报告题目：Exploring Sensitivity Limits in Cavity Ring-Down Spectroscopy

报 告 人：Haifeng Huang, Sandia National Laboratories, California

报告时间：10月26日下午4:00

报告地点：理科楼C-109

报告摘要：In cavity ring-down spectroscopy (CRDS), the traditional uncorrelated least squares fit (LSF) can be generalized to the case with data correlation, e.g. caused by data filtering which is essential to minimize noise. Analytical formulae of sensitivity limits of cavity ring-down spectroscopy (CRDS) for both white noise and shot noise dominated situations have been derived. In this talk, I will present several subtle noise sources in CRDS, which can deteriorate the sensitivity of CRDS by orders of magnitude.

In a high finesse (~105) optical resonator, very high order (~60th) transverse modes can be excited simultaneously with the TEM00 mode through mirror surface scattering coupling when both are near degenerate. The strength of the scattering coupling is on the order of 10−12 per reflection. This coupling leads to mode beating and hence produces extra noise in ring-down transients. Putting an intra-cavity aperture of right size suppresses the excitation of those high order transverse modes.

Residual or strain induced linear birefringence (10−7 ~10−6 rad) in high reflective (HR) coatings of super-mirrors will lift the degeneracy of TEM00 modes between x and y polarizations, generating two new eigen-states of polarization. These two new states 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 super-mirrors, these two new eigen-states are no longer orthogonal with each other, and produce mode beating noise in ring-down signal.

In continuous wave (CW) CRDS, for ring-down transients with S/N of 1400:1, the extinction ratio of the light modulator needs to be very high (>70dB) in order to reach fundamental sensitivity limit. The efficacy of using a semiconductor optical amplifier (SOA) as light modulator in CW-CRDS has been demonstrated. SOA has the highest extinction ratio (>80 dB).

Allan variance has been applied to characterize the long-term drift of the measured decay rate k in CRDS experiments. Final sensitivity to bulk cavity loss (1σ) of 4.4 × 10−12 cm−1 has been obtained during an optimum integration time of ~30 min with our CW-CRDS setup, which corresponds to the methane detection limit (3σ) in N2 of 29 parts in 1012 by volume (pptv) at 760 Torr pressure. The stability of our system is demonstrated by measuring sub-ppbv methane (190pptv at 3σ) in N2 at 760 Torr through recording the spectrum of methane lines with our setup.