The Principle and Advantages of Coherent Optical Communication

The key technology trend for achieving 400G and 100G transmission rates in data centers and network infrastructures today is coherent optical communication transmission technology. The transmission capacity is increased by using more dimensions, polarity, amplitude, phase, and frequency of the optical wave to transport more modulated information.

The main technologies used in coherent optical communication are coherent modulation and outlier detection.

The optical signal must be coherent light, such as a laser, for coherent modulation, which changes the frequency, phase, and amplitude of the optical carrier (rather than just the intensity of the light as in intensity detection).

Outlier detection is a method of obtaining an IF signal that varies in frequency, phase, and amplitude in the same pattern as the signal light by mixing a laser beam created by native oscillation with the input signal light in an optical mixer.

1. The working principle of coherent optical communication

At the transmitting end, the signal is modulated to the optical carrier by means of external modulation for transmission. When the signal optical transmission reaches the receiving end, it is first coherently coupled with an oscillating optical signal, which is then detected by a balanced receiver. Outlier detection, the optical signal is converted by photoelectricity to obtain an IF signal, which needs to be demodulated twice to be converted into a baseband signal.

2. Main advantages of coherent optical communication

(1) High sensitivity and long-relay distance

Coherent detection of coherent optical communication can improve the sensitivity of the receiver. Under the same conditions, coherent receivers outperform ordinary receivers by about 20dB, can achieve high performance close to the limit of scattered particles of noise, and can increase optical signal transmission distance without relaying.

(2) A high level of selectivity, and the ability to communicate.

The selectivity of the receiver can be improved by coherent optical communication. The receiving band in direct detection is large, and in order to suppress noise interference, the detector is usually placed in front of the narrow band filter, but the frequency band is still very wide.

The detection of coherent outliers in coherent optical communication is a mixture of signal light and fundamental light, allowing only noise in the IF band to enter the system, while other noise is filtered out by the microwave if amplifier's narrower bandwidth, resulting in good filtering performance.

At the same time, due to the high wavelength selectivity of coherent detection, coherent receivers can reduce the frequency interval of a frequency division multiplexing system, resulting in dense wavelength division multiplexing (DWDM), rather than the traditional large frequency interval optical multiplexing technology, with the potential benefit of higher transmission rates.

3. With a wide range of modulation techniques

In coherent optical communication, you can use PSK, DPSK, QAM, and other modulation formats in addition to amplitude modulation of light, which is ideal for flexible engineering applications.

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