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Optical transmitter working principle and use

Optical transmitter working principle and use

Optical transmitter working principle and use

Optical transmitter consists of input interface, light source, driving circuit, monitoring circuit, control circuit, etc. Its core is light source and driving circuit. In digital communication, the input circuit shaper the input signal (such as PCM pulse) and transform it into a code suitable for line transmission and then drive the light source of the circuit or send it to the light modulator to modulate the continuous light wave output of the light source. In order to stabilize the output average optical power and operating temperature, it is usually necessary to set up an automatic temperature control and power control circuit.

Light source modulation

As we all know, information processing is done in the field of electricity. In optical fiber communication, we have to convert electrical signals into optical signals so that they can be transmitted over optical fiber. In the optical fiber communication system, the information is carried by the light wave emitted by LED or LD, the light wave is the carrier wave, and the process of loading the information onto the light wave is modulation. An optical modulator is a device that converts electrical signals into optical signals.

Modulation is usually divided into two categories, namely analog modulation and digital modulation.

There are two types of analog modulation, one is to use analog baseband signal to modulate the intensity of the light source directly (d-im); The other USES continuous or pulsed radio frequency (RF) waves as the sub-carrier, analog baseband signal to its amplitude, frequency or phase modulation, and then the modulated sub-carrier to modulate the intensity of the light source. Analog modulation has the advantages of simple equipment and narrow bandwidth, but it has poor anti-interference performance and noise accumulation during relay.

Digital modulation is the main modulation mode of optical fiber communication. After the sampling of analog signals is quantized, the binary digital signal "1" or "0" is used to carry out on-off modulation on the optical carrier and pulse coding (PCM) is carried out. Digital modulation has the advantages of strong anti-interference ability, and the influence of noise and dispersion does not accumulate in the relay, so it can realize long-distance transmission. However, it has the disadvantages of wide frequency band and complex equipment.

According to the relationship between modulation and light source, there are direct modulation and external modulation. The former refers to the direct use of electrical modulation signal to control the oscillation parameters of semiconductor light source (light intensity, frequency, etc. The latter is to make the light output of the amplitude and frequency constant carrier of light through the optical modulator, optical signal by modulator optical carrier such as amplitude, frequency and phase modulation, the light source directly to the advantage of modulation is simple, but the modulation rate by the carrier lifetime and high rate of performance degradation under restrictions (such as frequency chirp, etc.). External modulation mode requires modulator, which is complex in structure, but can obtain excellent modulation performance, especially suitable for high rate applications.

According to the parameters of the modulated light wave: intensity modulation, phase modulation, polarization modulation and so on.

At present, baseband direct intensity modulation, subcarrier intensity modulation and digital modulation are mostly used in optical fiber communication.

Optical Transmitter

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