Investigations Of External-cavity Enhanced Frequency Doubling Of 1560 Nm Laser

The laser frequency doubling technics based on nonlinear optical effect are ways to extend the frequency range and to manufacture new laser. New laser combining the existed fiber laser or fiber amplifier and the frequency conversion devices is usually equipped with diodes of industrial grade and optical fiber devices for communication, which can be easily connected and sealed. And most parts can be air-cooled except the nonlinear crystal. Thus the new fiber laser is cheaper, more stable and not too much maintenance is required. This laser mainly benefits from the pump fiber laser of high power and narrow linewidth and the Quasi-Phase-Matching technic for the nonlinear crystals. The latter has made it possible to utilize the maximum nonlinear coefficient and it also makes up for the drawback brought by the refractive indexes of the nonlinear crystal, which cannot compensate the phase mismatch at regular temperature or for a particular pair of wavelengths. Progress on crystal growth and periodical poling technics also helps. All of these make the new fiber laser an competitor of diode laser and Ti:sapphire laser. We made some investigations on external-cavity enhanced frequency doubling of 1560 nm laser, aiming at realizing entanglement between 780 nm, which is the D2 line of rubidium atom, and 1560 nm at the window of fiber transmission.The major work of this thesis is listed as followed. Firstly, the frequency doubling technic and the relevant studies is introduced, including the basic concepts and experimental methods. Secondly, we doubled the 1560 nm laser with a bow-tie ring cavity and a piece of PPMgOrLN bulk crystal. A maximal output power of 1.5 W is realized. The corresponding conversion efficiency is 73%. The M2 factor of the second harmonic is 1.02 in the horizontal direction and 1.03 in the vertical direction, respectively. The RMS fluctuation of the output power in about 30 min is less than 1.2% and the continuous frequency tuning range is more than 950 MHz around 780 nm. To improve the mechanical stability and lower the linear loss of the enhancement cavity, we further designed and built a semimonolithic cavity made by a piece of PPKTP bulk crystal and a cavity mirror. The maximal output power of the harmonic is 1.6 W and the doubling efficiency is 77%. The M2 factor of the second harmonic of this doubler is 1.04 in the horizontal direction and 1.03 in the vertical direction. The output power of this doubler is more stable than the previous one. The RMS fluctuation of the power in about 30 min is less than 0.5%. Continuous frequency tuning range is similar to the result of the bow-tie four-mirror ring cavity.