fiber optics (optical fiber)

Fiber optics, or optical fiber, refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic strand or fiber. A fiber optic cable can contain a varying number of these glass fibers -- from a few up to a couple hundred. Surrounding the glass fiber core is another glass layer called cladding. A layer known as a buffer tube protects the cladding, and a jacket layer acts as the final protective layer for the individual strand.

 

How fiber optics works

Fiber optics transmit data in the form of light particles -- or photons -- that pulse through a fiber optic cable. The glass fiber core and the cladding each have a different refractive index that bends incoming light at a certain angle. When light signals are sent through the fiber optic cable, they reflect off the core and cladding in a series of zig-zag bounces, adhering to a process called total internal reflection. The light signals do not travel at the speed of light because of the denser glass layers, instead traveling about 30% slower than the speed of light. To renew, or boost, the signal throughout its journey, fiber optics transmission sometimes requires repeaters at distant intervals to regenerate the optical signal by converting it to an electrical signal, processing that electrical signal and retransmitting the optical signal.

 

Types of fiber optic cables

Multimode fiber and single-mode fiber are the two primary types of fiber optic cable. Single-mode fiber is used for longer distances due to the smaller diameter of the glass fiber core, which lessens the possibility for attenuation -- the reduction in signal strength. The smaller opening isolates the light into a single beam, which offers a more direct route and allows the signal to travel a longer distance. Single-mode fiber also has a considerably higher bandwidth than multimode fiber. The light source used for single-mode fiber is typically a laser. Single-mode fiber is usually more expensive because it requires precise calculations to produce the laser light in a smaller opening.

 

Multimode fiber is used for shorter distances because the larger core opening allows light signals to bounce and reflect more along the way. The larger diameter permits multiple light pulses to be sent through the cable at one time, which results in more data transmission. This also means that there is more possibility for signal loss, reduction or interference, however. Multimode fiber optics typically use an LED to create the light pulse.

 

While copper wire cables were the traditional choice for telecommunication, networking and cable connections for years, fiber optics has become a common alternative. Most telephone company long-distance lines are now made of fiber optic cables. Optical fiber carries more information than conventional copper wire, due to its higher bandwidth and faster speeds. Because glass does not conduct electricity, fiber optics is not subject to electromagnetic interference and signal losses are minimized.

 

In addition, fiber optic cables can be submerged in water and are used in more at-risk environments like undersea cable. Fiber optic cables are also stronger, thinner and lighter than copper wire cables and do not need to be maintained or replaced as frequently. Copper wire is often cheaper than fiber optics, however, and is already installed in many areas where fiber optic cable hasn't been deployed. Glass fiber also requires more protection within an outer cable than copper, and installing new cabling is labor-intensive, as it typically is with any cable installation.

 

Fiber optics uses

Computer networking is a common fiber optics use case, due to optical fiber's ability to transmit data and provide high bandwidth. Similarly, fiber optics is frequently used in broadcasting and electronics to provide better connections and performance.

 

Military and space industries also make use of optical fiber as means of communication and signal transfer, in addition to its ability to provide temperature sensing. Fiber optic cables can be beneficial due to their lighter weight and smaller size.

 

Fiber optics is frequently used in a variety of medical instruments to provide precise illumination. It also increasingly enables biomedical sensors that aid in minimally invasive medical procedures. Because optical fiber is not subject to electromagnetic interference, it is ideal for various tests like MRI scans. Other medical applications for fiber optics include X-ray imaging, endoscopy, light therapy and surgical microscopy.

Fibre optic receiver

Fibre optic receiver

Once data has been transmitted across a fiber optic cable, it is necessary for it to be received and converted into electrical signals so that it can be processed and distributed to its final destination. The fiber optic receiver is the essential component in this process as it performs the actual reception of the optical signal and converts it into electrical pulses. Within the fibre optic receiver, the photodetector is the key element

A variety of semiconductor photo-detectors may be used as fibre optic receivers. They are normally semiconductor devices, and a form of photo-diode. A variety of diodes may be used in fibre optic receivers, namely p-n photodiode, a p-i-n photodiode, or an avalanche photodiode. Metal-semiconductor-metal (MSM) photodetectors are also used in fibre optic receivers on occasions as well.

Overall receiver

Although the photo-detector is the major element in the fibre optic receiver, the are other elements to the whole unit. Once the light has been received by the fibre optic receiver and converted into electronic pulses, the signals are processed by the electronics in the receiver. Typically these will include various forms of amplification including a limiting amplifier. These serve to generate a suitable square wave that can then be processed in any logic circuitry that may be required.

Once in a suitable digital format the received signal may undergo further signal processing in the form of a clock recovery, etc. This will undertaken before the data from the fibre optic receiver is passed on.

Diode performance

One of the keys to the performance of the overall fibre optic receiver is the photodiode itself. The response times of the diodes govern the speed of the data that can be recovered. Although avalanche diodes provide high speed they are also more noisy and require a sufficiently high level of signal to overcome this.

The most common type of diode used is the p-i-n diode. This type of diode gives a greater level of conversion than a straight p-n diode as the light is converted into carriers in the region at the junction, i.e. between the p and n regions. The presence of the intrinsic region increases this area and hence the area in which light is converted.