Lithium niobate on insulator (LNOI) technology is revolutionizing the lithium niobate industry, enabling higher performance, lower cost and entirely new devices and applications. The availability of LNOI wafers has sparked significant interest in the platform for integrated optical applications, as LNOI offers the attractive material properties of lithium niobate, while also offering the stronger optical confinement and a high optical element integration density that has driven the success of more mature silicon and silicon nitride (SiN) photonics platforms. Due to some similarities between LNOI and SiN, established techniques and standards can readily be adapted to the LNOI platform including a significant array of interface approaches, device designs and also heterogeneous integration techniques for laser sources and photodetectors. In this contribution, we review the latest developments in this platform, examine where further development is necessary to achieve more functionalities in LNOI integrated optical circuits and make a few suggestions of interesting applications that could be realized in this platform. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Status and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated Circuits

Lithium niobate on insulator (LNOI) provides a platform for the fundamental physics investigations and practical applications of integrated photonics. However, as an indispensable building block of integrated photonics, lasers are in short supply. In this paper, erbium-doped LNOI laser in the 1550-nm band was demonstrated in microdisk cavities with high quality factors fabricated in batches by UV exposure, inductively coupled plasma reactive ion etching, and chemomechanical polishing. The threshold and conversion efficiency of the erbium-doped LNOI microdisk laser were measured to be lower than 1 mW and 6.5×10−5%, respectively. This work will benefit the development of integrated photonics based on LNOI.

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Microdisk lasers on an erbium-doped lithium-niobite chip

We have developed a rigorous phenomenological model for analyzing rare-earth doped waveguide lasers. The model is based on time-dependent laser rate equations for an arbitrary rare-earth-doped laser host with multiple energy levels. The rate equations are coupled with the laser signal and pump photon flux equations that have time-dependent boundary conditions. The formulation results in a large and stiff set of transcendental and coupled differential equations that are solved using finite difference discretization and the method of lines. Solutions for the laser signal power, pump power, and populations of ion energy levels as functions of space and time are obtained for waveguide lasers. We have used the model to predict the CW characteristics and Q-switched performance of waveguide lasers in lithium niobate pumped by a 980-nm source. Our analysis shows that hole burning can occur in erbium-doped lithium niobate lasers because of the intensity variation across guided transverse modes. We have predicted that Q-switch pulse peak powers can exceed 1 kW with pulsewidths less than 1 ns. Moreover, we have compared the CW and Q-switched performance of 980-nm pumped waveguide lasers and 1480 nm pumped waveguide lasers. An analysis of the effects of host- and fabrication-dependent parameters on CW 980-nm pumped lasers is included. These parameters include cooperative upconversion, excited state absorption, doping concentration, excess waveguide loss, cavity length, and mirror reflectance values. We demonstrate good quantitative agreement with waveguide laser experimental data obtained in our laboratory and with results from the literature.

Time-dependent modeling of erbium-doped waveguide lasers in lithium niobate pumped at 980 and 1480 nm

On‐chip light amplification with integrated optical waveguide fabricated on erbium‐doped thin‐film lithium niobate on insulator (TFLNOI) is demonstrated using the photolithography‐assisted chemomechanical etching (PLACE) technique. A maximum internal net gain of 18 dB is measured on a waveguide length of 3.6 cm, indicating a differential gain per unit length of 5 dB cm−1.

On-Chip Integrated Waveguide Amplifiers on Erbium-Doped Thin-Film Lithium Niobate on Insulator

We have investigated the polarization dependence of both the population inversion of the /sup 4/I/sub 13/2/ state and the green upconversion to the /sup 4/S/sub 3/2/ state in Er:LiNbO/sub 3/ optical fiber amplifier when codoped with Yb and pumped around 980 nm. The presence of Yb/sup 3+/ enhances the population of the /sup 4/I/sub 13/2/ state of the Er/sup 3+/ ions when pumped at /spl sigma/-polarization (>2.5 times) and at /spl pi/-polarization (>5.5 times), and introduces a broad useful pump band around 940-960 nm at /spl sigma/-polarization, relaxing fabrication tolerances on semiconductor pump sources. An enhancement of green upconversion is also observed by the Yb codoping. Lifetime measurements indicate that, as in glass substrates, nonradiative energy transfer is responsible for these enhancements.

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Polarization-dependent enhancement of population inversion and of green upconversion in Er:LiNbO 3 by Yb codoping

We have investigated by numerical modeling the relative efficacy of 980-nm and 1484-nm-pumped guided-wave optical amplifiers in Er-doped LiNbO/sub 3/. The shorter wavelength produces a larger differential gain, but the waveguide amplifier suffers from the photorefractive effect if no precautions are taken. Model calculations show that an optical gain of 2.5 dB/cm can be achieved at 80-mW 980-nm pump power, compared to 1.3 dB/cm when pumped at 1484 nm. Experimental results using 80 mW 980 nm give a gain of /spl sim/2.2 dB/cm (minus propagation loss). This gain, however, is degraded by photorefractive damage, which we find is not only due to the 980-nm pump light, but also to the green up-converted light, the latter having the larger effect. Several techniques were shown to reduce the photorefractive damage. ZnO indiffused waveguides in MgO-doped LiNbO/sub 3/ substrates proves to be the most effective. In bulk-doped Er:Mg:LiNbO/sub 3/ samples with ZnO waveguides, an optical gain of 1.6 dB/cm (minus propagation loss) was observed with 54 mW of out-coupled pump power.

C.-H. Huang

Papers

980-nm-pumped Er-doped LiNbO/sub 3/ waveguide amplifiers: a comparison with 1484-nm pumping