Optical system design for collimating beam combiner based on compound eye lens with free-form surface

Abstract

Limited by the theoretical output power limitation of single-mode fiber laser, achieving higher laser output power requires us to turn to using laser beam combination technology. The current beam combining methods can mainly be divided into spectral beam combining and aperture beam combining. The former method has the advantages of better optical quality, but the beam combining grating bears too much laser energy density and is limited by wavelength selection and scalability. The collimated beam combining method performs better at scalability and shows more uniform energy density, while the beam quality is seriously affected by decrease of optical axis uniformity caused by the thermal deformation of the mechanical cylinder shell, so the problem of laser heat deposition should be solved urgently. Considering that the laser is a Gaussian beam, there is still a small amount of energy beyond the waist radius, so an attempt was made to design a collimating beam scheme with the laser emitting end face as a whole continuous compound eye lens to replace the traditional discrete collimating unit. Zernike polynomial is a classical polynomial to describe optical surface figure. It is consistent with the description form of optical geometric aberration, and its unit orthogonality ensures that changing the number of polynomial terms will not change the original coefficients, which shows its excellent characteristics in optical surface fitting. Continuous compound eye lens is a freeform lens contains several aspheric collimating elements. In the design, according to the geometric characteristics of the distribution of each collimating unit, the basic Zernike polynomial structure can be optimized, and the best fitting number of terms which reaches the balance between the changing of Zernike polynomial terms and the accumulated calculation rounding error has been found. The simulation results of the fitting surface figure using seven channels laser beam show that when the diameter of a single lens is 30mm and the center distance is 80mm, there are 125 terms of optimized effective Zernike polynomials involved in surface fitting, equivalent to 1318 basic Zernike polynomials, and satisfying surface fitting results are obtained. Within each collimating unit, PV error is 53.6 nm, and RMS error is 1.1 nm, exceeding the requirements of manufacturing conditions. The analysis of the error distribution shows that the fairly large PV error is concentrated at the edge of the collimating unit, which has little influence on the laser energy transmittance, which shows the designed compound eye lens can meet the expected transmittance and optical surface figure fineness theoretically. In fact, using continuous compound eye lens as the laser emission end face of the collimator can keep the outgoing optical axis exactly consistent after continuous working with the thermal deformation, owing to its each collimating unit is measured and modified at the same benchmark, which ensured the whole system has higher stability, higher structural reliability and better laser tolerance. This design is recommended to be applied into wireless laser energy transmission, space laser communication and laser detection field.