OPTICAL FIBER. SINGLEMODE OR MULTIMODE It is important to understand the differences between singlemode and multimode fiber optics before selecting one or the other at the start of a project. Its different characteristics of bandwidth, light reflection, light source, etc. Make it suitable to use singlemode or multimode in different situations. Before going into the comparative let's take a look at some basic concepts about fiber optics and what elements make up a fiber optic cable. LIGHT CONCEPTS AFFECTING OPTICAL FIBER 1- Wavelength - Color of light. 2- - Loss of light. 3- - Transmission speed. 4- Reflectance - Amount of light that bounces in the fiber towards the transceiver. 5- Reflection - Amount of light reflected in the fiber. 6- Refraction - Amount of light absorbed by the cladding. Wavelength In conventional communication systems (radio or cable) frequency is often used, whereas in optical communications the wavelength expressed in nanometers is used. We refer to the wavelength as the color that the light has to different wavelength, different color. In the following graph we see how the light that circulates by a fiber optic cable is between nm and nm which is invisible infrared light. Ultraviolet - 400nm Violet - 455nm Blue - 490nm Green - 550nm Yellow - 580nm Orange - 620nm Red - 750nm Infrared - 800nm nm Optical fibers nm nm Microwaves RF + Frequency - Frequency First window MM Second window MM - SM Third window SM - Wavelength + Wavelength Visible spectrum Optical communications use parts of the electromagnetic spectrum that we call windows that correspond to certain wavelengths:, and nm.
It is the light loss along the cable expressed in db/km. Usually the optical fiber is made of silica and does not reflect all the wavelengths in the same way. The wavelengths that best reflect the silica are, and nm, which we have previously called as windows. Depending on the wavelength used, the optical fiber will have a certain propagation capacity of the light, and therefore a maximum applicable length. First window (nm) Second window (nm) Third window (nm) (dbm/km) 4 3 2 1 0 800 900 1000 1100 1200 1400 1 1600 (nm) An attenuation of more than 3dB (which means having at the output half of the input signal) is not permissible. It is the speed at which light is transmitted through the optical fiber and is expressed in MHz per km. A bandwidth of MHz-km indicates that at MHz the signal can be transmitted 1km away. The bandwidth for multimode fibers is typically Mhz per Km and in singlemode fibers it is in the range of GHz, usually 100GHz per Km. The bandwidth is inversely proportional to the distance. Reflectance When light incides a surface, reflection is generated In the direction and angle opposite to the light incident. In the optical fiber, when the light incides, a part is reflected on the surface material and returns to the transceiver. Therefore if the light reflected, it is at some time higher to the emitted, the transceiver will be blinded. The optical fiber reflectance is expressed in db. Incident ray Incidence Angle Reflection angle Reflected ray Reflective surface
Reflection and Refraction To understand these two terms imagine a fiber optic cable like a completely round tunnel with walls completely covered with mirrors. These mirrors have a 95% reflection capacity, that is, they reflect only 95% of the light that incides them. The other 5% is absorbed by the mirror, causing a reflection loss. Refraction is the way the light bounces in the tunnel of mirrors depending the light angle incides to the mirror. According to Snell's law, if the light incidence angle exceeds 45, the light will not be reflected. Therefore, depending on the angle of incidence, there are losses by refraction. In order for the light to propagate along the fiber, it must reach the end of the fiber within the boundary called "cone of acceptance". Light coming out of this cone is lost in the cladding. The half of the angle of the cone of acceptance respect to the fiber axis, is the maximum angle which the light rays still are accepted for the transmission through the fiber. This angle is called apex angle. Lost ray Fiber axis Cone of acceptance Core Propagated ray Critical ray Light ray outside the cone of acceptance It is essential to make the fiber splices as straight as possible and if connectors are used, that these are of the highest quality possible with the aim of aligning the two sides of the fiber to the maximum. FIBER OPTICS CABLE PARTS Coating Strength member Buffer Core
Core - The center of the fiber optic cable is a thin piece of glass that provides the path that light signals follow. The core is very small in diameter, both single-mode and multimode are 125 microns in outer diameter. The core is massive, not a tube. - The core is covered or surrounded by optical material which is also made of extremely pure crystal just like the core. The function of the cladding is to reflect the light signals back to the core. Having a different index of refraction to the core, the light passes through the core but bounces back in the cladding, so that it can travel through the core. Buffer - Generally plastic made that protects mechanically the previous two. Strength member - They can be aramid or polyamide fibers, depending on the type of cable. Coating - It is made of very strong polyurethane (PVC) and protects the interior components of the fiber optic cable. In order to increase the bandwidth of a fiber optic cable, the same cable can be formed by several buffers, which allow the assembly to carry very high amounts of information (TBytes/second). FIBER OPTIC CONNECTORS B A LC LC Duplex F-SMA ST FC SC SC Duplex SINGLEMODE OR MULTIMODE Singlemode It is a type of fiber, which by its construction (very small core), only allows the pass of one light beam, this light beam, does not bounce off the walls, and travels parallel to the length of the cable. For this reason, the losses by reflection (modal distortion) are smaller, and therefore the fiber can be longer than in the multimode. Multimode It is a type of fiber, which by its construction (larger diameter of the core) allows the pass of more than one light beam of different wavelength, and therefore allows several "modes" of light to enter and leave the fiber. It is based on reflection against its walls for the light propagation. Multimode 62,5 μm Multimode 50 μm Singlemode 9 μm The manufacturing cost of a singlemode fiber is higher because of the light transceiver used. They are laser diodes with greater power and directionality than the LED type diode that uses multimode fiber, which generates a more diffuse light.
Fiber types OM1, OM2, OM3, OM4, OM5, OS1, OS2 The meaning of OM applies to multimode (MM) optical fiber, the numbering refers to the type of core, maximum distance, operating window (wavelength) and bandwidth. For example, OM2 would be a multimode fiber with a 50μm core diameter, a 125μm cladding diameter, maximum distance 550m, wavelength nm, maximum attenuation 3.5dB/km and bandwidth Mhz- Km. The meaning of OS applies to singlemode optical fiber (SM). For example, OS1 would be a singlemode fiber with a 9μm core diameter, a 125μm cladding diameter, wavelength nm and attenuation 0.5 db/km. To greater number: OM1, OM2, OM3... higher the material quality is, and therefore, greater transparency has and further away can the light go. Optic fiber and cable type Wavelength nm Max. attenuation (db/km) overfilled modal (MHz Km) effective modal (MHz Km) 62,5/125μm Multimode TIA 492AAAA (OM1) 200 TIA 492AAAB (OM2) nm Laser optimized TIA 492AAAC (OM3) nm Laser optimized TIA 492AAAD (OM4) Singlemode indoor/outdoor Singlemode indoor Singlemode outdoor 1,0 1,0 1 3 2000 4700 Comparative Specification Singlemode Multimode Cost of fiber Less expensive Expensive Transmission equipment Wavelengths Application of use Distance More expensive (Laser diode) Low 1260nm to 1640nm Connections are more complex Medium and long networks (> 200 Km) Large (>1Tb/s) Basic and low cost (LED) High nm to nm Larger core, easier to handle Local networks (< 2Km) Limited (10Gb/s short distance)