Phase Conjugate Image Research Articles

Phase retrieval using hologram transformation with U-Net in digital holography

Digital holography is a method of recording light waves emitted from an object as holograms and then reconstructing the holograms using light wave propagation calculations to observe the object in three dimensions. However, a problem with digital holography is that unwanted images, such as conjugate images, are superimposed as the hologram is reconstructed to create an observed image. In particular, the superimposition of conjugate light on the observed image is caused by the imaging device’s ability to record just the intensity distribution of light rather than the phase distribution of light. In digital holography, it has been shown that unwanted light can be eliminated by the phase-shift method. However, it is difficult to apply the phase-shift method to digital holographic microscopy (DHM), which takes only one shot of light intensity. Alternatively, machine learning methods called deep learning have been actively studied in recent years for image-related problems, with image transformation as an example. Furthermore, a method that combines digital holography and deep learning has been proposed to perform image transformation to remove conjugate images using deep learning on the reconstructed image of a hologram. In this study, we generated a pair of holograms with only light intensity distribution and holograms with complex amplitude by simulating light wave propagation, trained U-Net to perform image transformation that adds phase information to the hologram with only light intensity distribution, and proposed a method for phase retrieval and conjugate image removal for holograms using the learned U-Net. To verify the effectiveness of the proposed method, we evaluated the image quality of the reconstructed image of holograms before and after processing by U-Net. Results showed that the peak signal-to-noise ratio (PSNR) increased by 8.37 [dB] in amplitude and 9.06 [dB] in phase. The amplitude and phase of the structural similarity index (SSIM) increased by 0.0566 and 0.0143, respectively. Furthermore, the results of applying the proposed method to holograms captured by actual digital holography optics showed the effectiveness of the proposed method in eliminating conjugate images in the reconstructed images. These results show that the proposed method is capable of phase retrieval of holograms in a single shot without the need for a complex optical system. This is expected to contribute to the field of portable DHMs and other applications that require compact and simple optical systems.

System design and error correction for 300 mm aperture vertical Fizeau spatial-temporal phase-shifting interferometer.

With the development of high-power lasers for aerospace, electronics, etc., the demand for large-aperture planar optical elements has become more urgent, along with the demand for measurement methods. In this paper, the design of a 300 mm aperture vertical Fizeau spatial-temporal phase-shifting interferometer is discussed. Based on position difference between laser sources, the spatial phase-shifting technique is achieved by generating a laser source array on the focal plane of the collimation lens, and four pairs of coherent beams with different phase shifts are integrated in a vertical Fizeau interference system. Combined with a tunable laser diode, a temporal phase-shifting technique can be realized in any pair of coherent beams through wavelength tuning. The key techniques, which include laser duplication to introduce different phase shifts, conjugate imaging, and separation for interferograms, and assembly for a transmission flat, are demonstrated. The systematic error and position mismatch error of interferograms are eliminated. Comparison experiments are conducted between spatial and temporal phase-shifting techniques. A dynamic water surface is also measured to verify its capacity for detecting dynamic objects.

Recovery of image distorted by turbulent atmosphere using phase-conjugate image generated by difference frequency generation

We demonstrate dynamic recovery of blurred images caused by atmospheric turbulence. In particular, using a phase-conjugate wave generated by a second-order nonlinear crystal or composite, we restore the original quality of the image after the optical radiation forming the image propagates through the turbulent atmosphere. One of the key elements for our experiment is a rotating phase plate being placed in the beam path for simulating turbulent atmosphere. Using the nonlinear composite, we demonstrate that the image recovery is insensitive to the polarization of the optical radiation forming the image.

Multiple Self-Pumped Phase-Conjugated Waves by Pulsed Laser

Multiple self-pumped phase-conjugated (SPPC) images with more different types of information than previously obtained SPPC images were obtained using a nanosecond-pulsed beam instead of continuous-wave beams. The minimum number of incident pulses, i.e., the minimum energy, required to generate one SPPC wave for one type of information strongly depended on the product of the number of incident pulses and the total power of incident beams. Both the time required for the generation (rise time) and the time required for the elimination (fall time) of SPPC images changed in the first-order exponential manner. These times due to the photorefractive effect were a sensitive function of beam intensity such that total energy transfer required long interaction length in BaTiO3 crystal.

Generation of Multi Self-Pumped Phase-Conjugated Waves by Pulsed Laser

Two self-pumped phase-conjugated (SPPC) images produced from two signal laser beams transporting different types of information were observed using a photorefractive barium titanate (BaTiO3) crystal. We have studied the smallest numbers of incident pulses required to generate one and two phase-conjugated images for one and two types of information, respectively, in order to transport much information. The lowest energy required for the generation of one phase-conjugated image depended strongly on the product of the numbers of incident pulses and the power of the incident pulsed beam.

Generation of Two Self-Pumped Phase-Conjugated Waves at 532 nm by Nanosecond Pulsed Laser

Self-pumped phase-conjugated (SPPC) images produced by two SPPC beams generated from a single nanosecond-pulsed laser beam are successfully obtained at beam power ratios of up to 4:1. The two SPPC beams are incident on opposite faces [(100) and (100)] of a photorefractive barium titanate (BaTiO3) crystal, and the SPPC images are reproduced on the respective crystal faces with high quality due to the reduced interference between the two SPPC beams owing to the delay between the nanosecond pulses of each beam. The quality of the images and the maximum power ratio at which both images are observable are superior to the continuous-wave configuration.

Generation of Two Self-Pumped Phase-Conjugated Waves at Different Wavelengths Using Single Photorefractive Crystal

Two self-pumped phase-conjugated waves at different wavelengths are successfully generated using an undoped barium titanate crystal. With one signal beam at a wavelength of 514.5 nm incident to the (100) face of the crystal and the other at a wavelength of 488.0 nm incident to the opposite (100) face with an offset of 2 mm, two phase-conjugated images are produced even at high power ratios between the signal beams. The use of two different wavelengths prevents interference between the two signal beams, resulting in the generation of phase-conjugated waves over a wide dynamic range.

Optical encryption and decryption using a sandwich random phase diffuser in the Fourier plane

We describe the encryption and decryption of a two-dimensional (2-D) image by using a sandwich diffuser in the Fourier plane of the first lens of a 4-f imaging system. The diffuser is made with two random phase diffusers, sandwiched together. The encrypted image is holographically recorded in a photorefractive BaTiO3 crystal. Decryption is done by generating a phase conjugate image through the phase conjugation technique. During decryption, if only one of the two random phase diffusers constituting the sandwich diffuser is used, it becomes impossible to retrieve the image. Also no decryption is possible if any of the diffusers constituting the sandwich is shifted in-plane with respect to the other. It is possible to generate the image only if the diffusers are sandwiched together again after matching point to point, and then the sandwich diffuser is kept at the same position and orientation in which it was kept during the encryption. Experimental method has been described to overcome the practical difficulty for the generation of the key in the case of the sandwich diffuser. Simulation and experimental results are presented in support of the proposed idea. To evaluate reliability of the technique, the mean-square-error (MSE) between decrypted and original image has been calculated.

Generation of Two Self-Pumped Phase-Conjugated Waves Transporting Different Information Using Single Photorefractive Crystal

Two optical configurations for generating two self-pumped phase-conjugated waves from two signal laser beams transporting different information were presented: one in which the two signal laser beams were directed at two opposite faces [(100) and (100)] of the crystal and the other in which both beams were directed at the same (100) face. Using a barium titanate (BaTiO3) single crystal as a self-pumped phase-conjugated mirror, the generation conditions of two discrete self-pumped phase-conjugated waves of high quality with reduced interference were investigated. Successfully self-pumped phase-conjugated images were observed to be more readily achievable by changing the incident beam power than by changing the incident position onto a single face.

Generation of Four Self-Pumped Phase-Conjugated Waves Using Single Photorefractive Crystal

A method of generating self-pumped phase-conjugated waves from four signal beams transporting different information using a single crystal of barium titanate is presented. With one pair of signal beams directed on the (100) face of the crystal and the other pair directed on the opposite (100) face, phase-conjugated images were produced according to the relative power of the two beam pairs. Images were produced even when the beams in each pair fully overlapped on the incident crystal face. At high power ratios (e.g., 1:5), no images of the lower-power beams were produced. The absence of fanning loops in the crystal for the lower-power beams indicates that the information was erased due to scattering of the higher-power beams and/or the interaction between the beams in the crystal. The generation of phase-conjugated waves could therefore be controlled selectively by adjusting the relative beam power of the input signal beams.

Selection from Complex Self-Pumped Phase-Conjugated Images

Two methods for the extraction of specific information from complex self-pumped phase-conjugated images are presented. Barium titanate is employed as a self-pumped phase-conjugated mirror. Extraction by adjusting the screen position to the desired sample is shown to be effective when the individual samples are spaced at discrete intervals, while vibration of the sample is effective when the samples are set in a compact sequence. These methods allow a single image to be extracted from complex images when coupled with digital post-processing.

Quantum noise properties of parametric amplifiers driven by two pump waves

In a parametric amplifier (PA) driven by two pump waves the signal sideband is coupled to three idler sidebands, all of which are frequency-converted (FC) images of the signal, and two of which are phaseconjugated (PC) images of the signal. If such a device is to be useful, the signal must be amplified, and the PC and FC idlers must be produced, with minimal noise. In this paper the quantum noise properties of two-sideband (TS) parametric devices are reviewed and the properties of many-sideband devices are determined. These results are applied to the study of two-pump PAs, which are based on the aforementioned four-sideband (FS) interaction. As a general guideline, the more sidebands that interact, the higher are the noise levels. However, if the pump frequencies are tuned to maximize the frequency bandwidth of the FS interaction, the signal and idler noise-figures are only slightly higher than the noise figures associated with the limiting TS interactions.

P2-40 Effect of a Waveguide in the Ultrasonic Phase Conjugate Imaging System(Short presentation for poster)

P2-40 Effect of a Waveguide in the Ultrasonic Phase Conjugate Imaging System(Short presentation for poster)

Phase Conjugate Image Enhancement using Photorefractive Media and Application to Surface Defect Detection

Phase Conjugate Image Enhancement using Photorefractive Media and Application to Surface Defect Detection

Image restoration through aberrant media by optical phase conjugation in a type II three-wave mixing interaction

Restoration of an image through an aberrant plate has been achieved by use of optical phase conjugation in a type II parametric interaction in which the amplified signal and idler waves are reflected back through the aberrant medium. The resolution of the restored phase-conjugate images is equal to 0.18 mm, and the amplification gain is ~4dB.

Phase conjugate imaging by type II parametric amplification

A method of imaging by optical phase conjugation using a three-wave degenerate parametric interaction is presented. The resolution in the phase conjugate image is about 90 μm and the image is amplified with a mean gain of 30 dB.

Sickle Mutually Pumped Phase-Conjugate Mirror: Theoretic and Experimental Demonstration

A new mutually pumped phase conjugator with a photorefractive copper-doped potassium sodium strontium barium niobate crystal is demonstrated, in which the beams travel along a geometric path similar to the outline of a sickle. The maximum reflectivity is as high as 168%. Phase-conjugate images are obtained. The two-interaction region theory, based on the coupled plane wave amplitude, is used to account for the main characteristics of the new phase conjugator. The threshold coupling strength which depends on the reflection loss and the incident beam ratio, is also investigated and calculated.

Experimental investigation of high resolution imaging using Brillouin-enhanced four-wave mixing

We present the results of an experimental investigation of a polarization-decoupled Brillouin-enhanced four-wave mixing (BEFWM) configuration used for short-pulse high-resolution imaging. In this geometry, the four-wave mixing medium is carbon disulphide (CS2) and the counterpropagating pump is generated from the forward pump by stimulated Brillouin scattering in a second CS2 cell. This configuration produced a good quality, high resolution (∼9 μm) phase conjugate image of a United States Air Force test chart, correcting for aberrations in the system.

Edge enhancement of a phase-conjugate image with a mutually pumped phase conjugator

Usually edge enhancement of optical images is produced by introduction of loss into the low spatial frequency components of the image-bearing beam in the Fourier plane of a lens. We report on edge-enhanced phase-conjugate images from a mutually pumped conjugator accomplished by spatial filtering in the Fraunhofer diffraction region of the input beams. High-resolution (128-160 lines/mm) edge-enhanced images are obtained through traditional spatial filtering in the Fourier plane and in the Fraunhofer regime. Amplification of these edge-enhanced images is observed with some loss of high spatial resolution (50 lines/mm).

Phase conjugation, edge detection and image broadcasting using two-beam coupling configurations in photorefractive KNbO 3

Two-beam coupling configurations are employed to obtain the phase conjugate image and edge extraction of an object using photorefractive KNbO 3: Fe. Also, the self-organization of a beam in the material into a hexagonal spot array is utilized to broadcast an input object to the location of each of the spots.