Single-shot wavelength-multiplexing for off-axis digital holography with a spectral filter.

We present a single-shot wavelength-multiplexing technique for off-axis digital holography based on a spectral filter. Only a spectral filter is inserted between beam splitter and mirror in reflection off-axis digital holography (RODH). The spectral filter can transmit a well-defined wavelength band of light, while reject other unwanted radiation. By adjusting the filter and mirror separately, the propagation orientation of different reference beams of two wavelengths can be separated, and thus two off- axis holograms with different fringe directions are simultaneously captured by a monochrome camera. The wavefront interference analysis of using a spectral filter is discussed in detail. Our scheme is available for real-time wavelength-multiplexing but requires fewer optical elements and system modifications. Numerical simulation and experiment results of different types of spectral filters demonstrate the validity of proposed method.

Ultrasensitive Biosensor with Hyperbolic Metamaterials Composed of Silver and Zinc Oxide.

We propose a hyperbolic metamaterial-based surface plasmon resonance (HMM-SPR) sensor by composing a few pairs of alternating silver (Ag) and zinc oxide (ZnO) layers. Aiming to achieve the best design for the sensor, the dependence of the sensitivity on the incidence angle, the thickness of the alternating layer and the metal filling fraction are explored comprehensively. We find that the proposed HMM-SPR sensor achieves an average sensitivity of 34,800 nm per refractive index unit (RIU) and a figure of merit (FOM) of 470.7 RIU-1 in the refractive index ranging from 1.33 to 1.34. Both the sensitivity (S) and the FOM show great enhancement when compared to the conventional silver-based SPR sensor (Ag-SPR). The underlying physical reason for the higher performance is analyzed by numerical simulation using the finite element method. The higher sensitivity could be attributed to the enhanced electric field amplitude and the increased penetration depth, which respectively increase the interaction strength and the sensing volume. The proposed HMM-SPR sensor with greatly improved sensitivity and an improved figure of merit is expected to find application in biochemical sensing due to the higher resolution.

Off-axis tilt compensation in common-path digital holographic microscopy based on hologram rotation.

We present a simple and effective compensation method for the off-axis tilt in common-path digital holographic microscopy (CPDHM) by introducing a rotating operation on the hologram. The proposed method mainly requires a digital reference hologram (DRH), which is a rotated version of the original one; it is assumed to be easy to obtain by rotating the specimen's hologram 180°. In this way, the off-axis tilt could be removed by subtracting the retrieved phase of DRH from the retrieved phase of the original hologram, but without any complex spectrum centering judgment, fitting procedures, or prior knowledge of the system. This highly automatic and efficient performance makes our approach available for real-time quantitative phase imaging (QPI), although it limits the field of view (FOV) of the specimen. Some experimental results of microlens array and phase plate demonstrate the feasibility and effectiveness of the proposed method.

Simple and flexible phase compensation for digital holographic microscopy with electrically tunable lens.

In a digital holographic microscopy (DHM) system, different microscope objectives (MOs) will introduce different phase distortions and thus lead to measurement errors. To address this problem, we present a simple and flexible method to compensate all phase distortions by introducing an electrically tunable lens (ETL) in the reference arm for a DHM system with multiple MOs. By exactly controlling the external currents of the ETL, we can change the reference wave front to match the wave front introduced by different MOs without complex alignment or additional numerical postprocessing manipulations. This method is suitable for quantitative real-time phase imaging especially when it refers to multiple MOs. To demonstrate the validity and effectiveness of our scheme, we did a series of simulations and carried out some real experiments with two different MOs (4× and 10×).

Improved performance of multi-view fringe projection 3D microscopy.

Fringe projection 3D microcopy (FP-3DM) plays an increasingly important role in micro manufacturing and measurement. In recent decades, research on FP-3DM has made considerable progress. Nevertheless, some disadvantages arising from the limited depth of field, local specular reflection and occlusion still exist and need to be further addressed. In this paper, a multi-view FP-3DM (MVFP-3DM) is presented. Four imaging branches with the Scheimpflug condition and one vertical projection branch are deployed to establish the system. The system is described with a general imaging model, which is independent of the system configuration. In system calibration, the edge of binary fringe is used to locate the benchmark, which takes advantage of the fact that the edge will keep its position whether it is in focus or out of focus. Furthermore, a group of experiments prove that our proposed MVFP-3DM system can extend measurable range in depth, improve precision in 3D reconstruction and reduce occlusion.

Suppression of projector distortion in phase-measuring profilometry by projecting adaptive fringe patterns.

In phase-measuring profilometry, the lens distortion of commercial projectors may introduce additional bending carrier phase and thus lead to measurement errors. To address this problem, this paper presents an adaptive fringe projection technique in which the carrier phase in the projected fringe patterns is modified according to the projector distortion. After projecting these adaptive fringe patterns, the bending carrier phase induced by the projector distortion is eliminated. Experimental results demonstrate this method to be effective and efficient in suppressing the projector distortion for phase-measuring profilometry. More importantly, this method does not need to calibrate the projector and system parameters, such as the distortion coefficients of the projector and the angle between the optical axes of projector and camera lenses. Hence, it has low computational complexity and enables us to improve the measurement precision for an arbitrary phase-measuring profilometry system.

Distortion correction for microscopic fringe projection system with Scheimpflug telecentric lens.

To increase the measurement range of 3D microscopy, Scheimpflug adjustment, in which the imaging plane is tilted with respect to the telecentric lens plane, is often employed. However, the inclined imaging plane will introduce certain distortion to the captured image, which further affects the accuracy of the 3D reconstruction result. In this paper, a distortion model was derived based on the geometric optics theory. With it, the imaging distortion caused by the Scheimpflug condition can be effectively corrected. Experimental results will be presented to demonstrate the feasibility and validity of the proposed method.