Focal length calibration of an electrically tunable lens by digital holography.

The electrically tunable lens (ETL) is a novel current-controlled adaptive optical component which can continuously tune its focus in a specific range via changing its surface curvature. To quantitatively characterize its tuning power, here we assume the ETL to be a pure phase object and present a novel calibration method to dynamically measure its wavefront by use of digital holographic microscopy (DHM). The least squares method is then used to fit the radius of curvature of the wavefront. The focal length is obtained by substituting the radius into the Zemax model of the ETL. The behavior curve between the focal length of the ETL and its driven current is drawn, and a quadratic mathematic model is set up to characterize it. To verify our model, an ETL and offset lens combination is proposed and applied to ETL-based transport of intensity equation (TIE) phase retrieval microscopy. The experimental result demonstrates the calibration works well in TIE phase retrieval in comparison with the phase measured by DHM.

Framework for gradient integration by combining radial basis functions method and least-squares method.

A framework with a combination of the radial basis functions (RBFs) method and the least-squares integration method is proposed to improve the integration process from gradient to shape. The principle of the framework is described, and the performance of the proposed method is investigated by simulation. Improvement in accuracy is verified by comparing the result with the usual RBFs-based subset-by-subset stitching method. The proposed method is accurate, automatic, easily implemented, and robust and even works with incomplete data.

Dynamic three-dimensional sensing for specular surface with monoscopic fringe reflectometry.

Dynamic full-field three-dimensional sensing of specular reflective surfaces can be conveniently implemented with fringe reflection technique. A monoscopic fringe reflectometric system can be adopted as a simple measuring setup. With the assistance of the windowed Fourier ridges method as an advanced fringe demodulation technique, only one cross grating is needed to reconstruct the three-dimensional surface shape changes. A suitable calibration enables determination of the actual three-dimensional surface profile. Experimental results of water wave variations are shown to demonstrate the feasibility of the proposed approach.

Tunable optofluidic aperture configured by a liquid-core/liquid-cladding structure.

Miniaturized and tunable optical components, such as the waveguide, lens, and prism, have been of great interest for lab-on-chip systems. This Letter reports an optofluidic aperture stop formed by the liquid-core/liquid-cladding flow. The aperture size can be tuned accordingly by adjusting the flow rates. Manipulation of the aperture size allows control of the amount of light passing through the corresponding optical system as well as the angular aperture on the image side. This optofluidic aperture enables lab-on-chip optical systems to have a greater flexibility and more functionalities.

Disposable flow cytometer with high efficiency in particle counting and sizing using an optofluidic lens.

Flow cytometers are widely applied to environmental monitoring, industrial testing, and biochemical studies. Integrating a flow cytometer into microfluidic networks helps to miniaturize the system and make it portable for field use. The integration of optical components, such as lenses, further improves the compactness and thus has been intensively studied recently. However, the current designs suffer from severe light scattering due to the roughness of the solid-based lens interface. In this Letter, we propose a flow cytometer using an optofluidic lens to focus the light beam. Benefiting from the smooth liquid-liquid lens interface and the refractive-index matching liquid as cladding streams, a light beam can be well focused without scattering. The variations of the signal peak values are reduced, owing to the small beam width at the beam waist. The device presents an efficient and accurate performance on both the counting and sizing of particles.

Tunable micro-optofluidic prism based on liquid-core liquid-cladding configuration.

The integration of optical components into microfluidic systems has the potential to reduce the amount of bulky external devices and thus reduce the cost. However, one of the challenges of this concept is the accurate alignment of the optical path among multiple optical components inside a chip. We propose a tunable micro-optofluidic prism based on the liquid-core liquid-cladding structure formed in a sector-shape chamber. The optical interface of the prism is maintained in a straight line shape by distributing a row of pressure barriers in the chamber. By adjusting the flow rate ratio between core and cladding streams, the apex angle of the prism can be tuned accordingly. As a consequence, the deviation angle of the light beam refracted by the prism can be changed continuously. This tunability of our optofluidic prism can be utilized for the alignment of the optical path inside a chip or for the development of optical switches.

Biconcave micro-optofluidic lens with low-refractive-index liquids.

One of the current problems of micro-optofluidics is the choice of a suitable liquid with a high refractive index (RI). We report the use of a low-RI liquid in a biconcave liquid-core liquid-cladding lens for focusing light. For the characterization of the lens, a telescope system was constructed from polydimethylsiloxane lenses to collimate and expand a light beam emitted from an optical fiber. The tunable optofluidic biconcave lens focuses the parallel beam. Fluorescent dye diluted in an index-matching liquid was used for the visualization of the light rays in a beam-tracing chamber. The focused beam is tuned by adjusting the flow rate ratio between core and cladding streams.

Modelling and optimization of micro optofluidic lenses.

This paper reports the modelling and experimental results of a liquid-core liquid-cladding optofluidic lens. The lens is based on three laminar streams in a circular chamber. The stream lines and the curvature of the interface can be predicted accurately using the theory of two-dimensional dipole flow in a circularly bounded domain. The model establishes basic relations between the flow rate ratio of the core/cladding streams and the radius of curvature and consequently the focal length of the lens. Compared to a rectangular chamber, this new circular design allows the formation of a liquid-core liquid-cladding lens with perfect curvatures. The circular design allows tuning a perfect curvature ranging from the chamber radius itself to infinity. The test device with a circular lens chamber with 1 mm diameter and 50 microm height was fabricated in PDMS. The lens shape as well as the stream lines were characterized using fluorescent dye and tracing particles. Experimental results agree well with the analytical results predicted by the model.

Pre-visual detection of iron and phosphorus deficiency by transformed reflectance spectra.

Reflectance spectroscopy and strategies for spectral analysis over the visible range from 380 to 780 nm were used to provide diagnostic information on iron (Fe) and phosphorus (P) status of Brassica chinensis L. var parachinensis (Bailey) grown under hydroponics conditions. Leaf reflectance (R) spectra were collected and normalized inner reflectance (NR(I)) spectra were calculated. The regression coefficients (B-matrix) and variable importance for projection (VIP) in partial least squares regression were used to determine important wavelengths that correlate with total chlorophyll (Chl) content. No single wavelength that showed good correlation with Chl content was found. Therefore, NR(I) was transformed into CIELAB color values, which simplified the whole visible spectrum into three values. Our results showed that upon Fe deprivation, plants entered into a deficiency state very rapidly, highlighting the importance of early diagnosis. The direct effect of Fe on leaf Chl content allowed CIELAB color values to be used for pre-visual detection of Fe deficiency 2 days before the appearance of visually distinguishable morphological changes. On the other hand, P-deprived plants showed a marked decline in cellular P levels but remained above critical threshold concentrations after 7 days. The Chl content was not affected by the leaf P content and CIELAB color values showed no difference with control plants.

Efficient implementation of a spatial light modulator as a diffractive optical microlens array in a digital Shack-Hartmann wavefront sensor.

A traditional Shack-Hartmann wavefront sensor (SHWS) uses a physical microlens array to sample the incoming wavefront into a number of segments and to measure the phase profile over the cross section of a given light beam. We customized a digital SHWS by encoding a spatial light modulator (SLM) with a diffractive optical lens (DOL) pattern to function as a diffractive optical microlens array. This SHWS can offer great flexibility for various applications. Through fast-Fourier-transform (FFT) analysis and experimental investigation, we studied three sampling methods to generate the digitized DOL pattern, and we compared the results. By analyzing the diffraction efficiency of the DOL and the microstructure of the SLM, we proposed three important strategies for the proper implementation of DOLs and DOL arrays with a SLM. Experiments demonstrated that these design rules were necessary and sufficient for generating an efficient DOL and DOL array with a SLM.