Automatic generation of a holographically shaped beam in an actual optical system for use in material laser processing.

In-system optimization involves designing a computer-generated hologram (CGH) in an actual optical system. An important advantage of this approach is automatic generation of a target shaped beam with compensation for imperfections in the actual optical system that would degrade the reconstruction performance. We developed a novel in-system optimization method for beam shaping based on our previous research where it had been applied only to generate parallel focused beams. The key point in the application to beam shaping is to accurately express the conditions and coordinates of the actual optical system in the CGH calculation.

Computational broadband imaging with laser-driven sequential light source arrays on a water film.

Imaging and computational processing fusion technologies have expanded the wavelength range that can be visualized. However, it is still challenging to realize a system that can image a wide range of wavelengths, including non-visible regions, in a single system. Here, we propose a broadband imaging system based on femtosecond-laser-driven sequential light source arrays. The light source arrays allow us to form ultra-broadband illumination light depending on the excitation target and irradiated pulse energy. We demonstrated X-ray and visible imaging under atmospheric pressure by using a water film as an excitation target. Furthermore, by applying a compressive sensing algorithm, the imaging time was reduced while maintaining the number of pixels in the reconstructed image.

Low-coherence optical profilometry with multi-reflection reference mirrors for large-depth object measurement.

A new, to the best of our knowledge, optical configuration for digital holographic profilometry for surface profile measurement of large-depth objects is proposed. Two multi-reflection mirrors were employed to extend the maximum axial measurable range by a factor of 2 without any degradation of the spatial resolution. By adjusting the distance and the position of the two multi-reflection reference mirrors, the system can be made more flexible for measuring different parts of the object. In addition to the axial extension, the two-mirror system also increases the visibility of the interference fringes so that the object profile can be measured with high accuracy.

Noncontact vibration measurement using three spatial-frequency shifting coherent digital holography.

A new digital coherent holographic system that works as a spatial-frequency shifter for measuring three-dimensional (3D) vibration of an object is proposed. The spatial-frequency shifter is constructed by a system of three mirrors inclined with different small angles to shift the object wave to three different frequencies in the spatial-frequency domain. By applying the Fourier transform method and appropriate filters to the hologram recorded by the camera of the system, a three-phase set of object waves corresponding to three shifted frequencies was obtained. From the relation between the phases and the relative position of the object, the position of each point on the surface of the object along the x, y, and z directions was extracted from each hologram. The same process was repeatedly applied to a series of holograms recorded by a fast camera, allowing the 3D vibration of the object to be precisely observed.

Fundamental-ray aberration analysis of off-axial optical systems: analytical formulae of first-order aberrations.

In our previous paper [Appl. Opt.59, 4466 (2020)10.1364/AO.389600], we presented a fundamental-ray aberration analysis that extends ray matrix analysis to the third-order aberration region. The analysis results shown in that paper were applicable to coaxial rotationally symmetric optical systems. This time, we have extended the fundamental-ray aberration analysis so that it can be applied to off-axial optical systems. Here we present new analysis formulae for fundamental-ray aberration analysis of the first-order aberration region. In addition, we newly present first-order aberration expansion formulae for four-element fundamental-ray aberrations and calculation formulae for the fundamental-ray aberration coefficients of the first order, which are necessary for this extension.

Unconventional Imaging: introduction to the feature issue.

This feature issue of Applied Optics is dedicated to the international meeting of Information Photonics 2020 (IP'20), which was held September 11-12, 2020, in Taipei, Taiwan. IP'20 covered a broad range of topics, including advanced display techniques, optical computing, and optical storage. This feature issue, however, limits topics to unconventional imaging techniques, such as digital holography, artificial-intelligence associated imaging, compressive imaging, and single-pixel imaging.

Volumetric bubble display with a gold-nanoparticle-containing glycerin screen.

A key issue in the development of volumetric bubble displays whose voxels are femtosecond laser-excited bubbles is to enlarge the size of displayed graphics. In our previous research in which used glycerin as a screen, this size was less than several millimeters. To increase the size, it is important to reduce the excitation energy, because increasing the display size leads results in a larger focus volume due to the use of laser scanning optics with a low numerical aperture and requires more laser energy to excite the material. The use of gold nanoparticles in glycerin has been proposed as one way of reducing the excitation energy, because such materials are commercially available with controlled shapes, and consequently a controlled absorption spectrum. It was found that glycerin containing gold nanoparticles (GNPs), including gold nanospheres (GNSs) and gold nanorods (GNRs), reduced the pulse energy required for bubble generation compared with the use of pure glycerin. Larger GNSs resulted in a smaller threshold energy and, in particular, GNRs resulted in a threshold energy one-quarter that of pure glycerin. It was also found that the density had almost no effect on the threshold energy, but did affect the bubble generation probability. Finally, it was demonstrated that the bubble graphics with a size on the order of centimeters were rendered in GNR-containing glycerin.

Low-coherence digital holography with a multireflection reference mirror.

A method for expanding the measurement range of low-coherence digital holography up to several times longer than the coherence length is proposed. The method was implemented with a multireflection reference mirror composed of partially and highly reflective mirrors, in conjunction with the Fourier transform method with spatial filtering for single-shot complex amplitude imaging, making it useful for observing a moving and deforming object. One of the features of the reference arm is that the measurement range is simply controlled by adjusting the position and angle of the highly reflective mirror. The measurement of objects with a general curved shape and a large step height was demonstrated.

In-system optimization of a hologram for high-stability parallel laser processing.

A method for optimizing a computer-generated hologram (CGH) for high-stability laser processing is proposed. The CGH is optimized during laser processing; therefore, unpredicted dynamic changes in the laser processing system, in addition to its static imperfections, are automatically compensated for by exploiting the rewritable capability of the spatial light modulator. Consequently, the short-term and long-term stability are improved, which will contribute to the realization of high-speed, high-precision laser processing. A CGH that generated 36 parallel beams was continuously optimized, and the maximum uniformity reached 0.98, which is higher than reported in previous research. To the best of our knowledge, this is the first demonstration of gradual improvement of parallel laser processing with in-process optimization of the CGH. Furthermore, it was also demonstrated that the performance of the laser processing system against unexpected disturbances was improved.

Area-coding method in frequency comb profilometry fused with optical interferometry for measuring centimeter-depth objects with nanometer accuracy.

Area coding masks in a frequency comb profilometer (FCP) based on a single-pixel imaging architecture are introduced for measuring a practical metal object that has weaker reflection than a specular object does. In such a case, it is important to increase the intensity of the encoded object light on the photodetector area because a photodiode operated at a high frequency of more than 1 GHz is generally small. The area-coding masks can concentrate more light on the focal point compared with random-coding masks that are commonly used. The increased intensity also increases the number of pixels in the FCP, and consequently accurate matching is achieved between the data obtained by optical interferometry and the FCP data. It was demonstrated that the introduction of area-coding masks increased the detected light intensity and allowed us to measure a practical metal object with 16 times more sampling points.

Fundamental ray aberration analysis: extension of ray matrix analysis to the third-order region using a four-element fundamental ray vector.

We have developed a new fundamental ray aberration analysis that extends conventional ray matrix analysis to the third-order region using a four-element fundamental ray vector. This analysis method can analyze the factors in the generation of the Seidel aberration coefficients by separating them into the transform characteristics of rays unique to the optical system and paraxial trace values representing the conjugate relationship. In establishing this analysis, we first introduce the fundamental ray aberration, and we present calculation formulae for the fundamental ray aberration coefficients of a co-axial rotationally symmetric optical system. Numerical examples employing these analysis results are shown, and it is confirmed that the causes of the Seidel aberration coefficients can be analyzed.

Phototropin perceives temperature based on the lifetime of its photoactivated state.

Living organisms detect changes in temperature using thermosensory molecules. However, these molecules and/or their mechanisms for sensing temperature differ among organisms. To identify thermosensory molecules in plants, we investigated chloroplast positioning in response to temperature changes and identified a blue-light photoreceptor, phototropin, that is an essential regulator of chloroplast positioning. Based on the biochemical properties of phototropin during the cellular response to light and temperature changes, we found that phototropin perceives temperature based on the temperature-dependent lifetime of the photoactivated chromophore. Our findings indicate that phototropin perceives both blue light and temperature and uses this information to arrange the chloroplasts for optimal photosynthesis. Because the photoactivated chromophore of many photoreceptors has a temperature-dependent lifetime, a similar temperature-sensing mechanism likely exists in other organisms. Thus, photoreceptors may have the potential to function as thermoreceptors.

Complex-amplitude single-pixel imaging.

A single-pixel camera can be represented using complex-amplitude. The complex-amplitude representation of input signals and output signals enables us to perform complex-amplitude imaging of an object, particularly profilometry with reflectance measurements or quantitative phase imaging with transmittance measurements. The complex-amplitude representation of optical coding masks and the coherent addition that is performed by interference can directly represent Hadamard patterns having positive and negative values. Furthermore, the residual area of the mask can be used for a reference beam with phase shifting. Such a complex-amplitude imaging system with a single-beam line structure is highly stabile against external disturbances.

Three-dimensionally structured voxels for volumetric display.

A three-dimensional (3D) volumetric display has been the goal of the display research field for many years. However, volumetric displays capable of rendering multicolor and updatable graphics that users can view with the naked eye are still a challenge. Here, we show a new volumetric display using three-dimensionally structured fluorescent voxels. The fluorescent voxels were generated by two-photon excitation with a femtosecond laser. To realize colorization, volumetric graphics were spatially rendered on a fluorescent screen in which structured voxels having different luminescent colors were arranged in each layer. The color of the fluorescent voxels was changed by a holographic color switching method using computer-generated holograms displayed on a liquid-crystal spatial light modulator. Because this display employed RGB fluorescent voxels that are accessed optically, it has a number of advantages, such as being observable with the naked eye, and being capable of multicolor rendering and refreshable graphics. This technology will open up a wide range of applications in 3D displays, augmented reality, and computer graphics.

Three-dimensional stimulation and imaging-based functional optical microscopy of biological cells.

A new type of functional optical microscope system called three-dimensional (3D) stimulation and imaging-based functional optical microscopy (SIFOM) is proposed, to the best of our knowledge. SIFOM can precisely stimulate user-defined targeted biological cells and can simultaneously record the volumetric fluorescence distribution in a single acquisition. Precise and simultaneous stimulation of fluorescent-labeled biological cells is achieved by multiple 3D spots generated by digital holograms displayed on a phase-mode spatial light modulator. Single-shot 3D acquisition of the fluorescence distribution is accomplished by common-path off-axis incoherent digital holographic microscopy in which a diffraction grating with a focusing lens is displayed on another phase-mode spatial light modulator. The effectiveness of the proposed functional microscope system was verified in experiments using fluorescent microbeads and human lung cancer cells located at various defocused positions. The system can be used for manipulating the states of cells in optogenetics.

Decomposition method for fast computation of gigapixel-sized Fresnel holograms on a graphics processing unit cluster.

A parallel computation method for large-size Fresnel computer-generated hologram (CGH) is reported. The method was introduced by us in an earlier report as a technique for calculating Fourier CGH from 2D object data. In this paper we extend the method to compute Fresnel CGH from 3D object data. The scale of the computation problem is also expanded to 2 gigapixels, making it closer to real application requirements. The significant feature of the reported method is its ability to avoid communication overhead and thereby fully utilize the computing power of parallel devices. The method exhibits three layers of parallelism that favor small to large scale parallel computing machines. Simulation and optical experiments were conducted to demonstrate the workability and to evaluate the efficiency of the proposed technique. A two-times improvement in computation speed has been achieved compared to the conventional method, on a 16-node cluster (one GPU per node) utilizing only one layer of parallelism. A 20-times improvement in computation speed has been estimated utilizing two layers of parallelism on a very large-scale parallel machine with 16 nodes, where each node has 16 GPUs.

Combining phase images measured in the radio frequency and the optical frequency ranges.

Phase images measured in the radio frequency (RF) and optical frequency (OF) ranges, whose difference was about 4×105, were combined on the basis of a pattern matching method. RF phase imaging was implemented with an optical frequency-comb femtosecond laser and a single-pixel camera to measure a meter-order depth object with micrometer-order accuracy. OF phase imaging was implemented with an optical interferometer using a low-coherence femtosecond laser pulse to measure the profile with nanometer-order accuracy and high spatial resolution. Combining the images obtained from both phase measurement systems enabled profilometry of a large depth object with high lateral and axial resolutions.

Temperature-dependent signal transmission in chloroplast accumulation response.

Chloroplast photorelocation movement, well-characterized light-induced response found in various plant species from alga to higher plants, is an important phenomenon for plants to increase photosynthesis efficiency and avoid photodamage. The signal for chloroplast accumulation movement connecting the blue light receptor, phototropin, and chloroplasts remains to be identified, although the photoreceptors and the mechanism of movement via chloroplast actin filaments have now been revealed in land plants. The characteristics of the signal have been found; the speed of signal transfer is about 1 µm min-1 and that the signal for the accumulation response has a longer life and is transferred a longer distance than that of the avoidance response. Here, to collect the clues of the unknown signal substances, we studied the effect of temperature on the speed of signal transmission using the fern Adiantum capillus-veneris and found the possibility that the mechanism of signal transfer was not dependent on the simple diffusion of a substance; thus, some chemical reaction must also be involved. We also found new insights of signaling substances, such that microtubules are not involved in the signal transmission, and that the signal could even be transmitted through the narrow space between chloroplasts and the plasma membrane.

Massively parallel femtosecond laser processing.

Massively parallel femtosecond laser processing with more than 1000 beams was demonstrated. Parallel beams were generated by a computer-generated hologram (CGH) displayed on a spatial light modulator (SLM). The key to this technique is to optimize the CGH in the laser processing system using a scheme called in-system optimization. It was analytically demonstrated that the number of beams is determined by the horizontal number of pixels in the SLM NSLM that is imaged at the pupil plane of an objective lens and a distance parameter pd obtained by dividing the distance between adjacent beams by the diffraction-limited beam diameter. A performance limitation of parallel laser processing in our system was estimated at NSLM of 250 and pd of 7.0. Based on these parameters, the maximum number of beams in a hexagonal close-packed structure was calculated to be 1189 by using an analytical equation.

Enhanced intensity variation for multiple-plane phase retrieval using a spatial light modulator as a convenient tunable diffuser.

In the multiple-plane phase retrieval method, a tedious-to-fabricate phase diffuser plate is used to increase the axial intensity variation for a nonstagnating iterative reconstruction of a smooth object wavefront. Here we show that a spatial light modulator (SLM) can be used as an easily controllable diffuser for phase retrieval. The polarization modulation at the SLM facilitates independent formation of orthogonally polarized scattered and specularly reflected beams. Through an analyzer, the polarization states are filtered enabling beam interference, thereby efficiently encoding the phase information in the axially diverse speckle intensity measurements. The technique is described using wave propagation and Jones calculus, and demonstrated experimentally on technical and biological samples.