1.Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China 2.Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230026, China
Fund Project:Project supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA17010104) and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2015264).
Received Date:11 January 2019
Accepted Date:23 April 2019
Available Online:01 June 2019
Published Online:20 June 2019
Abstract:Off-axis integrated cavity output spectroscopy (OA-ICOS) is a highly sensitive laser spectroscopy technique. However, due to the use of dense high-order modes for detection, OA-ICOS signal power is low, thus making the detection sensitivity highly dependent on the laser power. To this problem, we introduce an optical re-injection method to re-inject the laser back into the optical cavity again, improving the utilization of laser energy and the power of signal. In this paper, we use optical tracking software to design a re-injection structure, and study several factors affecting the signal gain. Then, we build a re-injection OA-ICOS device in the 2 μm band and also conduct a series of experimental researches. Our results show that the re-injection method enhances the OA-ICOS signal power 8 times and signal-to-noise ratio 4.6 times, which effectively improves the detection sensitivity and the absorption depth of the spectral signal, and alleviates the problem of low signal power in OA-ICOS detection. Keywords:reinjection/ off-axis integrated cavity/ mid-infrared/ high sensitivity
Mre上反射光斑的数量代表激光再入射的次数, 与增益系数有着重要关联. 为分析反射光斑分布情况, 对再入射OA-ICOS进行一系列模拟, 分别选择Mre和Mout截面进行观测, 模拟结果如图2—图4所示. 根据分析结果, 不同参数对光斑分布的影响如下. 图 2 在不同L和R时再入射镜Mre上光斑排布情况 Figure2. Distribution of spot on the re-injection mirror Mre with different L and R.
图 4 在不同入射角度时再入射镜Mre上的光斑排布情况 Figure4. Distribution of spot on Mre with different incidence angles.
1)再入射镜Mre的曲率半径R: 主要控制Mre上光斑分布密度和Mout上能量分布密度. Mre上光斑分布密度与R的大小呈正相关. 随着R变小, 反射光圈在Mout上变得难以按圆形排布, 能量密度降低, 反射光将超出光腔半径7 mm范围, 造成腔壁反射损耗和噪声; 随R着变大, 在Mout上的反射光圈越发聚拢, 能量密度不断增大; 当R > 800 mm后, 在Mre上光斑已无法产生成稳定光圈, 如图2和图3所示. 图 3 在不同L和R时腔镜Mout上光斑排布 Figure3. Distribution of spot on cavity mirror Mout with different L and R.
表1再入射结构中不同参数的影响 Table1.Effects of different parameters in reinjection.
24.3.增益系数分析与参数选择 -->
4.3.增益系数分析与参数选择
依据常理, 再入射次数越多, 入射光腔的能量也应该越多. 对图3中Mout截面光强进行计算, 可获得不同L和R条件下的模型的增益系数分布, 如图5所示. 当Mre上反射光斑稀疏时, 增益系数与光斑数量成正相关; 随着光斑排列紧凑, 有轻微重叠, 增益系数到达顶峰; 但随着光斑重叠面积继续增大, 增益系数与光斑数量开始呈现不符合常理的负相关. 针对负相关模型进行光线追踪模拟发现, 过分密集的光斑排布会使入射光的部分能量在初次反射后便沿再入射孔泄漏, 导致再入射次数即便增多, 信号增益也依然下降. 综上, 增益最优的光斑排布可总结为: 在离轴距离确定的情况下, 尽可能排布最多的反射光斑, 以保证高增益; 同时避免光斑间过度重合, 以减少入射能量泄漏和干涉噪声. 图 5 在不同L和R时Mout截面的增益系数 Figure5. Gain coefficients of Mout with different L and R.
如图5所示, 这类最优增益光斑排布不只有唯一解, 它们形成了连续的高增益区域, 可通过R和L的互补调节来维持最优增益系数. 但是在实际搭建过程中, 调节再入射距离L的代价远低于更换镜片曲率半径R的代价. 因此在高增益区域中选择参数时, 应首先依据光腔内反射光能量分布密度选择合适的R; 然后, 依据入射角可调节范围选择合适的L; 最后, 微调L以平衡增益系数与干涉噪声. 在高增益区域中, 选取多个最优增益点进行对比, 部分Mout能量分布如图6所示. 第一组(a)参数为R = 600 mm, L = 4 cm, 增益为18.74倍, 但光斑在光腔中单位面积功率密度低, 发散性强, 光线容易与腔壁接触产生噪声; 第二组(c)参数为R = 700 mm, L = 10 cm, 增益为18.04倍, 其单位面积能量密度高, 能量分布与(b)传统OA-ICOS相似, 且入射光可调节角度更大, 因此作为最优模拟结果. 图 6 Mout能量分布图 Figure6. Power distribution of Mout
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5.1.增益规律验证
为评估光学模拟的准确性, 需要对实验中对模拟增益进行验证. 依据设计定制再入射镜片Mre, 反射率实测为95%, 曲率半径R为734 mm, Min增透膜透射率实测有2%的吸收, 实验选取位于2003.57 nm的CO2吸收峰进行. 本节实验依据模拟设置搭建多组L不同的再入射OA-ICOS装置, 在常压下进行测量, 激光扫描频率为10 Hz, 对1000次扫描结果进行平均, 测得光谱吸收信号如图7. 图 7 在不同再入射位置L时系统输出信号 Figure7. Output signal at different re-injection position L.
为保证模拟增益准确性, 本文依据系统实际损耗和Mre曲率半径进行重新模拟, 然后进行对比, 结果如图8所示. 其中, 定义光束由Mre出发经Min的高反射镜面再回到Mre时的衰减为系统(单次再入射)损耗, 计算损耗时需要同时考虑到Mre反射率95%、Min增透膜存在2%吸收和高反膜反射率99.86%, 在这种情况下计算的单次损耗约为8.7%. 图 8 模拟与实验增益对比 Figure8. Gain comparison between simulation and experiment.