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.

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.

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.

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.

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.

Security-enhanced optical voice encryption in various domains and comparative analysis.

Optical voice encryption based on digital holography (DH), which uses the double random phase encoding (DRPE) in the Fourier transform domain, has been proposed [Opt. Lett.42, 4619 (2017)OPLEDP0146-959210.1364/OL.42.004619]. In the present work, we propose optical voice encryption in other optical domains such as fractional Fourier, Fresnel, and gyrator transform, which convert input information into different mixed space-frequency domains. We also analyze the recording conditions of human voice and some security aspects of the scheme. An optical setup based on off-axis DH is used to record the time-series of holograms, and these holograms can visualize the sound wave propagation. Then, the different encryption domains in double random phase encoding (DRPE) are used to encrypt voice information with enhanced security. With the numerical reconstruction and decryption procedure of DRPE with all correct keys and correct orders of optical domains, the original voice information can be retrieved. The extension of voice encryption in different domains will enhance the security level and analysis of recording, and security aspects will explore the possibilities and challenges of the optical voice encryption scheme. We present simulation results for encryption of an experimentally recorded human voice in various domains.

Space-variant video compression and processing in digital holographic microscopy sensor networks with application to potable water monitoring.

Water-related diseases affect societies in all parts of the world. Online sensors are considered a solution to the problems associated with laboratory testing in potable water. One of the most active research areas of such online sensors has been within optics. Digital holographic microscopy (DHM) has the potential to rival state-of-the-art techniques such as advanced turbidity measurement. However, its use as an online sensor is limited by the large data requirements typical for digital holographic video. In this paper, we provide a solution that permits DHM to be applied to a whole class of online remote sensor networks, of which potable water analysis is one example. The designed sensors incorporate a novel space-variant quantization algorithm to preprocess each frame of a video sequence before transmission over a network. The system satisfies the generally accepted requirements of an online system: automated, near real-time, and operating in a real environment. To verify the effectiveness of the design, we implemented and evaluated it in an active potable water facility.

Optical voice encryption based on digital holography.

We propose an optical voice encryption scheme based on digital holography (DH). An off-axis DH is employed to acquire voice information by obtaining phase retardation occurring in the object wave due to sound wave propagation. The acquired hologram, including voice information, is encrypted using optical image encryption. The DH reconstruction and decryption with all the correct parameters can retrieve an original voice. The scheme has the capability to record the human voice in holograms and encrypt it directly. These aspects make the scheme suitable for other security applications and help to use the voice as a potential security tool. We present experimental and some part of simulation results.

An optical encryption and authentication scheme using asymmetric keys.

We propose a novel optical information encryption and authentication scheme that uses asymmetric keys generated by the phase-truncation approach and the phase-retrieval algorithm. Multiple images bonded with random phase masks are Fourier transformed, and obtained spectra are amplitude- and phase-truncated. The phase-truncated spectra are encoded into a single random intensity image using the phase-retrieval algorithm. Unlike most of the authentication schemes, in this study, only one encrypted reference image is required for verification of multiple secured images. The conventional double random phase encoding and correlation techniques are employed for authentication verification. Computer simulation results and theoretical explanation prove the effectiveness of the proposed scheme.

Fresnel domain nonlinear optical image encryption scheme based on Gerchberg-Saxton phase-retrieval algorithm.

We propose a novel nonlinear image-encryption scheme based on a Gerchberg-Saxton (G-S) phase-retrieval algorithm in the Fresnel transform domain. The decryption process can be performed using conventional double random phase encoding (DRPE) architecture. The encryption is realized by applying G-S phase-retrieval algorithm twice, which generates two asymmetric keys from intermediate phases. The asymmetric keys are generated in such a way that decryption is possible optically with a conventional DRPE method. Due to the asymmetric nature of the keys, the proposed encryption process is nonlinear and offers enhanced security. The cryptanalysis has been carried out, which proves the robustness of proposed scheme against known-plaintext, chosen-plaintext, and special attacks. A simple optical setup for decryption has also been suggested. Results of computer simulation support the idea of the proposed cryptosystem.

Image encryption using polarized light encoding and amplitude and phase truncation in the Fresnel domain.

In this paper, an image encryption scheme based on polarized light encoding and a phase-truncation approach in the Fresnel transform domain is proposed. The phase-truncated data obtained by an asymmetric cryptosystem is encrypted and decrypted by using the concept of the Stokes-Mueller formalism. Image encryption based on polarization of light using Stokes-Mueller formalism has the main advantage over Jones vector formalism that it manipulates only intensity information, which is measurable. Thus any intensity information can be encrypted and decrypted using this scheme. The proposed method offers several advantages: (1) a lens-free setup, (2) flexibility in the encryption key design, (3) use of asymmetric keys, and (4) immunity against special attack. We present numerical simulation results for gray-scale and color images in support of the proposed security scheme. The performance measurement parameters relative error and correlation coefficient have been calculated to check the effectiveness of the scheme.

Known-plaintext attack-based optical cryptosystem using phase-truncated Fresnel transform.

In this paper, we propose a scheme for information security under the basic double random phase encoding framework but with enhanced complexity and immunity against the known-plaintext attack. Modified Gerchberg-Saxton algorithm is used to convert a primary image into a phase-only mask (POM). The POM is used as a Fresnel domain key for encrypting an arbitrary data, called random intensity mask (RIM) bonded with a random phase mask. With phase- and amplitude-truncation, asymmetric keys are generated from the RIM. For decryption, the main target is to get the POM, for which the concept of known-plaintext attack has been used. The conventional schemes for attack use known-plaintext for key generation, but in this study it refers to the asymmetric keys. Obtaining Fresnel transform with the same parameters of the POM gives the primary image. We present the computer simulation results of multiple gray-scale images without any cross talk and also for a color image. The decryption is simple and straightforward and can be done digitally or optically. An optical setup for decryption has been suggested.

Image encryption based on interference that uses fractional Fourier domain asymmetric keys.

We propose an image encryption technique based on the interference principle and phase-truncation approach in the fractional Fourier domain. The proposed scheme offers multiple levels of security with asymmetric keys and is free from the silhouette problem. Multiple input images bonded with random phase masks are independently fractional Fourier transformed. Amplitude truncation of obtained spectrum helps generate individual and universal keys while phase truncation generates two phase-only masks analytically. For decryption, these two phase-only masks optically interfere, and this results in the phase-truncated function in the output. After using the correct random phase mask, universal key, individual key, and fractional orders, the original image is retrieved successfully. Computer simulation results with four gray-scale images validate the proposed method. To measure the effectiveness of the proposed method, we calculated the mean square error between the original and the decrypted images. In this scheme, the encryption process and decryption keys formation are complicated and should be realized digitally. For decryption, an optoelectronic scheme has been suggested.

Asymmetric color cryptosystem using polarization selective diffractive optical element and structured phase mask.

A single channel asymmetric color image encryption scheme is proposed that uses an amplitude- and phase- truncation approach with interference of polarized wavefronts. Instead of commonly used random phase masks, wavelength-dependent structured phase masks (SPM) are used in the fractional Fourier transform domain for image encoding. The primary color components bonded with different SPMs are combined into one grayscale image using convolution. We then apply the amplitude and phase truncation to the fractional spectrum, which helps generate unique decryption keys. The encrypted image bonded with a different SPM is then encoded into a polarization selective diffractive optical element. The proposed scheme alleviates the alignment problem of interference and does not need iterative encoding and offers multiple levels of security. The effect of a special attack to the proposed asymmetric cryptosystem has been studied. To measure the effectiveness of the proposed method, we calculated the mean square error between the original and the decrypted images. The computer simulation results support the proposed idea.

Three-dimensional fluorescence imaging using the transport of intensity equation.

We propose a nonscanning three-dimensional (3-D) fluorescence imaging technique using the transport of intensity equation (TIE) and free-space Fresnel propagation. In this imaging technique, a phase distribution corresponding to defocused fluorescence images with a point-light-source-like shape is retrieved by a TIE-based phase retrieval algorithm. From the obtained phase distribution, and its corresponding amplitude distribution, of the defocused fluorescence image, various images at different distances can be reconstructed at the desired plane after Fresnel propagation of the complex wave function. Through the proposed imaging approach, the 3-D fluorescence imaging can be performed in multiple planes. The fluorescence intensity images are captured with the help of an electrically tunable lens; hence, the imaging technique is free from motion artifacts. We present experimental results corresponding to microbeads and a biological sample to demonstrate the proposed 3-D fluorescence imaging technique.