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激光的强度噪声是指激光器输出功率的随机起伏,区别于激光器的功率稳定性(随时间变化的功率起伏),强度噪声通常在频域上以频谱噪声的形式描述。在实验室条件下,为方便噪声分析,通常采用相对强度噪声(Relative Intensity Noise, RIN)描述激光强度噪声的大小,定义为:
$$ RIN=\frac{\mathrm{\Delta }P}{P}\left({\mathrm{H}\mathrm{z}}^{-1/2}\right) $$ (1) 其中,ΔP为单位频带内的噪声功率谱密度,P为激光器的平均功率。为了比较相对强度噪声的大小,定义理想激光场的相对强度噪声为:
$$ SQL = \sqrt {\frac{{2hc}}{{P\lambda }}} $$ (2) 其中,h为普朗克常数,c为真空中的光速,λ为激光波长,其结果对应于特定功率下的SQL,单位为Hz−1/2。实验中,对光场强度噪声进行测量,通常将待测光束直接注入高增益、低噪声光电探测器,将光信号转换为电信号,在傅里叶频谱分析仪上对其交流成份进行噪声分析,其中探测器输出电流信号表示为:
$$ {i}_{c}=P\frac{\text{η}e\text{λ}}{hc} $$ (3) 其中,e为电子的电荷量,η为光电二极管的量子效率。频谱分析仪测量到的交流信号对应的单位频带内的功率谱密度除以直流信号功率即为光场的相对强度噪声。
当前高精度计量学研究要求激光器具有极高的功率稳定性,例如,最先进引力波探测仪10 Hz分析频率处的强度噪声需达到2×10−9 Hz−1/2。经过几十年的发展,已形成多种不同的方法可以用来稳定激光功率噪声,其中技术最为成熟的是被动滤波和主动反馈控制降噪[36]。在实际应用中,由于激光器噪声表现出频率依赖的噪声特性,因而为了选择合适的降噪方法,首先需要对自由运转激光器的噪声进行频谱分析,针对频谱噪声的不同来源采取不同的抑噪方法达到预期的抑噪目标。
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在被动滤波降噪中,通常利用光学器件的滤波特性来减少激光束参数的波动。例如,通过光学谐振腔透射的激光束可以降低超过谐振腔带宽的频率波动,如图3所示;或者通过双折射晶体(偏振器)传输的激光束可以抑制偏振波动。大多数光学器件降噪受限于器件的频率特性,只能满足器件特定参数范围内的噪声抑制,抑噪水平有限,并且环境中的额外噪声也会通过器件耦合到光束中,从而限制了被动滤波降噪技术的应用范围。
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主动反馈控制降噪方案如图4所示,通过传感器(探测器)和执行器(AM或AOM)结合反馈控制环路抑制激光功率波动。其噪声抑制能力主要取决于两个因素:一是反馈控制环路的环路增益,二是传感器的灵敏度,一般而言,对于自由运转的光场噪声最终的噪声抑制水平是由控制环路增益决定。环路增益通常取决于控制环路的带宽,而环路带宽则受限于传感器或执行器的带宽。通常控制环路的带宽越大,在特定频率处环路的增益就越高。当环路增益足够高时,锁定控制环路剩余噪声的下限即为传感器的固有噪声。因此,在高环路增益的傅里叶频率处,主动反馈控制最终受限于传感器,此时,需要设计更低噪声的传感器实现进一步噪声抑制。除了控制环路带宽外,执行器的动态范围是补偿功率波动的另一个重要参数。因此,需依据自由运转激光器光场波动的峰-峰值选择合适动态范围的执行器。一般执行器不可能同时具备大的动态范围和大的带宽,通常将多个执行器组合使用:选取大带宽、小动态范围的执行器来增加控制环路带宽;通过慢速、大动态范围执行器补偿低频处功率的波动。最终,传感器的固有噪声成为反馈控制噪声抑制的下限;自由运转噪声特性决定了所需执行器的动态范围和控制环路的带宽。
通常反馈控制环路采用负反馈的方式实现噪声补偿,在控制环路中至少包含调制器、低噪声光电探测器和大带宽PID控制器。由上所述,这些光学和电子器件的固有噪声将决定反馈控制环路的抑噪水平。在前期研究中,重点考虑了位相调制器(用于光场位相调试,提取光学腔腔长锁定的误差信号)的剩余振幅调制,它是锁定环路误差信号零基线漂移的主要影响因素。实验中,笔者在调制晶体后端面切割一楔形角[37],当光束通过晶体后,由于晶体内部的双折射效应,剩余的非调制偏振光将偏离主光路,从而无法在下游偏振元件上与调制光场干涉,大幅降低了剩余振幅调制,误差信号零基线漂移由传统调制器的+3500/−3400 ppm/10 h降低至+70/−50 ppm/10 h,保证了位相锁定与PDH稳频锁定环路的长期稳定性,避免额外噪声的引入;同时在调制器中引入共振结构电路,放大输入信号,降低输入电压,解决半波电压较高的问题。在采用PDH稳频法锁定光学腔腔长时,为满足弱的光场信号的提取(对于阻抗匹配腔,反射光场信号较弱,反射端提取误差信号需要高增益探测器),设计了LC共振电路,并结合变压器输入耦合方式提升探测器的品质因子(Q),实现Q因子大于100的高增益低噪声探测器[38-39],与共振型电光位相调制器配合使用,有效抑制了调制边带之外的频率噪声,实现了弱功率(90 μW)的稳定反馈控制。同时,内环探测器的电子学噪声决定了反馈控制环路噪声抑制的能力,为了满足光场噪声的有效探测和反馈环路对低噪声探测器的需求,通过自举跨阻电路、结合双管平衡零拍探测自减探测方案,设计了低噪声、高增益和高共模抑制比(75.2 dB)的平衡零拍探测器,可有效抑制探测端技术噪声对反馈控制抑噪的限制,51 μW时探测到的散粒噪声仍高于电子学噪声13 dB@2 MHz[40-41]。下面主要回顾图4所述两种降噪效果最为显著的负反馈降噪技术的发展现状,不涉及具体控制环路中单个器件的介绍。
第一类是传统功率降噪(DC耦合),实验装置如图4(a)所示,光电探测器直接探测的光电直流信号与标准参考信号比较或探测器交流信号直接反馈至上游光路中的AM或AOM,构成内环反馈控制环路,抑噪后的信号通过AM或AOM调制光场幅度,实现外环光场噪声抑制。然而,光场散粒噪声反比于注入探测器功率的二次方根。为了获得满足引力波干涉仪2×10−9 Hz−1/2超低噪声水平的探测,光电管的输出电流至少需达到200 mA,对应注入功率约为390 mW。这种大电流光电探测器在技术上存在巨大挑战——光电二极管面临严重的热效应,是制约传统功率降噪的技术瓶颈之一。
如图5蓝色、红色和绿色图标所示,2004年,美国Jameson Rollins等人通过将一个大功率(250 mW,光电流142.5 mA,SQL 2.2×10−9)、低噪声的光电探测器结合电流分流驱动器应用于交流耦合强度噪声反馈控制技术[42],10 W Nd:YAG激光器10 Hz处的相对强度噪声降至1×10−8 Hz−1/2,100 Hz处噪声低至5×10−9 Hz−1/2。2006年,德国Frank Seifert等人通过在DC耦合控制环路中使用高功率、低噪声的单管光电探测器,在10 Hz处相对强度噪声达到5×10−9 Hz−1/2,在数kHz处达到3.5×10−9 Hz−1/2,已接近80 mA光电流的SQL[43]。2009年,德国Patrick Kwee等人通过采用低噪声多个光电二极管阵列串接的探测技术,减小单个光电管承受的光电流,分散光电流产生的热量,完成了低噪声、大电流探测器的设计(光电流189 mA,SQL 1.8×10−9),对连续波Nd: YAG激光器1 Hz到1 kHz的频带的强度噪声进行了有效抑制,10Hz处的相对强度噪声首次低至2.4×10−9 Hz−1/2,满足了先进LIGO引力波探测器对功率稳定性要求[44];2017年,德国Jonas Junker等人采用同样的技术(光电流128.8 mA,SQL 2.29×10−9)在100 Hz到1 kHz频带范围内实现2.6×10−9 Hz−1/2噪声抑制[45]。
图 5 激光器相对强度噪声传统抑噪(三角形、菱形、圆点)与AC耦合抑噪(方形)方案发展现状
Figure 5. Development status of conventional noise suppression (triangle, diamond, dot) and AC coupled noise suppression (square) schemes for laser relative intensity noise
第二类是AC耦合噪声抑制技术,实验装置如图4(b)所示,激光器输出激光经过一个高精细度的阻抗匹配光学谐振腔,反射光由反馈控制环路内环探测器转换为电信号直接反馈至上游光路中的AM或AOM,构成内环反馈控制环路,抑噪后的信号通过AM或AOM调制光场幅度,实现外环光场噪声抑制。如图3所示,其中光学谐振腔对透射光等效于强度噪声的光学低通滤波器,对反射光等效于高通滤波器。针对低频强度噪声采用腔的反射光反馈抑噪,实验要求光学谐振腔为阻抗匹配腔,反射光功率较弱(理想条件下功率为零),但因为低频噪声已传递至透射场,内环探测器探测到的弱功率噪声水平与传统降噪大功率探测器相当,该方法在弱功率条件下具有更高灵敏度的功率波动检测,是解决传统功率降噪方案热噪声问题的主要替代方案。
如图5 AC-coupling所示,2008年,德国Patrick Kwee等人提出了一种基于阻抗匹配光学谐振腔反射光探测的高灵敏度光功率特性检测方法,由谐振腔反射端提取到仅仅3 mA的光电流即完成了7.2×10−10 Hz−1/2相对强度噪声抑制(SQL 7×10−10),等价于传统探测器702 mA的抑噪效果[46]。2011年,德国Patrick Kwee等人同样利用光学AC耦合技术,在反射光束中额外插入一个模式清洁器,构成光学带通滤波反馈控制传感器,探测光电流为23 mA时,首次实现了1.1×10−10 Hz−1/2的相对强度噪声(SQL 1.1×10−10 Hz−1/2),等效于光电流为32 A (探测功率为67 W)传统抑噪技术[47]。2019年,德国Steffen Kaufer等人使用一个线宽为4 kHz的光学谐振腔,首次分析了阻抗匹配与光学腔传输函数增益的关系,并通过电动控制腔内安置的光阑孔径,精细调节阻抗匹配条件,为反馈控制环路提供了足够的增益,在4~50 kHz频率范围内、光电流50 mA时,噪声水平达到7×10−9 Hz−1/2[48]。
实际应用中,除了采用光电二极管阵列代替单个光电管的传统方案、高精细度光学腔反射端作为传感光束的AC耦合方案外,2018年,德国H. Vahlbruch等人利用量子压缩光(106 μW)与探测光耦合抑制内环传感光束的强度噪声,如图6所示,在5~80 kHz频带范围内,外环光束相比自由运转激光噪声降低9.4 dB;预期采用压缩注入方案,15 dB压缩光注入可达到2.3×10−10 Hz−1/2的噪声水平,可等效于传统探测6.4 W的抑噪水平[24]。此外,还包括非破坏测量方案,将外环光束功率噪声传递至另外一束具有更低噪声、高灵敏的待测光束——内环光场信号,然后反馈回外环光场实现噪声的抑制,这种方案原理上可实现突破散粒噪声极限的噪声抑制[3]。
Recent development of low noise laser for precision measurement (Invited)
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摘要: 激光精密测量的测量精度主要受限于光场噪声和各种技术噪声,在去耦合技术噪声后,光场量子噪声成为限制其测量精度的主要因素。文中针对全固态单频激光器强度噪声特性,阐述强度噪声的主要来源及其对功率噪声谱的影响,回顾了传统直流反馈控制、光学交流耦合反馈控制和量子压缩器三种强度噪声抑制技术。通过回顾相关技术的发展历程,总结了强度噪声抑制技术的当前发展水平和未来发展趋势——三种技术相结合的抑噪方案是解决高灵敏度探测的重要途径。Abstract: The measurement accuracy of laser precision measurement is mainly limited by optical field noise and various technical noises. After the de-coupling technical noises, quantum noise becomes the main factor limiting the measurement accuracy. Based on the intensity noise characteristics of solid-state single-frequency lasers, the main sources of intensity noise and their influence on the power noise spectrum were described, and three kinds of intensity noise suppression techniques, including traditional DC feedback control, optical AC coupled feedback control and quantum squeezer, were reviewed in this paper. By reviewing the development history of relevant technologies, the current development level and future development trend of intensity noise suppression technology were summarized-the noise suppression scheme combining three technologies is an important approach to solve high sensitivity detection.
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Key words:
- single frequency laser /
- intensity noise /
- precision measurement /
- noise suppression
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图 1 自由运转激光器泵浦过程中各种噪声源耦合原理图[33]。其中,G为受激辐射系数,Γ为泵浦速率,γ、γt为自发辐射速率,Vf为激光辐射场,Vvac为由输出端耦合镜引入的真空噪声,Vdipole为偶极起伏噪声,Vspont自发辐射噪声,Vloss内腔损耗引入噪声,Vp泵浦源引入噪声
Figure 1. Coupling schematic diagram of various noise sources during free-running laser pumping[33]. G is stimulated-emission coefficient; Γ is pump rate; γ and γt are the spontaneous-emission rates; Vf is field emitted from the laser; Vvac is noise from vacuum fluctuations entering the laser’s output; Vdipole is noise from dipole fluctuations; Vspont is noise from spontaneous emission; Vloss is noise from intracavity losses; Vp is noise entering the laser from its pump source
图 2 激光辐射过程中多种噪声源的频率依赖噪声谱[33]。(a)考虑全部噪声源;(b)真空噪声的贡献;(c)泵浦噪声的贡献;(d)自发辐射噪声的贡献;(e)偶极起伏噪声的贡献;(f)内腔损耗的贡献
Figure 2. Frequency dependent noise spectrum of the various noise sources during laser radiation[33]. (a) Noise of the laser with all contributions added; (b) Contribution from vacuum noise; (c) Contribution from pump-source noise; (d) Contribution from spontaneous-emission noise; (e) Contribution from dipole fluctuation noise; (f) Contribution from intracavity losses
图 7 压缩态光场噪声抑制基本原理及物理图像[49-50]。(a)理想相干态噪声分布;(b)振幅压缩态噪声分布,其中X1为振幅分量,X2为位相分量
Figure 7. Basic principle and physical image of noise suppression in squeezed state light field[49-50]. (a) Noise distribution in ideal coherent state;(b) Amplitude squeezed state noise distribution, where X1 is the amplitude component and X2 is the phase component
图 8 12 dB压缩态光场制备实验装置(a)与噪声功率谱((b)是随泵浦功率的变化趋势,(c)是随测量分析频率的变化趋势)测量结果
Figure 8. Experimental apparatus for 12 dB squeezed light field generation (a) and measurement results of noise power spectrum ((b) shows the change trend with the pump power and (c) shows the change trend with the analysis frequency of the measurement)
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