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The growing use of embedded and industrial computing brings with it increased demand for high-quality, long-distance, low-power and compact communication capabilities. Fiber optic technologies are answering this call thanks to the use of field-programmable gate arrays (FPGAs), and improvements to the transceivers, connectors and receivers. We have taken each area in turn, to help engineers pick appropriate components for their designs.
Meeting the Needs of Industry and Embedded Computing
Optical communications work by converting an electrical input signal to an optical—light—signal. The transmitter accomplishes the conversion in a fiber optic data link. After converting the electrical signal to an optical signal, the transmitter then sends—beams—the optical signals over an optical cable—fiber. The core of the fiber guide the light signals as they bounce off the cable cladding’s mirror-lined walls, which do not absorb any of the light from the core due to a principle, called total internal reflection. The fiber optic cable is the conduit that allows optical signals to propagate great distances. The quality of the cables and splice connections between cable sections is critical to the performance of the optical communications system. Although fiber optic cable is engineered to eliminate signal loss, some signals might degrade and require optical amplifiers to regenerate the signal depending on the amount of absorption, scattering, and dispersion they encountered while propagating through the fiber cable. Once the optical signals reach the receiver on the other end, they are once again converted back into the original electrical signal.
Optical communication speed comes down to how severely the signal gets distorted as it travels through the fiber with higher speed transmissions being more susceptible to get distorted, and when distortion is higher, you can get detection errors at the receiving end. Modern optical solutions exist partly to overcome the inherent limitations of radio frequency (RF) communications. Today’s optical communications technologies operate in a non-regulated spectrum, and are typically less power-hungry than RF, as well as being smaller, lighter, able to offer higher bandwidth and carry more data.
Pushing up bandwidth is vital, particularly in embedded and industrial applications. Fiber optics can carry wide-bandwidth signals, pushing into the GHz range, and you can multiplex lower-bandwidth signals onto the same cable. Moreover, the benefits for industrial use do not end there; fiber optics also offer a level of noise immunity, meaning cables do not necessarily require protective sheaths inside the conduit. Furthermore, fiber links do not contain enough energy to set off an explosion, which is ideal when operating in potentially volatile environments. In addition, fiber links do not require grounding, which makes them ideal in high voltage applications where arching might be an issue.
Fiber also has security advantages, including the fact that it does not produce an electromagnetic field that could be picked up by miscreants using external sensors or generates electrical interference issues to nearby sensitive electronics which could potentially compromise system security. Moreover, it is practically impossible to splice optical cable and steal the signal, in the way you can with copper cables.
From Telecoms and Networking to Embedded Computing And Industry, fiber optics have long been used in telecoms and networking. So given the advantages we touched on above, plus the fact that fiber optics can overcome the distance-limitations associated with traditional Ethernet, RS-232 and RS-422/485 cabling, it’s no surprise their use has spread into industrial systems, primarily for point-to-point connections.
Rugged, embedded systems, running data-intensive applications, are also making increased use of fiber optics for high-data-rate I/O, both for short and long links.
The Transceiver
Industrial and embedded equipment engineers can use transceivers to reduce the number of components they need, accelerate the design process and cut costs. A good example is the 650nm Broadcom AFBR-59FxZ range. These transceivers deliver 100Mbps Ethernet via a 2.2m jacketed standard polymer optical fiber (POF). The transceiver has a 650nm LED, for which the driver operates at 3.3V. These components can be used in a wide variety of industrial applications, including power-generation and distribution, factory automation and industrial vision.
Another option is the Finisar FTLX1x72x3BCL range. These are pluggable, SFF-8431-, SFF-8432- and 10GBASE-ER-compliant, multi-rate SFP+ transceivers. They support OTN, IEEE 802.3ae, 10G SONET, SDH and 8x/10x Fiber Channel over 40k links, as well as 6.144G/9.83 CPRI. They are intended for use in multi-rate 10Gb links using G.652 single-mode fiber, up to 40km long.
Figure 1: Finisar FTLX1672D3BCL. (Source: Finisar)
The FTLX1772M3BCL transceiver offers greater receiver sensitivity and optical transmit power than 1310nm 10GBASE-LR and OC-192 SR-1 options. The Finisar transceivers compensate for the greater fiber attenuation loss at 1310nm over 40km that you get with G.652 single-mode fiber, thanks to their optical link budget of 17dB. A two-wire serial interface provides digital diagnostics, as called out in SFF-8472. In addition, to improve the signal integrity of the host cards, as well as for SONET/SDH jitter compliance, these transceivers have internal transmitter and receiver re-timer integrated circuits. Applications include:
Adding an FPGA
Combining high-speed optical transceivers with FPGAs greatly cuts the length of the signal path between the chip’s I/O pad and the optical transceiver’s input. In turn, this helps reduce power usage, electromagnetic interference (EMI), jitter and data errors resulting from parasitic elements. It also enhances signal integrity.
In this space, Altera offers optical FPGA technology that helps overcome many of the challenges associated with power, port density, circuit board complexity, reach and cost. The Altera Arria® V Midrange FPGAs by Intel, for example, is a mid-range family of FPGAs. They feature 28nm process technology from TSMC, hard intellectual property (IP) blocks, and use 50% less power than earlier generations. Therefore, they are perfect for use in power-sensitive wireless infrastructure equipment, as well as HD video processing, image editing, intensive digital signal processing (DSP), switching and packet processing and 20G/50G bridging.
A single Arria V system-on-chip (SoC) gives you tight integration between a dual-core ARM Cortex-A9 MPCore chip, FPGA and hard IP blocks. Bandwidth can peak at over 128Gbps, thanks in part to the integrated data-coherency between the FPGA fabric and the processor.
Then there are the 28nm Altera Stratix® V High Bandwidth FPGAs also by Intel. These FPGA’s offer an enhanced core architecture, unique blend of integrated hard IP blocks and integrated transceivers, capable of delivering up to 28.05Gbps. As a result, these FPGAs are optimized for use in a completely new class of applications including:
On the Receiving End
So long as the signal levels in a fiber optic system are sufficient, the bit error rates (BERs) at the receiving end can be extremely low. Moreover, because fiber is not going to pick up EMI, signals on cables adjacent to one another are not coupled.
Component wise, Broadcom make the AFBR-25x1CZ Fiber Optic Receivers. These incorporate an integrated circuit with an integrated photodiode and provide TTL logic compatible output. These receivers support any signal from DC to 5MBd, up to 50m with 1mm 0.5 NA POF. Switch to 200µm 0.37 NA plastic-clad silica fiber (PCS), and the range jumps to 500m.
Figure 2: AFBR-25x1CZ recommended application circuit. (Source: BROADCOM)
Another benefit of these four-pin Broadcom receivers is the Versatile Link housing they come in, can be interlocked (N-plexed together) to minimize space and to provide dual connections with the duplex connectors.
These receivers are ideal for use in systems operating at 5MBd (or below), industrial control and automation, high-voltage insulation, extending RS-485 and RS-232, and to eliminate ground loops.
Connecting It Up
The world of fiber optic connectors is much improved. Creating a fiber optic termination used to involve cutting the fiber, polishing up the fiber’s end, and epoxying a connector onto it. It was labor intensive, and needed specialized equipment to create and test a connection.
The process itself has not really changed, but there are now good tools available to help with the cutting, aligning and joining processes. A typical connection loss nowadays would be between 0.2dB and 1dB, depending on the sort of connection you are creating.
If you are after an optical connector, consider the Ruggedized Optical Backplane Interconnect from TE Connectivity, which supports the VITA 66.1 standard. This product gives you a high-density blind-mate interconnect, in either a daughter card (mating plug) or backplane (receptacle) configuration. You can connect two MT ferrules, each with as many as 24 fiber paths.
Figure 3: TE Connectivity Ruggedized Optical Backplane Interconnect System. (Source: TE Connectivity)
As the name suggests, these connectors can be used in harsh environments and are also ideal where high-bandwidth is a must.
Addressing Other Challenges in Fiber Optic Communications
Despite all these advances and the abundant availability of components, there are still challenges when it comes to sending data using fiber optics.
For example, once you get beyond a certain threshold, increases in power can distort the data being transmitted through the fiber, rendering it useless when it reaches the receiver.
However, researchers are making progress. Recently a team of researchers from DTU Fotonik and others from Japan Fujikura Ltd. and NTT Corporation managed to successfully transmit 661Tbits/s of data through a single optical fiber from a chip-based optical frequency comb source. This colossal amount of data is analogous to more than the total internet traffic today.
Research organizations, universities, and private companies continue to explore ways to develop the next generation of faster, smaller, and more capable photonic devices and fiber cables as demand for higher bandwidths, higher data-rates, lower latency, lower energy consumption, and cost savings continue to increase on the road to 5G. So far some of this research points to some promising results with new devices capable of hundreds fold in increased fiber-optic speeds using a new light transmitting techniques and new specialized cables that allow spatial multiplexing along with specialized tri-mode fiber cables and new methods to reduce the power consumptions of the optics to ~5% compared to traditional techniques.
Looking Ahead
Much recent development in optical communications has focused around boosting the bandwidth of wavelength channels, increasing the number of wavelengths that can be sent through each fiber, boosting the overall distance optical fiber can be used, and more fiber integration in general.
Demand for creating solutions for use cases that require high volumes of real-time on-demand data while keeping costs and power-usage to a minimum is also on the rise.
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