Frequency Doubling With Periodically Poled KTP At The Fundamental Wave Of Cesium D₂ Transition

The interaction between light field and atoms is an important issue in quantum optics,atomic physics and quantum information science. Nonclassical light fields,known as the very important quantum resources, play an important role in light-atoms interaction.In theory,many quantum phenomena have been investigated and expected,such as nonclassical spectroscopy,nonclassical excitation,atoms-field nonclassical transfer and entanglement,etc.In experiment,there are also many researches have been done.Yet,all these experiments require the quantum state of light field which is exactly resonant on the specific atom transition.Previously,squeezed beams at Cesium D-line and Rubidium D-line either by KNbO₃ or PPKTP have already been generated.The usual method of generating nonclassical light is the optical parametric process.However,the usual way of obtaining the pump beam of optical parametric oscillator(OPO) is to use the frequency doubling.In our experiment,we obtained blue beam at 426 nm with PPKTP, which is the frequency doubling of the Cesium D₂ transition.This CW blue light can not only be used in the studies on nonclassical field-atoms interaction,but also has many applications in optical storage,laser spectroscopy,laser lithography,and material science,etc.So,it is very significant to study the blue laser at the resonance of the atom transitions.The main works of this dissertation are as follows:(1) We simply introduce the nonlinear interaction equations—coupling wave equations and the theory of second harmonic generation,then we discuss the important factor—phase-matching and introduce usual two kinds of phase-matching—Critical-Phase-Matching and Noncritical-Phase-Matching and discuss eventually a new kind of phase-matching technique—Quasi-Phase-Matching and compare these three kinds of phase matching techniques.(2) We discuss the configuration of frequency doubler.When frequency doubler is a bow-tie type ring cavity,the total round trip cavity length is 516 mm.The distance between the two curved mirrors is 58 mm and the waist radius between this two curved mirrors is 20μm.In order to enhance the frequency doubling conversion efficiency,we optimize the configuration of frequency-doubler according to spatial "mode-matching" and "optical impedance-matching".By sending the beam into the crystal(single-pass) we measured the nonlinear conversion coefficient E_NL is about 1.41%W^-1 with crystal length of 10mm at the phase-matching temperature.The intracavity linear loss L_loss is 2.65%.Finally,we choose the mode matching lens with focal length of 80mm.The transmission of input coupler is 7.6%.Based on the experimental parameters,we predict the experimental results theoretically.(3) We introduce our experimental system in details,including the set-up of the experiment,the procedures and the results.At first,frequency doubler is a bow-tie type ring cavity and the nonlinear crystal is PPKTP.We used a diode laser(SDL-TC40) as the pump source.The wavelength of the diode laser is at the Cs D₂ transition(852.356nm) which was checked by Cs glass cell and wavelength meter.The maximal output power is about 140 mW.We measured the maximal power of blue light at 426 nm,which is 54.4 mW and the conversion efficiency is about 40%when the pump power is 136.4 mW. This result is quite agreeable with the theory.The best phase matching temperature measured is 318.3 K(corresponding to 4.31 KΩ) at 852.356 nm, by a fine temperature controller.This CW blue light(426.178nm) can not only be used in atomic physics,quantum optics,but also can be used as the pump source of OPO to generate the quantum state at the Cs D₂ transition. (4) Based on our experimental system,we discuss the possibility of designing the frequency doubler by the standing-wave cavity.The motivation is that the bow-tie type ring cavity,due to four mirrors,may have some drawbacks of mechanical stability and intracavity losses.The standing-wave cavity by just two mirrors should have less losses and better stability but the flexibility.Other improve is to use the Ti:Sapphire laser as pump source.The Ti:Sapphire laser,compared to the diode laser(SDL-TC40),has high output power,low noise and good spatial mode.After these improvements,we hope higher conversion efficiency and more blue light can be obtained.We have discussed theoretically the possible results that could be reached experimentally.