| An optical waveguide structure is characterized by a region with relatively high refractive index surrounded by regions with lower index. Taking advantage of the short wavelength of the light, such a configuration allows confinement of the light in small volumes with dimensions of micrometer. Benefitting from their compact size, optical waveguides have been considered to be one of the key building blocks for on-chip photonic circuits, which are analogous to electronic systems but with higher information processing and transmission rate. As passive devices, waveguides can be used as optical switcher, splitter, multiplexer and coupler and so on. Meanwhile, many of the optical effects in the substrate materials has been realized in optical waveguides, such as laser oscillations from waveguide structures in laser media or frequency conversion from waveguides in nonlinear optical materials etc. Apart from being utilized as independent devices, active waveguides provide the possibilities of optical modulation, resulting in the integration of multiple functions in a single photonic circuit. Due to the high intra-cavity light energy, many performances could be considerably enhanced in the wave-guiding structures, such as low threshold for waveguide lasers, fast responses for frequency conversion in nonlinear waveguides, which is a promising feature for the practical application of waveguide devices as well as integrated optical circuits.Generally, waveguide stuctures are fabricated in materials including polymers, seniconductors, glasses, crystals and tansparent ceramics. Owing to good physical, chemical and optical properties, crystals play important roles in many optical devices. As a matter of fact, many efforts have been performed to fabricate waveguides in crystals. For instances, planar waveguides can be fabricated in crystals with ion-implantation and swift heavy ion-irradiation. During these processes, the incident ions will collide with the nuclear and electrons of the target atoms, producing a modification of lattice which leads to a change of refractive index. One of the most advantageous characteristics of these methods is the wide applicability of materials. Meanwhile, these methods in combination with, for example, photolithography could be used to fabricate channel waveguides. Recently, femtosecond laser inscription has emerged as a powerful technology for waveguide fabrication. During the inscription process, high optical energy at the laser focus would be deposited inside the materials due to the nonlinear multi-photon absorption, resulting in a highly localized structural modification to the materials, one example of which is refractive index change. This technology has attracted increasing attention for its ability of directly fabricating embedded3D waveguides in almost all kinds of materials without clean environment.The research works in this dissertation are based on the waveguide structures fabricated by using the methods mentioned above in a variety of optical materials, including laser crystals, transparent ceramics and nonlinear crystal. In order to assess the quality of these structures, a series of experiments on the waveguide prosperties are performed, which include the investigations about the m-line prosperities, refractive index distribution, propagation modes, losses, micro-luminescence or micro-Raman properties. Furthermore, waveguide lasing and frequency doubling effects are investigated to analyze their potential applications.As a result of the ion implantation works, planar or channel waveguides are fabricated in optical laser material including Nd-doped gadolinium gallium garnet (Nd:GGG) and Nd-doped langasite (Nd:LGS) by using hydrogen (H+) or carbon (C3+) ion implantation. The induced refractive index distributions of the waveguides are found to be "well"+"barrier". The channel waveguides exhibit excellent guiding performance, with propagation losses less than2dB/cm. The micro-luminescence investigations on the Nd:LGS channel waveguide indicate that the fluorescence properties are well preserved in the waveguide regions comparing with the substrate. Of particular significance is the demonstration of waveguide laser oscillation realized in the proton implanted Nd:GGG planar waveguide that exhibits a low lasig threshold of49.3mW and a slope efficiency of30%.As a result of the swift heavy ion-irradiation works, planar waveguides are fabricated in both laser media Nd-doped yttrium aluminum garnet (Nd:YAG) and nonlinear crystal Nd-doped gadolinium calcium oxoborate (Nd:GdCOB) as well as yttrium calcium oxyborate (Nd:YCOB). As for the fabrication of Nd:YAG waveguides60MeV Ar4+ions at fluence of2×1012ions/cm2and20MeV N3+ions at fluence2×1014ions/cm2are irradiated on the sample, respectively. Through a number of experimental studies and comparisons, it is concluded that the presence of accumulation effects due to the much larger fluence could lead to a larger lattice modification, resulting in a larger refractive index change. Both waveguides were demonstrated to be capable of laser generation with comparable performance.17MeV C5+ions at fluence2×1014ions/cm2are used to fabricate waveguide in Nd:GdCOB, with which second harmonic generations are realized through the Type I phase matching. The maximum output power of signal beam is～0.72mW with a conversion efficiency of6.8%W-1. Nd:YCOB planar waveguides are fabricated with170MeV Ar8+ions at fluence2×1012ions/cm2. As a result of Nd:YCOB works, continuous-wave (CW) waveguide laser at1061.2nm is generated at room temperature with a slope efficiency as high as-67.9%. In addition, the guided-wave second harmonics are realized through the frequency doubling and the self-frequency-doubling of the waveguides under the optical pumps at wavelengths of1064nm and810nm, respectively. Under the pulsed1064nm laser pump, the frequency conversion efficiency is measured to be0.12%. The maximum output power obtained from the intracavity self-frequency-doubling is36μW.As a result of the ULI studies, embedded waveguides were fabricated, including cladding structures in Nd:LGS, Nd:YCOB as well as Tm:YAG ceramic and double-line structures in Nd-doped lutetium vanadate (Nd:LuVO4). In the Nd:LGS works, the diameters of cladding waveguides are50μm and120μm, both of which show well-defined modal profiles and acceptable propagation losses at around1.06μm. The lasing threshold is as low as54mW for50μm waveguide, whilst the maximum slope efficiency (24.2%) was obtained from120μm waveguide. In the Nd:YCOB cladding waveguide structures, efficient laser emissions at1062nm are obtained. More importantly, guided green lasers at531nm are realized by self-frequency-doubling with the highest power level have ever been reported. As a result of Tm:YAG work, near-infrared (IR) to mid-IR multimode waveguiding in deep buried channel waveguides are achieved. CW waveguide laser operation at around2μm wavelength with multi-or single-transverse modes is also demonstrated from these waveguides, with the maximum slope efficiency of27%from multimode laser action and a minimum threshold of100mW (input power) from the single-mode one. A saturable absorber mirror is fabricated by coating a graphene film on an output coupler mirror. This is then used to obtain Q-switched mode-locking from a Tm:YAG ceramic single-mode waveguide, which produces Q-switched envelopes and mode-locked pulses at～2μm, with684kHz and7.8GHz repetition rate, respectively. Through the investigation on the channel waveguides in Nd:LuVO4, it is found that the luminescence properties of the bulk materials are well preserved in the waveguide core region. CW laser oscillation was observed from the waveguide with the absorbed pump power at threshold and laser slope efficiency of98mW and14%, respectively. |
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