| Optical waveguide is the basic element in integrated optical circuit used to confine and transmit light.It is composed of a waveguide region with a relatively high refractive index and a surrounding material with a relatively low refractive index.The light intensity obtained from the waveguides is much higher than that in bulks due to the compact geometry of waveguides.And some original optical properties of substrate materials can be greatly enhanced within the waveguides,such as nonlinear optical properties and laser properties.In practice,twodimensions(2D)waveguides are more preferred than one-dimensional(1D)waveguides.Second harmonic generation is a nonlinear optical process in materials.During the frequency doubling process,photons interacting with the non-linear material are effectively combined to form new photons with double frequency and extend the laser spectrum range.Second harmonic generation can convert infrared lasers into visible lasers.Pulsed lasers in the visible wavelength band have important applications in many fields.The main work of this thesis is to use different waveguide fabrication techniques to produce various types of waveguides in four different nonlinear crystals,and to carry out second harmonic generation experiments.This paper includes the following contents.Ridge waveguides were fabricated in Nd:GdCOB crystals by a combination of 15 MeV carbon ion irradiation and precise diamond blade dicing.The transmission characteristics of the prepared Nd:GdCOB waveguides were investigated by experiments and simulations.Transmission loss,mode distribution and polarization dependence of the waveguide were measured based on the end-face coupling system.We simulated the near-field modal distribution in waveguide by the RSoft software,and the simulations are very similar to the experimental results imaged by a CCD camera,indicating that the reconstructed refractive index is reasonable.The micro-second harmonic spectroscopic analysis shows that the nonlinear properties of Nd:GdCOB crystals are greatly enhanced within the waveguide region.We have achieved 1064→532 nm second harmonic generation in the waveguide by type I phase matching.Under the excitation of a 1064 nm pulsed laser,the maximum SH outputs of the planar waveguide and the ridge waveguide are～1.04 mW and～2.80 mW,respectively.And the corresponding conversion efficiencies are～8.32%W-1 and～22.36%W-1,respectively.Cladding waveguides and channel waveguides were fabricated by femtosecond laser direct writing technology in KTP crystal,and the second harmonic generation from 1550 nm to 775 nm in KTP crystal was realized.Based on the end-face coupling system,the output powers of fundamental wave and SHG wave along different polarization directions were measured.It was verified that the second harmonic process is based on type Ⅱ phase matching.Under 1550 nm femtosecond laser pumping,the maximum second harmonic peak powers in No.5 cladding waveguide and No.5 channel waveguide are～20.19 W and～21.75 W,respectively.And the corresponding conversion efficiencies are～0.35%and～0.38%.Dual-line waveguides were fabricated in PPLN crystal by femtosecond laser direct writing technology,and the propergation losses of the waveguides were measured by the Fabry-Perot method with a tunable laser.The corresponding relationship between the phase-matched wavelength and temperature in the waveguide is realized by temperature and wavelength tuning using an oven and a tunable laser,respectively.The second harmonic generation for 1.5 μm is realized under both pulsed and continuous configuration at room temperature.The maximum SHG output power is～3.52 mW,and the corresponding conversion efficiency is～1.06%under continuous configuration.Under pulsed configuration,the maximum conversion efficiency is～19.74%while the second harmonic peak output power is 732.95 W.The cladding waveguides were fabricated in Nd:GdCOB crystal by femtosecond laser direct writing technique.The end-face images of waveguides were taken by a metallographic microscope,and the loss and near-field mode distribution of waveguides at 1064 nm and 632.8 nm were measured based on the end-face coupling system.The channel waveguides were fabricated in CTGS crystal by femtosecond laser direct writing technique and the end-face images of the waveguides were taken by a metallographic microscope.Based on the end-face coupling system,the loss and near-field mode distribution of the waveguide at 1064 nm and 632.8 nm are measured as well as the output power along different polarization directions at 632.8 nm. |
