OMC’s CEO, Will Heath, explores the process of purchasing fibre optic datalinks, including a case study highlighting the financial and engineering benefits of bespoke solutions.
If the application is a new design, three basic questions need answering to supply the best fibre optic data link: what is the data rate; over what physical distance is the data being sent; and are there limitations on drive current. With this information, the fibre optic system manufacturer can calculate the optical budget which determines the transmitter’s power and receiver’s sensitivity.
Buyers often expect to be asked if the system is polymer or glass fibre. However, for a new design, OMC prefers to specify the whole system link—transmitter, type of cable and receiver—to manufacture exactly what is required, also considering temperature and space constraints. OMC’s job is to deliver the lowest cost, most readily available solution including parameters such as fibre core size.
Design-life is also important. If relatively short—< five years—the LED may tolerate higher currents. However, in high reliability applications in mass transit, power plants or heavy industry systems are typically specified for 30 or 40 years. Determining if the circuit requires an analogue or digital receiver output is also important.
Things get interesting when purchasing requests are for obsolete components for live designs or legacy systems undergoing refurbishment.
Many existing systems feature fibre optic data links installed several decades ago. It’s no surprise that the transmitters, receivers and optical fibres are no longer available. However, what if the requirement is renewing one end of a datalink and the datasheet for the original fibre and transmitter/receiver is still available. The temptation is to source the same part or one with identical specifications.
The problem is older systems used universal components designed to transmit over the longest feasible distance and down any type/size of multimode fibre. However, if such universal parts have gone obsolete or can’t be found, the option is to make a custom part. In this case, let’s make a specific device designed to perform the exact task. This avoids the cost of special optics and semiconductors with an unnecessary performance range.
This was exactly the case for a mass transit application requiring a replacement transmitter and the initial request was for a part identical to the original datasheet. The old part was a universal device with a chip and micro-optic arrangement designed to focus light into fibre cores from 50 to 1000μm.
A like-for-like replacement was costed at about £40 and the semiconductor was on six months’ lead time. OMC found the fibre had a 200μm core and produced a transmitter characterised for the task, for around £10, using available components. Consider this application uses many tens of thousands of transmitters over many years, the long-term savings run into hundreds of thousands, if not millions of pounds.
New parts will need re-qualification. However, if the system has been installed for many years, the parts—even if still available—are likely to have changed anyway. It’s rare for parts to exactly match an old datasheet, so requalification is almost always necessary. Another advantage of specifying readily available materials is that it’s straightforward to quickly produce a sample batch for evaluation which helps facilitate and speed-up the requalification process.