| The development of human civilization is to some extent the development of improving measurement accuracy.From defining units of length by feet,hands and steps to defining them by vernier calipers,microscopes and laser rangefinders,measurement accuracy has improved considerably.Precision measurement not only demonstrates the physical theory,but also helps to propose new theories and technologies.Relative displacement measurement has reached the sub-wavelength level and has been applied to nano-science,medical science and so on.However,compared with the quantum mechanical measurement method,the classical measurement method can not break the standard quantum limit.With the development of quantum mechanics,non-classical light field,including squeezed light and entangled light,can break through the standard quantum limit and has attracted more and more attentions.It has been widely used in gravitational wave detection,quantum radar,quantum imaging and quantum information processing etc.At present,there are three methods to generate squeezed light in the laboratory:four-wave mixing,nonlinear crystal parametric down-conversion and photodynamic interaction.A large number of experimental studies show that the nonlinear crystal parametric down-conversion has become the most effective method to generate squeezed light and has been studied by many experimental groups.The parameters of squeezed light mainly include squeezed level,squeezed bandwidth and stability.High squeezed level can improve the sensitivity of quantum precision measurement and the fidelity of quantum information processing.Squeezed lights at different frequency bandwidth are applied in different fields.Squeezed light in audio-frequency bandwidth is mainly used in gravitational wave detection,which is injected from dark port of interferometer to improve the sensitivity of measurement.The squeezed light in MHz bandwidth is mainly adopted in quantum radar,quantum information processing and other fields for improving the dynamic range of measurement in quantum radar and increasing the channel capacity in quantum information processing.High stability ensures long-term measurement.Therefore,it is very important and urgent to produce long-time stable and broad bandwidth squeezed light field with high squeezed level.It requires the system to have as low loss and phase jitter as possible.By reasonably designing optical parametric oscillator cavity,choosing high quality optical lenses and the phase compensation and so on,optical loss and phase jitter has been reduced as much as possible.But phase jitter introduced by phase locking and electrical loss produced by measurement system in the electrical part becomes the limiting factor of squeezed level further improvement.The key is the first stage resonant photodetector in the phase locked feedback loop and the balanced homodyne photodetector in the measurement system.This paper focuses on the research of resonant photodetector and balanced homodyne photodetectors.Firstly,the lossless error signal extraction is realized by optimizing the performance of the resonant detector in the locking loops.Secondly,a low noise,high SNR and high common-mode rejection ratio(CMRR)balanced homodyne photodetector is designed for squeezed state measurement in the low-frequency region.Due to the high SNR,the introduced measurement loss is negligible.Finally,the balanced homodyne photodetector with independent noise measurement and phase locking branches is developed and it can simultaneously reduce the measurement loss and phase jitter to the minimum.With the photodetector for the squeezed light measurement at 1550 nm,the squeezed light of high squeezed level is obtained stably for a long time.The main research contents of this paper include:1.A high-performance resonant photodetector is developed for extracting error signals in the cavity length and phase locking loops.According to the detected signal characteristics in locking cavity length and relative phase,the Q value of the resonance detector is increased from 100 to 320.83 on the basis of existing resonance detector so that the minimum detectable light power reaches-70 d Bm and the SNR of the error signal is increased by 15 d B.For the case of bright squeezed light of15 d B with the power of 10?W,the squeezed level is increased by 6.3%.It can greatly improve the locking accuracy and long-term stability.2.A high-performance detection system for bright squeezed state measurement in audio-frequency bandwidth is realized.By analyzing the characteristic of bright squeezed state in audio-frequency bandwidth,a capacitor is mounted at the input end of AC branch to prevent the DC photocurrent from entering the AC branch and meanwhile avoid AC branch saturation.A switch added to the DC branch can be used to calibrate the light beam,verify whether the light beam is completely detected by the photodiode and ensure the equal photocurrents generated by the two photodiodes.Furthermore the switch can isolate AC branch from the DC branch and prevent the adverse effect on the AC branch.As a result,the electronic noise of the AC branch is reduced to-125 d Bm,and the SNR with the incident power of 8m W reaches 48 d B and the CMRR is more than 59 d B in the range of 1k Hz-100 k Hz.3.The mechanism that the existing balanced homodyne detector can not be used to lock the phase of 0 between squeezed light and local oscillator in the process of measuring the squeezed light is studied.It is because the detector with the limited bandwidth can not demodulate the high modulated signal.Although the phase locking can be achieved by increasing the bandwidth to 100 MHz,due to the limitation of the gain bandwidth product of the operational amplifier,the increase of the bandwidth will lead to the reduction of the signal-to-noise ratio,which will introduce a large loss in the measurement of squeezed light.Hence,to solve the problem we extract the error signal for phase locking from the bias resistors.Based on the theoretical analysis and experimental validation,the phase locking between two beams and noise detection of squeezed light can be realized simultaneously.4.The squeezed state source at 1550 nm is constructed.By optimizing the locking loop and measurement system,the loss and phase jitter are reduced to the minimum.With the designed detector in audio frequency,the vacuum squeezed light with the quantum noise reduction of 8.76 d B at 1550 nm is detected at the analysis frequency of 5.2k Hz.Utilizing the detector with independent noise detection and phase locking branches,a bright squeezed light with quantum noise reduction of10.3±0.2d B at 10 MHz is detected by 1-h testing time.Innovative work includes:A.Based on theoretical analysis and experimental research,the Q value of resonant detector is optimized and increased from 100 to 320.83.The minimum operation power reaches-70 d Bm and the error signal SNR is improved by 15 d B.For the case of bright squeezed light of 15 d B with the power of 10?W,the squeezed level is increased by 6.3%.It can greatly improve the locking accuracy and long-term stability.B.By optimizing the circuit structure,a balanced homodyne detector for bright squeezed state measurement in audio-frequency bandwidth is developed.The electronic noise of the AC branch is reduced to-125 dbm,and the SNR reaches 48 d B within the range of 1k Hz-100 k Hz when the input optical power is 8m W.The CMRR of the AC branch is more than59 d B.Limited by the high audio-frequency noise of the laser,the bright squeezed state in low-frequency bandwidth can not be generated.With the detector,the vacuum squeezed light with the quantum noise reduction of 8.76 d B at 1550 nm is detected at the analysis frequency of 5.2k Hz.C.Based on the theoretical analysis and experimental validation,the balanced homodyne detector with independent noise measurement and phase locking branches is designed and applied to the 1550 nm bright squeezed light source.A bright squeezed light with quantum noise reduction of 10.3±0.2d B at 10 MHz is detected stably by 1-h testing time. |
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