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国内外的大型激光装置,在测量靶场波形时为降低成本多采用透镜(相位板)取样+光纤传输时分复用的架构[12-14]。我国的超大型激光装置在测量靶场三倍频波形时,为保证测量结果准确并降低建设成本,采用透镜取样+光纤传输并束时分复用的架构,每路波形测量取样光经一个3×3阵列透镜取样耦合到光纤集束,每4路波形经光纤集束传输后通过一个光学聚焦并束机构进入一个光敏面直径为Φ10 mm的真空光电管进行光电转换,示波器记录光电管的输出电信号获得测量波形[15]。
当时标脉冲能够通过光纤与取样光脉冲一同输入光学聚焦并束机构并通过真空光电管完成光电转换时,通过分析示波器上取样光脉冲与时标光脉冲的间隔即可对打靶光束的脉冲同步进行监测,时标光脉冲与波形测量系统相耦合的脉冲同步监测原理如图1所示。
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测量取样光脉冲的光电管在蓝紫波段灵敏度可达数V/W量级,时标脉冲处于这一波段时其功率需求并不高。为降低建设和维护成本,增加系统配置的灵活度,文中方案中时标光采用红外时间基准长程传输+光电转换+电光转换的方式产生,低功率的红外时标光经单模光纤长程传输到示波器位置后进行光电转换变为电脉冲,电脉冲驱动直接调制LD激光器产生符合输入要求的时标脉冲。该方案规避了复杂和敏感的激光放大与倍频过程,保证了时标波形输出的高度一致性,降低了建设成本,还增加了系统的可靠性(一个直接调制LD激光器损坏只影响一台示波器上光路的同步监测)。时标激光产生原理如图2所示。
红外时间基准信号由任意波形发生器驱动马赫-曾德尔型幅度调制器调制单纵模激光器并经放大、分束过程产生(如图3所示)。任意波形发生器输出半高全宽(FWHM)100 ps的电脉冲时,分束输出的红外时间基准信号能量大于等于5 pJ,能量稳定度优于1%(PV值),经带宽4 G的PIN半导体光电管探测模块进行光电转换,半导体光电探测模块的输出电脉冲上升沿为77 ps,FWHM为112 ps。
直调LD激光器是时标脉冲产生的关键器件,直调型LD激光器通过直接控制半导体激光器注入电流的大小来改变激光器的输出波形及强弱,其输出光功率与驱动电流的关系如公式(1)和图4所示[16-18]。
$$ P = \frac{{\eta E}}{e}(I - {I_{th}}) $$ (1) 式中:P为输出光功率;I为驱动电流;Ith为阈值电流;E为光子能量;$ \eta $为激光器的外微分量子效率;e为电子电荷。从图4中可以看出,当驱动电流小于阈值电流时,激光器基本不发光;当驱动电流大于阈值电流Ith时,激光器开始发射激光,且随着电流的增加线性增长,当增大到一定值时进入非线性饱和区。LD激光器输出激光的强弱直接与驱动电流大小有关。因此当半导体光电管输出信号注入LD激光器后,其形成的电流变化Ib可加载到半导体激光器上,其输出的激光波形将直接反映注入时间基准信号的波形特征。
靶场三倍频波形测量所用真空光电管带宽6 G,光阴极材料为蓝紫光灵敏型双碱阴极,响应波长190~680 nm,峰值响应度4 V/W@350 nm,在230 ~500 nm波段响应灵敏度较高(≥2 V/W),在<230 nm和>500 nm的波段,光电管的灵敏度呈断崖式下降。根据光电管的光谱响应和带宽特性,基于GaN基蓝光LD完成了直调型LD激光器的研制。LD激光器的输出波长为450 nm,3 dB响应带宽(9 K~3 G) Hz,激光器输出功率与注入电脉冲的幅值关系如图5(a)所示,注入电脉冲的幅值为1.2 V时,LD激光器达到最大输出功率为47 mW,此时真空光电管的输出幅值为115 mV;当注入电脉冲的幅值大于等于600 mV时,激光器稳定线性输出,输出功率波动为10%。
图 5 LD激光器的输出特性。(a)注入电脉冲幅值与输出光功率的关系;(b)输入电脉冲波形与输出时标光波形的比较
Figure 5. Output characteristics of LD laser. (a) Relationship of amplitude injected and laser power output; (b) Comparison of the electronic waveform injected and laser waveform output
搭配带宽6 G的真空光电管,LD激光器输出时标脉波形的上升沿为97 ps,脉冲FWHM为158 ps,注入LD激光器的电脉冲波形和LD输出时标光波形的测量结果如图5(b)所示。从图5(b)可知,在注入电脉冲幅值处于LD线性工作范围内时,激光器的输出波形较好地跟随了注入电脉冲的波形,能够将红外时基信号无畸变地转换为450 nm的时标光信号。
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当被监测的三倍频光束波形稳定并对三倍频波形测量结果做归一化处理后,基于光电-电光变换的时标激光系统其同步监测误差主要来源有3个部分:时标信号光功率波动引起的时标信号幅度起伏、示波器噪声、传输光纤延时的抖动。
1) 时标光脉冲功率波动引入的监测抖动:理论模拟显示,当时标脉冲能量波动为10%时,以脉冲后沿50%高度为判据,能量波动引入的同步监测误差δ1=4.7 ps(如图6所示)。
图 6 能量波动引入同步监测误差的理论模拟
Figure 6. Simulation result of synchronization moniting error caused by energy fluctuation
2) 示波器噪声引入的监测抖动:监测系统基于示波器判读同步误差,示波器的底噪会同时叠加到波形和时标信号上引起波形和时标信号幅度的变化,产生判读误差。实验所用示波器的噪声值为满度值的±1%,当波形和时标幅值在示波器上均位于满刻度的70%时,示波器噪声在时标信号和波形信号幅值测量结果中的最大比例为±1.4%,理论模拟显示,示波器噪声引入的波形判读误差δ2和时标信号判读误差δ3均为2.6 ps(如图7所示)。
图 7 示波器噪声引入同步监测误差的理论模拟
Figure 7. Simulation result of synchronization moniting error caused by oscilloscope noise
3) 温度变化导致传输光纤长度变化引起的同步监测抖动:同步监测系统中的红外时标信号在光纤中的传输长度为200 m,所用G655单模光纤每千米在1053 nm波段的温度系数为0.008 m/℃,时标系统工作环境的温度范围为24.5~25.5 ℃,温度变化引起的光纤长度变化为1.6 mm,引起的光程差和同步监测波动δ4为8 ps。
测量时标信号所用的真空光电管,其噪声与工作电压有关系,工作电压为1000~1600 V时,暗电流噪声在5×(10−4~10−3) mV之间,与示波器噪声相比为一小量,可忽略其对同步监测的影响,将红外时标光信号转换成电信号所用的PIN型半导体光电管,其暗电流噪声为2.5×10−4 mV,与示波器噪声相比为一小量,其对同步监测的影响也可忽略。
δ1、δ2、δ3、δ4互不相关,忽略光电管噪声对同步监测系统的影响,同步监测系统的最大监测误差$\delta = \sqrt {\delta _1^2 + \delta _2^2 + \delta _3^2{\text{ + }}\delta _4^2} = 10\;{\text{ps}}$。
Laser time fiducial system for high-power laser facility based on optic-electric and electric-optic conversion
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摘要: 提出了一种时标基准信号红外长程传输结合近程光电-电光转换的时标激光系统设计方案,该方案首先使用红外单模光纤完成时间基准信号的长程传输,然后使用光电探测器将红外时标基准信号转换为电信号驱动蓝光直调LD激光器,最终获得输出波长为450 nm,输出功率47 mW,脉冲宽度为120 ps的时标光信号。实验结果表明,所提方案可为激光驱动器提供窄脉宽,可长程传输的时标光信号用于甚多束紫外激光脉冲同步同发监测,同步监测精度可达10 ps。文中技术方案规避了敏感的固体激光放大与倍频过程,提高了时标光脉冲波形的稳定度与同步监测系统的可靠性。Abstract:
Objective Laser time fiducial system is needed in high power laser facility for tracking synchronization change in experiment caused by disturbances such as collimation and device replacement. Traditional time fiducial is an electrical trigger pulse input to the AUX channel of oscilloscope which is convenient but has obvious jitter (about 150 ps). Moreover, the electrical trigger sensitive to the electromagnetic interference. Another time fiducial form is an infrared fiducial can transfer a long distance in fiber and been send to an independent channel of oscilloscope. Infrared fiducial has a good accuracy but employ a measurement channel. In this article, we propose a new time fiducial scheme based on optic-electric and electric-optic transformation for getting a blue time fiducial signal @450 nm which can use the vacuum photodiode detecting together with UV pulse. The scheme provides a rapid fiducial signal with long transmission ability for UV pulse synchronization monitor and has a monitoring precision as high as 10 ps. Methods The study presents a fiducial system scheme based on optic-electric and electric-optic conversion. Firstly, arbitrarily waveform generator driven a M-Z modulator to modulate continuous infrared laser and infrared time fiducial signal been produced. Secondly, infrared time fiducial transferred a long distance by SM fiber and been converted to electrical signal by a photodiode. Thirdly, the electrical signal drive a direct-modulation LD and a blue time fiducial signal @450 nm is gotten. The final fiducial signal can be reach an index of 47 mW output power, 10% power jitter and 120 ps laser pulse width. Results and Discussions According to the scheme, the monitor jitter caused by fiducial power jitter, fiber length variation follow with temperature, oscilloscope noise and photodiode noise have been tested and analyzed. According to the analysis, the scheme can get a monitor accuracy of 10 ps with a 24.5-25.5 ℃ temperature variation. An experiment for examine monitor accuracy is done with 24.8-25.2 ℃ temperature variation, the monitor accuracy gotten in experiment is 7.2 ps which is coincide with the theoretical analysis result (7.03 ps). Conclusions In this study, a fiducial system scheme based on optic-electric and electric-optic conversion is proposed for high power laser facility synchronization monitor. In the scheme, infrared time fiducial signal firstly been transferred a long distance by SM fiber and been converted to electrical signal by a photodiode, after that the electrical signal drive a direct-modulation LD and a blue time fiducial signal @450 nm is gotten. The final fiducial signal can be reach an index of 47 mW output power, 10% power jitter and 120 ps laser pulse width. The experiment and theoretical analysis indicates that the method can providing a rapid fiducial signal with long transmission ability for UV pulse synchronization monitor, the monitoring precision can be as high as 10 ps. -
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