Optical centering method for lens assembly based on fiber point diffraction interferometry

Centering error in lens assembly reduces the image quality. We proposed an optical centering method based on fiber point diffraction interferometry (FPDI) in this paper. Two fiber point-light-sources (PLSs) interfere longitudinally. The intersection of the line connecting the two PLSs with an imaging plane occurs where the optical path difference (OPD) is an extremum. The line of these two PLSs defines a virtual reference axis. A lens group can be centered by mounting and adjusting lenses piece by piece between the two PLSs and keeping the OPD extremum point as unchanged as possible in this process. Introducing an auxiliary lens can enhance the sensitivity of this method to centering errors. We centered a lens group and showed the effectiveness of the proposed method. Compared with the autocollimation method, the FPDI method does not need to rotate lens elements and is always effective in testing centering errors. In contrast to the calculation of wavefront aberration in the computer-aided alignment (CAA) method, the FPDI method just determines the centering error through the coordinate offset of the OPD extremum point. It utilizes a reference axis that can be precisely positioned by interferometry. The theoretical assembly accuracy can reach micron and arcsec levels.