Optical Fiber Technology

- Jul 21, 2020-

With the rapid development of science and technology, optical fiber has been expanding in the fields of communication, electronics and electric power, becoming a promising new basic material. The accompanying optical fiber technology also wins people's favor with novelty and convenience.

Complete power transmission function
Larian Corporation of the United States successfully used optical fiber to complete the power transmission function, opening up a brand-new way in the power field. They use semiconductor laser diodes at the transmitting end to convert electrical energy into laser light for transmission in optical fibers, and use solar cells as the receiving end device. This device uses 300-micron thick gallium arsenide as an insulating substrate, covered with a 20-micron thick solar cell. It is divided into 6 independent areas, which are connected in series by gold-plated air bridges. When the laser light transmitted by the optical fiber hits the solar cell, the light energy immediately becomes electrical energy. The voltage generated by each area is exactly 1 volt, and the six areas in series have a voltage of 6 volts, which is sufficient for the control circuit of most sensors.

widely used
If the power of the laser diode is continuously increased and equipped with a complete power transmission system, optical fiber power transmission can be widely used in military, industrial, commercial and other aspects. France's Bogen laboratory, which specializes in computers, electronic equipment, signal processing and image technology, uses optical soliton and short pulses to achieve distortion-free transmission in optical fibers. This technology can solve the problems of chromatic dispersion and non-linear effects without requiring multiple regeneration devices along the optical cable. Only need to set up an amplifier every 100 kilometers or so when working. The solitary wavelets can pass through each other without interfering with each other. It is said that this new technology is used in submarine submarine within the range of 6450-12900 kilometers and can solve the problem of communication difficulties. An irregular carrier signal optical fiber communication technology developed by American communication security experts is specifically designed to deal with today's increasingly rampant and sophisticated eavesdroppers. This technology first converts useful information such as voice into digital pulse signals, and then encodes these digital pulse signals and modulates them on random microwave carriers that change irregularly. When sending, the laser transmitting device transmits the irregular carrier signal carrying information to the receiver via the optical fiber communication system. The receiver’s laser receiver uses special technology to synchronize and dynamically coordinate with the sending laser device, and finally completes the task of demodulating useful signals from irregular carriers. Using this technology, eavesdroppers will no longer be useful, they will only hear chaotic noises. Australia Pauline recently developed a fiber weighing scale that can weigh trucks with one fiber and one laser. This kind of fiber scale uses an optical fiber with very special resistance characteristics. When it is under pressure or tension, the optical fiber will be slightly deformed, causing the characteristics of the laser to change. At this time, the detector will immediately learn this change and convert it into an electrical signal change. This is reflected on the display panel of the instrument. Since the optical fiber is made of glass, it has moisture resistance and radiation resistance. More importantly, it is easy to install and maintain. It is suitable for installation on main roads in urban areas, around factories, airports and runways, warehouses, and ports. Continuous work for 24 hours. Therefore, in addition to weighing, it can also play a monitoring role, and the accuracy is far greater than that of existing electronic devices.

Plastic Optical Fiber
According to a recent report from the US Journal, a plastic optical fiber developed by Boston Optical Fiber Corporation, Massachusetts, has a transmission speed 30 times faster than the current standard copper wire, and is lighter, more flexible, and low cost than glass fiber. This kind of optical fiber uses the refraction of light or the jumping mode of light in the fiber to reach a higher transmission speed, and can transmit data at a speed of 3 megabits per second within 100 meters. At present, 370,000 kilometers of submarine optical cables have been laid all over the world. This length can almost circle the earth 10 times. Moreover, because lasers are used at both ends, repeaters for amplifying signals are no longer needed during transmission, which will greatly reduce the cost and call costs. According to reports, the world's largest capacity submarine optical cable connecting Europe and the United States is about to open. This submarine communication fiber optic cable connecting the world is being laid. This is the most magnificent project in the field of communication in the 20th century and is supported by 30 international telecommunications organizations around the world. It crosses the Atlantic Ocean, crosses the Mediterranean Sea, passes through the Red Sea and the Indian Ocean, and crosses the Strait of Malacca into the Pacific Ocean. With a total length of nearly 320,000 kilometers, it connects to 175 countries and regions, and can make 2.4 million phone calls or transmit hundreds of thousands of compressed images at the same time. The entire project cost 14 billion US dollars and is expected to be completed in 2003.

Composition principle
Optical fiber technology generally consists of three parts: the optical signal transmitting end, the optical fiber used to transmit the optical signal, and the optical signal receiving end.

The function of the optical signal transmitting end is to convert the electric signal to be transmitted into an optical signal through an electro-optical conversion device. Currently, the transmitting end electro-optical conversion device generally uses a light emitting diode or a semiconductor laser tube. The output light power of the light-emitting diode is relatively low, the signal modulation rate is relatively low, but the price is cheap. Its output light power and the drive current are basically linear within a certain range, which is more suitable for short-distance, low-speed, and analog signal transmission; The output power of the diode is large, the signal modulation rate is high, but the price is relatively high, and it is suitable for long-distance, high-speed, digital signal transmission. The function of the optical fiber is to transmit the optical signal at the transmitting end to the receiving end of the optical signal with as little attenuation and distortion as possible. At present, the optical fiber is generally used in the near infrared band. 0.84µm、1.31µm、1.55µmMulti-mode or single-mode quartz fiber with good transmittance. The function of the optical signal receiving end is to restore the optical signal to the corresponding electrical signal through the photoelectric conversion device. The photoelectric conversion device generally uses a semiconductor photodiode or an avalanche photodiode. The light emitting wavelength of the light source constituting the optical fiber transmission system must match the wavelength band of the low loss window of the transmission fiber and the peak response band of the photoelectric detection device. The transmitting end electro-optical conversion device adopts the central emission wavelength of0.84µmThe high-brightness near-infrared semiconductor light-emitting diode, the transmission fiber adopts multimode quartz fiber, and the receiving end photoelectric conversion device adopts the peak response wavelength of0.8µm-0.9µmSilicon photodiode. Each part will be further introduced below.

Folding optical signal transmitter
The driving and modulation circuit of the LED used in the system is shown in Figure 2. The signal modulation adopts the method of light intensity modulation, and the light intensity adjustment potentiometer is sent to adjust the static driving current flowing through the LED, thereby correspondingly changing the emitted light power of the LED , The set static driving current adjustment range is 0-20 mA, corresponding to the display value of the panel light transmission intensity driving display value 0-2000 units, when the driving current is small, the light emitting diode emission power and the driving current are basically linear, audio The signal is coupled to the negative input terminal of another op amp after being isolated by the capacitor, resistor network and op amp, and superimposed with the static driving current of the light-emitting diode to make the light-emitting diode send an optical signal that changes with the audio signal, and then through the optical fiber coupler This optical signal is coupled to the transmission fiber. The low end of the transmittable signal frequency can be determined by the capacitor and resistor network, and the low frequency response of the system is not greater than 20Hz

Folding optical signal receiver
It is the working principle diagram of the optical signal receiving end. The transmission fiber couples the optical signal from the transmitting end to the photoelectric conversion device photodiode through the optical fiber coupler. The photodiode converts the optical signal into a current signal proportional to it. The diode should be reverse-biased when in use, and the photocurrent signal is converted into a voltage signal proportional to it by the current-voltage conversion of the op amp. The audio signal contained in the voltage signal is coupled to the audio power amplifier to drive the speaker to sound through the capacitor and resistance. The frequency response of the photodiode is generally high, and the high frequency response of the system mainly depends on the response frequency of the operational amplifier.

Transmission fiber
At present, the optical fiber used for optical communication generally uses silica fiber. It is covered with a cladding layer with a small refractive index n1 inside the core with a large refractive index n2. The light is fully distributed on the interface between the core and the cladding. The reflection is restricted to propagate in the core of the fiber. As shown in Figure 5, the optical fiber is actually a kind of dielectric waveguide. The light is locked in the optical fiber and can only be transmitted along the optical fiber. The core diameter of the optical fiber is generally from a few microns to hundreds of microns. According to the transmission light mode, it can be divided into multi-mode fiber and single-mode fiber, and can be divided into refractive index step type and refractive index graded fiber according to the different ways of fiber refractive index distribution. The refractive index step type fiber contains two circularly symmetrical coaxial media, both of which are uniform in texture, but have different refractive indexes. The refractive index of the outer layer is lower than that of the inner layer.

A graded index fiber is a kind of fiber whose refractive index is graded along the cross section of the fiber. The purpose of changing the refractive index is to make the group velocities of various modes similar, thereby reducing modal dispersion and increasing communication bandwidth. Multimode refractive index step-type fibers produce inter-mode dispersion due to the different group velocities of each mode transmission, and the transmission bandwidth is limited. Multimode refractive index graded fiber increases the signal transmission bandwidth due to its special refractive index distribution, which makes the group speed of each mode transmission the same. Single-mode fiber is a fiber that only transmits a single optical mode, and single-mode fiber can transmit the highest signal bandwidth. At present, single-mode optical fibers are mostly used in long-distance optical communications.

The main technical indicators of silica fiber include attenuation characteristics, numerical aperture and dispersion. Numerical Aperture: Numerical aperture describes the characteristics of fiber coupled with light source, detector and other optical devices. Its size reflects the ability of the optical fiber to collect light. As shown in Figure 5, the light incident on the end face of the optical fiber within the solid angle 2θmax is totally reflected at the internal interface of the optical fiber to be transmitted, and the light incident on the end face of the optical fiber outside the 2θmax range is The internal interface of the fiber does not produce total reflection but is transmitted to the cladding and is immediately attenuated. The numerical aperture of the fiber is defined as: NA=Sinθmax, its value is generally between 0.1 and 0.6, and the corresponding θmax is between 90 and 330 , Multimode fiber has a large numerical aperture, and the numerical aperture of single-mode fiber is relatively small, so generally single-mode fiber needs LD semiconductor laser as its light source.

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