ObjectiveThe squeezed states can not only verify the fundamental principles of quantum physics, but also have important applications in the field of quantum information processing. In the field of quantum communications, the speed and fidelity of communication depend on the squeezing degree of the squeezed states. A degenerated optical parametric amplifier (DOPA) is efficient approach on the production of the squeezed states. In the paper, a coherent feedback controlled cascaded DOPA on the squeezing enhancement is discussed.MethodsBase on quantum Langevin equation, the input-output relation of the quadratures noise of the output field can be obtained. The output of the first DOPA is seeded to the second DOPA. Then, the noise of the light from the second DOPA can also be obtained. Further, the output of the second DOPA is partially feed back to the input of the first DOPA through a beam splitter. Based on the input-output relation of the beam splitter, the noise formula of the coherent feedback controlled cascaded DOPA is acquired.Results and DiscussionsBased on the noise formula, the dependence of squeezing from the coherent feedback controlled cascaded DOPA on feedback intensity, the phase delay of the feedback loop, pump parameter, the propagation loss of the feedback loop is numerically simulated and presented in the paper. These results show that the squeezing can be significantly enhanced with a certain parameter range.ConclusionsThis paper discusses a cascaded DOPA with coherent feedback, which uses a beam splitter as a feedback controller to feed the squeezed light output from the cascaded DOPA back to the input port. Our research results show that within a certain range of feedback intensity and specific feedback phase, coherent feedback can further enhance the quantum squeezing characteristics of the output light field from a cascaded DOPA. The paper has also studied the dependence of squeezing enhancement effect on system physical parameters. The research results of this paper can provide theoretical reference for improving the squeezing of the squeezed light for the short-wavelength range, laying the foundation for their application in quantum communication and quantum information fields.

ObjectiveThe single-photon source is the essential resource in the fields such as quantum communication, quantum computing, and quantum cryptography, while photon blockade is an important means of preparing single-photon sources and controlling single photon. The photon blockade effect in optomechanical systems has been receiving wide attention and was studied intensively. The Kerr nonlinear interaction between photons induced by optomechanics is just the main mechanism for generating photon blockade. This paper aims to realizing photon blockade in a fully coupled hybrid cavity opto-mechanical system consisting of a cavity opto-mechanical system and a two-level atom with three subsystems coupled to each other, where photon blockade effect occurs. It is of great significance for the preparation of single-photon and two-photon sources.MethodsThe Hamiltonian of the system consists of three parts: the free Hamiltonian, the interaction Hamiltonian and the driving Hamiltonian. We treat the Hamiltonian of the system by applying the Schrieffer-Wolff approximation to the Hamiltonian of the system, the three-body system can be approximated as two-body system of cavities and mechanical oscillators, and we obtain a fully diagonally effective Hamiltonian and its eigen systems. Although the three-body system is transformed into a two-body system, the transition frequency of the two-level atom plays an important role, which induces a shift in the cavity resonance frequency. There exist two detunings in the system, the cavity field detuning and the drive detuning, which affect the frequency of the cavity field, thus the system has two adjustable parameters to regulate the photon blockade effect. The Kerr nonlinear term leads to the anharmonicity of the energy levels which is the essential cause of the photon blockade.Results and discussionsWe perform accurate numerical simulation by means of the master equation comparing it with the analytical solution. We find that photon blockade occurs when both detunings are adjusted. The analytical results and the numerical results are in good agreement. At some cavity field detuning the two-photon blockade regime becomes wider when increasing the driving intensity. At some drive detuning the photon blockade has a small effect when increasing drive intensity. For weak driving the photon blockade depends mainly on the detuning rather than the driving intensity.ConclusionsIn this paper, we studied single- and two-photon blockade in the fully coupled cavity opto-mechanical system. After an unitary transformation a fully diagonal effective Hamiltonian can be obtained. Enhancement of the optical-mechanical coupling constant can be realized by adding a two-level atom to the system. A Kerr nonlinear term comes up in the Hamiltonian which constructs the conditions for the creation of photon blockade. It can be observed from the Hamiltonian that the system has two adjustable detuning respect to the resonant frequency of the cavity. Then we investigate the photon blockade of the system under the different detuning and driving strength using the probability amplitude equation method and the quantum master equation method, and the both results of which are in good agreement. The results of the study provide new options for obtaining single- and two-photon sources.

ObjectiveHigh performance balanced homodyne detectors with high bandwidth, high quantum efficiency, reasonable gain, and high common-mode rejection ratio are critical in the fields of high-speed quantum communication, broadband squeezed light and entangled light detection, and broadband quantum precision measurement. However, it is challenging to design and develop GHz bandwidth balanced homodyne detectors with overall high performance. For example, the actually achieved bandwidth and gain are usually limited by the distributed capacitance of the circuit board. Furthermore, high common-mode rejection ratio, linear response and low noise performance are also technically challenging due to fabrication and soldering processes of the printed circuit board (PCB).MethodsIn comparison with other high bandwidth balanced homodyne detectors developed in previous works, we have adopted a design that separates alternating current (AC) and direct current (DC). The DC output helps us monitor the optical power and ensures the balance of the two photodiodes, which is crucial for balanced homodyne detection in quantum optical experiments. Our circuit board design involves placing two photodiodes on opposite sides of the PCB. This makes it easier to ensure the circuit lengths of the two photodiodes are exactly the same and reduce the distributed capacitance of the circuit board. This is important for enhancing the detector bandwidth and improving the common-mode rejection ratio. Additionally, this design is more convenient for quantum optical experiment operations.Results and DiscussionsWe used two photodiodes with 95% quantum efficiency, low capacitance and dark current for our balanced homodyne detector. The DC gain of the balanced homodyne detector is 1003.6 V/W, which is at a reasonable level for optical power monitor. We also took the noise power spectra of the balanced homodyne detector at different optical powers. We demonstrate the detector response is linear at five different frequencies. An effective bandwidth of 1 GHz and a maximum signal-to-noise ratio of 12 dB were realized at 1550 nm. The common-mode rejection ratio of the homodyne detector was tested by modulating the input beam with an electro-optical modulator. By placing two photodiodes symmetrically on the front and back sides of the double-sided PCB board, a common-mode rejection ratio of 60.65 dB at 20 MHz was achieved. Subsequently, the balanced homodyne detector was used to measure 1550 nm quadrature amplitude squeezed light. Up to -2.5 dB squeezing was observed over a bandwidth range from 3.5 MHz to 71.5 MHz.ConclusionsIn this paper, we designed and developed a balanced homodyne detector with GHz bandwidth, high quantum efficiency, reasonable gain, and high common-mode rejection ratio. The results show that the balanced homodyne detector with high bandwidth and high common-mode rejection ratio are useful in broadband squeezed state measurement.

ObjectiveSqueezed states are an important resource in quantum information processing. Utilizing squeezed light can enhance the sensitivity of physical measurements and surpass the standard quantum limit, making it an indispensable element in quantum information science. Integrated photonics chips provide a reliable and easily scalable research platform for generating squeezed states, and the high stability and reproducibility of modern lithographic techniques offer hope for achieving large-scale quantum technologies. The four-wave mixing process based on silicon nitride micro-ring resonators is an important approach for generating squeezed states. The main factors limiting the generation of on-chip high-level-squeezing include micro-ring resonator design, nonlinear noise, and, especially, photonic loss. To further improve the level of squeezing generated on-chip, the preparation of low-loss or high-Q silicon nitride micro-ring resonators is particularly crucial.MethodsIn this work, we employed low-pressure chemical vapor deposition (LPCVD) to deposit high-quality silicon nitride films. To achieve micro-ring resonators with varying states of coupling, we designed a series of micro-ring resonators with different coupling gaps. Using scanning electron microscopy, we optimized both the deposition process of high-quality silicon nitride films and the etching process of micro-ring resonators on the silicon nitride platform. As a result, we successfully fabricated low-loss silicon nitride micro-ring resonators with various sizes and structures. We also set up a testing system for the micro-ring resonators, using lensed fibers for end-face coupling with the chip. Then, we experimentally characterized these micro-rings and systematically analyze their transmission spectra, coupling regimes, and quality factors.Results and DiscussionsResults show that the prepared micro-ring resonators have similar intrinsic quality factors. Notably, the intrinsic quality factor of the micro-ring resonator with a cross-sectional size of 2000 nm × 290 nm reaches 1.5×106, corresponding to a waveguide transmission loss of 0.23 dB/cm. Theoretical calculations indicate that this structure can optimally generate squeezed states with up to 9 dB of squeezing. When the total efficiency is 40%, approximately 2 dB of squeezing can be observed experimentally.ConclusionsIn this work, we fabricate silicon nitride micro-ring resonators with high quality factors. After optimizing the fabrication process, the quality factor of the micro-ring resonators can reach up to 1.5×106. Theoretical calculations suggest that this could enable the direct observation of squeezed states with approximately 2 dB of squeezing. This study provides an experimental foundation for on-chip generation of high-quality continuous variable squeezed states, thus paving the way for future advancements in integrated quantum photonics.

ObjectiveThe nitrogen-vacancy (NV) center in diamond, with its advantages of high sensitivity and high resolution, has been widely used in the measurement of magnetic fields. And the microwave frequency modulation technique can overcome the limitations of traditional ODMR in achieving high-sensitivity real-time magnetic field detection, and by reducing 1/f noise, it demonstrates higher sensitivity for magnetic field measurements, making it a research hotspot. However, most of the studies lack simulation-based theoretical model validation, and the modulation parameters in experiments are mostly set based on empirical values, constrained by hardware conditions. Therefore, to enhance the universality of the modulation-demodulation model and accurately improve the magnetic field measurement sensitivity of NV centers, we combine simulation and experimental methods to optimize the modulation parameters in order to determine the optimal modulation frequency and frequency deviation under the best sensitivity conditions.MethodsWe optimized the modulation frequency and frequency deviation parameters in the simulation to achieve the best sensitivity, based on ODMR and demodulation signal models. By analyzing the variations in the slope of the demodulation signal, we identified the optimal modulation parameters. An experimental platform was then constructed to validate the simulation results. During the experiments, we measured the slope of the demodulation signal and its amplitude spectral density under various conditions. Finally, the magnetic field sensitivity was calculated using the sensitivity formula, and the optimal modulation parameters were determined by comparing experimental data with simulation results.Results and DiscussionsThe simulation results show that the optimal modulation frequency deviation corresponding to the 10 MHz half-peak width ODMR signal is 3.55 MHz. However, due to limitations of the microwave source equipment, the best modulation frequency deviation obtained in the experiment is 8 MHz. Nevertheless, the trend of the curve's rise phase is generally consistent. For the modulation frequency parameter, the simulation cannot directly obtain its optimal value, while the experimental results show that the best modulation frequency lies within the range of 500 Hz and 1 kHz to 2 kHz. Therefore, in practical applications, the optimal modulation parameters should be referenced based on experimental results.ConclusionsBy developing the ODMR and demodulation signal models, simulations and experiments identified the optimal modulation parameters for the NV magnetic sensing system, resulting in the best sensitivity. The optimal modulation frequency deviation is 8 MHz, with the modulation frequency ranging from 500 Hz to 2 kHz. This study investigates the relationship between noise spectral density, the demodulation signal ratio, and sensitivity under different modulation parameters. Compared to previous studies, it provides a more accurate explanation of the impact of modulation parameters on sensitivity. However, future simulations require further optimization to improve the generality of the simulation model.

ObjectiveThe method of using Convolutional Neural Network (CNN) for identifying disturbance signals which acquired from distributed fiber optic sensing systems has become quite common. Based on the tremendous success in image processing, Convolutional Neural Network has widely applied to the analysis and feature extraction of time-series data. In the distributed fiber optic sensing systems, disturbance signals are often caused by various factors, leading to complex and varied patterns. Traditional machine learning methods usually require experts to manually design feature extractors based on experience, which is not only time-consuming but also has potentially biased, affecting the final identification results. Compared with machine learning methods, although Convolutional Neural Network can automatically learn deep features from the data, greatly simplifying the feature extraction process, but their performance in practical applications is largely determined by the configuration of parameters. Adjusting these parameters is typically a trial-and-error process that requires substantial time and computational resources. To address this issue, this paper proposes an improved Gray Wolf Optimizer (GWO) algorithm for the automated optimization of Convolutional Neural Network parameters.MethodsGray Wolf Optimizer is a intelligent optimization algorithms that simulates the hunting behavior of gray wolves. It has the advantages of simple structure, fast convergence speed and strong global search ability. The most important aspect of the Gray Wolf Optimizer is the selection of the objective function. In this paper, the accuracy on the training set is used as the objective function, and the convolution kernel size, batch size, and output dimensions of each convolutional layer and the first fully connected layer during the training process of the neural network are taken as the parameters to be optimized. These parameters are iteratively adjusted to find the parameter combination that maximizes the objective function as much as possible.Results and DiscussionsThe training result shows that the recognition accuracy on the test set can reach 96% after the Convolutional Neural Network optimized by the improved Gray Wolf Optimizer, while the accuracy before optimization is 94%. This indicates that the improved gray wolf optimizer for parameter optimization can indeed improve the accuracy of neural network training.ConclusionsThis paper proposes a method that uses the Gray Wolf Optimizer to optimize the parameters in Convolutional Neural Network. Through the automatic optimization of Gray Wolf Optimizer, it addresses the issues of slow manual tuning and the enormous computational resources required. This method is of great significance for the parameter tuning of Convolutional Neural Network, because it introduces an intelligent optimization algorithm into Convolutional Neural Network to adjust the parameters. By further improving the Gray Wolf Optimizer or adopting other intelligent optimization algorithms, we expect to achieve higher accuracy and faster convergence speeds of the algorithms in the future.

ObjectiveThe silicon-based micro ring resonator modulator is the core component of optical interconnect in ultra short distance transmission chips and an important device in applications such as quantum computing. Its modulation efficiency is generally around 1.88 V·cm, and the modulation rate can reach 50 Gbps. However, its modulation efficiency and modulation rate are significantly affected by temperature changes. Efficient control and stabilization of its modulation efficiency are of great value in improving its optical interconnect performance.MethodsIn this work, we propose an enhanced random sampling method, termed T-RANSCAC, to stabilize the efficiency of silicon-based micro-ring resonator modulators. We extracted the experimental system for the silicon-based micro-ring resonator modulator and collected data on the resonant wavelength and operating temperature through experiments. This data was analyzed to develop a model curve for wavelength control. Based on this model, we established a wavelength control system for the silicon-based micro-ring resonator modulator, which was subsequently validated through experimental verification.Results and DiscussionsIn this paper, we analyze the experimental data of the resonant wavelength and operating temperature of the silicon-based micro-ring resonator modulator, revealing the relationship between these two variables. This relationship is pre-stored in the FLASH memory of the wavelength control system and is utilized by an algorithm that automatically compensates for wavelength variations. We observe the modulation efficiency over the long-term operation of the system following wavelength compensation. Without this compensation, the modulation efficiency deteriorates over time. However, when wavelength compensation is performed based on the established relationship curve, the system's bit error rate can be maintained within an optimal range.ConclusionsWe analyze the test data of the silicon-based micro-ring resonator modulator, demonstrating a linear relationship between the resonant wavelength and changes in operating temperature, along with empirical values. Building on this analysis, we develop a wavelength control system for the modulator that enables stable long-term operation. This system offers a practical control method for applications, enhancing the overall feasibility and usability of the silicon-based micro-ring resonator modulator. It can stabilize the bit error rate of communication systems based on silicon-based micro ring resonator modulators at 10−7 when the transmission rate is 45 Gbps.

ObjectiveTo ensure communication security, it is necessary to verify the identities of the communicators. With the improvement of computing power, the classical identity authentication schemes based on the algorithmic complexity may be breached easily. The semi-quantum cryptographic communication realizes the quantum secure communication under the condition of reducing the communication equipment. Semi-quantum protocols serve as a bridge between quantum users and"classical" users with limited quantum capabilities, providing support for application scenarios that cannot afford the excessively high cost of quantum resources. Semi-Quantum Identity Authentication (SQIA) scheme is important part of Semi-quantum protocols. SQIA scheme based on Greenberger-Horne-Zeilinger-like states with quantum coding is proposed for the identity information security and efficiency. The GHZ-like state is composed of single quantum state and entangled quantum state. The single quantum state is measured by the authentication user. The entangled quantum state is encoded to realize the identity authentication process with the aid of quantum coding.MethodsThis paper explores a quantum identity authentication scheme based on GHZ-like states with quantum coding in the field of semi-quantum communication. By reducing the quantum communication equipment for authenticating the user, the process of authenticating the user's identity information is completed. In order to realize the semi-quantum authentication process, the GHZ-like state is used as the authentication information of the system. Then the thesis constructs a semi-quantum authentication scheme and authentication process. Under the authentication process planning, the authentication user first uses a semi-quantum operation to measure the single quantum information of a GHZ-like state, and according to the measurement results of the single quantum particles, the information is fed back to the trusted center by using the operation process of half quantum CTRL or SIFT operation, and the trusted center measures the feedback quantum entangled particles, complete the process of authenticating the user's legal identity.Results and DiscussionsOur results show that the authentication process of authenticated user and trusted center is realized in the shared encoded quantum entangled particles. The authenticator uses a semi-quantum authentication process to reduce the number of quantum communication devices. More efficient and secure communication network environment to complete the user identity authentication. The detailed safety analysis is indicated that the scheme is proven to be completely robust against an eavesdropping such as Trojan horse, impersonation attack and intercept-resend attacks. The scheme is simple, efficient and secure.ConclusionsThis paper presents a semi-quantum identity authentication scheme based on GHZ-like state with assisted of quantum coding, in which GHZ-like state is used as authentication information, authentication user adopt semi-quantum operation and realize the authentication process by comparing the classical information processing. The number of quantum devices required is reduced by the fact that the programmer does not require full quantum capabilities for all participants. The other semi-quantum authentication scheme can be extended to two-party authentication and multi-party authentication process, and can also be used to verify multi-user authentication of unmanned aerial vehicle without quantum capability, the application of the scheme can be further promoted.

ObjectiveThe theoretical and experimental studies on the polarizability and tune-out wavelength of Cesium atoms primarily concentrate on the 6S ground state and the 6P excited state. In recent years, experimental work involving highly excited atoms has progressed rapidly, particularly the quantum control based on Rydberg states. Rydberg atoms have emerged as a research focus in quantum computing, quantum information, precision measurement, and other fields owing to their unique transition and polarization properties, as well as the long-range interactions between atoms. The transition frequencies of Rydberg atoms encompass microwave and terahertz waves, demonstrating excellent coherence. Consequently, related spectroscopic techniques have been extensively utilized for high-precision measurements in the microwave and terahertz frequency bands.MethodsPolarizability is a physical quantity that quantifies the extent to which an electron cloud deviates from its normal distribution under the influence of an external field. When a laser of a specific frequency interacts with an atom, the dynamic polarizability of the atom momentarily becomes zero, effectively nullifying the interaction between the atom and the external field. The corresponding laser wavelength at this instant is referred to as the tune-out wavelength. At this wavelength, the atomic interactions can be precisely regulated, enabling the controlled activation and deactivation of interactions between multi-component atoms. Based on the theory of polarizability, the tune-out wavelengths for the 14S1/2 and 23S1/2 states of cesium atoms have been calculated within the terahertz frequency band.Results and DiscussionsBy theoretically calculating the tune-out wavelengths of the ground state of cesium atoms and comparing these results with existing experimental data, we validate the calculation accuracy. Subsequently, we compute the atomic polarizability of the 14S1/2 and 23S1/2 states within the terahertz transition band of Rydberg atoms, providing frequency points corresponding to five tune-out wavelengths for each of the 14S1/2 and 23S1/2 states, respectively. Furthermore, we analyze the impact of different energy level models on the tune-out wavelengths. It is found that atomic polarizability is more affected by external fields between energy levels with lower transition energies. For different energy levels model, the relative error of tune-out wavelength near the same resonant transition is about 0.001%, accuracy is about 10 MHz.ConclusionsHigh-precision tune-out wavelength calculations and measurements have been widely utilized in various applications, including the determination of transition matrix elements and atomic structure parameters. Furthermore, they can be applied to quantum precision measurements in the terahertz band. This work holds significant importance for the advancement of fundamental parameter measurements of Rydberg atoms within the terahertz frequency band, as well as for the development of sensing spectroscopy technology for electromagnetic field measurements.

ObjectiveImplementation of quantum network requires the transmission of information between the same or different quantum nodes through the fiber channel. Cesium atom nodes are significant quantum resources, and thus it is essential to connect cesium atom systems with optical fiber channels. The frequency of photons directly emitted by common quantum nodes has great loss in fiber channel and is not suitable for transmission in fiber, so quantum frequency conversion becomes the key technology. In quantum frequency conversion, due to the counts of noise photons are much higher than that of the target photons, signal-to-noise ratio cannot be ignored. Therefore, enhancing the signal-to-noise ratio in quantum networks is a paramount issue.MethodsWe intend to achieve 852 nm photons conversion to 1560 nm photons based on waveguide, and analyzed noise photons induced by the strong pump laser at different wavelength, such as spontaneous parametric down conversion, spontaneous Raman scattering and cascaded difference frequency generation. An acoustic-optical modulator and a strong attenuator are employed to chop and attenuate the 852 nm continuous-wave laser to 2.5 MHz repetition rate with low intensity. To enhance the signal-to-noise ratio of the target photons, a combination of changing polarization of noise photons in the difference frequency generation process and using the narrow-band filters is employed, the photonic wave packets before and after the conversion are character-ized by single photon detector. Finally, Connecting the optimized magneto-optical trap system to the frequency conversion device.Results and DiscussionsIn this paper, we first measured the optimal matching temperature of the waveguide is 36.6 ℃. Then filters with different bandwidths are employed, including a 50 nm bandwidth bandpass filter, a 12 nm bandwidth bandpass filter and a 0.3 nm bandwidth fiber bragg grating. The experimental results indicate that as the bandwidth of filters decreases, the signal-to-noise ratio significantly improves. When the 0.3 nm bandwidth fiber bragg grating and the 12 nm bandwidth bandpass filter are used, the signal-to-noise ratio can be increased to 91.3. If a filter with an even narrower bandwidth is selected, the noise photons near the target photons will be further suppressed. The internal conversion efficiency of the waveguide reaches 6.2% when the pump light power is 400 mW. Finally, the photonic wave packets before and after the frequency conversion are identical, indicating that the frequency conversion does not affect the photonic wave packets. Using feedback loop control, we load multiple atoms into the magneto-optical trap, the photons counts are 16000 counts/50 ms, the maximum SNR is approximately 2.ConclusionsBased on the above waveguide frequency conversion system, we aim to establish a connection between the cesium atom systems and the optical fiber communication band, ensuring that the photonic wave packets remain unchanged before and after the conversion. This allows photons to retain their original information after long-distance transmission, thereby providing a foundation for constructing quantum networks with hybrid nodes.

ObjectiveIn recent years, due to the proposal of the theory of optical soliton communication, more and more researchers are paying attention to optical solitons. When a beam propagates in nonlinear medium, there will be dynamic competition between the diffraction effect of the beam and the self-focusing nonlinear effect, when the two effects achieve balance, a stable transmission state-optical soliton will be generated. Due to the stable transmission characteristics of optical solitons, they have bright development prospects and application potential in fields, such as fiber optic communication, optical information processing, and all optical switching. Therefore, it is necessary to explore the transmission characteristics of beams in different nonlinear systems.MethodsThe beam described by a special function provides new ideas for beam propagation due to its unique properties. Pearcey beams have attracted widespread attention due to their self-focusing, self-acceleration, and self-healing characteristics. Based on the nonlinear Schrdinger equation, the propagation dynamics of one-dimensional symmetric Pearcey-Gaussian beams in photorefractive media are studied numerically by using the split-step Fourier method. The split-step Fourier method, also be known as beam propagation method, has been used to solve many optical problems, such as wave propagation in the atmosphere, unstable resonators, waveguide couplers, gradient index fibers, and so on. The central idea of this method is to assume that diffraction and nonlinear effects act independently, thus obtaining an approximate result. The transmission process from z to z+h can be divided into two steps. The first step only considers nonlinear effects, and the second step only considers diffraction effects. Due to the fact that this method converts derivative operations in the time domain into product operations in the frequency domain, it is one to two orders of magnitude faster than most finite difference methods.Results and DiscussionsPhotorefractive medium is a medium that can produce light induced refractive index changes and is a weak light nonlinear effect medium. When incident light propagates in a photorefractive medium, the photorefractive material absorbs the light energy, and the migration of charges changes the refractive index of the photorefractive medium. When the focusing effect of the refractive index on the beam is balanced with the diffraction effect of the beam, optical solitons are formed. The results show that the beam can form breathing-like soliton or soliton pair under the nonlinear effect of photorefractive medium. With the nonlinear coefficient increasing, stable single-breathing soliton is gradually formed in photorefractive media. The distribution factor also affects the beam propagation. The larger the distribution factor is, the smaller period of the breathing soliton is and the larger frequency is. The larger initial amplitude will cause the beam to split during the evolution process and produce multiple sub-beams. The linear chirp affects the deviation angle of solitons, and the sign of chirp coefficient affects the deviation direction of solitons. The larger absolute value of chirp coefficient is, the larger deviation angle of solitons will be. The quadratic chirp affects the diffraction effect of the beam. The larger absolute value of the quadratic chirp coefficient is, the stronger the diffraction effect is.ConclusionsTherefore, we can control the transmission characteristics of symmetric Pearcey-Gaussian beams by changing the nonlinear coefficient of the photorefractive medium, the distribution factor, the initial amplitude and the chirp parameter of the initial input beam. The relevant research results enrich the content of nonlinear optical soliton transmission, and provide certain theoretical guidance for beam controls in optical systems.

ObjectiveWith the continuous advancement of research on terahertz metamaterial broadband absorbers, tuning their absorption performance has become increasingly important. However, this tuning process often leads to a degradation of the original broadband absorption capabilities and makes the absorber more sensitive to polarization and incident angles. To address this issue, a terahertz broadband tunable metamaterial absorber based on graphene and vanadium dioxide (VO2) as tuning materials is proposed.MethodsThe absorber in this paper has four layers, which are the bottom metal plate made of gold, the dielectric layer made of PMI, the two resonant layers made of VO2 and graphene. The resonant structure is a split-ring to ensure the broadband absorption, and the VO2 is used to turn the absorption amplitude of absorber for its characteristic of phase transition. Furthermore, the turning of working frequency is achieved by graphene of which the conductivity can be changed by adjusting its chemical potential.Results and DiscussionsThe bandwidth of the proposed absorber with an absorption rate exceeding 90% can reach 6.35 THz, when the conductivity of VO2 is 200000 S/m and the chemical potential of graphene is and 0.01 eV. The absorption mechanism is analyzed by means of reflection theory and impedance matching principle, and the absorber's performance is tested by plotting absorption curves under various conditions, including different polarization states, conductivity of VO2 and Fermi energy levels of graphene. The study results will show that the absorption rate of the broad absorption band can be significantly adjusted by the conductivity of VO2 and the frequency of a absorption peak can changed by the Fermi energy of graphene. Meanwhile, the proposed absorber possesses the characteristics of polarization insensitivity and shows good absorption for waves under incident angles from 0° to 50°.ConclusionsThrough the comparison and analysis of the absorption curves under different conditions, it is found that the proposed metamaterial absorber possesses the characteristics of tunability, broadband absorption polarization insensitivity and wideangle absorption.

ObjectiveThe narrow-linewidth, continuously tunable single-frequency 319 nm ultra-violet laser system is of great significance for the single-step Rydberg excitation of cesium atoms. The use of high-precision ultra-stable optical cavity, combined with PDH (Pound-Drever-Hall) frequency stabilization technology, electronic sideband frequency stabilization technology, and HC (Hansch-Couillaud) frequency stabilization technology, which realize the frequency locking of the single-frequency ultraviolet laser system. However, the structure of the conventional feedback locking system is much complex, the cost is higher, the volume is larger, and the whole process requires more seperated instruments. Therefore, FPGA (Field Programmable Gate Array) is used to simplify and upgrade conventional feedback locking systems.MethodsFPFA has the advantages of low energy consumption, high efficiency, high flexibility, high integration, high stability, programmability, and it has application prospects in the whole scientific research, especially in the field of quantum optics and atomic physics. Frequency sweeping, modulation and demodulation, feedback control and monitoring through the Red Pitaya board can make the whole laser system simple. FPGA can not only greatly reduce the cost in the experiment, but also fully save the space, bring great convenience, and have high integration, high flexibility, high stability and simple operation.Results and DiscussionsBased on the Red Pitaya's FPGA board, the laser frequency is locked by using PDH frequency stabilization technology, electronic sideband frequency stabilization technology, and HC frequency stabilization technology. The frequency locking effect of PDH frequency stabilization technology and HC frequency stabilization technology of the four-mirror frequency-doubling ring cavity is compared, the frequency fluctuation after PDH locking is ±0.425 MHz within 10 minutes, and the frequency fluctuation after HC locking is ±0.61 MHz within 10 minutes, the locking effect of PDH scheme is better, and it has a wider range of continuous frequency scanning.ConclusionThe laser frequency locking of the entire 319 nm ultra-violet laser system is realized based on FPGAs. the frequency sweeping, modulation and demodulation, feedback control, and monitoring can be realized through FPGA, and the IQ module, ASG module, PID module, Switch module, and Scope module in FPGA board are used to replace the conventional seperated equipments. Based on the Red Pitaya's FPGA board, PDH locking technology and electronic sideband locking technology are used to achieve 1560 nm and 1077 nm laser frequency locking at the same time; and the frequency locking effect of PDH locking technology and HC locking technology on the four-mirror frequency-doubling ring cavity was compared. FPGA realizes the simplification of the entire laser system, which can not only save space and cost, but also has excellent performance, improves the integration of the system, reduces the complexity of instrument operation, and is easier to operate.