New optical cables, advanced modulation technology and upgraded high-density multi fiber connectors mean that we may be closer to the Shannon limit than ever before.

In high-speed data transmission, researchers continue to find ways to reduce signal attenuation, distortion and sensitivity to external interference on copper-clad PCBs and cables. In the long run, optical fiber will replace copper cable. Advanced signal conditioning, multi-level modulation and error correction technologies enable engineers to design 112gb / s operation on dual axis copper cable, which is far more than expected a few years ago.

Each technology has its limitations, and the high-speed copper channel may be approaching the limit specified by the laws of physics. As the bandwidth demand increases, attenuation reduces the effective length of the channel. In addition to extremely low attenuation, optical fiber links provide higher bandwidth capacity, making optical fiber an attractive alternative.

Switching from copper to optical fiber makes a huge leap in the scope of the system

Long distance communication lines have been using the advantages of optical fiber for many years. The cost and power consumption of the photoelectric conversion process, especially the internal optical fiber interconnection, are the main problems. The research progress of silicon photonics and the characteristics of optical fiber are changing this situation.

Basic structure of optical fiber

Optical fibers are usually divided into multimode and single-mode. Multimode fiber can use low-cost LED light source to transmit multiple modes of light. Single mode fiber usually needs to use modulated laser, but it is characterized by greatly increasing coverage and bandwidth. Low cost plastic optical fiber is being used in relatively short and low data rate applications. The international organization for Standardization (ISO) standardizes the performance of optical cables through a series of OM 1-5 names.

Optical fiber has experienced a continuous improvement process in bandwidth, strength, attenuation reduction, easy installation and cost reduction. Early optical cables were very prone to signal attenuation and fracture due to rough or sharp bends. New single-mode and multimode fibers expand the bending radius range.

Axon cable offers hybrid cable and multi-core solutions

The glass used to form the photoconductor continues to be optimized, thereby reducing scattering loss, dispersion, polarization mode dispersion and micro bending attenuation. At present, the length attenuation of 1550m optical cable produced is only 0.15db/km. The proliferation of parks and Metro data centers is an emerging trend. The high-capacity optical communication link up to 100 km has become a crucial condition for the operation of large-scale network systems. Improving the cost-effective optical link capacity is a necessary condition to support the exponential growth of network traffic. One solution is to use multi-core fiber. Multi core optical fiber allows different signals to be transmitted simultaneously along different cores in the same optical fiber, so as to increase the data transmission density on a single optical fiber. Advanced, extremely high fiber density optical cables are entering the market to support increasing traffic. Furukawa recently installed 6912 optical fibers in a 1.25 inch diameter pipe between two North American data centers. Hollow core fiber is another interesting variant. Light is transmitted not through glass or plastic, but through the core of the air center. Manufacturing improvements reduce loss and delay characteristics, making hollow fiber attractive in applications that need to transmit light or data with very short pulses or minimum delay.

Airborn's raoc active fiber is suitable for harsh military and commercial aerospace environments.

Active optical cables (AOCS) have become very popular because of their ability to expand traditional copper cable components. Using a standard copper interface, the signal is converted into an optical pulse within the connector strain relief and coupled to the optical fiber. The opposite process occurs at the receiving end. From the perspective of installers, the reach range of optical cables is increased and the volume of optical cables is reduced. Rugged optical fiber is characterized by internal strength and rugged external jacket, which can be used in harsh military, avionics and industrial fields. The packaging options of optical fibers continue to expand, including flat band configuration, simplified wiring and reduced resistance to cooling air flow. High density, multi fiber MPO and MXC connectors can terminate up to 72 fibers.

Optical fiber can be made into standard and customized planar components. A plurality of fibers are bonded to a flexible substrate to form an optical backplane.

Engineers can meet the growing demand for network capacity by laying more fiber to make the existing fiber infrastructure more efficient. Parallel optics provides an alternative to the traditional single fiber or pair fiber.

Optical fiber network

The transmitter at one end communicates with the receiver at the other end to propagate a single data stream over multiple optical fibers. With this configuration, a parallel optical link can use four 2.5gb/s transmitters to send a 10Gb / s signal. By using light of slightly different colors, multiple data streams can be sent simultaneously through the same fiber, rather than using light of a single color.

WDM optical fiber scheme

The multiplexer on the transmitting side encodes multiple data streams with different frequencies, which are embedded in a light beam and coupled to a single optical fiber. The opposite process occurs at the receiving end of the channel. Bidirectional optical signals can be transmitted through an optical fiber. Through dense wavelength division multiplexing, up to 80 data channels can be multiplexed into one optical fiber.

Advanced modulation technology enables designers to further improve optical transmission links. Quadrature amplitude modulation (QAM) combines multiple levels of amplitude and phase changes to increase the capacity of optical data communication links.

Using the combination of amplitude, phase and polarization, coherence technology is the most robust and effective modulation method to optimize optical data transmission. Coherent transmission combines four levels of amplitude modulation and phase modulation and vertical and horizontal optical polarization to maximize the data capacity of a single fiber. The next generation 800gb link using this technology has entered the market.

These technologies have pushed the capability of optical fiber to a higher level. The industry is now facing the theoretical limit of approaching a single communication channel. The Shannon limit, which dates back to 1948, is the largest calculated error free data rate. Until a few years ago, considering the capacity of existing optical channels, it was not particularly noteworthy.

The demand for high-speed data links is being driven by a variety of trends, including the growth of super large data centers, the movement of computing resources to the edge, the continued adoption of 5g, and the expansion of optical fiber to the home. The progress of optical fiber performance and advanced modulation technology, together with the improved high-density multi fiber connector, provide a roadmap to achieve the goal for future high-speed computing and communication.