Research on Laser Automatic Frequency Locking System Based on Modulation Transfer Spectroscopy
The quantum gravimeter has high requirements on the stability of laser frequency when it works in ship-borne,air-borne,space-borne and other external fields.However,due to changes in the external environment and its own instability factors,the center frequency of the laser output will change with time to produce MHz or even GHz magnitude drift,which will cause the laser mode hopping and affect the normal operation of the instrument.Therefore,it is necessary to lock the output frequency of the laser to suppress the frequency drift.The traditional laser frequency locking system needs to manually adjust the working parameters and judge the locking situation,which cannot meet the requirements of long-term stable operation of quantum precision measuring instruments without manual intervention.In this paper,a set of laser absolute frequency automatic locking system is designed by using the advantages of modulation transfer spectroscopy,such as flat zero background signal,large slope of error signal,high frequency stability(above 10-8 order of magnitude),small frequency line drift and easy locking.Firstly,based on the semiconductor laser,a frequency-locked optical path suitable for modulation transfer spectroscopy is built,and the 16-bit analog-to-digital converter is used to read the modulation transfer spectroscopy signal at high speed and send it to the automatic frequency locking operation system.The core of the laser automatic frequency locking operation system is the automatic frequency locking state machine,which contains six states:initialization state,scanning state,similarity recognition state,locking state,monitoring state and waiting for locking state.After the automatic frequency locking process is turned on,the convolution peak-seeking algorithm is used in the state machine to find the required characteristic spectrum information and obtain the reference voltage signal of the frequency point to be locked,and then the reference voltage signal is sent to the laser to be locked through the feedback control unit.Finally,the automatic locking of the laser frequency and the fast locking after the unlocking are realized.The test results show that after the system opens the automatic frequency locking process,the frequency ten thousand level stability is increased from 6.672×10-8@8 000 s in the free running state to 1.265×10-11@8 000 s in the frequency locking state,which is increased by three orders of magnitude.When the external factors cause the output frequency of the laser to be unlocked,the laser automatic frequency locking system can realize the fast relocking of the laser frequency within 1 s,and the consistency before and after the unlocking is good,and the frequency stability after the relocking reaches 6.651×10-11@1 s.The laser automatic frequency locking system developed in this paper has the advantages of high frequency locking accuracy,short locking time,simple structure and strong stability.It can meet the needs of laser frequency locking for a long time in an unattended environment.It is appropriate for cold atom gravimeter,cold atom gradient meter,cold atom gyroscope and cold atom clock and other quantum precision measuring instruments.Because the system needs to intercept the required characteristic spectrum and mark the frequency point to be locked before the state machine opens the automatic frequency locking process,it will increase the labor cost and time cost.It is hoped that the laser automatic frequency locking scheme based on artificial intelligence algorithms can be designed in the future.The neural network can be used to automatically identify the characteristic spectrum signal and locate the frequency point to be locked,completely realizing the laser automatic frequency locking under unmanned conditions.
Quantum gravimeterAutomatic frequency lockingConvolution peak-seekingLaserModulation transfer spectroscopyFrequency stabilityRelocking time
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