The traditional uncorrelated least squares fit can be generalized to the situation with data correlation (e.g. caused by data filtering which is essential to minimize noise), which has been applied to cavity ring-down spectroscopy (CRDS). The ideal accuracy limit of the extracted decay rate k was derived from the exponential decay signal for both white noise dominated and shot noise dominated CRDS. Different algorithms of single exponential fitting in CRDS were reviewed by comparing the bias and variance estimation of the extracted k, and computation efficiency of different fitting methods.;In continuous wave (CW) CRDS, it was found that the very high order (∼ 60th) transverse mode can be excited simultaneously with the TEM00 mode through mirror surface scattering coupling when they are near degenerate. This will lead to mode beating noise in ring-down transients even when the intensity coupling strength is extremely small, ∼ 10-12 per reflection. Putting an intracavity aperture of the right size will remove this noise by suppressing the excitation of high order transverse modes.;Linear residual or strain induced birefringence (10-7 ∼ 10-6 rad) in high reflective (HR) coatings of supermirrors will lift the degeneracy of TEM00 mode between two polarizations, generating two new eigen polarization states, which are very close to linear polarization and have a typical splitting of 0.1 kHz. Because of the polarization dependent loss (linear dichroism, ∼ 10-8) of HR coatings of supermirrors, these two new eigenstates are no longer orthogonal with each other, producing mode beating noise in ring-down signal. By controlling the polarization state of the incident light or in the detection, this noise can be removed.;Because of finite extinction ratio of the light modulator in CW-CRDS, the small light leakage will interfere with the exponential decay of intracavity light power, generating excess noise in the signal. A linear relation between (sigmak/k)2 and the extinction ratio has been found both theoretically and experimentally, with excellent agreement. In order to realize the ideal sensitivity (See Chapter 2) in CW-CRDS, light modulator with very high extinction ratio is needed. The efficacy of using a semiconductor optical amplifier (SOA) as light modulator in CW-CRDS was demonstrated. SOA has the highest extinction ratio (>80 dB). For ring-down transients with S/N of 1400:1, the SOA and acousto-optic modulator (AOM) are found to give equivalent performance.;The Allan variance was applied to characterize the drifting of the decay rate in CW-CRDS experiments. With differential measurements by two lasers between k and k0, the decay rates with and without absorption, the drifting between them will cancel out with each other greatly. A sensitivity of 5.6 x 10-12 cm -1 has been reached within an integration time of 15.4 minutes, which corresponds to trace methane detection limit (3sigma) of 0.3 parts per billion by volume (ppbv) at 20 torr or 15 parts per trillion by volume (pptv) at one atm measurement pressure around 1652 nm. By converting CO, CO2, ethane, propane and n-butane into methane with 100% conversion efficiency under the condition of excess H2 gas and nickel (or ruthenium) catalyst (for isobutane, the conversion efficiency is about 80%), each trace gas has been measured in nitrogen separately down to the level of sub parts per million by volume (ppmv), limited by ∼ 0.5 ppmv methane background in the conversion caused by the carbon contamination of both catalysts.
展开▼
机译：传统的不相关最小二乘拟合可以推广到具有数据相关性的情况（例如，由数据过滤引起的，这对于最小化噪声至关重要）已被应用于腔衰荡光谱学（CRDS）。对于白噪声为主的CRDS和散粒噪声为主的CRDS，从指数衰减信号得出提取衰减率k的理想精度极限。通过比较提取的k的偏差和方差估计以及不同拟合方法的计算效率，回顾了CRDS中的单指数拟合的不同算法;在连续波（CW）CRDS中，发现非常高阶（〜60当接近简并时，可以通过镜面散射耦合与TEM00模式同时激发横向模。即使强度耦合强度非常小，每次反射约为10-12，也会在振铃下降瞬态中导致模式跳动噪声。放置合适尺寸的腔内孔径将通过抑制高阶横向模的激发来消除这种噪声。;在超级反射镜的高反射（HR）涂层中线性残留或应变引起的双折射（10-7〜10-6 rad）将提升TEM00模式在两个极化之间的简并性，产生两个新的本征极化状态，它们非常接近线性极化，典型分裂为0.1 kHz。由于超级镜的HR涂层的偏振相关损耗（线性二向色性，约10-8），这两个新的本征态不再彼此正交，从而在振铃信号中产生模式跳动噪声。通过控制入射光的偏振态或在检测中，可以消除这种噪声。由于CW-CRDS中光调制器的消光比有限，小的漏光会干扰腔内光功率的指数衰减，在信号中产生过多的噪声。理论上和实验上都发现了（sigmak / k）2与消光比之间的线性关系，具有很好的一致性。为了在CW-CRDS中实现理想的灵敏度（请参阅第2章），需要具有很高消光比的光调制器。证明了在CW-CRDS中使用半导体光放大器（SOA）作为光调制器的功效。 SOA的消光比最高（> 80 dB）。对于信噪比为1400：1的振铃瞬变，发现SOA和声光调制器（AOM）具有同等的性能。;在CW-CRDS实验中，使用了Allan方差来表征衰减率的漂移。通过两个激光在k和k0之间的差分测量，衰减率在有吸收和无吸收的情况下，它们之间的漂移将相互抵消。在15.4分钟的积分时间内已达到5.6 x 10-12 cm -1的灵敏度，这对应于20托或十亿分之15的痕量甲烷检测极限（3sigma）为十亿分之0.3（十亿体积）（ppbv）。在约1652 nm的一个大气压测量压力下的体积体积（pptv）。在过量氢气和镍（或钌）催化剂（对于异丁烷，转化效率约为80％）的条件下，通过将CO，CO2，乙烷，丙烷和正丁烷转化为甲烷，转化效率为100％，每种微量气体在氮气中单独测得的总氮含量低至百万分之体积（ppmv），受两种催化剂碳污染导致的甲烷背景转化率限制为〜0.5 ppmv。
展开▼
