Single Frequency Laser Diodes
DFB Lasers / DBR Lasers / RWS Lasers
Single Frequency Laser Diodes have a wavelength-selective grating integrated in the laser chip. Thus they operate on a single resonator mode emitting quasi-monochromatic radiation with a very small linewidth and low phase noise. The lasers can have very low-intensity noise because of the lack of mode partition noise. Due to the Gaussian mode the output is diffraction-limited.
*please find more detailed explanation for variant references here