Volume 49 Issue 12
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Wang Yajun, Gao Li, Zhang Xiaoli, Zheng Yaohui. Recent development of low noise laser for precision measurement (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201073. doi: 10.3788/IRLA20201073
Citation: Wang Yajun, Gao Li, Zhang Xiaoli, Zheng Yaohui. Recent development of low noise laser for precision measurement (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201073. doi: 10.3788/IRLA20201073

Recent development of low noise laser for precision measurement (Invited)

doi: 10.3788/IRLA20201073
  • Received Date: 2020-10-23
  • Rev Recd Date: 2020-11-24
  • Available Online: 2021-01-14
  • Publish Date: 2020-12-25
  • 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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Recent development of low noise laser for precision measurement (Invited)

doi: 10.3788/IRLA20201073
  • 1. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan, 030006, China
  • 2. Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, 030006, China

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.

    • 全固态激光器以其高稳定性、长寿命、波长范围宽等优势,在工业制造、国防技术、生物医学、光谱测量、环境监测、精密测量和量子光学等领域具有重要的应用前景。近几年,全固态激光器及相关技术进入了快速发展期,尤其在精密测量的科学研究中,全固态单频(单纵模)激光器更是以窄线宽、低噪声的优良特性大幅提升了微弱信号测量的精度;例如,在地面引力波探测中,基于大型激光引力波干涉仪测量装置,美国国家激光干涉引力波天文台(LIGO)将微小位移量探测灵敏度提升至10−21 m,并于2015年9月圆满完成了人类首次引力波观测任务[1],打开了人类观测宇宙的新窗口。当前引力波探测频段处于10 Hz~10 kHz范围,在这一频段要继续提升测量灵敏度,实现每日观测任务,将面临诸多技术挑战,其中一项关键技术是实现高功率、高稳定、低噪声激光输出[2-3]


      在高精度精密测量中,光的量子特性尤为突出,对物理量的测量表现出统计分布的特性,比如,对电磁场振幅多次测量,会得到不同的结果,测量结果满足泊松分布。同时测量光场的两个非对易可观察量(如振幅与位相)时,满足海森堡不确定原理($ \Delta {X}_{1}^{2}\Delta {X}_{2}^{2}\geqslant 1 {\text{,}} \Delta {X}_{1}^{2} $正交振幅噪声方差,$ \Delta {X}_{2}^{2} $正交位相噪声方差,真空噪声方差为1),即两个可观测量不可能达到任意测量精度,这种不确定性限制了干涉仪的测量精度。理想的激光场等价于一个相干态,其正交分量(振幅与位相)的量子噪声方差为1,即标准量子噪声极限(Standard Quantum Limit,SQL)或散粒噪声极限(Shot Noise Limit,SNL)。对于自由运转的激光器,其噪声水平一般远高于SQL,尤其在音频段,多种环境噪声和激光产生过程中引入的噪声将湮没被测信号,从而限制了激光器在低频精密测量中的应用。海森堡不确定原理表明,通过主动反馈与被动滤波噪声抑制技术,强度噪声也只能接近或达到SQL,最终测量灵敏度将受限于SQL。1981年,C. M. Caves提出压缩光填补真空通道有望实现突破激光干涉仪SQL的更高精度测量[23],从此高压缩度压缩态光场的制备技术成为精密测量领域的一个重要研究方向。2018年,德国H.Vahlbruch等人利用量子压缩光(106 μW)与探测光耦合作为内环传感光束,通过光电负反馈控制环路加载到激光器驱动电流端,有效抑制了100 kHz频带范围内的强度噪声,相比自由运转激光强度噪声降低了9.4 dB,等效于传统探测76 mW的散粒噪声水平[24]。2019年10月,LIGO历时一个月将−7.2 dB@50 Hz压缩真空注入Advanced LIGO,实现中子星探测灵敏度提升14% (3 dB),探测速率提升40% (Hanford)/50% (Livingston),等价于提高注入激光功率65%/85%[25];同期,VIRGO采用10 dB@50 Hz压缩真空注入,探测灵敏度提高5%~8%,探测速率提高16%~26%[26]。可见,压缩光注入可有效解决低功率下强度噪声抑制的问题。


    • 全固态激光器(Diode Pumped All-Solid-State Laser,DPSSL)是指以固体材料为增益介质,半导体激光器作为泵浦源,实现结构紧凑、稳定可靠、较低噪声激光输出的一类激光器。当前发展最为成熟的低噪声全固态激光器包括NPRO激光器、薄片式激光器、块状晶体激光器、光泵半导体激光器、光纤激光器等。文中主要基于块状晶体单频激光器研究基础[27-32],回顾其强度噪声特性及其噪声抑制技术的热点研究方向。

      将激光场强度噪声归一化至SQL后,理想的光场噪声为1。然而,激光泵浦过程存在多种强度噪声源,同时弛豫振荡峰(Resonant Relaxation Oscillation, RRO)附近形成的大噪声尖峰[33-34],即${\omega _{RRO}} = \sqrt {\dfrac{1}{{\tau {t_c}}}} \sqrt {\dfrac{{{P_{pump}}}}{{{P_{th}}}} - 1} $ (激光能级自发辐射寿命$\tau $,腔内光子寿命${t_c} = \dfrac{L}{{c\delta }}$L为腔长,c为真空中光速,δ为内腔损耗),${P_{pump}}$为泵浦功率,${P_{th}}$为泵浦阈值功率。)[21, 35],均会伴随激光辐射过程耦合至输出场,导致输出光场噪声远高于SQL。除弛豫振荡噪声外,输出耦合镜引入的真空噪声、泵浦源的强度噪声、自发辐射噪声、偶极起伏噪声和内腔损耗引入的噪声成为强度噪声的主要来源,如图1所示,噪声耦合导致在一定的频谱范围内强度噪声远高于SQL,并且表现出频率f依赖的噪声特性,在不同的分析频段,噪声源贡献不同。以参考文献[33]分析的NPRO激光器强度噪声为例,如图2所示,分析频率f > 100 MHz时,真空噪声是激光强度噪声的唯一来源;分析频率RRO < f < 100 MHz、5 kHz < f < RRO,噪声谱中以真空、偶极起伏和内腔损耗引入的噪声为主,自发辐射噪声和泵浦噪声则无影响;分析频率f < 5 kHz,泵浦源的量子噪声则成为噪声的主要来源,此时,如果泵浦强度噪声突破SQL,则输出激光强度噪声将突破SQL。一般情况下,自发辐射噪声对噪声谱的贡献可以忽略,除非通过强度压缩光将光场噪声降至接近自发辐射引起的噪声水平时,需要考虑它对整个光场噪声的影响。通常,对于光场高频噪声可以通过一个窄线宽的光学滤波腔(等效为光学低通滤波器)有效抑制超出线宽的高频噪声,使其达到SQL。对于光场的低频噪声,当前发展最为成熟的技术是电光负反馈控制,通过操控泵浦噪声(主要的噪声源)或输出光场噪声,实现控制带宽范围内光场噪声的有效抑制[36]

      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

      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

    • 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:




      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。

    • 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。

      Figure 3.  Schematic diagram of noise suppression with optical resonator as a low pass filter[36]

    • 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。

      Figure 4.  Traditional intensity noise suppression technology (a) and AC coupled intensity noise suppression technology (b)[36]

      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。

      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]

      Figure 5.  Development status of conventional noise suppression (triangle, diamond, dot) and AC coupled noise suppression (square) schemes for laser relative intensity noise


      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]

      Figure 6.  A negative feedback intensity noise suppression scheme for injected squeezed light(a) and intensity noise measurement results(b)[24]

    • 由上所述,强度噪声抑制水平,受不确定原理决定所能达到的极限只能是SQL,即激光干涉仪的探测灵敏度将受限于SQL,利用经典降噪技术将无法突破这个限制。1981年,C. M. Caves提出压缩光填补真空通道有望实现突破激光干涉仪SQL的更高精度测量[23],从此量子压缩降噪技术并被应用于强度噪声的进一步抑制,实现突破SQL的超低噪声抑制。压缩态基本物理图像如图7所示,在满足不确定原理的前提下,相干态两正交分量的噪声方差为$ \Delta {X}_{1}^{2}= \Delta {X}_{2}^{2}= 1 $图6(a))。通过原子四波混频、光波导、光学参量过程和光机械等非线性耦合技术,操控泵浦光位相为π (对应于振幅),可将振幅噪声突破SQL限制(图7(b)),完成压缩态光场的制备,$ \Delta {X}_{1}^{2}<1 $。此时由于不确定性原理的限制,位相分量噪声则被放大,$ \Delta {X}_{2}^{2}>1 $。因此,压缩光突破SQL的特性,可用来完成突破SQL的高灵敏度噪声抑制。目前,以上四种压缩光制备方案中,最为有效的一种是利用低于阈值的光学参量振荡器制备压缩,下面将重点讲述基于该技术方案的压缩光制备技术的研究进展。

      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

      典型的光学参量过程制备压缩光的装置如图8(a)所示,激光器输出基频光分为两束,一束注入倍频腔(Second Harmonic Generator,SHG)产生二次谐波输出,用于泵浦光学参量放大器(Optical Parametric Amplifier, OPA),少部分作为种子光(seed beam)注入OPA;一束作为本地光(Local Oscillator,LO)注入平衡零拍探测,用来测量压缩噪声,图8(b)和(c)为典型的平衡零拍探测测量到的压缩光噪声谱,其中所有噪声均归一化至SQL (纵坐标0点)。如图9所示,压缩态制备过程通常是通过泵浦光与种子光相对位相实现对振幅或位相分量的噪声操控,π位相对应于振幅分量(图中压缩分量),0位相对应于位相分量(图中反压缩分量)[51];额外的光学损耗会引入真空噪声,位相的起伏会导致反压缩噪声耦合至压缩噪声分量,从而降低实测压缩度[52]。因此,高压缩度压缩态的制备难点在于克服光学损耗和位相起伏对压缩度的影响。下面主要回顾压缩光在解决噪声耦合问题所取得的成果和发展现状,如图10所示。

      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)

      Figure 9.  Relationship between phase and coherent state (a) amplitude squeezed state (b) influence of optical loss and phase fluctuation (c) on measured squeezing[51-52]

      Figure 10.  Research status of squeezed state light field at home and abroad

      图10(a)所示,在国际上,1986年,Texas大学Kimble研究组首次利用运转于阈值以下光学参量下转换过程,使输出场噪声功率相对于真空涨落降低超过50% (−3 dB)[53]。1986年,日本电报电话公司的Y. Yamamoto研究组利用负反馈控制半导体激光器的泵浦电流,使输出光电流起伏谱低于标准量子极限以下−7 dB[54];1991年,该组采用激光器与探测器直接耦合,在低温(66 K)条件下,将LD强度噪声压缩到散粒噪声极限之下−8.3 dB (85%)。1987年,法国国家科学研究中心E.Giacobino和C.Fable研究组,使用氩离子激光作为泵浦光,通过非简并参量下转换获得信号光、闲置光之间强度差噪声相对于SQL下降30%(−1.5 dB)[55]。1989年,美国加州理工学院Kimble研究组实现了 −3.6 dB压缩,进而完成了“EPR”佯谬实验研究,证明了两个光场的量子位相相关性[56]。1998年,德国康斯坦茨大学采用了Nd:YAG激光器泵浦的半整块简并MgO:LiNbO3光学参量放大器,在380 mW的泵浦功率下,制备得到−6.2 dB的1064 nm压缩[57]。2004年,澳大利亚国立大学Kirk McKenzie等人采用低于阈值的光参量振荡(OPO),获得了280 Hz到100 kHz宽带连续波压缩真空场,证明了宽带音频压缩真空场制备的可行性,在500 Hz处观察到−2 dB的真空噪声压缩。压缩真空的相位采用量子噪声锁定技术进行控制,为第一代引力波探测器测量频段噪声的抑制提供了可行方案[58]。2006年,德国汉诺威大学的H. Vahlbruch等人提出相干控制位相锁定方案,实现了对压缩真空场位相的稳定锁定,在10Hz到10kHz频率边带范围内实现了稳定的压缩真空场输出,通过两束与载波存在一定频率偏移的辅助光实现了压缩系统的长时稳定锁定,并在地面基引力波干涉仪的整个探测波段上实现了−4 dB的噪声压缩[59]。2007年,德国汉诺威大学的H. Vahlbruch等人在1 Hz到3.2 kHz范围内,直接探测到压缩真空噪声低于量子噪声极限6.5 dB[60]。2007年,日本东京大学Y. Takeno等人采用低于阈值的OPO("8"字环形结构),通过优化反馈控制回路及误差信号的信噪比(SNR)大幅抑制位相起伏引入的噪声;进一步减少腔内损耗,直接制备得到−9 dB的860 nm压缩光[61]。2008年,德国汉诺威大学的H. Vahlbruch等人采用整块非平面Nd:YAG环形腔激光源,通过整块7%掺杂的MgO:LiNbO3制成的I类简并光学参量振荡器(OPO),首次在实验上实现了−10 dB@1064 nm的压缩态光场输出[62]。2010年,该小组又将压缩度提高到了−12.7 dB[63],并注入Sagnac干涉仪,实现了突破SQL噪声极限的精密测量。同年,德国汉诺威大学的H. Vahlbruch等人通过将相干控制技术、平衡零拍探测技术和压缩真空技术集成于1.5平方米的光学面包板上,在10 Hz到10 kHz的探测波段内,完成了稳定输出的−9 dB的压缩真空态光场[64]。2011年,LIGO提出将−10 dB压缩真空态注入引力波激光干涉仪,可在10 Hz到几kHz频带内实现6 dB的噪声抑制[65]。2012年,澳大利亚国立大学采用双共振行波光学参量振荡器,通过设计高共模抑制比音频平衡零拍探测测量得到−11.6 dB的低频压缩,在探测频率10 Hz处压缩度仍可达到−10 dB,为地面引力波探测压缩光注入方案提供了重要的光源基础[66]。2013年,德国马普所的Tobias等人在1550 nm波段通过设计双模压缩真空态位相控制方案,实验制备出−10 dB的双模压缩真空态光场[67]。2016年,德国马普所采用整块非平面Nd:YAG环形激光器(1064 nm)泵浦双共振OPO,通过进一步降低光学损耗和位相起伏,首次将压缩真空态的压缩度提高到−15 dB@1064 nm[68]

      图10(b)所示,在国内,1998年,山西大学利用$ \alpha $切割的KTP晶体获得了−3.7 dB的正交位相压缩和−7 dB的强度差压缩[69]。1999年,山西大学利用频率锁定且偏振方向与KTP晶体两本征偏振方向夹角为45°的种子波,注入运转于阈值以下的非简并光学参量放大(NOPA),并将腔锁定在种子波频率上,得到了具有严格频率简并、偏振正交明亮的孪生光束,其耦合模为明亮正交位相压缩态光场,压缩度为−3.7 dB[70]。2010年,山西大学通过在光路中插入模式清洁器和改进锁定技术提高了系统稳定性,制备得到了−6 dB的双模压缩态[71]。2017年,山西大学采用单共振OPA腔,设计了一种不同锁定环路之间相位抖动相互补偿的方法,通过一个电光相位调制器为三个锁定环路提供锁定所用边带,最终获得在分析频率3 MHz处直接探测到了−12.6 dB的正交振幅明亮压缩和21.4 dB的反压缩[72]。2017年,山西大学在原有平衡零拍探测器的基础上减小跨阻,改进电路不再将交直流分开,使得探测器在kHz频段有较大的增益,经过改进平衡零拍探测器在1.4 mW的本底光注入时散粒噪声基准可以较电子学噪声水平抬高20 dB以上,最终在分析频率15.2 kHz处直接探测到了−9.9 dB音频真空压缩态[73]。2018年,山西大学在分析频率3 MHz处直接测量得到了−13.2 dB的真空压缩态(在泵浦功率为180 mW),校正电子噪声的影响后,最佳压缩因子达到−13.35 dB[74]。通过分析绿光诱导的红外吸收(GLIIRA)损耗,可以在整个泵浦功率范围内完美地拟合压缩因子。2019年,山西大学报道了压缩态产生过程中由腔失谐和相位不匹配引起的相移的补偿特性,实验中,通过微调晶体温度偏离共振,在5 MHz处正交振幅(或真空)噪声降低−12.3 dB[75];同年,通过使用辅助激光束,干涉效率提高到99.8%,通过在平衡零拍探测器中集成结晶型场效应晶体管(JFET)缓冲和JFET自举结构,将电子噪声的损耗降低至0.05%,直接探测到−13.8 dB压缩真空态[76]。2020年,山西大学光电研究所利用双色本振设计方案,解调光学参量放大器输出的多个高阶边模,滤除不需要的边模,在9 kHz的分析频率下测量到量子噪声方差突破SQL −10.2 dB[77];同年,提出了一种利用单个压缩光源实现类光梳分频技术,第一次实验完成了四组纠缠态光场的提取,四组边模关联噪声方差均突破SQL −8dB,并将之应用于多路密集编码量子通讯,实现了相同光子数下,当前国际上最高的光通讯信道容量[78]

    • 激光器强度噪声是限制其在精密测量领域应用的一个重要参数,主要来源于激光辐射过程中的弛豫振荡噪声、输出耦合镜引入的真空噪声、泵浦源的强度噪声、自发辐射噪声、偶极起伏噪声和内腔损耗引入的噪声,多种噪声呈现出频率依赖的功率噪声谱,在不同频段各种噪声源所占比重有所差别。在基于激光干涉仪的精密测量中,Hz~kHz频段是主要的测量频段,噪声主要来源于泵浦强度噪声,通常通过操控泵浦电流或外调制输出光场振幅实现控制带宽范围内光场噪声的有效抑制,或采用量子压缩光技术进一步突破散粒噪声基准的限制,实现突破噪声极限的更低噪声抑制。经过40多年的发展,低频降噪目前已形成了传统直流负反馈控制、高精细度光学腔反射端作为传感光束的AC耦合两种比较成熟的方案,通过优化内环控制环路的技术噪声和反馈增益系数,Hz~kHz频段已接近或达到散粒噪声基准,其相对强度噪声已达到10−9 Hz−1/2量级(达到散粒噪声基准),成为当前引力波探测仪抑噪的主要技术手段。在此基础上,利用量子压缩光技术可进一步突破这个噪声限值,以当前最高压缩度−15 dB@5 MHz为例,通过在反馈控制环路中引入压缩光,相对强度噪声可进一步降低至2.3×10−10 Hz−1/2。在精密测量研究中,经典降噪与量子降噪技术相结合的解决办法是提升激光精密测量精度的必然趋势。

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