Advantages of Optical Coherent Communication in Fiber Optic Networks

Optical coherent communication is a technology in the field of fiber optic communication, which has the technical advantages of longer transmission distance and larger transmission capacity. Therefore, it is widely concerned by all sectors of the industry and the research fever is rising. This article will introduce the advantages of optical coherent communication.

High Sensitivity and Long Relay Distance

One of the most important advantages of optical coherent communication is the ability to perform coherent detection, which improves the sensitivity of the receiver. In an optical coherent communication system, the magnitude of the output photocurrent after coherent mixing is proportional to the product of the signal optical power and the local oscillation optical power. Under the same conditions, the coherent receiver improves the sensitivity by about 18dB compared to the ordinary receiver, and can achieve the high performance close to the limit of quantum noise. This also increases the relay-free transmission distance of the optical signal.

Good Selectivity and Large Communication Capacity

One of the main advantages of optical coherent communication is that coherent detection enhances the selectivity of the receiver. In an optical coherent communication system, the magnitude of the output photocurrent after coherent mixing is proportional to the product of the signal optical power and the local oscillation optical power. Given that the local oscillation optical power is significantly higher than the signal optical power, the sensitivity of the receiver is greatly enhanced, enabling the detector to reach the quantum noise limit, and thus also extending the transmission distance of optical signals.

Dispersion Effects Can Be Compensated Using Electronic Equalization Techniques

In direct detection, to suppress the interference from noise, the received band is wide, and a narrow band filter is usually placed before the detector, though the band remains very wide. In coherent detection, the detector is a mixer of signal light and local oscillator light. Therefore, only noise in the IF band can enter the system, while all other noise is filtered out by the narrow bandwidth of the microwave IF amplifier. It is evident that coherent detection features superior filtering performance, which will play a significant role in the application of optical coherent communication. Moreover, due to the excellent wavelength selectivity of coherent detection, the coherent receiver can significantly reduce the frequency spacing in the frequency division multiplexing system, that is, dense wavelength division multiplexing (DWDM), replacing the traditional optical multiplexing technology of wide frequency spacing, with frequency division multiplexing to achieve the potential advantages of a higher transmission rate.

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