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Growth of Er3+‐doped 100‐nm thick LiNbO3(LN) films on Z‐cut LiTaO3(LT) single‐crystal by e‐beam evaporation and in‐situ anneal at 600–800°C was studied. Crystalline phase, crystallographic orientation, Er3+ spectroscopic properties, constituent element valence states, defects, Er3+ concentration and stoichiometry of the films were characterized. The results show that the films are in LN single‐crystal state and orient along with c crystallographic axis. Constituent elements Li, O and Er are present in their respective single valence states Li+, O2−, Er3+, while the Nb is in two valence states Nb5+ and Nb4+. The films have Li2O‐contents around congruent point. Antisite(NbLi4+) is the major defect. Its concentration decreases, while Li2O‐content, crystallinity degree, and fluorescence intensity increase with increased growth temperature. The presence of Er3+ in the grown films is in the form of LN crystalline phase. Its concentration is considerably lower than that in initial target material because of lower e‐beam evaporation efficiency of heavy Er3+. [ABSTRACT FROM AUTHOR]

Mode size converter enables to match two waveguides with different mode sizes. It has already become an indispensable component in modern integrated photonics. Here, we propose a strategy for implementing an on-chip mode size converter consisted of a gradient geometry metasurface and two cascaded waveguide tapers on the lithium-niobate-on-insulator platform. The metasurface, which is formed by a series of length-graded silica nanostrips embedded into LN waveguide, functions as focusing of incident wave and exaltation of conversion efficiency. A mode size converter that enables to match a fundamental TE 00 mode in a 10-μm-wide waveguide with that in a 0.8-μm-wide one was simulated to verify the feasibility of the strategy. We show that the device is not only compact with a length of 22.9 μm, but also shows an ultra-broad operation band as much as 1150 nm (from 1100 to 2250 nm), over which the conversion efficiency keeps >90 %. In addition, we show that the device has a good robustness to fabrication deviations. The strategy proposed here can be flexibly extended to other waveguide cross-section sizes and other guided modes. • A strategy is proposed for realizing an on-chip ultra-broadband mode size converter. • It uses gradient geometry metasurface and cascaded tapers on LNOI platform. • It is proved by simulating a device connecting a wide waveguide with a narrow one. • The device is valid for on-chip use and has an ultra-broad band as much as 1150 nm. • The strategy has a good scalability, the device has an excellent fabrication tolerance. [ABSTRACT FROM AUTHOR]

• Polarization beam splitter(PBS) based on LNOI triple-waveguide coupler is proposed. • A plasmonic waveguide is used to exalt modal birefringence and control TM mode coupling. • Good performance of high polarization extinction ratio and low insertion loss is expected. • The device has compact size for on-chip application and definite fabrication tolerance. • Present work offers a promising strategy for compact, efficient PBS based on LNOI. A compact polarization beam splitter based on a triple-waveguide plasmonic directional coupler is proposed and modeled. The directional coupler consists of two usual lithium-niobate-on-insulator (LNOI) ridge waveguides and a metal-capped plasmonic LNOI waveguide located between them. It utilizes stronger polarization dependence of modal birefringence of a plasmonic waveguide. By appropriately designing the plasmonic waveguide geometry, the TM component wave meets the resonant coupling condition and outputs at cross port. Instead, the TE component wave does not meet the resonant coupling condition, cannot be resonant in the triple-waveguide directional coupler and outputs at through port directly. Excellent performance is expected for an optimal device. First, the device shows an ultra-high polarization extinction ratio(PER) of 28.3 dB and an ultra-low insertion loss(IL) of 0.06 dB for the TE-polarized wave, and of 31.7 dB and of 0.31 dB in order for the TM-polarized wave. Second, the working bandwidth with PER > 25 dB and IL < 0.08 dB is 100 nm (1500–1600 nm) for the TE-polarized wave, and that with PER > 20 dB and IL < 0.4 dB is 30 nm (1534–1564 nm) for the TM-polarized wave. Third, the device has a compact structure of 33 μm in length and exhibits definite fabrication tolerance. Overall, the proposed device exhibits ultra-high PER and ultra-low IL, and has a compact size suitable for on-chip use. A comparison shows that present device exhibits advantage in IL and/or size over that based on other LNOI structures or the SOI platform. [ABSTRACT FROM AUTHOR]

A theoretical study has been performed on 1.76 μm on-chip amplification characteristics of Tm3+ in a rib waveguide of LiNbO 3 on insulator (Tm:LNOI) under 795 nm wavelength pumping. Steady-state rate equations and propagation equations of signal and pump waves are established on the basis of a three-level model of Tm3+ system with the cross relaxation process 3H 4 →3F 4 : 3H 6 →3F 4 included. The validity of the three-level model is demonstrated by referencing previously reported experiment result of Er3+-doped LNOI waveguide amplifier and by examining the effect of Tm3+ blue upconversion emissions on the 1.76 μm amplification. Besides the relations of signal gain to propagation length, launched pump power and input signal power, the effects of waveguide cross-section geometry and Tm3+ concentration on the gain characteristics are also studied. The results show that the waveguide cross-section geometry affects definitely the gain performance via its effect on mode field. The Tm3+ concentration affects the signal gain also via its effect on the 3H 4 →3F 4 : 3H 6 →3F 4 cross relaxation coefficient, besides the conventional effect that higher active ion concentration yields larger signal gain. A comparison with conventional Ti4+(Zn2+)-diffused Tm3+-doped LN [Ti(Zn):Tm:LN] waveguide shows that the Tm:LNOI waveguide has much better gain performance than the Ti(Zn):Tm:LN waveguide, including much stronger pump power and propagation distance dependences of signal gain, much higher signal gain and much lower threshold pump power. These features are associated with significantly increased Tm3+ population inversion extent due to ultra-compact mode field and larger overlapping factor of mode field and Tm3+ population profiles in the Tm:LNOI waveguide. • 1.8 μm amplification in Tm:LNOI waveguide is simulated based on three-level model. • Quantitative relations of signal gain to controllable parameters are established. • Tm:LNOI amplifier is more efficient than Ti(Zn):Tm:LN due to ultra-compact mode. • Waveguide cross-section geometry affects definitely gain via its effect on mode field. • Tm3+ concentration affects gain via its effect on cross relaxation process. [ABSTRACT FROM AUTHOR]