Determination of nonlinear refractive index by an iterative phase retrieval method.

Based on the chaos and phase retrieval algorithm, a hierarchical multiple binary image encryption is proposed. In the encryption process, each plaintext is encrypted into a diffraction intensity pattern by two chaos-generated random phase masks (RPMs). Thereafter, the captured diffraction intensity patterns are partially selected by different binary masks and then combined together to form a single intensity pattern. The combined intensity pattern is saved as ciphertext. For decryption, an iterative phase retrieval algorithm is performed, in which a support constraint in the output plane and a median filtering operation are utilized to achieve a rapid convergence rate without a stagnation problem. The proposed scheme has a simple optical setup and large encryption capacity. In particular, it is well suited for constructing a hierarchical security system. The security and robustness of the proposal are also investigated.

The most biological samples are transparent and have no amplitude modulation for the incident wave. It is hard to observe the sample without dyeing in traditional microscope. Phase retrieval algorithm can reconstruct complex-valued sample with several diffraction intensity patterns. In this paper, a lensless imaging method is designed by diffuser 1-D moving, which decreases acquisition dada time, simplifies the experimental equipment and reduces experiment cost compared to typical ptychography based on 2-D shifting. A diffuser is placed into imaging system to improve resolution, which brought higher frequency information into the captured patterns via scattering effect. In field reconstruction, the additional amplitude constraint is added into the iterative phase retrieval algorithm. The proposed algorithm achieved faster convergence and reconstructed high-quality images. For the biological sample image, a wide field of view was realized with 20 diffraction patterns.

Due to its low complexity and acceptable accuracy, phase retrieval technique has been proposed as an alternative to solve the classic optical surface measurement task. However, to capture the overall wave field, phase retrieval based optical surface measurement (PROSM) system has to moderate the CCD position during the multiple-sampling procedure. The mechanical modules of CCD movement may bring about unexpectable deviation to the final results. To overcome this drawback, we propose a new PROSM method based on spatial light modulator (SLM). The mechanical CCD movement can be replaced by an electrical moderation of SLM patterns; thus the deviation can be significantly suppressed in the new PROSM method. In addition, to further improve the performance, we propose a new iterative threshold phase retrieval algorithm with sparsity-constraint to effectively reconstruct the phase of wave field. Experimental results show that the new method provides a more simple and robust solution for the optical surface measurement than the traditional techniques and achieves higher accuracy.

An approach to adaptive optics utilizing a single-pixel camera (SPC) is proposed to maximize fiber coupling efficiency at the receiver side of an optical satellite-to-ground link perturbed by atmospheric turbulence. Using a single-pixel wavefront sensor enables operation at longer optical wavelengths, such as near and far infrared, which have advantageous propagation characteristics for free space optical communication. In this approach, a focal plane intensity image of the atmospheric-disturbed wavefront is taken via an SPC using a compressed sensing technique. An iterative speckle-based phase retrieval algorithm is then applied to infer the phase distortion corrected by a deformable mirror in a feedback loop. This computational approach to inferring the phase of the wavefront overcomes the limitations of traditional Shack-Hartman-based approaches, which are difficult to implement at high speed and at the long infrared wavelengths proposed for future optical satellite communication downlinks. It has been shown that fiber coupling efficiency is increased from less than 5% to 40%-50% in medium-to-strong turbulence scenarios with the phase retrieval algorithm proposed in this work.

We determine the absolute electron density of a lithographically grown nanostructure with 25 nm resolution by combining hard x-ray Fourier transform holography with iterative phase retrieval methods. While holography immediately reveals an unambiguous image of the object, we deploy in addition iterative phase retrieval algorithms for pushing the resolution close to the diffraction limit. The use of hard (8 keV) x rays eliminates practically all constraints on sample environment and enables a destruction-free investigation of relatively thick or buried samples, making holographic diffraction imaging a very attractive tool for materials science. We note that the technique is ideally suited for subpicosecond imaging that will become possible with the emerging hard x-ray free-electron lasers.

The iterative phase retrieval based on phase diversity technologies can solve the stagnation problem of Gerchberg–Saxton algorithm which performs Fourier transform to iterate back and forth between the object and spectral planes with known constraints. However, the application of phase diversity technologies in iterative phase retrieval methods will bring in multiple physical parameters such as distances or wavelengths. The measured accuracy of these physical parameters will ultimately affect the accuracy of the iterative phase retrieval methods. In this paper, a physical parameters estimation method which has the advantages of high global convergence and local convergence is proposed to improve the accuracy of iterative phase retrieval methods. Meanwhile, this method is introduced in the single-beam multiple-intensity reconstruction (SBMIR), termed PE-SBMIR, and its performance is verified by simulations and experiments. By simulating multiple sets of distance parameters with errors, the retrieved accuracy using PE-SBMIR can be improved by 2–4 orders of magnitude compared with SBMIR. Experimental results show that whether it is an amplitude-type object or phase-type object, the accuracy using PE-SBMIR is significantly higher than using SBMIR. The physical parameters estimation method proposed in this paper may be adopted in other iterative phase retrieval methods using phase diversity technologies.

A time resolved resonance Raman experimental set-up is described. It is based on the use of two independent lasers : a Q-switched Nd-YAG pumped dye laser and an excimer laser. A synchronization electronic device has been made to fire the two lasers with variable delays in the range .0 ns -+ 10 ms with a 10 ns step variation. A drift control unit is used to prevent any slow drifts of the excimer laser. The overall jitter between the two laser pulses is 4.5 ns. INTRODUCTION Time resolved resonance Raman spectroscopy is becoming a very powerful technique for the caracterisation of reactive excited states and transient intermediates in fast photochemical reactions (1,2). It provides the means of identifying the transient species, analysing their structures and bonding, and also of monitoring their kinetics of formation and decay. The two-color pump probe method is generally applied. The pump beam is used to start the photochemical reaction by providing energy to the system and promoting the molecules to high excited states. The probe, which is delayed with respect to the pump, is used to excite resonance Rarnan spectra of the transient species. Both have to be tuned to an electric transition : pump to the absorption wavelength of the molecules in the ground state and probe to the transient absorption of the species being analysed. In recent experiments, we have recorded the resonance Raman spectra of excited states and radicals of various molecules using a Q-switched Nd-YAG pumped dye laser (3,4,5). In these cases, the third or the fourth harmonic of the YAG laser was used as the pump beam and the resonance Raman spectra were excited by the dye laser pumped by either the second or the third harmonic of the YAG. The probe beam was delayed by an optical delay line (3 ns/m). Unfortunately, the low quality of the laser beams makes it impossible to use large delays and most experiments were performed in the range 20-50 ns. These experimental conditions did not allow kinetic measurements to be made. In order to overcome this problem, one has to use two independent lasers and synchronize their firing to increase the delays time between the probe and pump beams. TIME-RESOLVED EXPERIMENTS WITH TWO LASERS We have developed a new time-resolved Rarnan experimental set-up using the same Q-switched Nd-YAG pumped dye laser for the probe beam. An excimer laser was preferred for the pump beam because it is less expensive than a second Nd-YAG pumped dye laser and it emits many wavelengths in the UV region (193, 222, 248, Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19917123 C 7 4 6 4 JOURNAL DE PHYSIQUE IV 308, 351 nm) that are generally suitable to fit the absorption spectra of most organic compounds. We have used an YG 581 C QUANTEL YAG laser pumping a TDL 50 QUANTEL dye laser. The output of the dye laser can be continuously tuned from 220 nm to 750 nm by direct pumping of red dyes or coumarin using the second and the third harmonics of the YAG respectively, or by frequency-doubling and/or frequency mixing. The Excimer laser is a QUESTEK model 2420. The problem was to synchronize the two lasers with as low a jitter as possible. In all cases, the jitter time must be lower than the duration of the laser pulses, i.e. 7-9 ns for the dye and 20-30 ns for the Excimer. The repetition rate was fixed at 10 Hz because of the YAG laser. The YAG laser needs three trigger orders to be fired ; one to command the charge of the capacitors, the second to fire the flashlamps and the third to trig the pockels cell. The capacitors require about 80 ms to be charged. The excimer laser needs two trigger orders. One to command the charging of the capacitors and the second to fire the thyratron. For this laser, about 30 ms are needed to charge the capacitors. A digital pulse and delay generator has been designed to generate all the trigger orders, all the delays, and also to synchronize a gated multichannel detector with the YAG laser. The schematic diagram and the chronogram of this system are given in figures 1 and 2.

We have performed four wave mixing (FWM) measurements on a 2-section semiconductor optical amplifier (SOA) using CW and pulsed pump and probe beams. The FWM efficiency depends on the input pump power, the gain of the 2-section SOA and the wavelength separation between the pump and probe beams. A -19.1 dBm FWM signal power from the amplifier is achieved when the wavelength difference between the pump and probe beams is 2.5 nm. The pulse amplitude and pulse width of the FWM signal under the condition when the FWM signal is generated using pulsed pump and probe beams has been measured and analyzed. The results of the analysis are in agreement with the experimental data.

We calculated line-shapes of the coherent population trapping (CPT) for the open transition in 87 Rb by solving time-dependent optical Bloch equations and averaging results for the total excited state populations over atomic trajectories and velocities, while taking into account incident angles to the laser beam. Comparison with experiment produces good agreement for the laser intensity dependence of the CPT line-widths when the laser transverse beam profile is Gaussian. This theoretical model was used to calculate the probe laser CPT in the case of spatially separated pump beam. The probe beam is placed inside the cylindrically-shaped pump beam. The calculations as well as measurements show narrowing of the Hanle resonance due to Ramsey effect.

Self phase modulation is a nonlinear effect that is observed when a laser beam is focused on to a high-absorbing thermal medium. A regular tea sample in a plastic cuvette is used as the nonlinear absorbing sample. The change in the refractive index of the medium occurs due to the heat generated by the focused pump beam, which in turn changes the refractive index. In this paper, self phase modulation is investigated in different ways. An Ar-Ion laser of 514 nm is used as the pump beam and a 632 nm He-Ne laser is used as the probe beam. The probe beam is introduced from the opposite side of the pump beam. Ring patterns are observed from the each side of the sample. Regular far field ring patterns are observed from the pump beam, and two sets of rings are observed with the probe beam. The behaviors of these inner and outer rings are monitored for different pump powers. The steady state heat equation is solved to obtain an exact solution for the radial heat distribution and far field ring patterns are simulated using the Fresnel–Kirchhoff diffraction integral. Ring patterns are theoretically explained using simulations results, and compared with experimental observations. Finally, an interferometric setup using the low power He-Ne laser is also used to determine the induced change in refractive index. Results are compared with those obtained directly from self-phase modulation and from the probe beam method.

Transfer orthogonal sparsifying transform learning for phase retrieval

The problem of phase retrieval underlies various imaging methods from astronomy to nanoscale imaging. Traditional phase retrieval methods are iterative and are therefore computationally expensive. Deep learning (DL) models have been developed to either provide learned priors or completely replace phase retrieval. However, such models require vast amounts of labeled data, which can only be obtained through simulation or performing computationally prohibitive phase retrieval on experimental datasets. Using 3D X-ray Bragg coherent diffraction imaging (BCDI) as a representative technique, we demonstrate AutoPhaseNN, a DL-based approach which learns to solve the phase problem without labeled data. By incorporating the imaging physics into the DL model during training, AutoPhaseNN learns to invert 3D BCDI data in a single shot without ever being shown real space images. Once trained, AutoPhaseNN can be effectively used in the 3D BCDI data inversion about 100× faster than iterative phase retrieval methods while providing comparable image quality.

A novel optical image encryption system is proposed using password key based on phase retrieval algorithm (PRA). In the encryption process, a shared image is taken as a symmetric key and the plaintext is encoded into the phase-only mask based on the iterative PRA. The linear relationship between the plaintext and ciphertext is broken using the password key, which can resist the known plaintext attack. The symmetric key and the retrieved phase are imported into the input plane and Fourier plane of 4f system during the decryption, respectively, so as to obtain the plaintext on the CCD. Finally, we analyse the key space of the password key, and the results show that the proposed scheme can resist a brute force attack due to the flexibility of the password key.

We show that de-focused single particle micrographs recorded using a cryo-electron microscope (cryoEM) system may be processed like a Fresnel zone in-line hologram to obtain physically meaningful quantitative phase maps associated with individual particles. In particular, a region-of-interest (ROI) of the de-focused micrograph surrounding a particle can be numerically back-propagated, in order to determine accurate de-focus information for single particles based on the sparsity-of-gradient merit function. Further with the knowledge of de-focus information, an iterative Fresnel zone phase retrieval algorithm using image sparsity constraints can accurately estimate the quantitative phase information associated with a single particle. The proposed methodology which can correct for both de-focus and spherical aberrations is a deviation from the image processing chain currently used in single particle cryoEM reconstructions. Our illustrations as presented here suggest that the phase retrieval approach applies uniformly to de-focused image data recorded using the traditional CCD detectors as well as the newer direct electron detectors.

Generation of hollow beams by using phase filtering with multi-distance phase retrieval