1.School of Instrument Science and Opto-electronics Engineering, Hefei University of Technology, Hefei 230009, China 2.Beijing Engineering Research Center of Optoelectronic Information and Instruments, Beijing Key Laboratory for Optoelectronics Measurement Technology, Beijing Information Science and Technology University, Beijing 100016, China
Fund Project:Project supported by the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (Grant No. IRT_16R07), the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 61805017), and the Natural Science Foundation of Beijing, China (Grant No. 4184087).
Received Date:11 December 2018
Accepted Date:18 March 2019
Available Online:01 May 2019
Published Online:20 May 2019
Abstract:When the optical component works in a complicated working environment such as a large temperature difference or a multi-modal state, internal stress accumulation is likely to occur, which affects the overall performance of the optical system. The stress-optical constant of optical component in the infrared range is one of the concerns of many optical systems. In this paper, a method of measuring the internal stress-induced birefringence of optical glass based on the frequency splitting effect of 1556.16 nm erbium-doped fiber laser is proposed. The planar dielectric lenticular and fiber Bragg grating (FBG) are selected to form a linear semi-open resonator by using an erbium-doped fiber as a gain medium, and a 976 nm semiconductor laser (LD) with a single-mode pigtail output for pumping. At the pump power of 200 mW, a laser output with a spectral center wavelength of 1556.16 nm, a spectral 3-dB bandwidth of 0.018 nm, and a longitudinal mode spacing of 40.77 MHz is obtained. The birefringence and its fast axis introduced into the cavity by the fiber itself are analyzed. The force sensing structure of the optical glass to be tested is placed in the cavity, and the frequency splitting of the laser after the stress loading on the optical glass in the cavity is compared with the scenario in the empty cavity, and the bifurcation and cavity birefringence caused by the external load are obtained by combining the Jones matrix transfer equation. The load on the optical glass is gradually increased from 0 to 20 N. In the stress loading process, the direction of the stress birefringence in the optical glass is parallel to the direction of the applied force. The frequency splitting of the inner cavity increases from 35.59 MHz to 35.77 MHz, which corresponds to a 679.18 nm—682.62 nm optical path difference of the cavity. The correspondence between stress and frequency splitting is understood according to the birefringence superposition model and frequency splitting, and the result can be traced back to the basic physical quantity-wavelength. Continuous loading of the optical glass results in a system with measurement repeatability better than 0.0459 MHz. The experiment is designed to avoid the uncorrected systematic error of the system induced by the sub-cavity effect. The non-aligned error equation is obtained, and the error is calibrated. The experimental results show that the sensitivity of the instrument for K9 glass is 22060 Pa/nm and the linearity is 99.44%. The method has no damage to the surface of the optical structure, nor occlusion nor influence of its normal in-service work. It is of significance for optical lens, structural measurement and error correction, and can be widely used for accurately measuring the birefringence of optical components in the infrared band. Keywords:fiber laser/ frequency splitting/ stress measurement
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3.1.半外腔光纤激光器空腔频率分裂
单模光纤因温度、弯曲等因素易产生腔内双折射, 因此首先实验研究了无待测样品的光纤激光器空腔频率分裂特征. 当泵浦功率达到60 mW时, 调节反射镜角度, 获得稳定的激光输出, 光谱如图2所示. 光谱中心波长为1556.16 nm, 光谱3 dB带宽为0.018 nm. 图 2 半外腔频率分裂光纤激光器光谱图 Figure2. Optical spectrum of the half external cavity frequency splitting fiber laser.
在该线型腔激光器中, 根据激光谐振条件, 激光腔内纵模频率满足:
${\nu _m} = \frac{{cm}}{{2nL}},$
其中, νm为第m阶激光纵模频率, m为纵模序数, n为腔内有效折射率, L为几何腔长, c为光速. 在较长的光纤线型腔中, 具有不止一个纵模被激发, 其模式结构如图3所示. 不同级次纵模间隔为 图 3 空腔频率分裂频谱图 Figure3. Spectrum of cavity frequency splitting.
其中σ为中心处主应力, d为玻璃厚度, D为玻璃直径. 通过有限元分析法对圆形光学玻璃的应力分布进行仿真建模. 镜片应力大小与主应力方向有限元分析结果如图5所示, 分析表明中心部分主应力方向沿受力方向, 大小与加力成正比, 与(7)式对应. 图 5 镜片主应力大小与方向有限元仿真 Figure5. Finite element simulation of the main stress of the lens.
将加载应力的光学玻璃放置在激光器的谐振腔中, 使用微分头推动力传感器应力进行逐级加载以改变内腔中的双折射, 加载过程中拍频PMB2如图6(a)所示. 在加力过程中, PMB2的数值单调递增, 表明该频率分量为应力双折射所致的频率分裂ΔνB, 而Δ-ΔνB对应的PMB1单调递减, 与前文分析结果一致. 加载力从0 N以2.5 N为步长均匀增大到20 N, PMB2拍频信号从35.59 MHz增大到35.77 MHz. 图6(b)给出了PMB2拍频与加载力的关系. 图 6 (a)加载中PMB2拍频信号频谱变化; (b) PMB2拍频信号与加载力关系 Figure6. (a)Frequency spectrum change of PMB2 in loading; (b) relationship of the PMB2 and the force.