Introduction
Collimated and narrow laser beams are ideal for many industrial applications, including free-space communication, light detection and ranging (LiDAR), and material processing. Practical laser beams, however, are not perfectly collimated and always bear finite divergence angles. That is to say, the diameter of the laser beam changes along its propagation direction. Far-field pattern measurement serves to quantitatively characterize such imperfections of laser beams. In such measurement, the light emission intensity of a laser is measured for each individual emission direction, and an angular power spectrum, or far-field pattern, is obtained. This far-field pattern allows us to evaluate the quality of the laser beam. In this note, I will introduce the design of a measurement setup for characterizing the far-field patterns of lasers. This setup is designed for measurements within the near-infrared wavelength range, typically around 1300 nm or 1550 nm.