Contenido

Los sistemas modernos de fibra óptica generalmente incluyen: transmisores ópticos para convertir una señal eléctrica en una señal óptica que se envía por la fibra óptica; cables de fibra óptica que contienen múltiples haces de fibras ópticas que se instalan a través de conductos subterráneos y edificios; varios tipos de amplificadores y un receptor óptico para recuperar la señal como una señal eléctrica. La información contenida suele ser comunicación digital generada por computadoras, telefonía digital") y compañías de cable").

Transmitters

The most commonly used optical transmitters are semiconductor devices such as light-emitting diodes (LEDs or leds, plural) and laser diodes. The difference between LED diodes and lasers is that LEDs produce incoherent light, which is dispersed, and lasers produce coherent, non-dispersed light. For use in optical communications semiconductor optical transmitters must be designed to be compact, efficient and reliable, while operating in an optimal wavelength range and directly modulated "Modulation (telecommunication)") at high frequencies.

In its simplest form, an LED is a polarized PN semiconductor junction, emitting light through spontaneous emissions, a phenomenon known as electroluminescence. The emitted light is incoherent, with a relatively wide spectral width of 30-60 nm, although LED light transmission is also inefficient, with only 1% of the input power. However, due to their relatively simple design, LEDs are very useful for low-cost applications.

Communications LEDs are mainly produced from GaAsp") or GaAs. Because GaAsp LEDs operate at a longer wavelength than GaAs LEDs (1.3 micrometers "Micrometer (unit of length)" versus 0.81-0.87 µm), their output spectrum is wider by a factor of about 1.7 times. The broad spectrum width of LEDs causes high dispersion in the fiber, which considerably limits their bit rate-distance product (common utility measure). LEDs are primarily suitable for local area network applications with speeds of 10 to 100 Mbit/s, and transmission distances of a few kilometers. LEDs have been developed to use multiple quantum wells to emit light at different wavelengths over a wide spectrum, and are currently in use in wavelength division multiplexing local area networks.

A semiconductor laser transmits light through stimulated emission rather than spontaneous emission, resulting in high output power (~100 mW) as well as other benefits of coherent light. The laser output is relatively directional, allowing high-efficiency coupling (~50%) into single-mode fibers. The narrow spectral width enables high bit rates by reducing the chromatic dispersion effect. Semiconductor lasers can be directly modulated at high frequencies, due to short recombination time.

Laser diodes are often directly modulated, which is light output controlled by a current applied directly to the device. For very high data rates or very long distance links, a laser source may be continuous wave and the light modulated by an external device such as an electroabsorption modulator.

Receivers

The main component of an optical receiver is a photoelectric cell, which converts light into electricity through the photoelectric effect. The photodetector is generally a semiconductor-based photodiode. There are several types of photodiodes, including: PN photodiodes, PIN photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photodetectors are also used due to their suitability for the integration of wavelength multiplexers and regenerator circuits.

Electrical optical converters are usually the grouping of a transimpedance amplifier") and a limiting amplifier") to produce a digital signal in the electrical domain of the input optical signal, which may be attenuated and distorted as it passes through the channel. In addition to signal processing such as data clock recovery (CDR) carried out by a phase-locked loop, it can also be applied before data is transmitted.

Fiber

An optical fiber consists of a core, a cladding and a buffer&action=edit&redlink=1 "Buffer (optical fiber) (not yet written)") (an outer protective layer). The coating guides light along the core using the total internal reflection method. The core and cladding, which have a lower refractive index, are generally silica glass, although they may also be plastic. Fusion splicing (or mechanical splicing) is performed when connecting two optical fibers, and requires special skills and interconnection technology due to the microscopic precision required to align the fiber cores.

There are two types of optical fiber used in communications: multimode and singlemode optical fiber. Multimode has a larger core (50 or 62.5 micrometers), allowing for less precision but lower cost transmitters, receivers and connectors. However, multimode fiber introduces multimode distortion, which often limits bandwidth, and the link length exhibits greater attenuation. The core of a single-mode fiber is smaller (8-10 microns) and requires more expensive components and more precise interconnection methods, but allows for higher performance links, which increases transfer rate and distance.

Amplifiers

The transmission distance of a fiber optic communication system has traditionally been limited by fiber attenuation and fiber distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the optical signal into an electrical signal, and then use a transmitter to send the signal back at a higher intensity than the attenuated one received. Due to the high complexity with modern wavelength division multiplexing of signals (such as the fact that they have to be installed every few tens of kilometers) the cost of these repeaters is high.

An alternative approach is to use an optical amplifier, which amplifies the optical signal directly, without having to convert the signal to the electronic domain. Fiber amplifiers are optical amplifiers that use doped fiber "Doping (semiconductors)"), usually with rare earths. These amplifiers require external pumping with a continuous wave laser at an optical frequency slightly higher than the one they amplify. Typically, pumping wavelengths are 980 nm or 1480 nm and to get the best noise results, it should be done in the same direction as the signal.[2]​.

Wavelength multiplexing

Wavelength division multiplexing (WDM) is the practice of multiplying the available capacity of an optical fiber by adding new channels, each channel "Channel (communication)") at a new wavelength of light. The bandwidth of a fiber can be divided into 160 channels to support a combined bit rate in the terabit per second range. This requires a wavelength division multiplexer in the transmitting equipment and a demultiplexer in the receiving equipment.

Bandwidth-distance product

Because the effect of dispersion increases with fiber length, a fiber transmission system is often characterized by the product of its bandwidth "Bandwidth (computing)") and distance, often expressed in MHz·km. This value, product of bandwidth and distance, is due to the relationship between the bandwidth of the signal and the distance it can be transported.

Through a combination of advances in dispersion management, wavelength division multiplexing and optical amplifiers, optical fibers can carry information at around 14 terabits per second over 160 km of fiber.

Dispersion

For modern glass optical fiber, the maximum transmission distance is not limited by the absorption of direct materials, but by various types of dispersion "Scattering (physical)") or the propagation of optical pulses as they travel along the fiber. The dispersion of optical fibers is caused by a variety of factors. Intermodal dispersion, caused by the different axial velocities of different transverse modes, limiting the performance of multimode fiber. Because single-mode fiber only supports one transverse mode, intermodal dispersion is eliminated.

Single-mode fiber performance is primarily limited by chromatic dispersion, which occurs because the index of the glass varies slightly depending on the wavelength of the light. Polarization mode dispersion is another source of limitation, because although single-mode fiber can support only one transverse mode, it can be carried into this mode with two polarizations. This phenomenon is called fiber birefringence and can be counteracted by polarization and maintenance of the fiber.