Dual-polarization analog optical phase conjugation improves optical focusing through tissue

Light beams have a wide range of medical applications. For example, they can be used to investigate internal problems, to activate photosensitive agents, or as optical tweezers. However, when light passes through biological tissue thicker than one millimeter, the light scatters, making it hard to focus the light and use it effectively.

Existing solutions such as analog optical phase conjugation use optical phase conjugation based on photorefractive crystals to address the scattering problem. However, because these systems only phase-conjugate the scattered light in one polarization direction, half of the polarization information is lost when the tissue randomly scrambles the polarization of light.

Cheng et al. wanted to reclaim the lost polarization information to improve the quality of the optical focusing. After studying both the horizontal and vertical polarization components of the light field to understand the pattern of the focused light, the researchers redesigned the existing analog optical phase conjugation technique, and brought both orthogonal polarization components back into focus, effectively enabling focusing through tissues.

The new setup employs a photorefractive crystal that uses a holograph to record phase information, which helps conjugate the light beams, and should be able to refocus light passed through a scattering medium such as tissue and restore the intensity lost in scattering.

According to their test results, the new method yields an enhanced intensity of focused light by approximately a factor of four compared with the single-polarization method. The new setup also improves the quality and efficiency of the optical focusing, making it ideal for procedures on thick biological tissues in which the depolarization effect is severe.

Source: “Dual-polarization analog optical phase conjugation for focusing light through scattering media,” by Zhongtao Cheng, Jiamiao Yang, and Lihong V. Wang, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5097181 .