Design, fabrication and testing of a compact large-field-of-view infrared compound eye imaging system by precision glass molding

Highlights

• A 3D infrared compound eye system was designed and fabricated.

• Freeform microlenses are included in design to optimize imaging performance.

• Slow-tool-servo based diamond and molding are used to make freeform microlens arrays.

• The imaging tests shows a high performance larger field-of-view optical system.

Abstract

Bionic artificial compound eyes inspire a promising field of miniaturized imaging systems. In this research, a novel infrared (IR) three-dimensional (3D) compound eye imaging system, consisting of a double-side molded 3D microlens array and an aperture array, was designed and fabricated by combining modulated slow-tool-servo diamond turning and precision glass molding. To facilitate the complex profiles on the mold inserts, two novel slow-tool-servo strategies were adopted, namely virtual-axis based diamond broaching and adaptive diamond turning. This microlens array consists of 3 × 3 channels for a field of view of 48° × 48° with a thickness of 1.8 mm. The freeform microlens array on a flat surface was employed to steer and focus the incident light from all three dimensions to a two-dimension (2D) infrared imager. Using raytracing, the profiles of the freeform microlenses of each channel were optimized to obtain the best imaging performance. To avoid crosstalk among adjacent channels, a 3D printed three-dimensional micro aperture array was mounted between the microlens array and the IR imager. The imaging tests of the infrared compound-eye imaging system using the molded chalcogenide glass lenses showed that the asymmetrical freeform lenslets were capable of steering and forming images within the designed field of view. Compared to a conventional infrared camera, this novel microlens array can achieve a considerably larger field-of-view while maintaining low manufacturing cost without sacrificing image quality.

Introduction

Inspired by the ommatidia, the tiny independent photoreception units present in insects and crustaceans, artificial compound eyes are one approach towards more compact optical imaging systems from classical imaging concepts [1]. This leads to an enormous volume reduction and an increase in the field-of-view of the optical system. It is now attracting ever-increasing attention in a variety of fields, including wide-angle imaging [[2], [3], [4], [5]], biological sensing [6,7] and chemical analysis [[8], [9], [10]], to mention just a few. Moreover, artificial compound eyes with the capability of working in the infrared band can significantly extend novel applications and the performance of micro/nano-optical devices.

Currently, several fabrication techniques have been developed to fabricate microlens arrays, which can be classified according to material removal mechanism as physical [5,11,12], chemical [[13], [14], [15]], and mechanical in nature [[2], [3], [4],16]. A detailed review of recent fabrication methods for microlens can be found in Ref. [17] for the generation of these micro-optics. In general, most of the physical and chemical processes are often limited by: (a) specific materials and complex processes with high manufacturing cost [16]; (b) hard for these methods to obtain lenslets with well-defined intricate shapes, especially with superimposition of surface nanostructures; (c) a lack of manufacturing method for mass industrial replication [18].

More recently, the bottom up three-dimensional (3D) printing technique was adopted in fabrication of meso/micro/nano-optics with complex profiles [19,20]. Although the high precision process is superior to other fabrication methods in fast prototyping, the low production efficiency limited working dimension impedes its wider applications in optical manufacturing, especially in large-area micro-optics and quantum optics. Generally, mechanical machining is more universal and deterministic for the generation of complex, smooth, and successive profiles [21,22]. Among the popular methods, single-point diamond turning and high-speed diamond milling are the lead choices for the generation of microlens arrays on various engineering materials. With high-speed diamond milling, microlens arrays with complicated surface profiles can be rapidly generated, mainly because of the use of high-speed airbearing spindles and flexible programming. However, challenges still exist when it comes to fabrication of small features, such as microlens arrays. For instance, the high-speed milling spindle can cause degradation in surface finish due to the nature of milling operations [18]. Moreover, the size of diamond milling tools limits its use in the fabrication of smaller structures. Although AFM-tip based micro/nano-milling process can achieve very accurate sub-micron structures, it suffers from low efficiency and high tip wear rate [23]. Today such fabrication processes are still restricted to laboratory environments.

Compared with micro-milling, slow-tool-servo diamond turning, equipped with single-crystalline diamond tip, is more appropriate for complex optical profiles with or without micro/nano-structures. Additionally, advanced computer numerical servo control has enabled synchronization of the tool movements with the spindle rotation, thus allowing better flexibility and reliability. However, the inherent spiral trajectory of diamond turning makes it extremely difficult to generate discontinuous optical surfaces, especially for discrete elements. In general, an ideal strategy is to fabricate each discrete optical element separately, locally, and then combine them together, like in high-speed diamond milling process, which treats each element individually. In addition, the outline of each optical element is often changed into irregular shape instead of circular boundaries. Spiral trajectory is the best solution for scanning the entire circular area. Considering these requirements, a diamond broaching trajectory virtual-axis-based slow-tool-servo diamond turning is often selected for fabricating such complex discrete micro-optics. In this process, broaching trajectory is opted for polygonal boundary fabrication and virtual-axis-based slow-tool-servo was implemented in diamond turning process. However, due to the relatively low-efficiency and high cost of diamond turning, this method is typically only suitable for single piece or small batch production.

On the selection of optical materials, chalcogenide glasses are emerging as alternative infrared materials for their wide infrared transmission and ease of forming into micro/nano optical elements by precision glass molding (PGM), a near net-shape fabrication process [24]. For mass production, PGM of chalcogenide glass is preferred over other manufacturing methods due to its high efficiency, short cycle times and reusable tools. In a typical PGM process, a softened glass preform is pressed into the mold cavity and conformed to the desired shape. Around the molding temperature, the glass is in viscoelastic state, allowing it to be easily shaped under pressure. Precision molding of chalcogenide glasses provide the photonics industry with excellent material candidates for low-cost and high-performance devices.

Based on the properties of infrared materials at the molding temperature and available mold fabrication processes, a combined manufacturing approach was devised to fabricate infrared artificial compound eye imaging system. In this paper, both the micro aperture and profile of each lenslet were optimized by ZEMAX [25]. Then a virtual-axis-based diamond broaching trajectory and adaptive slow-tool-servo diamond turning technique was introduced to fabricate the mold inserts, which were subsequently employed in precision glass molding of chalcogenide glass lenses. In order to simplify the assembly process, the aperture together with the fixture structure was created by 3D-printing. Finally, the last part of the paper summarizes the optical performance of the artificial compound eyes.