The Impetus Behind 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.

Figure 1: Fiber optic cables with terminations.

Simply, optical communications consists 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 generated from their interactions with the molecules that make up the fibers. 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 communications, today’s optical solutions 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, fiber optics is able to carry very wide bandwidth signals, into the GHz range and lower bandwidth signals can be multiplexed onto the same cable. Fiber optics in industrial applications provides a noise immunity that once needed to be housed in protective sheaths inside conduit. And, within settings where potentially explosive atmospheres exist, fiber optic links do not store energy sufficient 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 fiber optics does not generate EMI fields that can be picked up with external sensors, it is virtually impossible to 'steal' signals by splicing into optical fibers compared to the ease of doing so with conventional copper wiring.

Addressing Industrial and Embedded Computing Needs

While used initially in telecommunications and wide area networking for many years, fiber optics have become increasingly prevalent in industrial data communications systems. As high data rate capabilities, noise rejection and electrical isolation became more important, fiber optic technology became increasingly ideal for use in industrial systems. In this segment, most often used for point-to-point connections, fiber 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, for which fiber optics are ideal. 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 Fiber (POF).

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

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 fiber attenuation loss at 1310nm over 40km of G.652 single mode fiber.

In this solution, digital diagnostics functions are available via a 2-wire serial interface, as specified in SFF-8472. The FTLX1772M3BCL transceivers use internal transmitter and receiver re-timer IC's 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 company’s Arria V GX 13688 LABs 704 IOs family, for example, is a comprehensive offering of mid-range FPGAs. The Arria V is ideal 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 28 nm process technology and hard intellectual property (IP) blocks, it has 50% lower power consumption than previous generations, and 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 128 Gbps peak bandwidth with integrated data coherency between the processor and FPGA fabric.

Altera’s 28-nm Stratix V FPGAs, in comparison, include such innovations as enhanced core architecture, integrated transceivers up to 28.05 Gbps and a unique array of integrated hard intellectual property (IP) blocks. This combination allow the Stratix V FPGAs to deliver a new class of application-targeted devices that are optimized 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 fiber optic system provides very low bit error rates (BER) as long as it is designed to provide adequate signal levels and since fiber 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 50 meters with 1mm 0.5NA POF and 500 meters with 200μm 0.37NA PCS are supported. The 4-pin device is packed in Versatile Link housing. Versatile Link components can be interlocked to minimize space while providing dual connections with the duplex connectors.

Figure 2: Avago AFBR-25x1CZ Fiber Optic Receivers Recommended Application Circuit. Source - Datasheet

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.

Connectors

In the past, fiber optic connections were labor intensive and involved cutting a fiber, epoxying a special connector, and polishing the end of the fiber. 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 fibers have been improved and simplified. Connection losses vary, depending on the type of connection, but typically range from 0.2 dB to 1 dB.

TE Connectivity (TE) Ruggedized 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 fiber paths. Typical applications are adverse environments and high-bandwidth computing applications requiring optical infrastructure. Supporting the VITA 66.1 Standard, the connectors maximize optical performance.

Breaking Through Remaining Barriers

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

Photonics researchers at the University of California, San Diego just announced that they have broken key barriers that limit the distance information can travel in fiber optic cables and still be accurately deciphered by a receiver. Published in the June 26, 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 fibers.

In a lab environment, researchers successfully deciphered information that travelled a record-breaking 12,000 kilometers (7456 miles) through fiber optic cables with standard amplifiers, but without using repeaters. The breakthrough removes this power limit and extends how far signals can travel via optical fiber 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 fiber 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

Recent optical communications advances concentrate on increasing the bandwidth of individual wavelength channels and number of wavelengths transmitted per fiber. 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 is 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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