Wavefront correction for spatial nonuniformity of the liquid crystal on silicon based spatial light modulator

Abstract

Liquid crystal on silicon (LCoS) based spatial light modulator (SLM) is widely used in many research areas, such as holography display, optical measurement, and optical tweezers, due to its programmable capability of phase modulation. However, a certain aberration term to the incident wavefront is introduced due to the spatial nonuniformity of the LCoS backplane, and the correction of this phase aberration is important to improve the accuracy of phase modulation by the SLM, which will then influence the SLM applications, e.g., the image quality in holography display, or the beam shaping for optical tweezers and free-space communications. Fortunately, the LCoS based SLM has the programmable capability of phase modulation; therefore, the spatial nonuniformity can be compensated by a wavefront correction. Conventionally, this aberration is corrected by adjusting the Zernike coefficients of the generated wavefront manually. However, this procedure is time-consuming and subjective, since there are dozens of coefficients to be adjusted, and the adjustment is evaluated manually by monitoring the interferogram. In this paper, we propose to use several phase retrieval techniques to measure the corresponding wavefront directly and employ the obtained wavefront to correct the aberration introduced by the spatial nonuniformity. A Twyman-Green interferometry is used, and an interferogram acquisition technique based on the self wavefront modulation of SLM is proposed. Besides, a method for determining the corresponding active area of the LCoS backplane and parameters of 2-D projective transformation is proposed to transform the active area to the fitted resolution of the SLM. RMS value of the aberration to approximately λ/32 can be obtained by the proposed method. Consequently, more accurate phase modulation by SLM for applications such as holography display and beam shaping is achievable. Effectiveness of the proposed method is validated by experiment, and the correction results by different phase retrieval techniques are evaluated and compared.