Theoretical And Experimental Investigation Of Doubly-Resonant Sum-Frequency Generation In An External Resonator

The laser plays an important role in many fields of industry and daily life. However, due to the lack of appropriate gain mediums, some special bands cannot be obtained directly by laser radiation. The nonlinear frequency conversion technique will be need. The nature of the nonlinear frequency conversion is a nonlinear optical effect produced in the process of the interaction between laser and medium. The nonlinear optical effect breaks with conventional limitations of linear superposition and independent propagation of lightwaves and reveals the exchanges of energy and phase association of lightwaves. Nonlinear frequency conversion technique includes second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), and optical parametric oscillator (OPO) and so on. Compared with SHG, SFG is a three-wave interaction because the wavelengthes of two fundamental lights are different. Therefore it is complex. However, SFG can achieve coherent radiation in the specific short-wavelength that can not be achieved by SHG and become a focus of research and exploration. The sum-frequency conversion efficiency is directly proportional to the product of two fundamental laser power densities. The doubly resonant technique in an external cavity is that the two fundamental lights should be frequency resonant with the external cavity simultaneously. In the application, the laser power in cavity will be amplified and the waist radius in cavity will be controlled, thereby improving the conversion efficiency of SFG.In this thesis, the example treated here is589nm generation by mixing938nm and1583nm fundamental lights. The basic theories of SFG in a doubly resonant external cavity were briefly analyzed, the phase matching angle and effective nonlinear coefficient of nonlinear cry stral were calculated, the mode matching and impedance matching in the process of coupling a laser beam to external cavity were detailedly analyzed. Then we introduce our works mainly around the serial locking of the sum-frequency system, the optimization of external cavity parameters and the wavelength tuning, frequency locking of sum-frequency output. The key parts contain following aspects:1. The serial locking of the sum-frequency system based on doubly resonant external cavity have been studied theoretically and experimentally. First, the common frequency locking techniques were compared. The Hansch-Couillaud (HC) frequency locking technique was selected for achieving the serial locking of the sum-frequency system based on doubly resonant external cavity. Then the HC frequency locking schemes based on a single KTP crystal and diffusion bonded KTP crystals were analyzed. The results show that two orthogonal linearly polarized components are seriously departed in a single KTP crystal and are not resonant with the external cavity simultaneously. That makes HC frequency locking schemes can not be used for type II phase matching SFG. The use of diffusion bonded KTP crystals (or periodically poled crystal) can overcome this drawback. In experiment, the cavity mode frequency of bow-tie cavity was locked the frequency of938nm laser firstly, and then the frequency of1583nm laser was locked to the cavity mode frequency of bow-tie cavity. The serial locking of three components were realized. In addition, in order to optimize the feedback circuit of the frequency locking system, the design principles of servo system were studied based on the basic control theory. The servo feedback circuit was design by measuring the transfer function of the laser-cavity system and the performance of frequency stabilization was improved.2. The cavity parameters of SFG in doubly resonant external cavity have been optimizated theoretically and experimentally. In theory, a theoretical model of SFG in a doubly resonant external cavity is established. The influence of BK factor and cavity enhanced factor on SFG conversion efficiency was analyzed. Then the factors that influence BK factor, the dependent relationship of BK factor and cavity structure parameters were detailed studied. Similarly, the factors that influence enhancement factors, the dependent relationship of enhancement factors and the reflectivities of the input couplers were detailed studied. Thereby the cavity structure parameters and the reflectivities of the input couplers were optimized by computational simulation and the conversion efficiency of SFG was improved. In experiment, the continuous-wave589nm emission with an output power of larger than0.2W has been obtained by mixing the938nm and1583nm fundamental lights in a bowtie-configured cavity containing a PPLN crystal and using the optimum cavity parameters that obtained by previous theoretical calculations. The experimental results and the prediction agree well considering the imperfect mode matching and reflection losses of cavity mirror and PPLN crystal.3. The wavelength tuning, frequency locking and parameter measurement of sum-frequency output generated in a doubly resonant external cavity have been studied theoretically and experimentally. First, the frequency scanning is analyzed theoretically when doubly resonant external cavity system is serially locked. In order to lock the frequency of sum-frequency output to reference frequency, the direct absorption spectrum and saturated fluorescence spectrum produced by leading the sum-frequency output to the gas cell was studied in detail. In experiment, a scanning signal was added to the port of frequency modulation of938nm laser source with large tuning range. The frequency of sum-frequency output can be scanned as soon as we scan the938nm frequency. The linewidth of sum-frequency output was measured by scanning the cavity length of a high-finesse F-P cavity. The sodium absorption spectrum and saturated fluorescence spectrum at different temperatures were measured by the design of the temperature control device. The frequency locking of589nm sum-frequency output to sodium D2a line was realize using wavelength modulation frequency locking technique. Finally, the variation of error signal with frequency locking was measured and was compared with the variation of error signal without frequency locking. The results show that the maximum scanning range of589nm frequency is about2.2GHz, the linewidth is7MHz. When the frequency of589nm sum-frequency output is locked to sodium D2a line, the frequency stability is improved from400MHz to4MHz.The creative works are as follows:1. The sum-frequency in doubly resonant external cavity system is studied in detail. When the fundamental beam is fixed, the factors that influence the SFG conversion efficiency is analyed. The optimum BK factor can be obtained by optimizing the cavity length parameters, the perfect impedance matching can be achived and the cavity enhanced factor can be improved by optimizing the reflectivities of the input couplers, thereby improving the SFG conversion efficiency2. The impact of walk-off effect of two orthogonal linearly polarized components in a single crystal on HC frequency locking technique is analyzed. This impact can be eliminated by using bonded crystal or periodically poled crystal. Therefore the type II phase matching SFG can be achieved in the doubly resonant external cavity based on HC frequency locking technique.3. The phase correlated locking of a doubly resonant external cavity are realized based on HC frequency locking technique. The cavity mode frequency of bow-tie cavity is locked the frequency of938nm laser firstly, and then the frequency of1583nm laser is locked to the cavity mode frequency of bow-tie cavity, thereby two fundamental beams resonant with the external cavity simultaneously.4. The wavelength tuning of sum-frequency output is achived when doubly resonant external cavity system is serially locked. The sum-frequency output is tuned as soon as tuning the frequency of the master laser5. The589nm sum-frequency output is generated by mixing938nm and1583nm fundamental lights in a doubly resonant external cavity. The1583nm laser source is a fiber laser, the938nm laser source is a semiconductor laser and the nonlinear crystal is PPLN. The sum-frequency conversion efficiency is42.8%...