Effective Improvement Of The Squeezed Optical Fields Using A High Signal-to-Noise Ratio Bootstrap Photodetector

As an important quantum resource in the field of quantum information,continuous variable squeezed state light field can break through the limit of quantum noise,and has important research value in the fields of quantum communication,quantum information storage,and quantum precision measurement.For example,in gravitational wave detection,injecting the squeezed state light field into the arm laser interferometer can reduce the introduction of the noise of the detection system from the particle noise,and thus effectively improve the sensitivity of gravitational wave detection.The index of the squeezed light field mainly includes squeezing degree,bandwidth,and long-term stability.Therefore,optimizing the optical system to improve the squeezing degree of the light field while taking effective measures to reduce the extra noise of the detection and locking system is an important basis for the preparation of high-pressure squeezed state light field and high-quality quantum information research.With the development of low-loss coating technology and nonlinear crystal growth technology,the parametric down-conversion process in optical parametric amplifier(OPA)has become one of the important ways to prepare a high-squeezed light field.It is necessary to obtain the error signal of high signal-to-noise ratio and complete the high-precision and long-term stable locking of an optical cavity.Because in the OPA locking process,the error signal in the resonator transmitted light has lower extra noise than the reflected light,using the transmitted light locking can obtain higher stability.However,because the squeezed light has extremely strict requirements on loss,the transmitted light is generally relatively weak in power,and the transmitted light that can be used for locking is very little,generally on the order of nanowatt.This puts forward a higher requirement for the signal-to-noise ratio of cavity-locked detectors.Based on the above research background,this paper first analyzes the factors that affect the stability of PDH locking technology.The results show that in a weak light environment,the use of a high-gain,low-noise detector to achieve OPA cavity locking is an effective way to improve its locking stability,and then can improve the squeezing of the light field.Here,we mainly carry out the following research work for the photodetector used for resonant cavity locking:1.The influence of the large junction capacitance of the photodiode on the detector noise is quantitatively analyzed,and the bootstrap amplification technology is proposed to greatly reduce the detector noise and effectively improve the signal-to-noise ratio of the detection signal.2.We design several suitable amplifying circuits and theoretically calculate their noise models,focusing on the element parameters that affect high-frequency noise.According to the calculated element parameters,we evaluate the characteristics and shortcomings of each circuit.Finally,the design method of the trans-impedance amplifier circuit is determined.Based on the above theoretical research,a low noise photodetector with high a signal-to-noise ratio(SNR)based on the combination of a trans-impedance amplifier(TIA)circuit and a two-stage bootstrap amplifier circuit is successfully developed.The experimental results show that when the injected optical power is 800 u W,the signal-to-noise ratio of the detector is 26.7d B when the analysis frequency is 3MHz.In the frequency range of 1～30MHz,the equivalent optical power noise level is less than2.4pW√(Hz),which is the highest index in the world.3.The detector is applied to the actual locking of the optical parametric amplifier,and the experimental results show that the detector can not only achieve high precision and long-term stable locking of the optical cavity but also effectively improve the squeezed of the light field.among them,the squeezing degree of the squeezed light field is increased by more than 34.9%.The photodetector has important research value in the study of continuous variable quantum information.The innovations of the above work are as follows:1.The effect of junction capacitance of photodiode is reduced by bootstrap amplification.It mainly connects the unit-gain buffer at both ends of the photodiode to reduce the potential difference between the two ends of the photodiode,so that the total input capacitance is reduced to reduce the noise at high frequency,thus improving the performance of the photodetector.2.The equivalent input optical power noise of the existing photodetector in 30 MHz is achieved 7.9 pW√(Hz),but the bootstrap low-noise photodetector designed in this paper uses bootstrap impedance enhancement technology,combined with low-noise chip selection and circuit design to reduce the noise of the detector,and the noise level of1-30 MHz low-noise photodetector is below 2.4 pW√(Hz).3.The trans-impedance amplifier circuit in this paper uses the amplifier chip LTC6268-10.Because of its extremely low input noise current and input capacitance,combined with bootstrap amplification technology,the photodiode junction capacitance is further reduced and the noise of the circuit is greatly reduced.