How Can the Dispersion Be Compensated in Optical Communication?

In optical fiber communication, there are three factors that can degrade the optical signal during transmission. They are attenuation, dispersion and nonlinear effects. Among these problems, attenuation has been solved with the improvement of fiber manufacturing and the invention of erbium-doped fiber amplifier (EDFA). But dispersion affects the system most and is tougher to overcome compared with the other two factors. How can the dispersion be compensated in optical communication?

What Is Dispersion?

In fiber optical transmission, dispersion is defined as the pulse spreading in optical fiber. In order to explain this phenomenon in a plain way, the most familiar example used is the rainbow, in which dispersion causes the spatial separation of a white light into components of different wavelengths (different colors). Dispersion increases along the fiber length. It is a consequence of the physical properties of the medium within which the light is propagating.

Why Is Dispersion a Problem in Optical Transmission?

Dispersion is regarded as a problem in fiber optic links because it limits the potential bandwidth and transmission performance of a fiber. The overall effect caused by dispersion on the performance of a fiber optical system is called Intersymbol Interference (ISI). ISI occurs when the pulse spreading causes the output pulses of a system to overlap, rendering them undetectable, which is known as a form of distortion of signal.

Nowadays, in high bit rate long-haul transmission systems, dispersion becomes an even more critical aspect, because of the higher the dispersion, the lower the bit rate. We can see the effects caused by dispersion to bit rate according to the following figure. The higher the dispersion is, the longer bit interval must be used, which means fewer bits can be transmitted per unit of time, i.e. lower bit rate.

Two Types of Dispersion in Optical Fibers

Dispersion is generally divided into two categories: chromatic dispersion and modal dispersion, the former of which occurs in all types of fibers while the latter of which occurs only in multimode fibers.

Chromatic Dispersion

Chromatic dispersion (CD) is due to the fact that different wavelengths are propagated at different speeds in the fiber because the index of refraction of glass fiber is a wavelength-dependent quantity. CD can be divided into two parts: material dispersion and waveguide dispersion.

CD

Material dispersion is due to the frequency dependency of a material to waves, i.e. The refractive index of the core varies as a function of the wavelength.

Waveguide dispersion is due to the physical structure of the waveguide. Waveguide dispersion has little effect in simple step-index fibers (large mode areas), but it will be significant in fibers with complex index profiles (small mode areas).

Modal Dispersion

Modal dispersion occurs in multimode fiber when the pulse spreading consists of different modes, caused by the time delay between lower-order modes and higher-order modes. Modal dispersion limits the bandwidth of multimode fibers.

Polarization mode dispersion (PMD) in one special form of modal dispersion. It occurs due to random imperfection and asymmetries that cause two different polarization modes, which normally travel at the same speed, to travel at different speeds. It will lead to rotation of polarization orientation along the fiber.

PMD

Different Dispersion Compensation Techniques

Dispersion compensation or dispersion management is the process of designing the fibers and compensating elements in the transmission path to control the total dispersion of a system to a small number. The most commonly employed techniques are as follows:

Dispersion-Shifted Fiber (DSF)

Dispersion-shifted fiber (DSF) is used to compensate dispersion at 1550nm wavelength (zero-dispersion wavelength). It is a type of single-mode optical fiber with a core-clad index profile tailored to shift the zero-dispersion wavelength from the natural 1300 nm in silica-glass fibers to the minimum-loss window at 1550 nm. But when it is used in wavelength division multiplexing (WDM) systems, DSFs have other effects such as high four wave mixing (FWM) and cross phase mixing (CPM). Thus non-zero dispersion shifted fiber is used.

Non-zero Dispersion Shifted Fiber (NZDSF)

Non-zero dispersion shifted fiber is designed to overcome the problems of DSF. Because of the small non-zero amount of dispersion that occurs in the 1550nm window, the FWM and CPM can be minimized so that it can be used in WDM systems.

Dispersion Compensation Fiber (DCF)

Dispersion compensation fiber is a special type of fiber that has large negative dispersion value equal to the transmitting fiber. Typically DCF dispersion can be in the range of -80 ps/(nm∙km). It can actually reverse the effects of chromatic dispersion suffered by the 1550nm signals that traverse standard single-mode fiber. It can be used as pre-compensation, post-compensation or in-line compensation fiber. One drawback of DCF is that it has high insertion loss. And because the length of DCF is determined by the length of the transport length, it leads to bulky terminal components together with high insertion loss. In addition, since DCF is only efficient for single wavelength, it is not a preferable choice for dispersion compensation in DWDM systems.

Fiber Bragg Grating (FBG)

Fiber Bragg grating (FBG) is used for chromatic dispersion compensation by recompression of the dispersed optical signals. It presents low insertion loss. Dispersion compensation using FBG is based on the wavelength-specific periodic variation by the way of chirped FBG. Together with a standard optical circulator, a highly effective dispersion-compensating module (DCM) can be achieved. Two types of FBG-based dispersion compensators are available in the market: multichannel (or channelized) and continuous. The latter type provides channel-spacing-specifc compensation. The former type provides continuous compensation through out the C band and L band, which is suitable for high bit rate systems.

Summary

In order to keep the bandwidth and transport performance at desired level, it is important to solve the dispersion problem in fiber optical communication systems. Thanks to the development of dispersion compensation technology, we are now able to employ different approaches to avoid the deterioration in performance of communication systems. And with the advance of dispersion management, more suitable dispersion modules will be introduced in the future.

Getting to Know Optical Circulator

The utilization of optical circulator starts from the 1990s, and now it has become one of the important elements in advanced optical communication systems. Similar to the function of an electronic circulator, an optical circulator is used to separate optical signals that travel in opposite directions in an optical fiber. Optical circulator has been widely applied to different fields, such as telecom, medical and imaging industries. Are you ready to know more about this optical device? This article will take you to explore the secrets of optical circulator.

optical-circulator

What Is Optical Circulator?

An optical circulator is built to pass light from one optical fiber to another. It is a non-reciprocal device routing the light based upon the direction of light propagation. Both optical circulator and optical isolator can be used to move light forward. However, there is typically more loss of light energy in the optical isolator than in the optical circulator. Optical circulator usually consists three ports: two ports are used as input ports and one port as output port. A signal is transmitted from port 1 to port 2, and another signal is transmitted from port 2 to port 3. Finally a third signal can be transmitted from port 3 to port 1. Many applications only require two, so they can be built to block any light that hits the third port.

optical-circulator-structure

Technologies of Optical Circulator Components

An optical circulator includes the components of Faraday rotator, birefringent crystal, waveplate, and beam displacer. The Faraday rotator uses the Faraday effect, which is a phenomenon that the polarization plane of an electromagnetic (light) wave is rotated in a material under a magnetic field applied parallel to the propagation direction of the lightwave. The light propagation in the birefringent crystal depends on the polarization state of the light beam and the relative orientation of the crystal. The polarization of the beam can be changed or the beam can be split into two beams with orthogonal polarization states. Waveplate and beam displacer are two different forms of birefringent crystal. A waveplate can be made by cutting a birefringent crystal to a particular orientation so that the optic axis of the crystal is in the incident plane and is parallel to the crystal boundary. Beam displacer is used to split an incoming beam into two beams with orthogonal polarization states.

Categories of Optical Circulator

According to polarization, optical circulator can be divided into polarization-dependent optical circulator and polarization-independent optical circulator. The former is used for the light with a particular polarization state, and the latter is not restricted to the polarization state of a light. Most of the optical circulators employed in fiber optic communications are designed to be polarization-independent.

According to functionality, optical circulator can be classified into full circulator and quasi-circulator. As mentioned before, full circulator makes full use of all ports in a complete circle. Light passes through from port 1 to port 2, port 2 to port 3, and port 3 back to port 1. About quasi-circulator, light passes through all ports sequentially but light from the last port is lost and cannot be transmitted back to the first port. For most applications, a quasi-circulator is enough.

Several Applications of Optical Circulator
  • Duplex Transmitter/Receiver System: Optical circulators can be used to enable 2-way transmission along a single fiber. Transmitter 1 sends signal through Port 1 of Circulator 1 and through the fiber to Port 2 of Circulator 2 so that it is directed to Receiver 2. The signal from Transmitter 2 follows the opposite path to Receiver 1.

duplex-transmitte-receiver-system

  • Double Pass Erbium Doped Amplifier: This technique allows high gain amplification of a signal through an erbium doped fiber amplifier. The signal passes through optical circulator and optical amplifier, returns from the fiber optic reflector and passes through the amplifier again. This amplified signal is directed through the return port.

double-pass-erbium-doped-amplifier

  • Wave Division Multiplexing System: Optical circulators in conjunction with Bragg gratings allow specific wavelengths to be reflected and sent down different paths.

wave-division-multiplexing-system

Conclusion

From this article, you may have a basic impression about optical circulator. It is an efficient and economical solution to use optical circulator for directing light signal with minimum loss. If you are interested in the optical circulator products, welcome to visit fs.com for more information.

Fiber Optic Cables Bring Great Communication Services

Fiber optic technology has paved the way for a new type of technology and its effects on home services. Everything from TV, phone, and even internet services have been positively altered due to the advancements brought on by fiber optic technology. With internet services in particular, this new form of connection allows for the internet to go in a direction that it has not always been able to go. Fiber Optic Internet is a step forward toward an unstoppable internet connection.

Optical communication motivation began with the invention of the laser in the early 1960s. Since then, the technology has evolved at the speed of light. Optical technology has advanced so fast that it has become the information conduit of the world. The transmission of data, voice and media is distributed at the speed of light over a mesh of glass fibers that span thousands of kilometers throughout the world. Fiber optic cables have developed to various types, mutimode fiber cable and single mode fiber cable are the basical one.

Multimode fiber allows multiple rays/modes to couple and propagate down the fiber at the same time. Large core fiber is attractive due to the ease in which light can be coupled into the fiber, greatly reducing transmitter design and packaging costs. Multimode fiber is sensitive to dispersion, which tends to limit an optical system’s distance and bandwidth. Multimode fiber can be stepped-refractive-index-profile, or graded-index-profile. While, single-mode fiber has an advantage of higher capacity/bandwidth and is also much less sensitive to the effects of dispersion than multimode fiber. It is also possible to incorporate wavelength division multiplexing techniques to further increase the transmission capacity of a single-mode fiber.

Fiber Optic Internet creates a different kind of online user experience as compared to other types of connections. No longer do users worry about losing connectivity during operations because of the quality of the transmission. Fiber optic technology also allows users to eliminate waiting for pages to load, messages to send, and images to appear. An overall more comfortable surfing experience is provided by fiber optic technology. With the increased popularity of social media sites and live content sites, a fiber optic connection allows users to more completely engage and interact. This type of internet connection is more able to meet the increasing demands of today’s internet-heavy society.

All fiber optic cable manufacturers diverse fiber cables but their item literatures should be cautiously studied so as to assess which variety of fiber cables they specialize in. Want to buy fiber optic cable, recommend you FiberStore, who provdes really high quality cables with reasonable price.

The Importance of Reliable Date Cabling

It is hard to imagine a world without the internet as it is so important in the modern business environment. We cannot stress enough the importance of reliable networking cabling. Some recent studies vindicated our evangelical approach to data cabling:

Data cabling typically account for less than 10 percent of the total cost of the network infrastructure.

The life span of the typical cabling system is upward of 16 years. Cabling is likely the second most long-lived asset or have. The first is the shell of the building.

Nearly 70 percent of all network-related problems are due to poor cabling techniques and cable-component problems.

Note: If you have installed the proper category or grade of cable, the majority of cabling problems will usually be related to patch cables, connectors, and termination techniques. The permanent portion of the cable such as the part of the wall will not likely be a problem unless it was damaged during installation.

Of course, these were facts that we already knew from our own experience. We have spent countless hours troubleshooting cabling systems that were nonstandard, badly designed, poor documented, and shoddily installed. We have seen much money wasted on the installation of additional cabling and cabling infrastructure support that should have been part of the original installation. No mater how you will think about it, cabling is the foundation of the network and it must be reliable!

The best way to ensure that your networking needs are met is by checking that the person installing the data cabling is registered with a cable registrar such as ACRS or one of the other five registrars in Australia. You should also make sure that they have the appropriate experience and qualifications in their background, possibly determining this via recommendations or terminations.

Another good thing to do is make sure you get two or three quotes in order to create an accurate idea of pricing, as some installed quote ridiculously high-but others quote too low indicating that they are using inferior quality products. Because the installation has been quoted cheaply, does not mean it’s a good idea. Properly priced instances are more likely to have the quality installation products from good fibre optic cable manufacturers.

Besides, installation can often have a warranty, usually between five and twenty years. The better the products, the longer the warranty period as a rule.

Costs that result from poorly planned and poorly implemented cabling systems can be staggering. One company that had recently moved into a new office space used the existing cabling, which was supposed to be Cat 5 cables. Almost immediately, 100Mbps Ethernet network users reported intermittent problems. These problems include exceptionally slow access time when reading e-mail, saving documents, and using the sales database. Other users reported that applications running under Windows 98 and Windows NT were locking up, which often caused them to have to reboot their PC.

After many months of networking annoyances, the company finally had the cable runs tested. Many cables did not even meet the minimum requirements of a Category 5 installations, and other cabling runs were installed and terminated poorly.

Contrary to most peoples thinking, faulty cabling cause the type of intermittent problems that the aforementioned company experienced. In additional to being vulnerable to outside interference from eletric-motors, fluorescent lighting, elevators, cellular phones, copies, and microwave ovens, faulty cabling can cause intermittent problems because of other reasons such as substandard components (patch panel, connectors, and cable) and poor installation techniques. LSZH cables are needed some safety advocates such as the plenum space.

Robert Metcalfe helped coin the term drop-rate magnification. Drop-rate magnification describes the high degree of network problems caused by dropping a few packets. Medicare estimates that a 1 percent drop in Ethernet packets can correlate to an 80 percent drop in throughput. Modern network protocols that send multiple packets and expect only a single acknowledgement are especially susceptible to drop rate magnification, as a single dropped packet may cause an entire stream of packets to be retransmitted.

Click to know fiber optic cable price per foot

Understanding Fiber Optic Based Light Source

Each piece of active electronics will have a variety of light sources used to transmit over the various types of fiber. The distance and bandwidth will vary with light source and quality of fiber. In most networks, fiber is used for uplink/backbone operations and connecting various buildings together on a campus. The speed and distance are a function of the core, modal bandwidth, grade of fiber and the light source, all discussed previously. Light sources of the fiber light source are offered in a variety of types. Basically there are two types of semiconductor light sources available for fiber optic communication – The LED sources and the laser sources.

Using single mode fiber for short distances can cause the receiver to be overwhelmed and an inline attenuator may be needed to introduce attenuation into the channel. With Gigabit to the desktop becoming commonplace, 10Gb/s backbones have also become more common. The SR interfaces are also becoming common in data center applications and even some desktop applications. As you can see, the higher quality fiber (or laser optimized fiber) provides for greater flexibility for a fiber plant installation. Although some variations ( 10GBase-LRM SFP+ and 10GBASE-LX4) support older grades of fiber to distances 220m or greater, the equipment is more costly. In many cases, it is less expensive to upgrade fiber than to purchase the more costly components that also carry increased maintenance costs over time.

Light sources of the fiber light source are offered in a variety of types. Basically there are two types of semiconductor light sources available for fiber optic communication – The LED sources and the laser sources.

In fiber-optics-based solution design, a bright light source such as a laser sends light through an optical fiber, called laser light source . Along the length of the fiber is an ultraviolet-light-treated region called a “fiber grating.” The grating deflects the light so that it exits perpendicularly to the length of the fiber as a long, expanding rectangle of light. This optical rectangle is then collimated by a cylindrical lens, such that the rectangle illuminates objects of interest at various distances from the source. The bright rectangle allows line scan cameras to sort products at higher speeds with improved accuracy.

The laser fiber-based light source combines all the ideal features necessary for accurate and efficient scanning: uniform, intense illumination over a rectangular region; a directional beam that avoids wasting unused light by only illuminating the rectangle; and a “cool” source that does not heat up the objects to be imaged. Currently employed light sources such as tungsten halogen lamps or arrays of light-emitting diodes lack at least one of these features.