As the name suggests, laser beam expansion is an optical system that expands the input beam to a larger diameter. The design concept of beam expander lens originates from the basic principle of telescope design. When a collimated laser beam is input to one side of beam expander lens, a collimated beam is output at the other end, that is, the light in object space and image space converges at infinity.
The beam expansion ratio is the basic performance parameter of beam expander lens. The beam expansion ratio is equal to the focal length ratio of output lens and input lens. When the laser beam is amplified by m, the divergence of the laser beam is multiplied by an inverse ratio of 1 / m. For example, if the beam expander lens magnification is 2, the output beam diameter will be twice the input beam diameter, and the divergence of the output beam will be half that of the input beam. Conversely, when used in reverse, the beam expander lens will reduce the output beam diameter.
1. Ensure that the required output diameter is less than the maximum output diameter of beam expander lens. Each beam expander lens has a maximum input diameter, which is usually related to the physical limitations of optical elements and housing. The main goal of using beam expander lens is usually to achieve a specific output diameter, so it is very important to ensure that the required output diameter is less than the maximum output diameter of beam expander lens.
2. Select a beam expander lens whose design wavelength is equal to or close to the wavelength of the laser source. Like any optical system, the performance of beam expander lens varies with wavelength. The material grade of internal optical elements and antireflection (AR) coating will affect the transmittance of beam expander lens, and the antireflection film reduces the loss at the design wavelength - in addition, the lens material and surface shape are optimized for a given wavelength, so beam expander lens whose design wavelength is equal to or close to the wavelength of laser source should be selected.
3. Please be sure to select the wavefront quality that meets the needs of the system. The transmitted wavefront error quantifies the beam quality at the output of beam expander lens, and the diffraction limiting performance is usually quantified as one quarter wave（ λ/ 4) transmitted wavefront. A higher quality transmitted wavefront is also possible and is usually specified as λ/ 8 even up to λ/ 10.
For the system that needs to adjust divergence or collimation, consider selecting beam expander lens with focus adjustment. Beam expander lens can provide a variety of different fixed and variable magnification options. Fixed magnification beam expander lens usually has a collimation adjustment, commonly referred to as "focusing" or "divergence" adjustment, which allows more compensation for the collimation and divergence of the laser beam leaving the beam expander lens. Variable magnification beam expander lens can be used to control the expansion ratio and collimation adjustment. This is particularly valuable in the prototype design process, helping to fine tune system requirements or compensate for changes between source beam diameters.