Dual–Wavelength Mode–Locked Erbium Fiber Laser Utilizing Ge–PCF Saturable Absorber

Reem Hanoon, Ali Abdulhadi, and Abdulla Abass

DOI: 10.1364/AO.538611 Received 05 Aug 2024; Accepted 08 Oct 2024; Posted 09 Oct 2024  View: PDF

Abstract: In this work, we demonstrate a mode lockederbium– doped fiber laser (EDFL) based on germanium–core doped photonic crystal fiber (Ge–PCF) as a passivesaturable absorber (SA). The proposed SA was incorporatedinside the PCF core during the manufacturing process. Theshort length of the proposed high nonlinear fiber can achievestable mode–locked pulse by controlling the fiber andcollapsing lengths. The proposed Ge–PCF has beenintegrated into erbium doped fiber ring laser to establish thedesired ultrashort pulse with the merits of a short interactionlength (< 3 cm), enhanced damage threshold, environmentalrobustness, ease of manufacture, and suitable massproduction. The obtained output pulses at FWHM have apulse width of 800 ps and a repetition rate of ~19 MHz.Furthermore, EDFL based on Ge–PCF achieved a dual–wavelength output spectrum with narrow bandwidthscentered at 1557.27 nm and 1558.81 nm. The short–pulsecapabilities of the proposed fiber laser make it highlysuitable for a wide range of photonic applications.

Mid-frequency contrast compensation for sparse aperture imaging systems by increasing the size of a sub-aperture

Tai Liu, Yang Zhao, Hongcai Ma, Peng Yu, Qingwen Wu, Zhenbang Xu, and Hasiaoqier Han

DOI: 10.1364/AO.539119 Received 14 Aug 2024; Accepted 08 Oct 2024; Posted 09 Oct 2024  View: PDF

Abstract: Sparse aperture imaging systems (SAISs) always suffer mid-frequency contrast loss because of dispersion and sparsity. This paper introduces an efficient method to enlarge the sub-aperture with the most significant effect on mid-frequency contrast compensation. This method is implemented by evaluating the mean square error between the modulation transfer function (MTF) of an SAIS and a monolithic aperture system with matching cutoff frequency. Two array configurations with different sub-apertures enlarged by various magnifications are simulated and their MTFs are analyzed. Subsequently, experiments are conducted by enlarging a sub-aperture within an array configuration at different magnifications. The results confirm the method’s effectiveness.

Low-cost, high-volume manufacturable 0.88 NA multi-wavelength diffractive lens array for optical document security

Rajesh Menon, Apratim Majumder, Tina Hayward, and Robert Stewart

DOI: 10.1364/AO.541419 Received 12 Sep 2024; Accepted 08 Oct 2024; Posted 09 Oct 2024  View: PDF

Abstract: We design, manufacture, and characterize a high-numerical-aperture (NA $=0.88$, f/$0.27$), multi-wavelength ($480\,\text{nm}$, $550\,\text{nm}$, and $650\,\text{nm}$) multilevel diffractive microlens array (MLA). This MLA achieves multi-wavelength focusing with a depth of focus (DoF) twice the diffraction-limited value. Each microlens in the array is closely packed with a diameter of $70\,\mu\text{m}$ and a focal length of $19\,\mu\text{m}$ in air. The MLA is patterned on one surface of a polymer film via UV casting, positioning the focal plane at the distal end of the polymer film. Each microlens focuses light at three design wavelengths into a focal spot with an estimated FWHM of $~\sim310\thinspace$nm. By placing this MLA directly on a standard high-resolution banknote print (minimum feature width of $10-15\,\mu\text{m}$), we demonstrate color-integral imaging for anti-counterfeiting. In contrast, refractive MLAs cannot achieve high NA, multi-wavelength focusing or extended DoF. The extended DoF of our MLA ensures reliable performance despite variations in the polymer film’s thickness. Our MLA, produced via UV casting, enables extremely low-cost, high-volume production, making it ideal for flat optics in banknotes and document security.

Harnessing Physics Guided Neural Networks for Tailoring the Orbital Angular Momentum Spectrum in Second Harmonic Generation

Saeed Ghavami Sabouri and Somayeh Sadat Hashemi

DOI: 10.1364/AO.538888 Received 07 Aug 2024; Accepted 07 Oct 2024; Posted 07 Oct 2024  View: PDF

Abstract: The management of orbital angular momentum (OAM) in frequency conversionprocesses is essential for numerous applications such as both quantum and classical opticalcommunications. This paper presents a wavefront modulation approach for the fundamentalbeam in second harmonic generation (SHG) to efficiently control the OAM spectrum. Weemploy an inverse design method to derive the necessary wavefront shape of the fundamentalbeam for achieving a desired SHG OAM spectrum. Specifically, we introduce an efficientinverse design technique based on physics-guided neural networks (PGNN) that incorporatesthe coupled equations governing SHG, aimed at tailoring the OAM spectrum of SHG. Utilizingthe proposed PGNN, we design the phase pattern for a spatial light modulator (SLM) to shapethe wavefront of the fundamental beam. Furthermore, we present a novel loss function thateffectively links the OAM of the SHG spectrum and efficiency to the SLM phase pattern andcrystal temperature, independent of empirical weight coefficients. The proposed PGNNfacilitates the purification of the SHG OAM spectrum, even when the fundamental beamcomprises mixed Laguerre-Gaussian (LG) modes. Additionally, we demonstrate the generationof desired SHG spectra using the proposed PGNN framework. This study introduces agroundbreaking inverse design method for developing photonic devices with customizedfunctionalities, addressing challenges associated with traditional data-driven deep learningtechniques.

Multifunctional Electrically Switchable Metalens

Yu Wang, Dajian Cui, Guohui Yang, Yue Wang, and Chunhui Wang

DOI: 10.1364/AO.542033 Received 17 Sep 2024; Accepted 07 Oct 2024; Posted 07 Oct 2024  View: PDF

Abstract: Here, we propose three all-solid-state, electricallyswitchable, transmissive, and multifunctional metalens arrays composed of tunable metal-oxide materialBaTiO3 (BTO) nanopillars with different structural parameters. To produce the required phase profile for eachmetalens, the rectangular nanopillars, as the unit cell ofthe BTO structure, are developed to support arbitrarycombinations of two independent phase shifts (0-2π)with bias voltage states of 0 V and 60 V at first. Secondly, the structure parameters of different functionalmetalens arrays are generated efficiently and accuratelybased on the dual-phase modulation characteristics of asingle structure. Finally, we use the finite-differencetime-domain method to simulate the three kinds ofswitchable metalenses. The results show that the threemetalenses can realize the function of adjustable focusposition, switchable beam focusing and beam deflection, and switchable beam focusing and beam splitting,respectively.

Design of freeform-mirror-based optical systems with an arbitrary number of stigmatic pairs

Rafael Gonzalez Acuña

DOI: 10.1364/AO.540494 Received 28 Aug 2024; Accepted 07 Oct 2024; Posted 08 Oct 2024  View: PDF

Abstract: This manuscript introduces a new method to design a freeform mirror-based optical system from scratch by adding segments and an additional mirror. Each segment forms a stigmatic pair for a respective given field. Then the segments of the last mirrors are concatenated with a numerical interpolation. The overall system is optimized to ensure a good performance in the fields not considered by the initial set of segments. The results are as expected by the theory and are explained with an illustrative example.

Structural design and cryogenic stability analysis for Long-wave Infrared Spatial Heterodyne Spectrometers

Yang Wu, Yutao Feng, Bin Han, Junqiang Wu, and Jian sun

DOI: 10.1364/AO.540922 Received 02 Sep 2024; Accepted 05 Oct 2024; Posted 07 Oct 2024  View: PDF

Abstract: Under cryogenic conditions, Long-wave infrared spatial heterodynespectrometers (LWIR-SHS) may exhibit variations in the inclination, frequency,and distortion of their interference fringes, which can affect the system'sperformance. To address the stability issues associated with cryogenic, an analysisand simulation of the opto-mechanical system's performance was conducted undersuch conditions. Employing a structural thermal optical performance (STOP)analysis, an integrated micro-stress dynamic flexible support structure for theLWIR-SHS was designed. The optimized system ensured stable operation undercryogenic conditions, maintaining the relative positional deviation of the key componentsbelow 0.00146 mm, the relative angular deviation within 25.984 ", and the inclination ofinterference fringes in both simulation and test is less than 21°. Furthermore, incomparison to the initial system, the interference fringe distortion also decreasedsignificantly. Cryogenic testing of the entire system validated the optimizedstructure's effectiveness in minimizing variations of interferogram, aligning withsimulation predictions. This outcome demonstrates the optimization strategy'ssubstantial impact on enhancing both the structural stability and operationalperformance of the cryogenic infrared optical system.

All-polarization-maintaining Figure-9 Erbium-doped modelocked fiber laser based on bidirectionally pumped weak asymmetric NALM

Yongjian Pan, Qiao Lu, Ting Ma, Dong Zhang, Hao Pan, and Leben Liang

DOI: 10.1364/AO.539186 Received 12 Aug 2024; Accepted 04 Oct 2024; Posted 04 Oct 2024  View: PDF

Abstract: This paper presents, for the first time, an all-polarization-maintaining Figure-9 erbium-doped mode-locked fiberlaser utilizing a weakly asymmetric nonlinear amplifying loop mirror (NALM) with bidirectional pumping. In the constructeddispersion-managed Figure-9 erbium-doped mode-locked fiber laser, with a fixed splitting ratio of 0.5, a linear phase shift of1.5π, and an approximately symmetric distribution of fiber within the loop, we investigated the impact of the bidirectionalpumping power ratio on the intracavity asymmetry. By precisely controlling the incremental bidirectional pumping power, andwhile ensuring the self-starting single-pulse operation of the laser, we achieved a mode-locked pulse with a repetition rate of103 MHz, a directly output pulse width of 284 fs, and an average output power of 58 mW, corresponding to a single-pulseenergy of 0.56 nJ. To the best of our knowledge, this represents the highest single-pulse energy achieved in a fully polarizationmaintaining erbium-doped Figure-9 mode-locked fiber laser solely by increasing the pump power, without the need for anyadditional adjustments. Moreover, we constructed a bidirectional pumped dissipative soliton type fully polarizationmaintaining Figure-9 erbium-doped mode-locked fiber laser without an actual filter, and investigated the changes in outputpulse spectral characteristics as the splitting ratio and bidirectional pumping power varied. This study provides significantreference value for future efforts to achieve higher single-pulse energies in Figure-9 mode-locked fiber lasers.

Efficient Pretraining Model based on Multi-Scale Local Visual Field Feature Reconstruction for PCB CT Image Element Segmentation

Chen Chen, Kai Qiao, Jie Yang, Jian Chen, and Bin Yan

DOI: 10.1364/AO.537767 Received 26 Jul 2024; Accepted 03 Oct 2024; Posted 04 Oct 2024  View: PDF

Abstract: Element segmentation is a key step in nondestructive testing of Printed Circuit Boards (PCB) based on Computed Tomography (CT) technology. In recent years, the rapid development of self-supervised pretraining technology can obtain general image features without labeled samples, and then use a small amount of labeled samples to solve downstream tasks, which has a good potential in PCB element segmentation. At present, Masked Image Modeling (MIM) pretraining model has been initially applied in PCB CT image element segmentation. However, due to the small and regular size of PCB elements such as vias, wires, and pads, the global visual field has redundancy for a single element reconstruction, which may damage the performance of the model. Based on this issue, we propose an efficient pretraining model based on multi-scale local visual field feature reconstruction for PCB CT image element segmentation (EMLR-seg). In this model, the teacher-guided MIM pretraining model is introduced into PCB CT image element segmentation for the first time, and a multi-scale local visual field extraction (MVE) module is proposed to reduce redundancy by focusing on local visual fields. At the same time, a simple 4-Transformer-blocks decoder is used. Experiments show that EMLR-seg can achieve 88.6% mIoU on the PCB CT image dataset we proposed, which exceeds 1.2% by the baseline model, and the training time is reduced by 29.6 hours, a reduction of 17.4% under the same experimental condition, which reflects the advantage of EMLR-seg in terms of performance and efficiency.

Design of dual-focal-plane head-up display system using double freeform mirrors

Tong Yang, HuiMing Xu, Lijun Zhou, Yinuo Cheng, zibo wu, SHAN SHANG, Dewen Cheng, and Yongtian Wang

DOI: 10.1364/AO.539415 Received 16 Aug 2024; Accepted 03 Oct 2024; Posted 08 Oct 2024  View: PDF

Abstract: This paper presents the design of a dual-focal-plane head-up display (HUD) systemutilizing double freeform mirrors. The system is specifically developed for automotiveapplications, aiming to enhance driving safety and experience by projecting vehicle informationwithin the driver's line of sight. Compared to traditional WHUD, ARHUD offers a broader fieldof view and a greater virtual image distance. If the ARHUD system has double focal planes, itcan separately display basic and interactive driving information to drivers, which is currently asignificant research direction in ARHUD system design. The proposed design only uses doublefreeform mirrors and uses a single display source for both focal planes, thus reducing the overallsystem complexity, assembly difficulty, and cost. Detailed design methods and results arediscussed, showcasing the system's good imaging performance and compact structure, whichprovide valuable insights for the development of next-generation dual-focal-plane HUDs forvehicles. The proposed system demonstrates significant advancements in reducing fabricationand testing difficulties compared to traditional dual-focal-plane HUD designs. The study’sfindings contribute to the future development of dual-focal-plane head-up display systems.

Enhancing the efficiency of wavelength-shift fiber-based detectors for optical wireless communications

Amjad Amjad, Mithilesh K Mane, Arfan Mahmood, Riffat Tehseen Amjad, and Jing Xu

DOI: 10.1364/AO.533015 Received 14 Jun 2024; Accepted 03 Oct 2024; Posted 03 Oct 2024  View: PDF

Abstract: A wavelength-shift fiber-based optical detector promises to revolutionize the deployment ofoptical wireless communication (OWC) due to its inherent advantages over traditionalreceivers. These advantages include a flexible structure, a wide field of view (FOV), and a largeactive area. Despite progress in previous studies, there remains a gap in optimizing the reutilization of unabsorbed light within wavelength-shift fiber (WSF) and maximizing theefficiency of light focusing onto photodetectors. To address these challenges, this studyexplores three innovative approaches to enhance the light conversion and detection efficiencyof WSF-based optical detectors. First, a reflective mirror is employed behind the WSF array toincrease the light absorption and re-emission probability. Second, a reflective mirror is placedat one end of the WSF array to direct the light toward the opposite end. Third, a tightly bundledWSF array configuration focuses the emitted light onto the photodetector's active area.Experimental results demonstrate that each approach significantly improves the peak-to-peakvoltage. This work presents an optical detector design featuring a large active area of 0.4 𝑐𝑚× 20 𝑐𝑚, based on a blue-to-green color-converting WSF and achieving a high 3-dBbandwidth of up to 48 MHz. This design enables real-time data transmission at rates of 275Mbps using non-return-to-zero on-off keying (NRZ-OOK) modulation over a distance of 1 m.Additionally, the transmission link operates at over 250 Mbps, with bit error rates (BERs) belowthe forward error correction (FEC) limit, under a wide FOV of 60°. This work opens excitingpossibilities for revolutionizing photodetection schemes in non-line-of-sight free-space opticalcommunications.

Decomposition and compensation of fringe harmonics errors by use of their partial orthogonality in phase-shifting fringe projection profilometry

Zhu Jianli, Shuai Lin, and Hongwei Guo

DOI: 10.1364/AO.538646 Received 05 Aug 2024; Accepted 02 Oct 2024; Posted 03 Oct 2024  View: PDF

Abstract: In phase-shifting fringe projection profilometry, fringe harmonics have been recognized as one of the main error-inducing factors. Generally, response of a phase-shifting algorithm to fringe harmonics strongly depends on the used phase shifts and is usually unpredictable, especially when using nonuniform phase shifts. For this reason, it is difficult to eliminate the phase-measuring errors caused by fringe harmonics in a general case, for example, when the phase shifts are not uniform but arbitrarily valued. To overcome this problem, this paper analyzes the phase error function related to each fringe harmonic under the condition of using arbitrary phase shifts, reveals the partial orthogonality of these functions, and then derives an algorithm for decomposing the harmonics-induced phase errors. In implementation, this algorithm calculates a coarse phase map first in the least squares sense from captured fringe patterns, and then estimates the coefficients of fringe harmonics from this calculated phase map by use of the partial orthogonality of the error functions. By using the estimated harmonic coefficients, the phase map is updated thus having improved accuracies, so that the harmonics-induced phase errors are compensated for through an iterative procedure. Simulation and experimental results demonstrate this method to be effective and efficient in measuring fringe phases.

Agar-made biodegradable ball lens

Lidia Rosa, Eduarda de Morais, Cristiano Cordeiro, and Eric Fujiwara

DOI: 10.1364/AO.534801 Received 05 Jul 2024; Accepted 01 Oct 2024; Posted 03 Oct 2024  View: PDF

Abstract: This work proposes the fabrication and characterization of ball lenses made of agar, a biodegradableand renewable material. Pouring a boiled solution of food-grade agar, glycerol, and water into cooledvegetable oil under agitation by a magnetic stirrer produces transparent and manipulable spheres. Theglycerol concentration and rotation speed define the refractive index and size distribution, respectively,yielding lenses with diameters between 1 and 8 mm and indices ranging from 1.33 to 1.46. Experimentsalso characterized the output beam profile with 632 nm laser illumination. The results corroborate withthe focal length simulated for a Gaussian beam model, suggesting controllable optical properties andanticipating potential applications of the agar ball lens as a biodegradable optical device for biomedicalimaging, illumination, and sensing.

A Study on Infrared Emissivity Measurement Methods Considering Target Reflective Characteristics

liu yin, Yiwen Li, Ma Simin, Puyousen Zhang, Li Yao, Fan Xiumei, and li weiqin

DOI: 10.1364/AO.535336 Received 11 Jul 2024; Accepted 01 Oct 2024; Posted 02 Oct 2024  View: PDF

Abstract: Emissivity measurements are of great significance for infrared thermal radiation andinfrared remote sensing. However, traditional methods often face challenges such as difficultiesin non-contact measurement, small measurement areas, and unsuitability for non-Lambertiansurfaces. To address these issues, we propose the Reflective Distribution Model IntegralMethod (RDMIM). This method is based on a reflective distribution model using theScattering-Reflective Deviation Angle (SRDA). By regressing and integrating the object'sreflective distribution model, it achieves accurate non-contact measurement of non-Lambertiansurfaces under normal temperature conditions. Additionally, the measurement scheme has beenfurther optimized to improve measurement efficiency while ensuring the accuracy of the modelregression. Finally, the proposed RDMIM method has been validated through experimentalmeasurements. The results have shown that this method has advantages in non-contact andlarge-area measurements. Moreover, the systematic error is smaller when the reflectivecharacteristics of the reference body and the target are relatively similar.

Correction algorithm for misalignment and magnification difference of focused and defocused images in phase diversity image reconstruction

Hua Bao, Shuqi Li, Qing Bian, Ying Zhang, Guoqing Gao, daiyin luo, Ning Jiang, and Changhui Rao

DOI: 10.1364/AO.537223 Received 23 Jul 2024; Accepted 01 Oct 2024; Posted 02 Oct 2024  View: PDF

Abstract: Phase diversity(PD) technology typically requires additional optical channels to capture images withknown aberrations. In real applications, defocus aberration is widely used, and a typical method is to addan imaging camera with known optical path differences from the focal plane. However, the method ofintroducing defocus aberration through optical path differences can lead to misalignment and differentmagnifications of focused and defocused images, resulting in significant decrease in reconstruction accuracyof PD algorithm. In this paper, we analyze the sources of magnification difference and misalignment,and propose the method based on image coordinate transformation. For simulation experiments of 400groups, the PV values of magnification and misalignment calculation errors are 0.0008 and 1.34 pixels,which verify the accuracy of our method for image correction. For the actual extended target images, byusing our method PD technology achieves a reconstruction result of 1.043 times the diffraction limit, andthe experimental results demonstrate the effectiveness and reliability of our method.

Research on 1×2 broadband polarization-independent beam splitter based on air annular photonic crystal designed by Particle swarm optimization algorithm

Yang Cao, Chen Yu, JiaQi Li, Shi Pengcheng, JiaQi Li, and Peili Li

DOI: 10.1364/AO.535143 Received 09 Jul 2024; Accepted 30 Sep 2024; Posted 01 Oct 2024  View: PDF

Abstract: We propose a 1×2 broadband polarization-independent beam splitter based on air annular photonic crystals. The inner and outer radii of the air annular photonic crystals at the V-shaped waveguide side and corners can influence the transmittance and broadband performance. To improve the design efficiency and transmittance of the broadband splitter, Particle Swarm Optimization was used to reverse-design the splitter. The performance of the designed splitter was studied, and the results show that the designed splitter has a wide bandwidth range of 1530~1565 nm. It exhibits a minimum transmittance of 84% and an average transmittance of 87.8% for TE polarization, and a minimum transmittance of 81% and an average transmittance of 83% for TM polarization. Response times for both polarizations are below 0.7 ps. This device has promising applications in all-optical communication networks and photonic high-density integration.

Modeling and experimental characterization of a compact Mach-Zehnder electro-optic modulator on SOI

Fanglin Xie, Hongqiang Li, Aijia Xue, MING Han, Lizhen Zhang, and Enbang Li

DOI: 10.1364/AO.537503 Received 25 Jul 2024; Accepted 30 Sep 2024; Posted 01 Oct 2024  View: PDF

Abstract: The Mach–Zehnder electro-optic modulator (MZM) plays a crucial role in photonics integration technology during signal transmission. We propose the design and fabrication of a silicon-based electro-optic modulator based on the M-Z structure and design a modulator using silicon as the waveguide core layer on a silicon dioxide substrate. The detailed design involves a 1 × 2 splitter, branch waveguides, modulating arm waveguides, and a 2 × 1 combiner. The MZM fabrication and characterization results reveal that the half-wave voltage of the silicon-based MZM is 2 V, with an optical loss of -2.464 dB and a device core size of 450 μm × 2800 μm. The experimental results indicate that the MZM with a low half-wave voltage, a low loss, and high integration has significant value. The proposed MZM exhibits considerable improvements, featuring a low half-wave voltage and a low loss, and this device may serve as a fundamental component in wearable large-scale photonic integrated circuits for weak ECG real-time detection.

Optimization of polarization-independent grating coupler on lithium-niobate-on-insulator

Xinke Xing, Bin Chen, kaixuan chen, and Liu Liu

DOI: 10.1364/AO.537895 Received 06 Aug 2024; Accepted 30 Sep 2024; Posted 01 Oct 2024  View: PDF

Abstract: We have designed a novel polarization-independent grating coupler with non-uniform periods, assisted by plasmonic modes on a lithium-niobate-on-insulator platform. By depositing metal into the LN grating trenches, we can excite plasmonic modes that adjust the effective refractive index difference between the TE mode and TM mode. In contrast to previous research, the etching depth of the LN is optimized to achieve high coupling efficiency for both polarization modes simultaneously in the C-band. The coupling efficiency, polarization-dependent loss (PDL), and back reflection were further optimized using inverse design. The simulated coupling loss of the proposed grating is -3.18 dB for the TE mode and -3.22 dB for the TM mode. The PDLs are less than 0.09 dB, and the back reflection to the waveguide is below -11.9 dB for both polarization modes over an 80 nm wavelength range. Additionally, we designed a taper that connects the gratings and single-mode waveguides to reduce transmission loss for TM modes by mitigating the influence of mode hybridization, resulting in a PDL of less than 0.03 dB. This novel grating coupler has potential applications in polarization multiplexing and polarization-independent systems on LNOI chips.

Improving Current- matching in Textured Perovskite/Silicon Tandem Solar Cells via Thickness Control Strategy

Mina Piralaee, Elmira Annabi Milani, and Asghar Asgari

DOI: 10.1364/AO.538341 Received 01 Aug 2024; Accepted 30 Sep 2024; Posted 01 Oct 2024  View: PDF

Abstract: This study presents an analysis of a 2-terminal tandem solar cell that integrates metal-doped, lead-freedouble Cs2AgBi0.75Sb0.25Br6 perovskite with silicon to enhance overall energy conversion efficiency. Thisstudy explores how the thicknesses of the top and bottom sub-cells affect current- matching in 2-terminaltandem perovskite/silicon solar cells with two separate planar and textured configurations. Using numericalmodeling in MATLAB, and considering dominant recombination effects, we calculated the performanceparameters of the device. We investigated the optical and electrical properties of textured tandem structures,focusing on current- matching and the influence of layer thickness on device performance. Given thecomplexity, time, and expense involved in constructing tandem solar cells, being able to analytically determinethe thickness at which current- matching occurs can be highly advantageous. This approach offers the benefitof providing a precise analytical relationship for this purpose. Our findings demonstrate that increasing the topcell thickness enhances current density and power conversion efficiency but at the cost of the bottom cell’sefficiency due to increased light absorption. Moreover, we discovered a nearly linear behavior between thethickness of the top and bottom cells for achieving current- matching. The study highlights the critical balancerequired to optimize layer thicknesses, thereby improving the design and performance of tandem solar cells.These insights are significant as they pave the way for more efficient and cost-effective tandem solar celldesigns in the future, potentially accelerating the adoption of advanced photovoltaic technologies. The resultsshow good agreement with experimental data, validating our model.

Super-resolution imaging by dual spherical-wave optical scanning holographic microscopy

Jung-Ping Liu and Chen-Hsiang Huang

DOI: 10.1364/AO.527739 Received 17 Apr 2024; Accepted 30 Sep 2024; Posted 30 Sep 2024  View: PDF

Abstract: Conventional optical scanning holographic microscopy (OSHM) is realized by using a plane wave and a spherical wave as the illumination. The resolution of the conventional OSH is limited by the numerical aperture of the spherical wave and cannot exceed the Rayleigh limit. In this paper, the OSHM by using dual spherical waves as the illumination is studied. The model of Gaussian wave together with the constraints on the minimum signal strength and on the minimum interference visibility are considered in the analysis of the dual-spherical-wave (DS) OSHM. For a specimen located at the symmetry plane of DS-OSHM, the resolution can slightly exceed the Rayleigh limit. In addition, the depth of field is extended significantly. If the specimen is off the symmetry plane, clear reconstructed image will be observed not only at the original object plane but also at its mirror plane. Although this phenomenon results in ambiguity of depth, it can further increase the depth of field of DS-OSHM.

Modeling and application of a bidirectional Mach-Zehnder electro-optic modulator with enhanced high-frequency modulation efficiency

Xuepei Ma, Yiwen Ou, Li Qian, Huiyong Guo, Chunfu Cheng, Ciming Zhou, and Hui Lv

DOI: 10.1364/AO.534791 Received 15 Jul 2024; Accepted 30 Sep 2024; Posted 30 Sep 2024  View: PDF

Abstract: We present a simple way to enhance bidirectional broadband modulation in a travelling wave Mach-Zehnder electro-optic modulator (EOM) by removing matched impedance and effecting a reflection of the microwave signal. A model was devised for determining the modulation efficiency (ME) in the presence of microwave reflection, without access to the physical parameters of the EOM. By using the model, the simulated MEs matched well with the measured ones, which verified the correctness of the model and also demonstrated that the internal parameters of the modulator could be inferred from the model. Compared with the traditional unidirectional EOM, the measured ME of the bidirectional modulator for the optical signal that counter-propagates with the incident microwave signal was improved by 10 times at least. When the velocities of microwave and optical signals were matched, the MEs at the frequency higher than 3.3 GHz was further improved. Our model was also used to derive the optical interference signal utilizing bidirectional modulation. It was found that the signal intensity was enhanced significantly, and further improved after introducing the velocity match. This work can be expected to open up many microwave photonics applications where bidirectional optical modulation is utilized in novel sensing and metrology.

Error analysis based on a tunable waveplate polarization interferometric imaging spectrometer

Feng Tang, Chunmin Zhang, Zhen Ma, Ke Ke, Yanqiang Wang, and ruixin zhao

DOI: 10.1364/AO.538907 Received 12 Aug 2024; Accepted 29 Sep 2024; Posted 03 Oct 2024  View: PDF

Abstract: Interference imaging spectroscopy combines modernimaging technology with spectral technology, holdingsignificant importance for object imaging and spectraldetection. This article introduces the principle of anadjustable wave-plate polarization interferometricimaging spectrometer. The example design specificationsare set for an observation wavelength range of 450 to780 nm and a maximum resolution of 2 nm at 450 nm,with a 0.5-inch detector as the base for calculating thespecific dimensions of the Soleil-Babinet compensator.An investigation was conducted on the issues of nonuniform sampling as well as three types of mechanicalerrors: flatness, wedge angle tolerance, and optical axisorientation accuracy. Emphasis was placed on discussingthe impact of these errors on the instrument's opticalpath difference and spectral reconstruction accuracy.This research provides theoretical guidance for thedesign and engineering of this miniaturized imagingspectrometer.

The Multi-view High Dynamic Range 3D Reconstruction and Point Cloud Quality Evaluation Based on Dual-frame Difference Images

Yun Feng, Rongyu Wu, Peiwu Li, Wenlei Wu, Jiahao Lin, xiaojun liu, and Chen Liangzhou

DOI: 10.1364/AO.533718 Received 26 Jun 2024; Accepted 28 Sep 2024; Posted 30 Sep 2024  View: PDF

Abstract: High dynamic range can easily lead to image saturation, making it a challenge for structured light 3D reconstruction. This paper proposes a multi-view 3D morphology measurement system, which consists of dual projectors and a high-precision rotating platform. The system uses single-frame images to achieve high dynamic range surface adaptive exposure. It then proposes using dual-frame difference images combined with multi-view imaging to identify high-reflective areas and complete the point cloud. This approach addresses the issues of shadows caused by occlusions of the object's geometric features and visual blind spots caused by local high reflectivity. Finally, the quality of the high dynamic range point cloud data is evaluated based on deep learning. Comparative experiments show that optimal exposure in single-frame images can achieve better imaging quality and shorten capture time. The dual-frame difference image algorithm can identify high-reflective areas and complete the point cloud data. The point cloud quality evaluation model based on IT-PCQA demonstrates the effectiveness of the proposed method for high dynamic range 3D reconstruction.

SiO2-capped ZnO quantum dots based highly sensitive optical fiber humidity sensor with potential applications in human breath monitoring and voice print recognition

Sunil Mohan and manish negi

DOI: 10.1364/AO.533760 Received 03 Jul 2024; Accepted 28 Sep 2024; Posted 30 Sep 2024  View: PDF

Abstract: This article describes the development and characterization of an optical fiber humidity sensor employing intensity modulation via evanescent wave (EW) absorption techanique. For the development of the sensor, SiO2-capped ZnO quantum dots (QDs) thin film is synthesized over the decladded portion of plastic cladding silica (PCS) fiber via the sol-gel method. A thorough experimental investigation was conducted by varying the thickness of the sensing film to optimize the sensor's response. The sensing probe with optimized film thickness of 891nm demonstrates a linear response over 30.5%-92.5%RH with an enhanced sensitivity of 46.2mV/%RH (0.0138RH-1). Very fast response and recovery times of 2s and 2.5s are observed during humidification and dehumidification for optimized sensing probe. The maximum resolution recorded during the short stability test is ±0.12%RH. Additionally, the proposed sensor demonstrates a very high degree of repeatability, reversibility, and stability. The proposed sensor has also been tested for human breath monitoring and voice print recognition. The result shows the sensor is able to detect minute humidity fluctuations in exhaled air during breathing and speaking.

Surface Roughness Metrology with Raster Scanning Single Photon LiDAR

Daniel Tafone, Luke McEvoy, Yong Meng Sua, and Yuping Huang

DOI: 10.1364/AO.537404 Received 22 Jul 2024; Accepted 27 Sep 2024; Posted 30 Sep 2024  View: PDF

Abstract: We explore a novel approach to surface roughness metrology utilizing a single pixel, raster scanning single-photons counting LiDAR system. It uses a collimated laser beam in picosecond pulses to probe a surface, capturing the changes of back-scattered photons across various incident angles into a single mode fiber, and counting them using a single photon detector. These back-scattered photons carry speckle noise produced by the rough surface, and the variation in photon counts over different illumination points across the surface become a good measure of its roughness. By analyzing the variation frequency as the LiDAR scans over the surface using machine learning techniques, we demonstrate general measurements of surface roughness between 1.21 (1.27 ± 4.51) to 102.01 (87.97 ± 10.55) microns.

Stochastic Topological Systems in Arrays of Optical Tweezers

Ming Gao, Nan Li, Xingfan Chen, Han cai, and Huizhu HU

DOI: 10.1364/AO.531281 Received 30 May 2024; Accepted 27 Sep 2024; Posted 27 Sep 2024  View: PDF

Abstract: Topology has become important in understanding condensed matter physics and photonics such as in the quantum Hall effect. Recently, interest has grown in applying topology to stochastic systems, including biological active matter. In this study, we explore stochastic topological physics using optically levitated particles in arrays of optical tweezers. We describe the particle dynamics with the Langevin and master equations, mapping the latter to a tight-binding model called the Markov network. By changing the escape rates between traps, we create the Su–Schrieffer–Heeger (SSH) model and show the existence of topologically protected edge states. Our theoretical and simulation results connect the decay rates of particles at certain sites to these topological states. This research provides a new way to study topological physics in stochastic systems with the precise control and measurement capabilities of optical tweezers.

TPF stitching imaging of rubber tree leaves.

Meng Liu, Guozhong Hou, Yuan Li, Yanyan Deng, Yuan Zhang, Kaiyuan Song, Chang Xu, Xinlong Liu, Yuanqin Xia, and yong zhang

DOI: 10.1364/AO.537026 Received 18 Jul 2024; Accepted 27 Sep 2024; Posted 10 Oct 2024  View: PDF

Abstract: This paper presents a two-photon fluorescence (TPF) microscopy platform based on a femtosecond oscillator. The system images rubber tree leaf samples by exciting and detecting TPF signals generated by the tissue using broadband femtosecond pulses. The imaging system utilizes a detection window of 565-615 nm to capture TPF signals from flavin adenine dinucleotide (FAD) in the rubber tree leaf tissue. Additionally, multiple TPF images were acquired through sample movement and Z-axis scanning, followed by image stitching to achieve large-area comprehensive visualization of the rubber tree leaf samples. This study provides detailed observations and analyses of various biological structures within the rubber tree leaf samples, contributing significantly to the understanding of plant growth, physiological functions, and environmental adaptability.

Multi-spectral radiation thermometry of space point targets based on spectral image pixel binning

Pengkai Dong, Liang Zhou, zhaohui Liu, and Kai Cui

DOI: 10.1364/AO.537027 Received 18 Jul 2024; Accepted 25 Sep 2024; Posted 26 Sep 2024  View: PDF

Abstract: The temperature characteristics of space point targets are essential indicators of theiroperational status and performance. To address the issue of significant temperaturemeasurement errors in space point targets caused by low temperatures and low imaging signalto-noise ratio (SNR), we propose a mathematical model for multi-spectral radiationthermometry, derived from the principles of dual-band radiation thermometry. Furthermore, amulti-spectral image pixel binning method is introduced to enhance SNR and minimizemeasurement errors. Experimental results indicate that the proposed multi-spectral radiationthermometry outperforms dual-band radiation thermometry. After merging 2 to 20 pixels,multi-spectral radiation thermometry in the 3.75-4.1 μm and 4.3-4.62 μm bands demonstratesenhanced SNR and reduced temperature measurement errors. For a 378.15 K blackbody, therelative errors decrease from 1.52% and 2.19% to 0.26% and 0.74%, respectively, after mergingsix and eight pixels in the two different bands, compared to unmerged images. This methodprovides a valuable reference for developing techniques to enhance the SNR and improvetemperature measurement accuracy for space point targets.

Turbulence Compensation based on pix2pixGAN for the Free Space Optical Communication of Orbital Angular Momentum Multiplexing

zhi zhang, Shuaiwei Jia, wen shao, Duorui Gao, Wei Wang, Jinhai Si, and Xiaoping Xie

DOI: 10.1364/AO.535452 Received 19 Jul 2024; Accepted 25 Sep 2024; Posted 26 Sep 2024  View: PDF

Abstract: Free space optical communication utilizing orbital angular momentum (OAM) multiplexing offers extremely high transmission capacity and speed. However, atmospheric turbulence causes mode crosstalk and power loss during OAMs decoding. Altough conventional adaptive optics (AO) makes up for the distortions, it introduces significant complexity and cost due to the need of wavefront-sensor. This paper proposes a novel sensor-less AO framework based on the image-to-image translation capabilities of pix-to-pix generative adversarial networks (pix2pixGAN). This approach directly recovers the compensation information by recognizing the distorted image. Laboratory experiments revealed that the proposed AO method effectively compensates the distortions of vortex beams in the 80Gbit/s communication system combining 4 wavelengths and 2 OAMs. Even in severe turbulence (D/r0=8), the power sensitivity minimally enhanced 3.7dBm under the bit-error-rate of 10-3.

Performance Prediction of Closely Spaced Dual Port Dual Band Silicon-Graphene Antenna with High Isolation for THz Wireless Applications using Machine Learning Algorithm

Gopal Kumar Thakur, Shridhar Kulkarni, Lov Kumar, and Rakhee Jain

DOI: 10.1364/AO.533749 Received 24 Jun 2024; Accepted 25 Sep 2024; Posted 26 Sep 2024  View: PDF

Abstract: This article explains the design and optimisation of twin port silicon-graphene radiator in THzfrequency regime. The suggested radiator's primary attributes are: (i) Plus-formed slot produces dualradiating mode (HEM11δ and HEM12δ) inside the ceramic (ii) presence of absorber wall in between thetwin port blocks the radiation between the ports and raises the separation level to above 35 dB as well; (iii)graphene sheet over ceramic and MS provides the tunability in proposed radiating structure; and (iv)machine learning procedures i.e. ANN and random Forest helps to expect the S-parameter of plannedantenna. Actual (HFSS/CST software) and projected value (using ML algorithm) confirms that the designradiator works effectively in twin working spectrum i.e. 3.3-3.98 THz and 4.9-5.45 THz. Good value of farfield and MIMO parameters confirms its applicability in THz built biomedical uses.

Spatial resolution limit for a solid immersion lens

Vladislav Zhelnov, Vladislav Ulitko, Maksim Skorobogatiy, Kirill Zaytsev, and Nikita Chernomyrdin

DOI: 10.1364/AO.536928 Received 17 Jul 2024; Accepted 25 Sep 2024; Posted 26 Sep 2024  View: PDF

Abstract: The solid immersion (SI) effect is widely used to increase the spatial resolution of optical focusing systemsand even overcome the Abbe diffraction limit. Resolution enhancement offered by a SI lens is mostlya function of its geometry and refractive index nSI. While SI lenses are relatively well understood, thescaling of the resolution enhancement by such lenses is still a subject of debate, with some works reporting≃ nSI and ≃ n2SI dependencies for the hemispherical and hyperhemispherical SI lens configurations,respectively. In this paper, we offer a general argument for a resolution limit for SI optics and, then, verifyit via the numerical analysis of the hemispherical and hyperhemispherical silicon SI lenses designedfor the terahertz (THz) range. In fact, we find that there is no contradiction in the reported resolutionenhancements ≃ nSI and ≃ n2SI, however, they happen in different operation regimes. We then demonstratethat the resolution values reported for the different SI lens arrangements in the visible (VIS), near- andmiddle-infrared (NIR and MIR), as well as THz bands obey the derived limit. Our findings will be usefulfor the further design and applications of SI optics.

Joint carrier phase tracking and IQ imbalance tolerance with the dynamic adaption of reference constellation for probabilistic shaped signals

Sameer Ahmad Mir, Lakshmi Narayanan Venkatasubramani, Liam Barry, and Deepa Venkitesh

DOI: 10.1364/AO.532756 Received 13 Jun 2024; Accepted 24 Sep 2024; Posted 24 Sep 2024  View: PDF

Abstract: Probabilistic constellation shaping (PCS) has played a pivotal role in facilitatingcapacity enhancement towards Shannon limits. However, the higher cardinality PCS is susceptibleto transmission system non-idealities such as transmitter IQ imbalance. In this paper, wedemonstrate a phase noise correction algorithm that simultaneously adapts the ideal constellationpoints and, hence, the decision boundaries, using the gradient descent adaption to toleratethe transmitter IQ imbalance for PCS signals. The equalizer uses a common error signalobtained using the least mean square approach to recover the carrier phase and adapt theideal constellation points. We numerically investigate the algorithm’s efficacy for 200 GBaudpolarization multiplexed PCS-64QAM signal for a range of imbalance and entropy values. Wealso validate the performance of the proposed algorithm at various optical signal-to-noise ratio(OSNR) values and at larger laser linewidth values. The proposed algorithm applies to anymodulation format and does not require any pilot symbols, hence improving the spectral efficiencycompared to traditional algorithms that employ pilot symbols.

Surface profile measurement of metal objects by use of fringe projection system with polarized dual projectors

Huijie Zhu and Hongwei Guo

DOI: 10.1364/AO.534147 Received 27 Jun 2024; Accepted 24 Sep 2024; Posted 26 Sep 2024  View: PDF

Abstract: When measuring surface profiles of metal objects by use of fringe projection technique, “high dynamic range” issueusually occurs and affects measurement accuracy and efficiency. To alleviate this problem, this paper suggests afringe projection measurement system that consists of dual projectors having orthogonal polarization directionsand a polarization camera having four polarized channels. The projectors simultaneously cast fringe patterns havingopposite phases onto the measured metal objects and the camera grabs four composite patterns at a single shot. Byanalyzing these composite patterns, two separate fringe patterns that correspond to the projected fringe patternscan be reconstructed. In this procedure, because redundant fringe patterns have been collected, the oversaturatedareas that are caused by “high dynamic range” issue and appear in some of the four composite patterns can beexcluded and will not affect the reconstructed fringes. As a result, the reconstructed fringe patterns can have a largerdynamic range over the camera capacity, thus helping to alleviate effects of the “high dynamic range” issue. By usingphase-shifting technique, the fringe phases and, further, the depth map of the measured object are calculated.Besides, this method has an improved measurement efficiency in comparison with others using a single projectorbecause we reconstruct two phase-shifted fringe patterns from a single shot. The validity of this method isdemonstrated by using experimental results.

Exit pupil uniformity improvement of holographic waveguide displays based on genetic algorithm

Qibin Feng, Yusong Guo, Chen Zheng, Min Guo, Wang Zi, and Guoqiang Lv

DOI: 10.1364/AO.535457 Received 11 Jul 2024; Accepted 21 Sep 2024; Posted 23 Sep 2024  View: PDF

Abstract: Waveguide displays based on volume holographic grating (VHG) have the benefitof expanding the exit pupil. Exit pupil size and exit pupil uniformity are two importantindicators of waveguide displays. Most current optimizations for exit pupil uniformity arebased on the central viewing angle. However, this does not ensure satisfied exit pupiluniformity for other viewing angles. In this paper, a method based on a genetic algorithm tooptimize the exit pupil uniformity under the whole horizontal field of view (HFOV) isproposed. Firstly, the ray-tracing-based mathematical model to analyze the uniformity of thewhole HFOV at all wavelengths is established. Based on the mathematical model, theobjective function can be defined, which can optimize the refractive index modulationdistribution on several regions of the out-coupling VHG. The simulation results show that fora holographic waveguide display with the exit pupil size of 16 mm × 12 mm, the average exitpupil uniformity can reach 86.87% at a diagonal field of view (DFOV) of 30°, 9.16%improvement than the previous method. The genetic-algorithm-based method proposed in thepaper can effectively improve the exit pupil uniformity.

Off-Axis Freeform Optical Design for Large Curved Field of View Imaging

Xingyue Jia and Hongbo Jia

DOI: 10.1364/AO.536986 Received 18 Jul 2024; Accepted 19 Sep 2024; Posted 26 Sep 2024  View: PDF

Abstract: Recording neuron activities is pivotal for elucidating the functionality of the nervous system. However, the curved cortex surface of experimental mice presents a significant challenge for optical systems, particularly when a larger field of view (FOV) is required. To address this challenge, we have designed an off-axis three-mirror system that incorporates freeform surfaces on both the primary and secondary mirrors. This system achieves a large imaging FOV of 18°×9°, delivering near-diffraction-limit imaging quality across a curvature spectrum of -14.5 mm to -15.5 mm. Manufacturability analysis indicates that the freeform surfaces are straightforward to produce, and the overall system demonstrates low sensitivity to tolerance and measurement errors. This study introduces a novel solution to the field curvature constraints in optical imaging of cortical activity, providing substantial technical support for in vivo neuronal imaging endeavors.

Enhanced upconversion luminescence of Er3+/Y3+co-doped tellurite glasses for volumetric three-dimensional display

miao gang ajd, yan deng, Qiang Zhao, Wencai Li, jikang jikang, and dongcheng han

DOI: 10.1364/AO.531048 Received 28 May 2024; Accepted 19 Sep 2024; Posted 19 Sep 2024  View: PDF

Abstract: A large-size Er3+/Y3+ co-doped tellurite glasses with high image quality were preparedby conventional melt-quenching method for volumetric three-dimensional display. The effect of theconcentration of Er3+ and Y3+ on the two-frequency upconversion luminescence properties andvolumetric three-dimensional display were investigated. Under dual-wavelength excitation at 850nm and 1550 nm, the two-frequency two-step upconversion luminescence intensity at 546 nm in theEr3+/Y3+ co-doped tellurite glass was significantly enhanced by about 44% comparing with that inthe Er3+ doped tellurite glass. However, the upconversion luminescence mechanism and its dynamicprocess were further obtained. A range of the high-resolution and high-contrast volumetric threedimensional images can be achieved in the Er3+/Y3+ co-doped tellurite glass with optimal dopingconcentration, utilizing a coordinated control system of galvanometer scanner and digital micromirror devices. The results indicate that Er3+/Y3+ co-doped tellurite glass has promising potentialfor widely volumetric 3D displays.

Relieving photobleaching impacts on fluorescence thermometry via neural network predictions

jiahao wang, Binhe Wu, Chunrui Wang, Jian Zhou, Hao Sun, Wenhan Cao, and Huimei Yu

DOI: 10.1364/AO.533586 Received 27 Jun 2024; Accepted 19 Sep 2024; Posted 19 Sep 2024  View: PDF

Abstract: The thermal sensitivity of luminescence intensities enables fluorescence thermometry for remote temperature probing with high spatial and temporal resolutions. However, its accuracy suffers from factors such as nonlinear thermal response and the photochemical stability ofthe fluorescence sensors. In this work, we realized thermometric measurements with high spatialresolution at micrometer scale using thin films with europium(Eu) complexes and microscopicmeasurements. We identified the tris(dibenzoylmethane)phenanthroline europium(III)/polystyrene (Eu(DBM)3Phen/PS) thin film as an optimal choice for not only its linear-dependence offluorescence intensity for temperatures of biological interest, but also its stronger resistance tophotobleaching effect. More importantly, we show that the latter effect can be effectively compensated via neural network methods. This approach has been validated for surface temperaturemapping at thermal equilibrium, where better uniformity as compared with results without correcting the photobleaching effect was achieved. The temperature elevation of resistive wires dueto Joule heating can be clearly identified. This work shows that neural network models are powerful tools in improving the accuracy of fluorescence thermometry and beneficial for applicationsranging from biology to nanotechnologies.

Transparent Porous Medium for Optical Fluid Flow Measurement using Refractive Index Matching

Happiness Imuetinyan, Paul Fruton, Cedric Giraudet, and Fabrizio Croccolo

DOI: 10.1364/AO.536805 Received 16 Jul 2024; Accepted 19 Sep 2024; Posted 19 Sep 2024  View: PDF

Abstract: We present a method for obtaining a transparent porous medium for optical fluid flow measurement using refractive-index matching. We report on the design of three transparent porous media, one of which was used in a study about the dynamic spreading of convection. The method relies on shadowgraphy, a well-known imaging technique, that enables index matching between a porous material and a saturating fluid. The method was tested using borosilicate glass beads as the porous material, while the refractive index of the fluid mixture was determined using a calibrated refractometer operating at multiple wavelengths. The best matching condition is obtained by merging two criteria: the maximum intensity ratio and the minimum standard deviation. The previous knowledge of the fluid refractive index additionally allows us to recover the refractive index of the solid phase, which is 1.471 at λ=670 nm. The paper discusses the importance of understanding the fluid flow through porous media and optical measurement techniques in studying these processes.

A multiwavelength Raman lidar system for profiling the CCN number concentrations

Jiandong Mao, Jun Bao, and Qiang Wang

DOI: 10.1364/AO.538248 Received 01 Aug 2024; Accepted 16 Sep 2024; Posted 17 Sep 2024  View: PDF

Abstract: Cloud condensation nuclei (CCN) play an important role in research of cloudmicrophysical and aerosol–cloud interactions. This study employ a multiwavelength Ramanlidar for measuring CCN concentration. First, the multiwavelength Raman lidar was usedtomeasure the atmospheric relative humidity profile, and the combination of relative humidityand the aerosol backscattering coefficient was used to retrieve the hygroscopic growth factor. By fitting the hygroscopic growth factor using the κkappa parameter model, the hygroscopicparameter κkappa that characterizes the hygroscopicity of aerosols was obtained. Then, thecritical activation radius of aerosols was derived using the κ–Κöhler theory andhygroscopicity parameter κkappa. Finally, the CCN number concentration was obtained throughcombination with the aerosol particle size distribution. Experiments were conducted to verifythe feasibility of the multiwavelength Raman lidar. Results showed that the effectivedetection range of the lidar is approximately 0–4 km. The error of the temperature measuredby the lidar at the height of 0.3–3.8 km is approximately ±1 K. When the relative humiditychanges is 0.77–0.87, the range of the hygroscopic growth factor changes is 1.06–1.10, thehygroscopic parameter γ is 0.065, and the hygroscopic parameter κkappa was 0.009. The CCNnumbers concentration decreases with height, but increases closer to the cloud. Themultiwavelength Raman lidar is an important tool for detection of cloud microphysical andaerosol–cloud interactions, and could have scientific importance and research value both forimproved understanding of the formation of clouds and precipitation and for enhancedaccuracy of weather modification.

Gain Performance and Thermal Effects of Nd:Glass and Nd,Y:SrF2 crystal

Yenan Zhang, Jiangfeng Wang, Jiangtao Guo, Xinghua Lu, Xiaochao Wang, zhen zhang, Dapeng Jiang, Liangbi Su, Wei Fan, and Xuechun Li

DOI: 10.1364/AO.539514 Received 16 Aug 2024; Accepted 15 Sep 2024; Posted 16 Sep 2024  View: PDF

Abstract: The gain performance and thermal effects of Nd:glass (N31) and Nd,Y:SrF2 crystal areinvestigated and compared. The results show that under the same pump conditions, Nd,Y:SrF2can provide a small-signal gain similar to that of N31. The main advantages of Nd,Y:SrF2 areits weaker thermal effects and greater thermal-fracture limitations compared with those of N31.In this paper, the two gain media are compared in the form of rods whose diameter is 5mm andlength is 60mm. The small-signal gain coefficients of N31 and Nd,Y:SrF2 are 1.39 and 1.46,respectively, at a pump energy of 1.5 J/1 Hz. The pump energy is set at 1.5 J/8 Hz toexperimentally investigate their thermal effects. The thermal wavefront of N31 is 0.987λ at arepetition rate of 10 Hz, whereas that of Nd,Y:SrF2 is 0.679λ. In thermal destructiveexperiments, N31 fractured at an average pump power of 15 W (1.5 J/10 Hz), whereasNd,Y:SrF2 fractured at a higher power of 27 W (1.5 J/18 Hz) owing to its excellent thermalconductivity, which is 7.28 times that of N31. These results indicate the potential of Nd,Y:SrF2crystal as a gain medium for high-repetition-rate laser amplifiers.