Study On The Effect Of Polarization Distortion On The STED Microscope PSF

Due to the far-field diffraction limit of optical microscopes,traditional optical microscopes cannot clearly observe that the size of object is below 100 nm.However,STED(stimulated emission depletion)can clearly observe it.In STED systems,the use of many optical components or film systems introduces uncertain phase delay that causes the distortion of polarization state.Based on this,the influence of polarization distortion on the focused light field of the STED system is analyzed in this paper,and a method to compensate the effects of polarization distortion is introduced.In this paper,the effect on polarization distortion on the focusing plane of the STED system is studied by using a dichroic mirror(DM).Firstly,the influence of the polarization distortion introduced by DM of the two-dimensional STED system is studied.The polarization distortion introduced by DM is analyzed for the excitation light,the depletion light and the effective PSF peak intensity or the central intensity and shape.Under the influence of polarization distortion,the two-dimensional STED system of excitation light intensity FWHM 7.54 are not changed.The variation range of the excitation light within a period of 214.6 nm ~ 298 nm,the amplitude is 41.7 nm.Compared with the FWHM of the excitation light without polarization distortion,the maximum change(increased or decreased)is 16.3%.The intensity of the center point of the depletion light varies from 0 to 2.08.When ? is constant,the depletion light is increased with ?,and the intensity of the center point is sinusoidal.The trend of the normalized peak intensity of the effective PSF is the same as that of the depletion of the center of the light.For the FWHM of effective PSF: When the saturation coefficient S=3,? changes by 15°,the FWHM of the effective PSF of the STED system is reduced by 2.3%.When the saturation coefficient is S=3,? also changes by 15°,the FWHM of the effective PSF of the STED system is reduced by 8.9%.Therefore,when introducing polarization distortion,in the two-dimensional STED system,the intensity of the excitation light is not affected by polarization distortion,but the intensity of the depletion light changes,and the change of the effective PSF intensity is derived from the depletion light.The change of ? does not affect the ellipticity angle,in other words,the shape of the depletion light is not affected by ?.Based on the research of two-dimensional STED system,this paper further expands the research object to the three-dimensional STED system.The intensity and shape change of the excitation light,the depletion light and the effective PSF in the XZ direction are analyzed.The peak intensity of the excitation light of the 3D STED system is the same as that of the 2D STED system,the value is 7.54.The FWHM in the z direction does not change at 617.2 nm,the intensity of the center point of the depletion light varies from 0 to 2.08,and when ? is constant,the intensity of the center point of the depletion light increases with ?.In the three-dimensional STED system,the peak intensity of the effective PSF changes with the depletion light,and as ? increases from 0° to 180°,the FWHM in the z direction increases from 214.4 nm to 254 nm and then decreases to 214.4 nm,the period is 180°.Therefore,the polarization distortion introduced by ? and ? have the same effect on the center intensity of depletion light and effective PSF peak intensity of the three-dimensional STED system as the two-dimensional STED system,and the shape change in the z direction is different from the two-dimensional STED system: The shape of the excitation light in the z direction is unaffected by polarization distortion.Finally,based on the analysis of the influence of polarization distortion on the focused light field of two-dimensional and three-dimensional STED systems,a method of compensating for polarization distortion using a combination of HWP and QWP is proposed,and a detailed calculation process is given.And the function expression of the angle at which the quarter wave plate needs to be rotated,to provide a theoretical calculation basis for constructing the actual light path.