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Electrically controlled holographic varied line-spacing grating based on polymer dispersed liquid crystal
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摘要:
本文报道了一种基于聚合物分散液晶的电控全息变间距光栅。采用柱面波和平面波干涉得到具有变间距的干涉条纹，并将此条纹记录于聚合物分散液晶材料中。实验分析研究了该光栅的空间频率、衍射特性和电场调控特性。光栅的空间频率变化范围和趋势与理论计算公式相匹配，实验结果表明光栅衍射效率与曝光光强和时间存在一定的关系。空间频率在530 mm-1~650 mm-1内的光栅衍射效率能达到70%以上。光栅的阈值电压为2.4 V/μm，上升沿和下降沿时间分别为300 μs和750 μs。该光栅不但具备了普通变间距光栅的优点，而且还具备了聚合物分散液晶的电场调控的特性，在光纤通信，光电探测及光谱探测等领域具有一定的应用前景。
Abstract:Electrically controlled holographic varied-line-spacing (VLS) grating based on polymer dispersed liquid crystal (PDLC) is reported. Varied-line-spacing interference pattern is generated through interference between cylindrical wave and plane wave, and recorded in PDLC. Characteristics, such as spatial frequency, diffraction and electric-optic, are analyzed by experiments. The results show that the trend and range of grating period match well with the theoretical simulation. The relationships between diffraction efficiency and exposure intensity as well as exposure time are studied. The grating diffraction efficiency can be achieved more than 70% with spatial frequency from 530 mm-1 to 650 mm-1. Meanwhile, the grating has good electrically controlled property. The threshold voltage is 2.4 V/μm, and the rise time and fall time are 300 μs and 750 μs, respectively. The grating not only has advantages of ordinary VLS grating but also has electric-optic characteristics of PDLC. It has potential applications in the fields of optical fiber communication, photoelectric detection and spectrograph.
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Key words:
- liquid crystals /
- holographic gratings /
- electric control /
- varied line-spacing /
- optical devices
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Abstract:With the development of laser manufacturing technology, people can fabricate VLS plane gratings by holographic lithography. Compared with traditional gratings, the light incident on different positions of the VLS grating can diffract light with different angles, which leads to astigmatism and focusing effect. The application of VLS plane gratings reduces the number of optical elements inside the instrument and improves the resolution of the instrument. It is widely used in vacuum ultraviolet, soft X ray, optical fiber communication, sensor and many other fields.
The fabrication of VLS plane gratings has been reported mainly by mechanical scribing and holographic lithography. Adjacent lattice spacing in VLS grating is always in nanometer scale. It is difficult to use the ruling machine to make grating and to guarantee the accuracy in the actual operation process. Holographic lithography use photosensitive material to record spherical wave or non-spherical wave interference that can form varied distance of interference fringes and further fabricate VLS spherical or plane gratings. It has the advantages of simple operation, low cost, and easy to control the line-space, which is a common method to fabricate VLS grating. The holographic polymer dispersed liquid crystal (H-PDLC), as a new photoelectric information functional device, has high diffraction efficiency, fast response, and simple preparation. Its electric-optic characteristics have been greatly improved.
In this paper, H-PDLC material is used as a photosensitive material to record VLS interference pattern. The grating not only has advantages of ordinary VLS grating but also has electric-optic characteristics of H-PDLC. First, VLS interference pattern is generated through interference between cylindrical wave and plane wave. Second, the PDLC material is produced and put into the interference field for exposure. During polymerization, the prepolymer absorbs intense optical energy and polymerizes at the bright region, while the liquid crystal molecules are forced to diffuse from the bright region to the dark region, which forms the periodic alternating LC-rich region and polymer-rich region, corresponding to the interference optical pattern. Characteristics such as spatial frequency, diffraction and electric-optic, are analyzed by experiments. The results show that the trend and range of grating period match well with the theoretical formula. The relationship between diffraction efficiency and exposure intensity, as well as time is studied. The grating diffraction efficiency can be achieved up more than 70% with spatial frequency from 530 mm-1 to 650 mm-1. In addition, the grating has good electrically controlled property. The threshold voltage is 2.4 V/μm, and the rise time and fall time are 300 μs and 750 μs, respectively.
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图 1 变间距干涉条纹模拟图.
Figure 1. Simulated interference pattern of varied-line-spacing stripe.
图 5 不同参数下的光栅空间频率对比曲线. (a) D=43.5 cm，α=17.55°和20.18°. (b) α=17.55°，D=37.2 cm和43.5 cm.
Figure 5. Line-spacing variation with different fabrication parameters. (a) D=43.5 cm, α=17.55° and 20.18°. (b) α=17.55°, D=37.2 cm and 43.5 cm.
图 6 光栅衍射效率及光电特性测量光路图.
Figure 6. Measurement setup for diffraction efficiency and electricoptic characteristic of grating.
图 7 不同曝光强度的光栅衍射效率图.
Figure 7. Diffraction efficiency of gratings with different exposure intensities.
图 8 不同曝光时间的光栅衍射效率图.
Figure 8. Diffraction efficiency of gratings with different exposure time.
图 9 衍射效率随驱动电压的关系曲线.
Figure 9. Measured diffraction efficiency as a function of driving voltage.
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