## 1 Introduction  
The microlens array is an optical device with a micron-sized structure and unique optical characteristics. With high-tech industry development, the microlens array has become an essential element in the optical system owing to its excellent geometric characteristics and optical performance. The microlens array, relying on its small structure, wide field of view, and high resolution, plays an essential role in optical sensing, optical communication, fiber coupling, and non-imaging optics [1]. Microlens arrays can be divided into flat microlens arrays and curved microlens arrays [2]. As a bionic compound eye lens (micro-optical structure inspired by insect eyes), the microlens array plays a significant role in multi-channel imaging and machine vision [3]. It can also be used to design compact imaging systems in medical imaging, monitoring equipment, and other fields [4]. Many studies have focused on manufacturing high-precision, high-quality microlens arrays through different methods, including grinding [5], ultra-precision diamond turning [6], and micro-milling [7]. Although these processing methods can achieve good processing quality, their disadvantages are conspicuous--low efficiency and high processing costs. Recently, new methods have been popular to fabricate microlens arrays such as microinjection molding [8], electrochemical etching [9], local grayscale oxidation [10], direct laser writing [11], photolithography [12], and laser catapulting [13]. Although these new methods can process the microlens array with good surface quality and small element size, they are still challenging for mass production. Some methods require complicated manufacturing processes, such as etching or photolithography technology. Besides, there are some problems including complicated processing steps, high cost, and low processing efficiency. [PERSON] et al. [14] and [PERSON] et al. [15] used ultra-precision milling to process silicon-based molds combined with injection molding to process high-quality microlenses, but the processing cycle is too long. [PERSON] et al. [16] realized non-contact rapid rolling on the glass substrate based on the rolling technology and produced a microlens array with high surface quality; however, it is difficult to control each microlens' dimension accuracy. [PERSON] et al. [17] fabricated a polymer refractive microlens array using a stainless steel mold through non-contact hot embossing technology. By contrast, hot embossing has the advantages of low cost, high efficiency, mass production, and environmental friendliness [18; 19; 20]. Therefore, hot embossing is considered to be one of the most effective and promising methods for creating optical microlens arrays. In recent years, many studies on microlens array hot embossing have focused on polymer filling performance, morphology deviation, interface friction and stickiness, geometrical morphology, hot embossing process parameters, birefringence deviation, replication fidelity, and hot-embossing process simulation. [PERSON] et al. [21] used the focused ion beam technology to process microlens cavities on pure nickel, stainless steel, and silicon wafers. Then, they replicated the micro-lens array structure on the polycarbonate surface by hot embossing. The experimental results showed that the silicon wafer array structure processed by the ion beam has an excellent optical surface, so that the lens array replicated by the silicon mold has the optimal quality. Meanwhile, polymer viscosity and hot embossing pressure were the most critical factors for high fidelity and good surface replication. [PERSON] et al. [22] used micro-milling technology to process the micro-groove array structure and thermo-forming technology to replicate high-precision polymer micro-arch array lenses. [PERSON] et al. [23] used induction heating to achieve effective hot embossing of polymer array structures on silicon-based molds. The experimental results showed that rapid local-induction hot press forming technology can improve replication and morphology replication.