Holographic femtosecond laser processing for biological tissue

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Femtosecond laser processing (FsLP) provides high spatial resolution and causes reduced thermal destruction to a target as a result of the ultra-short interaction time between the laser pulse and the material. Furthermore, when a femtosecond laser pulse is tightly focused in a transparent material, a void and a refractive index change are formed inside. To fabricate an optical device, a huge number of processing points is required. Therefore, the processing speed must be sufficiently high to enable the desired throughput. We have demonstrated a holographic FLP (H-FsLP) with parallel, arbitrary, and variable patterning features, which were achieved by using a liquid crystal spatial light modulator (LCSLM) displaying a computer-generated hologram (CGH). In such parallel laser processing methods, the uniformity of the diffraction peaks is presently a key issue to be resolved.We have demonstrated the H-FsLP with a multiplexed-phase Fresnel lenses (MPFL), which were optimized by regulating the center phase and the diameter of the constituent PFLs while taking account of the LCSLM properties and the spatial profile of the irradiated laser pulse. By the optimization, the uniformity of the diffraction peaks in optical system was improved to the maximum value of 95%. To realize further improvement of the uniformity, an optimization method with an optical estimation was applied. The uniformity was improved to the maximum value of 98%. A f-FsLP with a target tracking feature was developed for the processing of biological tissues. The block matching method based on a sum-of-squared-difference metric was used to measure the transverse movement of the target. Surface position was detected with an astigmatic sensor to measure the axial movement. The maximum tracking speeds of the target in the transverse and axial directions were 255 μm/s and 125 μm/s, respectively.