Low Loss High-performance On-chip Lithium Niobate Photonic Devices

As a multitude of groundbreaking technologies incessantly materialize,including artificial intelligence,5G communication,lidar,sensitive light sensing,and photonic quantum information,the demand for high-bandwidth,low-power consumption,and cost-effective information processing solutions has surged exponentially.Electronic integrated chips,which serve as the underpinning platform for high-speed information processing,experienced rapid development during the latter half of the 20th century.As electronic integrated chips continuously advance,the linewidth of integrated circuits fabricated through traditional lithography processes gradually is approaching to the atomic scale,resulting in electronic chip performance approaching limits in terms of bandwidth and power consumption.Due to photons’unique advantages,including high bandwidth,low loss,and parallel transmission,the development of integrated photonic chips（abbreviated as PICs,for Photonic Integrated Circuits）has emerged as the crux of next-generation information processing key technologies,presently becoming a cutting-edge research hotspot domestically and internationally.To realize high-density integrated photonic chips,it is essential to integrate various optical components with different functionalities on a photonic platform,including light sources,electro-optic modulators,optical amplifiers,and photodetectors.Lithium niobate（Li Nb O₃）crystals exhibit stable physical and chemical properties,substantial nonlinear coefficients(d₃₃=?41.7±7.8 pm/V),and large electro-optic coefficients,with a wide optical transparency window（0.4～5.5μm）,Since the birth of bulk crystals,lithium niobate has been highly regarded in the field of photonics device manufacturing and has become one of the most promising platform materials for photonics chips.However,lithium niobate is difficult to micro-nano etch,conventional lithium niobate optical waveguide fabrication methods often employ proton exchange or ion diffusion processes,resulting in low refractive index contrast and large waveguide cross-sectional dimensions,impeding high-density integration and affecting lithium niobate’s application as a photonic chip material.Recently,with the advent and commercialization of lithium niobate on insulator（LNOI）thin films and the maturation of micro-nano processing techniques,numerous high-performance photonic devices based on LNOI thin film tightly bound waveguide systems have been successfully fabricated and demonstrated,including ultra-low loss optical waveguides,ultra-high quality factor optical microcavities,and high-performance electro-optic modulators.Simultaneously,the reduced size and optical mode of tightly bound waveguide systems present substantial challenges for the fabrication process of large-scale,high-quality,high-efficiency,and low-energy consumption LNOI photonic chips.The most common fabrication methods for LNOI photonic chips involve electron beam lithography or ultraviolet lithography combined with reactive ion etching technology.Electron beam lithography offers high processing precision but suffers from relatively complex processes,lower processing efficiency,expensive equipment,high maintenance costs,and limited writing field size,hindering the efficient fabrication of large-scale lithium niobate photonic chips.Ultraviolet lithography boasts a large exposure area and low manufacturing costs,but the fabricated optical waveguide devices exhibit low sidewall smoothness and relatively higher optical transmission losses.To address the challenges in LNOI photonic chip fabrication,the femtosecond laser lithography assisted chemical mechanical polishing technique,which the author participated in improving,features a large writing field,high flexibility,high fabrication efficiency,high processing consistency,and extremely low sidewall roughness.This technique enables the rapid and efficient fabrication of low-loss,high-performance lithium niobate photonic devices.This thesis work is based on the femtosecond laser direct writing-assisted chemical mechanical polishing technique,investigating the principles,design,fabrication,and testing characterization of LNOI thin film integrated photonic chips.The innovative achievements of the thesis are as follows:1.The feasibility of fabricating Mach-Zehnder（MZ）electro-optic modulators on LNOI using femtosecond laser direct writing-assisted chemical mechanical polishing technology has been verified.Through the optimized design of multimode interference（MMI）beam splitters and tapered edge couplers,an MZ modulator with a 4 cm long modulation arm has achieved a low half-wave voltage of 0.84 V,a fiber-to-fiber insertion loss of 7.6 d B,and an extinction ratio of 25 d B.Subsequently,by optimizing the design of LNOI waveguide configurations,edge couplers,and traveling-wave electrodes,a high-performance MZ electro-optic modulator featuring a half-wave voltage-length product of 2.16 V?cm,a fiber-to-fiber insertion loss of 2.6 d B,and an electro-optic modulation bandwidth of 50 GHz has been realized based on femtosecond laser direct writing-assisted chemical mechanical polishing technology.2.Using femtosecond laser direct writing-assisted chemical mechanical polishing technology,highly smooth surface and sidewall optical waveguide structures were fabricated on erbium-doped lithium niobate thin films.Drawing inspiration from the double-cladding structure of fiber amplifiers,a waveguide amplifier was created by depositing a tantalum oxide thin film with a similar refractive index on a 10 cm long erbium-doped lithium niobate waveguide.Pumped by a semiconductor laser with a center wavelength of 980 nm,a small signal on-chip net gain of over 20 d B was achieved at a signal wavelength of 1532 nm.Compared to waveguides fabricated on the same wafer without the deposition of the tantalum oxide cladding layer,waveguides with the tantalum oxide cladding layer exhibited higher gain characteristics.This experimental phenomenon has been explained by a theoretical model based on waveguide mode field distribution and the steady-state response of erbium ions.By optimizing the cladding layer to control the mode field distribution,the adverse effects of erbium ion quenching can be reduced,effectively enhancing the interaction between the optical field and erbium ions in the waveguide.3.By optimizing the process parameters,the challenge of fabricating coupler devices with waveguide gaps smaller than 1μm using femtosecond laser direct writing-assisted chemical mechanical polishing technology was successfully overcome.On erbium-doped lithium niobate thin films,a single-frequency microring laser with an output wavelength of 1531 nm was fabricated using femtosecond laser direct writing-assisted chemical mechanical polishing technology.Experimental measurements showed that the laser threshold power of the microring laser was approximately 24.5mW.