| Laser spectroscopy has been widely used in environmental monitoring,combustion diagnosis,medicine,industry,agriculture and other fields.Based on the Lambert-Beer law,the measurement sensitivity of laser absorption spectroscopy is directly related to the effective optical path length(OPL)of the interaction of laser and substance.The amplitude of the gas absorption signal can be increased by enhancing the effective OPL,which is conducive to improving the detection sensitivity.The common approach to improve the OPL is to use a multipass cell(MPC).The traditional multipass cells includes the Herriott MPC,the White MPC and the Chernin MPC.Compared with the traditional cells,the toroidal MPC has the advantages of miniaturization,easily adjustable OPL and high utilization of the inner surface.Therefore,in this paper,some researches on the optical design and related theories of toroidal MPC were mainly conducted.The main work is as follows:1.Based on the off-axis incident beam,a toroidal multipass absorption cell was developed for achieving multi-layer spot patterns.The inner surface of toroidal multipass absorption cell is a single ring.Furthermore,the utilization of the inner surface of the cell is high,and the processing difficulty is low and the effective OPL is increased.2.The exact analytical equations for the design of toroidal MPC were presented based on the vector calculations,which can quickly derive the unique incident angle corresponding to the maximum of the effective OPL.In addition,according to the light transmission in the toroidal MPC,the vector relation could be calculated.The relation is between the direction vector of the nth reflection beam and the normal vector and the direction vector of the n-1th reflection beam.Therefore,the coordinates of the spatial position of spots formed by each reflection could be obtained.In addition,Matlab,Tracepro and Solidworks software were used for the numerical simulation of the two-layer and the threelayer spot patterns in the MPC with an inner radius of 50 mm.Theoretically,under the condition of dense and non-overlapping spot patterns,a maximum of 300 reflections was achieved in the MPC.In this case,spots were divided into three layers and each layer was evenly distributed,and the maximum OPL reached 30 m.Through experimental verification,two practical spot patterns were demonstrated.When the effective volumes corresponding to the two-layer and three-layer spot pattern were 63 mL and 94 mL,the effective OPLs of 8.3 m and10 m were achieved in the toroidal multipass absorption cell,respectively.3.In the process of experimental verification,an optical mask that could effectively suppress stray light was added to the toroidal MPC.To prove the fringe suppression of the cell,we used the triangular wave signal with the frequency of5 Hz generated by a signal generator to scan the laser frequency.The laser beam exiting from the cell was detected by a photo detector to obtain the baseline measurement results.The fringe effect is substantially reduced to less than 0.5%by the usage of our designed mask.4.To enhance the performance of the cell and further improve the ratio of effective OPL to effective volume(OPL/V),an optical multipass absorption cell with small volume and long OPL can be designed.Our research proposed two methods to improve the ratios of OPL to volume.The first method is increasing the number of reflections by reducing the size of the spot to increase the OPL.In the other approach,without affecting the normal transmission of the beam,a cylinder is added at the empty position in the center of the MPC.This cylinder can also be further extended for assembling the required sensor.The toroidal MPC proposed in this thesis has the advantages of strong optical stability,low volume,simple and convenient adjustment,rapid response and high utilization of its inner surface.It can be widely used in trace toxic gas detection,atmospheric environment monitoring,medical diagnosis,industrial process control and other fields. |
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