Single-photon detectors(SPDs)exhibit high sensitivity and strong anti-interference capability,and are often integrated with the traditional pulse position modulation(PPM)technique for long-distance laser communication.However,this integration suffers from low communication rates.To address the rate per-formance limitations of PPM modulation,a broadband reconfigurable pulse sampling data transmission scheme based on single-photon detectors is proposed.In addition,an adaptive pulse-width algorithm tailored to this modulation method is designed to achieve optimal pulse-width selection for the data acquired by single-photon detectors.At the transmitter,the FPGA GTX high-speed transceiver and real-time serial port transceiving are adopted to optimize the transmitted code pattern;at the receiver,a bit error rate(BER)mon-itoring and adaptive algorithm module is developed.Link simulations and adaptive simulations are conduc-ted to evaluate the impacts of actual channel scenarios,and a 1 550 nm single-photon detection experimental system is built for validation.Experimental results show that this modulation scheme enables single-photon laser communication with a rate range from kbps to Mbps.Meanwhile,preliminary tests based on avalanche photodiodes(APDs)demonstrate the feasibility of Gbps-level high-speed communication using this scheme.At the kbps and Mbps rate levels,compared with the default pulse width,the adaptive pulse-width modula-tion algorithm reduces the communication BER by one and two orders of magnitude,respectively.In com-parison with traditional single-photon laser communication systems,this modulation scheme supports wide-rate-range adjustment from kbps to Gbps and optimal pulse-width selection at kbps-Mbps rates,thus provid-ing a novel solution for single-photon detection devices based on different technical routes.
This research reported a novel integrated strategy based on microfluidic technology for the syn-thesis and surface ligand modification of CdSe quantum dots(QDs).The strategy aims to achieve precise and efficient regulation of the luminescence properties of QDs to meet the specific requirements for their optical characteristics in fields such as display,imaging,and optical sensing.Firstly,a microfluidic platform suitable for QDs synthesis was developed,enabling high-throughput and precise control of multiple reaction condi-tions for efficient synthesis.We systematically investigated the influence of reaction temperature,time,and precursor ligand ratios on the growth process and luminescence properties of CdSe QDs.Benefiting from the efficient mass and heat transfer of the microfluidic platform,the reaction time was significantly reduced from the 1 hours required by traditional ligand modification methods to just 5 minutes in our design.To address the issue of spectral shifts in luminescence properties(such as emission color and full width at half maxim-um,FWHM)during functional ligand modification,this research introduced oleic acid(OA)as a surface modification ligand in the microfluidic system.Through efficient and stable anchoring of OA ligands,the lu-minescence efficiency of the quantum dots was enhanced by threefold,while successfully suppressing re-growth and agglomeration behaviors,thereby maintaining the stability of the emission wavelength and FWHM.The innovative use of microfluidic technology in this research not only provides a reproducible and scalable platform technology for precise regulation of QD size and luminescence color but also achieves syn-ergistic optimization of QD luminescence efficiency and stability.Our strategy paves a technical avenue for the practical application of QDs materials in fields such as luminescent displays and quantum light sources.
This study investigates the performance degradation and underlying damage mechanisms of silicon PIN photodiodes under xenon lamp irradiation. To this end, detectivity is defined and operationalized. A 50 kW xenon lamp irradiation test platform was established, where the S5106-type silicon PIN photodiodes were selected as the representative test device. Real-time monitoring of output photocurrent and surface temperature enabled systematic analysis of the factors governing detectivity degradation, as well as characterization of the damage threshold. A damage threshold model for silicon PIN photodiodes was developed based on the one-dimensional heat diffusion equation. Model accuracy was verified by comparing with the experimentally measured threshold data. Silicon PIN photodiode damage was categorized into two regimes-soft damage and hard damage-based on the recoverability of detectivity. Under soft damage conditions, the de tectivity of the device exhibited a nonlinear negative correlation with both irradiation time and surface temperature. The hard damage irradiance thresholds followed an inverse-square-root dependence on irradiation time, a trend fully consistent with the damage threshold model. Hard damage was observed at a minimum irradiance of approximately 6.6 W/cm2, corresponding to an irradiation time of about 382 s. Under this threshold condition, the surface temperature ranged within (385.77 +/- 4.16)degrees C. Theoretical analysis indicated that soft damage primarily arose from thermally induced degradation of carrier mobility and increased leakage current. Conversely, hard damage resulted from melting and cracking of the silicone rubber optical window, as well as thermally induced functional failure of the PN junction. The findings provide a quantitative basis for performance evaluation and protection design of silicon PIN photodiodes employed in broad-spectrum high-intensity optical detection scenarios.
Multilayer coatings are widely applied to high-precision imaging optics to improve throughput. In short-wavelength systems, however, coatings not only alter transmittance/reflectance but also introduce pronounced phase effects and coating-induced lateral shifts, which collectively manifest as additional wavefront aberrations at the system level. This work systematically investigates coating-induced full-field degradation in short-wavelength imaging systems operated at small angles of incidence. A multilayer-coating break-point ray-tracing algorithm is used to incorporate coating-induced phase and lateral-shift effects into the geometrical ray-tracing workflow, enabling a comparative evaluation of coating-induced wavefront aberrations in the visible, infrared, and extreme ultraviolet (EUV) bands. A six-mirror EUV projection system (NA = 0.25) is then analyzed to quantify the wavefront changes introduced by a uniform 40-bilayer Mo/Si multilayer coating. Furthermore, a full-field wavefront analysis method based on Gram-Schmidt orthogonalization (GSO) is developed to characterize the field dependence of Fringe-Zernike aberration coefficients over a curved image field. The results indicate that coating-induced wavefront aberrations are negligible for long-wavelength systems but become significant in the short-wavelength regime. In the EUV example, the coating introduces strong tilt and defocus, increasing the RMS wavefront error from 0.016 lambda to 0.842 lambda. Full-field analysis shows a 0.727 lambda field-dependent tilt component and a 0.034 lambda field-independent defocus component; the tilt terms primarily correspond to image translation, magnification variation, and low-order distortion. These results demonstrate that multilayer coatings can induce severe image-plane deformation in EUV systems and therefore must be accounted for during the optical design stage.
This paper presents a continuous fine-tunable terahertz radiation source based on L-band laser dif-ference frequency.A fully polarization-maintaining fiber link,including polarization-maintaining isolators,polarization-maintaining couplers,and polarization-maintaining erbium-doped fiber amplifiers,was designed to keep the polarization states of the two beams consistent.By using the difference frequency of L-band dual lasers to excite the InGaAs high-performance photoconductive antenna,continuous terahertz radiation ran-ging from 0.1 to 2.7 THz was generated within the wavelength range of 1 568.8 to 1 589.6 nm.The power and frequency of the terahertz waves were respectively tested using a Golay cell detector and a terahertz scan-ning Fabry-Perot interferometer.The results show that the power instability of the terahertz wave within 25 minutes is within 4%,and the frequency measurement results at 0.5 THz and 1 THz are highly consistent with the frequency interval of the L-band dual lasers.Additionally,within the range of 0.9 to 1 THz,a high-precision tuning of 1 GHz was achieved,corresponding to a wavelength interval of 0.008 nm.This continu-ously fine-tunable terahertz radiation source has high application potential in high-precision spectral detec-tion and other fields.
We propose a generalized adjoint-optimization method enabled by a phase-convergence mechanism. Central to this method is a gradient-to-structure mapping model that translates complex-valued adjoint gradients into physically realizable structural updates, establishing a stable iterative relation between structural perturbations and the resulting phase response. This mechanism ensures monotonic phase convergence at the device plane, enabling meta-element-level control of arbitrary phase profiles. Within this formulation, the adjoint simulation employs a single electric dipole excitation, independent of the desired metasurface function. Functional diversity is achieved solely by adjusting the update mapping rather than redefining the ad joint source or modifying the simulation model. This establishes a unified and computationally efficient inverse-design framework capable of handling multiple types of wavefront-shaping functionalities. As proof of concept, numerical validations are performed on diverse metadevices. Specifically, a 2D nanopillar metalens and a linear phase gradient metagrating achieved efficiencies of 83.9% and 72.4% (at 30 degrees deflection), respectively. For arbitrary wavefront shaping, a bifocal lens showed a focusing efficiency of 67.2% and a holographic metasurface generated a hollow triangle pattern with 60.3% energy efficiency. Our results confirm that the proposed method features simplified source construction, high computational efficiency, and strong adaptability, providing a unified and viable framework for the engineering of metasurfaces in imaging, wave-front engineering, and ultraviolet detection.
Carbon fiber-reinforced polymer(CFRP)composites are usually covered with paint in practical applications.To achieve the nondestructive removal of the surface paint on CFRP and improve the recycling rate of CFRP,we investigated the cleaning effect and mechanism of infrared high-repetition-rate pulsed lasers on the surface paint of CFRP under different defocus distances.First,when the defocus distance was 40 mm,the surface paint of CFRP was cleaned by adjusting the power and scanning speed,and the cleaning effect was observed to obtain relatively optimized process parameters.On this basis,the paint was treated by varying the defocus distance.Finally,the cleaned samples were characterized and analyzed in terms of sur-face morphology,elemental composition and contact angle,so as to obtain the cleaning effect,the contact angle variation law and the cleaning process under different defocus distances.The results show that with an increase in the defocus distance,the cleaning process transforms from vaporization to a heat accumulation ef-fect,and proper heat accumulation contributes to the complete removal of paint and resin.When the power,scanning speed and defocus distance are 25 W,720 mm/s and 40 mm respectively,the paint can be com-pletely removed without damaging the carbon fibers,while effectively improving the surface wettability of the substrate.
With the rapid development of spectral imaging, on-chip communication, and multifunctional integrated photonics, modern optical systems require multi-band beams to be independently controlled in three dimensions within compact platforms. However, conventional multi-band solutions typically rely on multiple optical components or complex folded beam paths, resulting in bulky structures and inter-band crosstalk that limit device integration and scalability. To address this challenge, we propose a dual-band dual-phase-gradient design strategy for independent wavefront control. This approach enables mutually independent phase-gradient units for different wavelengths to coexist on a single-layer metasurface, achieving spatially independent beam deflection. To validate the strategy, a Si/SiO2 all-dielectric metasurface is designed for the 800 nm and 1 150 nm bands. The meta-units provide full 0-2 pi phase coverage at their respective bands. By imposing phase gradients and arranging the two types of units in a 90 degrees rotationally interleaved configuration, a composite periodic structure is formed in which dual-band phase profiles coexist without interference. Simulations show that under the condition of normal incidence, the device deflects the transmitted beams in the 800 nm wavelength band by approximately 14 degrees in one lateral direction relative to the incident normal, while deflection that in the 1 150 nm wavelength band by about 24 degrees in another lateral direction, achieving clear spatial separation and independent beam manipulation. The device also exhibits polarization-insensitive and stable wavefront control, confirming the reliability of its dual-band orthogonal operation. This design strategy provides a compact, efficient, and generalizable pathway toward highly integrated multi-band on-chip photonic devices, with broad potential applications in spectral imaging, dual-band communication, and integrated photonic systems.
In order to achieve comprehensive, highly efficient, and multi-objective precise optimization of fiber structural parameters and further enhance the transmission capacity of optical communication systems, a homogeneous weakly coupled seven-core fiber based on trench-assisted structures is designed. Particle Swarm Optimization (PSO) is introduced to replace traditional empirical designs or local scanning methods. First, a multi-objective fitness function incorporating constraints such as dispersion, cutoff wavelength, effective mode field area, and coating loss is established. Then, the algorithm performs a global search to precisely determine the optimal structural parameters under standard dimensional constraints. Simulation results demonstrate that with a fiber core pitch of 45 mu m, the optimized fiber achieves an ultra-low inter-core crosstalk of below -90 dB/km at a wavelength of 1 550 nm. This design scheme not only effectively resolves the conflict between crosstalk suppression and spatial utilization in multi-core fibers but also proves the efficiency and reliability of the PSO algorithm in complex fiber structural design, providing an important theoretical basis and technical support for the research and manufacturing of ultra-large-capacity optical communication systems.
Frequency-sweep amplitude-modulation laser ranging (FSAMLR) is a ranging method that determines the target distance by solving for the in-phase frequency, characterized by high measurement accuracy and low system complexity. To address issues such as the low signal-to-noise ratio in sampled waveforms containing in-phase frequencies and the resulting limitations in solving accuracy, a method based on singular spectrum analysis combined with local parabolic fitting (SSA-LPF) is proposed. The principle of FSAMLR is outlined, emphasizing that ranging accuracy depends on the precision of the in-phase frequencies. Subsequently, simulations compare the solving accuracy of in-phase frequencies among the swing method, parabolic fitting, cubic fitting, and quartic fitting, using identical sampled waveforms filtered via the SSA method. Parabolic fitting is verified to enhance solution accuracy. Simulation results demonstrate that parabolic fitting achieves a 95.7% reduction in mean absolute deviation relative to the swing method and a 65.6% improvement over other least-squares fitting methods. Experimental analysis indicates that the SSA-LPF method yields a ranging standard deviation below 30 mu m across varying distances and sweep steps. Adopting the SSA-LPF method in FSAMLR enhances ranging efficiency while maintaining high ranging accuracy.
To mitigate shot noise and background interference in single-photon depth imaging, alongside single-axis image deviation induced by UAV attitude fluctuations during flight, this paper proposes a robust depth reconstruction method. Building upon the SPIRAL-TAP framework, the proposed approach integrates multi-scale image features with an adaptive thresholding strategy. Firstly, an image weighting matrix is constructed via multi-scale gradients and local variance to effectively characterize texture complexity. Subsequently, a dynamic threshold adjustment mechanism, guided by Rough Order Map (ROM) estimation, is implemented to enhance noise robustness. In the screening phase, an adaptive strategy merges scale-space smoothing with weighting matrix soft-tuning to stabilize the filtering process. Experimental results demonstrate that the proposed method significantly outperforms the conventional SPIRAL-TAP algorithm under varying signal-to-background ratios (SBR) and photon intensities. Specifically, at tilt angles of 10 degrees and 15 degrees, the RMSE is reduced from 0.32 to 0.14 and from 0.43 to 0.21, respectively. This method provides an effective solution for UAV-borne single-photon depth reconstruction and exhibits significant potential for highspeed airborne imaging systems.
Electro-optic beam deflection technology possesses advantages such as low power consumption,miniaturization,and good controllability.Compared with mechanical beam deflection,acousto-optic beam deflection,and liquid crystal beam deflection technologies,it is more easily able to meet the practical applic-ation requirements of rapidity and stability in space laser communication.This paper systematically summar-izes the domestic and international research progress of several widely applied novel electro-optic materials(such as lithium niobate,lead lanthanum zirconate titanate,and potassium niobate tantalate)in the field of beam deflection.Based on the intrinsic deflection characteristics of different electro-optic materials,the fea-tures of corresponding beam deflection technologies are comprehensively analyzed and compared from the perspectives of application configurations and key performance metrics.Furthermore,the application pro-spects of these electro-optic material-based beam deflection technologies in space optical communication are discussed,the urgent challenges that need to be addressed currently are highlighted,and the directions for fu-ture research endeavors are clarified.
To achieve simultaneous lightweight design and high-energy output under special environmental conditions,a compact,water-cooling-free high-energy pulsed laser system based on high-temperature laser diode array(LDAs)side-pumped zigzag Nd:YAG crystals is demonstrated for operation in demanding envir-onments.The zigzag beam propagation increases the effective gain length,while symmetric LDAs pumping of two Nd:YAG crystals improves gain uniformity.Thermal isolation between the crystals and LDAs is im-plemented,with independent temperature control achieved using thermoelectric coolers(TEC)for the Nd:YAG crystals and forced air cooling for the LDAs.A potassium dideuterium phosphate(DKDP)crystal is employed for electro-optic Q-switching.At a repetition rate of 100 Hz without water cooling,a maximum pulse energy of 129.2 mJ with a pulse duration of 9.0 ns is obtained,corresponding to an optical-to-optical efficiency of 9.6%and a slope efficiency of 13.1%,with energy stability better than 2.26%.An output en-ergy of 87.6 mJ is achieved at 150 Hz.This system provides a compact and environmentally robust light source for laser ranging and illumination applications.
This paper presents comprehensive theoretical and experimental investigations on the transmission spectral characteristics of an integrated photonic structure consisting of a microring resonator coupled with a Fabry-Perot (FP) cavity. The FP cavity is realized by introducing a grating reflector into the straight waveguide of a single-side-coupled microring. Within this dual-resonator configuration, novel multi-cavity coupled transmission spectra are achieved. A systematic theoretical model is established to analyze the conditions under which these multi-cavity coupled spectral profiles appear, and the structural parameters are subsequently optimized. A grating-type Fabry-Perot-microring coupled resonator device was successfully fabricated on a silicon-on-insulator (SOI) platform. For the first time, multi-cavity coupled transmission spectra consistent with theoretical predictions were experimentally observed, including nested electromagnetically induced transparency (EIT)-like and double Fano resonance line shapes. Experimental measurements indicate that, under a waveguide loss of 3.43 dB/cm, the EIT central peak exhibits a quality factor of 1.40 & times;10(4), while the slope of the double Fano resonance reaches 37.70 dB/nm. These results provide new insight into the underlying mechanisms of integrated photonic coupled resonator systems and demonstrate a viable approach toward highly integrated, high-performance photonic device platforms. The proposed structure shows strong potential for applications in high-sensitivity optical sensing, narrowband filtering, and high-speed modulation.
To address the challenge of temperature rise control during the coating process for thermally sens-itive substrates(e.g.,epoxy adhesive-bonded structural components),this paper proposes a low-temperature electron beam evaporation coating process.Through a dynamic thermal management strategy featuring seg-mented deposition-cooling cycles,the performance of this process in terms of the core properties(i.e.,stress,adhesion,and optical performance)of metallic reflective films,with silver films as the research subject,was systematically investigated,and the deposition process was optimized by integrating the thermal failure threshold of the epoxy adhesive.Experimental results demonstrate that under strictly controlled substrate temperature conditions,this process not only significantly reduces the residual stress of the reflective film,but also ensures that the interfacial adhesion meets the strictest Class 03 severity level specified in the nation-al standard,the average reflectivity in the visible wavelength range is comparable to that of the traditional continuous coating process(>99%@450-900 nm),and the substrate temperature rise remains consistently below the critical threshold of the epoxy adhesive.Through the synergistic effect of Ion-Assisted Deposition(IAD)and dielectric encapsulation,the oxidation resistance and environmental durability of the silver film are significantly improved,satisfying the long-term service requirements of aerospace optical devices under extreme multi-physics field coupled environments.Further theoretical analysis reveals that the thermal relax-ation mechanisms and structural regulation principles of this process exhibit cross-scenario applicability,providing an innovative solution for high-performance coating of low-temperature-sensitive substrates that balances aerospace reliability and industrial universality.
To address the issues of structural complexity and high cost in high-performance optical systems,this study proposes an optical system simplification and aberration correction method oriented towards com-putational correction.On the optical design side,a simplification design criterion based on aberration correct-ability analysis is constructed:priority is given to suppressing aberrations that are difficult for neural net-works to compensate,while retaining portions amenable to computational correction,thereby simplifying the optical system structure while ensuring imaging quality.On the computational processing side,a multi-mod-ule progressive collaborative correction network is designed,comprising four modules:distortion correction,chromatic aberration compensation,monochromatic aberration correction based on physically-constrained Point Spread Function,and frequency-domain enhancement.This network is driven by a Temporal Stage Controller(TSC),which utilizes its dynamic weight scheduling mechanism for progressive stage-wise pro-cessing,effectively suppressing the mutual interference between different aberration types.Experimental res-ults demonstrate that images from a simplified dual-lens system corrected by this network achieve a Peak Signal-to-Noise Ratio(PSNR)of 31.47 dB and Structural Similarity(SSIM)of 0.95,with imaging quality comparable to conventional six-lens double-Gauss systems,while significantly reducing optical system com-plexity.Ablation studies validate the effectiveness of the TSC and multi-module correction architecture.This research provides a novel technical pathway for achieving high-quality imaging with simplified optical sys-tems.
To enhance the imaging performance of arthroscopes in clinical surgery and broaden their potential for clinical applications, a wide-spectrum arthroscopic optical system featuring a large field of view, high resolution, and parfocal imaging capability in both visible and near-infrared bands was designed. The objective lens uses a high optical power negative lens to compress the chief-ray angle and reduce the optical path difference between off-axis and on-axis rays. Through conjugate aperture imaging, an equivalent virtual stop is formed inside the turning prism, which allows the system to maintain both high transmission efficiency and high image quality under a limited aperture. The relay lens adopts a near-symmetric structure, and by distributing optical power and Abbe numbers appropriately, it effectively suppresses the accumulation of axial chromatic aberration across the broad spectral range, thereby achieving parfocal imaging. Tolerance analysis shows that the system has good manufacturability and assembly feasibility. Experimental results verify that the designed wide-spectrum arthroscope achieves parfocal imaging in the visible and near-infrared bands with a 95 degrees field of view, and angular resolutions of 4.34 C/(degrees) and 2.74 C/(degrees), respectively. The optical system provides a feasible solution for achieving low-cost, high-performance fluorescence arthroscopy and has significant application value.
To address the challenge of achieving high-precision registration between visible light(RGB)and photoluminescence(PL)images in Micro LED defect inspection,which arises from substantial modality dif-ferences,this study introduces a robust multimodal image registration approach capable of attaining sub-pixel accuracy,aiming to establish a direct mapping between the physical structure and electrical characteristics of the chips.We propose a registration method that integrates structural feature constraints with bidirectional re-sidual optimization.First,leveraging the geometric regularity of Micro LED arrays,a tailored feature detec-tion strategy is employed:electrode centers in RGB images are accurately extracted via ellipse fitting and Density-Based Spatial Clustering of Applications with Noise(DBSCAN),while chip centers in PL images are localized using an enhanced watershed algorithm with sub-pixel refinement.Second,during the registration optimization stage,a bidirectional residual constraint framework is constructed,incorporating a confidence weighting mechanism derived from residual distribution analysis.The optimal affine trans-formation parameters are then estimated using an iterative reweighted least squares method.Experimental results demonstrate that the proposed method achieves sub-pixel-level accuracy,with a mean absolute error(MAE)of 0.823 pixels,representing a 94.2%reduction compared to baseline methods.The root mean square error(RMSE)is 0.996 pixels,the maximum error remains below 2.839 pixels,and the inlier rate at-tains 75.0%.Each registration process takes only 0.036 seconds on average,achieving an order-of-mag-nitude improvement in computational efficiency over traditional mutual information(MI)methods.By effect-ively mitigating feature mismatch and outlier interference in multimodal images,the proposed method out-performs conventional approaches in terms of registration accuracy,robustness,and efficiency,thereby providing a reliable technical foundation for precise defect detection and multimodal analysis of Micro LED chips.
The structure of multilayer dielectric film will modulate the optical field,thus in the study of laser-induced damage characteristics,it is necessary to consider the energy distribution within the film system and the resulting changes in material optical properties.Taking the HfO2/SiO2 multilayer dielectric film structure as an example,and based on the laser-induced ionization/electron multiplication process,the Drude model is introduced into the optical field calculation,extending the film layer refractive index from a static constant to a dynamic complex refractive index driven by the free electron density.Based on this,a thermal conduction(and thermal stress)model is coupled to calculate the evolution of thermal effects inside the thin film under nanosecond pulsed laser irradiation,and the corresponding damage threshold is determined to be 13.65 J/cm2,and the damage characteristics of the film are studied experimentally.The verification experiment observed that the damage appearance of the HfO2/SiO2 multilayer dielectric film is a round hole type,which is a typic-al thermal melting damage,and is consistent with the conclusion of the theoretical model.The measured damage threshold is 13.75 J/cm2,which is only higher than the theoretical analysis result.The improved mod-el established is helpful to further analyse the interaction between strong laser and multilayer dielectric film from the theoretical level,and to better study the damage resistance of optical thin film.
Addressing the critical challenge of thermal radiation noise suppression in infrared systems for long-range dim target detection, we present a composite detection system with an optimized cooling-based thermal radiation suppression scheme. A common-aperture optical configuration capable of simultaneous long-wave infrared and laser dual-band detection is achieved through a Ritchey-Chretien (R-C) optical structure and a dichroic-secondary mirror with a hollow design. To mitigate thermal radiation noise, the thermal emission characteristics within the temperature range of 230 K to 320 K were analyzed using Planck's law and non-sequential ray tracing. An improved detection range model incorporating noise terms was developed. The cooling strategy was optimized via dynamic programming, leading to an optimal solution where the main mirror and folding mirror baffles are cooled to 220 K. Experimental results demonstrate that the detection range at 300 K ambient temperature increases from 300 km to 430 km, and remains above 400 km across the entire 230-320 K range. The proposed dual-band composite detection scheme and zoned cooling methodology provide a valuable reference for the design of cold optical systems and long-range weak target detection.
