Research On The Application Of Special Optical Fiber Technology In STED Microscopy Imaging System

With the development of science and technology,various of frontier scientific issues pointed to the exploration of basic laws in the world of micro-size or nano-size.Due to the diffraction limit,traditional optical microscope cannot clearly distinguish the detailed information at this scale.On the other hand,electron microscope or other non-optical microscope tends to damage the samples,which makes it difficult to carry out real-time observation on living samples.Against this background,the invention of Stimulated Emission Depletion(STED)microscope provided a powerful tool for the development of modern science.As a genius design,STED microscope adopted optical method to break the diffraction limit.Therefore,it not only has the advantages of optical microscopes such as non-contact,non-destructive and real-time observation,but also can meet the requirement of resolution in many cutting-edge scientific problems.STED microscopy has been widely used in many fields such as biology and medicine within a few decades since its invention.And it has won the Nobel Prize in chemistry in 2014.Nowadays,most of STED microscopy systems reported at home and abroad use methods of spatial light modulation to achieve the scanning beam.Since the spatial light paths are very complex and precise,it brings various of difficulties in the operation and adjusting of the microscope.In view of the requirements of scanning beam in the STED microscope system,this paper reported researches on special optical fibers which can generate annular STED beam,and output the STED beam and the excitation beam concentrically.The main work of this dissertation includes the following aspects:Firstly,by analyzing the mode theory of three-layer step-index special fiber,it was proposed that when the refractive index decreases layer by layer from inside to outside in the special fiber,due to the interference effect between the first three order scalar modes in the special fiber,the output beam can be a doughnut-shaped dark hollow beam.In order to meet the requirements of scanning beam in STED microscopy system,some limitations for the design parameters of the special fiber are deduced.Secondly,based on the above design principle,a special fiber which can generate the scanning beam in STED microscopy system was achieved.Experimental results showed that a normal laser beam at the wavelength of 633nm can be transformed into a dark hollow beam by the special fiber.And a normal laser beam at the wavelength of 475nm can be transformed into a beam with bright center at the same time.Furthermore,the two beams can keep a high concentricity while propagating.The dark hollow beam generated by the special fiber can maintain a high quality while propagating in space.Even focused by lens,the dark center of the beam is still visible in the micro-scale focal spot.Based on the special fiber,a simplified scanning beam generation system for STED microscope was designedThirdly,another kind of three-layer step-index special fiber which can also generate the scanning beam in STED microscopy system was achieved.This kind of special fiber can only support the first two order scalar modes for the incident light at the wavelength in a specific range(including from 450nm to 532nm).When the incident light is concentrated into the inner-cladding rather than the core,only the second order scalar modes can be excited in the fiber,and the output is a dark hollow beam.When the incident light is concentrated into the core rather than the inner-cladding,only fundamental modes can be excited in the fiber,and the output is a Gaussian-shaped beam.Moreover,for longer wavelengths(633nm)and shorter wavelengths(405nm),even if the incident light is concentrated into the inner-cladding rather than the core,the output will not be an annular beam,but a beam with bright center.Based on the above experimental results,with some other fiber components,an all-fiber scanning beam generation system for STED microscope was designedFinally,for the particularity of incident light conditions required by the second special fiber mentioned above,a method to induce power-coupling from fundamental mode to second order scalar mode in the special fiber was proposed by writing Long Period Fiber Grating(LPFG)on it.In experiments,when the special fiber with LPFG was applied,for the light whose wavelength is in the specific range,a dark hollow beam can be generated even under an ordinary incident light condition.For the light whose wavelength is longer than the range,the output is a beam with a bright center.The influence of the pitch of LPFG on the output optical field distribution was studied.The spectral loss of the special fiber induced by LPFG were analyzed.