Study On The Adjustable Polarization State And The Vector Light Of Q - Phase And

Polarization is one important and essential property of light. Unlike those scalar beams (e.g., linearly polarized beam, circularly polarized beam, elliptically polarized beam) with spatially homogeneous states of polarization, cylindrical vector beam (CVB), which possesses an axisymmetric or rotational symmetric polarization mode, is drawing more and more attention in recent years. Due to its intriguing properties after tight focusing (e.g., beyond the diffraction limit and strong longitudinal electric field), it can be widely applied for various applications, such as high-resolution microscopy, electron acceleration, optical trapping, and material processing.Since its discovery in 2004, graphene has become one of the most popular nanomaterials. As a novel form of saturable absorbers, graphene has opened a new door for laser Q-switching and mode-locking due to its outstanding properties over traditional saturable absorbers, such as broad operation bandwidth, fast recovery time, controllable modulation depth, and simple fabrication with low cost.In this dissertation, we combine CVB with graphene creatively and did a series studies both theoretically and experimentally to solve the shortcomings in the present study in this field. Details are presented as follows.(1) Generation of CVB with tunable states of polarizationFirstly, we deduced the mathematical description of CVB from Helmholtz equation. Then we theoretically analyzed the vector superposition process of two orthogonal linearly polarized TEM01 modes by using the Stokes parameters with its Poincare sphere. The theoretical analysis indicates that CVBs with diverse polarization distributions can be synthesized by changing the initial angle-φ0 between the mode direction and polarization direction of the two TEM01 beams and the phase difference-δ between them. Based on the above theoretical analysis, experimentally, a Mach-Zehnder interference arrangement was constructed to realize such a superposition process. The φ0 and δ can be changed from [0,π] and [-π,π] by rotating the discontinuous phase plates and adjusting the voltage uploaded on the electro-optical phase-modulator respectively without changing the structure or element of the arrangement. As a result, the CVB with tunable states of polarization was produced in our experiments.(2) Preparation and characterization of graphene saturable absorber mirrorFirstly, we theoretically analyzed the nonlinear saturable absorption of graphene from its energy band structure. Then the high quality graphene raw material was produced by the oxidation-reduction and chemical vapor deposition methods. Finally, we designed and fabricated two different types of graphene saturable absorber mirror (i.e., GSA and GOC) by choosing appropriate film transfer technology (mainly include spin-coating method and wet chemistry method) to transfer graphene onto a piece of K9 glass substrate of which both sides have been coated with SiO2/Ti2O5 films with different transmittance. The optical properties of the GSA and GOC were also characterized in detail. This part of our studies provided the key element for the follow-up laser Q-switching and mode-locking experiments.(3) Characterization of graphene Q-switched and mode-locked laserFirstly, we theoretically analyzed the mechanism of graphene Q-switched pulse generation and the threshold condition of graphene continuous-wave (CW) mode-locking operation. Based on the above theoretical analysis, we proposed the factors that should be taken into consideration in designing the graphene-based passively Q-switched and mode-locked lasers. Then, experimentally, we achieved the stable Q-switching and mode-locking in the diode end pumped Nd:YVO4 and diode side pumped Nd:YAG lasers with graphene respectively. This part of our studies proved that our fabricated graphene saturable absorber mirror can be applied as an effective Q-switching and mode-locking element in the laser resonant cavity.(4) Generation of pulsed CVB with narrow pulse widthFirstly, we theoretically analyzed the principle of polarization control in the cavity based on the thermal birefringence and bifocal thermal lensing of the Nd:YAG laser rod. Then, on the basis of previous studies, experimentally, we demonstrated the passively Q-switched and mode-locked CVB all-solid-state lasers by using GOC and GSA for the first time. Our lasers generated the Q-switched radially polarized CVB with a pulse width of 192 ns,2.7 W average output power,167 kHz repetition rate and mode-locked radially polarized CVB with a 2 W average output power,15.6 ps pulse width, respectively, the corresponding mode-locked peak power is up to 1.08 kW.