Tilted wavefront coding system to eliminate the retroreflection with superior imaging property: publisher's note.

This publisher's note corrects information in the author affiliations in Appl. Opt.58, 7205 (2019).APOPAI0003-693510.1364/AO.58.007205.

Tilted wavefront coding system to eliminate the retroreflection with superior imaging property.

Since some previous methods sacrifice too much imaging quality to achieve the valid retroreflection reduction, a novel image plane tilted system combined with the wavefront coding is proposed in this paper, which could improve the privacy and security of the imaging equipment by eliminating the retroreflection without suffering significant degradation in imaging quality. First, the theoretical model of the tilted wavefront coding system is established based on the wave-optics theory. According to the derived point spread function and the modulation transfer function, the maximum tilt angle of the image plane is obtained. Second, the target system is designed and optimized by Zemax, and the later simulation analyzes the effects of the tilting on both the retroreflection and the imaging property. Finally, the experiments verify the simulation results. It shows that when the tilt angle equals 5°, there is no longer significant retroreflection on the observation plane with the total receiving power decreasing to nearly 4 orders of magnitude compared with the state without tilting. Moreover, within the maximum tilt angle, the final decoded images of the wavefront coding system still maintain superior clarity and uniformity for both the artificial and the natural objects. Consequently, the BRISQUE index is reduced by 15.92% and 28.19%, respectively, compared with the conventional imaging system, which indicates that the image quality is significantly improved by the proposed system. In conclusion, the technique of wavefront coding could effectively compensate for the high-order aberrations induced by the tilting and preserve a large field of view for precise imaging with no retroreflection, which shows us a broad prospect.

Experiment on the temporal evolution characteristics of a CCD multilayer structure irradiated by a 1.06 µm continuous laser.

In this study, an experiment of a CCD detector irradiated by a 1.06 µm continuous laser is established. Factors including the microlens focusing beam, microlens carbonization at high temperature, shading aluminum film opening rate, low shading aluminum film absorption rate, and different materials' absorption to the laser are comprehensively considered. By combining the irradiation time, the output image of the CCD detector, surface morphology of the damage area, the optical micrograph, and regional energy spectrogram, the damage mechanism of the CCD is explored. In addition, the threshold time and the damage morphology of the multilayer structure of the CCD detector are investigated. The results show that when the irradiation time increases, the damage starts from the microlens due to melting, which is represented as point damage. Subsequently, the aluminum film melts and is separated from the SiO2 by stress and melting damage, causing vertical bright linear damage. Without the protection of the shading aluminum film, the silicon electrode heats up and reaches the melting point, causing damage to the wiring circuit, which is represented as horizontal dark linear damage. Eventually, the N-Si layer in the silicon substrate melts and the clock signal is destroyed, which means that the optical signal is not converted into an electrical signal. The CCD detector gets completely damaged.

Laser filamentation in air via Mathieu modulation: ranging from trajectory-predesigned curved filament to quasi-soliton and ring light bullet.

We propose theoretically various kinds of filaments via the Mathieu modulation. Our results indicate curved filaments, in-phase and out-of-phase quasi-solitons and nonlinear light bullets can be formed successfully in air. Through calculated initial Mathieu accelerating beam (MAB), curved filament is able to propagate along a predesigned elliptical trajectory. By transforming the MAB into an axial symmetrical structure with in-phase and out-of-phase modulations, we obtain two kinds of quasi-solitons in air, respectively. The latter case can even propagate in a breathing fashion. With a ring structure of MAB, we successfully form a light bullet in air that generates a chain of intensity peaks over extended distances. These unique filaments can offer significant advantages for numerous applications, such as micro engineering of materials, THz radiation generation and attosecond physics.

Method to measure the phase modulation characteristics of a liquid crystal spatial light modulator.

The universal liquid crystal spatial light modulator (LC-SLM) is widely used in many aspects of optical studies. The working principles and applications of LC-SLM were introduced briefly. The traditional Twyman-Green interference method, which was used to measure the phase modulation characteristics of a liquid spatial light modulator, had some obvious disadvantages in practice. To avoid these issues, the traditional Twyman-Green interference method was improved. Also, a new method to process interference fringes and measure the shift distances and cycles automatically by computers was proposed. The phase modulation characteristics of P512-1064 LC-SLM produced by the Meadowlark Company were measured to verify the validity of the newly proposed method. In addition, in order to compensate and correct the nonlinear characteristics of the phase modulation curve, three universal inverse interpolation methods were utilized. The root mean squared error and residual sum of squares between the calibrated phase modulation curve and the ideal phase modulation curve were reduced obviously by taking advantage of the inverse interpolation methods. Subsequently, the method of shape-preserving subsection cubic interpolation had acquired the best performance with high computation efficiency. Experiments have been performed to verify the validity of the interpolation method. The experimental results showed that the phase modulation characteristics of LC-LSM could be acquired and calibrated automatically with convenience and high efficiency by utilizing the newly proposed processing method.

Multi-layered black phosphorus as saturable absorber for pulsed Cr:ZnSe laser at 2.4 µm.

A high-quality black phosphorus (BP) saturable-absorber mirror (SAM) was successfully fabricated with the multi-layered BP, prepared by liquid-phase exfoliation (LPE) method. The modulation depth and saturation power intensity of BP absorber were measured to be 10.7% and 0.96 MW/cm(2), respectively. Using the BP-SAM, we experimentally demonstrated the mid-infrared (mid-IR) pulse generation from a BP Q-switched Cr:ZnSe laser for the first time to our best knowledge. Stable Q-switched pulse as short as 189 ns with an average output power of 36 mW was realized at 2.4 µm, corresponding to a repetition rate of 176 kHz and a single pulse energy of 205 nJ. Our work sufficiently validated that multi-layer BP could be used as an optical modulator for mid-IR pulse laser sources.

Dynamic optical sampling by cavity tuning and its application in lidar.

Optical sampling by cavity tuning (OSCAT) enables cost-effective realization of fast tunable optical delay using a single femtosecond laser. We report here a dynamic model of OSCAT, taking into account the continuous modulation of laser repetition rates. This allows us to evaluate the delay scan depth under high interferometer imbalance and high scan rates, which cannot be described by the previous static model. We also report the demonstration of remote motion tracking based on fast OSCAT. Target vibration as small as 15 µm peak to peak and as fast as 50 Hz along line-of-sight has been successfully detected at an equivalent free-space distance of more than 2 km.

Atmospheric transfer of a radio-frequency clock signal with a diode laser.

Remote transfer of a radio-frequency clock signal over a 60 m open atmospheric link has been experimentally investigated using a diode laser as the clock carrier. Phase-noise spectra and Allan deviations are both measured to characterize the excess clock instability incurred during the transfer process. Different detection schemes are used to assess the contributions from different noise sources. With an 80 MHz clock frequency, the total root-mean-square noise amplitude is measured to be about 5×10(-3) rad, with fractional frequency instability on the order of 1×10(-10) at 1 s. The majority of this excess noise is attributed to the transmitter noise, with the amplitude fluctuations of the diode laser identified as the main source. The excess phase noise caused by air turbulence is at the level of 10(-4) rad under the current experimental conditions. Our finding suggests that suppressing the transmitter noise is critical for improving the clock-transfer fidelity.