光学反馈线性腔衰荡光谱技术不确定性

Uncertainty of optical feedback linear cavity ringdown spectroscopy

腔衰荡光谱技术是一种高灵敏的腔增强分子吸收光谱测量技术,由于激光频率噪声大,导致激光到腔耦合效率低,严重限制了其对痕量气体的探测灵敏度.光学反馈可以有效压窄半导体激光器的线宽,提高激光到外部谐振腔的耦合效率.本文基于精细度为7800的Fabry-Pérot腔,发展了光学反馈线性腔衰荡光谱技术.首先从光场相位的角度给出了线性腔光学反馈的原理,然后分析了影响测量不确定性的因素,包括光学反馈率、衰荡信号触发阈值、探测器相对透射汇聚光斑位置等.实验结果表明,通过设置低反馈率(3%自由光谱区间)、高衰荡信号触发阈值(90%腔模幅度)以及将光斑聚焦到探测器有效面中心等措施,结合光学反馈效应,可将空腔衰荡时间的相对不确定度提升至0.026%,远优于传统腔衰荡技术获得的典型值.系统在积分时间为180 s时,获得探测灵敏度为1.3×10^(-10)cm^(-1),对应甲烷的最小可探测吸收体积浓度为0.35×10^(-9),从而满足了碳监测的要求.

Cavity ring-down spectroscopy(CRDS)is a highly sensitive molecular absorption spectroscopic technology,which has been widely used in mirror reflectance measurement,atmospheric trace gas detection,molecular precision spectroscopy and other fields.It deduces the intracavity absorption by measuring the rapid variation of the ringdown signal.As a result,detector with high linearity,broad bandwidth and low electrical noise is indispensable.Additionally,owing to the large noise in laser frequency,low laser-to-cavity coupling efficiency is obtained.Consequently,the cavity transmission is faint,which deteriorates the detection sensitivity.Optical feedback can address this problem by locking the laser to the cavity longitudinal mode.Then,the laser frequency noise is suppressed and hence better detection sensitivity is expected.Optical feedback CRDS with Vshape cavity has been widely studied.Compared with Fabry-Perot cavity,this cavity geometry is very sensitive to mechanical vibration and possesses low degree of fineness due to an additional mirror.In this paper,optical feedback linear cavity ring-down spectroscopy based on a Fabry-Perot cavity with a degree of fineness of 7800 is presented.The principle of the combination of optical feedback and linear cavity is explained from the perspective of the light phase,which shows that the reflection will not generate efficient optical feedback if the feedback phase is appropriately controlled and laser to cavity locking can be therefore realized.And then,the factors influencing the stability of ring-down signal are analyzed,including the feedback ratio,the trigger voltage for the ringdown event,and the distance between the light spot and the detector center.The experimental results show that a superior fractional uncertainty of the empty ringdown time of 0.026% can be obtained with a low feedback rate(3% FSR),a high ringdown signal trigger threshold(90% cavity mode amplitude)and superposition of the light spot with the detector center.With Allan variance analysis,the white noise response of 1.6×10^(-9) cm^(-1)·Hz^(-1/2) and the detection sensitivity of 1.3×10^(-10) cm^(-1) for trace gas detection can be achieved in an integration time of 180 s,corresponding to the lowest CH4 concentration detection of 0.35×10^(-9) at 6046.9 cm^(-1).This robust spectroscopic technique paves the way for constructing high-sensitive and stable-cavity based instrument for trace gas detection.