Center for Relativistic Laser Science

2019 IBS AOI Conference

In the field of optical imaging and therapy, where light is used to diagnose or treat disease, it is important to deliver sufficient light energy to a target within the biological tissues. However, the tissues give rise to multiple light scattering and spatial spread of the propagating waves as they are complex media composed of structures with various refractive indices. For this reason, the light energy delivered to a target object is steeply decreased with the increase of depth.

 In previous studies, we proposed methods of focusing light energy on the target inside the scattering medium by controlling multiple scattering waves [1, 2]. We measured the transfer matrix from the time-gated signals reflected from the target, found eigenchannels maximizing the intensity of the multiple scattering waves from the matrix, and then illuminated the eigenchannel pattern to increase the light energy delivered to the target in the scattering medium. In particular, we delivered more than 10 times light energy to the target under the mouse skull without damaging to the skull. However, this eigenchannel method requires thousands of images to obtain the matrix, and the computation time increases exponentially with the increase of view field. In addition, it is very vulnerable to vibrations and movements occurring during measurements and eigenchannel computation.

 Here, we combine the phase conjugation with time-gated measurement to maximize the intensity of multiple scattering waves at the target object. We send the phase-conjugation of the backscattered waves arriving at a certain flight time back to the sample. By repeating this process iteratively, we find the pattern of illumination that can deliver light energy similar to the eigenchannel method with only a few rounds of image acquisitions. In doing so, the measurement time is shortened by hundreds times. In addition, since the pattern is updated at each step, the pattern changes adaptively to the vibrations or movements of the sample. These advantages are expected to make the proposed method useful for real-time optical imaging or therapy.

References

[1] S. Jeong, Y. Lee, W. Choi, S. Kang, J. Hong, J. Park, Y. Lim, H. Park, and W. Choi, “Focusing of light energy inside a scattering medium by controlling the time-gated multiple light scattering”, Nat. Photonics 2018, 12(5), 277-283.

[2] S. Jeong, D. Kim, Y. Lee, W. Choi, and W. Choi, “Iterative optimization of time-gated reflectance for the efficient light energy delivery within scattering media”, Opt. Express 2019, 27(8), 10936-10945.