What is Coherent Optical Communication System?

Coherent optical communication systems have many advantages and are widely used in long-distance optical transmission networks. In recent years, with the explosive growth of network traffic, data centers have also put forward new requirements for data transmission speed. Coherent optical communication has also shown many advantages in the short-distance optical interconnection of data centers, and has been widely used. research and concern.

This paper will introduce the basic composition of coherent optical communication system, three coherent detection methods and the digital signal processing algorithms that need to be used in coherent optical communication.

1. Basic structures of the coherent optical communication system

The basic components of a coherent optical communication system are the sending end is composed of ECL (external cavity laser), IQ modulator and AWG (arbitrary waveform generator) for modulating signal generation. The signal at the transmitting end reaches the receiving end after being amplified by the fiber transmission link and EDFA (Erbium fiber amplifier), the receiving end mixes the LO (local oscillator) signal and the receiving signal through a 90-degree optical mixer, then balances the detection, and finally performs DSP (digital signal processing) demodulation and recovery to obtain the transmitting end signal.

We can also use the dual-polarization feature of light to let the x-polarization and y-polarization of light carry information respectively, forming a polarization multiplexing coherent optical communication system, which can double the communication rate.

The basic structure is similar to the above. The only difference is that the X and Y polarization of the transmitting laser are modulated respectively, then the polarization beam is combined, and then the polarization beam is divided at the receiving end, and then mixed with the X and Y polarization of the vibrating light respectively, and then detected, demodulated in the digital domain, etc.

2. Coherent detection methods of the coherent optical communication

According to the relationship between the wavelengths of the transmitter laser and the receiver local oscillator laser, the coherent detection methods can be divided into three methods: homodyne, intradyne and heterodyne, and the two most commonly used detection methods are homodyne and intradyne.

Among the above three methods, the two methods, homodyne detection and intradyne detection, are the most commonly used because they bring less noise and reduce the power overhead of digital signal processing and the requirements for related devices.

3. Digital signal processing algorithms required for coherent optical communication systems

At the transmitting end, the modulation algorithm of the signal is mainly required, mainly including the generation of different modulation formats (such as OOK, PAM, QPSK, QAM, OFDM and other modulation formats), pulse shaping, etc.

There are two main algorithms for quadrature imbalance compensation: one is the Gram-Schmidt quadrature algorithm (GSOP), and the other is the ellipse correction method (EC).

For dispersion compensation, the frequency domain dispersion equalizer is mainly used.

For polarization equalization, the constant modulus algorithm (CMA) is mainly used.

The carrier frequency offset estimation algorithm for coherent optical communication system can be divided into two categories, one is the estimation algorithm based on the phase difference, and the other is the FFT algorithm based on the symbol or symbol phase.

For the carrier phase estimation algorithm, there are mainly constellation transformation (CT) algorithm, blind phase search algorithm (BPS), maximum natural estimation (ML) algorithm and the combination of these algorithms.

In addition, related nonlinear compensation algorithms include Voltera algorithm and some neural network nonlinear compensation algorithms. The relevant channel error correction coding algorithms mainly include LDPC coding, Turbo coding and so on.