The C-band, ranging from 1530 nm to 1565 nm, stands for the conventional band (see Figure 1). It is the primary wavelength band used for optical communication especially in a long-distance (i.e. inter-continental and trans-oceanic) transmission system, because the attenuation of optical fiber has its minimum in the C-band. The current lowest loss of optical fiber is 0.1419 dB/km at 1560 nm, which was reported in 2017 .
Figure 1: O/E/S/C/L-bands
Another important factor, that has made the C-band the primary wavelength band for optical communication, is the invention of erbium-doped fiber amplifier (EDFA). An EDFA enables high-gain optical amplification with low noise, and has enabled long-distance optical transmission without using an O-E (optical-to-electronic) and E-O converter.
Figure 2: EDFAs used in optical transmission system
Fiber chromatic dispersion in C-band
The conventional single-mode fiber possesses a chromatic dispersion coefficient of around 15-17 ps/nm/km in the C-band (see Figure 3), and chromatic dispersion compensation is necessary for high-speed and long-distance transmission systems. Optical dispersion compensation devices, such as dispersion compensation fibers and fiber Bragg gratings, have long been used in an intensity-modulated, direct-detection (IM-DD) transmission systems.
Figure 3: Chromatic dispersion (CD) characteristics of conventional single-mode fiber (G.652.D ),
showing five optical transmission bands and max/min CD coefficients.
Digital-coherent detection
Digital-coherent detection has now become common in many state-of-the-art, long-haul and submarine optical transmission systems. In a digital-coherent detection system, the signal is sent by changing the phase of light, and is detected by measuring the phase. The phase modulation/detection allows electronic dispersion compensation using a digital signal processor (DSP), and optical dispersion compensation devices can be eliminated from the system.
Increasing bandwidth outside C-band
Now optical communication using the C-band has become very mature, and demand for further transmission capacity increase never ends. In order to meet such capacity demand, optical transmission using the L-band has been considered and now is commertially available. Even DWDM transmission in the S-band has attracted attention in R&D stage.
FiberLabs products related to this article
FiberLabs offers the following products related to this article. Please visit these pages if you are interested in this article.
Y. Tamura et al., “Lowest-Ever 0.1419-dB/km Loss Optical Fiber,” in Optical Fiber Communication Conference Postdeadline Papers (2017), paper Th5D.1, p. Th5D.1, Optical Society of America (2017) [http://doi.org/10.1364/OFC.2017.Th5D.1].
