A fiber network
is a network which is consisted of optical components, i.e fiber patchcord, fiber connectors, optical patch panels, light sources, photodetectors etc.
The transmitter converts the electrical signal into light signal, which is suitable for transmission through a fiber network cable
. It has low electrical power consumption and high optoelectronic conversion efficiency. The modulation bandwidth has range from 200MHz (for LED source) to 10GHz (for SLD source). Light source can be either LED (light emitting diode) or SLD (semiconductor laser diode).
LED is a forward-biased p-n junction which emits light by spontaneous emission, the combination of holes and electrons. The emitted light is incoherent with a wide spectral width of 30-60nm. LED s have low launch efficiency with only 1-5% of input power converted into launch power. Semiconductor lasers use stimulated emission, instead of spontaneous emission, which results in high output power (10mW max).
SLD sources allow a certain directivity to the laser beam, increasing the launch efficiency up to 50% into singlemode fiber . They also have narrow spectral width, which allows higher data rates, and high modulation bandwidth, from 1 to 3GHz (typical). In most applications LED sources have been replaced mostly by VCSEL (Vertical Cavity Surface) lasers which offer better transmission properties at a similar cost.
Optical splice is a permanent joint between two individual fibers. Fiber splicing
is done for various reasons for example, adjusting the length of optical network, replace a destroyed fiber part due to trenching etc. The cutting and mating of the two fibre parts is a very delicate procedure because it requires precise alignment between the mated fiber cores. In that case Alfaphonet offers special tools and accessories like polishing disks, polishing and cleaning liquids, cleaving tools, microscopes etc, in order to achieve a perfect and without transmission impairments Fiber splicing.
In long transmission distances, optical losses and attenuation cause gradually deterioration of signal's quality. In that case Optoelectronic repeaters and optical amplifiers are used to overcome this problem.
Optoelectronic repeaters are optical fiber converters which convert the optical signal into electrical signal and then re-generate it using a transmitter. Optical amplifiers directly amplify the optical signal using, mostly, erbium doped fibre amplifiers
In doped fiber amplifier the input signal is doped by a, suitable wavelength laser, which provides gain in region of 1510 to 1600nm and between 1530 and 1565nm allowing dense wavelength division multiplexing.
In receiver's side, the optical signal is converted into an electrical signal using a photodetector, an amplifier and a signal restorer. The optical signal in the photodetector is converted, due to photoelectric effect, into current which is amplified and finally its data are recovered and processed to the user.
Patchcords and connectors for fiber network
Unlike data communication systems where twisted pairs are mainly used, in fiber networks fiber patch cords are used to convey messages. Fiber patch cords act similar to circular waveguides where EM wave (light) is confined inside them and propagates through successive reflections to their internal surface. Each optical cable is consisted of three layers: the buffer, the clading and the core.
The core is made of glass or plastic and has a larger index of refraction from the cladding. The propagation of light inside the core is based on Snell's law: Φc = arcsin(n2/n1)
where n2 and n1 is the index of refraction of cladding and core respectively. Any light ray which incident on glass surface with angle greater than Φc is totally reflected back into the glass. This is called total internal refraction.
Modes in optical cables
Electromagnetic waves, like light, are composed from two fields the magnetic and the electric. These two fields are propagating in two different planes: The electric in the vertical plane and the magnetic in the horizontal plane. In waveguides and optical cables the EM waves can have several modes. The term mode describes the way a EM wave is distributed along the transmission medium, in our case in a fibre cable
The typical core diameter for multimode fibers is 50μm, for Europe and 62.5μm for USA and they operate to 850nm and 1300nm wavelengths. For singlemode fibers the core diameter is 9 or 10μm depending on the manufacturer. The operating wavelengths are from 1310nm to 1550nm.
have larger core diameter and thus the light is propagated through successive reflections. The signal is more susceptible to noise and losses and cannot reach long distances The laser source doesn't need to be precise so the multimode fibers are used in optical fiber networks where transmission distances are limited and inexpensive equipment is needed.
have small core diameter and therefore the light is propagated directly, without reflections. The coupling of light though the fiber core requires precise laser source with small tolerance making the singlemode fibers more expensive. On the other hand the light signal is affected less from noise and other negative effects like modal dispersion and can travel faster and in longer distances than in multimode fibers.
The OM1, OM2 and OM3 are categories of multimode fibers
while OS1 and OS2 are for singlemode fibers. The class OF-XXX shows the minimum distance the light signal can traverse in meters.
Like copper cables, fiber optic cables can be terminated in connectors or adaptors, resulting in fiber patch cords. The most used and known FO connector types are ST connectors
, SC connectors
and LC connectors
A fiber patch cord can be described by the code format XX/125 XXX, e.g E9/125 OS2 or G62.5/125 OM1. “E” and “G” describe the type of fibre. “E” stands for singlemode fibers and “G” for multimode fibers. The first two numbers of XX/125 describe the diameter of the core while the 125 is the diameter of the cladding and it is the same for both multimode and singlemode fibers
Apart from typical fibre cables, there are other type of cables which are called fiber pigtails
. FO pigtails have the same structure as the other optical cables, except they lack the buffer coating. Optical patch panels and splice boxes offer an ideal solution for termination of FO pigtails by providing protection from environmental conditions and contaminants.