When modulating laser light, a distinction is made between direct light mod ulation and indirect or external modulation. In direct modulation, the Direct Modulated Laser( DML), the light from the laser diode is modulated directly, whereas in indirect light modulation the modulation unit is connected downstream of the laser diode.
There are electro-optical modulation units that work on the basis of absorption or on a method of wave substitution. The electro-optic modulator (EOM) is based on the change of the refractive index. In this process, the refractive index changes by applying an electric field to the doped material. This has the effect of changing the phase position of the laser light, which means that the light waves shift.
If two light waves with the same phase position meet, then the levels of both waves add up, resulting in a higher light intensity at the output. However, if the phase positions are opposite, they cancel each other out. Mach-Zehnder mod ulators and electro-absorption modulators are electro-optical modulators.