Investigation Of Optical Properties On Ridge Waveguides In Crystals Produced By Energetic Ion Beam Irradiation

Integrated optics is a discipline researching the optical phenomenon in the medium film and the optical components integration. The concept of integrated optics was first proposed by Dr. S.E.Mille at Bell Labs in 1969, and then was gradually developed as a new discipline as well as an important branch of modern optoelectronics. The main task of integrated optics is to miniaturize the traditional optical components and systems, and then "integrate" these components or systems in a certain way to form integrated optical system which has a variety of functions. As the basic components in integrated optics, optical waveguides consist of high-refractive-index regions surrounded by low-refractive-index regions. Relying on the principle of total reflection of light at the interface between two kinds of materials with different refractive indexes, optical waveguide could confine light propagation within very small volume with size of micrometer order, in which the optical intensities could reach a much higher level. According to the geometric structures, optical waveguides can be classified into two types:one-dimensional waveguides and two-dimensional waveguides. One-dimensional waveguides, which are also called planar waveguides or slab waveguides, could only confine light propagation in one dimension; while two-dimensional waveguides including channel waveguides or ridge waveguides could confine light propagation in two dimensions, resulting in much higher optical intensities and integration level with respect to one-dimensional waveguides. As the shape of ridge waveguide device is easy to be connected with other elements in integrated optics to construct complex "integrated photonic chips", realizing more functions, it has a very broad application prospect and is an important research topic in the field of integrated optics.At present, several techniques have been utilized to fabricate optical waveguides, including energetic ion beam irradiation, femtosecond (fs) laser writing, pulsed laser deposition, molecular beam epitaxy and ion exchange and thermal diffusion, etc. Among these methods, energetic ion beam irradiation is a mature and widely used technique for optical waveguides fabrication in recent years, which is applicable to a wide range of materials. Energetic ion beam irradiation is an efficient method for crystal material surface modification. When the charged ions with certain energy go into the substrate material, they will collide with the nuclei or electrons of the atoms in target material and lose their energies. This process will cause damage to the lattice structure of the substrate material, and change the refractive index distribution of the specific region of the material, forming optical waveguide structure at the surface of the material. As the ion irradiation fluences required for fabricating optical waveguide are comparatively low, there is almost no doping effect and the original composition of the material could be preserved. In addition, through appropriate thermal annealing treatment, the color centers and point defects in the waveguide could be eliminated, and the maximum original properties of the material could be preserved. As this technique is not affected by temperature during the fabrication process, precise control of dimensions and refractive index distribution of the waveguide could be achieved by selecting suitable substrate material, ion species, ion dose and ion energy. In this way, high-quality optical waveguide devices with complex structures could be realized by collocating ion irradiation parameters reasonably. So far, optical waveguide structures have been fabricated in more than 80 kinds of insulating materials utilizing energetic ion beam irradiation technique. Combined with additional techniques such as femtosecond laser ablation and precise diamond blade dicing, two-dimensional ridge waveguide structures can be fabricated.In this dissertation, energetic ion beam irradiation technique was utilized to fabricate planar waveguide in a variety of optical materials, including mid-infrared (MIR) crystals, rare-earth ion doped laser crystals and non-linear crystals, and then femtosecond laser ablation or precise diamond blade dicing was employed to micro-manufacture two-dimensional ridge waveguide structures. A series of experiments have been carried out to measure and investigate the guiding properties of optical waveguides, including microscopic structure, refractive index distribution, propagation mode, propagation loss, Raman spectrum and fluorescence spectrum, etc. Furthermore, waveguide lasing and frequency doubling effects were carried out to analyze their potential applications. Depending on the types of substrate materials, the research work and results in this dissertation can be summarized as follows:ZnSe crystal, as an important mid-infrared material, possesses high thermal conductivity and excellent mechanical property, and is a preferred material to fabricate infrared lenses, laser windows and thermal infrared imagers. Planar waveguide structure is fabricated in ZnSe crystal using swift heavy ion irradiation, and then two ridge waveguides with different widths are constructed by using precise diamond blade dicing on the planar waveguide. The ridge waveguide structures support multi-mode guidance at MIR wavelength of 4 μm. The minimum propagation loss of the ridge waveguide is measured to be～1.1 dB/cm at 4 μm. The micro-Raman spectra indicate that the lattice structure of the waveguide region in ZnSe crystal has no significant change along the ion tracks after Kr8+ ion irradiation. This experiment provides a research basis for the application of optical ZnSe ridge waveguide devices in the field of mid-infrared integrated optics.LiNbO3 crystal is an important multifunctional material, which has excellent electro-optic, acousto-optic, piezoelectric, ferroelectric and nonlinear optical properties. We have fabricated optical ridge waveguides in MgO:LiNbO3 crystal by using combination of O5+ ion irradiation and precise diamond blade dicing. The ridge waveguide structures show good guiding properties at MIR wavelength of 4 μm along TM polarization. After a series of thermal annealing treatments the guiding properties of the ridge waveguides have been significantly improved and the minimum propagation loss of the ridge waveguide is reduced to 1.0 dB/cm. The experiment indicates that combination of swift heavy ion irradiation and precise diamond blade dicing is an efficient method for fabricating mid-infrared optical ridge waveguides in MgO:LiNbO3 crystal.Nd:YAG crystal is the most excellent solid-state laser material so far, which has high laser gain, low laser threshold, high laser power, low absorption of 1064 nm light and good thermal conductivity. Planar waveguide structure is fabricated in Nd:YAG crystal using swift Kr8+ ion irradiation, and then a ridge waveguide structure is constructed by precise diamond blade dicing on the planar waveguide. The propagation losses of the planar and ridge waveguides at 632.8 nm are measured to be-2.1 dB/cm-3.7 dB/cm, respectively. Under optical pumping of 808 nm light, continuous waveguide lasers at 1065 nm have been realized. In the planar waveguide structure, the maximum output power is 49.3 mW with the slope efficiency of 45.6%. For the ridge waveguide structure, the maximum output power is 71.5 mW with the slope efficiency of 60.8%. The laser performance of ridge waveguide is more outstanding than that of planar waveguide, which provides an efficient method for fabricating high-performance miniature waveguide laser devices at near-infrared wavelength.Yb:YAG crystal is a kind of solid-state laser material with excellent performance, which has high doping concentration, high quantum conversion efficiency, long fluorescence lifetime and large absorption bandwidth, etc. Firstly planar waveguide is formed in Yb:YAG crystal by using swift C5+ion irradiation, and then four ridge waveguides with different fabrication parameters are constructed on the planar waveguide by femtosecond laser ablation. We investigate the guiding properties of the ridge waveguides at 632.8 nm through end-face coupling arrangement, finding that the ridge waveguide with the lowest scanning speed and maximum width has the lowest propagation loss. By measuring the maximum incident angular deflection, we estimate the maximum refractive index contrast of the waveguide △n≈+0.004. Based on the reconstructed refractive index profile, near-field modal profiles for the ridge waveguides are simulated by using (FD-BPM), which is in agreement with the experimental results. This work provides a practical basis for optimizing the guiding properties of the ridge waveguides and searching the optimal fabricating parameters of high-quality Yb:YAG ridge waveguides.LGS is a new type of multi-functional crystal, which has excellent electro-optic properties, larger electromechanical coupling coefficient and piezoelectric constant, and no phase change from room temperature to melting point (1470℃). We have fabricated optical ridge waveguides in Nd:LGS crystal by using combination of swift C5+ion irradiation and precise diamond blade dicing. After 200℃ thermal annealing treatment, the ridge waveguide structures could confine light propagation efficiently both at 632.8 nm and 1064 nm along TM polarization. The lowest propagation losses of the ridge waveguide at 632.8 nm and 1064 nm are measured to be 1.6 dB/cm and 1.2 dB/cm, respectively. The micro-fluorescence spectra indicate that the Nd3+ luminescence features have been well preserved in the waveguide region. The micro-Raman spectra indicate that the microstructure of the waveguide region has no significant change after swift heavy ion irradiation. The experimental results have guiding significance for using swift C5+ ion irradiation to fabricate ridge waveguides in Nd:LGS crystal. The experimental results show that the Nd:LGS ridge waveguides have excellent guiding properties and a great application potential in the field of visible light and near infrared integrated optics.KTP crystal is a kind of nonlinear optical material with excellent properties, including large nonlinear optical coefficient, high photoelectric conversion efficiency, low dielectric constant, good mechanical properties and high heat conduction coefficient. We have fabricated optical ridge waveguides in KTP crystal by using combination of swift O5+ ion irradiation and femtosecond laser ablation. A prism coupler is used to investigate the dark-mode-line spectrum at 632.8 nm and the refractive index profile is reconstructed through reflectivity calculation method (RCM). Second harmonic generation (SHG) of 1064→532 nm has been realized both in the planar and ridge waveguide structures under pulsed 1064 nm laser configuration. Specifically, in the planar waveguide structure, the maximum conversion efficiency of SHG 1064→532 nm is 11.5%. For the ridge waveguide structure, the maximum conversion efficiency of SHG 1064→532 nm is 25.4%. The experimental results show that the fabricated KTP ridge waveguides have superior SHG performance to the planar waveguide, and it provides a method for fabricating compact high-performance waveguide fruency doubling devices.YCOB is a very versatile nonlinear functional material, which has good chemical stability, high laser damage threshold and broad optical transparency range, and cannot be easily deliquesced. We have fabricated optical ridge waveguides in Yb:YCOB crystal by using swift C5+ ion irradiation combined with precise diamond blade dicing. We calculated the energy deposition process of the C5+ ion irradiation on Yb:YCOB crystal through the software SRIM. After 260℃ thermal annealing treatment, the ridge waveguide structures show good guiding properties at 1064 nm along TM polarization and the lowest propagation loss of the ridge waveguide is measured to be 1.7 dB/cm. By measuring the maximum incident angular deflection, we estimate the maximum refractive index contrast of the waveguide to be △n≈ +0.004. Based on the reconstructed refractive index profile, near-field modal profile for the ridge waveguide is simulated by using (FD-BPM), which is in agreement with the experimental result. The micro-Raman spectra indicate that C5+ ion irradiation does not cause significant damage to the lattice structure in the waveguide region. The micro-fluorescence spectra imply that the Nd3+ luminescence features in the waveguide region have been well preserved. The experimental results show that swift heavy ion irradiation combined with precise diamond blade dicing is an effective method for fabricating high-efficiency compact ridge waveguide devices in Yb:YCOB crystal.