Light and Technology: Where does light come from in optical communications?

What is the nature of sunlight and light, and how is it different from the light in fiber optic communications? With these questions in mind, we begin our journey in this issue of “Light and Technology”.

The study of light in ancient China is relatively long. In the “Mo Jing” compiled by the scientific sage Mozi, there is a saying: “The person with light is as warm as a shot. People who are below are also high, and those who are high are also below. The light is enough to block the light below, so the scenery is above; The head covers the upper light, so the scenery is below. There are ends in the distance and the light, so the scenery is in the library.” The general meaning is that “when light shines on a person (a person of light), the light reflected by the human body advances in a straight line like an arrow (Xu Ruo shoots). In this way, the lower part of the person is imaged at the height of the screen; the upper part of the person is at the top of the screen. There is an image on the bottom of the screen (the person below is also high, and the person above is also below). That is to say, the human image is inverted.

Why? This is because the lower part of the light from the feet is blocked; the upper part of the light from the head is blocked (the feet block the lower light, so the scene is above; the head blocks the upper light, so the scene is above) Viewed below). But precisely because there are small holes near or far along the light path that allow light to penetrate, the image formed in the camera obscura is a bright image (there are ends in the far and near and in the library of light scenes).

It can be seen from the above description that the Mo Jing not only explains the situation of small hole imaging, but also points out the nature of the straight forward movement of light, but it is limited to these. The research on the light of future generations is more or less repetitive and does not go deeper.

It can be seen from the above description that the Mo Jing not only explains the situation of small hole imaging, but also points out the nature of the straight forward movement of light, but it is limited to these. The research on the light of future generations is more or less repetitive and does not go deeper.

The in-depth study of the nature of light requires the first industrial revolution. And this started the debate between “the wave nature of light” and “the particle nature of light”.

The in-depth study of the nature of light requires the first industrial revolution. And this started the debate between “the wave nature of light” and “the particle nature of light”.

There is a famous experiment on the wave nature of light, “Young’s Double Slit Interference Experiment”. I believe everyone is aware of its weirdness. Later, Maxwell predicted that light, like radio, is also a very high-frequency electromagnetic wave and is essentially an electromagnetic phenomenon. The confirmation of this phenomenon was accomplished by Hertz, who used experimental methods to measure the speed of electromagnetic waves, which is approximately the speed of light. Many phenomena can be explained by using the wave nature of light, but sometimes light does not behave like a wave. Hence the subsequent study of the particle nature of light.

The particle nature of light was first recognized by scientists led by Newton. They believed that the essence of light is particles. It was Hertz who really discovered this phenomenon. It was during his experiment to prove the wave theory that he found that if one of the two zinc balls was irradiated with ultraviolet light, it would be very easy for the electric spark to jump between the two balls. Considered the “photoelectric effect”. The explanation for this phenomenon was proposed by Planck, who believed that light is energy and should be emitted part by part. Later, Einstein founded the photoelectric effect theory and proved the particles of light.

The process of understanding the nature of light can be described as twists and turns. In the 20th century, it finally came to an end with the conclusion that “ light has wave-particle duality “ . Here we will not elaborate too much on the historical “battle” between them.

Through their in-depth understanding of light, scientists successfully demonstrated the laser for the first time in 1960. What we usually call light in optical communications refers to the light in lasers , that is, the lasers in optical modules. The laser can be said to be the heart of the optical module, and its cost accounts for about 50% of the total cost of the optical module.

Let’s take a look at the English word “laser”, which evolved from the abbreviation of LASER (Light Amplification by Stimulated Emission of Radiation), which is the optical amplifier of stimulated emission of radiation. The laser is composed of three parts: the working material, the pump system and the resonant cavity. The pump system delivers energy to the working material to achieve particle inversion, while the resonant cavity continuously amplifies the stimulated radiation light and outputs it. Stable laser.

Pump sources can take many forms, the two most common being optical energy and electrical energy. For optical pumping, it can be a lamp. Electrical pumping can be achieved by direct current (such as laser diodes), discharge (noble gas lasers and excimer lasers) or radio frequency discharge. The figure below lists the classification of some lasers.

The spectra of some lasers are listed for reference, as follows:

However, when lasers were first invented, there were no semiconductor lasers that could work at room temperature. There were only light-emitting diodes (LEDs) (which may be different from the LD diode laser in the picture above), and their spectrum was around 850nm. It is the first window of optical fiber communication as we know it. The light wavelength of this window requires a multi-mode optical fiber to ensure that the light emitted by the LED can be effectively injected into the optical fiber. With the continuous reduction of optical fiber attenuation and the diversification of optical fiber types, other windows for optical fiber communication are gradually opening.

The lasers currently used in optical modules in fiber optic communications are mainly tunable lasers such as VCSEL (vertical cavity surface emitting laser), FP (Fabry-Perot laser) and DFB (distributed Bragg feedback laser). kind of laser.

Among them, DFB lasers are more expensive than FP lasers, and FP lasers are more expensive than VCSEL lasers. VCSEL lasers are used for transmission distances of tens of meters , FP lasers are generally used for optical modules within 40km, and DFB lasers are generally used for transmission distances greater than 40km.

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