Far-field Spot Research Articles

Pulse compression via cascaded SBS and α-Al2O3 nanosheet enhanced LIB

The leading-edge based stimulated Brillouin scattering (SBS) pulse compression technology is increasingly being recognized as a significant advancement for the generation of high-energy short-pulse lasers. Trailing-edge based laser-induced breakdown (LIB) compression is anticipated to be an effective supplement to SBS, helping to overcome its compression limitations. In this study, we integrated SBS and LIB to generate a high-power, short-pulse laser, thereby surpassing the compression constraints of SBS. Furthermore, we enhanced LIB by doping the LIB medium with α-Al2O3 nanosheets obtained through molten salt synthesis. As a result, we have successfully compressed the pulse duration from 8ns to 241ps, achieving relatively symmetrical leading and trailing edges. Moreover, compared to pure water, we obtained a higher quality far-field light spot.

Multi-line-of-sight Hartmann-Shack wavefront sensing based on image segmentation and K-means sorting

Multi-line-of-sight wavefront sensing, crucial for next-generation astronomy and laser applications, often increases system complexity by adding sensors. This research introduces, to the best of our knowledge, a novel method for multi-line-of-sight Hartmann-Shack wavefront sensing by using a single sensor, addressing challenges in centroid estimation and classification under atmospheric turbulence. This method contrasts with existing techniques that rely on multiple sensors, thereby reducing system complexity. Innovations include combining edge detection and peak extraction for precise centroid calculation, improved k-means clustering for robust centroid classification, and a centroid filling algorithm for subapertures with light loss. The method’s effectiveness was confirmed through simulations for a five-line-of-sight system and experimental setup for two-line and three-line-of-sight systems, demonstrating its potential in real atmospheric aberration correction conditions. Experimental findings indicate that, when implemented in a closed-loop configuration, the method significantly reduces wavefront residuals from 1 λ to 0.1 λ under authentic atmospheric turbulence conditions. Correspondingly, the quality of the far-field spot is enhanced by a factor of 2 to 4. These outcomes collectively highlight the method’s robust capability in enhancing optical system performance in environments characterized by genuine atmospheric turbulence.

Spot Invalid Point Repair Algorithm of Detector Array Measurement System Based on Image Correlation Coefficient

The detector array method has been widely used in the field of high-energy laser far-field spot parameter measurement due to its ability to directly measure the far-field spot of high-energy lasers, wide dynamic range of the detectors, high system sampling frequency, good real-time performance, and suitability for various testing environment requirements. However, during the measurement process, the irradiation of strong lasers or damage to other hardware systems can result in invalid points in the acquired spot images, thereby reducing the measurement accuracy of the system. In order to achieve accurate measurement of laser far-field spot parameters, this paper establishes an experimental model based on the analysis of the sampling spacing of the detector array target and proposes a laser far-field spot invalid point repair algorithm based on image correlation coefficient. Experimental results demonstrate that the algorithm proposed in this paper effectively reduces the impact of invalid points in the measurement system on the measurement accuracy, and achieves accurate measurement of high-energy laser measurement systems.

Quality and spatial intensity distribution characteristics of short-exposure spot images of a supercontinuum laser in a turbulent atmosphere.

In this study, considering the combined effects of atmospheric attenuation and turbulence, we examine the spot quality and spatial intensity distribution characteristics of a supercontinuum (SC) laser propagating in a turbulent atmosphere using the multi-layer phase-screen method. An increase in turbulence strength or a decrease in the initial beam radius resulted in a greater impact of the atmospheric turbulence on the SC laser. A decrease in source coherence results in the deterioration of the image quality of the far-field spot. Under moderate to strong turbulence, the scintillation index decreased as the source coherence decreased; however, the opposite trend was observed under weak turbulence. These results are significant for the advancement and optimization of SC laser systems.

Design of a real-time automatic resonator alignment method for unstable resonators

The auto-alignment method for unstable resonators proposed in this paper innovatively replaces the secondary mirror of traditional unstable resonators with a double-convex lens coated with a high reflective film on one side, and installs the primary mirror on a designed electrically adjustable bracket. During the laser emission process, the computer calculates the relative displacement of the centroid of the far-field spot on the camera detection surface to determine the magnitude of the resonator misalignment, and simultaneously controls the electrically adjustable bracket to complete the compensation for the resonator misalignment. Finally, the experimental results show that when using the real-time automatic resonator alignment method, the output power of the laser remains very stable with a relative standard deviation of the normalized power less than 1.1% (if the auto-alignment method is not adopted, the output power will decay to 89.8% of the peak value). Therefore, this method can effectively solve the resonator misalignment problem of unstable resonator lasers. In addition, this method has the advantages of simple optical path structure and high detection speed while ensuring high accuracy.

Fourier Ptychography-Based Measurement of Beam Divergence Angle for Vertical Cavity Surface-Emitting Laser

The Vertical Cavity Surface-Emitting Laser (VCSEL) has led to the rapid development of advanced fields such as communication, optical sensing, smart cars, and more. The accurate testing of VCSEL beam quality is an important prerequisite for its effective application. In this paper, a method for measuring the divergence angle of the VCSEL far field spot based on transmissive Fourier ptychography is proposed. First, a single CCD multi-angle VCSEL far-field spot acquisition system is designed. Second, based on the proposed Fourier ptychographic algorithm with synchronous optimization of embedded optical transfer function, a resolution-enhanced phase image of the spot is reconstructed and the boundary extracted by the Sobel operator of the phase image is defined as the boundary position of the beam waist. In this way, the beam waist radius of the laser beam is calculated. Finally, the divergence angle of the laser beam is measured via the radius of the beam waist. Compared with the traditional Gaussian beam definition method, the method proposed in this paper has higher accuracy in divergence angle measurement. The experimental results show that this method can improve the divergence angle measurement accuracy by up to 9.7%.

Integrated lithium niobate optical phased array for two-dimensional beam steering.

Optical phased arrays (OPAs) with high speed, low power consumption, and low insertion loss are appealing for various applications, including light detection and ranging, free-space communication, image projection, and imaging. These OPAs can be achieved by fully harnessing the advantages of integrated lithium niobate (LN) photonics, which include high electro-optical modulation speed, low driving voltage, and low optical loss. Here we present an integrated LN OPA that operates in the near-infrared regime. Our experimental results demonstrate 24 × 8° two-dimensional beam steering, a far-field beam spot with a full width at half maximum of 2 × 0.6°, and a sidelobe suppression level of 10 dB. Furthermore, the phase modulator of our OPA exhibits a half-wave voltage of 6 V. The low power consumption exhibited by our OPA makes it highly attractive for a wide range of applications. Beyond conventional applications, our OPA's high speed opens up the possibility of novel applications such as high-density point cloud generation and tomographic holography.

Multilevel synergically controlling wavefront correction of a high-power slab laser system.

We present a multilevel synergically controlling wavefront correction method that can apply in a slab laser system. To fully utilize the response frequency and the stroke of actuators of the single deformable mirror (DM), we design a set of multilevel wavefront correction devices to reduce the root-mean square of wavefront aberration before the DM. As the wavefront of slab geometry solid-state lasers mainly consists of fourth and longitudinally distributed aberration, such as 5th, 9th, and 14th orders of Legendre polynomials. We design a precompensating level of the aberration with a slow-drift mirror, fast-steer mirror, one-dimensional adjustable slab-aberration compensator, and beam-shaping system to reduce these orders of wavefront aberration with low spatial resolution and large stroke. As the controlling bandwidth of different devices is diverse, the coupling oscillation between the precompensating level and adaptive optics (AO) level occurs, then we develop the multilevel synergically control to address the coupling. With the precompensating level, the experimental result shows the residual wavefront aberration of the slab laser is compensated well by the AO level effectively within the compensating capability. We clean up a 9.8kW slab laser system with the beam quality β of far-field focus spots improved from 17.71 to 2.24 times the diffraction limit.

High-speed modal analysis of dynamic modal coupling in fiber laser oscillator

Up till now, the spatial and temporal dynamics of transverse mode instability (TMI) in fiber laser oscillator have increasingly attracted a worldwide attention. Here, we develop a high-speed modal decomposition (MD) system to analyze the modal coupling for fiber laser oscillator above the TMI threshold. A set of angular-multiplexing transmission functions (TFs) are designed for simultaneous MD and monitoring the far-field beam profile. The TMI threshold of the deployed fiber laser oscillator is 181 W at a co-pumping power (CPP) of 279 W. As the CPP increases from 318 W to 397 W, the power fluctuations of the output laser become more drastic. The changes of the far-field beam profile and the centroid of far-field spot (COFFS) indicate an increased velocity of energy transfer between modes. The high-speed MD verifies above process and analyzes the modal components, indicating that the single cycle of modal coupling decreases from 11 ms to 4 ms. Otherwise, the strong mode coupling occurs between modes with relatively large weights. The high-speed MD provides a powerful tool to research the TMI effect.

THE STUDY ABOUT RELATIONSHIP BETWEEN CO-APERTURE SPLITTING RATIOS AND LASER FAR-FIELD SPOT MORPHOLOGY AND POWER

To address the problem of difficult to determine the beam splitting ratio of the spectroscopic system which results from the overlapping optical paths for laser emission and imaging in laser countermeasure shared aperture system, this paper conducts a field experiment which investigates the relationship between laser spot size control and the far-field spot morphology and the energy density magnitude at the central spot using the 3.7𝝁𝒎 mid-wave infrared laser band as the light source. After setting up the field test conditions the laser pattern and energy in the far field is measured steadily by changing the stop size at the front of the laser near the conventional 86.5% laser energy. By analysis in the case of 30% to 70% laser energy outer ring blocking light, the maximum change in spot size is 1/3, the energy in the centre of the spot is affected considerably, in a non-linear relationship, especially after the blocking of light in more than 30% energy drops sharply.

THE STUDY ABOUT RELATIONSHIP BETWEEN CO-APERTURE SPLITTING RATIOS AND LASER FAR-FIELD SPOT MORPHOLOGY AND POWER

To address the problem of difficult to determine the beam splitting ratio of the spectroscopic system which results from the overlapping optical paths for laser emission and imaging in laser countermeasure shared aperture system, this paper conducts a field experiment which investigates the relationship between laser spot size control and the far-field spot morphology and the energy density magnitude at the central spot using the 3.7𝝁𝒎 mid-wave infrared laser band as the light source. After setting up the field test conditions the laser pattern and energy in the far field is measured steadily by changing the stop size at the front of the laser near the conventional 86.5% laser energy. By analysis in the case of 30% to 70% laser energy outer ring blocking light, the maximum change in spot size is 1/3, the energy in the centre of the spot is affected considerably, in a non-linear relationship, especially after the blocking of light in more than 30% energy drops sharply.

Focusing of a Laser Beam Passed through a Moderately Scattering Medium Using Phase-Only Spatial Light Modulator

The rarely considered case of laser beam propagation and focusaing through the moderately scattering medium was researched. A phase-only spatial light modulator (SLM) with 1920×1080 pixel resolution was used to increase the efficiency of focusing of laser radiation propagated through the 5 mm layer of the scattering suspension of 1 µm polystyrene microbeads in distilled water with the concentration values ranging from 105 to 106 mm−3. A CCD camera with micro-objective was used to estimate the intensity distribution of the far-field focal spot. A Shack-Hartmann sensor was used to measure wavefront distortions. The conducted experimental research demonstrated the 8% increase in integral intensity and 16% decrease in diameter of the far-field focal spot due to the use of the SLM for laser beam focusing.

Calculation of spot entroid based on physical informed neural networks

To determine the centroid of far-field laser beam spot with high precision and accuracy under intense noise contamination, a positioning algorithm named centroid-PINN is proposed, which is based on physical information neural network. A U-Net neural network is utilized to optimize the centroid estimation error. In order to demonstrate this new method, Gaussian spots polluted by two kinds of noises, i.e. ramp noise and white noise, are generated by simulation to train the neural network. The neural network is tested by two kinds of spots, i.e. Gaussian spot and Sinc-like spot. Both are predicted with high accuracy. Compared with traditional centroid method, the centroid-PINN needs no parameter tuning, especially can cope with ramp noise interference with high accuracy. This work will be conducive to developing the far-field laser beam spot measurement device, and can also serve as a reference for developing the Shack-Hartmann wavefront sensor.

Measurement for far-field focal spot of high power laser based on the diffraction inversion of sidelobe beam

Measurement for far-field focal spot of high power laser based on the diffraction inversion of sidelobe beam

Defining speckle-based spectral metric from a linear array detector for characterizing the laser focusing spot

Speckle modulation in the laser beam projection scenarios such as adaptive optics, remote sensing, and active imaging has been a long-standing challenge that causes intensity fluctuation and further degrades the performance of the system. Speckle statistical characteristics coupled with the feature of the laser beam intensity distribution can be obtained by analyzing the speckle variation through a single point detector or an image sensor. A single point detector has the advantage in high sampling rate but may lose speckle spatial information, while the image sensor has a high spatial resolution but slow sampling rate. In this paper, we defined a dynamic speckle metric based on a linear array detector to estimate the encircled energy of the far-field spot on the diffuse target, which well balanced the trade-off between sampling rate and spatial resolution. The potential of the metric is analyzed using a physics-based speckle simulation and experimental verification. Both results agree with each other, showing that the metrics based on the speckle-field spatiotemporal spectrum analysis are monotonically dependent on the target focused-spot size.

Indirectly coherent beam combining of pulsed lasers based on active control of continuous carrier

High-power and high-quality pulsed fiber lasers with low repetition frequency are widely applied to various fields ranging from basic science to industrial applications. Coherent beam combining (CBC) is a significant method to obtain that beam, but few methods used for large-scale CBC of pulsed fiber lasers with low repetition frequency were presented. To realize it, a new method based on a continuous carrier was designed, where the continuous wave worked as the beacon signal, and the stochastic parallel gradient descent algorithm was employed for phase locking and tilt correction. The beam combining experiment revealed that the combining efficiency of two lasers with a repetition frequency of 15 kHz and a pulse width of 100 ns was 95%, and the fringe contrast in the center of the far-field spot was improved about three times. This method promises to be furtherly applied to combine the pulsed lasers with lower repetition frequency and narrower pulse width. These results pave the way for large-scale CBC of high-power and high-quality pulsed fiber lasers.

Optimization of continuous phase plate for reducing near-field diffraction modulation of back-stimulated Brillouin scattered light

A continuous phase plate (CPP) is often employed to shape and smooth laser beams to improve focal spot quality in inertial confinement fusion facilities; however, near-field modulation resulting from back-stimulated Brillouin scattering (SBS) and the CPP may damage optical devices. In this study, we first explored the relationship between the back-near-field diffraction intensity and the CPP phase structure based on SBS and CPP diffraction models. Subsequent theoretical analysis and simulations showed that strong modulation of the back-near-field diffraction occurs near the extremum of the CPP phase Laplacian. Then, the probability distribution of the CPP phase Laplacian was calculated to determine optimize threshold in real-time, and CPP phase local spatial filtering was performed to reduce phase fluctuation to constrain the near-field modulation in the iteratively designed CPP. The improved CPP can effectively reduce the back-near-field modulation, including the local maximum intensity and overall contrast over a long diffraction distance, while the uniformity of the far-field focal spot is slightly reduced within an acceptable range of 2% to satisfy the high requirements for the focal spot uniformity. The optimization of the CPP is of great significance for reducing the damage of high-power laser devices and practical application of CPP.

Research and Realization of the Minimized Far-Field Spot Size of Laser Weapons Variable-Focus Emission System

Research and Realization of the Minimized Far-Field Spot Size of Laser Weapons Variable-Focus Emission System

Method of inverting wavefront phase from far-field spot based on deep learning

In the adaptive optics system, the accuracy and robustness of wavefront sensor greatly affect the ability of aberration detection and closed-loop correction. In the condition of the nonuniformity of amplitude distributions or insufficient of beacon light energy, it will cause accuracy decrease of Hartmann wavefront sensing due to the lack of sub-aperture light. Meanwhile, the real-time performance of the wavefront sensing-free adaptive system based on far-field spot inversion cannot meet the practical requirements. The method of the wavefront inversion based on deep learning is to directly obtain aberrations by inputting the far-field light intensity image, which can be used as an effective supplement to the adaptive optical system. Through numerical simulation, this paper proved that the deep residual neural network could directly predict the Zernike coefficient of the wavefront phase through the far-field spot. And experimental demonstrated the corrected residual RMS between input and reconstructed wavefront phase was 0.08 waves, the average computation time was less than 2 ms by GPU acceleration. This method can predict the Zernike coefficient of incident wavefront distortion more accurately, and has a good aberration correction capability, suitable for measuring and correcting the main components of wavefront distortion in traditional adaptive optics method, or providing a good initial wavefront estimation for optimized adaptive optics.

Laser beam focusing through a moderately scattering medium using a bimorph mirror.

The rarely considered case when the optical radiation passes through the weakly scattering medium, e.g. mid-density atmospheric fog with the number of scattering events up to 10 was investigated in this paper. We demonstrated an improvement of focusing of a laser beam (λ=0.65 µm) passed through the 5 mm-thick layer of scattering suspension of 1 µm polystyrene microbeads diluted in a distilled water. For the first time the low-order aberration corrector - wide aperture bimorph deformable mirror with 48 electrodes configured in 6 rings was used to optimize a far-field focal spot. We compared efficiencies of the algorithm that optimized the positions of the focal spots on Shack-Hartmann type sensor and the algorithm that optimized the peak brightness and the diameter of the far-field focal spot registered with a CCD. We experimentally demonstrated the increase of the peak brightness of the far-field focal spot by up to 60% due to the use of the bimorph deformable mirror for beam focusing through the scattering medium with concentration values of scatterers ranged from 105 to 106 mm-3.