A developed spectrometer based on optical-feedback cavity ring-down spectroscopy (OF-CRDS) has been demonstrated with a distributed feedback laser diode and a V-shaped glass ceramic cavity. The laser is coupled to the V-shaped cavity, which creates an absorption path length greater than 2.8 km, and resonance between the laser frequency and the cavity modes is realized by modulating the cavity length instead of tuning the laser wavelength to obtain a higher resolution. A noise-equivalent absorption coefficient of ∼2.6 × 10⁻⁸ cm⁻¹Hz^−1/2 (1σ) is determined with spectral resolution of ∼0.003 cm⁻¹ and spectral range of 1.2 cm⁻¹. As an application example, the absorption spectrum measurement of water vapor in the spectral range of 6590.3∼6591.5 cm⁻¹ is demonstrated with this spectrometer.

We describe the wavelength calibration method of a narrowband laser diode in continuous-wave (CW) cavity ringdown spectroscopy (CRDS). The method uses known spectral lines as wavelength markers to calibrate and refine the wavelength-current relation of laser diodes, and their spectral positions are taken directly from the HITRAN 2004 database. We built a compact CW CRDS apparatus with a $1.517\mathrm{\mu m}$ ($\sim 6594{\mathrm{cm}}^{-1}$) distributed feedback (DFB) laser diode as the light source and a $25\mathrm{cm}$ long glass ceramic as the cavity in which to demonstrate the method. A wavelength precision of $\sim 0.8\times {10}^{-3}{\mathrm{cm}}^{-1}$ was obtained by comparing the HITRAN 2004 database, which was approximately four times more precise than that of the conventional method.

The inhomogeneity of high-reflectivity mirror coatings is a potential error source in the application of the cavity ringdown technique. Here, the ringdown times for different transverse modes were recorded. Together with the observed spatial distribution of these modes the ringdown times can be used to approximately locate the position of coating defects. A simple model based on a weighted sum of Hermite–Gaussian mode functions is used to explain the experimental results.

Optical surface scattering is an important subject in the field of optics. Previous studies of surface scattering mainly focused on the influence of surface topography and often ignored the influence of the mechanical property’s change caused by polished surface densification. In this paper, we study the mechanical property of fused silica glass in detail and analyze the scattering behaviour of actual fused silica glass’s surface with sub-angstrom roughness, considering the topography and the change of refractive index. Experimental results show that there is a negative correlation between the height and modulus on the surface of roughly polished fused silica glass, and the correlation coefficient γ = −0.29 was determined. After super−polishing, the mechanical properties of the sample surface become significantly uniform with a roughness of Rq = 0.06 nm and Ra = 0.05 nm, and the correlation coefficient becomes γ = −0.02. Moreover, the nanoindentation test proves that silica glass surfaces have been densified during polishing. Based on the densification characteristics, the bidirectional reflectance distribution function (BRDF) was simulated by the finite element method. The result indicates that the densification characteristics will increase the scattering intensity. This work not only deepens the understanding of the properties of polished optical surfaces but also the surface scattering characteristics of optical elements.

A compact absorber composed of an optical cavity with an ultrathin dielectric metasurface, which can detect ultrasonic signals with extreme sensitivity.