Research On Laguerre-Gaussian Solid-State Lasers And Optical Angular Momentum

As an eigen-solution of the paraxial wave equation deduced from Maxwell's equations by means of the paraxial approximation in cylindrical coordinates,Laguerre-Gaussian(LG)mode has been widely studied.In particular,the unique spiral phase distribution of LG mode,namely optical orbital angular momentum(OAM),makes LG mode stand out among other modes.A large number of researches show that the high order LG beams can be used in the fields of optical tweezer,particle control,space communication and super-resolution imaging,etc.The LG modes can be divided into two types,one is the non-spiral LG mode,and the other is the spiral LG mode with OAM.In recent years,LG mode lasers are more and more popular among scientists and technicians because of their high power,efficiency and stability.With the rapid development of optical OAM communication and optical tweezer,the OAM measurement of vortex beam has become a research hotspot.However,there are few existing methods for measuring OAM,and most of the experimental devices are complex.As well as,the simple OAM measurement methods have poor applicability and low accuracy,especially for measuring higher order OAM.Therefore,how to obtain a simple and accurate method to measure OAM has become the focus of research on OAM.In optical angular momentum,there are OAM and spin angular momentum(SAM).Different from the spatial phase distribution of OAM,SAM is an independent property of each photon and does not depend on other photons.That means if applied SAM for quantitative phase imaging(QPI),each photon can carry information independently,which can greatly improve the spatial resolution of the imaging system.Therefore,Laguerre-Gaussian solid-state lasers,orbital angular momentum measurement and quantitative phase imaging are studied in this thesis.The main academic results of this work are as follow.1.Non-spiral radial Laguerre-Gaussian solid-state laserBased on a diode-pumped Pr:YLF laser with hemispherical linear resonator,a thin Nd:YVO4 crystal was inserted into the laser cavity to introduce a thermal effect,which realized the non-spiral LG mode laser output.The central wavelength of the output LG laser is 639.5 nm,meanwhile,the maximum power reaches 150 mW with slope efficiency of 7.0%.In additional,the self-reconstructing characteristic of the LG laser was also demonstraded.2.Vortex Hermite-Gaussian solid-state laserVortex and vortex Hermite-Gaussian(vHG)laser beams can be obtained in a hemispheriacal linear resonator by inserting a small tilted angle on the output mirror.The maximum output power of the red vortex laser is up to 320 mW with the slope efficiency of 9.4%.In addition,we achieved twin-vortex and triplet-vortex Hermite-Gaussian laser beams,the property of OAM was also verified by the interference fringes.This method for generating vortex and vHG beams have potential applications in laser processing and optical manipulation,etc.3.Orbital angular momentum measurementBased on an improved Fizeau interferometer,we realized determining topological charges of vortex beams by using a suitable coated flat-concave mirror.Because of the two reflected surfaces are relatively static,the interference is very stable and not sensitive to external condition.Vortex,Hermite-Gaussian,and elliptical vortex beams are commendably verified by this method.Moreover,the topological charge of l=50 is also determined in experiment.This method may have some potential applications in free space optical communication.4.Quantitative phase imaging based on spin angular momentumWe demonstrated that the phase and amplitude information can be translated effectively to the state of the polarization of an electromagnetic field.This polarization encoding method requires neither spatial interrogation of interference firings,nor an external controllable component such as a SLM or PPM.The interferometric nature of the measurement provides high sensitivity information,which can be further improved by taking advantage of remarkable advancements in image processing.Moreover,because the polarization is a point property of the optical field,the encoding procedure can be applied to the examined field without compromising the spatial resolution afforded by the optical imaging system.The high spatial resolution provided for both phase and amplitude,the simplicity and versatility of the practical implementation,make our polarization encoding technique suitable for broad range of applications,especially in the field of bio-medical optics,including optical diagnosis and prognosis.In experiment,we realized QPI of different types of cells based on Mach-Zehnder interferometer,the internal structure of cells could be well observed.The accurate of phase measurement can be reach ?/125,which is of great significance for the study of biological cells and tissues.Moreover,QPI of nano-scale polystyrene spheres was performed by using a common-path intereferometric phase imaging device.The reliability and sensitivity of the system are both well verified by comparing the experimental results with the phase theory of subwavelength particles.By detecting the small change of phase information(?0.1 rad),we can accurately distinguish the sizes of subwavelength polystyrene spheres.