Parallel Phase-Shifting Digital Holography Using Femtosecond Laser Pulse

We propose a parallel two-step phase-shifting digital holography technique capable of instantaneous measurement of three-dimensional objects, with a view toward measurement of dynamically moving objects. The technique is based on phase-shifting interferometry. The proposed technique carries out the two-step phase-shifting method at one time and can be optically implemented by using a phase-shifting array device located in the reference beam. The array device has a periodic two-step phase distribution, and its configuration is simplified compared with that required for three-step and four-step parallel phase-shifting digital holographies. Therefore the optical system of the proposed technique is more suitable for the realization of a parallel phase-shifting digital holography system. We conduct both a numerical simulation and a preliminary experiment in the proposed technique. The results of the simulation and the experiment agree well with those of sequential phase-shifting digital holography, and results are superior to those obtained by conventional digital holography using the Fresnel transform alone. Thus the effectiveness of the proposed technique is verified.

Parallel phase-shifting digital holography is a powerful technique for recording motion picture of holograms with an image sensor and numerically reconstructing the motion picture of the high-quality three-dimensional (3-D) images of the object in a computer. This technique provides high-quality images of light intensity and phase of the object at a time instant at arbitrary depth position. Therefore, the technique achieves 3-D motion-picture imaging of a dynamic object. The technique provides not only large depth of field and high temporal resolution at the same time but also a motion picture of an invisible object. It is difficult for other 3-D imaging techniques to achieve these features of parallel phase-shifting digital holography. The authors review two experiments demonstrating the 3-D imaging by the technique and the features of the technique. One experiment demonstrated the motion-picture 3-D imaging of a minute crystal, sinking down in solution, by refocusing the amplitude images of the crystal. The result was obtained for the first time, to the best of our knowledge. The other demonstrated the motion-picture 3-D imaging of refractive indices of dynamic invisible gas flow by applying the Abel inversion to the phase images of the flow obtained by the technique.

An encryption system that combines compressive sensing (CS) and two‐step parallel phase shifting digital holography (PPSDH) using double random phase encoding (DRPE) is presented in this paper. The two‐step PPSDH is a linear inline holographic scheme and is much suitable for encrypting the 2D/3D information in a single exposure. The distribution of random phase mask (RPM) in the DRPE is implemented using circular harmonic key which increases the security of the encryption process. In this system, the keys used to encrypt are spatial positions of the planes, wavelength, and rotation of the circular harmonics in RPMs, and CS acts as an additional key that makes the system more secure than the conventional optical encryption methods. At the transmission end, two‐step PPSDH is applied to encrypt the object information in single hologram. The digital mirror device (DMD) is placed between the object and a single‐pixel detector for acquiring fewer hologram measurements. At the receiver end, the single digital hologram is numerically recovered by using a CS optimization problem. The original complex object field is decrypted from the CS recovered holograms by the inversion of two‐step PPSDH process with the help of the correct keys. The numerical simulations are presented for complex 2D and 3D objects to test the feasibility of the proposed encryption and decryption system. The proposed method carried out intensity and phase reconstruction of the original object field using single‐pixel compressive imaging. The computer simulation results demonstrated that the encrypted information is highly secured with the rotation of the circular harmonic key. The sensitivity of the decrypted intensity and phase images is also studied with variations of the encrypted keys. The obtained results show that the proposed encryption scheme is feasible and has better security performance and robustness.

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.

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.

Parallel phase-shifting digital holography (PPSDH) has been introduced. Movies of high-speed 3-D imaging and those of high-speed phase imaging obtained by PPSDH using a high-speed camera will be demonstrated at the presentation. PPSDH will contribute for 3-D measurement of moving object in such the applications as biological microscope, particle and flow measurement, and so on. This study was partially supported by Next Generation World-Leading Researchers, Japan Society for the Promotion of Science (JSPS).

The authors review recent advances in high-speed imaging of dynamic object with parallel phase-shifting digital holography. 3-D imaging of temperature of the air surrounding a heated object was demonstrated as an example of the advances.

Minimum optical energy required for parallel four-step phase-shifting digital holography is evaluated numerically by using photon-counting method. One of the attractive features of parallel phase-shifting digital holography is the instantaneous recording of fast 3D events where only the complex amplitude distribution of an object wave is obtained. The reconstruction is executed by numerical wave propagation such as angular spectrum propagation or Fresnel propagation. Numerical results indicate that required optical energy of an input image with 512 × 512 pixels is about 11 pJ. Under the criteria used in the evaluation, the required optical energy is independent of the image size.

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.

Development of the Imaging System for the Curing Process in UV Curing Resin Using Parallel Phase-Shifting Digital Holography

Using parallel phase-shifting digital holography with a high-speed camera, the authors succeeded in phase-shifting interferometry at the rate of 180,000 fraames/s. Motion pictures of three-dimensional image of dynamically moving objects were demonstrated.

Two-photon holographic microscopy (TPHM) is a powerful technique for stimulating biological cells three-dimensionally. TPHM requires an imaging technique to detect fluorescence lights from the three-dimensionally distributed cells. In this study, parallel phase-shifting incoherent digital holography is applied to TPHM. Owing to the parallel phase-shifting method, the lights from the cells are detected with the frame rate of an image sensor. The feasibility of the proposed method is experimentally verified.

Biological hard tissues, such as those found in bone and teeth, are complex tissues that build a strong mineral structure over an organic matrix framework. The laser-matter interaction for bone hard tissues holds great interest to laser surgery and laser dentistry; the use of short/ultrashort pulses, in particular, shows interesting behaviors not seen in continuous wave lasers. High laser energy densities in ultrashort pulses can be focused on a small irradiated surface (spot diameter is 10-50 μm) leading to rapid temperature rise and thermal ablation of the bone tissue. Ultrashort pulses, specifically those in the picosecond and femtosecond ranges, impose several challenges in modeling bone tissue response. In the present paper we perform time-dependent thermal simulations of short and ultrashort pulse laser-bone interactions in singlepulse and multipulse (set of ultrashort pulses) modes of laser heating. A comparative analysis for both radiation modes is discussed for laser heating of different types of the solid bone on the nanosecond, picosecond and femtosecond time scales. It is shown that ultrashort laser pulses with high energy densities can ablate bone tissue without heating tissues bordering the ablation creator. This reaction is particularly desirable as heat accumulation and thermal damage are the main factors affecting tissue regrowth rates, and thus patient recovery times.

As an ultrafast 3D imaging technique, an improved light-in-flight recording by holography using a femtosecond is presented. To record 3D image of light propagation, a voluminous light-scattering medium is introduced to the light-inflight recording by holography. A mode-locked Ti:Sapphire laser are employed for the optical source. To generate the 3D image of propagating light, a voluminous light-scattering medium is made of gelatin jelly and set in the optical path of the object wave of holography. 3D motion picture of propagation of a femtosecond light pulse was achieved for 260ps with 220fs temporal resolution. Digital recording of 3D image of light propagation is also presented. To record the 3D image of the light propagation, digital holography is combined with the light-in-flight recording by holography using a voluminous light-scattering medium. The hologram is recorded with an image sensor such as CCD image sensor. The image of the light is reconstructed from the digitally recorded hologram by computer. To obtain the motion picture of the 3D image of the light propagation, a set of pieces of holograms consisting of 512 × 512 pixels are extracted from the whole area of the digitally recorded hologram. The position of the extracted piece on the recoded hologram is shifted along the direction in which the reference optical pulse swept on the image sensor, piece-by-piece of the hologram. The set of the pieces are reconstructed sequentially, then the 3D digital motion picture of propagation of femtosecond light pulse is achieved. The recordable time of the motion picture was 60 ps.

\n Direct laser writing (DLW) techniques with ultrashort laser pulses have been extensively used for materials processing. The nonlinear nature of the interaction between ultrashort pulses and matter confers high spatial localization to these techniques and allows the fabrication of compact devices in a single step with resolution in the micro/nanoscale. In this context, we can highlight the fabrication of microstructures in transparent materials, like polymers and glasses, since they allow for the realization of three-dimensional devices. For instance, high-quality direct laser-written waveguide circuits have been fabricated and applied to different fields, such as telecommunications, quantum optics, sensors, etc. As photonic circuits often guide high light intensity, the characterization of the nonlinear optical properties of these structures becomes essential to direct their technological application. In this work, we realized the third-order nonlinear optical characterization of fs-laser written waveguides, which were fabricated in special glasses: Corning® Gorilla® and Eagle XG® Glass. The nonlinear characterization was performed through the Dispersive-scan (D-scan) technique, the temporal analog of Z-scan that can be applied to waveguides. The results obtained for the Gorilla® Glass waveguides indicate that the fs-laser writing process is responsible for reducing the magnitude of the nonlinear refractive index, when compared to the bulk material. This effect depends on the writing parameters, and the reduction is stronger when higher energy pulses are employed. Raman spectroscopy measurements revealed that this reduction could be related to the generation of non-bridging oxygen hole centers (NBOHCs) upon fs-laser irradiation. The characterization of Eagle XG® Glass waveguides, fabricated in different writing regimes (repetitive and cumulative), confirms the reduction of the nonlinear refractive index in the repetitive regime. However, it also revealed that little to no modification occurs when the cumulative regime is used, meaning that heat accumulation can be responsible for erasing NBOHCs and restoring the optical nonlinearities. Our results bring new light to what is currently known about the light-matter interaction when ultrashort pulses are involved. Furthermore, they can help tailor the application of fs-laser written glass waveguides, according to the desire for high or low optical nonlinearities.\n