Integrated optical-disk pickup that uses a focusing grating coupler with a high numerical aperture

An integrated optical-disk pickup that uses a focusing grating coupler with a numerical aperture of 0.45 (a focal length of 1.0 mm and an aperture of 1.0 mm × 0.8 mm) was developed, and the readout signal on an International Standards Organization (ISO) -formatted 90-mm optical disk was detected by the pickup. The signal-amplitude ratio of the minimum-limit data pattern to the maximum was 0.53 on the innermost track. It meets the requirement on the signal resolution for optical-disk storage devices.     

Polarization-dependent circular Dammann grating made of azo-dye-doped liquid crystals

A polarization-dependent circular Dammann grating (CDG) was generated from an azo-dye-doped liquid crystal (LC) cell. A simple multiexposure photo-alignment process was used to fabricate a binary phase LC CDG zone plane, which was composed of an odd zone with a twisted nematic LC structure and an even zone with a homogenous LC structure. A two-order CDG with equal-intensity rings was produced through a Fourier transform. The maximum zeroth and first diffraction orders of obtained CDG can be separately achieved by rotating the analyzer's polarization direction. The CDG using an azo-dye-doped LC cell can be used to generate diffractions by lasers in a broad wavelength range, hence expanding possible device applications.     

Three-dimensional complex image coding using a circular Dammann grating

Recently, optical image coding using a circular Dammann grating (CDG) has been proposed and investigated. However, the proposed technique is intensity based and could not be used for three-dimensional (3D) image coding. In this paper, we investigate an optical image coding technique that is complex-amplitude based. The system can be used for 3D image coding. The complex-amplitude coding is provided by a circular Dammann grating through the use of a digital holographic recording technique called optical scanning holography. To decode the image, along the depth we record a series of pinhole holograms coded by the CDG. The decoded reconstruction of each depth location is extracted by the measured pinhole hologram matched to the desired depth. Computer simulations as well as experimental results are provided.     

Use of the circular Dammann grating in angle measurement

We describe a novel method of angle measurement by borrowing the concept of the circular Dammann grating (CDG). A three-order CDG is employed in this experiment. The displacement of the tilted angle can be determined accurately by measuring the projection from the distorted CDG image. This method is controlled only by the initial radius of the image and the converging ratio of the lens. Compared with conventional techniques, this technique has the advantages of a simple design with superior resolution to within 1 degree, low cost, and compactness. A theoretical analysis together with experimental results is presented.     

Simple focal-length measurement technique with a circular Dammann grating

A novel technique for focal-length measurements with a circular Dammann grating is presented. In the back focal plane of the lens under test, a one-order circular Dammann grating with limited aperture will produce double-humped radial rings. The separation between the two lobes varies with the displacement of the observed plane from the focal plane of the lens. By searching for the position at which the separation is minimal, the focal point of the lens can be located and hence the back focal length can be determined. Experimental results demonstrated that this method is efficient and can be used effectively for a quick check of focal length.     

Structured light spots projected by a Dammann grating with high power efficiency and uniformity for optical sorting

We report a method for microfluidic multiple trapping and continuous sorting of microparticles using an optical potential landscape projected by a Dammann grating, enabling a high power-efficient approach to forming a composite two-dimensional spots array with high uniformity. The Dammann grating is fabricated in a photoresist by optical lithography. It is employed to create an optical lattice for multiple optical trapping and sorting in a mixture of polymer particles (n = 1.59) and silica particles (n = 1.42) with the same diameters of 3.1 μm. In addition to the exponential selectivity by the projected optical landscapes, the proposed microfluidic sorting system has advantages in terms of high power efficiency and high uniformity due to the Dammann grating.     

Synthetic aperture focusing technique signal processing

The synthetic aperture focusing technique (SAFT) is briefly reviewed and addressed as a heuristic digital ultrasonic imaging scheme which exploits the idea of back-propagating a set of measured and digitally stored A-scans. It is shown that for a far-field experimental set-up, ie for small, isolated defects remote to the transducer, SAFT reduces to the filtered back-projection imaging scheme which is well known within the framework of conventional X-ray computer tomography. Therefore, alternative data processing via Fourier transforms only, similar to the Fourier slice theorem of tomography, is possible, which sheds considerable light upon the heuristic SAFT pixel-space envelope-detection scheme. The resulting imaging identity has been termed POFFIS (physical optics far-field inverse scattering). The far-field assumption is then omitted yielding a Fourier-transform-SAFT algorithm (FT-SAFT) whose results are identical to back-propagation imaging with the definite advantage of fast processing capabilities based upon standard hardware and allowing immediate implementation of high resolution procedures as well as inclusion of mode-conversion effects; the theoretical background is pulse-echo diffraction tomography. The above results are supported and illustrated by application of all three algorithms — SAFT, POFFIS, FT-SAFT — to experimental data obtained from scanning a line aperture for several test specimens.     

Two-dimensional wavelet transform achieved by computer-generated multireference matched filter and Dammann grating

A two-dimensional wavelet transform is optically performed in real time by use of a new multichannel system that processes the different daughter wavelets separately. The system, which is able to handle every wavelet function, relies on a Dammann grating for generating a multichannel array. All channels are processed in parallel by a conventional two-dimensional correlator. Experimental results applying Morlet-wavelet decomposition are presented.     

Surface plasmon resonance imaging using a high numerical aperture microscope objective

We designed, constructed, and tested a surface plasmon resonance (SPR) microscope using a high numerical aperture objective from a commercially available inverted optical microscope. Such a configuration, combined with various methods to shorten the surface plasmon propagation length, achieves diffraction-limited spatial resolution in the transverse direction and near-diffraction-limited resolution in the longitudinal direction. A virtue of the objective-type SPR imaging is that we achieve distortion-free angle-resolved SPR imaging, allowing the angle-dependent reflectivity of the sample to be examined on a pixel-by-pixel basis, thus offering high-resolution information about surface properties.     

Splitting of femtosecond laser pulses by using a Dammann grating and compensation gratings

When a Dammann grating is used to split a beam of femtosecond laser pulses into multiple equal-intensity beams, chromatic dispersion will occur in beams of each order of diffraction and with different scale of angular dispersion because the incident ultrashort pulse contains a broad range of spectral bandwidths. We propose a novel method in which the angular dispersion can be compensated by positioning an m-time-density grating to collimate the mth-order beam that has been split, producing an array of beams that are free of angular dispersion. The increased width of the compensated output pulses and the spectral walk-off effect are discussed. We have verified this approach theoretically and validated it through experiments. It should be highly interesting in practical applications of splitting femtosecond laser pulses for pulse-width measurement, pump-probe measurement, and micromachining at multiple points.     

Three-dimensional sensing by using a lenslet, a Dammann grating, and a combination

In the last two decades, three-dimensional sensors based on misfocusing have been suggested. This research addresses the question of measuring several focal depths simultaneously. Two options for generating the necessary array of spots are analyzed: the use of a lenslet array and the use of a Dammann grating. Finally, a combination of the two approaches is proposed. Such a combination enables tailoring the system performance to the exact needs of the user.     

Coupling characteristics of laser diodes to high numerical aperture thermally expanded core fibers

We numerically calculate the coupling loss coefficients of Laser Diode (LD) to high numerical aperture (HNA) thermally expanded core (TEC) fiber coupling in terms of lateral, longitudinal and angular misalignments. The propagating fields in the HNA-TEC fiber are obtained by Galerkin’s method and the coupling loss coefficients are calculated by the overlap integral technique. Our numerical results are compared with recently available experimental data. It is demonstrated that, in HNA-TEC fibers the lateral and longitudinal tolerances were effectively enlarged in comparison with those of conventional SMF-28. It is also found that TEC fibers are good candidates for free-space LD-to-fiber optical coupling scheme with long working distance. However, the angular tolerance of the HNA-TEC fiber is lower than that of the SMF-28.     

Coupling characteristics of laser diodes to high numerical aperture thermally expanded core fibers

We numerically calculate the coupling loss coefficients of Laser Diode (LD) to high numerical aperture (HNA) thermally expanded core (TEC) fiber coupling in terms of lateral, longitudinal and angular misalignments. The propagating fields in the HNA-TEC fiber are obtained by Galerkin’s method and the coupling loss coefficients are calculated by the overlap integral technique. Our numerical results are compared with recently available experimental data. It is demonstrated that, in HNA-TEC fibers the lateral and longitudinal tolerances were effectively enlarged in comparison with those of conventional SMF-28. It is also found that TEC fibers are good candidates for free-space LD-to-fiber optical coupling scheme with long working distance. However, the angular tolerance of the HNA-TEC fiber is lower than that of the SMF-28.     

Optical head employing a concentric-circular focusing grating coupler

An optical head employing a concentric-circular grating coupler (CGC) and a concentric-circular focusing grating coupler (CFGC) is proposed, and its operating principle and characteristics are reported. Satisfaction with a prerequisite for the head, i.e., the removal of aberrations caused by deviations in wavelength and the effective index, is theoretically achieved by application of the concept of optimization of an annular aperture. With CGC and CFGC fabricated by an electron-beam-writing method, we experimentally confirmed its fundamental characteristics of light input, waveguiding, output, and convergence, with an elliptical focusing spot converging at half-intensity widths of 1.8 and 4.0μm.     

Theories for the design of a hybrid refractive-diffractive superresolution lens with high numerical aperture

By geometrical optics and the Rayleigh-Sommerfeld diffraction formula, theories for the design of a hybrid refractive-diffractive superresolution lens (HRDSL) with high numerical aperture are constructed. Differences between the profile of the diffractive superresolution element (DSE) with high numerical aperture and that with low numerical aperture are indicated. Optimization theory can obtain a globally optimal solution through a linear programming much more simplified than the corresponding one in Liu et al. [J. Opt. Soc. Am. A 19, 2185 (2002)]. The rules of the structure of the designed DSE are both theoretically proved and numerically verified. Comparison of this optimization theory with the other design theories and examples of designing the HRDSL with high numerical aperture are provided. Last, some limits of optical superresolution with high numerical aperture are set and compared with those for low numerical aperture.     

Three-dimensional imaging using a frequency-domain synthetic aperture focusing technique

A frequency-domain method for implementing the synthetic aperture focusing technique is developed and demonstrated using computer simulation. As presented, the method is well suited to reconstructing ultrasonic reflectivity over a volumetric region of space using measurements made over an adjacent two-dimensional aperture. Extensive use is made of both one- and two-dimensional Fourier transformations to perform the temporal and spatial correlation required by the technique, making the method well suited to general-purpose computing hardware. Results are presented demonstrating both the lateral and axial resolution achieved by the method. The effect of limiting the reconstruction bandwidth is also demonstrated.     

Effects influencing focusing in synthetic aperture vector flow imaging

Previously, a synthetic aperture vector velocity estimation method was proposed. Data are beamformed at different directions through a point, where the velocity is estimated. The flow direction is estimated by a search for the direction where the normalized cross-correlation peaks and the velocity magnitude along this direction are found. In this paper, different effects that influence the focusing in this method are investigated. These include the effect of phase errors in the emitted spherical waves, motion effects, and the effect of various interpolation methods in beam-forming. A model based on amplitude drop and phase error for spherical waves created using the virtual source concept is derived. This model can be used to determine the opening angle of a virtual source. Simulations for different virtual source placements are made, and it is recommended that the virtual sources be placed behind the aperture when shallow structures are imaged, and when deeper-lying structures are imaged the virtual sources be placed in front of the aperture. Synthetic aperture methods involve summation of data from numerous emissions. Motion between these emissions results in incoherence and affects resolution, contrast, and the signal-to-noise ratio. The effects of motion on the synthetic aperture vector velocity estimation method are investigated, and it is shown that for both axial and lateral motion, the contrast and signal-to-noise ratio can be seriously affected. A compensation method using the previous vector velocity estimate, when new data are beamformed, is implemented and tested. It is shown from a number of flow phantom experiments that a significant improvement with respect to bias and standard deviation of the velocity estimates can be obtained by using this compensation. Increased performance is gained at the expense of computation time. Different interpolation methods can be used for beam-forming the data. In this paper, the velocity estimation performance using various more complex interpolation schemes are compared to that using linear interpolation. No significant difference in the performance of the method is seen when other interpolation methods are used.