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图1为弹性加力激光器频率差漂移的测试装置,它由激光器系统、频率差数据采集系统以及稳频系统组成。
选用谐振腔长为135 cm,功率为1 mW,出光波长为0.6328 μm的双折射-塞曼氦氖双频激光器[6]作为实验测试的光源,当其平面输出镜内存在应力时,各向同性的腔镜相当于一个双折射元件,当激光穿过腔镜,沿腔内两个主应力方向偏振的光的折射率就有了微小的偏差,折射率之差如公式(1)所示:
$$\Delta {{n}} = {n_x} - {n_y} = ({c_x} - {c_y})(\sigma _x^F - \sigma _y^F)$$ (1) 式中:nx和ny为激光通过应力各向异性介质沿沿两个主轴方向偏振的折射率;cx、cy分别是沿两个主轴方向材料的光学系数;σxF、σyF分别是两个方向上的应力大小。由此带来的频差值可由公式(2)表示:
$$\Delta \upsilon = \frac{\upsilon }{L}({c_x} - {c_y})\frac{{8P}}{{\pi D}}h = \frac{\upsilon }{L}\frac{{8\lambda }}{{\pi D}}\frac{F}{{{f_0}}}$$ (2) 式中:
$\Delta \upsilon $ 为频差大小;P为施加在单位宽度上的外力;D为输出平面镜的直径;h为施加外力的宽度;F为施加外力的大小;ƒ0=λ/(cx−cy)为材料条纹值。调节腔镜应力分布即可以实现频率差的赋值。激光器管体外加有横向磁场,这样做的目的是分离o光和e光的增益原子,减小频率的模竞争[7]。
实现加力功能的弹性加力元件由使用磷青铜材料的薄片经线切割工艺制得,包含底座B、加力片SFE和悬臂CL三部分(如图2所示):底座B按照激光器管尺寸被折弯贴合在管壁上,实现了元件加力与固定的分离,互不干扰;加力片SFE为上下两个半圆环,由两对螺母和螺钉连接,通过调节螺钉的松紧程度可以改变施加在平面镜上基片上的外力,进而修正激光器的频率差;悬臂CL用于连接底座B和加力片SFE。实验测试时,采用营口高新电源研究所制造的GL-04型激光电源对激光器供电,其正常工作电流为4 mA,电压是4 V。调节磁条间距,使纵模分裂频率o、e光的塞曼增益曲线交于斜率最大处。为了减小空气扰动和机械振动对实验测试的影响,激光器需要使用灌胶工艺封装,同时也能使管体温度更稳定。
激光器输出光经分光镜分为两路,透射光用于激光器稳频。基于所用激光器为全内腔结构且横向塞曼增益线存在等增益点(如图3所示的A点和B点),选择了一种结构简单、成本较低的等光强热稳频法[8],其中
$ \pi {\text{和}}\sigma $ 曲线分别是正交偏振模式o光和e光的增益曲线,等增益点位置两模式的光强度相等。图4为稳频系统的控制回路。两分裂频率的光强信号由光电池接收并转换成电信号,再经过光电检测模块的滤波和求差放大处理得到差分信号e。其中一路信号e先通过PID调节,再和稳定频率的三角波叠加后输入比较电路的一端。另一路差值信号e经过模数转换芯片变为数字信号,通过串行外设接口传递给单片机,单片机计算后,将反馈量通过数模转换芯片转换成电压,并作为比较电路的参考电平。软硬件相互配合得到占空比与光强差相关的PWM信号,PWM信号通过开关电路控制加热丝的加热和断开,进而实现腔长的调节。当差值信号e为0或者恒定,也即两正交偏振模式的光强相等时,激光分裂频率稳定在等光强点对应的频率位置。
稳频系统通过伺服控制改善谐振腔长的漂移,进而减小腔长调谐时频率差的变化。采用热稳频的方法能将激光器温度变化控制在±0.2 ℃以内,减小热胀冷缩对人工应力的影响,提高了频率差的稳定性。
由分光镜反射的一路光用于频率差数据采集。反射光先经过632 nm的偏振片滤光,再由PIN光电二极管将光信号转化为电信号,电信号经过信号处理电路由频率计采集数据,并通过电脑上的上位机程序记录频率差。
Thermal drift of frequency difference of frequency splitting laser with force-exerting
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摘要: 频率差可调的加力型频率分裂氦氖激光器具有广阔的应用前景,目前鲜有关于该激光器频率差热漂移的具体报道,但它是一个重要的应用指标,特别是在光刻机用干涉仪中需要重点关注。采用弹性加力法对双折射-塞曼氦氖双频激光器进行频率差赋值,详细观察了频率差在开机漂变阶段、过渡阶段及稳定阶段状态的变化过程, 给出了频率差对开机时间的曲线。实验表明稳频作用下的频率差的稳定性优于23 kHz/h,重复性优于130 kHz。此外还就不同结构或材料的弹性加力元件对频率差的影响进行了对比分析,实验表明系统温度分布的均匀性和稳定性对频率差热漂移起重要作用。Abstract: The force-exerting laser with tunable frequency difference has a broad application prospect, there are few reports on the thermal drift of frequency difference of this laser, but it is an important application index, especially for the interferometer of lithography machine. The frequency difference of birefringence Zeeman dual frequency laser was assigned through elastic force method, and its states of frequency difference in the drift stage, transition stage and stable stage were observed. The experiments prove that the stability of frequency difference is better than 23 kHz/h, and its repeatability is better than 130 kHz; In addition, this paper also comparatively analyzed the influences of elastic force-exerting elements with different structures or materials on frequency difference. The experiments indicate that the uniformity and stability of the temperature distribution play an important role in the frequency difference thermal drift.
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Key words:
- elastic force-exerting /
- He-Ne laser /
- frequency difference /
- thermal drift
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