High-spectral-resolution Lidar Based On Field-widened Michelson Interferometer

Atmospheric aerosols refer to the solid and liquid particles such as dust,fog,etc.with diameters among 0.001?100 microns suspended in the atmosphere.Although the proportion of aerosols in the atmosphere is very small,they strongly affect the conversion and spatial distribution of atmospheric radiation energy,thereby affecting the earth's climate and environment.Because of its high signal-to-noise ratio and less a priori assumptions required in the data inversion compared with standard Mie lidar,high-spectral-resolution lidar(HSRL)technique has been paid more and more attentions in aerosol remote sensing applications.In spite of having been developed for more than 30 years,the applications of HSRLs in the global aerosol observations are still not popular.This may be largely due to the fact that the spectral discrimination filters with wide adaptability for HSRL are not available.Currently,there are two types of spectral discrimination filters adopted in HSRL,i.e.,the iodine absorption cell and Fabry-Perot(FP)etalon.The iodine absorption cell has high rejection to aerosol scattering signal.However,its available gaseous absorption lines for convinient laser wavelengths only exist at 532 nm.The FP etalon has flexible wavelength tuning ability while it suffers from small angular acceptance,making it only usable in short wavelengths(such as 355 nm)that are with strong backscattering.In addition,the spectral discrimination ability of FP etalon is not so satisfactory as well.Investigating new frequency discrimination techniques would be of great significance to promote the development of HSRL towards generalization and high performance.A new type of spectral discrimination filter,called field-widened Michelson interferometer(FWMI),is proposed in this dissertation.Also,a proof-of-concept HSRL system based on the FWMI is established to verify the feasibility of this new spectral discrimination filter technique.Because of the great adaptability and good spectral discrimination performance,it is very potential that the FWMI would be the third kind of spectral discrimination filter along with the iodine absorption cell and FP etalon.The main contents of this dissertation include:A general HSRL theoretical model is established.Most HSRLs adopt iodine absorption cell filters that feature with extremely high rejection to aerosol scattering,thus it is easy to conduct the data retrieval.When using the FP etalon as the spectral discriminator,the crosstalk between aerosol scattering and molecular scattering should be considered seriously.Focused on an important science problem "to what extent the spectral discrimination filter should separate the aerosol and molecular spectrum in lidar return in order to satisfy the requirements of HSRL detection?" the requirements that the HSRL technique imposes on the spectral discrimination filter are reexamined by means of general modeling and error propagation approach.Many important but not widely realized conclusions are obtained.These contents provide fundamental and general guidelines for developments of all kinds of spectral discrimination filters and HSRL instruments.A theoretical framework about the application of the FWMI is estabilished.The necessity of using the field widening concept in the interferometric spectral discriminator is presented,and the design method for the FWMI is proposed.Moreover,a comprehensive performance estimation model for the proposed FWMI is introduced.This model can be employed to optimize the design of a practical FWMI and it also provides a useful way to evaluate the practical parameter of each component in the FWMI,thus guiding the fabrication and adjustment of this interferometer.At last,the performance of FWMI and FPI as the HSRL spectral discriminator is quantitatively compared based on the established HSRL general model and FWMI theoretical framework,which indicates the superiority of the FWMI further.An experimental approach,which can accomplish the optimal adjustment and evaluate the field widening characteristic of the FWMI simultaneously,is also proposed.This approach is very helpful to convert the FWMI concept to a real FWMI device,and can optimize the augular ability of an FWMI to its theoretical limitation.Frequency locking technique is crucial to lock the resonant frequency of an FWMI to the central freqeucny of the HSRL transmitter.A novel technique,called optimal multi-harmanics heterodyning locking,is proposed and experimentally demonstrated,which can complete the desirable FWMI frequency locking with very high accuracy even when the fineness of the FWMI is low.A proof-of-concept HSRL system based on the FWMI is developed.The optical layout,main parameters,working mode,as well as the data retrieval of the HSRL system are introduced.The proposed calibration approach and results for this system are also discussed in detail.Experimental results in atmospheric remote sensing by this HSRL are given to verify the feasibility of FWMI technique and FWMI-based HSRL.