Engineering of optical alignment tolerance between a laser source and a Fabry–Perot (FP) cavity is of great importance in optimizing a laser analysis system and saving costs in the additional alignment task. We propose a modification of the conventional confocal FP (CFP) cavity–a symmetric nonconfocal FP (SNFP) cavity with two identical spherical mirrors separated by a distance equal to half of the common radius of curvature. Numerical analysis of the two cavities reveals that the SNFP cavity has better off-axis stability and much better angular misalignment tolerance than the CFP cavity, even with a longer optical path length (OPL). The SNFP cavity provides a six-transit path length for a basic cavity mode that supports a 1.5-fold long OPL within the same volume as the CFP cavity, which is favorable for applications requiring limits on the footprint and weight of the cavity. In addition, a ray-tracing simulation with a disk-type source having an angular Gaussian distribution (α = β = 5 deg) reveals that the emission beam profile from the SNFP cavity is narrower than that from the CFP cavity. These results are expected to help reduce cavity loss and to improve alignment efficiency between the laser source and the FP cavity.
Low-level light therapy, which is a red or near-infrared light-employed therapeutic methodology, can act to help the brain repair in cases of traumatic brain injury and stroke. In this paper, we investigate the effects of near-infrared light therapy (NILT) for the recovery of blood flow of mice with cerebral hypoperfusion, which is a key mechanism leading to vascular dementia, induced by bilateral common carotid artery stenosis (BCAS). The mice are divided into three groups of 4-5 mice per group: a normal group (without BCAS operation), a BCAS group (without NILT), or a BCAS/NILT group. The LED with a peak wavelength of 810 nm and a power of 20 mW is arranged to illuminate on the top of the mouse head. The mice receive treatment from the LED source 3 times per week for one month. After the treatment, positron emission tomography is used to quantify the effects of NILT on whole brain and regional cerebral blood flow in the cortex, striatum, and hippocampus.
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