With the continuous advancement of social informatization, the germination and development of emerging services such as ultra-high-definition video, cloud computing, and the Internet of Things have increased demand for bandwidth. Business needs drive technological progress. The single-channel transmission rate of the optical transmission system has experienced an improvement from 2.5Gbit/s→10Gbit/s→40Gbit/s→100Gbit/s, and the next-generation ultra-100G optical transmission system is also brewing breakthroughs.
How does the ultra-high-speed optical communication system of 100G and above obtain a great increase in the transmission rate? Compared with the low-speed optical communication system, what is the innovation of the 100G optical system?
ZR Cable Optical Communication System
Traditional optical communication systems use intensity modulation/direct detection (IM/DD), that is, the transmitter modulates the intensity of the optical carrier, and the receiver performs envelope detection on the optical carrier. This kind of system is simple in structure, easy to integrate and low in cost, but it can only use amplitude modulation, the frequency band utilization rate is very low, and the transmission capacity and relay distance of the system are also greatly limited.
Drawing on the idea of modulating electromagnetic waves in various ways to improve spectrum utilization in wireless communication systems, can we also modulate the frequency and phase of the optical carrier? The answer is yes, but the premise is that this optical carrier must be There is a definite frequency and phase (unlike natural light, which does not have a definite frequency and phase). So the researchers turned their attention to coherent light. Coherent light refers to light with the same frequency, non-vertical vibration direction, and constant phase difference.
Using this feature, the transmit signal can be modulated onto the optical carrier by means of amplitude modulation, frequency modulation, and phase modulation at the transmitting end using external optical modulation technology. At the receiving end, a laser beam generated by local oscillation and the input signal light are mixed in an optical mixer to obtain an intermediate frequency signal whose frequency, phase and amplitude change according to the same rules as the signal light. This is the coherent optical communication technology, that is, the key technology behind the ultra-high-speed optical communication system of 100G and above.
Since the modulated dimensions (amplitude, phase, and polarization state) of the optical signal are fully utilized to carry data, replacing ordinary light sources with coherent light can greatly improve the spectral efficiency, and further improve the performance of a single fiber while the available frequency band resources remain unchanged. transfer capacity.
To sum up, the main advantages of coherent optical communication technology are: it can use various modulation formats such as PSK, DPSK, QAM, etc., which is conducive to flexible engineering applications; under the same communication conditions, the sensitivity of coherent detection receivers is higher than that of ordinary receivers. The increase is about 20dB, which can extend the relay distance of the optical transmission system;
Coherent detection has excellent wavelength selectivity, and the coherent receiver can greatly reduce the frequency interval of the frequency division multiplexing system, namely dense wavelength division multiplexing (DWDM), which replaces the large frequency interval of traditional optical multiplexing technology and has the advantages of frequency division multiplexing. with the potential advantage of achieving higher transfer rates.
In practical application, based on the consideration of cost and compatibility, the laid fiber optic cable can be fully utilized, and the existing optical transmission system can be improved by upgrading and transforming the optical transceiver unit to improve the transmission data rate of a single wavelength channel. system capacity. The 100G optical communication system transformed by the above method has the best cost performance and feasibility.
