Proc. SPIE. 11438, 2019 International Conference on Optical Instruments and Technology: Optoelectronic Imaging/Spectroscopy and Signal Processing Technology
In the optical interferometry fields, the phase extracted by the arctangent function is a 2π-wrapped phase, it is necessary to carry out the phase unwrapping to obtain a correct continuous phase distribution. However the undersampled phase occurs due to too low sampling frequency and higher fringe density, thus the common unwrapping algorithms will fail. Aimed at the undersampled problem, a phase unwrapping algorithm based on lateral shearing and zooming is presented in the paper. The algorithm combines least square phase unwrapping based on second lateral shearing and bicubic interpolation to obtain larger anti-undersampled range. Taking peaks function as object, the anti-undersampled ranges are analyzed for different phase unwrapping algorithms. It can be shown that the presented algorithm retrieves the continuous phase of 2750 times the peaks function. The algorithm can provide a phase unwrapping solution for the serious undersampled phase, and the analyses of anti-undersampled capability for different phase unwrapping algorithms also are as a reference for future measurement.
The coherent noise appears in the constructed image of digital holographic microscopy due to the laser source; thus, the imaging quality is degraded. A method of coherent noise reduction using a laterally shifting hologram aperture is presented. An original hologram with coherent noise is captured by a camera first. A series of holograms are sampled by laterally shifting the digital aperture in the original hologram. Instead of extracting the specimen’s part information, each sampled hologram, which includes the whole specimen, is reconstructed. The coherent noise is reduced by averaging the different reconstructed images. The experiment demonstrates the feasibility of the approach. The presented approach with a single recorded hologram realizes the coherent noise reduction without loss of spatial resolution, which is useful for real-time measurement.
A T-shaped cavity dual-frequency Nd:YAG laser with electro-optical modulation is proposed, which consists of both p- and s-cavities sharing the same gain medium of Nd:YAG. Each cavity was not only able to select longitudinal mode but also tune frequency using an electro-optic birefringent filter polarization beam splitter + lithium niobate. The frequency difference of dual frequency was tuned through the whole gain bandwidth of Nd:YAG, which is far above the usually accepted free spectral range value in the case of a single-axis laser. As a result, the simultaneous operation of orthogonally and linearly polarized dual-frequency laser was obtained, which coincides with the theoretical analysis based on Jones matrices. The obtained frequency difference ranges from 0 to 132 GHz. This offers a simple and widely tunable source with potential for portable frequency reference applications in terahertz-wave generation and absolute-distance interferometry measurement areas.
The adaptive spatial filtering method is commonly adopted to extract the +1 term spectrum in digital holography for real-time dynamic analysis. However, the typical filtering method is not satisfactory for automatic analysis, because the reset of the filtering window is needed to extract the area of the +1 term spectrum. Therefore, an adaptive spatial filtering method based on region growing and the characteristic of the spectrum separation is proposed. Its filtering window is automatically formed by region growing. The key parameters, including threshold and seed point, are set by the intensity distribution of the hologram spectrum. Then the adaptive filtering extracting the +1 term spectrum is realized by multiplying the hologram spectrum by the filtering window. Compared to the typical filtering method, the experimental results of a microhole array and a phase step show that the proposed method has better adaptability and a higher precision. Moreover, the applicability of this method for different uses is also demonstrated by experiments with a microhole array and a phase step.
The telecentric arrangement in digital holographic microscopy (DHM), considered to be a pure-physical compensation for defocus aberration introduced by microscope objective (MO), shows shift-invariant behavior. Its optical arrangement requires precise adjustment of the distance between MO aperture stop and collimated lens. However, it is difficult to measure and quantify the distance even by monitoring the spatial frequency spectrum of recorded hologram in the absence of object. Thus the misalignment results in the residual defocus aberration in the telecentric arrangement. The total aberrations compensation for misalignment of telecentric arrangement in DHM is presented, in which a posteriori surface fitting method based on Zernike polynomials is performed to eliminate the residual defocus aberration as well as other primary aberrations. The approach reduces the difficulty in precise alignment of the telecentric arrangement and decreases the measurement error caused by aberrations in construction. Three-dimensional retrieval of the height for micro-hole arrays with high-spatial-frequency content demonstrates the feasibility of the method.
Digital holographic microscopy (DHM) has been widely applied for the topography measurement of microscopic
specimen. A total surface fitting method based on Zernike polynomials is presented to remove aberrations in DHM, in
which Zernike polynomial coefficients enable to provide quantitative measurement of primary aberrations. The phase
free of aberrations is obtained by subtracting out the surface fitting result from the reconstructed phase. The method
carries out the total phase aberrations compensation automatically by only one hologram, instead of knowing the
physical parameters of optical setup and the aberration mathematical model in advance. The optical system of off-axis
DHM is set up and the experiment results are given. Compared with the double-exposure method, the Zernike surface
fitting method obtains better phase information owing to removing residual tilt aberration.
In order to obtain the non-overlapping and high-quality reconstructed image, this paper analyzes the system parameters
in digital holographic microscopy. Nowadays a few scholars have analyzed the system parameters which need to satisfy
the sampling theorem and spectrum separation conditions. In this paper, not only the sampling theorem and spectrum
separation but also the size relationship between the reconstructed plane and the magnified image are studied. Then
relationships of system parameters are proposed. First, the maximum object size is directly proportional to the
wavelength and microscope objective focal length, inversely proportional to the sampling interval. Second, the minimum
magnification is described accurately. Finally, the paper gives the range of recoding distance. Experiments further
demonstrate the proposed conclusion’s validity.
When a new birefringent filter consisting of a polarizing beam splitter (PBS) and a half wave-plate (&lgr;/2), i.e.,
PBS-&lgr;/2 was included in a 1064nm Nd:YAG laser cavity, the laser was enforced to oscillate in single longitudinal
mode. The single longitudinal mode selecting ability of the intra-cavity filter of PBS-&lgr;/2 had been studied
experimentally by rotating the half wave-plate around the laser cavity axis, and the tuning characteristics of the
single-frequency laser output power versus the rotation angle of the half wave-plate had also been studied. An
orthogonally and linearly polarized dual-frequency Nd:YAG laser at 1064nm had been designed and demonstrated,
which included two standing-wave cavities sharing the same gain medium of Nd:YAG crystal and the birefringent filter
of PBS-&lgr;/2, the p-and s-components of the 1064nm laser light simultaneously oscillated in single longitudinal mode in
each cavity. The frequency-difference of the dual-frequency laser at 1064nm was measured to be approximately
1.87GHz, limited by the free spectral range of the scanning Fabry-Perot interferometer. It is predicted theoretically that
the frequency-difference of the dual-frequency laser at 1064nm can be tuned in a range from zero up to the lasing
bandwidth of the Nd:YAG laser.
A novel displacement sensor based on diode-end-pumped solid-state laser technology has been investigated theoretically and experimentally. The investigation results indicate that provided the average radius of the pump beam in the gain medium is much smaller than the radius of the waist of the TEMoo laser beam, the exponential of the laser output power will change in a manner of a Gaussian function when the waist of the pump beam is displaced axially. Both the measurement range and the sensitivity of the displacement sensor depend on the pump power, the measurement range will be enlarged and the sensitivity be enhanced when the pump power is increased. For the experimental system of the diode-end-pumped 1064-nm Nd:YAG laser sensor, the measurement range and the sensitivity are 13.045-mm and 0.148-mW/μm, respectively, when the input optical power is 7.24-Watt (corresponding to a maximum output power of 1.926-Watt). Several main error sources that affect measurement accuracy of the displacement sensor have also been analyzed.
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