| Squeezed state is an important resource for many applications,including precision measurement,such as gravitational wave detection and quantum information technique.For instance,quadrature squeezed states are applied to improve the sensitivity of laser interferometers,realize quantum teleportation which is a fundamental protocol in quantum information processing.The sensitivity and fidelity of such applications are limited by the squeezing level.So it is very important to detect strongly squeezed light.Optical parametric oscillation(OPO)belongs to the most successful approaches of squeezed light generation.So far,the generation of squeezed states by OPO has mainly concentrated on squeezed vacuum states,which has good noise performance at low-frequency band.On the contrary,the generation of bright squeezed light is poor noise performance at low-frequency band.But bright squeezed light is simple to implement the control of the cavity length and relative phase.However,limited by the classical noise of the seed wave,the quantum fluctuations of the pump wave,its maximum squeezed degree is far below that of squeezed vacuum state.The research work of this thesis is to carry out the generation of the squeezing light with strong quantum correlation based on the optical parametric down conversion.The main accomplished works in the thesis are as follows:(1)A compact and low-noise single-frequency Nd: YVO4 laser is developed.The intensity noise characteristics of the laser is analyzed experimental and theoretical.By reducing the transmission rate at 1064 nm of the output coupling mirror,optimizing the parameters of the optical resonator.As a result,the output power of 2.5 W at 1064 nm singlefrequency laser is manufactured,and its intensity noise is reduced to shot noise limit above 1.5 MHz with a mode clear filter resonator.We redesigned the OPO,with the semimonolithic cavity structure.The eigen mode of cavity waist spot location on the center of the crystal,by optimizing the cavity length and radius of curvature of output coupling mirror.The beam waist size is more appropriate,so as to improve the eigen mode volume of the cavity,which is conducive to more efficient nonlinear transformation.And then the temperature control structure with better insulation performance is designed to ensure the PPKTP phase matching accuracy.Therefore,the nonlinear loss is reduced and the effective optical parametric down-conversion is realized.(2)We optimize the light path of balanced homodyne detection system,the signal light and local light after 50/50 beam splitter propagate the same distance,transmission through the same optical element.The signal light and local light produce an equal loss in the propagation of the two arm,ensuring that the signal light power and the local light power are equal after the 50/50 beam splitter is separated.In addition,we designed the incident angle of the signal light and the local light into the photodiode at 20 degrees(the diode is coated with 20 degrees of incident and anti-reflection film).A set of concave mirrors is used to collect the residual reflected light reflected from the photodiodes and refill the photodiode.We designed a new balanced homodyne detector with lower electronic noise and higher gain.When injected with 11 mW local light,the shot noise limit is 28 dB higher than the electronic noise.The common mode rejection ratio at audio frequency is over 67 dB.Through the improvement of the above,the detection efficiency of the whole balanced homodyne detection system is improved.(3)We calculate the focal length and location of the lens through the computer program to implement mode matching and gaussian beam shaping.Make mode matching and beam transform simple and efficient.In the experiment,the size and position of the initial beam waist are measured with the beam quality analyzer.Then the focal length and position of the lens group are calculated according to the parameters of the target beam.Accordingly,the lens group is placed into the corresponding position in the light path to reshape the initial beam and verify the theoretical calculation results.By fine tuning and experimental verification,the pattern matching efficiency of gaussian beam and resonant cavity is nearly 100 %.In addition,PDH locking technique is used for each cavity length and relative phase in the squeezing generation system.In order to improve the stability and bandwidth of the locking system,we have designed a more stable phase shifter and the cavity length feedback structure of the high-gain resonant lock cavity detector and the response frequency higher mechanical structure.In order to further reduce the fluctuation of the locked cavity length and relative phase.We found that the error signal extracted from the front and back of the OPA cavity produced the opposite phase fluctuation direction.Based on this feature,we designed a scheme to compensate the phase jitter between different locking loops,which caused by residual amplitude modulation.The relative phase jitter of the pump light and seed light is compensated by the relative phase jitter of the local light and the signal light.By this method reduces the total phase fluctuation of the whole experimental system.Finally,the output of the squeezed state light field of 12.6 dB is more than three hours,and the fluctuation is ±0.2 dB.(4)In order to improve the reliability and stability of the whole squeezing light generation system.We have improved the compression generation system from the following aspects: First,a compact low noise high power 1064 nm laser designed for squeezing light source is developed.Its output power is up to 2.5 W,and the power fluctuation of 8 hours is just ±0.32 %.In order to improve the stability of the single frequency operation of the laser,we insert a nonlinear crystal in the cavity of the laser to introduce nonlinear optical loss.The loss ratio between the mode of oscillation and the non-oscillating is one to two,and the main oscillation mode is more advantageous in the mode competition,and the single frequency stability of the laser is improved.Secondly,in order to improve the stability of the system,all the frames in the optical path are made of a custom frame.In addition,we have improved the mechanical structure of the phase shifter to reduce the mechanical jitter of the scanning phase,thus reducing the uncertainty of the beam phase.Third,optimize the optical path design,by inserting the mode cleaner,the spatial distribution of the pattern is optimized,and the laser intensity noise is reduced,and the beam pointing is reduced.After careful design system light path makes the structure more compact,the optical path is shortened,the loss is reduced.The balanced homodyne detector has been redesigned to make its electronics noise lower and gain greater.The common mode rejection ratio at audio frequency is higher.After a series of improvement and optimization,a bright squeezed light with non-classical noise reduction of 12.6 dB is obtained at the analysis frequency of 3 MHz,and 9.9 dB of vacuum squeezing light was measured at the analysis frequency of 15.2 kHz.In the thesis,the work that belongs to innovation has a few points:A: The main factors of optical loss and phase jitter that influence the squeezing degree are analyzed theoretically,the method of reducing loss and jitter is proposed,and it was verified in the experiment.B: A mathematical model is established to match the mode of gaussian beam to resonant cavity eigenmode,so that the determination of focal length and position of the lens group in the mode matching and the beam transform are simple and fast.C: The compensation scheme of relative phase jitter between different locking loops is designed,and the total phase jitter of the system is reduced.D: The advantages and disadvantages of the standing wave cavity and the traveling wave cavity are compared,which lays a theoretical foundation for the realization of efficient optical parametric conversion in the external cavity second harmonic generation.E: The one-piece structure of the phase shifter,optical parametric oscillator and mode cleaner is designed,guarantees the mechanical stability of the system,and improve the installation method of piezoelectric actuator can enhance the response frequency of the lock loop.F: We investigate the dependence of the measured squeezing level on the local oscillator(LO)intensity noise and balance homodyne detector common mode rejection ratio and gives the calculation error of the method in actual situation. |
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