Direct modulation of semiconductor lasers is to modulate the laser by superimposing the pre-modulated (AM, FM, PCM) signal current on the bias current of the laser. Because it is to change the intensity of light, it is also called intensity modulation. For digital transmission, although the speed is very high, only two working states of optical signal or non-optical signal are available, so long as the two states can be judged at the receiving end. This modulation method has several disadvantages:

1. When LD (Semiconductor Laser) is directly modulated, when the intensity changes, the wavelength also changes, that is, the frequency modulation (FM) of light is produced. This phenomenon is called additional frequency modulation (Chirping), which will further increase the modulation sideband.

2. Broad-spectrum signals with modulation, whose phase shifts due to the dispersion characteristics of optical fibers, will exhibit compound second-order distortion (CSO) at the receiving end.

3. The linear relationship between the output and the current is static when the direct light intensity modulates the LD laser. When the direct light intensity modulates the LD laser at high frequency, the frequency of the laser will change due to the interaction between the internal current and the light. In this case, a large composite second-order distortion (CSO) will also occur.

Although direct modulation is simple and low cost, due to the above problems, people are also studying other modulation methods. This is the technology of modulating the output light of the laser with an external modulator instead of the internal modulation of the laser.

External modulator technology is a kind of external modulator which injects a certain intensity of light emitted by laser (LD) into LiNbO 3 (lithium niobate) crystal. That is to say, the processed radio frequency modulation signal is added to the electrode of the modulator. Under the action of electric field, the intensity and phase of the beam are modulated. The output intensity of the modulator is the light signal that changes with the modulation signal. Unlike direct modulation, the modulation signal is added to the modulator, not to the light source. The light output is direct current light, and the light source can be either LD light source or solid-state laser. Because the external modulator does not have the additional frequency modulation of the light source, the combined second distortion (CSO) can be effectively overcome.

Although there are several external modulation modes of light in principle, the current external modulator used in CATV network is mainly Mach-Zehnder effect (M-Z) interferometer. Its structure is based on electro-optic crystallization as a substrate, and then a single-mode optical waveguide as a channel on the substrate. The input light is divided into two paths by the optical waveguide path, and the M-Z interferometer is the one that produces interference when synthesizing. In the unilateral path of Mach-Zehnder interferometer, when the phase is changed by electro-optic effect, the output of the interfered light will change with the phase difference. In order to produce electro-optic effect, the principle is to mount the electrodes on both sides of the unilateral waveguide path and add the signal voltage. If the length of the two paths is equal and the applied voltage is zero, no phase difference will form an additive synthesis. When the phase difference of the applied half-wavelength voltage is, the cancellation will occur and the output will be zero.

Direct modulation has a modulation effect, too long line will affect the indicators, and can not be secondary amplification, short distance lines can also be. External modulation is much better, optical amplification can be cascaded, but the price will be slightly higher, there are also external modulation, but the main device is certainly foreign. Mainly the difference in transmission distance and price!