At the sending end, just as the cargo needs to be placed in the container to facilitate the transportation of goods, the 40 G/100 G signal on the client side also needs to be installed in the OTU3/OTU4 signal "container". OTU3/OTU4 is actually signal containers of different sizes. OTU3 can hold 40 G signals, and OTU4 can hold 100 G signals. The laser light emitted by the laser is divided into two vertical directions of X and Y by the polarization beam splitter.

For the 100G coherent optical transmission systems, the OTU4 signal is converted into 4 signals, and the laser signals in the two polarization directions are respectively modulated by PM-QPSK. The modulated polarized light is combined into a laser beam through the polarization combiner and transmitted to the optical fiber line and sent to the remote. Similarly, for a 40G coherent optical transmission system, the OTU3 signal is converted into two signals, and the laser signals in the two polarization directions are respectively modulated by PM-BPSK. PM-QPSK (Polarization-multiplexed Quadrature Phase Shift Keying) and PM-BPSK (Polarization Multiplexed Binary Phase Shift Keying) both convert information signals into a suitable way for line transmission of signals. I won‘t discuss it in depth here. Readers who are interested in understanding can refer to related technical books in digital modulation technology.

At the receiving end, the received signal light is divided into the X and Y polarization directions by the polarization beam splitter. The local oscillator laser also separates the polarized light in the X and Y directions to be coherent with the received signal light. The coherent signal enters the DSP (Digital Signal Processing, digital signal processing) module after photoelectric conversion and analog to digital processing by the ADC (Analog to Digital Converter) module. The DSP module digitally compensates signal distortions such as chromatic dispersion and polarization mode dispersion on the optical path, and restores the original signal later.

Adopting the coherent receiving technology, the 40G/100G coherent optical transmission system does not need to be equipped with a fixed dispersion compensation module (DCM) and a tunable dispersion compensation module (TDCM), which reduces the configuration of fiber amplifiers in the system and does not require fiber link length and dispersion The precise measurement not only reduces the system configuration cost and manpower investment, but also improves the performance of the optical fiber transmission network.

40G/100G coherent signals can be mixed and transmitted with traditional 10G/40G wavelength signals, realizing the smooth upgrade of traditional networks equipped with DCM to 40G/100G coherent networks.