Miniature Optoelectronic Compound Eye Camera

Insect compound eyes (CEs) are remarkably efficient imaging systems, characterized by their small size, wide field-of-view (FOV), distortion-free imaging, and sensitive motion tracking. These features inspire the development of artificial CEs to overcome limitations in existing imaging technologies and enhance performance in applications such as medical endoscopy, paramedic imaging, microneedle navigation, and robot vision. Previous attempts to create artificial CEs have involved integrating microlens arrays (MLAs) with commercial CCD/CMOS detectors. Planar CE cameras, while achieving resolution imaging, suffer from limited FOV due to their planar structure. Three-dimensional (3D) CEs, mimicking insect-like structures, have been produced using advanced micro-nano fabrication techniques. However, these often lack integrated photodetectors, limiting portability and real-time capabilities. Challenges arise from the mismatch between the complex 3D configuration of natural CEs and the planar nature of readily available imaging sensors. This paper addresses these challenges by presenting a novel miniature optoelectronic integrated CE camera.

Significant efforts have been devoted to creating artificial compound eyes. Early approaches focused on planar structures by integrating MLAs with CCD/CMOS detectors, but these designs had limited FOV. To increase FOV, 3D CEs with insect-like structures were developed using techniques such as microlens templating to integrate curved MLAs, polymer cones, and waveguide cores. Other fabrication methods include reconfigurable micromachining and self-writing in photoresist, and 3D miniature CEs with hundreds of ommatidia have been created using femtosecond laser additive/subtractive manufacturing. However, limitations include dependence on a single 3D MLA and the need for microscope-based image acquisition systems. Previous efforts towards optoelectronic integration have involved attaching micro-optical components to individual photodetectors, creating curved camera arrays. Examples include artificial CE cameras constructed by assembling microlenses and photodetectors, or flexible lens arrays combined with deformable silicon photodetector arrays. While these approaches improved FOV, they resulted in complex assemblies with relatively low resolution and larger sizes compared to insect CEs. The integration of complex CE structures with standard imaging sensors at an insect-like scale remained a significant hurdle.

The authors address the challenge of integrating complex 3D micro-optics with planar sensors by utilizing a logarithmic lens profile for the ommatidia in their artificial compound eye. This design significantly increases the depth-of-field and focus range, allowing for direct integration with a commercial CMOS detector (OY9374). The compound eyes were fabricated using femtosecond laser two-photon polymerization (FL-TPP) of a polymer, a technique that allows precise control over the 3D structure of the ommatidia. The logarithmic profile is described mathematically, and comparisons are made between the optical properties of the logarithmic lenses and traditional spherical lenses. Both theoretical simulations and experimental results using a He-Ne laser demonstrate the increased focus range and depth-of-field of the logarithmic lenses. The fabrication process involved using an organic-inorganic hybrid photoresist (SZ208) with a photosensitizer. A commercial gray-scale printer-based FL-TPP system was employed for the fabrication, with the 3D structure characterized using a laser scanning confocal microscope (LSCM). The optical properties of the fabricated compound eyes were evaluated using a transmission optical microscope and a spectrometer. The optoelectronic integration involved directly mounting the fabricated compound eye onto the CMOS sensor.

The study demonstrates a significant improvement in imaging performance compared to previous designs that utilized spherical ommatidia. The logarithmic lens design mitigates the defocusing issues inherent in integrating curved image planes with planar detectors. The fabricated miniature compound eye camera (approximately 400 µm in size) successfully integrates 19-160 ommatidia with a commercial CMOS detector, achieving a large FOV (90°). Experimental results confirm its ability to perform large-FOV imaging, spatial position identification, and sensitive trajectory monitoring of moving targets. The camera's performance was tested by tracking the movement of a beetle, demonstrating the reconstruction of its trajectory based on real-time video. The high-quality imaging was also validated by imaging various objects such as the letter 'F' and different insects. The angular sensitivity was measured to be 12.1°, showing a high sensitivity to moving objects. The integration of the miniature camera with a microfluidic chip enabled real-time monitoring of living microorganisms like Paramecium. A machine learning approach, using a backpropagation neural network, was employed for camera calibration, allowing for accurate 3D reconstruction of the Paramecium's trajectory.

The successful creation of this miniature optoelectronic compound eye camera addresses a critical challenge in the field of bio-inspired imaging. The logarithmic lens design provides a practical solution to the defocusing problem often encountered when integrating complex 3D micro-optical elements with planar sensors. The results demonstrate that this approach enables the development of compact, high-performance imaging systems with capabilities comparable to those of natural insect eyes. The ability to accurately track moving targets and monitor microorganisms in real-time opens up exciting possibilities for applications in various fields, particularly robotics and microscopy. The achieved miniaturization, combined with the high-resolution imaging and large FOV, represents a significant advancement in the development of artificial compound eyes.

This research successfully developed a miniature optoelectronic compound eye camera with a 90° field of view using logarithmic profile ommatidia. This design overcomes the defocusing limitations of previous systems, enabling effective integration with a standard CMOS sensor. The camera's ability to perform spatial position identification and trajectory monitoring, particularly demonstrated with the tracking of a beetle and Paramecium, showcases its potential for various applications, including miniature robotics and microfluidic-based biological imaging. Future work could explore expanding the number of ommatidia for higher resolution, integrating more sophisticated signal processing for improved image quality, and investigating applications in areas such as minimally invasive surgery and environmental monitoring.

While the study demonstrates the effectiveness of the logarithmic lens design and the successful integration of the compound eye with a CMOS sensor, there are some limitations. The current prototype uses a relatively small number of ommatidia (19-160), potentially limiting the overall resolution. The calibration process relies on machine learning, and the accuracy of the 3D reconstruction might be affected by the training data and model parameters. Further research could explore techniques to improve the calibration accuracy and expand the number of ommatidia for better resolution.