Design of diffractive optical elements for subdiffraction spot arrays with high light efficiency.

Spot arrays beyond the diffraction limit are required in many optical applications, and the shaping of a light beam into subdiffraction spot arrays can be implemented by diffractive optical elements (DOEs). However, the low light efficiency of spot arrays is undesired in many applications. In this paper, a modified Gerchberg-Saxton algorithm is presented for generating DOEs to realize subdiffraction spot arrays with higher light efficiency. In the simulation, the spot size is reduced to approximately 70%-90% of the diffraction-limited spot, and the corresponding light efficiency is within the range of 20% to 50%. The experimental results are also shown to demonstrate the effectiveness of the proposed algorithm.

Interferometric spherical surface testing with unknown phase shifts.

A general method is presented for spherical surface testing with unknown phase shifts based on a physical model of the interferometer cavity, which describes the phase shifts taking into account the rigid cavity motions and the radial imaging distortion of the interferometer. The captured interferograms are processed frame by frame with the regularized frequency-stabilizing method, so as to get the phase shifts between the frames. These phase shift data are subsequently fitted, and the initial estimations for the wavefront, direct current and interference contrast terms are calculated by the least-squares method. Specially, a simple way is proposed to find reasonable initial guess for numerical aperture (NA) of the test beam (when NA is unknown), so as to ensure the effectiveness of the above phase shift fitting procedure. Then, the wavefront result is further refined in an iterative way, by fitting the sequence of interferograms to the physical model of the interferometer cavity with the linear regression technique. Finally, the wavefront result related to the actual surface profile is retrieved after removing the aberrations due to the surface misalignment and the imaging distortion. Both simulations and experiments with the ZYGO interferometer have been carried out to validate the proposed method, with experimental accuracies better than 0.004λ RMS achieved. The proposed method provides a feasible way to spherical surface testing without the use of any phase-shifting devices, while retaining good accuracy and robust convergence performance.

Phase registration based on matching of phase distribution characteristics and its application in FDOCT.

Phase fluctuations in a two-transverse-dimensional scanning Fourier domain optical coherence tomography (FDOCT) seriously affect in vivo phase related applications. The phase difference between two A-scans sampled at the same scanning position or adjacent scanning position is acquired by matching of the phase distribution characteristics on the surface of two A-scans. Finger and palm scanning experiments are performed and defocused images of finger and palm are recovered based on Fresnel scalar diffraction algorithm by using phase compensated OCT complex signals. To further prove the performance of the proposed method, human eye scanning experiments are also performed and blood flow images of retina are extracted from the phase registration results. The accurate, fast and simple phase compensation method is critical for in vivo phase related applications.

Holographic Optical Tweezers That Use an Improved Gerchberg-Saxton Algorithm.

It is very important for holographic optical tweezers (OTs) to develop high-quality phase holograms through calculation by using some computer algorithms, and one of the most commonly used algorithms is the Gerchberg-Saxton (GS) algorithm. An improved GS algorithm is proposed in the paper to further enhance the capacities of holographic OTs, which can improve the calculation efficiencies compared with the traditional GS algorithm. The basic principle of the improved GS algorithm is first introduced, and then theoretical and experimental results are presented. A holographic OT is built by using a spatial light modulator (SLM), and the desired phase that is calculated by the improved GS algorithm is loaded onto the SLM to obtain expected optical traps. For the same sum of squares due to error SSE and fitting coefficient η, the iterative number from using the improved GS algorithm is smaller than that from using traditional GS algorithm, and the iteration speed is faster about 27%. Multi-particle trapping is first achieved, and dynamic multiple-particle rotation is further demonstrated, in which multiple changing hologram images are obtained continuously through the improved GS algorithm. The manipulation speed is faster than that from using the traditional GS algorithm. The iterative speed can be further improved if the computer capacities are further optimized.

Analysis and comparison of machine learning methods for blood identification using single-cell laser tweezer Raman spectroscopy.

Raman spectroscopy, a "fingerprint" spectrum of substances, can be used to characterize various biological and chemical samples. To allow for blood classification using single-cell Raman spectroscopy, several machine learning algorithms were implemented and compared. A single-cell laser optical tweezer Raman spectroscopy system was established to obtain the Raman spectra of red blood cells. The Boruta algorithm extracted the spectral feature frequency shift, reduced the spectral dimension, and determined the essential features that affect classification. Next, seven machine learning classification models are analyzed and compared based on the classification accuracy, precision, and recall indicators. The results show that support vector machines and artificial neural networks are the two most appropriate machine learning algorithms for single-cell Raman spectrum blood classification, and this finding provides essential guidance for future research studies.

Theoretical and experimental studies on tightly focused vector vortex beams.

The high-NA focusing properties of vector vortex beams are studied theoretically and experimentally. The vector vortex beams are generated by space-variant segmented subwavelength metallic gratings first. Then the mathematical expressions for the focused fields are derived based on the vector diffraction theory, and some numerical simulations are presented that show that the focused fields are not dark at the center and the focusing spot size of vector vortex beams with high topological charges approaches the diffraction limitation at high NA. Finally, to verify the theoretical analysis, the tightly focused fields are measured based on a confocal microscopy system when the NA of the objective lens is 0.90. The research results confirm the potential of vector vortex beams in some applications, such as optical trapping, laser printing, lithography, and material processing.

Blood identification at the single-cell level based on a combination of laser tweezers Raman spectroscopy and machine learning.

Laser tweezers Raman spectroscopy (LTRS) combines optical tweezers technology and Raman spectroscopy to obtain biomolecular compositional information from a single cell without invasion or destruction, so it can be used to "fingerprint" substances to characterize numerous types of biological cell samples. In the current study, LTRS was combined with two machine learning algorithms, principal component analysis (PCA)-linear discriminant analysis (LDA) and random forest, to achieve high-precision multi-species blood classification at the single-cell level. The accuracies of the two classification models were 96.60% and 96.84%, respectively. Meanwhile, compared with PCA-LDA and other classification algorithms, the random forest algorithm is proved to have significant advantages, which can directly explain the importance of spectral features at the molecular level.

An encryption method with multiple encrypted keys based on interference principle.

An encryption and verification method with multiple encrypted keys based on interference principle is proposed. The encryption process is realized on computer digitally and the verification process can be completed optically or digitally. Two different images are encoded into three diffractive phase elements (DPEs) by using two different incident wavelengths. Three DPEs have different distances from output plane. The two wavelength parameters and three distance parameters can be used as encryption keys, which will boost security degree of this system. Numerical simulation proves that the proposed encryption method is valid and has high secrecy level.

Diffractive superresolution elements for radially polarized light.

An optimization method for diffractive superresolution elements (DSEs) for radially polarized light is proposed. Only the longitudinal component of the focused field of radially polarized light is considered for optimization, and the results are 0, pi two-phase distributed DSEs. A series of such DSEs are designed, and the corresponding superresolution performances are calculated with both longitudinal and transverse components of the focused field of radially polarized light. Simulation results show that good superresolution performance can be obtained by the optimization method considering only the longitudinal component of the focused field of radially polarized light. Simulation results also show that such DSEs realize better superresolution performance with radially polarized light than with linearly polarized light.

Achromatic generation of radially polarized beams in visible range using segmented subwavelength metal wire gratings.

We propose an efficient method to achromatically transform a circularly polarized beam to a radially polarized beam in the visible range using segmented subwavelength metal wire gratings. We present a theoretical analysis of the relationship between the polarization purity of the transmitted beams and the number of segments. To verify our analysis, we fabricate a device composed of four quadrant sectors of subwavelength metal wire gratings and measure the transformation properties of the device at visible wavelengths of 488 nm, 532 nm, and 633 nm, which show a good agreement with theoretical results and the broadband achromatic property of the generation method.

Parallel confocal microscopy using high-order axially symmetric polarized beams.

A novel scheme of parallel confocal microscopy using high-order axially symmetric polarized beams (ASPBs) is proposed. The basic concept of ASPBs is introduced first, then the principle of the scheme is presented, finally some numerical results are shown to verify the feasibility of the scheme. Seen from the results, multiple imaging spots are obtained and the size of spots is about 70% of the spot size in the single lens microscopy, and a kind of high temporal and spatial resolution parallel confocal microscopy is achieved, which may find wide applications in the fields of 3D profile measurement and biomedical imaging.

Anterior segment optical coherence tomography evaluation of ocular graft-versus-host disease: a case study.

To explore ocular graft-versus-host disease (GVHD), anterior segment optical coherence tomography (AS-OCT) imaging of eyelids, tear meniscus, cornea and conjunctiva is performed in subsequent sessions on a patient who has ocular GVHD after allogeneic related donor stem cell transplant. The OCT results are presented together with those from a normal subject. OCT imaging is promising in visualizing several ocular GVHD manifestations, such as abnormal meibomian gland orifice (MGO), conjunctival keratinization, conjunctival hyperemia and chemosis, corneal epithelium opacification, thinning and sloughing. This case study demonstrates the capability of AS-OCT in the imaging and monitoring of ocular GVHD, which may be useful in the development of current ocular GVHD staging system and the clinical management for GVHD treatment.

Platform to investigate aqueous outflow system structure and pressure-dependent motion using high-resolution spectral domain optical coherence tomography.

The aqueous outflow system (AOS) is responsible for maintaining normal intraocular pressure (IOP) in the eye. Structures of the AOS have an active role in regulating IOP in healthy eyes and these structures become abnormal in the eyes with glaucoma. We describe a newly developed system platform to obtain high-resolution images of the AOS structures. By incorporating spectral domain optical coherence tomography (SD-OCT), the platform allows us to systematically control, image, and quantitate the responses of AOS tissue to pressure with a millisecond resolution of pulsed flow. We use SD-OCT to image radial limbal segments from the surface of the trabecular meshwork (TM) with a spatial resolution of ∼5 µm in ex vivo nonhuman primate eyes. We carefully insert a cannula into Schlemm's canal (SC) to control both pressures and flow rates. The experimental results demonstrate the capability of the platform to visualize the unprecedented details of AOS tissue components comparable to that delivered by scanning electron microscopy, as well as to delineate the complex pressure-dependent relationships among the TM, structures within the SC, and collector channel ostia. The described technique provides a new means to characterize the anatomic and pressure-dependent relationships of SC structures, particularly the active motion of collagenous elements at collector channel ostia; such relationships have not previously been amenable to study. Experimental findings suggest that continuing improvements in the OCT imaging of the AOS may provide both insights into the glaucoma enigma and improvements in its management.