Optical transmitter consists of input interface, light source, driving circuit, monitoring circuit and control circuit. Its core is light source and driving circuit. In digital communication, the input circuit shapes the input signal (such as PCM pulse) and converts it into a code suitable for line transmission, and then drives the circuit light source or sends it to an optical modulator to modulate the continuous light wave output by the light source. In order to stabilize the output average optical power and working temperature, automatic temperature control and power control circuits are usually set up.
As we all know, information processing is completed in the field of electricity. In optical fiber communication, we must convert electrical signals into optical signals in order to spread on optical fibers. In optical fiber communication system, information is carried by light waves emitted by LED or LD, which is the carrier wave, and the process of loading information onto light waves is modulation. Optical modulator is a device to realize the conversion from electric signal to optical signal. Modulation methods are usually divided into two categories, namely analog modulation and digital modulation. There are two kinds of analog modulation, one is to directly modulate the intensity of light source with analog baseband signal (D-IM); The other uses continuous or pulsed radio frequency (RF) waves as subcarriers. The analog baseband signal first modulates its amplitude, frequency or phase, and then uses the modulated subcarrier to intensity modulate the light source. The advantages of analog modulation are simple equipment and narrow bandwidth, but poor anti-interference performance and noise accumulation during relay. Digital modulation is the main modulation mode of optical fiber communication. After sampling and quantizing the analog signal, the optical carrier is switched and modulated with binary digital signal "1" or "0" and pulse coded (PCM). The advantage of digital modulation is that it has strong anti-interference ability, and the influence of noise and dispersion will not accumulate during relay, so it can realize long-distance transmission. Its disadvantage is that it needs a wider frequency band and the equipment is complicated. According to the relationship between modulation mode and light source, there are two kinds of direct modulation and external modulation. The former means oscillation parameters (light intensity, frequency, etc. ) directly controlled by the electric modulation signal to obtain the amplitude modulation wave or frequency modulation wave of the optical frequency, also called internal modulation; The latter is to let the optical carrier with constant amplitude and frequency output by the light source pass through the optical modulator, and the optical signal modulates the amplitude, frequency and phase of the optical carrier through the modulator. The advantage of direct modulation of light source is simplicity, but the modulation rate is limited by carrier lifetime and performance degradation (such as frequency chirp) at high speed. External modulation requires a modulator, which is complex in structure, but can obtain excellent modulation performance, especially suitable for high-speed operation. According to the parameters of modulated light wave: intensity modulation, phase modulation, polarization modulation, etc. At present, baseband direct intensity modulation, subcarrier intensity modulation and light source digital modulation are widely used in optical fiber communication, and external modulation is used at high speed.