We propose a white-light interferometric demodulation algorithm for high-finesse fiber-optic F-P sensors, in order to improve the demodulation accuracy and the dynamic range encountered in traditional demodulation techniques. The interferometric spectral signal of the high-finesse F-P cavity was converted to the frequency domain and then a more accurate cavity length was estimated based on full phase on higher-order components. A detailed theoretical analysis was operated. A high-finesse F-P temperature sensor based on a silicon diaphragm was fabricated and tested to verify the proposed method. The demodulation accuracy increases with the increase of order, and the anti-noise performance is improved. For the third-order component, the optical path difference sensitivity obtained by this algorithm is 0.231 ± 0.0188 μm / ° C, and the average error rate of cavity length demodulation value is 0.0152%. The proposed algorithm is applicable to demodulate the high-finesse F-P cavities in the light source bandwidth of 1525 to 1575 nm, providing high accuracy and improved anti-noise performance.
In-line digital holography is wildly used to measure object field. While it might on one hand is outstanding in non-contact, simple structure and high resolution, on the other hand its zero-order terms, real and conjugated images are aliased together, also the information of real image is not legible. Compressive holography applies optimization algorithms to eliminate the interference from in-line digital holography imaging, its algorithm model requires zero-padding to avoid cyclic convolution artifacts. Based on the equally divided block compression holography model, this paper illustrates a combination of zeropadding and data-padding, that fulfils a certain amount of redundant information sub-hologram based on the equidistant hologram. The redundant information protects the boundary in the compression holographic model operation, and achieves the high-resolution reconstruction of the in-line hologram. In addition, the method can reconstruct in-line hologram, plus the lateral resolution can reach 1 μm in bubble sample experiment. This is a thirtyfold increase compared to the national standard GBT 15489.2-1995. The relative error of bubble diameter measurement is only 0.48%, which is substantially lower than the traditional techniques.
In this research process, in order to improve the ability of interference-based optical image encryption system to resist chosen plaintext attacks, we propose an optical pseudo-image encryption algorithm to enhance the correlation of plaintext. In this work, we introduce an addictional key, which is highly correlated with the secret image. The key can dynamically update the random phase plate and break the linear relationship between the input and output of the optical encryption system, so that the system will not be cracked by chosen plaintext attacks. Furthermore, the number of keys is reduced through improved sine chaos mapping, so that the encryption system can reduce the amount of key storage data while improving the security of the system.
The parameter information of particle size and position in particle field is of great significance in scientific research, engineering and other fields. In this paper, after the particle hologram is obtained by in-line digital holography, the angular spectrum algorithm is used to reconstruct the particle field, and the amplitude and phase information of the particle field are obtained; The gray gradient compound method is used for particle recognition, and the particles are separated from the background; Then the watershed segmentation algorithm is used to segment the overlapping particles, and the Hough transform is used to extract the particle size, transverse position and other information; Finally, the depth information of particles is determined by Laplace operator function method, and the three-dimensional field distribution of particle field is obtained. The experimental results show that the measurement error of polystyrene particles with standard diameter of 50μm is 1.53μm and the measurement error rate is 3.06%, which proves the feasibility of this method.
Digital holography can record and display three-dimensional information of objects, and is widely used in many fields. Due to the coherence of the light source, the digital hologram is destroyed by speckle noise, which seriously influences the quality of the reconstructed image. In order to remove speckle noise, in this paper, we combine random overlapping masks and 3D block matching filtering. This method uses only one hologram, which is simple and time-saving. The experimental results show that when the image noise is large, the proposed method can still achieve higher peak signalto-noise ratio (PSNR) and structural similarity (SSIM), especially the structural similarity, reaching 0.99, and meanwhile the relative deviation and visual effect both are excellent.
Dual-wavelength digital holography has advantages over single-wavelength digital holography in resolving phase discontinuities at high aspect-ratio. However, the operations are very time-consuming and cannot achieve real-time processing. We realized the phase reconstruction of dual-wavelength off-axis holograms on Java platform, and used GPU to accelerate the computation-intensive part. Preliminary experiments show we can reconstruct 1 mega pixel holograms continuously at a speed of 41 fps, which can satisfy the stable video-rate. Through Java, the system can be easily combined with numerous plugins of ImageJ, such as filters, LUT for pseudo-color, 3D tools, etc. This is of great help to the subsequent image analysis and processing.
Off-axis dual-wavelength digital holography (oaDWDH) can enable quantitative phase imaging on thickness samples without numerical phase unwrapping in a single shot. However, the traditional oaDWDH is huge and unstable owing to its separated-path geometries. In this paper, we presented a compact oaDWDH using wavefront-splitting in the quasi common-path. In our approach, a dual-wavelength spherical wave is split into two parts to act as the reference wave and the object wave, respectively. Only a few such optical elements as a mirror and a beam splitter are employed to adjust and recombine the two waves, and a hologram containing two-wavelength information is then captured by a monochromatic CCD camera. The information of a specimen, including phase and height, can be reconstructed through a division algorithm with the help of a specimen-free multiplexed interferogram. In order to verify the feasibility of the system, observations were performed on the step samples. The height of the sample is obtained quantitatively, and finally compared with the measured height result of the step sample by AFM to prove the accuracy of the measurement result.
An autofocusing method using correlation coefficient (CC) is proposed for dual-wavelength off-axis digital holography. The complex amplitudes of the object wave relative to the two wavelengths are first retrieved at different reconstruction distances, and the correlation degrees are then calculated between the two complex amplitudes. Considered the diffraction independency between the two wavelengths, the maximum CC is employed to automatically determine the focus plane. Our method can be applicable for the amplitude, phase, or both mixed sample. The experimental results demonstrate that the proposed method can enable automatically autofocusing with higher resolution in contrast to the state-of-the-art method.
An extrinsic Fabry-Perot interferometric fibre microphone based on polydimethylsiloxane, or PDMS, diaphragm was proposed. The large free-standing PDMS diaphragm, with a diameter of 4 mm, is prepared by a simple “spin-strip” process. The experimental result shows that the fabricated sensor has a high dynamic pressure sensitivity of about -136 dB re 1 rad/μPa in the range of 100~2000Hz. The noise equivalent acoustic signal level of the microphone, limited by the environmental noise, is about 1000 μPa/Hz1/2. And the dynamic range is tested to be more than 47.48 dB. The proposed microphone is expected to be used in the field of weak acoustic pressure testing.
We build a two-wavelength off-axis quasi-common-path digital holography for quantitative phase imaging (QPI) using polarization-multiplexing and flipping. The interference is performed by flipping the relative position of a sample and reference beam, and the dual-wavelength information is spatially multiplexed onto a monochromatic CCD camera simultaneously using polarization-multiplexing. Due to orthogonal interference fringes of two-wavelengths, the unwrapped information on the phase and thickness for the sample can be extracted from a single interferogram. Our setup requires no pinholes, gratings or dichroic mirror with straightforward alignment. Additionally, a division algorithm for dual-wavelength off-axis digital holography with the help of a specimen-free multiplexed interferogram is proposed to extract the phase of a specimen. We demonstrate the operation of the setup with step target and circular pillar.
We propose a security-enhanced optical interference-based multiple-image encryption (IBMIE) using a modified multiplane phase retrieval algorithm (MPPRA) in the Fresnel transform domain. In this IBMIE scheme, while a phase-only mask (POM) distributing randomly between [ 0 , 2π ] is fixed, the other POM is iteratively extracted based on a modified mMPPRA, and thus, multiple plaintexts are simultaneously encrypted into two POMs with different distances to the image planes. At last, the retrieved POM is pixel scrambled by chaotic pixel scrambling (CPS). During image decryption, the decrypted images can be obtained at their preset positions by an intensity detector directly. The silhouette of the original images cannot be obtained using either of the two POMs. The parameters of both CPS and Fresnel transform can serve as security keys to enhance the security. Numerical simulation is presented to demonstrate the validity of the proposed mMPPRA-based IBMIE.
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