Study On Digital Holographic Microscopy Based On LED Illumination

With the rapid development of science and technology,micro-nano devices such as MEMS and micro-lens arrays have been widely used,and hence precise topographic measurement of such structures has become particularly important.Digital holographic microscopy?DHM?is a measurement technique,which takes use of CCD or CMOS cameras to record holograms and performs numerical reconstruction in a computer,to obtain the complex amplitude of the measured object.It is a combination of digital holography and optical microscopy.DHM has the advantages of non-contact,non-destructive,high-resolution and quantitative measurement of phase map.And it has been widely used in three-dimensional topographic measurement of micro-nano structures and some other fields.In digital holographic microscopy,usually a coherent laser is adopted as the illumination source.The high coherence of laser will lead multiple reflections on optical element's surface and thus introduce parasitic interference fringes in holograms.As a result,the signal-to-noise ratio of the hologram is decreased and the reconstructed phase map is degraded.In order to overcome these drawbacks,here a light emitting diode?LED?,as partially coherent light source is used as the illumination light source in DHM.The topographic measurement scheme is presented,and results of both single-wavelength and dual-wavelength interferometry are studied.The thesis mainly includes three parts:1.The generation and development of DHM were reviewed.The basics of DHM were briefed,including interferometric recording theorem,on-axis and off-axis configuration,phase shifting interferometry technology,etc.,as a theoretical foundation for subsequent experiments.2.In single-wavelength DHM,an LED with central wavelength of 630 nm was selected as the illumination light source.And a common-path interferometer was designed by using an objective with working distance of 33.5 mm.By taking advantage of its long working distance,a micro-Michelson interferometer was constructed,with which interference between two beams can be easily generated.In the experiments,we used the slightly off-axis phase-shifting interferometry to record phase-shifted-holograms.Afterwards,two-step blind phase-shifting algorithm was adopted for reconstruction.A reflective USAF 1951 resolution target with a height of 200 nm was measured experimentally.The measured height map was consistent with the nominal value,which verified the correctness of the results.Compared with the results obtained from a He-Ne laser illumination,the noise in the phase map under LED illumination was reduced by 70%.Therefore,illuminating with an LED can effectively reduce coherent noise and improve imaging quality.3.In dual-wavelength DHM,two LED beads with central wavelengths of?₁=630 nm and ?₂=517 nm were selected,respectively.A color CCD camera was used to record two-color multiplexed holograms simultaneously.After recording two multiplexed holograms,two set of phase-shifted holograms corresponding to ?1 and ?2 can be extracted,respectively.In the experiment,rectangular staircase steps with heights of 1000 nm and 200 nm were measured.The reconstructed results were also in good agreement with its nominal value.It demonstrated that the scheme can tackle the measurement of stepped samples with optical path difference larger than one wavelength.The comparative experimental results between laser illumination and LED illumination showed that the noise of phase map in the first case was only 21% of that in the latter case.It showed that the phase noise can be effectively reduced when LEDs were used in dual-wavelength digital holographic microscopy.