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Use the optical network to spread the information sent by everyone to all over the world. Next, we will introduce technologies that can quickly transfer massive amounts of information at once.

1. What is optical communication?

In short, optical communication is a technology that uses light to transmit information to each other.

1. Basic structure of optical communication

Computers and mobile phones around us send information through electrical signals "0 and 1". Optical communication consists of a "transmitter" that converts electrical signals into optical signals, a "receiver" that converts optical signals into electrical signals, and a circuit "optical fiber" that transmits light.

2. Advantages of optical communication

Long transmission distance, economical and energy-saving

Transfer massive amounts of information at once

fast communication speed

1) Long transmission distance, economical and energy-saving

Assuming that 10Gb of information (10 billion signals) is to be transmitted within 1 second, if electrical communication is used, the signal must be adjusted every 100 meters. In contrast, if optical communication is used, it is necessary to adjust the interval to more than 100 kilometers. The fewer times to adjust the signal, the less the number of machines is used, so it has the effect of economical energy saving.

For example, when talking with foreign friends or chatting online, it feels no different from calling in China. The sound doesn't lag like before. In the era of only electronic communication, the distance that can be transmitted at one time is short and the amount of information transmitted is small. International communication is mainly transmitted through artificial satellites as relays.

However, optical communication requires a long distance and a large amount of information to be transmitted at one time. Therefore, by using optical fiber cables laid on the seabed, natural and smooth communication with overseas can be realized.

Electric waves and light travel at the same speed. However, since the transmission path becomes longer via the satellite, the signal arrives slower. Submarine cables have a much shorter distance, so the signal will arrive faster.

2) Transmit massive amounts of information at one time

A large number of users can simultaneously receive desired information (movies or news, etc.). In 1 second, electrical communication can only transmit up to 10Gb (10 billion 0 and 1 signals) of information, compared to optical communication that can transmit up to 1Tb (1 trillion 0 and 1 signals) of information.

3) Fast communication speed

Electrical communication can be erroneous due to electrical noise, resulting in a decrease in communication speed. However, optical communication is not affected by noise, so signals can be transmitted quickly.

2. Where is optical communication used?

1. Optical communication exists around and around the world

Devices that use the Internet, such as mobile phones and IP phones, connect everyone to their region, to an entire country, and even to the global communication network. For example, the signals sent by computers and mobile phones are gathered at the base stations of local communication operators and network providers, and then transmitted to all parts of the world through optical fibers in submarine optical cables.

2. Various devices connected to the network

All kinds of devices we use in daily life can be connected to the Internet. The emergence of the Internet has made our lives more comfortable and convenient.

3. Why do you need optical communication technology?

1, communication volume

Our traffic is increasing every year. We usually exchange information when using mobile phones, text messages, receiving images, and online (virtual) stores. The performance of equipment improves year by year, and the method of use changes with it. We can imagine that the amount of communication will continue to increase in the future. Optical communication technology is used in information exchange.

2. Transmission volume

As the amount of communication throughout society increases, technologies that can transmit more information with just one optical fiber continue to emerge.

3. The unit of transmission volume

The transmission unit is bps, which is the abbreviation of bit per second, indicating the number of bits that can be transmitted within one second. For example, 1bps means that 1 bit of data can be transmitted within 1 second.

4. Devices used in optical communications (optical transmission devices)

1. What does the optical transmission device do?

The key parts of the optical communication network are equipped with optical transmission devices. This device serves many purposes.

Signal conversion (transmitting a signal): Converting an electrical signal into an optical signal.

Signal multiplexing: Multiple narrow signals are combined into one wide signal.

Signal relay: Long-distance transmission, relaying signals in the middle.

Signal steering: change the transmission direction of the signal.

Signal demultiplexing: decomposing multiplexed signals into their original individual signals.

Signal conversion (received signal): converts an optical signal into an electrical signal.

2. Optical transmission device

Various components are installed in the unit.

1) Convert (send signal), convert the received electrical signal into an optical signal.

2) Multiplexing, multiplexing multiple signals to send at the same time.

3) Relay, during the transmission process, the waveform and intensity of the signal deteriorate, so it is necessary to restore the waveform to the neat waveform of the original signal and increase the light intensity. If the waveform is severely degraded, it is necessary to temporarily convert the optical signal into an electrical signal, and after the waveform error is corrected, it is converted back into an optical signal for transmission.

4) Steering, according to the whereabouts of the signal, the optical switch switches the transmission direction of the optical signal.

5) Demultiplexing, which decomposes the multiplexed signal into the original separate signal.

6) Convert (receive signal). Convert the received optical signal into an electrical signal.

5. Communication methods (present and future)

The following describes the communication method through the car and the driveway. Assume that the car represents the time of occupying the lane (1 interval), the goods represent the amount of information (number of bits) transported each time, and the lane represents a wavelength of light.

1. Current communication speed: 10Gbps and 40Gbps per wavelength.

・Time Division Multiplexing (TDM: Time Division Multiplexing)

Because the information that can be transmitted each time is limited, it needs to be transmitted in different periods of time. For example, when multiple users send information at the same time, there is only one lane for carrying information, so trucks loaded with different information packages need to be lined up for transportation. When lanes are congested, transmissions slow down.

・Wavelength Division Multiplexing (WDM: Wavelength Division Multiplexing)

The amount of information that can be transmitted at one time is large, and the information of multiple users can be transmitted at the same time by changing the wavelength. For example, even if multiple users send information at the same time, as long as there are multiple lanes distributed, it is not easy to cause congestion, and the goods (number of bits) can be transported smoothly, and the transmission speed is relatively stable.

・Multi-level Modulation (MM: Multi-level Modulation)

A method of transmitting multiple signals in one section of one wavelength. By changing the waveform of light, the information of multiple users is transmitted on the same wavelength. A representative technique is the quadrature differential phase shift keying modulation method (DQPSK: Differential Quadrature Phase-Shift-Keying). Normally, each truck loads 1 bit of cargo, but when using "DQPSK", each truck can load 2 bits of cargo.

2. Future communication speed: 100Gbps transmission per wavelength

100Gbps is equivalent to the speed of transferring a DVD in about 0.4 seconds. (assuming converted to a DVD with a capacity of 4.7GB)

・Polarization multiplexing

The light moves forward while vibrating. The direction of vibration is called "polarized wave", which is divided into light that vibrates vertically (vertically polarized wave) and light that vibrates horizontally (horizontally polarized wave). The information contained in the polarized waves will not interfere with each other, and a large amount of information can be transmitted. For example, if two trucks are driving on one lane at the same time, the two trucks will not collide when transmitting information.

6. The supernatural optical network (example introduction)

With fiber optics all over the world, we can get high quality service in every occasion. Related examples are presented next.

Spoiler:

1. Why is the sky blue and the sunset red?

Have you ever wondered why the sky looks blue? The sky is blue for a reason. Different wavelengths of light produce different colors. When sunlight enters the Earth's air layer (atmosphere), it collides with dust (molecules) in the air, thereby changing the direction of the light.

That is, because the shorter wavelengths of light (compared to red and blue light) are more likely to collide with dust, the light tends to spread around. The reason why the sky looks blue is because the shorter wavelength blue light is diffused throughout the sky.

2. The wavelength of sunlight

The sun's light looks white, but it's actually a mixture of light from red to blue. That is, the wavelengths of light in sunlight vary.

3. The reason why the sky appears blue (sunlight collides with dust)

1) Sunlight enters the air layer. There is a lot of dust floating in the air layer.

2) The blue light with shorter wavelength is easy to collide with dust, and the light will spread around.

In addition, red light with a longer wavelength shuttles freely among the dust.

3) The blue light diffuses to the entire sky, and the sky appears blue from a distance.

4. The reason why the sunset is red

Why does the blue sky appear red in the evening? This has to do with how far the sun's rays travel through the air layer. As the sun sets, the sun's position moves horizontally from directly above us.

As a result, the distance that sunlight travels through the air layer becomes longer in the horizontal direction than when it is directly above, and the longer-wavelength red light that previously passed through the dust gaps begins to collide with the dust. Also, because blue light has a shorter wavelength and cannot reach far, only the sky covered in red light comes into view, so the sunset looks red.

The reason why the sunset appears red is that light with a longer wavelength is not easily scattered even after passing through the thick atmosphere, and has the characteristic of being able to reach a long distance. Optical communication uses this principle. In order to reduce the scattering in the optical fiber and achieve long-distance transmission, light with a slightly longer wavelength is used.

5. Bo's partner

There are different wavelengths and different types. The frequency used in optical communication is 1.3 microns or 1.55 microns, which is a kind of infrared rays.