Why Is Single-mode Fiber So Attractive?

Since the invention of optical fibers in the early 1970s, the use of and demand for optical fiber today are quite numerous. With the explosion of information traffic due to the Internet, electronic commerce, computer networks, multimedia, voice, data, and video, the need for large amount of signal transmission is paramount. Fiber optics has proven to be the best solution. Single-mode fiber is one of optical fibers which is designed for the transmission of a single ray or mode of light as a carrier and is used for long-distance signal transmission.
A typical single-mode fiber has four parts: the core, cladding, buffer and jacket. In the center, it’s called the core where the light is “guided” down in the fiber. The core is surrounded by an optical material called the “cladding” that traps the light in the core using an optical technique called “total internal reflection”. The core and cladding are usually made of ultra-pure glass. The fiber is coated with a protective plastic covering called the “primary buffer coating” that protects it from moisture and other damage. More protection is provided by the cable which has the fibers and strength members inside an outer covering called a “jacket”.
Single-mode fiber has characteristics of low dispersion, high frequency and high bandwidth. First, the high dispersion rate will make the signal worse during its transmission over long distances. When the light travels through the core, the core doesn’t retain all of the light. As a result, the dispersion will be caused when some of the light travels along the fiber cladding. Single-mode fiber could erase the dispersion. Second, the frequency at which the fiber optic signal will be transmitted can influence the signal transmission distance. The higher the frequency, the greater distance the system will be able to support. Single-mode systems have 1300 and 1550 nanometers. Third, bandwidth of fiber is described in MHz per kilometer. Typical fiber bandwidth of single-mode fiber has an inherently higher bandwidth and can reach thousands of MHz per km.
Due to the special favorable characteristics of single-mode fiber, it could transmit data with high speed over long distances. And it’s usually used for connections over large areas, such as college campuses and cable television networks. So that’s why single-mode fiber is attractive especially for long distance signal transmission.

How to choose the right Single-mode Fiber?

by http://www.fiber-mart.com

As we all know, multimode fiber is usually divided into OM1, OM2, OM3 and OM4. Then how about single-mode fiber? In fact, the types of single-mode fiber seem much more complex than multimode fiber. There are two primary sources of specification of single-mode optical fiber. One is the ITU-T G.65x series, and the other is IEC 60793-2-50 (published as BS EN 60793-2-50). Rather than refer to both ITU-T and IEC terminology, I’ll only stick to the simpler ITU-T G.65x in this article. There are 19 different single-mode optical fiber specifications defined by the ITU-T.
Each type has its own area of application and the evolution of these optical fiber specifications reflects the evolution of transmission system technology from the earliest installation of single-mode optical fiber through to the present day. Choosing the right one for your project can be vital in terms of performance, cost, reliability and safety. In this post, I may explain a bit more about the differences between the specifications of the G.65x series of single-mode optical fiber families. Hope to help you make the right decision.
EThe ITU-T G.652 fiber is also known as standard SMF (single-mode fiber) and is the most commonly deployed fiber. It comes in four variants (A, B, C, D). A and B have a water peak. C and D eliminate the water peak for full spectrum operation. The G.652.A and G.652.B fibers are designed to have a zero-dispersion wavelength near 1310 nm, therefore they are optimized for operation in the 1310-nm band. They can also operate in the 1550-nm band, but it is not optimized for this region due to the high dispersion. These optical fibers are usually used within LAN, MAN and access network systems. The more recent variants (G.652.C and G.652.D) feature a reduced water peak that allows them to be used in the wavelength region between 1310 nm and 1550 nm supporting Coarse Wavelength Division Multiplexed (CWDM) transmission.
G.653 fiber was developed to address this conflict between best bandwidth at one wavelength and lowest loss at another. It uses a more complex structure in the core region and a very small core area, and the wavelength of zero chromatic dispersion was shifted up to 1550 nm to coincide with the lowest losses in the fiber. Therefore, G.653 fiber is also called dispersion-shifted fiber (DSF). G.653 has a reduced core size, which is optimized for long-haul single-mode transmission systems using erbium-doped fiber amplifiers (EDFA). However, its high power concentration in the fiber core may generate nonlinear effects. One of the most troublesome, four-wave mixing (FWM), occurs in a Dense Wavelength Division Multiplexed (CWDM) system with zero chromatic dispersion, causing unacceptable crosstalk and interference between channels.
The G.654 specifications entitled “characteristics of a cut-off shifted single-mode optical fiber and cable.” It uses a larger core size made from pure silica to achieve the same long-haul performance with low attenuation in the 1550-nm band. It usually also has high chromatic dispersion at 1550 nm, but is not designed to operate at 1310 nm at all. G.654 fiber can handle higher power levels between 1500 nm and 1600 nm, which is mainly designed for extended long-haul undersea applications.
G.655 is known as non-zero dispersion-shifted fiber (NZDSF). It has a small, controlled amount of chromatic dispersion in the C-band (1530-1560 nm), where amplifiers work best, and has a larger core area than G.653 fiber. NZDSF fiber overcomes problems associated with four-wave mixing and other nonlinear effects by moving the zero-dispersion wavelength outside the 1550-nm operating window. There are two types of NZDSF, known as (-D)NZDSF and (+D)NZDSF. They have respectively a negative and positive slope versus wavelength. Following picture depicts the dispersion properties of the four main single-mode fiber types. The typical chromatic dispersion of a G.652 compliant fiber is 17ps/nm/km. G.655 fibers were mainly used to support long-haul systems that use DWDM transmission.
As well as fibers that work well across a range of wavelengths, some are designed to work best at specific wavelengths. This is the G.656, which is also called Medium Dispersion Fiber (MDF). It is designed for local access and long haul fiber that performs well at 1460 nm and 1625 nm. This kind of fiber was developed to support long-haul systems that use CWDM and DWDM transmission over the specified wavelength range. And at the same time, it allow the easier deployment of CWDM in metropolitan areas, and increase the capacity of fiber in DWDM systems.
From the passage above, we know that different kind of single-mode fiber has different application. Since G.657 is compatible with the G.652, some planners and installers are usually likely to come across them. In fact, G657 has a larger bend radius than G.652, which is especially suitable for FTTH applications. And due to problems of G.643 being used in WDM system, it is now rarely deployed, being superseded by G.655. G.654 is mainly used in subsea application. According to this passage, I hope you have a clear understanding about these single-mode fibers, which may help you make the right decision.

Singlemode fiber and multimode fiber different and selection method(1)

1.What is singlemode and multimode fiber? What is the difference between them?

The concept of single-mode and multi-mode is to classify fibers according to the propagation mode—the concept of multi-mode fiber and single-mode fiber propagation mode. We know that light is an extremely high-frequency (3×1014Hz) electromagnetic wave. When it propagates in an optical fiber, it is found from theories of wave optics, electromagnetic fields, and Maxwell equations.

When the fiber core has a geometric dimension much larger than the wavelength of the light, the light will propagate in the fiber in dozens or even hundreds of propagation modes, such as TMmn mode, TEmn mode, HEmn mode, etc. (where m, n=0, 1, 2, 3, …).

Among them, the HE11 mode is called the basic mode, and the rest are all called high-order modes.

Multimode fiber

When the fiber’s geometric size (mainly the core diameter d1) is far greater than the wavelength of light (about 1μm), there will be dozens or even hundreds of propagation modes in the fiber. Different propagation modes have different propagation speeds and phases, resulting in delays and widening light pulses after long-distance transmission. This phenomenon is called the modal dispersion of the fiber (also called inter-modal dispersion).

Mode dispersion can narrow the bandwidth of multimode fiber and reduce its transmission capacity. Therefore, multimode fiber is only suitable for smaller-capacity fiber communication.

The refractive index distribution of a multimode fiber is mostly a parabolic distribution, ie, a graded index profile. Its core diameter is about 50μm.


Single Mode Fiber

When the fiber’s geometry (mainly the core diameter) can be similar to the wavelength of light, if the core diameter d1 is in the range of 5~10μm, the fiber only allows one mode (base mode HE11) to propagate in it, and all other high-order modes are all cut off. This kind of fiber is called single-mode fiber.
Since it only has one mode to propagate and avoids the problem of mode dispersion, single-mode fiber has a very wide bandwidth and is particularly suitable for large-capacity optical fiber communications. Therefore, in order to achieve single-mode transmission, the parameters of the fiber must satisfy certain conditions. Through formulae calculations, for a fiber with NA=0.12, single-mode transmission above λ=1.3 μm, the radius of the fiber core should be ≤ 4.2 μm, ie its core diameter d1 ≤ 8.4 μm.
Because the core diameter of a singlemode fiber is very small, more stringent requirements are imposed on its manufacturing process.

2.What are the advantages of using optical fiber?

1) The passband of the fiber is very wide and the theory can reach 30T.
2) The length of non-relay support is up to tens to hundreds of kilometers, and the copper wire is only a few hundred meters.
3) Not affected by electromagnetic fields and electromagnetic radiation.
4) Light weight and small size.
5) Optical fiber communication is not powered, and the use of safety can be used in flammable, volatile and other places.
6) The use of a wide range of ambient temperatures.
7) Long service life.


3.how to choose the optical cable?

In addition to selecting the number of optical fiber cores and optical fibers, the optical cable must be selected according to the use environment of the optical cable to select the structure of the optical cable and the outer sheath.

1) Optical cable for outdoor use When loosely buried, it is better to use loose-sheathed cable. When overhead, a loose PE cable with a black PE sheath with two or more ribs can be used.
2) Optical fiber cables used in buildings should use tight-fitting optical cables and pay attention to their fire-retardant, toxic and smoke characteristics. The type of flame-retardant but smoke (Plenum) or flammable and non-toxic type (LSZH) can be used in the pipeline or in forced ventilation. The type of flame-retardant, non-toxic and non-smoking (Riser) should be used in the exposed environment.
3) When vertical or horizontal cabling is installed in a building, it can be used when using tight-fit optical cable, distribution optical cable or branch optical cable that are common in the building.
4) Select single-mode and multi-mode optical cables based on network applications and optical cable application parameters. Usually, indoor and short-distance applications use multimode optical cables, while outdoor and long-distance applications use single-mode optical cables.

4.In the connection of optical fibers, how to choose different applications of fixed connection and active connection?

The active connection of the fiber is achieved through a fiber optic connector. An active connection point in the optical link is a clear split interface. In the choice of active connection and fixed connection, the advantages of fixed connection are reflected in lower cost, light loss, but less flexibility, and the active connection is the opposite. When designing the network, it is necessary to flexibly select the use of activities and fixed connections according to the entire link situation to ensure flexibility and stability, so as to give full play to their respective advantages. The active connection interface is an important test, maintenance, and change interface. The active connection is relatively easy to find the fault point in the link than the fixed connection, which increases the convenience of replacement of the faulty device, thereby improving system maintenance and reducing maintenance costs.


5.Fibers are getting closer to user terminals. What do you need to pay attention to when it comes to the meaning of “fiber to the desktop” and system design?

“Fiber-to-the-desktop” in the application of the horizontal subsystem, and the relationship between copper and copper cable is complementary and indispensable. Optical fiber has its unique advantages, such as long transmission distance, stable transmission, free from electromagnetic interference, high support bandwidth, and no electromagnetic leakage. These characteristics make the optical fiber play an irreplaceable role in some specific environments:
1) If the information point transmission distance is greater than 100m, if you choose to use copper cable. Replicators must be added or network equipment and weak rooms must be added to increase costs and hidden troubles. Using fiber can easily solve this problem.
2) There are a large number of sources of electromagnetic interference in specific work environments (such as factories, hospitals, air-conditioning rooms, power equipment rooms, etc.), and optical fibers can be operated stably without electromagnetic interference in these environments.
3) There is no electromagnetic leakage in the fiber. It is very difficult to detect the signal transmitted in the fiber. It is a good choice for places where the security level is relatively high (such as military, R&D, auditing, government, etc.).
4) The environment with high demand for bandwidth has reached more than 1G. Optical fiber is a good choice.

There are many differences between single-mode fiber and multi-mode fiber, and the selection method is not the same. Let’s talk about it today. For more details, please keep an eye on Singlemode fiber and multimode fiber different and selection method(2).