That is, the distributed feedback laser, which is different from the built-in Prague Grating (Bragg), is a side emitting semiconductor laser. At present, DFB laser is mainly based on semiconductor materials, including InP (GaSb), gallium arsenide (GaAs), indium phosphide (INP), zinc sulfide (ZnS) and so on. The biggest characteristic of DFB laser has very good monochromaticity (i.e., the width of spectral purity) it generally can do less than 1MHz, and has very high side mode suppression ratio (SMSR), the current can reach up to 40-50dB.