Alternative Title正弦波位相変調法を用いた光渦の位相分布計測に関する研究
Note (General)The vortex beam (VB) with a helix wavefront has a wide range of applications in optical manipulation and information processing. In the past decades, some research on the vortex phenomenon had remarkable development and the research on VB has gradually become an important branch of modern optics. The spiral wavefront of VB is characterized by characteristics phase of exp(ilθ) with the topological charge of l and the azimuth angle of θ. The unique spiral phase structure presents a hollow bright ring in the intensity distribution of VB and yields a phase singularity in the optical axis with the undefine phase and amplitude of zero. Because of the helix wavefront, each photon of VB possesses the orbital angular momentum (OAM), given by hl, related to the topological charge, where the h is the Planck constant. Considering the unique property of OAM, the VB has a wide range of applications in optical tweezers and information processing. For example, the focused VB generating the electric field gradient force can be used for capturing particle, making it rotate around the optical axis of VB. In addition, the property of OAM can be used for fabricating the chiral micro-scale structures materials as well. In the optical communication field, the OAM can have been used as an alternative degree of freedom for multiplexing modulation, thereby enlarging the capacity of optical communication, which is also referred to as mode/spatial-division multiplexing. Given these applications using the feature of the VB, the phase distribution of a wavefront of VB, which is related to the topological charge, is the one of most crucial parameters. Therefore, investigation of the property of VB and proposing the phase measurement method of VB are significant for these applications using the VB. Main research and innovations of this thesis are as follows: (1) Investigation of the property of vortex beam. The VB with the spiral phase structure is derived from wave equation under the condition paraxial approximation. Due to the spiral wavefront, each photon of VB possessing the OAM that has many applications in optical tweezers and optical communication. In the experiment, the common VB has mainly two types for Laguerre-Gaussian and Bessel beams. Considering these applications, the generation of high quality of VB is essential and the general methods of generating VB is mainly classified to Laser mode modulation and phase element. (2) Sinusoidal phase modulation method Phase measurement of the VB as well as estimation of the topological charge are significant for these applications. Some methods of phase measurement of VB have been proposed. Nevertheless, these methods have some obvious disadvantage due to the complex experimental configuration and data processing. Therefore, the Sinusoidal phase modulation method is proposed to measure the phase profile of vortex wavefront based the interferometric measurement technique with the high measurement accuracy and the simple data processing method. Due to more convenient configuration and straightforward calibration, the SPM is excellent method for phase measurement of VB. (3) Estimation method of topological charge The topological charge of VB is the one of most crucial parameters as well and defined by the maximum variation in phase distribution of the VB along to the azimuth angle. Hence, the continuous variation of phase along to azimuth angle in wrapped phase was used for estimating the topological charge. However, due to the discontinuous phase distribution in wrapped phase distribution of VB, it may lead to the enormous measurement error. For obtaining the continuous phase distribution of VB, the phase unwrapping method based on polar coordinates used for estimating the topological charge can distinguish the position of phase singularity and phase step, with high measurement accuracy. (4) Reduction of effect of phase singularity The phase singularity with an undefined phase and amplitude of zero must be reduced or eliminated in advance. Because of the intensity in the position of phase singularity being far small from other pixel of intensity of VB, the suitable threshold value of measured intensity of VB was defined to reduce the part of phase singularity only that the above the threshold value of intensity is highlighted.
新大院博(工)第541号
Collection (particular)国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
Date Accepted (W3CDTF)2023-06-02T22:06:20+09:00
Data Provider (Database)国立国会図書館 : 国立国会図書館デジタルコレクション