Fiber-optic cables

Fiber-optic cables

Data transmission via fiber-optic cables (FO) has many advantages. It allows data rates of up to 40 Gbps over routes that are many kilometers long, does not have a negative affect on parallel cables, and at the same time is resistant to electromagnetic interference. The different fiber types (POF, PCF, GOF) and fiber categories OM1 to OM5 plus OS2 allow cabling concepts to be tailored to specific requirements.

Principle of optical data transmission

Principle of optical data transmission

Advantages of fiber optics transmission

Fiber-optic cables transmit data in the form of light across long routes. To do so, the electrical signals at the transmitter are converted into optical signals and sent to the receiver via plastic or fiberglass. There, the transmitted light signals are converted back into electrical signals and then evaluated and processed. The cables and lines are up to 90% lighter and thinner than copper cables, and yet enable longer transmission routes and higher data rates of up to 40 Gpbs or more. At the same time, elaborate shielding concepts are unnecessary since the system is absolutely resistant to EMC and ESD interference due to metal-free transmission.
The materials used and the associated costs for passive cabling are typically lower compared to copper cabling. And wide transmission bandwidths with higher signal density provide the option to transmit several signals in different wavelengths via the same fiber-optic cable (multiplexing).

Data transmission in data centers

Fiber-optic cabling optimizes data transmission in data centers

Fiber-optic cables in use

Whether it’s short, medium, or long distances, with speeds of less than 100 Mbps or up to 40 Gbps, or in bus or Ethernet structures, the right cable for fiber-based data transmission exists for virtually any requirement in industrial and semi-industrial automation. Even when used under  harsh conditions such as those on wind farms, fiber-optic cables reliably complete their task. That is why their applications range from use in vehicle technology and industrial cabling to Local Area Networks (LAN) in data centers and wide area networks. The key to cabling is selecting the right type of fiber and fiber category.

Fiber core and cladding diameters in fiber-optic cables

Comparison of various fiber core and cladding diameters

The right fiber for all applications

Each type of fiber has its own specific application. The smaller the outer diameter of the fiber, the more delicate the fiber behavior is during assembly. For physical reasons, higher data rates and longer distances are attained with lower fiber core diameters.

- POF (polymer optical fiber): In POF cables, both the core and the cladding are made of plastic. The typical core diameter is 980 µm, and for cladding, it is 1,000 µm. With short transmission routes of up to 70 m and data rates of a maximum of 100 Mbps, depending on the active components, POF cables are used for automotive engineering or industrial cabling. The robustness and size of the fiber make it easy to assemble in the field. Due to high attenuation and dispersion, this fiber type is not suitable for high data rates or long distances.

- PCF (polymer-clad fiber): PCF is plastic-coated fiber-optic cable made of glass. Known under various designations such as PCS (polymer-clad silica), HCS (hard-clad silica), and HPCF (hard polymer-clad fiber), these cables are robust and can be assembled easily. PCF fibers with a typical core diameter of 200 µm and a cladding diameter of 230 µm are often encountered in industrial cabling with medium lengths of up to 300 m and data rates of ≤ 100 Mbps. Other areas of application include automotive, sensors, and medical technology.

- GOF multimode (glass optical fiber): This glass fiber has a core made of quartz surrounded by a coating of reflective glass. Multimode cables have core diameters of 50 µm or 62.5 µm. The larger diameter allows more light energy to be coupled at the beginning of the fiber, but attenuation is higher along the length of the fiber. That is why multimode fibers are mainly used in Local Area Networks (LANs) and data centers, where they can handle transmission routes of up to 550 m with 10 Gbps.

- GOF singlemode: The singlemode fibers have a much smaller core diameter of approximately 8 µm. For singlemode fibers, we differentiate between core diameter and mode field diameter. The mode field diameter depends on the wavelength. The larger the wavelength, the larger the mode field diameter. Since only one light mode can be transmitted in the fiber, a great deal of signal light can be fed into the fiber and transmitted. The fiber’s coefficient of attenuation in the transmission range is very low. Low attenuation and low dispersion are the ideal conditions for using singlemode fibers for distances of up to 50 km and data rates of say 40 Gbps.

In accordance with ISO/IEC 11801, fiber categories OM1, OM2, OM3, and OM4 have been internationally established for multimode fibers and OS1 and OS2 have been established for singlemode fibers. They indicate which transmission bandwidths and attenuation values a fiber has. Due to the constant increase in transmission bandwidths, the number of future categories, such as OM5 for transmission rates of up to 400 Gbps, is also growing.

Attenuation in fiber-optic cables

Possible causes leading to attenuation in fiber-optic cables

Losses and fiber-optic cables

Attenuation is a loss of signal light intensity that occurs as light is being transported from the transmitter to the receiver. The goal is to transport signal light to receivers with as little attenuation as possible. There is a difference between the attenuation that occurs at a specific location and attenuation in relation to length, the attenuation coefficient. The attenuation coefficient for fiber-optic cables refers to a length of 1 km.

- Insertion and coupling losses: They can occur when the light is fed into the fiber by the transmitter or also through plug and splice connection along the route and at the receiver. Many things can cause these types of losses. One frequent cause is contamination on connector faces. Coupling different core diameters in one link also leads to losses. Splice connections created with fusion splicing are very low-attenuation, with values below 0.1 dB. Longitudinal, transverse, and angled fiber end offsets can also lead to attenuation. Scratches and cracks on face surfaces not only increase attenuation, but can also damage the coupled face surface on the opposite side. Assembly errors such as a notch from the outside on fiberglass during assembly can also lead to attenuation or even breakage at a later point in time.

- Bending losses: Minimum bending radii are listed in the fiber-optic cable data sheets. Values below them cause losses, and the attenuation increases accordingly. Part of the light escapes from the core. Several years ago, GOF fibers that can be bent to very small angles were developed for the multimode and singlemode fiber-optic cables. Bending radii of under 10 mm can be achieved with fibers that are less sensitive to bending. The fibers are specified in the corresponding international series IEC 60793-x and ITU-Tx standards. The advantage is that they can be routed under poor installation conditions in buildings, residential units, and industrial environments.

- Manufacturing losses: The material used to manufacture fiber-optic cable and the manufacturing process itself could cause attenuation. Causes can be material-specific or the result of contaminants, for example. Fiberglass is manufactured such that its is optimized for specific wavelength ranges. In these wavelength ranges, attenuation is as low as possible. The attenuation coefficients that apply for these wavelengths are listed in the data sheets accordingly. The fiber-optic cables should be operated within those ranges.

Dispersion in fiber-optic cables

Signal distortion during the runtime from transmitter to receiver

Effects of dispersion

Dispersion also limits the data rates and transmission bandwidths of fiber-optic cables. Dispersion is when signals are deformed. During the runtime from transmitter to receiver, the signal loses amplitude and the edges continuously drop off. When two signals are sent back to back, the recipient can no longer tell if there is one signal or two. This results in transmission errors. The higher the transmission bandwidth and the longer the link length, the more important it is to focus on low dispersion. For long singlemode routes in particular, this is a key factor for reliable, error-free transmission quality.

FO portfolio from Phoenix Contact

Comprehensive product portfolio for fiber-optic cabling

Products for FO-based data cabling

Phoenix Contact offers an extensive product portfolio for FO cabling. Alongside a comprehensive selection of cables and the matching connection technology, device connections, patch panels, couplings and distributors for DIN rails round out the portfolio.

  • Transmission rates of up to 40 Gbps
  • Solutions for IP20, IP65/IP67, and IP68
  • For all common fiber types
  • For all common interfaces
  • Maximum protection against the effects of EMC and ESD