Research On Beam Steering Technology Of High-efficiency Broadband Liquid Crystal Polarization Gratings

Broadband liquid crystal polarization gratings(LCPGs)non-mechanical beam steering technology has a wide application prospect in lidar,laser communication,laser weapons,and augmented reality due to its advantages of low power consumption,high precision,light and small size,fast response,and wide angle.By controlling the rodlike liquid crystal molecules to fan out in the plane,the LCPG forms a spatially varying birefringent layer,and realizes the beam deflection like the blazed grating with the help of geometric phase distribution.The LCPG can deflect the laser beam with almost 100%diffraction efficiency when applied to the circularly polarized incident light.And the light can be switched between the two directions by changing the rotation of the circularly polarized incident light.When the LCPGs and circular polarization switchers(CPSs)are alternately cascaded,the beam deflection system with large scanning angle can be realized.However,the core components of the system,the LCPG and the CPS,can only work at a single wavelength,and the efficiency decreases sharply in a wide band range.In order to extend the wide band application of the beam steering system,this dissertation focuses on the structure design and performance optimization of LCPGs and CPSs.The existing numerical optimization methods are faced with the problem of multiple design parameters and difficult optimization when designing the wider-band grating structure.To solve this problem,we firstly proposed a simplified design model of multi-layer mirror symmetry of broadband LCPGs.LCPGs was disassembled into mirror-symmetric broadband half-wave plates with circular polarization conversion function,which simplified the design structure and reduced the optimization parameters by 50%.In order to combine with the traditional LCPG fan-shaped pattern photo-alignment technology of liquid crystal molecules,we selected a self-aligned multilayer twisted structure and proposed a Poincaré Sphere visualization method to design the structure of the simplified grating model.Finally,we successfully implemented the mirror-symmetric four-layer twisted LCPG,and the working waveband range was expanded to 420-945 nm.In the wave-band range,the normal incidence diffraction efficiency of LCPG with period 10 ?m is 99%,and the diffraction efficiency of ±30° is90%,while the reported waveband in references can only do 470-670 nm.The CPS needs to arbitrarily switch between left-and right-handed circularly polarized states,and it also faces the problem of low conversion efficiency in a wide band range.To solve the above problems,we analyzed the reason why the structure mentioned in references is inefficient at oblique incidence.That is,in the rotation switching mode and the rotation maintaining mode,the delay changes in the two are opposite under oblique incidence,so it is difficult to achieve good FOV compensation at the same time.Therefore,we proposed a solution that electrically suppressed helix ferroelectric liquid crystal cell(ESH-FLC)as dynamic switching layer.Then,we proposed a model for designing the broadband polarization evolution path with PoincaréSphere,and designed a four-layer structure.Further,analyzing the polarization evolution path,the structure was simplified into three layers.In the middle layer,a half wave retardation ESH-FLC was used with the optical axis switching between-90° and-45°.And a positive uniaxial quarter wave retardation film with the optical axis of 0° was used in both front and rear layers.And two positive uniaxial delay films with optical axis vertically arranged is used to compensate the oblique incidence delay.Finally,we realized the large FOV and broad-band CPS working at 450-650 nm.The polarization conversion efficiency can reach 96% at the incidence angle of 49°,while the reported structure can only achieve 13°.Both above devices work in circularly polarized light mode,and the LCPG has high diffraction efficiency only for the circularly polarized incident beam,which is extremely sensitive to the incident polarization state.Therefore,polarization state switching devices are usually needed to convert the polarization state of incident light from linear to circular.In order to make the LCPG realize single order high diffraction efficiency modulation of linearly polarized light,we revealed working principle of the LCPG to trans-form the polarization state of beam at the condition of linear polarization incident beams based on a finite element simulation model of the LCPG.Then,a composite polarization grating was proposed,which was composed of two layers of traditional LCPGs whose periods satisfy the double relation and whose thicknesses satisfied half wave and quarter wave retardation,respectively.The incident beam of the composite polarization grating is linearly polarized,which is easy to be used in combination with the laser beam.It can realize the beam deflection with high diffraction efficiency from linear polarization state to circular,and the experimental results are as high as 96%,while the traditional LCPG has only 50% single order diffraction efficiency for linearly polarized incident light.As a result,the restriction is broken that the LCPG-based beam steering system has high diffraction efficiency only for the circularly polarized incident beam,and the requirement is expanded about polarization state at the incident end of the beam deflection system.After the above research,the beam energy efficiencies of the LCPG and the CPS are greatly improved in the wide band range.It is expected to accelerate the engineering application of broadband LCPG-based non-mechanical beam steering technology.