X-cut Lithium Niobate Thin Film Research Articles

Scalable and Reconfigurable Continuously Tunable Lithium Niobate Thin Film Delay Line Using Graphene Electrodes

We propose a novel continuously tunable delay line in X-cut lithium niobate thin film driven by graphene electrodes, featuring low power consumption and low half-wave voltage-length product. The use of the graphene electrodes combined with the air slots makes the device quite low power consumption. Our designed device, which has a footprint of 8.9 mm × 1.9 mm, is capable of providing a continuously tunable delay range from 0 to 100 ps with minimum 3 dB bandwidth of >20 GHz. By optimizing the width of the waveguide core and the distance between the graphene and the waveguide core, we can obtain the low power consumption and low voltage - length product <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> = 1.12 V·cm at 1550 nm when <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</i> = 2 μm and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> = 0.3 μm. In addition, the graphene electrode has negligible light absorption with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</i> = 2 μm and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> = 0.3 μm, the switching power is 151 mW without air slots, and the switching power is 95.36 mW with air slots etching depth of 5.3 μm formed on both sides of the waveguides. And further, the air slots formed on both sides and at the bottom of the waveguides make a low switching power of 2.85 mW.

High-Performance Electro-Optical Mach–Zehnder Modulators in a Silicon Nitride–Lithium Niobate Thin-Film Hybrid Platform

We analyzed a Mach–Zehnder electro-optical modulator based on a silicon nitride strip–loaded waveguide on 0.5 μm thick x-cut lithium niobate thin film. The optical and radio frequency parameters for two different modulator structures (Type I: packaged with 2 μm thick SiO2 and Type II: unpackaged) were simulated, calculated, and optimized. The Optical parameters included the single-mode conditions, effective indices, the separation distance between the electrode edge and the Si3N4-strip-loaded edge, optical power distribution, bending loss, optical field distribution, and half-wave voltage. The radio frequency parameters included the characteristic impedance, attenuation constant, radio frequency effective index, and −3 dB modulation bandwidth. According to the numerical simulation and theoretical analysis, the half-wave voltage product and the −3 dB modulation bandwidth were, respectively, 2.85 V·cm and 0.4 THz for Type I modulator, and 2.33 V·cm and 1.26 THz for Type II modulator, with a device length of 3 mm.

Optimization of periodic poling of x-cut lithium niobate thin film

Periodic poling of lithium niobate thin film (lithium niobate on insulator, LNOI) is a critical step in nonlinear optics applications using quasi-phase matching. The electric field method is widely used to achieve periodic poling. A problem frequently encountered in congruent LNOI periodic poling is some regions which were not reversed. In this paper, repeated poling process was used to deal with the problem mentioned above. When the poling process was repeated for 6 times, the poling fidelity was nearly 100% for a periodicity of 3.8 μm. The results provide useful information for achieving high-fidelity poling in LNOI.

Second harmonic generation by quasi-phase matching in a lithium niobate thin film

Lithium niobate thin film becomes a promising material platform for integrated nonlinear optics. Quasi-phase matching is an important way to realize optical frequency conversion. Due to the strong dispersion of the high-refractive-index-contrast lithium niobate thin film waveguides, a small poling period (several micrometers) is needed, which becomes a challenge for the poling process. In this paper, a periodic reversed domain with a period of 3.8 µm was realized by poling with an external electric field in the x-cut lithium niobate thin film. The reversed domains were characterized by piezoresponse force microscopy and the confocal Raman spectroscopy. The second harmonic generation was realized in the ridge waveguides, and the normalized conversion efficiency was 684%W−1cm−2. The waveguide loss, the irregularity of the reversed domain, and the inhomogeneities effective index along the waveguide were analyzed to be the main sources of the conversion efficiency decrease. The simulations showed that the phase matching was very sensitive to waveguide geometry. This work provided useful information for studying second harmonic generation in the lithium niobate on insulator.

Two-dimensional grating coupler on an X-cut lithium niobate thin-film.

A two-dimensional grating coupler for coupling light between a standard single-mode fiber and ridge waveguides on an X-cut lithium niobate thin-film is designed and demonstrated. Using circular holes for grating cells, simulated coupling losses reach -3.88 dB at 1550 nm and -5.78 dB at 1563 nm with 1-dB bandwidths of 49 nm and 45 nm for P-polarized and S-polarized light inputs, respectively. Experimentally, peak coupling losses of -5.13 dB at 1561 nm and -7.6 dB at 1568 nm are obtained for P-polarized and S-polarized light inputs, respectively, and corresponding 1 dB bandwidths are about 30 nm. An approach to improve the coupling performance of the grating coupler is also proposed using two crossing ellipses as grating cells as well as a bottom metal reflector. The coupling loss and the polarization dependent loss are decreased to around -3.4 dB and 0.44 dB, respectively.

Polarization Coupling of $X$-Cut Thin Film Lithium Niobate Based Waveguides

Thin film lithium niobate (LN) shows great potentials for highly compact passive and active devices. As LN is an anisotropic material, waveguides made on it exhibit different mode properties from those on conventional isotropic materials. We study the effective refractive indices of fundamental modes of two polarizations in etched ridge waveguides on an X-cut LN thin film. Mode hybridization phenomenon, where the effective refractive indices of the two polarizations are close, is analyzed in detail with different structural parameters. Transmission through a 90° bend, which is a typical routing element for a photonic chip, is simulated. Significant polarization coupling related to the mode evaluation through the bend is observed, and becomes the dominant fact limiting the performance of this element. In order to ensure a low bending loss, the required bending radius is much larger than that for waveguides on an in-plane isotropic material, e.g. a Z-cut LN thin film. Mode hybridization also plays an important role in the performance of the 90° bend, which should be avoided. Generally, decreasing the thickness of the LN thin film, working at a longer wavelength, or confining the propagation angle on a chip would help to decrease the polarization coupling.

Integrated electro-optic modulators in x-cut lithium niobate thin film

Integrated optics is poised to revolutionize several data communication applications. The development of high performance electro-optic modulators is essential for the technology to be viable. In this paper, single-mode electro-optic modulators in x-cut lithium niobate thin film are designed, simulated, and analyzed. The single-mode conditions of lithium niobate ridge waveguides are analyzed in detail. The design parameters of Y-branch and separation distances between electrodes and ridge waveguides are optimized. The half-wave voltage of the electro-optic modulator with device lengths of 10 mm is determined. The half-wave voltage-length product is about 2.2 V·cm indicating the efficient electric-optical modulation. This study is helpful for understand of electro-optic modulator structures in lithium niobate thin film, as well as for the fabrication of high-performance and multifunctional electro-optic modulation devices.

Design and Optimization of Proton Exchanged Integrated Electro-Optic Modulators in X-Cut Lithium Niobate Thin Film

In this study, we designed, simulated, and optimized proton exchanged integrated Mach-Zehnder modulators in a 0.5-μm-thick x-cut lithium niobate thin film. The single-mode conditions, the mode distributions, and the optical power distribution of the lithium niobate channel waveguides are discussed and compared in this study. The design parameters of the Y-branch and the separation distances between the electrodes were optimized. The relationship between the half-wave voltage length production of the electro-optic modulators and the thickness of the proton exchanged region was studied.

Monolithic $1\times4$ Reconfigurable Electro-Optic Tunable Interleaver in Lithium Niobate Thin Film

We propose and experimentally demonstrate a monolithically integrated $1\times 4$ reconfigurable electro-optic tunable interleaver in X-cut lithium niobate thin film by cascading three asymmetrical Mach-Zehnder interferometers into two stages. Over a wide wavelength range from 1530 nm to 1605 nm, our fabricated device exhibits polarization-insensitive center wavelengths and channel spacing of ~50 GHz, slightly different switching voltages of 4.5 V and 6.7 V, and extinction ratios of 8–15 dB and 7–13 dB for the transverse electric (TE) and the transverse magnetic (TM) polarized input light, respectively.

GHz Broadband SH0 Mode Lithium Niobate Acoustic Delay Lines.

We present the first group of GHz broadband SH0 mode acoustic delay lines (ADLs). The implemented ADLs adopt unidirectional transducer designs in a suspended X-cut lithium niobate thin film. The design space of the SH0 mode ADLs at GHz is first theoretically investigated, showing that the large coupling and sufficient spectral clearance to adjacent modes collectively enable the broadband performance of SH0 delay lines. The fabricated devices show 3-dB fractional bandwidth ranging from 4% to 34.3% insertion loss between 3.4 and 11.3 dB. Multiple delay lines have been demonstrated with center frequencies from 0.7 to 1.2 GHz, showing great frequency scalability. The propagation characteristics of SH0 in lithium niobate thin film are experimentally extracted. The demonstrated ADLs can potentially facilitate broadband signal processing applications.

Gigahertz Low-Loss and Wideband S0 Mode Lithium Niobate Acoustic Delay Lines.

We present the first group of gigahertz S0 mode low loss and wideband acoustic delay lines (ADLs). The ADLs use a single-phase unidirectional transducers (SPUDT) design to launch and propagate the S0 mode in an X-cut lithium niobate thin film with large electromechanical coupling and low damping. In this work, the theoretical performance bounds of S0 mode ADLs are first investigated, significantly surpassing those in state-of-the-art. The design tradeoffs of S0 mode ADLs, when scaled to the gigahertz frequency range, are also discussed. The fabricated miniature ADLs show a fractional bandwidth (FBW) of 4% and a minimum insertion loss (IL) of 3.2 dB, outperforming the incumbent surface acoustic wave (SAW) counterparts, and covering a wide range of delays from 20 to 900 ns for digitally addressable delay synthesis. Multiple ADLs with center frequencies from 0.9 to 2 GHz have been demonstrated, underscoring their great frequency scalability. The propagation properties of S0 waves in lithium niobate at the gigahertz range are experimentally extracted. The demonstrated ADLs can potentially enable wide-range and high-resolution delay synthesis that is highly sought after for the self-interference cancellation in full-duplex radios.

Compact and electro-optic tunable interleaver in lithium niobate thin film.

We propose and experimentally demonstrate a compact and electro-optic (EO) tunable interleaver in X-cut lithium niobate thin film using an asymmetrical Mach-Zehnder interferometer configuration. Our typical fabricated device has an EO interactive length of ∼1.35 mm and a total length of ∼4.0 mm. Over a wide wavelength range from 1528 to 1605nm, the device exhibits polarization-insensitive center wavelengths and channel spacing of ∼49.7 GHz, but a slightly different extinction ratio of 10-20 and 12-23dB, and electrical wavelength tuning sensitivity of ∼18 and ∼16 pm/V for the transverse electric- and transverse magnetic-polarized input light, respectively. The proposed interleaver also has the potential to be used as a tunable filter or a wavelength-selective switch.

Periodic domain inversion in x-cut single-crystal lithium niobate thin film

We report the fabrication of periodically poled domain patterns in x-cut lithium niobate thin-film. Here, thin films on insulator have drawn particular attention due to their intrinsic waveguiding properties offering high mode confinement and smaller devices compared to in-diffused waveguides in bulk material. In contrast to z-cut thin film lithium niobate, the x-cut geometry does not require back electrodes for poling. Further, the x-cut geometry grants direct access to the largest nonlinear and electro-optical tensor element, which overall promises smaller devices. The domain inversion was realized via electric field poling utilizing deposited aluminum top electrodes on a stack of LN thin film/SiO2 layer/Bulk LN, which were patterned by optical lithography. The periodic domain inversion was verified by non-invasive confocal second harmonic microscopy. Our results show domain patterns in accordance to the electrode mask layout. The second harmonic signatures can be interpreted in terms of spatially, overlapping domain filaments which start their growth on the +z side.

Channel waveguides and y-junctions in x-cut single-crystal lithium niobate thin film.

Proton exchanged channel waveguides in x-cut single-crystal lithium niobate thin film could avoid optical leakage loss which existed in the z-cut case. Indicated by simulations, the mechanism and condition of the optical leakage loss were studied. The light energy in the exchanged layer and the mode sizes were calculated to optimize the parameters for fabrication. By a very short time (3 minutes) proton exchange process without anneal, the channel waveguide with 2 μm width and 0.16 μm exchanged depth in the x-cut lithium niobate thin film had a propagation loss as low as 0.2 dB/cm at 1.55 μm. Furthermore, the Y-junctions based on the low-loss waveguide were designed and fabricated. For a Y-junction based on the 3 μm wide channel waveguide with 8000 μm bending radius, the total transmission could reach 85% ~90% and the splitting ratio maintained at a stable level around 1:1. The total length was smaller than 1 mm, much shorter than the conventional Ti-diffused and proton exchanged Y-junctions in bulk lithium niobate.