Communications are vital in military affairs. Submarine communication, however, has great difficulties relative to other platforms. Because the biggest advantage lies in the submarine concealment, so most of the time are submerged in the water, and the most commonly used military shortwave communication means it cannot be used for submarine communication of underwater.

The reason is that the shorter the wavelength of the electromagnetic wave and the higher the frequency, the greater the attenuation in the sea. Short wave attenuation in water is so rapid that it is almost impossible to travel through the sea. However, the longer wavelength of very long wave and ultra long wave attenuation in sea water is much smaller, and it can enter tens of meters to hundreds of meters of water, so it has become the main means of submarine communication.

Very long wave communication is a radio communication with a wavelength of 100 thousand to 10 thousand meters (3~30 kHz), also called very low frequency communication. Long wave propagation attenuation in sea water is small, and the depth of water can reach 20 meters, which is mainly used for one-way transmission of submarine. Long wave radio is composed of transmitter, antenna system, power supply equipment and so on. It is mainly used for sending letters to submarine and ocean surface ships, and is the most important communication equipment for submarine.

But very long wave communication in water depth and can not meet the requirements of submarine combat, long wave communication and people and the development of the longer wavelength, which is 1 million ~ 100 thousand M wavelength (frequency 30 ~ 300 Hz) radio communication, also known as the elf communication.

The attenuation of ultra long wave propagation in sea water is very small, and the depth of water entering is more than 100 meters. Super long wave transmitter can be used to send submarine to submarine, and its communication distance can reach thousands of kilometers. However, the long wave to submarine communication rate is very low, generally can only use the predetermined meaning of several letters combination for signal communication, send 3 words combination of signal about 15 minutes.

Although very long and very long wave to solve the problem of submarine communication, but these two kinds of radio communication scale is huge, the transmitter and antenna is very large, long wave antenna like hundreds of kilometers, to the transmitter power in a few megawatts, large investment, high operation cost, survival ability, and only can not accept that rate is very low, so generally used only for the high level of combat readiness under fire command.

The modern submarine is also equipped with satellite communication system and integrated communication buoy, but not easily, because they take the boat to the surface or near the sea surface is not only easy to intercept the enemy, signal, but also easy to expose the position of submarine.
In order to improve the communication ability of submarine, people are constantly exploring new technologies for submarine communication.

At present, the blue and green laser submarine communication is the most promising. In the last century in 60s, found that people in the study of wave propagation characteristics in the ocean, the sea of 450-550 nm band blue light attenuation ratio of attenuation to many other small optical band, which confirmed that in the sea there is a similar to that in the atmosphere light window. On the basis of this physical phenomenon, the use of laser operating in the blue and green bands, can develop new physical mechanism of communication equipment based on water, to solve the long troubled countries of the Navy diving under the communication problem has brought new hope.

After measuring, the blue-green laser with a wavelength of 450-530 nm has the least attenuation in the sea water, and the penetration depth can reach 600 meters. Therefore, when the blue-green laser is used for submarine communication, the submarine does not need to go up to the dangerous depth or surface. The blue and green laser submarine communication has high data rate, large transmission capacity, can transmit data, voice and image signals, narrow beam width, good direction, small and light equipment, has the advantages of anti-jamming and anti interception and anti damage ability and is not affected by electromagnetic and nuclear radiation.

With the development of quantum communication technology, there is a new hope for submarine submarine communication. In fact, quantum communication is still based on blue-green laser communications, and its greatest advantage lies in good communication security. In communication, satellites and submarines transmit photons to each other via blue-green laser, and quantum entanglement is realized by quantum entanglement.

In 90s of last century, China began to study blue and green laser submarine communication technology. As China's laser technology is world class, quantum communications technology is now at the world's leading level, so theoretically, China will be very advanced in the submarine blue, green laser and quantum communications technology.
According to Hongkong "Ta Kung Pao" reports, Chinese has begun in the submarine test of quantum communication technology, and achieved success. Although it is still not practical, but with advanced technology base, the future into practical, it should not be too long in time.


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