Research On Diffractive Optical Elements Arrays With Continuous Relief For Parallel Laser Direct Writing

Laser direct writing (LDW) is one of the important technologies used to fabricate diffractive optical elements (DOEs). With the development of DOEs for large aperture and/or array, the requirements for writing efficiency and accuracy of LDW get more and more strict. However, the throughput of traditional LDW technology with one exposured spot only is very low. With the development of multi-spots parallel laser direct writing (PLDW) technology in recent years, it is believed that multi-spots PLDW has more potential to improve the writing efficiency. However, there are still such scientific and key technique problems as it can't take both writing resolution and diffraction efficiency into consideration, the theoretical model of diffraction focusing characteristics of microlens arrays is not precision, and the separation of writing system and defocus-detecting system makes it can't detect focus during exposure in real time.The subject"Research on Diffractive Optical Elements Arrays with Continuous Relief for Parallel Laser Direct Writing"is to solve the scientific and key technique problems of multi-spots PLDW, and focus on the model, analysis and experiment of DOEs arrays with continuous relief. The subject provides a powerful theoretical foundation and preparation work for multi-spots PLDW with high efficiency and high accuracy.In order to study the effect of diffraction focusing characteristics of microlens arrays on the PLDW quality, we use nonparaxial approximation to analyze the focus characteristic of a single microlens, take into consideration the cross talk effect of a number of microlenses on the focusing characteristics of an array, and establish a theoretical focusing intensity model of a microlens array to describe the influence of a change in F-number (F/#) and/or center distance on the diffraction focusing characteristics of a PLDW system while incident writing laser is normal. The study would help the design of an array for PLDW use. In order to overcome the shortcomings between the writing resolution and diffraction efficiency of zone-plate arrays and/or refractive microlens arrays used for traditional PLDW technology, a PLDW method based on DOEs arrays with continuous relief is proposed. The method takes the DOEs arrays with continuous relief as PLDW objective array to take both the writing resolution and diffraction efficiency into consideration, and design the array with deep relief to make the fabrication of arrays easy, which provides a novel design method for PLDW array and system.A focusing theoretical model with nonparaxial approximation is established for DOEs with continuous relief fabricated by LDW by taken the deep fabrication error and convolution effect into consideration, and then analyzes the effect of the structure parameters, the diameter of the writing spot and the interscan distance on the diffraction focusing characteristics of low F/# DOEs with continuous relief, which establishes a firm theoretical foundation for the application of DOEs array with continuous relief for PLDW technique.In order to make writing laser and defocus-detecting laser focus synchronously into one spot, a method to design PLDW defocus-detecting system with harmonic theory is proposed. It designs the writing array for defocus detecting laser with harmonic theory to integrate the defocus-detecting array into the writing array. A harmonic focusing theoretical model with nonparaxial approximation is then established for harmonic DOEs, and further analyzes the effect of different defocus-detecting laser on the harmonic focusing characteristics.Finally, two arrays with F/# of F/7.5 and F/4 are fabricated by LDW, and then an experimental system is established to measure the focusing characteristics at normal incidence and laser wavelength of 441.6 nm and 670nm. The measurements made indicate that the experimental results agree well with theoretical results, the spot size of the array coincides well with the diffraction-limited value, and the diffraction efficiency obtained by the array excels 70%, which is much higher than the diffraction efficiency of 41% for a zone-plates array used for conventional PLDW and make it possible to further increase NA at a high diffraction focusing efficiency. Further, the DOEs arrays realize the writing laser and defocus-detecting laser focusing into synchronization.