Improvement of scattering suppression effect of time-reversal propagation using digital phase-conjugate light

Strong scattering in turbid medium is a severe difficulty for optical imaging and measurement in an animal body. Therefore, the suppression of scattering effect is crucially important in many applications of biomedical optics. In the optical transillumination imaging of an animal body, this effect appears as strong blurring of the image. This blurring poses a fundamentally important difficulty restricting the practical application of transillumination imaging. Therefore, we have attempted to suppress scattering effect using the time-reversal ability of phase-conjugate light. For our previous study, we constructed a digital system to generate light not only with conjugated phase but also with the same intensity distribution as non-scattered signal light. Using this system, we were able to restore the pattern of incident light from the blurred image because of time-reversal propagation of the phase-conjugate light. In comparison to a case with phase information only, we found that addition of the intensity information greatly improves the scattering suppression capability of the time-reversal principle. However, our pilot study showed this ability was valid only for the scattering medium with the optical distance OD less than 1. This report describes the improvement of our measurement system to make the scattering suppression possible for turbid media with OD of more than 1 using a light source with a longer coherence length. Scattering suppression was effective for spatial frequencies of 0.4-1.0 lp/mm through the scattering medium up to OD=1.8.