Femtosecond Laser Inscribed Optical Channel Waveguides In Laser Crystals And Ceramics

Integrated circuits can realize processing electrical signals and information in a highly integrated space, but recently, its development faces the challenges, such as much smaller size, less power consumption and other bottlenecks. Integrated optical circuits, by contrast, possess the capability of high-speed transmitting and processing optical signals with higher stability and lower power consumption in micro-region, and fields like optical communication and optical information processing has broad application prospects in the future. Since the concept of integrated optical was brought forward in the late 1960s, people had began the research on integrated optical circuits. Compared with the traditional optical devices, which owns large volume, low stability, and difficult for beam collimation, integrated optical circuits are mainly depended on the optical devices and optical waveguide devices to achieve integrated, miniaturized integrated photonics systems, meanwhile, the devices show stable performance with high efficiency and low manufacturing cost. Optical waveguide devices can fulfill confinment and guidance of the light transmission, and they are indispensable basic elements of integrated photonics systems by connecting different functional photonic devices. As the shortest board mentioned in the"bucket theory", quality of optical waveguide devicse will directly determine the overall quality of integrated photonics systems.Optical waveguide is a structure composed of higher-refractive-index region surrounded by a region with low refractive index of refraction higher region, and light can be confined and propagated in a space with volume in a micro-or nano-scale size through the full reflection, and thus optical density is greatly enhanced, and optical properties, such as nonlinear optics and laser characteristics has been strengthened. Up to now, photo-couplers, optical switches, waveguide lasers, waveguide frequency converter have been achieved in passive or active waveguide devices. According to the structural characteristics, optical waveguide can be divided into planar optical waveguide and channel optical waveguide. Channel optical waveguides (such as tubular optical waveguide and ridge optical waveguide) can confine optical propagation and guidance in two or even three dimensional scale, rather than planar optical waveguide can only fulfill it in one dimensional optical transmission, which means channel optical waveguides have much higher research value and much broader application prospects.As a widel used optical waveguide substrate material, dielectric crystal and transparent ceramics have excellent physical and chemical characteristics and optical properties, and are widely used in several fields. Laser crystals and ceramics are the most common operating mediums for all-solid-state lasers, and they show great features such as high gain, good thermal conductivity, and lower lasing threshold compared with glass materials. Stable ground wave-guided laser output can be achieved through laser materials combined with high integration, high efficiency and low loss optical waveguide devices. With the development of integrated photonic circuits, demand for the integration and multifunction, miniaturization of devices increases, however, the implementation of high power waveguide laser lays a solid foundation for the combination with non-linear optical elements and obtaining multifunction integrated photonic devices in the future.So far, people have utlized a variety of techniques to fabricate optical waveguides in laser crystals and ceramics, such as ion implantation, ion-exchange, focused proton beam writing, and femtosecond laser writing technology. Femtosecond laser writing technology utilize ultrashort pulse laser to realize three dimensional micro-processing in micro-spatial scale. Peak power of femtosecond laser could up to 1015W, and it can be used for processing of metal, transparent crystal material, organic compounds and other substances, moreover, the resolution of the three-dimensional processing can be less than 10 nm. Now, femtosecond laser micromachining are widely used in the military, bio-medical, research, manufacturing and other industries fields. In this thesis, femtosecond laser micro-processing optical waveguides in transparent dielectric crystal and ceramic materials is based on that the pulse laser could be focused on material surface or inside, and then the pulse energy can be absorbed during the scanning through nonlinear optical effect, including multiphoton absorption, avalanche ionization and tunnel ionization, at the same time, the refractive index near the focus will be changed. Finally, by setting reasonable processing parameters (e.g., writing speed, writes energy), optical waveguide structures can be obtained through multiple scans. Due to its high efficiency, low cost, non-pollution, strong adjustability, and fit for processing of aser crystals and cermaics, femtosecond laser micromachining show excellent advantages compared with other technologies.The content mainly includes femtosecond laser micromaching laser crystal and ceramics to achieve channel optical waveguides, μ-PL and μ-Raman measurements for analysis the waveguide formation mechanism, and research about the guiding and lasering characteristics of the optical waveugides. According to the used crystal or ceramic materials and the different types of optical waveguide devices, the main work of this thesis can be summarized as follows:We report on the fabrication of depressed cladding waveguide lasers in Nd:YAG ceramics microstructured by femtosecond laser pulses. Under optical pump at 808 nm, cladding waveguides showed continuous wave efficient laser oscillation. The maximum output power obtained at 1064.5 nm is-181 mW with a slope efficiency as high as 44%, which suggests that the fabricated Nd:YAG ceramic waveguides are promising candidates for efficient integrated laser sources. We report on the design and implementation of a prototype of optical waveguides fabricated in Nd:YAG crystals by using femtosecond-laser irradiation. Compared with single-cladding waveguides, the concentric tubular structures, benefiting from the large pump area of the outermost cladding, possess both superior laser performance and nearly single-mode beam profile in the inner cladding. Double-cladding waveguides of the same size were fabricated and coated by a thin optical film, and a maximum output power of 384 mW and a slope efficiency of 46.1% were obtained. We demonstrate a new design of waveguiding microstructure, which contains both cladding and inner dual-line configurations, in Nd:YAG laser crystals inscribed by femtosecond laser processing. Based on this prototype, a few "cladding+dual-line" hybrid structures with different parameters have been successfully manufactured in Nd:YAG. The waveguide lasing at 1.06 lm has been realized in the hybrid structures. Compared to the single dual-line waveguides, the dual-line core in hybrid configuration benefits from the large-area pump from the external cladding, reaching an enhancement of 26% on the maximum output power and of 24% on the slope efficiency of the waveguide lasing.Depressed tubular cladding waveguides have been produced in Nd:GGG crystals by using multiple inscription with femtosecond (fs) laser pulses. At room temperature continuous wave (cw) laser oscillations at wavelength of-1063 nm have been realized through the optical pump at 808 nm. The slope efficiency of the cladding waveguide lasers is as high as 44.4% and the maximum output power at 1063 nm is 209 mW.We report on the buried channel waveguide laser at 1065 nm in Nd:KGW waveguides fabricated by femtosecond laser writing with dual-line approach. The fluorescence emission spectra of Nd3+ions measured shows that the fluorescence properties were well preserved in the waveguide region. A stable continuous wave laser at 1065 nm has been obtained at room temperature in the buried channel waveguides by optical pumping at 808 nm. A maximum output power of 33 mW and a slope efficiency of 52.3% were achieved in the Nd:KGW waveguide laser system.We report on the fabrication of depressed cladding waveguides in Nd:GdVO4 laser crystal by using femtosecond laser inscription. The cross section of the structure is a circular shape with a diameter of 150 μm. Under the optical pump at 808 nm, the continuous wave (cw) as well as pulsed (Q-switched by graphene saturable absorber) waveguide lasing at 1064 nm has been realized, supporting guidance of both TE and TM polarizations. The maximum output power of 0.57 W was obtained in the cw regime, while the maximum pulse energy of the pulsed laser emissions was up to 19 nJ (corresponding to a maximum average output power of 0.33 W, at a resonant frequency of 18 MHz). The slope efficiencies achieved for the cw and pulsed Nd:GdVO4 waveguide lasers were as high as 68% and 52%, respectively. We report on waveguide lasers at 1064.5 nm in femtosecond laser-written double-cladding waveguides in Nd:GdVO4 crystals. The detailed structure of the single and double claddings has been imaged by means of μ-Raman analysis, and the observed slight fabrication asymmetries with respect to an ideal circular cladding are in well agreement with the observed differences in TE/TM propagation losses. Raman imaging shows the complete absence of lattice defect at the laser active volume. Under the optical pumping at 808 nm, a maximum output power up to 0.43 W of the continuous wave waveguide laser with a slope efficiency of 52.3% has been achieved in the double-cladding waveguide.Rectangular Y-branch cladding waveguides have been fabricated in Nd:YAG crystal by femtosecond laser inscription. Such novel configurations are fabricated with depth of 50 μm, supporting multimode guidance in both TM and TE polarizations. Continuous wave laser oscillations at wavelength of 1.06 μm have been achieved under the optical pump at 808 nm. The maximum output power is 0.2 W with a slope efficiency of 20%in the device with splitting angle of 0.5°. We report on Q-switched waveguide lasers on the graphenebased crystalline Y-branch platform. The Q-switched lasing operation at 1064 nm is achieved in transmission mode, by attaching a two-layer graphene on the resonator output mirror, as well as by using interaction between the evanescent field and a few-layer graphene that was positioned right above the Y-type waveguide. Q-switched laser with a maximum average power of 173 mW, pulse energy and duration of 63 nJ and 90 ns is obtained.