November 13, 2019
Non-circular optics for lidar and other cutting-edge technologies
By Sanjay Gangadhara, Chief Technology Officer
As innovative technology companies strive to make smaller and lighter optical devices, there is a growing trend in incorporating non-circular components into the designs. The use of such components can address volume constraints raised by design requirements for tighter packaging, as well as improve the thermomechanical stability of designs exposed to variable operational and environmental loads. Non-circular optics also play a key role in working with asymmetric sources of illumination that are becoming more commonplace in the industry. Following is a look at a few applications for non-circular optics, and the benefits of, and challenges associated with, designing such optics—as well as some specifics about designing them in OpticStudio.
Technology benefits of using non-circular optics
Non-circular optics can be useful for any design that incorporates laser diode arrays and requires uniform far-field illumination. One practical application is lidar systems.
According to Lumotive Senior Director of Engineering Apurva Jain, “Cylindrical optics are generally used for fast and slow axis collimation of edge emitting semiconductor lasers. In my product development experience, this is by far the most common use of non-circular optics. I’ve also employed cylindrical optics for beam shaping purposes at times to achieve non-circular far-field beam profiles.”
In general, non-circular optics are useful in systems used for beam-shaping, whether that involves creating a non-circular light pattern from a symmetric illumination source or vice versa. For example, when designing optics for interior lighting, incorporating non-circular optics can allow for better flexibility and control in creating uniform lighting within the typically non-circular spaces that are being illuminated. Similarly, non-circular optics can be useful for holographic applications in which designers are interested in generating specific light patterns in the far-field, often from small non-uniform sources. Other applications in which non-circular optics can be used include medical lasers, optical communications, bar-code scanners, and handheld laser devices such as laser pointers or cosmetic lasers for home use.
Edmund Optics offers examples of non-circular lenses for miniature spectrometers and fiber-coupled diode lasers in this article.
Manufacturing benefits of using non-circular optics
There are three key manufacturing benefits to using non-circular components in an optical product design. First, because optical engineers can cut out the parts of the lens that aren’t going to be used, non-circular optics can take up less space. The resulting lens often has a square or rectangular cross-section with flat edges, allowing it to be mounted flat with fewer mechanical components for a much smaller footprint.
Second, the flat edges achieved when constructing non-circular optics can provide for increased thermomechanical stability of the overall product assembly, leading to better performance and reliability of the system under operational and environmental loads. This increased stability comes from the fact that an optomechanical mounting structure can be simplified when using optics with flat edges, in many cases allowing multiple optics in the assembly to be mounted to the same mechanical platform.
Finally, the use of non-circular optics can make it simpler to detect flaws in the lens, because of the non-rotational symmetry of the optic. The break in rotational symmetry provides a key means by which manufacturing or assembling defects can be analyzed (other methods for breaking rotational symmetry include using freeform surfaces, as described here), allowing engineers to pinpoint flaws more efficiently.
It is worth nothing that the use of cylindrical optics—when needed to meet performance requirements—also offers benefits in the manufacturing and assembly of the lens system. Lumotive’s Jain says, “Cylindrical optics split the horizontal and vertical axis, making the optic much less susceptible to drifts in one or more directions. For instance, a cylindrical lens collimating in the horizontal direction has no impact if it moves in the vertical direction—the tolerances in that direction are huge. Any perturbation in this direction (whether thermal or mechanical in nature) will have minimal impact on performance. This can result in more stable assemblies with lower labor costs, sometimes traded off with increased component count.”
Challenge of non-circular optics
Non-circular optics are more expensive than off-the-shelf circular lenses because they take more effort to manufacture and test. However, we see that the gap between manufacturing costs for circular and non-circular optics is going down as technologies are continuing to evolve. The trade-offs in designing with non-circular optics involve weighing the performance and cost benefits associated with the use of these components against the cost of the lenses themselves. Modern manufacturing capabilities allow these trade-offs to be meaningful.
Another challenge in designing products with non-circular optics is the ability to accurately simulate these optics in the virtual prototyping of the design. However, that challenge is overcome with the use of OpticStudio for virtual prototyping.
Designing non-circular optics in OpticStudio
OpticStudio offers everything engineers need to design optical systems, whether they’re working with circular or non-circular optics. Optical engineers can analyze, optimize, and tolerance their systems to create effective designs.
A key benefit of OpticStudio is that it offers geometry descriptions suitable for most any application. Geometries that can be modeled in OpticStudio include rectangular (such as square lenses offered by Edmund Optics), cylindrical, and toroidal.
“OpticStudio offers a wide variety of surfaces to give designers freedom in choosing the best type of surface for each application,” says Shu-i Wang, Senior Staff Optical Engineer, Northrup Grumman. “For example, for a niche application that uses a surface type not often used, OpticStudio offers surfaces that allow the designer to quickly design a system. Most lens designers want to spend our time designing optics, not writing programs for user-defined surfaces.”
Another key benefit of OpticStudio is that it allows engineers to work in both sequential and non-sequential mode, allowing designs to be taken from idealized descriptions to real-world models. One challenge in designing non-circular optics is the ability to optimize the design in the simulation process, since most virtual prototyping tools leverage rotational symmetry of the lens for efficient design optimization. In OpticStudio, the initial lens design could still be done in sequential mode using a circular geometry. Then, with space constraints in mind, users can convert the model to non-sequential mode and start trimming the lens using tools such as the Boolean Native to see if this has any impact on the optical performance.
Finally, coming soon to OpticStudio is TrueFreeform, the latest method in designing freeform optics, which inherently is best-suited to a rectangular geometry. These geometries are becoming more common in applications where freeform surfaces are being used, such as AR/VR headsets and heads-up displays.
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