The cavity mirror has curvature, the curvature radius of front cavity mirror is shorter, and the one of rear cavity mirror is longer, so the front cavity mirror is shorter than the rear cavity mirror in length. The two cavity mirrors are mounted in alignment on one side, and the laser beam is output from the other side of the front cavity mirror. As shown below:
Each time the laser beam passes through this space, it shows different transmission laws in the height and width directions: the size in the width direction is much larger than the size of the laser beam, so the laser beam passes freely in this direction without restriction; the size of the height direction is similar to the size of the laser beam, so the laser beam is constrained to pass through in this direction each time. Because the laser beams oscillate in the cavity for many times, pass through the flat space between the two plates for many times, and is output on one side of the front mirror, the final output laser beams are asymmetric in the width and height directions.
In addition, the divergence angle and spot size of the laser beam in the two directions are different: the divergence angle is small in the width direction and large in the height direction, which means that the beam quality in the two directions is different. The schematic diagram of laser beam transmitted in free space after output from the resonator is as follows:
To obtain laser beams with the same divergence angle and the same spot size in two directions, it is necessary to shape above-mentioned asymmetric laser beams to obtain the same beam quality factors in the two directions, so that the circular symmetrical Gaussian beams of quasi-fundamental mode can be obtained. The circular focused spot can be gained by laser processing equipment such as marking machine and cutting machines.
To shape the laser beam into a complex system composed of a series of lens, asymmetric optical elements, such as cylindrical lenses or cylindrical mirrors, are generally introduced in two directions.