Significance   Micro-optics theory is a new discipline for the study of the design and manufacturing of micron-sized and nano-sized optical components, as well as the use of such components to achieve the theory and technology development of light waves. As a research field of optics, diffractive optics is based on the diffraction principle of light waves developed microoptics. Diffractive optical technology is of great significance in realizing lightweight, miniaturization, integration, high efficiency and low cost of optoelectronic systems, and the development of diffractive optical technology has also become one of the important ways to develop modern optical systems. As a typical micro-optical element, diffractive optical elements have broad application prospects in industrial and civil fields such as optical imaging, laser technology, and biomedicine due to their small size, light weight, multiple degrees of design freedom and good imaging quality. The processing methods of optical element can be summarized into two types of mechanical processing and optical processing, both of which have their own advantages and disadvantages. The advent of laser provides a new idea for the preparation of diffractive optical elements. Laser processing is a non-contact wear-free technology with high precision and high flexibility, which can process complex contours and has the characteristic of environmental friendliness and simple production process, so the study of laser processing technology in the application of diffractive optical elements is of great significance.

  Progress   With the continuous development of modern optical systems, higher requirements are put forward for the processing efficiency and preparation accuracy of diffractive optical elements. Laser direct writing technology does not need mask plate in the process of preparing diffractive optical elements, simplifies the steps, shortens the production cycle (Fig.1(a)). There are many factors affecting the preparation quality of diffractive optical elements, the article summarizes the main factors affecting the surface quality of diffractive optical elements (Fig.1(b)), and explains the influence of focusing system (Fig.2), laser energy (Fig.3) and scanning speed on the preparation of diffractive optical elements, which is very important for improving the preparation accuracy and surface quality of optical components. Different types of laser direct writing systems should also be considered in the preparation of diffractive optical element with different structures (Tab.1). From the aspects of process and system, the research progress of femtosecond laser direct writing system based on Cartesian coordinate system and polar coordinate system in processing diffractive optical element is discussed (Fig.4, Fig.6). Besides, in order to solve the problems of low energy utilization and poor processing efficiency in the process of laser preparation of diffractive optical element, a multi-beam parallel processing method based on laser direct writing technology is proposed (Fig.7). Diffractive optical elements have a variety of functions in optical systems due to their unique characteristics, and the article summarizes the typical applications of diffractive optical elements, such as infrared imaging (Fig.8), chromatic aberration correction, beam shaping, laser processing (Fig.9), image display, etc.

  Conclusions and Prospects   In the field of optics, the development of micro-optics theory technology continues to promote the advancement of diffractive optics theory. The application of diffractive optical element has also been expanded in more fields. As a high-precision, programmable, short cycle and flexible processing method, laser direct writing technology has incomparable advantages in the preparation of diffractive optical element. But in the actual processing process, there are problems of limited processing materials, insufficient utilization of laser energy, and the complexity of the system caused by the alignment mechanism in the preparation of curved element, so the research on expanding materials, simplifying equipment, optimizing processes and seeking applications is a continuous and important topic.