Diffractive Achromat with Freeform Slope for Broadband Imaging over a Long Focal Depth

Area of Science:

• Optics and Photonics
• Optical Engineering
• Materials Science

Background:

• Achieving extended depth of field (DOF) in optical systems, particularly achromatic ones, remains a significant challenge.
• Diffractive optical elements offer miniaturization potential but often suffer from chromatic aberrations and limited DOF.
• Existing fabrication methods for diffractive elements can be complex, costly, and prone to errors.

Purpose of the Study:

• To propose and demonstrate a novel method for designing a long-focal-depth diffractive achromat (LFDA).
• To develop a fabrication-friendly design process that minimizes errors and costs.
• To experimentally validate the performance of the designed LFDA for achromatic imaging over an extended DOF.

Main Methods:

• Rotational symmetric parameterization was applied to design the LFDA.
• The binary variable slope search (BVSS) algorithm was employed to optimize the element's surface profile, ensuring smooth slope transitions.
• Grayscale laser direct writing lithography was used for fabrication, with a focus on minimizing deviation from the designed profile.

Main Results:

• An LFDA with a 10.89 mm diameter was successfully designed for three wavelengths (654 nm, 545 nm, 467 nm) and six focal planes.
• The fabricated LFDA showed a profile deviation of 9.68% from the design.
• The LFDA achieved a significant DOF of 500 mm and produced focused beams with a full-width at half-maximum (FWHM) close to the diffraction limit.

Conclusions:

• The proposed LFDA design method, utilizing BVSS and rotational symmetry, is fabrication-friendly and cost-effective.
• The LFDA demonstrates excellent performance in generating high-quality chromatic images over an extended depth of field.
• This LFDA technology paves the way for compact achromatic optical systems for applications in imaging, sensing, and 3D displays.