Noise Analysis Of The Atomic Superheterodyne Receiver Based On Flat-top Laser Beams

In recent years,with the rapid development of laser technology,it has become possible to precisely prepare high-energy level Rydberg states.Since the heterodyne electrons of highly excited Rydberg atoms are far away from the nucleus,they are very sensitive to weak electric field.Therefore,based on Rydberg radio wave sensor has become a current research hotspot.And its theoretical measurement sensitivity is only limited by quantum noise,showing wide application prospects in various fields.However,the actual measurement sensitivity of atomic radio wave sensors is still far from the quantum noise limit sensitivity.In order to identify the influencing factors that limit the further improvement of the sensitivity of atomic radio wave sensors,this paper analyzes and accurately traces the noise sources of the Rydberg atomic superheterodyne receiver,focusing on the contribution of the crossing noise to the noise of the atomic superheterodyne receiver.The precise traceability of noise points the way to further improve the sensitivity of atomic radio wave sensors,and the research work in this paper is of great significance for quantum precision measurements of microwave electric fields.The structure of this paper is as follows:The first chapter mainly introduces the research background.Firstly,the characteristics of the Rydberg atom and the wide application of the Rydberg atom radio wave sensor in various fields are introduced in detail.Afterwards,the working principle of the atomic superheterodyne receiver is introduced.The realization of the atomic superheterodyne receiver improves the sensitivity of atomic-based measurement of radio waves by three orders of magnitude.However,there is still a certain gap between its measurement sensitivity and theoretical sensitivity.In order to further improve its measurement sensitivity,the relevant progress in the research of atomic superheterodyne receiver noise is introduced.The second chapter mainly introduces the theoretical principle of noise measurement of Rydberg atomic superheterodyne receiver based on flat-top beam.Firstly,the coordinate correspondence of the Gaussian beam to the flat-top beam shaping process is calculated mathematically.According to the corresponding relationship,a flat-top beam model consistent with the shaper is constructed in ZEMAX,and the experimental process is simulated using this model.Next,the total Hamiltonian and the optical Bloch equation of the atomic superheterodyne receiver under the rotational wave appearance are obtained according to the actual energy level structure of the noise measurement.Corresponding simplification of the calculated density matrix elements to calculate the relationship between the probe laser transmission spectrum and the beam area of the Rydberg atomic superheterodyne receiver based on the flat-top beam.The third chapter mainly introduces the experimental system of atomic superheterodyne receiver noise measurement.Firstly,the principle of frequency stabilization based on beat frequency locking,experimental locking process and frequency locking results are introduced in detail.Afterwards,the preparation method of the flat-top beam,the optimization of the shaping system,and the phase pre-compensation of the flat-top beam are introduced in detail.Through a series of optimizations,the beam incident into the cesium vapor is a flat-top beam with uniform intensity distribution.Finally,the precise adjustment method of atomic number based on electric rotary displacement stage and the precise characterization experiment device of Rydberg atomic superheterodyne receiver noise are introduced.The fourth chapter mainly introduces the analysis process and results of the atomic superheterodyne receiver noise data.Firstly,the shot noise limit of the probe laser is determined by measuring the noise power spectrum of the probe light under different optical powers,and the diameter of the excitation beam is calibrated according to the linear relationship between noise and power.Afterwards,the relationship between the EIT signal amplitude and the beam area of the Gaussian beam and the flat-hat beam under different diameters was studied experimentally,and the high uniformity of the flat-hat beam was proved experimentally.And noise analysis based on noise power spectrum is used to analyze the white noise,1/f_n noise,intensity noise caused by laser frequency noise and phase noise in the atomic superheterodyne receiver.Finally,an analysis method based on analyzing the power-law relationship between the noise power and the number of atoms to trace the source of the noise of the atomic superheterodyne receiver in different frequency ranges is proposed.After analysis,we conclude that the measurement sensitivity of the current atomic superheterodyne receiver is only limited by quantum noise under the experimental conditions that the read-out frequency range is greater than 70 k Hz and the beam diameter is less than 2 mm.The innovations of this paper:First,a tunable,low-noise Rydberg atomic excitation laser system has been realized,and a size-tunable flat-top beam has been prepared.Based on this laser system,the linear relationship between the number of Rydberg atoms and the area of the flat-top beam was precisely controlled.Second,a precise and efficient noise analysis traceability method based on the noise power spectrum is proposed:the contribution of different kinds of noise in complex noise is evaluated by analyzing the power-law relationship between noise power and atomic number.Third,based on the noise traceability of the atomic superheterodyne receiver,it is concluded that the measurement sensitivity of the current atomic superheterodyne receiver is limited only by quantum noise in the experimental conditions of read-out frequency range greater than 70 k Hz and spot diameter less than 2 mm.