Moving picture recording and observation of three-dimensional image of femtosecond light pulse propagation

Scalable Multistep Imprinting of Multiplexed Optical Anti-counterfeiting Patterns with Hierarchical Structures

Multiplexed optical techniques with multichannel patterns provide powerful strategies for high-capacity anti-counterfeiting. However, it is still a big challenge to meet the demands of achieving high encryption levels, excellent readability, and simple preparation simultaneously. Herein, we use a multistep imprinting technique, leveraging surface work-hardening to massively produce multiplexed encrypted patterns with hierarchical structures. These patterns with coupled nano- and microstructures can be instantaneously decoded into different pieces of information at different view angles under white light illumination. By incorporating perpendicular nano- and microgratings, we achieve four-channel encoded patterns, enhancing anti-counterfeiting capacity. This versatile method works on various metal/polymer materials, offering high-density information storage, direct visibility, broad material compatibility, and low-cost mass production. Our high-performance anti-counterfeiting patterns show significant potential in real-world applications.

Unlocking stray light mysteries in the CoRot baffle with the time-of-flight method

Stray light (SL) has emerged as a primary limiting factor for space telescopes. Pre-launch testing is essential for validating performance and identifying potential issues. However, traditional methods do not enable the decomposition and identification of individual SL contributors. Consequently, when problems arise, resolving them often involves a cumbersome and risky trial-and-error approach. The time-of-flight (ToF) method was recently introduced, employing a pulsed laser source and ultrafast sensor to characterize individual SL contributors. A proof of concept was achieved using a simple three-lens system. In this paper, we apply the ToF method to a real space optical system: the spare model of the CoRoT baffle. We successfully measured individual SL contributors over a dynamic range of 10-11, identifying direct scattering on vane edges and two-step scattering paths. Our results provide a performance breakdown, differentiating intrinsic baffle SL from contributions arising from experimental conditions. Notably, the ToF method allowed us to discriminate air scattering, eliminating the need for expensive vacuum testing. The ToF provides unparallel insights, including defects identification. For instance, we identified the presence of localized dust particles causing significant SL. These results confirm the utility of the ToF method even for the most challenging space systems.

Swept coded aperture real-time femtophotography

Single-shot real-time femtophotography is indispensable for imaging ultrafast dynamics during their times of occurrence. Despite their advantages over conventional multi-shot approaches, existing techniques confront restricted imaging speed or degraded data quality by the deployed optoelectronic devices and face challenges in the application scope and acquisition accuracy. They are also hindered by the limitations in the acquirable information imposed by the sensing models. Here, we overcome these challenges by developing swept coded aperture real-time femtophotography (SCARF). This computational imaging modality enables all-optical ultrafast sweeping of a static coded aperture during the recording of an ultrafast event, bringing full-sequence encoding of up to 156.3 THz to every pixel on a CCD camera. We demonstrate SCARF's single-shot ultrafast imaging ability at tunable frame rates and spatial scales in both reflection and transmission modes. Using SCARF, we image ultrafast absorption in a semiconductor and ultrafast demagnetization of a metal alloy.

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.

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.

Spatiotemporal observation of light propagation in a three-dimensional scattering medium

Spatiotemporal information about light pulse propagation obtained with femtosecond temporal resolution plays an important role in understanding transient phenomena and light–matter interactions. Although ultrafast optical imaging techniques have been developed, it is still difficult to capture light pulse propagation spatiotemporally. Furthermore, imaging through a three-dimensional (3-D) scattering medium is a longstanding challenge due to the optical scattering caused by the interaction between light pulse and a 3-D scattering medium. Here, we propose a technique for ultrafast optical imaging of light pulses propagating inside a 3D scattering medium. We record an image of the light pulse propagation using the ultrashort light pulse even when the interaction between light pulse and a 3-D scattering medium causes the optical scattering. We demonstrated our proposed technique by recording converging, refracted, and diffracted propagating light for 59 ps with femtosecond temporal resolution.

Intensity-corrected 4D light-in-flight imaging

Light-in-flight (LIF) imaging is the measurement and reconstruction of light's path as it moves and interacts with objects. It is well known that relativistic effects can result in apparent velocities that differ significantly from the speed of light. However, less well known is that Rayleigh scattering and the effects of imaging optics can lead to observed intensities changing by several orders of magnitude along light's path. We develop a model that enables us to correct for all of these effects, thus we can accurately invert the observed data and reconstruct the true intensity-corrected optical path of a laser pulse as it travels in air. We demonstrate the validity of our model by observing the photon arrival time and intensity distribution obtained from single-photon avalanche detector (SPAD) array data for a laser pulse propagating towards and away from the camera. We can then reconstruct the true intensity-corrected path of the light in four dimensions (three spatial dimensions and time).

Stray light characterization with ultrafast time-of-flight imaging

Understanding stray light (SL) is a crucial aspect in the development of high-end optical instruments, for instance space telescopes. As it drives image quality, SL must be controlled by design and characterized experimentally. However, conventional SL characterization methods are limited as they do not provide information on its origins. The problem is complex due to the diversity of light interaction processes with surfaces, creating various SL contributors. Therefore, when SL level is higher than expected, it can be difficult to determine how to improve the system. We demonstrate a new approach, ultrafast time-of-flight SL characterization, where a pulsed laser source and a streak camera are used to record individually SL contributors which travel with a specific optical path length. Furthermore, the optical path length offers a means of identification to determine its origin. We demonstrate this method in an imaging system, measuring and identifying individual ghosts and scattering components. We then show how it can be used to reverse-engineer the instrument SL origins.

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.

Compressive Coded Rotating Mirror Camera for High-Speed Imaging

We develop a novel compressive coded rotating mirror (CCRM) camera to capture events at high frame rates in passive mode with a compact instrument design at a fraction of the cost compared to other high-speed imaging cameras. Operation of the CCRM camera is based on amplitude optical encoding (grey scale) and a continuous frame sweep across a low-cost detector using a motorized rotating mirror system which can achieve single pixel shift between adjacent frames. Amplitude encoding and continuous frame overlapping enable the CCRM camera to achieve a high number of captured frames and high temporal resolution without making sacrifices in the spatial resolution. Two sets of dynamic scenes have been captured at up to a 120 Kfps frame rate in both monochrome and colored scales in the experimental demonstrations. The obtained heavily compressed data from the experiment are reconstructed using the optimization algorithm under the compressive sensing (CS) paradigm and the highest sequence depth of 1400 captured frames in a single exposure has been achieved with the highest compression ratio of 368 compared to other CS-based high-speed imaging technologies. Under similar conditions the CCRM camera is 700× faster than conventional rotating mirror based imaging devices and could reach a frame rate of up to 20 Gfps.

Toward the Super Temporal Resolution Image Sensor with a Germanium Photodiode for Visible Light

The theoretical temporal resolution limit tT of a silicon photodiode (Si PD) is 11.1 ps. We call “super temporal resolution” the temporal resolution that is shorter than that limit. To achieve this resolution, Germanium is selected as a candidate material for the photodiode (Ge PD) for visible light since the absorption coefficient of Ge for the wavelength is several tens of times higher than that of Si, allowing a very thin PD. On the other hand, the saturation drift velocity of electrons in Ge is about 2/3 of that in Si. The ratio suggests an ultra-short propagation time of electrons in the Ge PD. However, the diffusion coefficient of electrons in Ge is four times higher than that of Si. Therefore, Monte Carlo simulations were applied to analyze the temporal resolution of the Ge PD. The estimated theoretical temporal resolution limit is 0.26 ps, while the practical limit is 1.41 ps. To achieve a super temporal resolution better than 11.1 ps, the driver circuit must operate at least 100 GHz. It is thus proposed to develop, at first, a short-wavelength infrared (SWIR) ultra-high-speed image sensor with a thicker and wider Ge PD, and then gradually decrease the size along with the progress of the driver circuits.

Motion-picture recording of ultrafast behavior of polarized light incident at Brewster's angle

Observing light propagation plays an important role in clarifying ultrafast phenomena occurring on femtosecond to picosecond time scales. In particular, observing the ultrafast behavior of polarized light is useful for various fields. We have developed a technique based on Polarization Light-in-Flight Holography, which can record light propagation as a motion picture that can provide information about the polarization direction. Here we demonstrate motion-picture recording of a phenomenon, which is characteristic of polarized light, by using the proposed technique. As a phenomenon, we adopted the behavior of a light pulse incident at Brewster’s angle. We succeeded in recording the light reflection of specific polarized light by the proposed optical setup. The method of recording the motion-picture, reconstruction procedure, and the quantitative evaluation of the results are demonstrated.

Light-In-Flight Imaging by a Silicon Image Sensor: Toward the Theoretical Highest Frame Rate

Light in flight was captured by a single shot of a newly developed backside-illuminated multi-collection-gate image sensor at a frame interval of 10 ns without high-speed gating devices such as a streak camera or post data processes. This paper reports the achievement and further evolution of the image sensor toward the theoretical temporal resolution limit of 11.1 ps derived by the authors. The theoretical analysis revealed the conditions to minimize the temporal resolution. Simulations show that the image sensor designed following the specified conditions and fabricated by existing technology will achieve a frame interval of 50 ps. The sensor, 200 times faster than our latest sensor will innovate advanced analytical apparatuses using time-of-flight or lifetime measurements, such as imaging TOF-MS, FLIM, pulse neutron tomography, PET, LIDAR, and more, beyond these known applications.

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.

Wavelength-Selective Phase-Shifting Digital Holography: Color Three-Dimensional Imaging Ability in Relation to Bit Depth of Wavelength-Multiplexed Holograms

The quality of reconstructed images in relation to the bit depth of holograms formed by wavelength-selective phase-shifting digital holography was investigated. Wavelength-selective phase-shifting digital holography is a technique to obtain multiwavelength three-dimensional (3D) images with a full space-bandwidth product of an image sensor from wavelength-multiplexed phase-shifted holograms and has been proposed since 2013. The bit resolution required to obtain a multiwavelength holographic image was quantitatively and experimentally evaluated, and the relationship between wavelength resolution and dynamic range of an image sensor was numerically simulated. The results indicate that two-bit resolution per wavelength is required to conduct color 3D imaging.

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.

A trillion frames per second: the techniques and applications of light-in-flight photography

Cameras capable of capturing videos at a trillion frames per second allow to freeze light in motion, a very counterintuitive capability when related to our everyday experience in which light appears to travel instantaneously. By combining this capability with computational imaging techniques, new imaging opportunities emerge such as 3D imaging of scenes that are hidden behind a corner, the study of relativistic distortion effects, imaging through diffusive media and imaging of ultrafast optical processes such as laser ablation, supercontinuum and plasma generation. We provide an overview of the main techniques that have been developed for ultra-high speed photography with a particular focus on 'light-in-flight' imaging, i.e. applications where the key element is the imaging of light itself at frame rates that allow to freeze its motion and therefore extract information that would otherwise be blurred out and lost.

Single-shot ultrafast optical imaging

Single-shot ultrafast optical imaging can capture two-dimensional transient scenes in the optical spectral range at ≥100 million frames per second. This rapidly evolving field surpasses conventional pump-probe methods by possessing the real-time imaging capability, which is indispensable for recording non-repeatable and difficult-to-reproduce events and for understanding physical, chemical, and biological mechanisms. In this mini-review, we survey comprehensively the state-of-the-art single-shot ultrafast optical imaging. Based on the illumination requirement, we categorized the field into active-detection and passive-detection domains. Depending on the specific image acquisition and reconstruction strategies, these two categories are further divided into a total of six sub-categories. Under each sub-category, we describe operating principles, present representative cutting-edge techniques with a particular emphasis on their methodology and applications, and discuss their advantages and challenges. Finally, we envision prospects of technical advancement in this field.

Single-shot real-time femtosecond imaging of temporal focusing

While the concept of focusing usually applies to the spatial domain, it is equally applicable to the time domain. Real-time imaging of temporal focusing of single ultrashort laser pulses is of great significance in exploring the physics of the space-time duality and finding diverse applications. The drastic changes in the width and intensity of an ultrashort laser pulse during temporal focusing impose a requirement for femtosecond-level exposure to capture the instantaneous light patterns generated in this exquisite phenomenon. Thus far, established ultrafast imaging techniques either struggle to reach the desired exposure time or require repeatable measurements. We have developed single-shot 10-trillion-frame-per-second compressed ultrafast photography (T-CUP), which passively captures dynamic events with 100-fs frame intervals in a single camera exposure. The synergy between compressed sensing and the Radon transformation empowers T-CUP to significantly reduce the number of projections needed for reconstructing a high-quality three-dimensional spatiotemporal datacube. As the only currently available real-time, passive imaging modality with a femtosecond exposure time, T-CUP was used to record the first-ever movie of non-repeatable temporal focusing of a single ultrashort laser pulse in a dynamic scattering medium. T-CUP's unprecedented ability to clearly reveal the complex evolution in the shape, intensity, and width of a temporally focused pulse in a single measurement paves the way for single-shot characterization of ultrashort pulses, experimental investigation of nonlinear light-matter interactions, and real-time wavefront engineering for deep-tissue light focusing.

High-speed image-reconstruction algorithm for a spatially multiplexed image and application to digital holography

We propose a high-speed image-reconstruction algorithm for a spatially multiplexed image that is obtained by spatial frequency-division multiplexing. The algorithm utilizes smoothing and does not require any Fourier transform (FT) or iterative procedure to extract the desired information selectively from a single image. Numerical and experimental results show its validity and color holographic imaging ability. Calculation time of the proposed is less than a tenth of that of the FT method when using a central processing unit and an image sensor with four megapixels. Furthermore, throughput that is three times that of the FT method can be achieved in the case of using a graphics processing unit.

Digital holography and its multidimensional imaging applications: a review

In this review, we introduce digital holographic techniques and recent progress in multidimensional sensing by using digital holography. Digital holography is an interferometric imaging technique that does not require an imaging lens and can be used to perform simultaneous imaging of multidimensional information, such as three-dimensional structure, dynamics, quantitative phase, multiple wavelengths and polarization state of light. The technique can also obtain a holographic image of nonlinear light and a three-dimensional image of incoherent light with a single-shot exposure. The holographic recording ability of this technique has enabled a variety of applications.

Dual-wavelength phase-shifting digital holography selectively extracting wavelength information from wavelength-multiplexed holograms

Dual-wavelength phase-shifting digital holography that selectively extracts wavelength information from five wavelength-multiplexed holograms is presented. Specific phase shifts for respective wavelengths are introduced to remove the crosstalk components and extract only the object wave at the desired wavelength from the holograms. Object waves in multiple wavelengths are selectively extracted by utilizing 2π ambiguity and the subtraction procedures based on phase-shifting interferometry. Numerical results show the validity of the proposed technique. The proposed technique is also experimentally demonstrated.

Single-photon sensitive light-in-fight imaging

The ability to record images with extreme temporal resolution enables a diverse range of applications, such as fluorescence lifetime imaging, time-of-flight depth imaging and characterization of ultrafast processes. Recently, ultrafast imaging schemes have emerged, which require either long acquisition times or raster scanning and have a requirement for sufficient signal that can only be achieved when light is reflected off an object or diffused by a strongly scattering medium. Here we present a demonstration of the potential of single-photon detector arrays for visualization and rapid characterization of events evolving on picosecond time scales. The single-photon sensitivity, temporal resolution and full-field imaging capability enables the observation of light-in-flight in air, as well as the measurement of laser-induced plasma formation and dynamics in its natural environment. The extreme sensitivity and short acquisition times pave the way for real-time imaging of ultrafast processes or visualization and tracking of objects hidden from view.

High-frame-rate observation of single femtosecond laser pulse propagation in fused silica using an echelon and optical polarigraphy technique

We have demonstrated high-frame-rate observations of a single femtosecond laser pulse propagating in transparent medium using the optical polarigraphy technique and an echelon. The echelon produced a spatially encoded time delay for the probe pulse to capture directly four successive images of an intense propagating pulse with picosecond time interval and femtosecond time resolution. Using this method, we observed the propagation process of a single femtosecond laser pulse in fused silica. The influence of pulse-energy fluctuation on the spatial and temporal distribution of the single laser pulse was visualized using the single-shot measurements.

Digital holography based on multiwavelength spatial-bandwidth-extended capturing-technique using a reference arm (Multi-SPECTRA)

Single-shot digital holography based on multiwavelength spatial-bandwidth-extended capturing-technique using a reference arm (Multi-SPECTRA) is proposed. Both amplitude and quantitative phase distributions of waves containing multiple wavelengths are simultaneously recorded with a single reference arm in a single monochromatic image. Then, multiple wavelength information is separately extracted in the spatial frequency domain. The crosstalk between the object waves with different wavelengths is avoided and the number of wavelengths recorded with both a single-shot exposure and no crosstalk can be increased, by a large spatial carrier that causes the aliasing, and/or by use of a grating. The validity of Multi-SPECTRA is quantitatively, numerically, and experimentally confirmed.

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.

Toward 100 Mega-frames per second: design of an ultimate ultra-high-speed image sensor

Our experience in the design of an ultra-high speed image sensor targeting the theoretical maximum frame rate is summarized. The imager is the backside illuminated in situ storage image sensor (BSI ISIS). It is confirmed that the critical factor limiting the highest frame rate is the signal electron transit time from the generation layer at the back side of each pixel to the input gate to the in situ storage area on the front side. The theoretical maximum frame rate is estimated at 100 Mega-frames per second (Mfps) by transient simulation study. The sensor has a spatial resolution of 140,800 pixels with 126 linear storage elements installed in each pixel. The very high sensitivity is ensured by application of backside illumination technology and cooling. The ultra-high frame rate is achieved by the in situ storage image sensor (ISIS) structure on the front side. In this paper, we summarize technologies developed to achieve the theoretical maximum frame rate, including: (1) a special p-well design by triple injections to generate a smooth electric field backside towards the collection gate on the front side, resulting in much shorter electron transit time; (2) design technique to reduce RC delay by employing an extra metal layer exclusively to electrodes responsible for ultra-high speed image capturing; (3) a CCD specific complementary on-chip inductance minimization technique with a couple of stacked differential bus lines.