Abstract
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.
Highlights
Multi-pass cells (MPCs) with a long effective optical path length (OPL) in a compact volume are of increasing importance in laser absorption spectroscopy for trace gas sensing, as these properties enhance the absorption spectroscopy signals of gaseous samples with extremely low concentrations or weak absorption line strengths, in addition to reducing the instrumental footprint
A new version of a circular multi-pass cell is proposed for the first time, which consists of two identical high-reflectance coated mirrors with spherical inner surfaces
A DR-circular multi-pass cells (CMPC) can be uniquely defined by the parameter set of R, p, q, c and φ, where R, p and q are the radii of the curvature of the internal spherical surface, number of the edges and density of the p-sided star polygon pattern, respectively, which are the same as in the traditional CMPC; c determines the longitudinal distance between the two rows of reflection spots; and φ is the angle between the entrance and exit port in the x-O-z plane, which determines the effective OPL (Figure 2b)
Summary
Multi-pass cells (MPCs) with a long effective optical path length (OPL) in a compact volume are of increasing importance in laser absorption spectroscopy for trace gas sensing, as these properties enhance the absorption spectroscopy signals of gaseous samples with extremely low concentrations or weak absorption line strengths, in addition to reducing the instrumental footprint. The key advantage of a separated entrance port and exit port relies on the avoidance of overlapping of the pre- and post-transfer optical systems, but it limits the adjustable range of the incident angle and the range of effective OPL that can be achieved within the cells. The reflection spots only appear circularly in the same plane on the internal wall (with the beam reflection trajectory forming an entire regular star polygon pattern) in the CMPC This means that if one wants to increase the effective OPL of the CMPC, the only way is to decrease the incident angle of the beam, θ. An spaced adjustment of effective OPL can be implemented by rotating one of the mirrors around the axis of rotational symmetry with a relatively rough precision of only a few degrees without any change in incident angle and pattern of spots. 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
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