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Analysis of temperature-induced liquid crystal phase control beam quality deterioration
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摘要:
液晶光学相控阵是下一代光束控制技术的核心器件，提高其耐受激光阈值是当前研究的热点之一。针对较高功率激光入射场景下评测液晶光学相控阵相位调制性能恶化程度的问题，本文基于传统四分之一波片法，实现快速、直接测量液晶对入射激光的相位调制量。验证试验发现，当中心温度为33 ℃时，对应的最大畸变相位为3.6 rad。同时，本文基于该实测相位调制结果，研究出射光的光束质量恶化过程。分析结果表明：当液晶移相器的中心温度变化小于10 ℃时，光束质量恶化小于20%。
Abstract:The liquid crystal optical phased array (LCOPA) is the core device of next-generation beam control technology. Improving its laser-induced damage threshold is one of the current research hot spots. Aiming at the scene of higher power laser incidence, the degradation degree of LCOPA phase modulation performance should be evaluated. Based on the traditional quarter-wave plate method, this paper realizes fast and direct measurement of the phase modulation of the liquid crystal to the incident laser. The verification test found that when the core temperature is 33 ℃, the corresponding maximum phase aberration is 3.6 rad. At the same time, based on the measured phase modulation results, this paper studies the deterioration process of the beam quality of the outgoing light. Analysis results show that the deterioration of beam quality is less than 20% when the core temperature of the liquid crystal phase shifter changes less than 10 ℃.
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Overview: The liquid crystal optical phased array (LCOPA) is the core device of next-generation beam control technology. For beam deflection control, it has the advantages of high precision, fast response, low threshold voltage, small size, etc. Increasing its laser-induced damage threshold is one of the current research hot spots. Temperature distribution will be formed on the surface of the device, due to the absorption of the device when a high-power laser is illuminated. Since the liquid crystal is a temperature-sensitive optoelectronic material, its phase modulation to incident light will deteriorate due to temperature rise. To investigate the performance of LCOPA under high-power laser incidence, the relationship between temperature and phase modulation must be established. Aiming at the scene of high-power laser incidence, the degradation degree of LCOPA phase modulation performance should be evaluated. The traditional method is to calculate directly through theory, but the accurate liquid crystal characteristic parameters must be known first, and for unknown liquid crystals, this method is difficult. Based on the traditional quarter-wave plate method, this paper realizes fast and direct measurement of the phase modulation distribution of high-power incident light with a LCOPA, and then the temperature-voltage-phase correspondence of the liquid crystal is established.
The LCOPA is loaded with a specific periodic voltage value to make the ideal deflection angle of 0.5 mrad. At the same time, 4 kinds of one-dimensional Gaussian temperature distributions with different core temperatures (42 ℃, 39 ℃, 37 ℃, 33 ℃) are added to the device. The actual phase distribution of LCOPA can be obtained according to the above temperature-voltage-phase correspondence relationship. The result show that the phase distortion distribution under the influence of temperature has a Gaussian envelope related to the external temperature distribution, and there is also a sawtooth distribution related to the external voltage distribution on the Gaussian envelope. Meanwhile, the phase distortion reaches the maximum at the core temperature. Taking the center temperature of 33 ℃ as an example, the corresponding maximum distortion phase amount is 3.6 rad. Then, based on the phase modulation results at the above four temperatures, starting from the Helmholtz equation and taking the efficiency ratio in the barrel as the criterion, the quality deterioration process of the beam within 1000 m from the device is studied. The MATLAB simulation results show that as the transmission distance increases, the beam quality of the emitted light slowly decreases and finally stabilizes. The deterioration of beam quality is less than 20% when the core temperature of the liquid crystal phase shifter changes less than 10 ℃.
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图 4 (a) 高斯温度分布；(b) 理想相位分布；(c) 实际调制相位分布；(d) 相位畸变量
Figure 4. (a) Gaussian temperature distributions; (b) Ideal phase distribution; (c) Actual modulation phase distributions; (d) Phase distortion
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