Coaxial holographic encoding based on pure phase modulation

Combination Compensation Method to Improve the Tolerance of Recording Medium Shrinkage in Collinear Holographic Storage

Holographic optical storage has great potential for enormous data storage, although the recording medium can cause dimensional change, which can deteriorate the quality of the reconstructed hologram. Compensation in traditional off-axial holographic storage systems is sensitive to vibration and requires high precision. In contrast, a collinear system is more compact with better stability, and its compensation would be different. In this paper, the combination compensation method for compensating for the dimensional change of the recording medium by simultaneously adjusting the reading light wavelength and the focal length of the objective lens is established, which was implemented in a collinear system for a high dimension-change-rate (σ) of the medium condition. Its compensation effects for the lateral dimension change and the vertical dimension change were compared as well. The results show that the bit error ratio of the reconstructed hologram could be decreased to 0 for both of the dimensional change conditions with a large adjustment scope under σ = 1.5%. Compared with the compensation method, in which only the focal length or the wavelength are adjusted, this combination compensation method can enlarge the compensation scope and improve the tolerance of the recording medium shrinkage in a collinear holographic storage system.

Recent Advances in Generation and Detection of Orbital Angular Momentum Optical Beams-A Review

Herein, we have discussed three major methods which have been generally employed for the generation of optical beams with orbital angular momentum (OAM). These methods include the practice of diffractive optics elements (DOEs), metasurfaces (MSs), and photonic integrated circuits (PICs) for the production of in-plane and out-of-plane OAM. This topic has been significantly evolved as a result; these three methods have been further implemented efficiently by different novel approaches which are discussed as well. Furthermore, development in the OAM detection techniques has also been presented. We have tried our best to bring novel and up-to-date information to the readers on this interesting and widely investigated topic.

See-through display combined with holographic display and Maxwellian display using switchable holographic optical element based on liquid lens

We report a switchable holographic optical element based on a liquid lens for a see-through display. For the switchable holographic optical element, we recorded two optical components in the holographic film in two steps. A numerical simulation was also done to define the recording and reconstruction conditions. After the recording process, the entire system was changed from 4f optics to Maxwellian optics by changing wavefront of the reference wave using a liquid lens. The diffraction efficiency was 0.46 for a single element recording and around 0.14 for a double element recording. The holographic display and the Maxwellian display were successfully switched without any crosstalk. The field of view and eye box of the holographic display were 1° and 4.36 mm, respectively, and the field of view and the eye box of the Maxwellian display were 3.8° and 23.2 um, respectively. In the proposed system, spatial frequency filtering by the liquid lens and image shape distortion seriously affected the hologram image. However, we successfully verified the feasibility of our proposed switchable holographic optical element using a liquid lens.

Fabrication of diffraction based security elements using direct laser interference patterning

Product piracy impacts wide areas of the global economy resulting in multi-billion dollar losses per year. Therefore, product protection technologies are required to produce security elements with high flexibility and complexity. In this work, the fabrication of diffraction-based security elements, so called grating cell arrays, using direct laser interference patterning is presented. Ultraviolet two-beam interference patterning is used to structure line-like gratings into 80 µm in diameter spots with spatial periods ranging from 0.7 µm to 2.2 µm on PET substrates. The developed grating cell arrays are generated by the combination of several holographic surface gratings with variable spatial period and pattern orientation. The stored security information can be visualized by illumination with a coherent light source such as a laser pointer.

Linear phase encoding for holographic data storage with a single phase-only spatial light modulator

A linear phase encoding is presented for realizing a compact and simple holographic data storage system with a single spatial light modulator (SLM). This encoding method makes it possible to modulate a complex amplitude distribution with a single phase-only SLM in a holographic storage system. In addition, an undesired light due to the imperfection of an SLM can be removed by spatial frequency filtering with a Nyquist aperture. The linear phase encoding is introduced to coaxial holographic data storage. The generation of a signal beam using linear phase encoding is experimentally verified in an interferometer. In a coaxial holographic data storage system, single data recording, shift selectivity, and shift multiplexed recording are experimentally demonstrated.

Orthogonal reference pattern multiplexing for collinear holographic data storage

An orthogonal reference pattern multiplexing (ORPM) method for collinear holographic data storage (CHDS) is investigated to increase the data storage density and realize parallel optical image superimposition. Holograms are multiplexed in the same volume of the recording medium with multiple orthogonal reference patterns (RPs). The physical principle of this method is analyzed based on scalar diffraction theory. The orthogonal condition of the RPs is derived in order to suppress the interpage cross talk. The parameters of the radial-line RP have significant influence on the signal-to-noise ratio (SNR) of the reconstructed data page. They are optimized to reduce the intrapage cross talk. With a random binary phase mask (RBPM) located closely before the spatial light modulator, SNR of the reconstructed data page is seven times the SNR without the RBPM. Three data pages are multiplexed in the same volume of the medium using the ORPM method. The reconstructed data pages for the CHDS system show the effectiveness of the RBPM in suppressing the intrapage and interpage cross talk.

Binary image encryption based on interference of two phase-only masks

Optical image encryption based on interference has attracted a lot of attention recently. The technique employs two pure phase masks derived from the complex field of the image in the Fresnel diffraction domain. The image decryption procedure can be carried out by inverse Fresnel transformation of the summation of two pure phase masks. However, the silhouette of the original image, which is recovered by either of the two phase-only masks, impedes the application of this technique. In this paper, a very simple method for binary image encryption based on interference of two phase-only masks is proposed without any silhouette problem. The binary image in combination with a random phase mask is separated into two phase-only masks directly, and the decryption by summation of the two masks can be performed digitally or optically. In this paper, the encryption and decryption processes are analyzed, after which both the optical simulation and the experimental results based on single-beam holography are given to demonstrate the feasibility of the encryption method. As information nowadays is mainly digitized into binary codes, the proposed encryption method may find applications in the information processing field.