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.

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.

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.

Back-light color holography system using a reflection hologram.

We propose a system that utilizes reflection holograms to realize color holography that illuminates from the back site. In this configuration, two types of polarizers and two quarter-wave plates are used. By combining these elements and controlling the polarization direction of the transmitted beam that passes through them, it is possible to avoid the drawbacks of the conventional method of illuminating from the front. The reconstructed image has a wide viewing angle, and a clear color image can be observed. The effectiveness of this configuration in the proposed color holography system is discussed with respect to the dependence of the polarization characteristics of the elements on the angle of incidence. We also constructed a benchtop prototype with this configuration and evaluated its effectiveness.

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.

Superresolution of interference fringes in parallel four-step phase-shifting digital holography.

A superresolution method for interference fringes obtained by parallel four-step phase-shifting digital holography is proposed. A complex amplitude distribution of an object wave is derived from a recorded hologram by parallel phase-shifting interferometry using two pixels without any interpolation procedures. Multiple distributions are derived by changing one of the two pixels when conducting phase-shifting interferometry. The angular spectrum distribution of the object wave is obtained by both the Fourier transforms and synthesis of the spectrum distribution from the Fourier-transformed images in the spatial frequency domain. Available space bandwidth is extended to half of that of an image sensor.

Space-bandwidth extension in parallel phase-shifting digital holography using a four-channel polarization-imaging camera.

We propose a method for extending the space bandwidth (SBW) available for recording an object wave in parallel phase-shifting digital holography using a four-channel polarization-imaging camera. A linear spatial carrier of the reference wave is introduced to an optical setup of parallel four-step phase-shifting interferometry using a commercially available polarization-imaging camera that has four polarization-detection channels. Then a hologram required for parallel two-step phase shifting, which is a technique capable of recording the widest SBW in parallel phase shifting, can be obtained. The effectiveness of the proposed method was numerically and experimentally verified.

Multiwavelength parallel phase-shifting digital holography using angular multiplexing.

We propose a single-shot digital holography for recording multiwavelength and complex amplitude information by using a single monochromatic image sensor. The zeroth-order wave and conjugate image in each wavelength are removed from a recorded single hologram by applying parallel phase-shifting interferometry. Angular multiplexing is utilized to record the complex amplitude of an object wave in each wavelength separately, and no color filter is required. The effectiveness of the proposed technique was experimentally verified.

Image reconstruction algorithm for recovering high-frequency information in parallel phase-shifting digital holography [Invited].

We propose an image reconstruction algorithm for recovering high-frequency information in parallel phase-shifting digital holography. The proposed algorithm applies three kinds of interpolations and generates three different kinds of object waves. A Fourier transform is applied to each object wave, and the spatial-frequency domain is divided into 3×3 segments for each Fourier-transformed object wave. After that the segment in which interpolation error is the least among the segments having the same address of the segment in the spatial-frequency domain is extracted. The extracted segments are combined to generate an information-enhanced spatial-frequency spectrum of the object wave, and after that the formed spatial-frequency spectrum is inversely Fourier transformed. Then the high-frequency information of the reconstructed image is recovered. The effectiveness of the proposed algorithm was verified by a numerical simulation and an experiment.

Algorithm for reconstructing wide space-bandwidth information in parallel two-step phase-shifting digital holography.

We propose an image-reconstruction algorithm of parallel phase-shifting digital holography (PPSDH) which is a technique of single-shot phase-shifting interferometry. In the conventional algorithms in PPSDH, the residual 0th-order diffraction wave and the conjugate images cannot be removed completely and a part of space-bandwidth information is discarded. The proposed algorithm can remove these residual images by modifying the calculation of phase-shifting interferometry and by using Fourier transform technique, respectively. Then, several types of complex amplitudes are derived from a recorded hologram according to the directions in which the neighboring pixels used for carrying out the spatial phase-shifting interferometry are aligned. Several distributions are Fourier-transformed and wide space-bandwidth information of the object wave is obtained by selecting the spectrum among the Fourier-transformed images in each region of the spatial frequency domain and synthesizing a Fourier-transformed image from the spectrum.

Single-shot femtosecond-pulsed phase-shifting digital holography.

Parallel phase-shifting digital holography is capable of three-dimensional measurement of a dynamically moving object with a single-shot recording. In this letter, we demonstrated a parallel phase-shifting digital holography using a single femtosecond light pulse whose central wavelength and temporal duration were 800 nm and 96 fs, respectively. As an object, we set spark discharge in atmospheric pressure air induced by applying a high voltage to between two electrodes. The instantaneous change in phase caused by the spark discharge was clearly reconstructed. The reconstructed phase image shows the change of refractive index of air was -3.7 × 10(-4).

Spatial-carrier phase-shifting digital holography utilizing spatial frequency analysis for the correction of the phase-shift error.

We propose a single-shot digital holography in which the complex amplitude distribution is obtained by spatial-carrier phase-shifting (SCPS) interferometry and the correction of the inherent phase-shift error occurred in this interferometry. The 0th order diffraction wave and the conjugate image are removed by phase-shifting interferometry and Fourier transform technique, respectively. The inherent error is corrected in the spatial frequency domain. The proposed technique does not require an iteration process to remove the unwanted images and has an advantage in the field of view in comparison to a conventional SCPS technique.

Single-shot dual-illumination phase unwrapping using a single wavelength.

We propose a single-shot phase unwrapping technique using a single wavelength. In the proposed technique, an object is illuminated by two laser beams, which are emitted from the same laser, whose illumination angles and polarizations are different. Then two types of the object waves generated by the two beams are separately and simultaneously recorded by a polarization imaging camera. We conducted an experiment and a 2.5 mm height object, which was 9400 times height of the wavelength of the laser, was reconstructed without wrapping.

Single-shot polarization-imaging digital holography based on simultaneous phase-shifting interferometry.

We propose single-shot digital holography which is capable of simultaneously capturing both the information of multiple phase-shifted holograms and the distribution of the polarization. In this technique, a single image sensor records both the information required for phase-shifting interferometry and that of the polarization states of objects using an array of polarizers. The essence of the technique is the capability of imaging the distribution of the polarization of three-dimensional objects with a single-shot exposure by using the space-division multiplexing of holograms. The validity of the proposed technique was confirmed by the preliminary experiments.

High-speed phase imaging by parallel phase-shifting digital holography.

Parallel phase-shifting digital holography can obtain three-dimensional information of a dynamically moving object with high accuracy by using space-division multiplexing of multiple holograms required for phase-shifting interferometry. We demonstrated high-speed parallel phase-shifting digital holography and obtained images of the phase variation of air caused by a compressed gas flow sprayed from a nozzle. In particular, we found the interesting phenomenon of periodic phase distributions. Reconstructed images were obtained at frame rates of 20,000 and 180,000 frames per second.

Compensation algorithm for the phase-shift error of polarization-based parallel two-step phase-shifting digital holography.

We propose an algorithm for compensating the phase-shift error of polarization-based parallel two-step phase-shifting digital holography, which is a technique for recording a spatial two-step phase-shifted hologram. Although a polarization-based system of the technique has been experimentally demonstrated, there had been the problem that the phase difference of two phase-shifted holograms had been changed by the extinction ratio of the micropolarizer array attached to the image sensor used in the system. To improve the performance of the system, we established and formulated an algorithm for compensating the phase-shift error. Accurate spatial phase-shifting interferometry in the system can be conducted by the algorithm regardless of phase-shift error due to the extinction ratio. By the numerical simulation, the proposed algorithm was capable of reducing the root mean square errors of the reconstructed image by 1/4 and 1/5 in amplitude and phase, respectively. Also, the algorithm was experimentally demonstrated, and the experimental results showed that the system employing the proposed algorithm suppressed the conjugate image, which slightly appeared in the image reconstructed by the system not employing the algorithm, even when the extinction ratio was 10:1. Thus, the effectiveness of the proposed algorithm was numerically and experimentally verified.

Improvement of color reproduction in color digital holography by using spectral estimation technique.

We propose a color digital holography by using spectral estimation technique to improve the color reproduction of objects. In conventional color digital holography, there is insufficient spectral information in holograms, and the color of the reconstructed images depend on only reflectances at three discrete wavelengths used in the recording of holograms. Therefore the color-composite image of the three reconstructed images is not accurate in color reproduction. However, in our proposed method, the spectral estimation technique was applied, which has been reported in multispectral imaging. According to the spectral estimation technique, the continuous spectrum of object can be estimated and the color reproduction is improved. The effectiveness of the proposed method was confirmed by a numerical simulation and an experiment, and, in the results, the average color differences are decreased from 35.81 to 7.88 and from 43.60 to 25.28, respectively.

Hypertensive disorders of pregnancy increase the risk of future menopausal hot flashes in Japanese women: results from the Japan Nurses' Health Study.

OBJECTIVE: We aimed to investigate whether hypertensive disorders of pregnancy (HDP) are associated with problematic hot flashes in Japanese women. METHODS: In this study, we included participants in the Japan Nurses' Health Study who were women aged 41 to 55 years at baseline, parous, and completed a 4-year follow-up questionnaire. The main outcome was self-reported problematic hot flashes. At the 4-year follow-up survey, we investigated hot flashes using the Climacteric Symptom Checklist for Japanese Women, which was developed by a subcommittee of the Japan Society of Obstetrics and Gynecology. RESULTS: At the baseline survey, of the 4,627 women included in the analysis, 610 (13.2%) reported a history of HDP. At the 4-year follow-up survey, 394 women (8.5%) reported problematic hot flashes, 529 (11.4%) were diagnosed with premenopausal hypertension, and 2,389 (51.5%) were postmenopausal. Multivariable logistic regression analysis revealed a multivariable-adjusted odds ratio (95% confidence interval) of problematic hot flashes for women with a history of HDP of 1.42 (1.04-1.94), compared with women without this history. Among women without premenopausal hypertension, the odds ratio increased to 1.55 (1.10-2.19) among women with HDP as compared with those without these disorders. CONCLUSIONS: In this prospective study, we found that women with a history of HDP have a significantly increased risk of problematic hot flashes, compared with their counterparts without a history of HDP.

Image quality improvement of parallel four-step phase-shifting digital holography by using the algorithm of parallel two-step phase-shifting digital holography.

We propose an algorithm that can improve the quality of the reconstructed image from the single hologram recorded by the optical system of the parallel four-step phase-shifting digital holography. The proposed algorithm applies the image-reconstruction algorithm of parallel two-step phase-shifting digital holography to the hologram so as to reduce errors in the reconstructed image and eliminate ghosts. We numerically and experimentally confirmed that the proposed algorithm decreased 25% in terms of root mean square error in amplitude, and eliminated the ghosts, respectively.

Experimental demonstration of parallel two-step phase-shifting digital holography.

Parallel two-step phase-shifting digital holography is a technique for single-shot implementation of phase-shifting interferometry and requires only the intensity distribution of the reference wave and spatial two phase-shifted holograms. We constructed a system of parallel two-step phase-shifting digital holography and experimentally demonstrated the technique, for the first time. The system uses an originally fabricated image sensor having an array of 2 × 1 micro polarizers. Each micro polarizer was attached on pixel by pixel. In the experiment, the unwanted images, the zero-order diffraction wave and the conjugate image, are removed from the reconstructed image of objects by the system, while the images superimpose on the image of objects reconstructed by Fresnel transform alone. Also the capability of single-shot and three-dimensional imaging is demonstrated by the system.