Several factors should be considered when selecting lenses best suited for particular laser applications. Once a lens material and coating are selected, a laser engineer decides whether the application needs a simple plano-convex, best form (see Note 1), or aspheric lens shape.
Answering this question properly requires knowledge of the system’s laser beam parameters (wavelength, diameter, mode, or beam parameter product [BPP]); the lens parameters (diameter, thickness, surface radii); and the application’s requirements (focus diameter, power level, power density requirements, etc.). Once the laser and lens parameters are known, focused beam metrics can be calculated and used to determine if they will meet the application’s requirements.
High-speed diamond turning machines, magnetorheological finishing (MRF) machines, and CNC asphere polishing machines, along with increased competition in the industry, are making aspheric lenses more cost-effective and commonplace. But costs are still relatively high compared to simple plano-convex and best form lenses, so it’s crucial for laser engineers to choose the appropriate lens shape for the application.
Main laser beam types
To determine whether a lens is suitable for a specific laser application, it’s important to know the laser beam type and its parameters. Laser wavelength, power, and beam profile are the three key factors needed for proper analysis of a lens. Laser power and wavelength are easy parameters to identify for most laser systems, but the beam profile can be confusing to properly parameterize.
Theoretically, Gaussian profiles have an infinite diameter. It’s not realistic to use in a lens design or analysis program, so many engineers use a beam size of 1.5X the e-2 power point diameter. Some even use the Gaussian beam diameter at the e-2 power points, clipping ~13.5% of the beam’s energy. These diameters are often used to set the clear aperture of the lens system, too. To ensure best performance when designing aspheric lenses, a beam diameter of 1.7X the e-2 power point diameter should be used. During the design phase, most analysis types trace rays through the lens and analyze the rays at the image plane, so it is important that the rays cover the full beam diameter. There are exceptions to this rule, but for most cases, 1.5–1.7X the e-2 power points will provide adequate results.
Spherical vs. aspherical focusing lens
How are these analyses useful to determine whether a focusing lens is good enough for the job? Often, applications will dictate system requirements such as a final spot size. But theoretical spot size is determined by system parameters like laser beam characteristics and focal length of the optic(s). Wavefront error analysis is a particularly useful tool to explore whether the focusing lens is introducing aberration into the system and how this might affect overall performance (see Fig. 9).
