As the telecommunication industry slowly migrates to 100Gb/s transmission in long distance terrestrial optical networks and plans for even higher data rates of 400Gb/s and beyond, improving system OSNR and spectral efficiency are some of the challenges to be surmounted. While system manufacturers are resorting to coherent detection and digital signal processing, super channels, and advanced modulation formats to address these challenges, many fiber optic manufacturers are focusing on developing lower attenuation and/or higher effective area fibers. In this article, the importance of low loss and large effective area in optical networks are summarized. A list of leading low loss optical fiber products currently available on the market is also presented.
The growing demand for bandwidth (and the need for lower bandwidth cost/bit) is compelling transmission system developers to increase the data rate per channel of DWDM systems. 40Gb/s and 100Gb/s have already been standardized and many operators have already migrated to the higher data rate systems. Meanwhile 400Gb/s standards (400GbE and OTU-5) are expected soon and forward-looking engineers are already exploring 1Tb/s.
These developments come with a lot of challenges that make it difficult to use current technologies and processes to meet the required higher optical signal to noise ratio (OSNR) and spectral efficiency. To address these challenges, system manufacturers are resorting to using advanced modulation formats such as PM-QPSK and PM-16QAM and advanced digital signal processing. With these innovations, systems can now handle electronic compensation of chromatic and polarization mode dispersion.
With electronic dispersion compensation, non-zero dispersion shifted optical fibers remain only relevant for 10Gb/s and 100Gb/s direct detection systems. For 100Gb/s coherent systems and higher data rates, many fiber optic manufacturers are developing G.652 and G.654 compliant optical fibers with reduced attenuation and/or higher effective area to contribute to OSNR in terrestrial long-haul networks.
Since the conceptualization of transmission of light in optical fibers, the number one focus was the reduction of its attenuation. In 1965, Charles Kao determined that the fundamental threshold for optical fibers to be used in telecommunication was an attenuation below 20dB/km. It was not until 1970 that Corning scientists developed a process to fabricate optical fibers that met this threshold.
Since then fiber optic manufacturers have been trying to perfect their processes to lower attenuation and enable more capacity and extend the fiber reach. Typical single mode fiber attenuation in uncabled optical fibers has been reduced to about 0.2dB/km.
In response to higher data rates, each of the leading fiber optic manufacturers has developed at least one terrestrial single mode fiber optic product with even lower attenuation. Table 1 lists some of the leading low loss fiber optic products and their suppliers. Most of these suppliers have reduced their attenuation from their standard single mode fiber offering by at least 0.02dB
While 0.02dB may sound like a small number, it is a significant attenuation reduction leading to significant improvement in OSNR or fiber reach. In an experiment comparing the performance of 256Gb/s, PM-16QAM, 40 Channel DWDM signals over fibers with different attenuation values, a 0.02dB difference in attenuation resulted in >500km increase in reach from a baseline of 1,500km.
If optical power is lost along the fiber, why not launch light with enough power to overcome OSNR or reach limitations? When light with too much power is launched into a fiber, the power intensity (Power/Effective Area) alters the properties of the fiber leading to non-linear impairments. Self-phase modulation, cross phase modulation, four wave mixing and stimulated Brillouin scattering are some of the non-linear impairments resulting from high power intensity in optical fibers.
By increasing the effective area of the fiber, more power can be launched into the fiber while keeping the intensity below the non-linear effect threshold. Many submarine fiber optic products have larger effective areas than that of standard single mode fibers (typically 80μm2). In response to higher data rates, some fiber optic manufacturers are developing terrestrial single mode optical fibers with larger effective area. Table 1 also shows effective areas for some of the products.
These topics are covered in more detail in our Certified Optical Network Associate training course.
|Product||Standards Compliance||Supplier||1550nm Attenuation|
|Effective Area (μm2)|
|SMF-28 ULL||G.652.B||Corning||0.17||Not specified|
|Tera Wave ULL||G.654.B||OFS||0.17||125|
|SMF-28 Ultra||G.652.D/G.657.A1||Corning||0.18||Not specified|
|FullBand Ultra||G.652.D/G.657.A1||YOFC||0.18||Not specified|