Interconnection
Sources of inconsistency and non-reliability in fibre-optic systems for industrial applications
By William Heath, MD, OMC I
ndustrial datalinks are among the most challenging types of fibre optic systems to specify, design and manufacture consistently. Well-designed systems can achieve 100 per cent yield at the point of installation, last for several decades with little or no maintenance, and exhibit zero field failures. However, commodity fibre optic components designed primarily with consumer-type applications in mind rarely meet the performance, robustness and consistency requirements needed for long-term industrial use. OMC has developed a range of unique features into its SMA-based fibre-optic interconnect systems specifically to meet the most exacting of high-reliability requirements, and as a result achieves a typical design life of up to forty years in many applications. Industrial fibre-optic data-communications links typically use LED sources, solid-state receivers and glass or plastic fibre to transmit data reliably in an industrial application such as a mass-transport, high voltage, petrochemical or CNC machining system, or in processing plants for transmitting data and command signals to and from equipment like industrial weighing scales and control devices. The Fibre SMA (FSMA) connector system was first introduced several decades ago and has an excellent track record for long-term reliability. While consumer and IT networking applications have largely moved away from the connector, opting instead for the convenience of sprung and latched plastic connectors that are quick to plug in and remove, FSMA remains widely used in the industrial sector due to the aforementioned track record, alongside its robust construction, suitability for a wider range of fibre sizes, and the ‘fit & forget’ nature of its thread-and-nut design.
But in challenging environments where a design life of decades is required, even within the standard FSMA interconnect system, there are several sources of inconsistency which can compromise datalink integrity in the medium to long term. These include: ● Transmitter launch power and receiver sensitivity variation from device to device ● The particular (and in most cases,
60 December/January 2024
random) way in which transmitters, receivers and optical cables are ‘paired up’ during installation. For example, a transmitter on the lower end of the output tolerance range, if paired with a receiver at the similar end of the sensitivity range, will fail early if those tolerances are too wide. ● Installation errors ● Variability in connector insertion loss between matings caused by rotational variation of the connector ● Electromagnetic interference ● Thermal issues, and ● Fine particulates in the environment that may contaminate the optical path. OMC has developed unique enhancements to the FSMA connector system which can be incorporated into its datalinks (transmitter, receive and fibre-optic cable), helping to address each and every one of these issues. Available enhancements include: ● Keyed SMA connectors. These are transmitter and receiver housings and corresponding mating fibre-optic cable connectors which have all the benefits of the FSMA connector; robustness of design, simplicity of installation and a secure
Components in Electronics
locking nut; but also feature a keyway system which ensures the connector is always installed in the same orientation, eliminating rotational variation. ● In addition, through the use of multiple keyways, OMC can help to eliminate installation errors by designing out the possibility of misconnection. ● Further, each transmitter and receiver device is actively aligned, meaning that it is powered up during the manufacturing process and the launch power or receiver sensitivity measured by machine, aligned, and fixed in place with the device characteristics recorded. This allows much tighter performance tolerances to be achieved, leading to greatly enhanced long-term reliability and ensured link integrity irrespective of how components are matched in the field. ● To address thermal issues, three approaches are employed. Firstly, active alignment as described above allows for better optical coupling, meaning lower drive currents can be used to achieve the same transmitter launch power, leading to reduced heat dissipation. Secondly, the transmitter and receiver housings have zero plastic components
and are instead precision-machined to rugged, all-metal designs, which act as a thermal heatsink. Thirdly, the transmitter and receiver devices are encapsulated with a proprietary thermally-conductive, electrically insulating compound which provides excellent thermal transfer between the semiconductor device and the housing. ● The all-metal connector design, which can be grounded, also helps screen the active device against EMI. ● To guard against physical stresses, the all-metal device housings are designed to be rugged and robust, with base flanges that feature secure screw-mounting points to provide a firm fixing to the PCB and to help to resist vibration and strain, while adding only minimal real-estate to the design. These mounting points are tapped to accept a permanent screw-mount connection, rather than the common approach of relying on press-fit or soldered pins. ● Lastly, when the application environment is expected to contain very fine airborne particulates such as dust, flour, paint or oil spray, there is the real danger that these particulates find their way into the optical path between the transmitter/receiver and the cable, building up over time and ultimately disrupting the link. To tackle this, all the device receptacles have been designed to allow OMC’s fine-particulate ingress shield to be optionally incorporated. With this in place, once the connector is inserted, the integral ingress-shield forms an automatic, air- tight barrier between the optical interface and the outside world, eliminating particulate ingress.
Summary
FSMA remains a connector system of choice in industrial fibre-optic applications and OMC continues to invest in developing this connector style further. The range of available enhancements has been well-received by industrial customers looking to achieve unparalleled system design life in a wide range of sectors.
http://www.omc-uk.com/
www.cieonline.co.uk.uk
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