How can you tell if you really need an aspheric lens?

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.

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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.

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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.

Fiber tip diameter and fiber numerical aperture are very important when designing and analyzing lenses for multimode fiber laser beams. If a manufacturer claims a full beam diameter of X mm after collimation, then X is the value that should be used to analyze a focusing lens. This specification should also be verified for the same reasons when designing collimator lenses.

If you don’t know the laser beam parameters, then at a minimum, a beam size that fits the full aperture of the lens should be used. For example, if using a 30 mm lens with a usable clear aperture of 26 mm, a beam size of 26 mm should be used to analyze lens performance.

Lens and beam analysis types

Spherical aberration is traditionally one of the main methods used to analyze single lenses in monochromatic systems (lasers, in this case). There are two types of spherical aberrations: longitudinal (LSA) and transverse (TSA)—these terms are used to differentiate between a ray error that’s transverse to the central axis and measured at the image plane, and a ray that intersects the central axis at some point before or after the image plane. Figure 1 illustrates these two measurements. Typically, only one of these parameters is used to evaluate a lens. Calculating this parameter is simple with Snell’s Law for refracting a ray through the lens, as well as simple trigonometry to trace the ray to the image plane.

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As demonstrated in Figure 2, the edge ray intercept (+10 mm from the center of our lens) strikes the image plane at -0.6 mm. The minus sign indicates that the ray strikes the image plane below the central axis, and rays starting at ±3 mm strike the image plane close to zero. But for input heights greater than this, the deviation at the image plane increases exponentially.