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Optical Channel Waveguides In Dielectric Crystals: Fabrication, Lasing And Second Harmonic Generation

Posted on:2016-01-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y C GuFull Text:PDF
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Optical waveguides, being defined as high refractive index cores surrounded by low-index layers, are the basic component for multi-functional integrated photonic systems.Generally, optical waveguide structures could confine the light propagation within very small regions of orders of several micrometers, in which the light intensities could reach very high levels with respect to the bulk materials. Benefiting from this exceptional advantageous, capability of some features (e.g. nonlinear response, lasing performances, etc.)of many materials may be considerably enhanced to a certain extent in waveguide structures, enabling the implementations of highly efficient miniaturized and integrated platforms for a variety of phtonic applications. In practice, channel (two dimensional,2D) waveguides are more attractive than planar (one dimensional,1D) waveguide structures because the former could restrict the light propagation in two dimensions, reaching higher optical density in more compact geometries, and are easily for constructions of more compact functional integrated devices.Dielectric crystals play important roles in many fields of optics and photonics. For example, nonlinear crystals are important frequency converters of light at different wavelength regions. Laser crystals are the favorite and attractive gain mediums for solid-state laser systems with comparable low lasing thresholds and excellent thermal properties with respect to glasses. Electro-optic crystals are ideal platforms for the modulations of light phase, energy and polarizations.Up to now, various optical applications have been realized through a broad variety of crystal-based devices and components.Benifiting from this, crystalline waveguides, with the combination of compact geometry of channel waveguides and valuable features of crystals, could become unique platforms for versatile miniature and integrated photonic applications.As of yet, a few methods have been employed to fabricate channel waveguides in crystals, including femtosecond (fs) laser micromachining, focus proton beam writing, ion implantation/irradiation, etc. Among these, fs laser micromachining has recently emerged as one of the most efficient techniques for direct three-dimensional microfabrication of transparent optical materials.During this process, fs laser pulses focused beneath the surface of a crystal are absorbed through nonlinear photoionization mechanisms, giving rise to a permanent localized modification with dimensions on the order of micro or submicrometre. By a suitable adjustment of the irradiation parameters, one could induce a localized refractive index modification, enabling direct channel waveguide fabrication by simple scanning the bulk materials, showing wide applicability in a large number of materials.Ion implantation/irradiation is also a powerful and efficient method for waveguide construction, which has been proved successful in more than 100 materials.Generally, ion implantation/irradiation influences the refractive indices of the target materials mainly via physical mechanisms, which are independent to the chemical properties of the target materials.Benefiting this unique feature, ion implantation/irradiation has a wide applicability to most optical materials.Particularly, by combinations of fs laser micromachining or precise diamond dicing,2D channel waveguide structure could be efficiently fabricated on the surface of the substrate.The research work in this thesis mainly focus on the fabrication of optical channel waveguides in dielectric crystalline materials and the realizations of continuous wave (CW) and Q-switched pulsed waveguide laser in laser crystals/ceramics as well as second harmonic generations (SHG) in nonlinear waveguides. The fabrication methods mainly cover fs laser micromachining, ion implantation/irradiation, precise diamond dicing and photolithography. Based on the geometries of the channel waveguides, this thesis includes the following studies:We proposed two novel and efficient methods by combining fs laser writing or precise diamond blade dicing and ion implantation/irradiation for producing ridge waveguide lasers (at both CW and Q-switched pulsed regimes) in laser crystals/ceramics (Nd:YAG crystal, Nd:YAG ceramic, Nd:GGG crystal) and SHG in nonlinear crystals (Nd:GdCOB crystal). The resultant ridge waveguide lasers and SHG show superior performances than the planar ones in these materials, suggesting these innovative approaches are efficient and potential for producing integrated ridge laser sources and frequency convertors.In details, the ridge waveguide lasers at 1.06 μm have been produced in Nd:YAG crystals, Nd:YAG ceramics and Nd:GGG crystals by combining fs laser micromachining and 17 MeV oxygen (O5+),2.5 MeV helium (He+) and 17 MeV carbon (C5+) ion implantation/irradiation, respectively. The lasing thresholds and slope efficiencies for these cases are 39.6,64.9, and 71.6 mW and 35%, 42.5%, and 41.8%, which exhibit superior laser performances to the planar configurations. Besides, based on another Nd:YAG crystal sample, the CW and passively Q-switched ridge waveguide lasers have been fabricated by a combination of 670 MeV krypton (Kr+) ion irradiation and femtosecond laser ablation. The maximum output power of 182 mW was obtained in the CW regime. The pulsed waveguide laser has a range of repetition rate from 0.9 MHz to 4.2 MHz, with pulse energy of up to 26.5 nJ and pulse duration of 90 ns at pump power of 851 mW. In addition, ridge waveguides with smooth side walls have been realized in Nd:YAG crystals by using diamond dicing of planar waveguides fabricated by 15 MeV carbon (C5+) ion irradiation. Efficient waveguide lasers with maximum output power of ~84 mW and a slope efficiency as high as ~43% at 1064 nm have been realized through optical pumping at 808 nm. For nonlinear waveguides, ridge waveguides have been fabricated in Nd:GdCOB crystals by using fs laser micromachining of 17 MeV carbon (C5+) ion irradiated planar waveguide. The SHG has been achieved through the ridge structures for the 1064→532 nm conversion.We developed the applications of fs laser writing for single and double cladding waveguide fabrications in crystalline materials and realized efficient waveguide lasers and SHGs in laser (Yb:YAG, Nd:YVO4) and nonlinear crystals (BiB3O6, Nd:GdCOB). The experimental results indicate fs laser micromachined cladding waveguides as highly efficient integrated laser sources in the integrated photonic systems and ideal platforms for unpolarized pumping as light sources or frequency-conversion-based phase-matching mechanisms. In addition, the double cladding waveguides show superior laser and SHG properties to the standard single claddings. Particularly, we have fabricated cladding waveguides with diameter of ~100 and 120 μm in Nd:YVO4 crystals, supporting good guidance in both of the two polarizations.The CW waveguide lasers at~1064 nm were realized under the optical pump at 808 nm, reaching the slope efficiency as high as 65% and the maximum output power of as high as 335 mW. In BiB3O6 cladding waveguides, the SHG based on birefringent Type I phase matching from 1064→532nm and 800→400 nm has been realized under CW and pulsed laser configuration. The μ-SH images of the waveguide’s cross section have revealed that the nonlinear response of original BiB3O6 crystal have been well preserved within the waveguide.Under 1064 nm (CW and pulsed) and 800 nm (CW) laser pumping, the guided-wave second harmonic generation of green light at 532 nm and violet light at 400 nm has been realized, with conversion efficiencies of 0.083% (green) and ~0.98%/W (violet) at CW regime and 25%(green) at pulsed regime. Double cladding structures consist of tubular central structures with 30 μm diameter and concentric larger size tubular claddings with diameters of 100-200 μm have been realized in Yb:YAG and Nd:GdCOB crystals. Single-mode CW laser oscillations at wavelength of 1030 nm have been realized in Yb:YAG double cladding waveguides through optical pump at 946 nm. The obtained maximum output power of the double-cladding waveguide lasers is ~80.2 mW with a slope efficiency as high as 62.9%.For Nd:GdCOB double cladding waveguides, confocal μ-PL and ~-SH images have revealed that both the fluorescence properties and the nonlinear response of the Nd:GdCOB crystal have been well preserved within the waveguide volume, which suggests the formed structures are promising candidates for the near future development of efficient SFD laser waveguides operating in the visible. The SHG based on birefringent Type I phase matching from 1064 to 532 nm has been realized under pulsed laser configuration, with conversion efficiency of 5.1% and maximum output peak power of 184 W from the inner core of double cladding, showing superior SHG performance to the standard single-cladding waveguides, which is benefited from the large area pump of outermost cladding.By using mask-assisted 500 keV protons implantation at a fluence of 6×1016 ions/cm2 we fabricated optical channel waveguides in ZnSe single crystal. The formed waveguides are with typical "enhanced well"+"negative barrier" refractive index profile. The numerical calculated modal profiles are in good agreement with the measured near-field intensity distribution of the guided light. The propagation loss of the channel waveguide is determined to be ~4 dB/cm after thermal annealing treatment in air.In addition, we demonstrate a novel family of 3D photonic microstructures monolithically integrated in crystal wafer produced by fs laser writing, capable of simultaneous light waveguiding and beam manipulation. In these guiding systems, tailoring of laser modes by both passive/active beam splitting and ring-shaped transformation are achieved by an appropriate design of refractive index patterns. Integration of graphene thin-layer as saturable absorber in the 3D laser structures allows for efficient passive Q-switching of tailored laser radiations which may enable miniature waveguiding lasers for broader applications. The results pave a way to construct complex integrated passive and active laser circuits in dielectric crystals by using fs laser written monolithic photonic chips.
Keywords/Search Tags:Optical channel waveguide, Dielectric crystal, Femtosecond laser micromachining, Ion implantation/irradiation, Waveguide lasers, Second harmonic generation
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