Miniature Lens Types and Production for Optical Devices

The convenience and size of modern electronics make it easy to overlook the amount of engineering crammed into each device. For instance, most individuals do not consider what went into loading multiple tiny lenses into the slim device(s) they carry on daily. Today, most smartphones are equipped with cameras packing anywhere from four to nine miniature lenses per device. Multiple lenses are needed to correct for aberrations and properly focus the light to form a quality image. The capabilities of smartphone cameras have increased dramatically over the past decade. The improvement has progressed so much so that the camera specifications are one of the most prioritized considerations of consumers purchasing a new smartphone. Instead of hauling a separate DSLR or digital cameras to outings, the average person now simply grabs their phone to shoot satisfactory photos.

Miniature lens used in a CD drive. Courtesy of Wikipedia.

Smartphones, obviously, are not the only products requiring small or miniaturized lenses. Miniature lenses are found in a variety of devices. They are used for both imaging and illumination. For example, Blu-ray players and other optical disk drives contain at least one small lens. Many medical instruments, like endoscopes, contain a small lens or lenses. While some applications—due to space restrictions or otherwise—require miniature lenses, the advantages and disadvantages of using a larger or smaller lens should be considered when deciding between the two.

With smaller lenses, less material is needed for production. This may incline someone to use a more expensive, higher quality material than they would normally select. Miniature lenses weigh less than larger lenses. Changes in temperature affect smaller lenses less than a larger lens. Likewise, aberrations (besides for distortion) will scale down when using a smaller lens. However, the tolerances on thickness and decenter are tighter for smaller lenses than larger ones.

Furthermore, different types of lenses are used in optical devices. Spherical, aspherical are cylindrical are all lens types that come to mind. Spherical and aspherical lenses tend to be the types of lenses used most frequently. Aspheric elements mitigate or even cancel the effects that spherical aberration has on the system. Aspheres can also combat astigmatic effects. Since aspheric surfaces have varying radii of curvature, they take more resources to manufacture than spherical lenses. The radii must be adjusted to meet specifications during the grinding and polishing process. Cylindrical lenses, on the other hand, focus incident light into a line rather than a point as a spherical lens would. Cylindrical lenses are found in devices from semiconductor manufacturing machines to medical imaging systems. In semiconductor manufacturing, cylindrical lenses are sometimes used for inspecting wafers or circularizing elliptical beams.

Miniature Lens Production Techniques

Different techniques are used to create the correct surface curvature. Some techniques are better than simply cutting, grinding, and polishing when it comes to miniature lenses. One technique is magnetorheological finishing, or MRF. MRF uses a magnetic fluid to polish the lens surface. The fluid responds to an applied magnetic field making it possible to control both the amount and viscosity of the fluid.

Diamond turning is another option to manufacture lenses—especially small lenses. Since diamond turning only has contact with a very small portion of material at a time, it is a precise method for forming the proper lens curvature. It is not necessary for additional polishing to be done when diamond turning is used for production.

Molding tends to be cheaper when there is a larger number of units being produced/with increasing frequency of mold usage. The initial mold can be pricy but can be worth the expense in terms of efficiency and production time with larger volume orders. Injection molding is quicker than the other named fabrication methods, though it can be restrictive in terms of which materials can be implemented effectively.

Regardless of lens size and production technique, the lenses should be inspected for quality control. Metrology is used to check for irregularities in the lenses being produced. This ensures the lenses meet specifications within the reported tolerances for categories like surface quality, center of thickness, and radius of of curvature.

Metalenses

As previously mentioned, it can be difficult to properly cut and shape the glass of smaller lenses. As those in the optical design space knows, it is not often that only a single lens is used on its own. More often, stacked lenses, entire multi-lens systems, or a combination of the two, are needed. This poses a problem when there is limited space available in the optical housing. A space-saving solution in the small lens space has been developed through the creation of metalenses. Metalenses are created out of dielectric metasurfaces. These types of surfaces are manipulated when created so they interfere with light at the nanoscale level causing a phase delay. These lenses act somewhat like a diffractive lens. Some of the advantages of a metalens is that they can be designed to influence the polarization of light, as well as exhibit resonant behavior.

Different types of metalens surface structures, magnified. Courtesy of Nature Communications.

One of the most useful reasons to use a metalens is to combat chromatic aberration. Chromatic aberration, or dispersion, occurs when different wavelengths of light get separated from refracting at different angles. This leads to light focusing at multiple locations. Newton himself thought this was inherent to a system and could not be eliminated. This was later demonstrated to be incorrect with the creation of the achromatic doublet. An achromatic doublet uses a high dispersion glass (called the flint) in conjunction with a low dispersion glass (the crown) to focus the wavelengths at a more uniformly positioned focus. Metalenses exist that have been structured to combat chromatic dispersion. This allows very thin lenses to correct the chromatic dispersion rather than needing the standardly implemented lenses that are bulky and thick by comparison.

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