The discussion of the transit broadening with a Gaussian beam and beam expanding in high resolution laser spectroscopy

It is pointed out that,in high resolution laser spectroscopy,when a particle passes through a Gaussian beam,the total broadening called as transit broadening should include the transit-time broadening and curvature broadening,and this broadening only has a connection with the beam waist,and is independent of the opsition.Therefore,the beam expanding just means to expand the waist and not the beam size.Some laws obeyed by the beam expanding by use of the thin lens are discussed.