Aspheric Lenses Information
Aspheric lenses have surfaces which do not resemble segments of a sphere or cylinder. Instead, aspheric lenses have a profile that changes in curvature across the surface. This type of lens is designed to reduce spherical and optical aberrations without the need for a multi-lens assembly. Small glass and plastic aspheric lenses can be molded, but large glass lenses require grinding and polishing. Finishing techniques to reduce insertion loss and improve surface quality are common.
All spherical lenses suffer from spherical aberration independent of errors in alignment or manufacturing, thus a non-spherical, or aspheric surface is needed to correct for it. By changing the conic constant and aspheric coefficients, an aspheric lens can be optimized to minimize aberration.
Aspheric lenses can be manufactured in a variety of ways. Subtractive methods polish the optics with small-surface-area tools such as computer numerically controlled (CNC) grinding and polishing or MRF, which uses a ribbon of magnetically responsive fluid to polish the surface. Finally diamond turning uses even smaller single-point tools to shape the optical surface.
Molding methods can produce surfaces without removing material. Both precision glass molding and plastic injection molding can be used to produce aspherical lenses. Precision glass molding heats glass to its softening point and compresses it between two molds; plastic injection molding forces liquid plastic into a mold then cools it to a solid. No matter which manufacturing method is used, proper metrology to the necessary accuracy will ensure the parts meet required surface accuracies.
The most common measurement techniques fall into either profilometry or interferometry. Profilometry measures the changes in height of a part by moving a probe across the part. Interferometry measures the difference between a reference wavefront and a wavefront reflected off the optic. Profilometry tends to be simpler to set up and more flexible in the shapes it can measure but also tends to be less accurate. The exact aspheric wavefront of the designed surface can be much more difficult to achieve and generally limits where an interferometer can be used for testing aspheres.
Comparison of spheric and aspheric lens profiles. Image credit: Edmund Optics
Aspheric lenses are useful for any optical application in which spherical aberration is an issue. In applications that require large acceptance angles, spherical lenses are unsuitable due to spherical aberration. Aspheric lenses maintain constant focal length or very high NA, avoiding the need for multiple elements to correct for spherical aberration. This simplifies system design by reducing weight and component count. It also results in less transmission loss, and less ghosting due to having fewer surfaces. The primary drawback of aspheric lenses is off-axis performance is poor. This is not a problem for coupling to and from optical fibers or collimating light sources, but aspheric lenses are not usable over a wide field of view.
Spherical aberration in a spheric lens (left) compared to no aberration in an aspheric lens (right).
Image credit: Edmund Optics
When specifying an aspheric lens, typical optics specs related to the physical dimension of the optic such as lens diameter, center thickness, and edge thickness are of course important. Focal length and the surface quality of the optic is important as well. Whether or not the optic is coated and what kind of coating it has (laser resistant, antireflection) can be quite important as well depending upon the application.