glass fiber deigned to carry light only directly down the transverse mode. Its core diameter is 8.3 to 10 microns. Compared to multimode fiber whose diameter is 50 to 100 microns, single mode fiber has a smaller core, which allows fewer signals to be transmitted simultaneously on a fiber. So the light can only travel on a single path not multimode patch. With only one mode of transmission, single mode fiber has no intermodal dispersion. Multimode fiber transmit signals through multimode paths for its larger core. Those multimode fiber can cause signal distortion at the receiving end in long-distance transmission, making signals unclear and incomplete. In addition, the index of refraction between the core and cladding of single mode changes less than it does for multimode fiber, creating little dispersion.

fiber_mode

Conditions for Launching Light to Single Mode Fiber

As one type of optical fiber, single mode fiber carry lights to transmit information. But what should we take care of when launching light into single mode fiber?

Efficiently launching light into a single mode fiber requires the transverse complex amplitude profile of that light at the fiber’s input ends matches that of the guided mode. This means the light source have a high beam quality and a focus at the fiber’s input end and the light should be precisely aligned. A light into a single mode fiber needs well designed mechanical conditions. Taking lasers for example, the laser beam should has the correct size, allowing to precisely align and keep the fixed the focusing lens. It is usually easy to align correctly at the target position, but angular alignment is more critical.

Applications of Single Mode Fiber

Single mode fiber can achieve a high beam quality of the output together with WDM technology and optical amplifiers. In the optical links, single mode fiber can be used to connect different components such as connectors by fusion splicing. Due to little signal dispersion, single mode fiber is widely used for long-distance transmission. Optical media converters are used to do the fiber to fiber conversion to connect two different transmission modes, making single mode fiber efficient in long-distance transmission.

LC-LC Single-mode Fiber Patch Cable

For more information about single mode fiber cable, please visit WWW.FS.COM. Various single mode fiber solutions for you to choose.