Security Encyclopedia Optical Fiber Welding Engineering Technology

With the development of optical fiber communication technology, optical fiber fusion welding technology is the most extensive and basic specialized technology in the field of optical fiber applications. It is one of the basic skills indispensable for security engineering technicians.

Under this background, Xiao Bian has conducted detailed exploration on optical fiber fusion splicing technology in order to provide reference for relevant research.

1 Fiber Overview

1.1 The concept of optical fiber

Optical fiber is a medium that transfers information from one end to the other. Is a glass or plastic fiber as a transmission medium for passing information. Fiber and coaxial cable are similar except that there is no mesh shielding. The center is a light-transmitting glass core. In a multimode fiber, the diameter of the core is 15 μm to 50 μm, which is roughly equivalent to the thickness of human hair. The single-mode fiber core has a diameter of 8 μm to 10 μm. The core is surrounded by a glass envelope having a lower refractive index than the core so that the optical fiber is held within the core. On the outside is a thin plastic jacket that protects the envelope. Optical fibers are usually bundled and protected by outer shells.

1.2 Differences Between Optical Fibers and Fiber Optic Cables

In general, the terms optical fiber and optical fiber cable will be confused. The optical fiber is covered by several layers of protection structure before it is actually used. The coated cable is called optical fiber cable. The outer protective structure can prevent bad environment from damaging the optical fiber, such as water, fire, electric shock and so on. Optical cables include: optical fibers, buffer layers, and coatings.

2 Optical fiber transmission characteristics

Since optical fiber is a transmission medium, it can be used like a normal copper cable to transmit data such as telephone conversations or computer data. The difference is that optical fiber transmits optical signals rather than electrical signals, and optical transmission is incomparable with coaxial cables. Advantages make it the preferred device for long distance information transmission. Therefore, the optical fiber has many unique advantages.

(1) Low transmission loss Loss is an important characteristic of the transmission medium. It only determines the distance required for transmitting the signal.

(2) Transmission frequency bandwidth The bandwidth of the optical fiber can reach 1 GHz or more.

(3) Strong anti-interference performance The carrier wave in optical fiber transmission is light wave, which is an electromagnetic wave with extremely high frequency, which is much higher than the frequency used in general radio wave communication, so it is free from interference, especially strong electric interference.

(4) High safety performance The glass material used in optical fiber is non-conducting and lightning proof. The optical fiber cannot be eavesdropped like a cable. Once the optical cable is damaged, it will be discovered immediately, and therefore the security is stronger.

(5) Light weight and good mechanical properties Fibers are as small as silk, and their weight is very light. Even with multi-core optical cables, the weight does not increase exponentially with the increase in the number of cores, and the weight of the cables is generally proportional to the outside diameter.

(6) Long life optical fiber transmission The average video cable length is 10-15 years. The service life of optical fiber cable is as long as 30-50 years.

3 Transmission Principle and Work Process

3.1 Principle of Optical Transmission

The propagation process of light waves in an optical fiber is performed using the principle of light refraction and reflection. In general, the diameter of an optical fiber core is more than several ten times higher than the wavelength of the propagation light. Therefore, the qualitative analysis using the method of geometric optics is sufficient. And the understanding of the problem is also very concise and intuitive.

When a bundle of fibers is projected onto the interface of two media with different refractive indices, refraction and reflection occur. For a series of interfaces formed by a multilayer dielectric, if the refractive index n1>n2>n3... >nm, the incident angle of incident light at each interface gradually increases until total reflection is formed. Due to the change of refractive index, incident light is deflected and the direction of propagation changes.

Undefined

The fiber consists of a core, a cladding, and a jacket. The role of the jacket is to protect the optical fiber and have no effect on the propagation of light. The refractive index of the core and the cladding is different, and the distribution of the refractive index of the helium mainly has two forms: a continuous distribution type (also referred to as a gradient distribution type) and an intermittent distribution type (also referred to as a step distribution type).

3.2 Fiber Transmission Process

First, a light emitting diode (LED) or an injection laser diode (ILD) emits an optical signal that propagates along the optical medium. At the other end, a PIN or APD photodiode is used as a detector to receive the signal. The modulation of the optical carrier is amplitude-shift keying, also known as intensity modulation (Intensity Modulation).

The typical practice is to represent two binary digits with the appearance and disappearance of light at a given frequency. The signals of the light emitting diode LED and the injection laser diode ILD can be modulated in this way. The PIN and ILD detectors directly respond to the brightness modulation. Power amplification - placing the optical amplifier in front of the optical transmitter to increase the optical power into the fiber. Improve the optical power of the entire line system. On-line repeater amplification - When the building group is larger or the distance between buildings is farther, it can play the role of relay amplification and increase the optical power. Preamplifier ─ Amplifies the micro signal after the photodetector at the receiving end to improve the receiving capability.