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March 15, 2023
Optical designers face an ongoing challenge to meet consumer demands for smaller, more lightweight devices, like cameras, while continuously improving image quality. As a rule, getting to optimal quality is a function of lens count: the more lenses you can fit into the device, the better resolution and color precision you can achieve.
In the case of smartphone cameras, devices currently on the market have up to eight lenses. As of 2022, patent applications showed designs that included at least two more.
To reach this point, manufacturers had to first abandon older methods using spherical glass lenses (shown in Figure 1) from a time when extreme precision and miniaturization were not a requirement. In place of these, aspherical plastic lenses became the norm. The plastic ensures thinness and a lighter weight. The aspherical shape enables complex lens configurations that use the "brute force" of powerful optical software calculations to steer and focus the ray paths until they produce a desirable image, as shown in Figure 2.
No matter how small they get or what material you use, more lenses will always add weight to the device. Not only is this added weight inconvenient for consumers, but it also has an impact on power consumption and manufacturability. Smartphones use features like autofocus mechanisms and voice-coil motors to physically manipulate lenses by moving them up and down within the device housing. But the heavier the lens system, the harder it is to perform these manipulations in such a limited space.
Making smaller, lighter lens systems can only be achieved through advances in optical simulation software, along with parallel manufacturing advancements in precision tooling. Simulation makes it possible to identify ray paths through a proposed lens system design and predict and correct any unwanted effects the rays have on image quality. Tooling advancements enable optical companies to manufacture the tiniest and most complex optical systems their engineers can design.
We are persistently seeking ways to leverage the latest evolutions in optics and opto-mechanics to help our customers design and produce the best, most efficient, and most powerful optical devices. Cell phone companies and lens vendors regularly use Ansys Zemax solutions for building these simulations with the speed and accuracy needed to keep pace in a competitive market.
To help meet the performance challenges of miniaturized systems, some optical teams are looking to use diffractive gratings in place of certain lenses. Diffractive gratings are surfaces composed of a repeated pattern of tiny grooves or bumps with a constant period. When light hits these surfaces, it diffracts the spectrum into various visible colors at different angle directions, producing a rainbow effect.
On the scientific level, diffractive gratings are technically spectrometers, splitting visible light into a set of wavelengths in precise directions. This capability makes diffractive gratings useful for optical design because of the ways the resulting rays can be manipulated in and out from a light guide.
Diffractive optics are mostly helpful for providing enhanced color correction — a critical component of image quality that's especially challenging to achieve with plastic lenses — which have strict limits on their capacity for color control within a single lens. To get a good image with plastic lenses, you need more of them, which is what led to the propagation of lenses inside our modern devices. And if additional lenses will always add cost and weight to your final product, diffractive gratings provide the service of acting as multiple lenses within one photonic element, reducing the number of lenses you need overall. A design that might otherwise have required eight lenses to achieve superior image quality can be achieved using half that number or fewer when the design includes a diffractive element.
The decision of whether to use diffraction technology depends on the business trade-offs a company is willing to make. Diffraction gratings are complex optics and generally much costlier to produce than lenses. Part of this cost comes from the fact that the gratings must be designed and manufactured with a high level of precision to avoid the "rainbow" color flare typically associated with them. Picture the dazzle you see when holding a CD upside-down and moving it around slightly in your hand — this is an everyday experience of the diffractive grating effect. It's a fun quality to add for aesthetic purposes in certain products, but as an optical system feature, it's unwanted.
Still, if the right diffractive element for your optical product is worth the investment, the simplification it provides to your design will pay a variety of benefits. Not only do you achieve image quality requirements with less weight in less space, but if you get the optics just right, you also get a level of color correction that's not possible with lenses alone. This is because, by using a single element in place of multiple lenses, you don't need to "cancel out" color by comparing diffractive indexes between elements; it's all in one. The diffractive element itself corrects for color without requiring additional calculations. Diffractive elements are especially useful with plastic lenses, in which limited variations in the index of diffraction make color correction a unique challenge.
Another advantage of simplifying your design using diffractive optics is reduced placement error during manufacturing. The fewer the elements, the higher your threshold for introducing problems with correct placement — and the less you need to worry about tolerancing the system.
Overall, however, the biggest advantage is that fewer parts means that teams can save time and money. Material investments are reduced, as are work cycles themselves, in part due to the decrease in tolerancing considerations. Having fewer pieces to align and build makes manufacturing easier and decreases the time required to build.
These savings are potentially offset by the cost of producing the diffractive grating, as mentioned above, but the net savings can still make diffraction a viable option in many scenarios. There are even ecological benefits in that lighter devices are more energy efficient at the battery level, and having fewer design and production cycles translates to a smaller environmental footprint with less material waste.
For more insights and details about diffractive gratings, including how they can be simulated in Ansys products, read our recent blog article on using the Ansys Lumerical Sub-wavelength Model (LSWM) with Ansys Speos.
The next blog post in this series will discuss the role of rigorous coupled-wave analysis (RCWA) in increasing diffractive efficiency in diffractive grating systems, along with other innovative benefits of RCWA technology in optical design.
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