Conventional Band (C-band)

Why C-band is so common in optical communication?

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.

• (*) HP stands for “High Power”.
• (*) 4-λ CWDM uses 1531, 1551, 1571, and 1591 nm.
• (*) 8-λ CWDM uses 1471, 1491, 1511, 1531, 1551, 1571, 1591, and 1611 nm.

Reference

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