TECH FOCUS FREEFORM OPTICS
Automating optical design
Benqi Zhang, Guofan Jin and Jun Zhu
Researchers have developed a program for outputting high- quality freeform optical systems that simply requires basic optical design knowledge
Figure 1: Some of the 59 outputs from the researcher’s automated design programme, based on input specifications that took 35.3 hours to compute. Top left, the curve of the number of running jobs, versus the running time
O
ptical imaging systems have been playing an essential role in
scientific discovery and societal progress for several centuries. Since the early days of optical design, scientists and engineers have relied on aberration theory and performing large amounts of complex calculations to describe and quantify the deviation of light rays from the ideal focusing of an imaging system. Thus mathematical skills have previously been a very important requirement in the field of optical design. Computers have since
freed people from such heavy calculation tasks, and have helped realise fast ray tracing and the solving of complex aberration equations, leading to the application and development of optimisation algorithms, and software that greatly improve the speed and efficiency of optical design. Today, the design of optical
systems therefore relies largely on efficient ray tracing and optimisation algorithms. A successful and widely used optimisation-based optical design strategy thus consists of choosing a well-known optical system as a starting point, and steadily achieving incremental improvements. Such a ‘step- and-repeat’ approach to optical
30 Electro Optics July 2021
design, however, still requires considerable experience, intuition and guesswork, which is why optical design is sometimes referred to as both an art and a science. This applies especially to freeform optical systems. Until recently, most imaging
systems included spherical and aspherical refractive lenses or reflective mirrors, or a combination of both. With the introduction of new ultra-precise manufacturing methods, however, it has become possible to fabricate lenses and mirrors that lack the common translational or rotational symmetry about a plane or an axis. Such optical components are called freeform optical elements, and they can be used to greatly extend the functionalities, improve the performance, and reduce the volume and weight of optical imaging systems.
Lightening the load Although there are an increasing number of automated tools emerging, optical design without human guidance is still generally considered impossible, according to a team of scientists from the Department of Precision Instruments at Tsinghua University, China.
For them, the future of optical design features three main aspects: Firstly, human operators will
no longer need to participate or make decisions in the design process. Only primary knowledge about optical design will be required, with the designer only needing to provide the system specifications and constraints. Secondly, all the output results will satisfy the
“The main job of the designer will be to browse the output results... and then select the preferred design”
specifications and constraints given, and the imaging quality merits will meet the given requirements. A variety of optical systems of different types will be obtained to provide an overview of the solution space of the optical system under the given specifications and constraints. And lastly, the main job of
the designer will be to browse the output results, consider factors such as manufacturing and system structure comprehensively, and then select the preferred design.
Towards achieving this, in Light: Science & Applications, the team of scientists have reported their development of a result-diversified automatic design method for freeform optics. Using just the system’s specifications (field-of-view, focal length and entrance pupil diameter etc) as the only input, a variety of three-mirror freeform imaging systems can be obtained automatically. The proposed method
is able to perform a coarse search of the solution space of three-mirror freeform systems and design a wide variety of imaging systems, working in the visible and long-wavelength infrared bands. The user can then focus on specific designs and conduct finer searches to obtain solutions that are diffraction limited, or have optimal imaging quality.
Computing power The design method is composed of five phases (detailed in the paper1 and summarised here): One, construct a series of coaxial spherical systems with various optical power distributions. Two, for every coaxial spherical system, find out a series of unobscured systems that meet the given constraints. Three, based on the unobscured off-
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