A methane sensor based on dispersion spectroscopy is presented in this paper. A standard Mach–Zehnder modulator working in carrier suppression mode is adopted to generate a spectrum of a carrier and two sidebands. We aim at detecting the phase shift of the beatnote generated by the two sidebands in a methane concentration evaluation process. We put forward an analytical model to describe the dual-sideband heterodyne scheme and carry out experiments to demonstrate the model. Long-term tests show that the sensor has a minimum detection limit of $0.4\mathrm{ppm}·{\mathrm{mHz}}^{-0.5}$ at an average time of 1 s. And in the condition of 1 atm and room temperature, a linear measurement range from 0.4 to $44955\mathrm{ppm}·\mathrm{m}$ is achieved.

To advance the practical application of optical coherence tomography (OCT) in the field of biomedical imaging, the imaging depth must be extended without sacrificing resolution while maintaining sufficient sensitivity. However, there is an inherent trade-off between lateral resolution and depth of field (DOF) in OCT. To address this shortcoming, this article proposes a multifocal Bessel beam spectral-domain optical coherence tomography (MBSDOCT) capable of increasing the DOF with unchanged lateral resolution and a high signal-to-noise ratio. The proposed technique is demonstrated by simulation and experiment. A three-focal MBSDOCT with an axicon lens theoretically achieved a DOF of ∼6 mm with a lateral resolution of ∼13 μm. In imaging experiments performed on the acinar cells of orange tissue, a measured DOF of ∼4 mm was demonstrated with a sensitivity penalty of ∼18.1 dB, relative to the Gaussian beam spectral-domain OCT, with a 9-mW light source.

We proposed a six-gas simultaneous detection system for methane (CH ₄ ), ethane (C ₂ H ₆ ), acetylene (C ₂ H ₂ ), ethylene (C ₂ H ₄ ), carbon monoxide (CO) and carbon dioxide (CO ₂ ) based on wavelength modulation spectroscopy (WMS). Five different near infrared distributed feedback (DFB) lasers combined with a custom $5\times1$ wavelength division multiplexer were used to detect the six gases. The lasers were modulated at five different sinusoidal signals and the detection light intensity signals were demodulated with corresponding sinusoidal signals, which can be named frequency division multiplexing (FDM). Among them, CH ₄ and C ₂ H ₆ were measured with one laser based on time division multiplexing (TDM). The principle of four multi-gas detection based on FDM was also derived. To miniaturize the device, a Raspberry Pi was used for data processing based on python, and the detected gas concentrations can be displayed directly on the screen. The six-gas sensing system exhibits detection limits of 247 ppb, 1.28 ppm, 126 ppb, 982 ppb, 2.55 ppm and 4.87 ppm with average time of 1 s for CH ₄ , C ₂ H ₆ , C ₂ H ₂ , C ₂ H ₄ , CO and CO ₂ , respectively. With an average time extended to 100 s, the measurement precision achieves 24 ppb, 135 ppb, 14 ppb, 141 ppb, 495 ppb and 608 ppb, respectively. With a fast response time, compact size and high detection sensitivity, the detection system is promising for dissolved gas analysis (DGA) in transformer oil and other material aging process.

A new scheme for methane remote sensing is presented. Unlike a standard published remote sensor based on wavelength modulation spectroscopy (WMS), a reference cell is inserted into the measuring optical path. This scheme inherits the merits of WMS and can achieve high signal-to-noise ratio especially in a low concentration environment. Experimental results show that the presented remote sensor can detect ambient methane with a detection limit of 5 ppm m (parts per million meter) at a distance of 10 m and 16 ppm m for 20 m. A methane leak test shows the sensor can detect a methane leak of 15 ml min⁻¹ within a range up to 37 m.

High path-to-volume ratio (PVR) and low-aberration-output beams are the two main criteria to assess the performance of multipass absorption cells. However, no substantial progress has been reported for large-numerical-aperture-coupled multipass cells, which is due to the accumulated aberrations caused by a large number of off-axis reflections. Based on Chernin’s design, in this study, we modified Chernin’s four-objective multipass matrix cell by using slicer mirrors to eliminate alignment difficulty and decrease the system volume. A generalized design routine based on user requirements is also proposed. Based on the automatic modeling tool package (Pyzdde) connected with Zemax and boundary conditions of the parameters selection proposed, a low-aberration-output beam and a high PVR are easily obtained compared with other multipass cells schemes. In one demo design, 108 passes ($5\times 7$ matrix spots) in a base length of 300 mm are presented. The PVR and peak-to-valley value wavefront errors are 67.5 m/L and 0.92 μm, respectively. Finally, a tolerance analysis of this optical multipass system is also presented. This work may provide better broadband optical absorption cells in terms of response time and a better detection sensitivity in versatile applications.

A new design of circular multi-pass cells (CMPC) with two rows of reflection spots on mirrors is presented. The effective optical path length (OPL) of this novel CMPC is double that of traditional CMPC with the same diameter and interval of spots. This OPL can be readily adjusted to have regular intervals by rough rotation adjustment. We achieved a spatial separation of pre- and post-transfer optical systems that was adequately large even in the cases with a large number of passes. Analytical chief ray tracing analysis and a generalized method for parameter determination for designing the cell are presented in detail. The stable condition of the double-row CMPC (DR-CMPC) is also derived by the ABCD matrix method. Designs with maximum effective OPL of 74.72 m, 48.67 m and 24.57 m are demonstrated and verified by ray tracing simulations within a 25 cm diameter DR-CMPC. An adjustment of the regular intervals to 1 m can be achieved in both designs. The overall astigmatism of the design with an effective OPL of 74.72 m is only 9.30 × 10⁻⁶ mm, which is four orders of magnitude smaller than that of the traditional CMPC with similar geometric parameters.