Design and fabrication of multifunctional holographic optical elements in laser Doppler velocimeter

Replacing the bulky traditional optical elements in the optical system with a holographic optical element (HOE) is conducive to the functional integration and volume miniaturization. However, when the HOE is used in the infrared system, the mismatch between the recording wavelength and the working wavelength will lead to the reduction of diffraction efficiency and the introduction of aberration, which will seriously affect the performance of the optical system. This paper proposes a design and fabrication method of multifunctional infrared HOEs that can be used in laser Doppler velocimeter (LDV), which can reduce the effect of wavelength mismatch on HOE performance while integrating the functions of the optical system. The restriction relationship and selection method of parameters in typical LDV are summarized; the decrease of diffraction efficiency due to the mismatch between recording and working wavelengths is compensated by designing the angle of signal and reference wave of the HOE; and the aberration caused by wavelength mismatch is compensated by cylindrical lens. The optical experiment shows that the HOE can produce two groups of fringes with opposite gradient, which proves the feasibility of the proposed method. Moreover, this method has a certain degree of universality, and it is expected to design and fabricate HOEs for any working wavelength in the near-infrared band.