单频激光宽频段频率和强度噪声测量技术 
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Objective In recent years, single-frequency lasers have been widely used in the fields of fiber communication, lidar, and fiber sensors due to their narrow linewidth and good stability. However, in high-precision applications, such as precise spectral measurement, gravitational-wave detection, and high-precision frequency transfer, noise characteristics are also important parameters and worth further optimization. High-precision measurement of single-frequency laser noise is the basis of laser noise analysis. So far, the current noise measurement method usually evaluates noise performance in a specific frequency band and hence it becomes difficult to cover the whole mHz to MHz frequency range. To realize broadband noise measurement, it is necessary to adopt segmented measurement schemes and use spectrum splicing techniques. However, there is still a lack of standard measurement schemes for obtaining the broadband noise spectrum. In this paper, a standard measurement technique for mHz to MHz broadband laser noise is reported.
Methods Among the various methods of frequency noise measurement, the optical heterodyne beat-frequency method is suitable for measuring low frequency noise, but the maximum measurement range is limited, and the correlation delay self-heterodyne measurement technique based on a fiber-type interferometer is quite suitable for high-frequency noise measurements above 1Hz. Therefore, a correlation delay self-heterodyne measurement system based on a fiber-type Michelson interferometer [Fig. 1(a)] is designed and developed for measuring high-frequency noise by suppressing environmental noise through passive control techniques such as sound insulation, vibration isolation, and temperature control, where the 10Hz--1MHz spectral range can be successfully measured. The optical heterodyne beat-frequency method [Fig. 1(b)] is used to measure low frequency noise by the beating of two self-developed DBR fiber lasers with similar performance, where a reference laser with calibrated frequency noise can be obtained. The laser to be tested is beating against the reference laser to achieve frequency noise measurements in the low frequency range of 1 mHz--100Hz. By using the direct average method, the spectra measured by these two methods can be spliced together smoothly, and the frequency noise measurement in the frequency range of 1 mHz--1 MHz can be ultimately realized.
For measuring relative intensity noise, the direct measurement method based on the electric spectrum analyzer is used in the high-frequency range, and the digital measurement method based on the digital multimeter and FFT analyzer is used for frequencies lower than 10kHz. Combining these two noise measurement techniques and using the gradual in and out spectrum splicing method, the relative intensity noise spectrum in the frequency range of 1 mHz--50MHz is obtained.
Results and Discussions Using the measurement schemes mentioned above, we evaluate the noise spectral characteristics of a single-frequency DBR fiber laser from NP photonics. We measure the frequency noise spectra with the developed correlation delay self-heterodyne method (
Conclusions A standard technique for measuring single-frequency laser frequency and intensity noise in an ultra-wide Fourier-frequency range from mHz to MHz is presented in this paper. We successfully measure both the frequency noise and intensity noise spectral characteristics of single-frequency lasers in a Fourier-frequency range of mHz to MHz, using two established measurement systems in conjunction, namely, a correlation delay self-heterodyne frequency noise measurement system based on a fiber-type Michelson interferometer and an optical heterodyne beat-note measurement system with calibration function, combined with common analysis instrument. The accuracy of our measurement results has been verified. This presented method may be used for laser noise evaluation in various applications such as gravitational-wave detection and precision measurement.
1 引言
随着单频激光在精密测量[1]、相干通信[2]、激光雷达[3]和光学传感[4]等领域越来越广泛的应用,人们逐渐开始重视其噪声性能。在高灵敏光学测量中,需要激光源具有极低的强度噪声,以获取高信噪比[5];在相干精密测量中,激光频率噪声则直接决定了探测灵敏度[6]。精确测量与评估激光噪声性能是噪声抑制的基础和前提,迄今,人们已发展出了多种噪声测量方案[7-14]。对于强度噪声测量,通过采用低噪声光电探测器、由电学频谱分析仪(ESA)直接分析探测到的电信号[7-8]、由快速傅里叶变换(FFT)分析仪[9]或数字万用表(DMM)记录电信号的时间变化[10]等方法,可获得相对强度噪声(RIN)的功率谱。对于频率噪声测量,一般通过构建的高稳定光学干涉仪,记录外差干涉信号的相位随时间的波动,再由干涉仪测量系统的相位传递函数,反推出待测激光的相位或频率噪声[11]。为精确测量激光的相位波动,人们一直在着力提升干涉仪的稳定度,还发展出了多种相位解调技术,以抑制环境扰动对干涉仪测量稳定性的影响。例如,在相位载波解调[12]技术中,通过主动反馈控制压电陶瓷(PZT),使干涉仪工作在正交状态,从而补偿环境扰动对干涉仪臂长差的影响;也可将干涉仪中分光合束器件改成3×3耦合器[13-14],其三路耦合输出端口之间的相位差稳定在120°的特点,使得无需利用主动反馈控制就可滤除环境扰动对干涉仪臂长差的影响;还可采用所谓的相关延时自外差技术[15],通过在干涉仪中引入声光调制器(AOM),将干涉信号的频率移动至射频(RF)波段,以抑制低频环境扰动对干涉仪稳定性的影响。但是,在Hz以下的分析频率段,频率噪声测量要求干涉仪臂长差长达数千米。而随着干涉仪臂长的增加,环境噪声对干涉仪稳定性的影响将难以被有效消除,基于干涉仪的频率噪声测量方法只能评估出Hz以上频段的噪声水平[16]。随着引力波探测和新型精密测量应用需求的发展[17],需要精确评估激光源低至mHz频段的频率噪声行为[18],为此,人们将传统外差拍频方法应用到低频段激光频率噪声测量中[15],但是,该方法要求被测激光源为两台特性几乎完全相同的激光器,或者至少一台为已知低频率噪声的参考激光源,这给采用该方案精确测量频率噪声带来了困难。不仅如此,这种光外差拍频技术受测试仪器的采样速率的限制,只适合评估100Hz以下频段的频率噪声,不能满足高频段噪声的测量需求。因此,为准确测量出mHz至MHz宽频段的完整频率和强度噪声性能,必须综合运用多种噪声测量方案。但是,采用不同方案进行分段测量时所用仪器的性能存在差异,因此需要将由不同方法分段测量获得的强度和频率噪声谱进行拼接,以便高精度地评估出单频激光的完整强度和频率噪声谱特性。
本文将展示一种旨在获取mHz至MHz宽频段激光频率和强度噪声特性的分频段噪声测量规范。在该测量规范下,将研制基于迈克耳孙光纤干涉仪的相关延时自外差频率噪声测量装置和能对噪声进行定标的光外差拍频测量装置,结合ESA和FFT分析仪等标准仪器,最终实现对单频激光的宽频段频率(mHz至MHz)和强度噪声的测量。此外,对该噪声测量规范的有效性和准确度也进行了讨论和验证。
2 测量方案与仪器研制
高精度频率噪声测量需要高度稳定的干涉仪。
图 1. 频率噪声测量系统示意图。(a) 基于迈克耳孙光纤干涉仪的相关延时自外差测量;(b) 外差拍频测量
Fig. 1. Schematic diagrams of frequency noise measurement system. (a) Correlation delay self-heterodyne measurement based on fiber-type Michelson interferometer; (b) heterodyne beat-frequency measurement
针对10Hz以下频段的频率噪声测量,设计研制了两台性能几乎完全相同的分布Bragg反射(DBR)单纵模光纤激光器。这两台激光器的增益光纤、光纤光栅以及腔长几乎完全一致,其制作工艺和封装温控也完全相同,因此二者具有相似的噪声行为。利用
在激光的RIN测量中,利用低噪声光电探测器(PD,PDA-10CF-EC,Thorlabs)将RIN转换成电信号后,由ESA直接测量RIN功率谱。所用的ESA在GHz高频段的噪声极低(-171dBm@1GHz),但其在10kHz以下频段的噪声相对较高,从而适用于10kHz以上频段的RIN分析。所用探测器的带宽为150MHz,对于大多数单频激光,其RIN在150MHz分析频率处时早已接近散粒噪声极限。10kHz以下频段的RIN谱由FFT分析仪或基于DMM的方法测量。所用 FFT分析仪(SR770,Stanford Research Systems)带有高精度的16位模数转换器(ADC)和具有滤波、外差以及傅里叶变换等运算功能的数字信号处理器(DSP),可方便地实现低至mHz(最低约476μHz)处的RIN测量,但受其ADC采样速率的限制,难以实现100kHz以上频段的RIN评估;同样地,DMM则通过记录电信号电压随时间的变化,再由FFT算法计算出RIN功率谱,这与频率计数器对低频频率噪声的测量相似,从而确保在较长的总测量时间内可评估低至1mHz处的RIN功率谱,但受DMM(GDM-9060,Gwinstek)电压计数速率的限制,可测量的最大RIN功率谱范围为100Hz。RIN中的直流分量大小由DMM长期观测得到的电压均值确定。实际测得的RIN功率谱均已扣除测量系统本身的本底噪声。
在上述噪声测量方案中,频率和强度噪声均由不同测量方案的分段测量获得,因此,需对测得的不同频段噪声谱进行频谱拼接。基于干涉仪的相关延时自外差测量系统有效抑制了环境噪声,且利用性能几乎完全相同的两台激光器定标了光外差拍频测量所需的已知参考激光器,这使得这两套系统测得的高低频段噪声谱在重叠区域的幅度差异小,且无明显阶跃。因而,在频率噪声谱拼接中可采用直接平均法进行频谱拼接,即保留非重叠区域实测的各自频谱值,重叠区域频谱值由直接平均值替代。而对于强度噪声谱的拼接,所用仪器的测量精度不同,导致了测得的噪声谱在重叠区域存在一定的差值,频谱拼接时选用了渐入渐出拼接法[21],这时可保留不同方法实测的非重叠区域频谱值,但在重叠区域,则由该区域对应的频率上下限fmax和fmin分别构造出对应低频段和高频段噪声谱RINLO(f)和RINHI(f)的加权因子σ1(f)=(fmax-f)/(fmax-fmin)和σ2(f)=1-σ1(f),再由RIN(f)=σ1(f)·RINLO(f)+σ2(f)·RINHI(f)计算出重叠区域的频谱数据。
3 测试结果与讨论
选取NP Photonics公司商售的1064nm单频DBR光纤激光器作为待测光源,厂家已给出该激光源在若干典型傅里叶频率处的噪声数据,从而便于通过比对来考查本文测量系统所测结果的可信度。首先,利用研制的基于迈克耳孙干涉仪的频率噪声测量系统,测量出[10Hz, 1MHz]频段内的频率噪声谱,结果如
图 2. 基于相关延时自外差法测得的频率噪声谱,其中菱形点为厂家提供的数据
Fig. 2. Frequency noise spectrum measured with correlation delay self-heterodyne method. Diamond points show data provided by supplier
图 3. 由光外差拍频法测得的低频段频率噪声谱
Fig. 3. Frequency noise spectra measured with optical heterodyne beat-frequency method
通过比较
图 4. 1mHz~1MHz频率范围内的频率噪声谱
Fig. 4. Measured frequency noise spectrum in frequency range of 1mHz--1MHz
图 5. 相对强度噪声谱。(a)基于ESA测得的RIN谱,其中圆点为厂家提供的数据;(b)基于FFT分析仪与DMM测得的RIN谱
Fig. 5. RIN spectra. (a) Measured RIN spectrum with ESA. Dots show data provided by supplier; (b) measured RIN spectra with FFT analyser and DMM
FFT分析仪相较DMM方法所能测量的RIN谱的最大傅里叶频率更高,从而利于实现FFT分析仪测得的RIN谱与ESA方法测得的RIN谱之间的高精度频谱拼接。通过在104~105Hz重叠区域应用前述渐入渐出的拼接算法,容易得到待测激光器在1mHz~50MHz宽频段内的相对强度噪声谱,如
图 6. 1mHz--50MHz频段范围内的相对强度噪声谱
Fig. 6. Measured RIN spectra in frequency range of 1mHz--50MHz
4 结论
宽频段激光频率和强度噪声难以由单一测量技术进行评估,必须运用不同测量方案的分段测量和频段拼接来获取。一种有效的宽频段噪声测量规范是:采用基于参考激光的光外差拍频法和基于光纤干涉仪的相关延时自外差法分别测量出低频段和高频段频率噪声谱,再通过直接平均法进行频段拼接,从而获得mHz至MHz宽频段的频率噪声特性;通过采用FFT分析仪和ESA分别测量低频段和高频段RIN谱,再由渐入渐出拼接法获得mHz至MHz宽频段RIN谱。根据该测量规范,研制了一种基于迈克耳孙光纤干涉仪的高精度相关延时自外差频率噪声测量装置,并研制出两只性能相同的光纤激光器,经噪声定标后构建出了外差拍频测量装置,结合ESA和FFT分析仪等标准仪器,实现了对单频激光的[mHz,MHz]宽频段频率和强度噪声的测量。通过将测量结果与厂家数据进行对比,验证了该宽频段噪声测量规范的有效性和准确度,这为引力波探测和新型精密测量应用中的激光噪声评估提供了一种参考。
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Article Outline
张骥, 魏珊珊, 刘昊炜, 刘元煌, 姚波, 毛庆和. 单频激光宽频段频率和强度噪声测量技术[J]. 中国激光, 2021, 48(3): 0301002. Ji Zhang, Shanshan Wei, Haowei Liu, Yuanhuang Liu, Bo Yao, Qinghe Mao. Measurement Technique for Broadband Frequency and Intensity Noise of Single-Frequency Laser[J]. Chinese Journal of Lasers, 2021, 48(3): 0301002.
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