At trade shows, people from the Telecoms Industry often ask us what makes a ‘Specialty Fiber’ special? This question has given us the idea of featuring one range of Fibercore products each month in a short article designed to provide the answers – starting with SM.
Q. What makes Fibercore SM fibers special?
A. Three things – operating wavelength, resistance to bend-induced attenuation and strength
Standard telecommunication fibers are typically designed to operate in the second and third telecommunications windows between 1310 nm and 1550 nm. Fibercore SM-series fibers are designed for various wavelength ranges from 476 – 532 nm (SM450) to 1550 nm + (SM1500) to enable different visible and near infra-red optical sources (lasers, LEDs, SLDs etc.) to be used. These different operating wavelength ranges are achieved by changing the diameter of the core – smaller diameters optimize transmission for shorter wavelengths.
Resistance to Bend-Induced Attenuation:
Standard telecommunications fibers are typically used in a straight line – with bend diameters less than about 30 cm rarely encountered. Fibercore SM fibers, on the other hand, are often used in very small diameter coils. For example the biggest use of SM is fiber hydrophones, which can be as small as 10 mm in diameter – they need to be to reduce hydrodynamic drag when they are pulled through the water and to pack the longest optical path length into the smallest possible volume. Light just cannot travel through a telecoms fiber bent to 10 mm – but some of our SM fibers (mainly the SM1500 series) have very high numerical apertures (NAs) to provide exceptionally strong guidance and virtually lossless transmission under these conditions. Typical telecoms fibers have NAs of around 0.13 – compared with 0.3 for SM1500(4.2/80).
The small bend-diameters that Fibercore SM-series can experience puts them under enormous stress which, over time, can cause them to break – a phenomenon called static fatigue. We prevent this in two ways – by reducing the fiber diameter and by increasing the proof-test level, relative to telecoms fibers. Standard Corning SMF28 telecoms fiber, with an outside diameter of 125 μm and proofed to 1% strain (around 100 kpsi stress) could fail immediately if coiled to 10 mm. By comparison, SM1500(4.2/80) at 80 μm diameter and proofed to 3% strain (300 kpsi stress) could last 25 years and SM1500(4.2/50) even longer.
Fibercore’s SM product range can be found at http://www.fibercore.com/