Advances in industrial and embedded optical communications

There is an insatiable demand for ever-greater communications bandwidth in industrial and embedded computing settings where distance, low power and small configurations matter. Specific enabling technologies such as FPGAs and advances in transceivers, connectors and receivers support the rapid evolution in optical communications.

Put simply, optical communications consist of a transmitter that encodes messages into optical signals, a channel to carry the signal to its destination and a receiver that reproduces the message from the optical signal. The speed of optical communications depends greatly on the distortions of the information signals, with the distortions generated from the signals' interactions with the molecules making up the fibres. The higher the speed of transmission, the more likely the signal will be distorted. When distortions are large, detection errors occur at the receiving end.

Spurred by the limitations inherent in radio frequency (RF) communications, today's optical communications operate at higher bandwidth and carry a greater amount of data from a package that is smaller, lighter and less power-hungry than RF, while operating in a non-regulated spectrum.

Given the critical nature of increasing bandwidth in industrial and embedded settings, fibre optic technology is able to carry very wide-bandwidth signals, into the GHz range, and lower-bandwidth signals can be multiplexed onto the same cable. Fibre optics in industrial applications provide a level of noise immunity that would require copper cables to be housed in protective sheaths inside conduit. And, within settings where potentially explosive atmospheres exist, fibre optic links do not store sufficient energy to ignite an explosion.

In both industrial and embedded applications there is a need for improved security - and optical communications has its benefits. Given that fibre optics do not generate EMI fields that can be picked up with external sensors, it is virtually impossible to 'steal' signals by splicing into optical fibres compared to the ease of doing so with conventional copper wiring.

Industrial and embedded computing needs

While used initially in telecommunications and wide area network (WAN) applications for many years, fibre optics have become increasingly prevalent in industrial data communications systems. As high data rate capabilities, noise rejection and electrical isolation became more important, fibre optic technology became increasingly ideal popular for industrial systems. In this segment, most often used for point-to-point connections, fibre optic links are being used to extend the distance limitations of RS-232, RS-422/485 and Ethernet systems.

Rugged embedded computing systems also require high-data-rate input/output signals, for which fibre optics are excellent. The I/O could be a relatively short link, connecting two plug-in modules, or it could be a longer run. In numerous data-intensive applications, the advantages of optical computing pay dividends.

Transceivers are used in embedded and industrial high-speed applications where they eliminate components, speed design and save money. The Avago AFBR-59FxZ compact 650nm transceivers, for example, implement Fast Ethernet (100Mbps) communications over 2.2mm jacketed standard Polymer Optical fibre (POF).

Applications for the AFBR-59FxZ transceivers include factory automation, industrial vision systems and power generation and distribution systems. The transceiver features a 650nm LED that is driven by a fully integrated driver IC operating at 3.3V. The IC is a linear integrated LED driver with differential input signals, converting input voltage into an output current for the LED.

In contrast, Finisar's FTLX1x72x3BCL pluggable Multi-Rate SFP+ transceivers are compliant with SFF-8431 and SFF-8432, 10GBASE-ER and support 10G SONET, SDH, OTN, IEEE 802.3ae, 8x/10x fibre channel over 40k links and 6.144G/9.83 CPRI. The transceivers are designed for use in 10-Gigabit multi-rate links up to 40km of G.652 single-mode fibre.

Finisar FTLX1772M3BCL transceivers also have higher optical transmit power and better receiver sensitivity than 1310nm 10GBASE-LR and OC-192 SR-1 transceivers, and they support an optical link budget of 17dB to compensate for the higher fibre attenuation loss at 1310nm over 40km of G.652 single-mode fibre.

In this arrangement, digital diagnostics functions are available via a two-wire serial interface, as specified in SFF-8472. The FTLX1772M3BCL transceivers use internal transmitter and receiver re-timer ICs for SONET/SDH jitter compliance and to enhance host cards' signal integrity. Applications include 10GBASE-ER/EW and 10G Fibre Channel (FTLX1672D3BCL), OTN G.709 OTU1e/2/2e FEC bit rates, 6.144G/9.83G CPRI, 8.5Gb/s Fibre Channel, 10G NRZ SONET, SDH, 10G Ethernet and Fibre Channel and G.709 OTN FEC bit rates.

FPGA integration

Addressing important power reduction and electrical signal path length requirements, the integration of high-speed optical transceivers and programmable devices dramatically reduces the signal path from the I/O pad of the chip to the input of the optical transceiver. The shorter path also lowers EMI and jitter, improves signal integrity and reduces data errors caused by parasitic elements.

Altera's Optical FPGA technology breaks through recent reach, power, port density, cost and circuit board complexity limitations. The Altera Arria V GX 13688 LABs 704 IOs family, for example, is a comprehensive offering of mid-range FPGAs. The Arria V is an excellent choice for power-sensitive wireless infrastructure equipment, 20G/50G bridging, switching and packet processing applications, high-definition video processing and image manipulation, and intensive digital signal processing (DSP) applications. Featuring TSMC's 28nm process technology and hard intellectual property (IP) blocks, it has 50 per cent lower power consumption than previous generations, and are said to be the lowest-power transceivers of any midrange family.

The family provides tight integration of a dual-core ARM Cortex-A9 MPCore processor, hard IP and an FPGA in a single Arria V system on a chip (SoC). It supports more than 128Gbps peak bandwidth with integrated data coherency between the processor and FPGA fabric.

Altera's 28nm Stratix V FPGAs, in comparison, include such innovations as enhanced core architecture, integrated transceivers up to 28.05Gbps and an innovative array of integrated hard intellectual property (IP) blocks. This combination is claimed to enable the Stratix V FPGAs to deliver a new class of application-targeted devices that are optimised for bandwidth-centric applications and protocols, including PCI Express (PCIe) Gen3, data-intensive applications for 40G/100G and beyond, and high-performance, high-precision digital signal processing (DSP) applications.

Receivers

At the receiving end, a fibre optic system provides very low bit error rates (BER) as long as it is designed to provide adequate signal levels and, since fibre does not pick up electromagnetic interference (EMI), signals on adjacent cables are not coupled together. AFBR-25x1CZ fiber optic receivers from Avago Technologies consist of an IC with an integrated photodiode providing TTL logic families that have compatible output. Along with Avago's AFBR-15x9Z or AFBR-16x9Z transmitter, any type of signal from DC to 5MBd at distances up to 50m with 1mm 0.5NA POF and 500m with 200um 0.37NA PCS are supported. The four-pin device is packed in a Versatile Link housing. Versatile Link components can be interlocked to minimise space while providing dual connections with the duplex connectors.

Applications include optical receivers for 5MBd systems and below, industrial control and factory automation, extension of RS-232 and RS-485, high-voltage insulation, elimination of ground loops and it reduces voltage transient susceptibility.

Fibre optic connectors

In the past, making fibre optic connections was labour-intensive and involved cutting a fibre, epoxying a special connector, and polishing the end of the fibre. This operation required specific tools and test equipment to ensure a good connection. While this technique is still used, devices used to cut, align and join fibres have been improved and simplified. Connection losses vary, depending on the type of connection, but typically range from 0.2 to 1dB.

The TE Connectivity Ruggedised Optical Backplane interconnect system, for example, delivers a high-density, blind-mate optical interconnect in a backplane/daughter card configuration. TE offers the optical system in both receptacle (backplane) and mating plug (daughter card) connectors that interconnect up to two MT ferrules, each accommodating up to 24 fibre paths. Typical applications are adverse environments and high-bandwidth computing applications requiring optical infrastructure. Supporting the VITA 66.1 standard, the connectors maximise optical performance.

Overcoming remaining barriers

Optical communications, however, are still not without their challenges. With fibre optics, for example, beyond a threshold power level, additional power increases irreparably distort the information travelling in the fibre optic cable.

Photonics researchers at the University of California, San Diego have recently announced that they have broken key barriers that limit the distance information can travel in fibre optic cables and still be accurately deciphered by a receiver. Published in the 26 June 2015 issue of the journal Science, their research has increased the maximum power - and therefore distance - at which optical signals can be sent through optical fibres.

In a laboratory environment, researchers successfully deciphered information that travelled a record-breaking 12,000km (7456 miles) through fibre optic cables with standard amplifiers, but without using repeaters. The breakthrough removes this power limit and extends how far signals can travel via optical fibre without a repeater. Removing periodic electronic regeneration when dealing with 80 to 200 channels saves substantial cost and enables a more efficient transmission of information.

The breakthrough uses wideband "frequency combs' to ensure that the signal distortions, or crosstalk, between bundled streams of information travelling long distances through the optical fibre are predictable and, most important, reversible at the receiving end. The frequency comb prevents the random distortions that make it impossible to reassemble the original content at the receiver.

The future for industrial and embedded optical communications

Recent optical communications advances concentrate on increasing the bandwidth of individual wavelength channels and the number of wavelengths transmitted per fibre. Ongoing advances will concentrate on supporting a variety of emerging applications that provide real-time, on-demand and high data-rate capabilities in a flexible, low-power and cost-effective way.

Optical communications are not without challenges. Work continues on bandwidth expansion, distance, power and integration. As the industrial and embedded segments continue to demand more rapid communications capabilities, mixed with greater security and lower price tags, optical technologies will continue to provide answers.

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