Integrated lithium niobate microwave photonic processing engine.

Integrated microwave photonics (MWP) is an intriguing technology for the generation, transmission and manipulation of microwave signals in chip-scale optical systems1,2. In particular, ultrafast processing of analogue signals in the optical domain with high fidelity and low latency could enable a variety of applications such as MWP filters3-5, microwave signal processing6-9 and image recognition10,11. An ideal integrated MWP processing platform should have both an efficient and high-speed electro-optic modulation block to faithfully perform microwave-optic conversion at low power and also a low-loss functional photonic network to implement various signal-processing tasks. Moreover, large-scale, low-cost manufacturability is required to monolithically integrate the two building blocks on the same chip. Here we demonstrate such an integrated MWP processing engine based on a 4 inch wafer-scale thin-film lithium niobate platform. It can perform multipurpose tasks with processing bandwidths of up to 67 GHz at complementary metal-oxide-semiconductor (CMOS)-compatible voltages. We achieve ultrafast analogue computation, namely temporal integration and differentiation, at sampling rates of up to 256 giga samples per second, and deploy these functions to showcase three proof-of-concept applications: solving ordinary differential equations, generating ultra-wideband signals and detecting edges in images. We further leverage the image edge detector to realize a photonic-assisted image segmentation model that can effectively outline the boundaries of melanoma lesion in medical diagnostic images. Our ultrafast lithium niobate MWP engine could provide compact, low-latency and cost-effective solutions for future wireless communications, high-resolution radar and photonic artificial intelligence.

Design and resonator-assisted characterization of high-performance lithium niobate waveguide crossings.

Waveguide crossings are elementary passive components for signal routing in photonic integrated circuits. Here, we design and characterize two multimode interferometer (MMI)-based waveguide crossings to serve the various routing directions in the anisotropic x-cut thin-film lithium niobate (TFLN) platform. To address the large measurement uncertainties in traditional cut-back characterization methods, we propose and demonstrate a resonator-assisted approach that dramatically reduces the uncertainty of insertion loss measurement (< 0.021 dB) and the lower bound of crosstalk measurement (-60 dB) using only two devices. Based on this approach, we demonstrate and verify TFLN waveguide crossings with insertion losses of < 0.070 dB and crosstalk of < -50 dB along all three routing directions at 1550 nm. The low-loss and low-crosstalk waveguide crossings in this work, together with the simple and efficient characterization strategy, could provide important layout design flexibility for future large-scale classical and quantum TFLN photonic circuits.

Cost-effective fiber-to-lithium niobate chip coupling using a double-side irradiation self-written waveguide.

In recent years, integrated lithium niobate (LN) chips have been widely used for developing a variety of photonic devices, such as high-speed electro-optical (EO) modulators and frequency comb generators. A major challenge for their practical applications is the high coupling loss between micrometer-scale LN waveguides and optical fibers. Lensed fibers and special taper structures are commonly used to tackle the coupling issue. However, in some situations, these approaches may increase the overall complexity and cost of design, fabrication, and alignment. Here, we propose using the self-written waveguide (SWW), an optical waveguide induced by light irradiation, to cope with this coupling issue. The approach can apply in connecting a single-mode fiber (SMF) to any waveguide surface in principle, even with a large mode-field mismatch, significantly alleviating the tight alignment requirements typically needed for end-fire coupling into integrated waveguides. Our study demonstrates that the coupling loss between a SMF with a mode-field diameter (MFD) of 4.4 µm and a sub-micrometer LN rib waveguide could be dramatically reduced from an initial value of -14.27 dB to -5.61 dB, after double-side irradiated SWW formation. Our proposed approach offers a potential solution for achieving a cost-effective and flexible fiber-to-LN chip optical interconnect.

Spectral Engineering of Optical Microresonators in Anisotropic Lithium Niobate Crystal.

On-chip optical microresonators are essential building blocks in integrated optics. The ability to arbitrarily engineer their resonant frequencies is crucial for exploring novel physics in synthetic frequency dimensions and practical applications like nonlinear optical parametric processes and dispersion-engineered frequency comb generation. Photonic crystal ring (PhCR) resonators are a versatile tool for such arbitrary frequency engineering, by controllably creating mode splitting at selected resonances. To date, these PhCRs have mostly been demonstrated in isotropic photonic materials, while such engineering can be significantly more complicated in anisotropic platforms that often offer more fruitful optical properties. Here, the spectral engineering of chip-scale optical microresonators is realized in the anisotropic lithium niobate (LN) crystal by a gradient design that precisely compensates for variations in both refractive index and perturbation strength. Controllable frequency splitting is experimentally demonstrated at single and multiple selected resonances in LN PhCR resonators with different sizes, while maintaining high quality-factors up to 1 × 106. Moreover, a sharp boundary is experimentally constructed in the synthetic frequency dimension based on an actively modulated x-cut LN gradient-PhCR, opening up new paths toward the arbitrary control of electro-optic comb spectral shapes and exploration of novel physics in the frequency degree of freedom.