Fiber Basics and Connectors (Part 1)

Fiber cables used in network systems are divided into two types: Singlemode and Multimode. This article will help you distinguish the types of fiber optic cables currently used in the network.

To identify these two types, we first need to understand a few concepts. the following basic.

1. Mode

The term Mode used in fiber optic cables comes from the Latin word for light (path). When propagating in an optical fiber, light will follow many paths, each of which can be considered as a ray of light (Mode).
Light rays propagating in an optical fiber are divided into 3 types:

  • Axial Mode: light rays propagate along the central axis of the fiber. The time it takes for a light ray to travel in an optical fiber from A to B is the fastest.
  • High Order Mode: light rays propagate in an optical fiber has a large number of reflections. Light rays travel longer in fiber from A to B than light rays Low Order Mode.
  • Low Order Mode: light rays propagating in an optical fiber have a small number of reflections. Light rays travel faster in fiber from A to B than light rays High Order Mode.

2. Dispersion

  • Axial, High Order, and Lower light rays propagate in an optical fiber along paths of different lengths. Thus, light rays – transmitted at the same time with the same velocity – propagate to the end of the fiber at different times. The difference in time between these rays is called Dt. This phenomenon is called dispersion.
  • Limiting dispersion. bandwidth of fiber optic cable and affects data transmission (the larger the Dt  the greater the impact on data transmission).
    • For signals Analog signals distort the signal form
    • For digital signals, it causes overlapping of bits.

3. Differentiate Singlemode and Multimode fiber

Multimode fiber: The feature of Multimode fiber is that it propagates all 3 light rays simultaneously (Axial Mode, High Order Mode and Low Order Mode).

There are 2 types of Multimode fiber:

A. Step Index Multimode Fiber (Step Index Multimode) 

It is an optical fiber with a uniform core with refractive index n1 and a cladding layer around the core with refractive index n2 (n2 < n1)

  • Order refractive index Multimode fiber is highly affected by dispersion, so the bandwidth is not high and is not used in network systems.
  • In order to limit the influence of dispersion phenomenon, manufacturers have launched a variable refractive index Multimode fiber cable (Graded Index Multimode).
Light rays in Fiber Optic Step Index Multimode
  • Graded Index Multimode: The fiber core is composed of multiple layers of glass with refractive index n1, n2, n3…   li>
  • Different point instead of only one refractive index as in the core of Step Index Multimode fiber. The inner glass layer has a higher refractive index and the outer layer has a lower refractive index.
  • Light rays will propagate slower when encountering a high refractive index and faster when encountering a low index of refraction.
  • Therefore, Graded Index Multimode fiber reduces the time difference Dt, limits dispersion, helps increase bandwidth usage and becomes popular variables in today’s network.
Light rays in Fiber Optic Graded Index Multimode

B. Singlemode fiber

  • A fiber in which only one ray of Axial light is propagated. Therefore, light dispersion does not occur in Singlemode fiber. This makes Singlemode fiber have a larger bandwidth and transmit data farther than Multimode fiber.
  • Light-based fiber optic cable transmits the signal, so it has less loss and is commonly used. for long distance connection.
  • The fiber optic cable is made up of three main components: the core, the reflective layer, and the primary protective covering (also known as coating, primary buffer).  ;
    • Core made of fiberglass or plastic used to transmit light. Wrapping the core is 
    • Cladding – a layer of glass or plastic – to protect and reflect light back to the core. 
    • Primary coating  is a PVC cover that protects the core and cladding from dust, moisture, and scratches.  Two popular types of optical fiber are GOF (Glass Optical Fiber) – optical fiber made of glass and POF (Plastic Optical Fiber) – optical fiber made of plastic. POF has a rather large core diameter of about 1mm, used for short distance signal transmission, low speed network. On technical documents, you often see GOF fiber optic cable recording parameters 9/125µm, 50/125µm or 62.5/125µm, this is the diameter of the core/cladding; and the primary coating has a default diameter of 250µm.
    • The protection of a fiber optic cable is an outer layer consisting of many different layers depending on the structure and nature of each type of cable. But there are three main layers of protection: the tensile layer (strength member), the outer protective layer (buffer) and the armor layer (jacket) – depending on the document will have different names. 
    • Strength member  is a heat-resistant, stretch-resistant layer, usually made of Kevlar fibers. 
    • Buffer usually Made of PVC, protected from shock and moisture. The outermost layer of protection is the Jacket. Each type of cable, depending on the requirements of use, will have different jacket layers. 
    • Jacket  is resistant to impact, heat and abrasion , protect the interior from moisture and environmental influences
  • There are two different design ways to protect fiber optic cables: loose-tube and tight buffer.
    • Loose-tube.   often used outdoors, allowing to contain many optical fibers inside. Loose-tube helps fiber optic cable “expand” before temperature changes, stretch naturally, not stretch, bend in bends.
    • Tight-buffer commonly used indoors, tightly encapsulate fiber optic cables (such as power cables), making it easy to install during construction
  • On some documents, you will come across two IFC acronyms, OSP. IFC (Intrafacility fiber cable) is a type of cable used in the home, with little availability. physical protection layer and flexible installation. OSP (Outside plant cable) is a type of cable for outdoor use, resistant to extreme conditions of temperature, moisture, dust… this cable has multiple layers of protection.
  • The light rays inside the fiber optic cable have two modes of transmission: Singlemode and Multimode. Optical cable Singlemode (SM) has a rather small core diameter (about 9µm), uses a laser source that transmits light rays throughout, so the signal is less attenuated and has a good speed. great. SM usually operates at 2 wavelengths (wavelength) 1310nm, 1550nm. Optical cable Multimode (MM) has a larger core diameter than SM (about 50µm, 62.5µm). MM uses LED (Light Emitting Diode) or laser light source to transmit light and usually operates at 2 wavelengths 850nm, 1300nm; MM has a smaller connection distance and transmission speed than SM. 
  • MM has two types of transmission: step extraction (Step index) and continuous extraction (Graded index). Light rays of the type Step index  propagate in many different directions so they have high attenuation and relatively slow speeds. Step index is less common, usually used for optical fiber POF. Light rays of the type Graded index curve and converge at a point. Therefore, Graded index has less loss and has higher transmission rate than Step index. Graded index is used quite commonly. 
  • Signal transmission on fiber optic cable has two forms of simplex and duplex. Simplex transmits signals only in one direction. Duplex can transmit and receive signals in either half-duplex or full-duplex at the same time depending on the configuration. 

To connect fiber optic cables to the patch panel or to the input/output ports on optical transmitting and receiving devices, people often use a single-ended optical cable with a connector. (pigtail) or both ends have connectors (pathcord). 

4. Some types of fiber optic cable

  • Ribbon (Left): ribbon fiber optic cable, containing multiple optical fibers inside.
  • Zipcord (Right): two optical fibers with a contiguous outer sheath (like a wire) .
  • Any optical communication consists of three components: a source, a medium (optical cable) and a receiver. The source converts the electronic signal into light and transmits it via optical fiber. The receiver converts the light into an electronic signal.
  • There are two types of emitters, laser and LED. Laser low dispersion, allowing for fast data transmission, long distance (over 20km) , can be used for both Singlemode and Multimode but high cost, difficult to use. LED high dispersion, slower transmission, low cost, easy to use, Commonly used for multimode fiber optic cables. LED for shorter distance systems, can be used for both glass fiber and plastic fiber.

5. Optical parameters need attention

  • Optical loss: The amount of optical power lost during transmission over fiber optic cable, coupling point. Symbol dB.
  • Optical Return loss: light is reflected at coupling points, optical connectors.
  • Insertion loss: reduces optical power at both ends of the coupling. Typical values ​​are from 0.2dB – 0.5dB.
  • Attenuation: The level of optical power attenuation during transmission over a range determined way. Symbol dB/km. For example, with Multimode fiber at 850nm, the attenuation is 3dB/km, while at 1300nm, the attenuation is only 1dB/km. Singlemode fiber: 0.4dB/km attenuation at 1310nm, 0.3dB/km at 1550nm. Connector attenuation 0.5dB/connector pair. The splice attenuates 0.2 dB/point.
  • Wavelength: is the period of travel of the electromagnetic wave. The symbol nm (nanometer). The light we see has wavelengths from 400nm to 700nm (violet to red). Fiber optic cables use light in the infrared region with a wavelength greater than the wavelength we see – between 850nm, 1300nm and 1550nm. Optical transmission wavelengths are determined based on two factors to overcome the loss caused by energy and transmission materials: wavelengths located in the infrared region and wavelengths not located in the absorption and interference region. resistance to light energy transmission (absorption) due to impurities mixed in the fiber optic cable from the manufacturing process.

So why don’t we use longer wavelengths? Infrared wavelength is the transition between light and heat. The longer the wavelength, the hotter the ambient heat, the more turbulent the signal. Therefore, usually POF has wavelength 650nm, 850nm. GOF with Multimode operates at 850nm and 1300nm, Singlemode at 1310nm, 1550nm. There is no difference between the two wavelengths 1300nm and 1310nm, just a conventional way to distinguish whether to use Singlemode or Multimode fiber.

  • Optical connector: comprised of many components, they come in many styles such as SC/PC, ST/UPC, FC/APC… But there are the two components you need to take care of are the SC, ST, FC…and PC, UPC, APC contact points. 

SC (subscriber connector), ST (straight tip), FC (fiber connector) are types of optical connectors with square, round shape…

Inside the connector is a ferrule, which protects and holds the fiber optic cable straight. Ferrules are made of glass, metal, plastic or ceramic – where ceramic is the best.

The top of the ferrule is smoothed (polish) with three main contact forms PC (Physical Contact), UPC (Ultra Physical Contact) and APC (Angled Physical Contact), which helps to ensure that the junction has less light being lost. lost or most reflected. 

Curved PC form, used with FC, SC, ST connector types. PC, has an optical return loss of 40dB. Since this value is quite high, it has motivated manufacturers to keep looking for better solutions. UPC is the next solution, it is also curved like PC but reduces return loss more.

UPC has a return loss value of 50dB. UPC for use with FC, SC, ST, DIN, E2000 connectors. The APC is crossed by 8 degrees, eliminates most reflections at the coupling point, and has a return loss value of 60dB. You should be aware that reading optical specifications that mention attenuation can make it easy to misinterpret the “+” and “-“ signs. For example, with the calculation results, measuring the loss level is -40dB. The spec can write loss values ​​as 40dB or reflectance as -40dB or gain as -40dB. All are the same, so you need to pay attention to the spelling to avoid misunderstanding.

Currently, the cost of fiber optic cables and optical accessories is lower than it was a few years ago. Along with the application of many solutions such as IP Camera, VoIP, Online video conferencing, gigabit network connection between buildings, offices, and factories; fiber optic cable is gradually becoming the number one choice for network infrastructure deployments that require a lot of bandwidth and high speed. 

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