Theoretical And Experimental Study On External Resonant Frequency Doubling Based On PDH (Pound-Drever-HaⅡ) Technology

Nonlinear optics is a new discipline arising from the development of laser technology, which makes it possible to generate the output of more laser wavelengths on the basis of available laser material, and it’s attracting more and more attention nowadays. Frequency doubling is the most widely used technique in the field of nonlinear optics, but the conversion efficiency is always low as it dependent highly on the intensity of fundamental light. The use of resonant extra-cavity in frequency doubling improves the efficiency greatly, especially in the condition of low power, cw fundamental light. Compared with the single pass situation, the conversion efficiency is two orders of magnitude higher in the way of resonant external-cavity frequency doubling, and it has showed great superiority.The study of this thesis focuses on the theoretical and experimental research of resonant extra-cavity frequency doubling. After the full investigation on the research in domestic and overseas, the scheme of resonant external-cavity frequency doubling based on PDH technique is designed. Firstly, the scheme of resonant external-cavity frequency doubling based on PDH technique is designed. The BK focusing factor and multi-beam interference theory are introduced to analyze the conversion efficiency in the case of resonant extra-cavity frequency doubling, when the length of LBO is 30mm, and the power of the fundamental light is 10W, the optimum conversion efficiency of more than 80% is calculated. The ring cavity for resonant frequency doubling is designed and optimized. Using transfer matrix method to analyze the impact of the radius and the distance of mirrors on the waist radius of cavity mode, the parameters of the ring cavity is optimized to realize the mode matching and impedance matching condition.Secondly, The theory of cavity length lock based on PDH technique is investigated, with the analysis of change relation between cavity mode signal, error signal and cavity length offset, it is verified by locking the cavity length with PDH technique. The PDH cavity length feedback control system used in extra-cavity frequency doubling is designed and the cavity length lock experiment is carried out. The Electro-optical phase modulator, modulation and scan signal which will affect the error signal are optimized and set. After changing the Electro-optical phase modulator from spatial to optical, setting the frequency and amplitude of modulation signal to 12.5MHz and 0.2V, setting the frequency and amplitude of scan signal to 10Hz and 5V, setting the parameter of PID controller G, P, I, D to 21,9500,215 and 0, the desirable locking state is achieved. The circulating power of fundamental laser in cavity is 152.69 W by monitoring the leak power of the cavity, that is, the resonant enhancement factor is 15.87. The distribution of beam profile is uniform as a result of fundamental mode in cavity.Finally, the experiment of resonant external-cavity frequency doubling is done by inserting the LBO crystal into the cavity. Using the size of 8mm×8min×30mm and I type noncritical phase match LBO which is placed in a heat sink working at the temperature of 150℃. During the experiment, the impact of LBO temperature, the power of fundamental laser, the beam waist, the central wavelength and the reflectivity of input coupling mirror on the efficiency of frequency doubling is investigated. The results show that while the temperature of LBO is 150℃, the power of the fundamental laser is 8.5W, the beam waist is 50μm, and the reflectivity of input coupling mirror is 93%, the highest power of harmonic light 4.59 W is achieved and the efficiency of frequency doubling is 54%, which is 200 times of single pass condition.Above all, the results achieved in the paper can be concluded in the following three aspects:1) Combine the focused Gaussian light beam theory and the cavity theory to establish the resonant extra-cavity frequency doubling model, analysis the impact of input power Pm, radius of beam waist ω0 and the reflectivity of input coupling mirror r1 on frequency doubling efficiency to guide the design of the experiment system of the external-cavity frequency doubling. The results of experiment and the theoretical calculations agree well.2) Use the PDH control system, compared with the traditional technique based on intensity type error signal, the sensitivity of phase demodulation is high, as well as avoiding effect of the amplitude noise on the system. Improve the reliability of the system.3) Establish the extra-cavity frequency doubling experimental system. Using the ring cavity to realize the lock of the cavity length, the resonant enhancement factor is more than 15 and the conversion efficiency is 54% with the output of green laser power of 4.59W, which is 200 times of single pass condition. This experiment shows the feasibility of high conversion efficiency in the case of low incident CW fundamental power.