The Scheme Of Deep Ultraviolet Cooling Laser In Hg~+ Ion Optical Frequency Standard

Ultraviolet (UV) or deep ultraviolet (DUV) laser is in high demand for various applications including semiconductor lithography, inspection, optical disk mastering, fabrication of fiber Bragg grating, and high resolution microscopy. But it is difficult to generate ultraviolet laser by pumping the laser rods directly, because it requires a more shorter pumping laser than the ultraviolet laser and the pumping rate should be higher. Therefore it is more practical to generate ultraviolet laser by harmonic generation. Especially, the continuous-wave running DUV coherent light sources operating below 200nm is widely applied in high-resolution spectroscopy of atomic and molecular lines.In Hg+ ion optical frequency standard, the clock transition is 5d₆ s S_1/2â†'5d 6s D_5/2, wavelength at 282nm. In order to cooling down the Hg⁺, we need two different lasers around 194nm with 47GHz frequency difference. In this dissertation, we decide to generate the 194nm radiation by sum-frequency mixing of 718nm and 266nm radiation. The light at 718nm is derived from a titanium sapphire (Ti:S) laser and the light at 266nm is harmonically generated in a BBO crystal pumped by two 532nm laser with 23.6GHz frequency difference, which is a well-developed commercial product and could also be used to pump titanium sapphire laser.In the second harmonic generation of 532nm to 266nm, we choose the BBO crystal as the nonlinear crystal, and calculate the phase matching angle. Considering the requirement of double waves resonating in the cavity at the same time, we design a particular cavity and estimate some property parameters. Similarly, in the process of generating 194nm by sum-frequency of 718nm and 266nm, we compare several common optical nonlinear crystals working at ultraviolet wave band, and find it is BBO crystal which is most suitable choice while adopting the type I phase matching. And the cavity is also designed.Thermal lens effect is one of the thermal effects in nonlinear crystal, and plays an important role in the nonlinear process in the crystal. It comes from the different refractive index distribution caused by multiform temperature distribution, just like lens. Generally speaking, thermal lens effect is determined by three elements: 1. The temperature dependence of refractive index. The change of temperature can directly change the refractive index distribution. 2. The refractive index change caused by optical- elastic effect. 3. The strain-induced change of the curvature of the end of the crystal. In this dissertation, we made a numerical simulation of the temperature distribution and the thermal lens effect in BBO crystal in the process of second harmonic generation. The result shows that the waist of the laser in the crystal and the power of the pumping laser have something to do with the thermal lens effect. We can change the length of the cavity to avoid the laser energy concentrate at one point or control the crystal at a low temperature to decrease the effect of the thermal lens.In the end, we have done an experiment to generate 266nm by second harmonic generation in BBO crystal, which is controlled at 25oC. We use the"Hansh-Couillaud"way to lock the cavity, and with a little mechanical vibration we get a continuous 266nm ultraviolet laser output.