The experimental results show that the proposed multi-scale progressive DTM construction method can effectively realize the filtering in complex terrain environments. The proposed method is less affected by terrain changes and can effectively protect terrain details in areas where the slope of the terrain changes drastically. Four backpack LiDAR point clouds in complex terrain areas are used for testing. The average total error of the filtering algorithm in this study is 7.88%, while the average Kappa coefficient is 79.48%. When compared with the three other filtering methods, namely LAStools, MCC, and CSF, the average total error of the proposed method is the lowest, while the average Kappa coefficient is the highest. Thus, it can be concluded that the proposed method can obtain better filtering performance for different terrain environments.

Based on a vehicle-mounted laser track point cloud, this paper develops a rail surface and sleeper detection algorithm. Multiple sets of different grid size comparison experiments are carried out in the process of extracting the rail surface, based on rail data in different regions, and the optimal grid size range is 0.080.1 m. The effect is shown in Fig. 6. On this basis, in contrast with the method proposed by Yang, the rail surface extraction effect of this algorithm is slightly better than that of Yang’s algorithm, and the robustness is better. The extraction quality averages 97.8% and 96.3%, respectively. Several different basic threshold segmentation experiments are carried out during the extraction process to extract sleepers for different areas. In bridge areas and non-bridge areas, the optimal basic thresholds are 0.25 m and 0.28 m, respectively. The maximum value of the z-axis of the point cloud in each segment is used as the upper limit of segmentation, and the difference between it and the corresponding basic threshold value is used as the lower limit of segmentation, which has a better extraction effect. Fig. 7 depicts the effect. The extraction quality is 93.6%. In conclusion, the algorithm proposed in this paper is effective and feasible, has some practical applications, and can provide efficient and accurate measurement data for track maintenance.

In this paper, the beam quality optimization of an elliptical Gaussian beam under wind-dominated thermal blooming is studied analytically and numerically. The expression of distortion parameter of an elliptical Gaussian beam propagating in the atmosphere is derived, and its correctness is proved. Due to the astigmatism of an elliptical Gaussian beam, the influence of atmospheric thermal blooming on its propagation depends on the wind direction. Thermal blooming can be weakened by making the short axis of a focused EGB along the general wind direction. In the atmosphere, as the beam width in the windward direction of the source plane is large (the same spot area), the thermal blooming effect on the EGB is weak, which results in a better symmetrical spot and energy focus ability at the target, i.e. better beam quality at the target. The time required to achieve steady-state thermal blooming for a focused elliptical Gaussian beam is proportional to the beam width along the wind direction in the source plane.

The propagation model for the laser quads in the hohlraum based on broadband laser beam smoothed using ISI and de-DLA has been built in this study, aiming at the optical path arrangement scheme and cylindrical target hohlraum structure of the NIF, and then the irradiation characteristics of the laser quads on the hohlraum wall have been analyzed and optimized. The results show that the mismatch between the focal length of the primary lens and parameters of the cylindrical target hohlraum will cause varying degrees of damage to the envelope of the laser spots on the hohlraum wall for beams with different incident angles, as a result of the laser spots overlapping on the hohlraum wall, the irradiation uniformity on the hohlraum wall greatly diminishes. The effect of the principal lens focal length, the number of DLA sub-lens, and the long and short axes ratio of the sub-lens on the intensity distribution of the laser quads on the hohlraum wall are discussed on this basis. The optimization of these parameters, such as the principal lens focal length, number of DLA sub-lens, the long and short axes ratio of the sub-lens, and incident angle of the laser quads, achieves the improvement of the irradiation uniformity on the hohlraum wall. The results show that increasing the focal length of the primary lens appropriately can effectively retain the envelope of the laser spots on the hohlraum wall for beams with different incident angles, minimizing the overlap of the laser spots on the hohlraum wall. Besides, the duty ratio of laser spots on the hohlraum wall increases, and the time required for beam smoothing reduces when the number of DLA sub-lens and the long and short axes ratio of the sub-lens are optimized. Furthermore, after optimizing the incident angle of the inner cone laser quads, the overlap of the inner and outer cones laser quads on the hohlraum wall is eliminated.

This paper proposes an ultrafast beam smoothing scheme based on the rotation arrangement of phase plates. By rotationally arranging the phase plates with a rotational asymmetric distribution in the laser quad, different spatial phase modulations are first provided for each sub-beam in the laser quad. Then the dynamic interference of the sub-beams with a certain wavelength difference on the target plane makes the speckles within the focal spot sweep rapidly in multiple directions and multiple dimensions, so as to achieve the goal of improving the uniformity of the focal spot in the picosecond time scale. On this basis, the effects of parameters of phase plates, beam arrangement, spatial wavefront distortion, and spatial deviation of laser beams on beam smoothing are analyzed. The results show that in the ultrafast beam smoothing scheme based on the rotation arrangement of phase plates, only the same phase plates with a rotational asymmetric distribution are required to be processed at the same time, which can lower the design and processing difficulty of phase plates. In addition, this scheme is little affected by spatial wavefront distortion, beam arrangement, and spatial deviation of laser beams. Combining the beam smoothing scheme based on the rotation arrangement of phase plates proposed in this paper with the traditional beam smoothing schemes can significantly improve the uniformity of focal spots within a few picoseconds, which can be an effective supplement to the traditional beam smoothing schemes.

In this study, we investigate the dynamics of laser-induced bubble pairs with variable relative interval. The bubble pair oscillation process significantly varies with relative interval. For a noncoalesced bubble pair, the oscillation is nearly spherically during the first period, but both of their oscillation processes are prolonged. For a coalesced bubble pair, the smaller the relative interval, the more spherical the bubble shape during its first period. The first oscillation period is longer than that of every single bubble and unaffected by the relative interval when it is less than 0.75. Besides, the evolution of coalesced bubble could still be described by Rayleigh-Plesset model. However, the relative interval of bubble pairs significantly influences the collapse shock wave emission and rebound bubble generation after the collapse of the coalesced bubble. The findings of this study are expected to facilitate the applications of laser-induced bubbles in microfluidic operations, such as rapid mixing and cell sorting.

In this study, we propose a high-speed secure key distribution scheme based on symmetric-phase-shift-keying chaotic synchronization. Using the delayed self-interference of an unbalanced M-Z interferometer, the correlation between the drive and response signals is reduced to 0.25. The eavesdropper cannot directly obtain part of the relevant key from the chaotic drive signal to improve the key distribution security. An open-loop chaotic synchronization structure is constructed by a commonly driven semiconductor laser without external cavity feedback, which avoid multiple oscillations of chaotic signals in the feedback external cavity in the closed-loop structure, shorten the recovery time of chaotic synchronization to 1.8 ns, and improve the key distribution rate. Finally, when the BER is 3.8×10^-3, a high-speed secure key distribution at a rate of 1.28 Gbit/s is realized.

This study investigated the effect of atmospheric turbulence on laser communication systems and proposed a communication detection and reception method based on closed-loop regulation of the APD gain factor. Building a wireless laser communication experiment verifies that the APD gain closed-loop regulation method significantly improves the system performance under the turbulence fading channel. The above experiments show that the APD gain control can effectively suppress the optical power fluctuations generated by atmospheric turbulence jitter; thus, significantly improving the communication performance. Under weak turbulence conditions, the power spectrum curve with AGC is more consistent with the 5/3 power-law spectrum. The variance of light intensity flicker is reduced from 0.057 to 0.023; the system R_BE is reduced from 1.75×10^-8 to 3.99×10^-10, a reduction of two orders of magnitude. Finally, the turbulence intensity and corresponding system average R_BE variation curves under different periods are calculated. This shows that the adaptive gain control algorithm effectively suppresses turbulence-induced signal power fluctuations and achieves high interference immunity for wireless laser communication.

Based on the mechanism of the IOFDR technique, this study proposed a low-cost, high-precision and distributed fibre quality detection method by taking Rayleigh backscattering light in the fibre as the signal light combined with its light wave conduction equation. Additionally, its numerical model is derived in detail. The system structure is designed. A high-precision online health monitoring system for optical fibre networks is developed, which realises the quality and health monitoring of optical fibre networks by frequent-spatial transformation. The experimental verification shows that the system can realise distributed detection of 10 km optical fibre with very low optical power (<10 mW). It can also guarantee the spatial resolution of 0.1 m with no difference along the optical fibre. The dynamic range is better than 34.5 dB, and the event blind area is very low; thus it accurately reflects the position and quality of fibre failure points or joints. In future studies, we will improve the range of frequency modulation and frequency modulation rate, focussing on the suppression and solution of nonlinear problems in frequency modulation. The spatial resolution and measurement distance index of IOFDR will be further improved, which is a low-cost and high-precision optical fibre sensing and detection technology with great development potential and promotion value.

We propose a method to acquire respiration and heartbeat signals based on FBG sensors. A combined variational mode decomposition with improved wavelet threshold function (VMD-IWT) noise reduction algorithm is used to remove noise interference existing in signals. The simulation results show that our proposed algorithm realizes the best SNR, RMSE, and other indicators, and makes actual signals smoother after noise reduction. We use a band-pass filter to separate signals and calculate their frequencies. The maximum error rate of heart rate is 8.75% with respect to the reference value, and the maximum deviation of respiration rate from the reference value is 1 bpm, which proves the better accuracy of the proposed method. This provides a more convenient and economical way to monitor health at home.

This study describes the underwater OOK channel model and analyses the correspondence between BER and SNR of underwater OOK modulation methods in different water quality conditions. To cope with the impact of an underwater channel on the optical signal transmission, an underwater optical communication transmitter-transceiver based on hardware circuits and field-programmable logic gate devices is designed. The digital signal processing modules such as FIR filtering (to improve the SNR of the system), adaptive judgement threshold and sliding mean filtering are designed to improve the communication BER performance. The communication performance of the underwater communication transmitter and transceiver is verified under different water quality conditions. The experimental results show that the terminal can achieve a sensitivity of -38 dm at a transmission rate of 5 Mbps and BER of 10^-6. The transmission distance can reach 20, 10 and 4.5 m in class I, class II and class III waters, respectively. In the class III water test, the communication distance of 5 m and BER of 10^-5 can meet the demand of voice transmission. The distortion-free image and SD video transmission function can be realised at the communication distance of 4.5 m and BER of 10^-6, verifying the feasibility of underwater optical communication based on digital signal processing.

In this study, the transmission characteristics of a double Sagnac loop filter, composed of two segments of PMF in parallel, are analyzed, simulated and measured in detail, which proves that the proposed double Sagnac loop filter has the polarization-independent characteristic and the tunable channel spacing. According to the results of simulation and measurement, a channel spacing tunable multi-wavelength erbium-doped fiber laser utilizing a double Sagnac loop filter with the assistance of the FWM effect is designed. The laser can output multi-wavelength lasing lines with channel spacings of 0.35 nm and 0.9 nm. Owing to the polarization-independent characteristic of the double Sagnac loop filter, the polarization-dependent devices are not interfere with the transmission spectra of the filter. Thus, the double Sagnac loop filter exhibits the simple structure, polarization-independence, the tunability of channel spacing, and a great potential application in the field of multi-wavelength fiber lasers.

Tabletop 3D light-field display with full parallax based on direction-turning films and views-segmented voxels is demonstrated. In this system, the collimated backlight and the lenticular lens array reversely attached to an LCD panel with a resolution of 7680 pixel×4320 pixel are designed to achieve a 100° horizontal viewing angle, and the direction turning films and the holographic functional screen are applied to reconstruct 3D light-field images within a 90° frontal viewing angle. Views-segmented voxels are first proposed to separate the frontal viewing range into some disparate viewing zones according to the spatial coordinates of different viewers, then the views-segmented image coding method and the mapping function between the views-segmented voxels and sub-pixels on LCD are also deduced. Finally, after the real-time adaptive perspective correction in each viewing zone, a full parallax, high-definition tabletop 3D scene is presented to multiple viewers.

To achieve the fusion display effect of real and virtual scenes in a holographic stereogram, an image coding method based on instance segmentation and depth value determination is proposed. A theoretical analysis and the experimental results show that the proposed image coding method can effectively add some virtual 3D elements in the real scene to enhance the visual experience. The combination of virtual and real scenes fully considers the occlusion relationship, which is not a simple scene superposition. The key to fully considering the occlusion relationship is to assign and determine the pseudo depth after scene layering. There is still a significant gap between the virtual reality fusion method discussed in this paper and the latest work in the AR field. However, the proposed method provides a basic idea for further research into holographic stereogram AR displays. For example, by continuously improving the efficiency of instance segmentation, the scene is more accurately layered. The accurate depth calculation method is studied, and the accurate depth value is used as the basic processing data, which can better show the effect of virtual real scene fusion display in a holographic stereogram. This paper only considers the simple and small scenes used in an experiment and tends to assume ideal conditions. In theory, the proposed method is suitable for scenes with a clear occlusion relationship between different instances in the sampled image; however, it cannot effectively deal with the occlusion between the same instances. Therefore, further analysis of complex scenes is required.

Here, we propose and verify the possibility of using the Raman field as an intermediate process to excite the Brillouin OFC in a free-space oscillator. As far as we know, this is the highest reported power for any Brillouin OFCs, which is four orders of magnitude higher than that of the micro-resonator-based OFC. This free-space approach provides a new path for realizing high-power OFCs in specific wavelengths.

A new edge detection technique based on iodine molecular absorption cells is proposed. This technical path has the advantages of signal intensity detection independent of incident angle, fast measurement speed, not being easily affected by laser intensity jitter, and not requiring salinity information. It is expected to be used on airborne, spaceborne, and other large mobile platforms, and it has a promising future application.

In this paper, a method for measuring laser wavelength stability with the FMCW interferometric cavity is proposed, which uses the advantages of high precision, high resolution, and real time measurement of FMCW interference to solve the quantification problem of laser wavelength stability. By deducing the theory underlying the interferometric cavity measurement method, the demodulation algorithm of wavelength drifts is designed to realize the real time measurement of laser wavelength drifts and effectively improve the measurement resolution of wavelength drifts. The experimental results show that the measurement resolution of wavelength shift of the proposed method is 0.016 pm, the calculation speed of wavelength shifts is up to 50/s, and the measurement time is 0.02 s. Compared with those of the optical beating method and interference comparator method, the measurement speed is greatly improved. The laser shows an average wavelength stability of 0.19×10^-6 within 1 h. This stability is better than that of most dual-frequency lasers on the market . The proposed method is of great significance to the research on improving the accuracy of FMCW interferometry and has good application value in the field of laser wavelength stability measurements.

This study proposes a detection method for the 248 nm depolarizer independent of laser intensity fluctuation. We selecte an excimer KrF laser with a stable output wavelength of 248.372 nm as the light source. The adverse effects caused by the light intensity fluctuation are overcome through pre-polarizing and polarization splitting design. The error analysis shows that the proposed method is suitable for performance detection of the depolarizer. The absolute system error is less than 0.1226%, and the random error is ~0.2000%. The verification experiments show that the measured mean DOP is highly consistent with the theoretical value. It also shows that the measurement system error is only 0.0423%, confirming the validity of the proposed method. The stable test shows the repeatability level (3σ=0.2316%) is consistent with the analysis expectation (~0.2000%), and the reproducibility level (3σ=0.3090%) is only slightly worse than the repeatability level, indicating the stability of the device. Therefore, the proposed method provides a stable and effective tool for offline performance detection of the depolarizer used in the 248 nm lithography tool illumination system. Furthermore, the method is essential and should be further developed to meet the requirements of shorter wavelength lithography systems.

The experimental results show that both wavelength modulation and cavity length modulation can complete the periodic acquisition of ring-down signal and repeated measurement of ring-down time. However, from the comparison, the cavity length modulation method had higher measurement accuracy and lower limit detection of concentration, while the wavelength modulation exploits a tunable laser, and its structure is relatively simple without additional mechanical structures such as PZT. Therefore, in practical application, the selection can be made between measurement performance, device complexity, and cost according to the demand.

To further understand the aging behavior of microplastics in nature influenced by environmental factors, we investigate the aging mechanism of micron-sized PS particles at a single-particle level by simulating UV radiation. The spectral characteristics of PS particles under particle size and aging time are characterized using the single-particle Raman spectroscopy, and the principal component analysis further reveals the changes in the chemical structures of PS particles under UV exposure. UV radiation is found to damage the structures of PS polymers and PS molecules, and the small particle size, which is more significantly damaged, is degraded to a higher extent. These results suggest that the microplastics with different sizes exhibit different behavioral processes when facing with the same environmental factors, and that small size microplastics are more likely to be decomposed into small particles or small fragments that are potentially more threatening after UV radiation. This study provides new insights into the aging dynamics of microplastics in the environment from the single particle level.