Generation of arbitrary vector vortex beams on a higher-order Poincaré sphere using a double-exposure polarization-multiplexed hologram.

The existing methods for the generation of arbitrary vector vortex beams often involve complex optical setups or intricate fabrication methods. In this Letter, we propose a novel, to the best of our knowledge, and simplified approach for the efficient generation of vector vortex beams using a polarization-multiplexed hologram fabricated on an azo-carbazole polymer using a simple double-exposure technique. The hologram generates a vector vortex beam when simply illuminated by a collimated beam and also allows for a seamless traversal across the entire higher-order Poincaré sphere (arbitrary vortex beam generation) just by modulating the polarization of an illuminating beam.

Multiple motion picture recording in light-in-flight recording by holography with an angular multiplexing technique.

Light-in-flight recording by holography (LIF holography) is an ultrafast imaging technique for recording light pulse propagation as a motion picture. In this study, we propose and demonstrate multiple motion picture recordings of light pulse propagation by use of LIF holography with angular multiplexing. We set incident angles of reference light pulses to remove the difficulty in adjusting the optical path length difference between an object light pulse and reference light pulses and the complexity of the optical system. In the experiment, by using LIF holography with angular multiplexing, we succeeded in recording a propagating light pulse as two motion pictures with durations of 129.6 ps without an inseparable superimposition of the reconstructed images. In addition, cross talk between the recorded images, noise caused by cross-terms in an image plane, and the number of motion pictures that can be recorded are discussed.

Three-dimensional dynamic measurement of unstable temperature fields by multi-view single-shot phase-shifting digital holography.

A multi-view phase measurement system based on single-shot phase-shifting digital holography is proposed to dynamically obtain three-dimensional (3-D) information of an unstable temperature field. The proposed system consists of a laser, three polarization imaging cameras, and the corresponding optical components. The laser beam emitted from the laser is separated by the fibers into three pairs that contain three object beams and three reference beams. The object beams pass through the object in three different directions and interfere with the reference beams at the image sensor plane respectively. The recording of the three cameras is triggered simultaneously, which enables the phase measurement of dynamic objects from different viewpoints. We successfully measured the 3-D distributions of an unstable temperature field in the experiments with the proposed system.

Ultrafast double motion-picture recording technique for propagating light pulses with an ultrashort time difference.

Ultrafast imaging techniques involving light propagation, which can record light-pulse propagation as a motion picture, are commonly applied in various fields. However, conventional ultrafast imaging techniques cannot obtain multiple motion- pictures with an ultrashort time difference. In this Letter, we propose an imaging technique to obtain double motion-pictures of propagating light pulses with an ultrashort time difference. To record double motion-pictures of propagating light pulses without superposition of the motion pictures, we employ a space-division multiplexing technique for recording holograms. Also, we construct and introduce an optical delay setup for generating light pulses with an ultrashort time difference. In the experimental demonstration of the proposed technique, we observe the first and second light propagations for a duration of 6.9 ps with a 1.78-ps time difference.

FFT-based simulation of the hologram-recording process for light-in-flight recording by holography.

We propose a numerical simulation method of the hologram-recording process for light-in-flight recording by holography (LIF holography) based on fast Fourier transform (FFT) to improve the efficiency of the simulation. Because it is crucial to consider the difference in the optical-path length between the object and reference light pulses, we modify a point-spread function by considering the optical-path lengths of the object and reference light pulses and whether both pulses interfere with each other in LIF holography. The computational time was shortened by 5.5×105 times for the 4,096×4,096 resolution of the hologram using the proposed method. We evaluate the proposed method by calculating the root mean square error (RMSE) of the reconstructed holographic images. The RMSEs were relatively small considering the effect of speckle noise; these results effectively demonstrate the validity of the proposed method. Moreover, we reconstruct the moving pictures of light pulse propagation from the hologram generated by the proposed method. We compare the simulation and experimental results, and succeed in qualitatively demonstrating the validity of the proposed method.

Motion picture of magnified light pulse propagation with extending recordable time of digital light-in-flight holography.

We experimentally demonstrate a motion picture imaging technique that can record a magnified image of light pulse propagation with extending the recordable time of digital light-in-flight recording by holography. We constructed an optical system that achieves a recordable time extension and an observation of a magnified image of light pulse propagation. As a result, we experimentally succeeded in recording light pulse propagation with a 7.45 magnification rate with extending the recordable time. The recordable time of the motion picture was 714 fs, which is twice that of the conventional one.

Simultaneous imaging of sound propagations and spatial distribution of acoustic frequencies.

We propose a simultaneous imaging technique of both sound propagations and spatial distribution of acoustic frequencies. We experimentally demonstrated the proposed technique for the acoustic waves of frequencies 39,500 and 40,500 Hz, which have close sound pressure. The sounds were recorded at the framerate of 100,000 fps by parallel phase-shifting digital holography. To obtain the distribution of the acoustic frequencies, the short-time Fourier transform analysis was applied. The simultaneous imaging was carried out by assigning the frequencies and the pixel values of the phase-difference images to the components of HSL color space. The images obtained by the proposed technique represent the frequencies with the hue in addition to the sound propagations with the luminance. We succeeded in imaging the spatiotemporal evolution of the spatial frequencies of the sounds.

Influence of the lateral size of a hologram on the reconstructed image in digital light-in-flight recording by holography.

Digital light-in-flight recording by holography is a promising technique for observing a propagating ultrashort light pulse as a motion picture. A typical reconstruction process of digital light-in-flight recording by holography, we extract holograms without considering the relationship between the lateral size of the extracted hologram (sub-hologram) and the size of an area where the propagating ultrashort light pulse and an image sensor overlap. The area records the image of the ultrashort light pulse at a certain moment. In this study, by considering the size of the small interference fringe image, we assessed the influence of the lateral size of the hologram on the reconstructed image. We defined the size of the area in which the interference fringe image at a moment is recorded. Then, we examined the reconstructed images by changing the lateral size of the sub-hologram. As a result, we found that the lateral size of the hologram does not affect the size of the reconstructed image but the spatial resolution of the reconstructed images.

Single-shot common-path off-axis digital holography: applications in bioimaging and optical metrology [Invited].

The demand for single-shot and common-path holographic systems has become increasingly important in recent years, as such systems offer various advantages compared to their counterparts. Single-shot holographic systems, for example, reduce computational complexity as only a single hologram with the object information required to process, making them more suitable for the investigation of dynamic events; and common-path holographic systems are less vibration-sensitive, compact, inexpensive, and high in temporal phase stability. We have developed a single-shot common-path off-axis digital holographic setup based on a beam splitter and pinhole. In this paper, we present a concise review of the proposed digital holographic system for several applications, including the quantitative phase imaging to investigate the morphological and quantitative parameters, as a metrological tool for testing of micro-optics, industrial inspection and measurement, and sound field imaging and visualization.

Multimodal sound field imaging using digital holography [Invited].

Sound is an important invisible physical phenomenon that needs to be explained in several physical and biological processes, along with visual phenomena. For this purpose, multiparameter digital holography (DH) has been proposed to visualize both features simultaneously due to the phase and amplitude reconstruction properties of DH. In this paper, we present a brief review on sound field imaging techniques with special focus on the multiparameter imaging capability of DH for visualizing sound and visual features. The basic theory and several experimental results with very high-speed recordings are also presented to demonstrate sound field imaging for the audible range as well as in the ultrasound range.

High-speed imaging of the sound field by parallel phase-shifting digital holography.

Sound field imaging techniques have been found very useful for acoustic designs. Building on this idea, innovative techniques are needed and presented in this paper, where we report on developed imaging of the sound field radiated from speakers by parallel phase-shifting digital holography. We adopted an ultrasonic wave radiated from a speaker for an object. The phase distribution of the light wave was modulated by the sound field radiated from the speaker. The modulated phase distribution was recorded in the form of multiplexed phase-shifted holograms at the frame rate of 100,000 fps. A 40,000 Hz sound field radiated from a speaker is used as an observation target. Our proposed method can implement the imaging of the sound field successfully. Also, in order to demonstrate the digital refocusing capability of digital holography, we set two speakers, whose difference in depth positions was 6.6 cm, as a long-depth object. We demonstrated the digital refocusing on the two speakers along with the capability of measuring the positions of the objects. Furthermore, we succeeded in imaging of 40,000 Hz and 41,000 Hz sound fields radiated from the two speakers. The presented experimental results showed that parallel phase-shifting digital holography is very useful and suitable for sound field imaging.

Single-shot common-path off-axis dual-wavelength digital holographic microscopy.

A single-shot common-path off-axis self-interference dual-wavelength digital holographic microscopic (DHM) system based on a cube beam splitter is demonstrated to expand the phase range in a stepped microstructure and for simultaneous measurement of the refractive index and physical thickness of a specimen. In the system, two laser beams with wavelengths of 532 nm and 632.8 nm are used. These laser beams are combined to transilluminate the object under study, then the object beam is divided into two beams by using a beam splitter oriented in such a way that both the beams propagate in almost the same direction, with an appropriate lateral separation between them. One of the object beams is spatially filtered at its Fourier plane, using a pinhole to generate a reference spherical beam free from the object information. The reference beam interferes with the object beam to form a digital hologram at the faceplate of the image sensor. The phase information is extracted from a single recorded digital hologram using the phase aberration compensation method that is based on principal component analysis (PCA). Owing to the common-path configuration, the system shows high temporal phase stability and it is less vibration-sensitive compared to counterparts such as a Mach-Zehnder type DHM. The performance of the dual-wavelength DHM system is verified in two different application fields by conducting the experiments using microsphere beads and living plant cells.

Holographic multi-parameter imaging of dynamic phenomena with visual and audio features.

In this Letter, a concept of new multi-parameter imaging that can acquire visual and audio data of dynamic object phenomena simultaneously by a holographic technique is proposed. Temporal intensity distributions give us visual information of the dynamic events. The temporal profile of the phase distribution can give different information of the dynamic events, such as audio data. These two physical data can express the dynamic events with multi-parameters in various dimensions. The proposed imaging approach has potential in several applications in physics and life science research. Two experimental demonstrations such as static object and broken glass with visual and sound features are provided to show the effectiveness of the proposed concept.

Spatiotemporal observations of light propagation in multiple polarization states.

Real-time imaging techniques involving light propagation are commonly applied in the fields of physics, chemistry, and biomedicine. However, conventional techniques provide only the intensity change associated with light propagation. Here, we propose an imaging technique to visualize the ultrafast behavior of the polarization state of a propagating light pulse with four different linear polarization components. This approach provides ultrahigh temporal resolution to observe the light in motion. We recorded a motion picture of a three-dimensional image of a light pulse propagating through a diffuser and a calcite crystal at 56.8 and 75.4 ps, respectively. This technique can contribute to revealing the polarization state of propagating light pulses in a medium and ultrafast phenomenon.

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.

Extending recordable time of light-in-flight recording by holography with double reference light pulses.

Light-in-flight (LIF) recording by holography is a powerful technique for observing ultrashort light pulse propagation. However, the recordable time of the technique has been limited by the lateral length of the holographic plate. Then, to extend the recordable time of LIF recording by holography, we proposed a space-division multiplexing technique of holograms, which divides the holographic plate longitudinally and uses double reference light pulses. We experimentally demonstrated that the recordable time becomes twice as long as before for the first time, to the best of our knowledge, using the proposed technique. Specifically, we recorded the motion picture of the ultrashort light pulse propagation for 236 ps.

Three-dimensional imaging of distribution of refractive index by parallel phase-shifting digital holography using Abel inversion.

Although digital holography is a powerful technique obtaining a phase image of a transparent object, the image reconstructed by the technique merely expresses phase distribution of the light wave after transmitting through the object. Phase variation of inside of the object is difficult to be obtained. Then, we applied Abel inversion method to the high-speed phase image of a dynamic transparent object assumed axially symmetric. The phase is accurately recorded by phase-shifting method. We experimentally recorded transparent dynamic gas flow, assumed axially symmetric along the direction in which gas flowed, at 3,000 frame/s and reconstructed motion picture of 3D distribution of the refractive index of the gas from the high-speed phase motion picture obtained by parallel phase-shifting digital holography.

Single-shot incoherent digital holography using a dual-focusing lens with diffraction gratings.

A new optical configuration of incoherent digital holography is presented to improve the quality of reconstructed images when the random polarization state of incoherent light is used. The proposed system improves the signal-to-noise ratio of the holograms by suppressing the unmodulated terms of a spatial light modulator. To generate the self-interference of a quasi-incoherent point-like source, we use a dual-focusing lens with diffraction gratings. The preliminary experimental results confirm the validity of the proposed method by reconstructing two point-like sources generated by a LED light source. When the pixel pitch of the phase-mode SLM is small enough, the off-axis hologram can be generated. The single-shot recording of the incoherent digital holography is expected.

Image recovery from defocused 2D fluorescent images in multimodal digital holographic microscopy.

A technique of three-dimensional (3D) intensity retrieval from defocused, two-dimensional (2D) fluorescent images in the multimodal digital holographic microscopy (DHM) is proposed. In the multimodal DHM, 3D phase and 2D fluorescence distributions are obtained simultaneously by an integrated system of an off-axis DHM and a conventional epifluorescence microscopy, respectively. This gives us more information of the target; however, defocused fluorescent images are observed due to the short depth of field. In this Letter, we propose a method to recover the defocused images based on the phase compensation and backpropagation from the defocused plane to the focused plane using the distance information that is obtained from a 3D phase distribution. By applying Zernike polynomial phase correction, we brought back the fluorescence intensity to the focused imaging planes. The experimental demonstration using fluorescent beads is presented, and the expected applications are suggested.

Optical voice recorder by off-axis digital holography.

An optical voice recorder capable of recording and reproducing propagating sound waves by using off-axis digital holography, as well as quantitative visualization, is presented. Propagating sound waves temporally modulate the phase distribution of an impinging light wave via refractive index changes. This temporally modulated phase distribution is recorded in the form of digital holograms by a high-speed image sensor. After inverse propagation using Fresnel diffraction of a series of the recorded holograms, the temporal phase profile of the reconstructed object wave at each three-dimensional position can be used to reproduce the original sound wave. Experimental results using a tuning fork vibrating at 440 Hz and a human voice are presented to show the feasibility of the proposed method.