基于双晶匹配的低压铌酸锂电光削波器的研究

Low Half-Wave Voltage Lithium Niobate Electro-Optic Slitter Based on Dual Crystal Matching

分析了电光削波器半波电压的关键影响因素,发现使用具有较大有效电光系数的电光晶体、合理设计晶体的横纵比是降低器件半波电压的有效手段;在此基础上,设计并制备出低半波电压的铌酸锂电光削波器。采用沿铌酸锂晶体x轴通光、z轴加电的横向电光工作模式,并利用双晶匹配的方式消除自然双折射的影响。匹配后两块晶体通光面的光学质量良好,直流高压下测得的半波电压约为900 V,动态消光比达200∶1。使用幅值为800 V、脉宽为0.95 ns、重复频率为1 Hz的脉冲高压驱动电光削波器工作时,从1064 nm连续激光中获得了脉宽为1.46 ns、重复频率为1 Hz的激光脉冲输出。

Objective Electro-optic slitter is a type of laser pulse shaping device based on electro-optic effect.It is widely usedin laser amplification systems,nonlinear optical systems,laser fine machining,and other applications.At present,there are commercial slitters made of DKDP,BBO,RTP,KTP,and other electro-optical crystals.These slitters areaffected by crystal cutting type and working mode,whose aperture is between 2 and 20 mm,and the half-wavevoltage is several thousand volts or even ten thousand volts.The extremely high half-wave voltage greatly affects thepreparation and performance of the driving power supply.However,it is relatively easy to obtain an electrical pulsewith fast-rising time and narrow pulse width at low voltage for the driving power supply,thereby improving therepetition frequency of the device,reducing the power dissipation of the whole device,and reducing electromagneticinterference to other electronic devices in the system.Therefore,reducing the half-wave voltage positively improvesthe overall performance of the electro-optical slitter.The advantages and disadvantages of DKDP,BBO,RTP,KTP,and LN crystals are compared.From the requirements of the electro-optical slitter,LN crystal,as the corecomponent of the electro-optical slitter,is selected for designing and preparing the slitter.In this study,we designand fabricate a low half-wave voltage LN electro-optic slitter.Methods The electro-optic effect theory provides the electro-optic characteristics of LN crystal when an electric field is applied along each axis.The electro-optic effect is maximized using 90°x-cut crystals with radiation propagating and control voltage applied along the x-and z-axes,respectively.Also,we achieve a compensation of natural birefringence using two LN crystals whose z-axes are rotated relative to one another by 90°.A low half-wave voltage LN electro-optic slitter is fabricated and the optical quality,half-wave voltage,extinction ratio,and slitting performance of the slitter are measured and characterized.Results and Discussions After processing and coating,LN crystals are packaged in an elastic holder(Fig.3).The electro-optic slitter consists of two Glan-laser lenses with orthogonal polarization directions and a set of LN crystals(Fig.4).By observing the conoscopic interference pattern of two matched LN crystals without electricity and applying a DC high voltage,the optical quality of the x faces of the LN crystal is good(Fig.6).Also,the extinction ratio of the slitter is measured in the orthogonal polarized and parallel polarized systems.In the orthogonal polarization system,the output optical power increases with increased voltage and reaches the maximum value at the half-wave voltage.In the parallel polarization system,the output power is minimum at the half-wave voltage(Fig.7).The half-wave voltage is measured at~900 V under DC high voltage with a dynamic extinction ratio of 200∶1(Table 1).When the electro-optic slitter is driven by a pulsed high voltage of 800 V,0.95 ns,and 1 Hz(Fig.8),a 1.46 ns,1 Hz laser pulse output is obtained from a 1064 nm continuous wave laser(Fig.9).Conclusions In this study,we investigate the key factors affecting the half-wave voltage of an electro-optic slitter.Results show that an effective method for reducing the voltage is applying the crystal with large effective electrooptic coefficient and designing the transverse/longitudinal ratio of the crystal.Thus,a low half-wave voltage LN electro-optic slitter is designed and fabricated.Furthermore,the electro-optic effect is maximized using 90°x-cut crystals with radiation propagating along the x-axis and the control voltage applied along the z-axis.Also,compensation of natural birefringence is achieved using two LN crystals whose z-axes are rotated relative to one another by 90°.The optical quality of the z-faces of the LN crystal is good.The half-wave voltage is measured at~900 V under DC high voltage,and the dynamic extinction ratio is 200∶1.Upon driving the electro-optic slitter using a pulsed high voltage of 800 V,0.95 ns,and 1 Hz,we obtain a 1.46 ns,1 Hz laser pulse output from a 1064 nm continuous wave laser.The output laser pulse waveform of the electro-optic slitter is related to the electrical pulse waveform of the driving power supply.For future work,we hope to obtain a laser pulse with a narrower pulse width by reducing the width of the electrical pulse.