For lithium niobate, electro-optics and acousto-optics are the most attractive areas due to the excellent electro-optic, piezoelectric, and photoelastic properties of the material. The past decade has witnessed tremendous breakthroughs of nanophotonic devices based on the emerging thin-film lithium niobate, making it a highly promising platform for new-generation integrated photonics. In this perspective, we focus on the electro-optics and acousto-optics in lithium niobate, reviewing the most recent advances and major challenges, meanwhile providing our insight on the exciting prospect of this emerging field. 

Perspective on Lithium-Niobate-on-Insulator Photonics Utilizing the Electro-optic and Acousto-optic Effects

We propose and experimentally demonstrate a polarization multiplexed silicon optical phased array (OPA) with a wide scanning range. The two polarization states share the same power splitter tree and the phase shifter array. A polarization switch is introduced in front of the power splitter tree to manipulate the polarization state of the light in OPA. Through a polarization splitter-rotator (PSR), the light of two polarization states propagates into the superlattice grating antenna array. The wavelength tuning efficiency could be doubled by optimizing the parameters of the waveguide grating. We demonstrate the scheme on the commonly used 220 nm silicon-on-insulator (SOI) platform. Experimental results indicate that the 24.8° vertical scanning range could be realized with a high wavelength tuning efficiency of 0.31°/nm. The measured field of view (FOV) is 24.8 × 60°. 

Polarization multiplexing silicon photonic optical phased array with a wide scanning range.

A high-performance lithium-niobate photonic filter for 50G passive optical networks is proposed and demonstrated for the first time. The present photonic filter consists of three wavelength channels of 1270, 1300, and 1342 nm, realized by introducing two-mode (de)multiplexers as well as multimode waveguide gratings (MWGs) in cascade. In experiments, the presented photonic filters are made of a 400 nm-thick X-cut lithium-niobate-on-insulator, exhibiting box-like spectral responses with ultralow excess losses of <0.2 dB for the wavelength channels. The crosstalk between the two channels of 1270/1300 is as low as ∼−30 dB and the measured sidelobe suppression ratio for the channel of 1342 nm is about 20 dB. Furthermore, all the channels have achieved the channel bandwidths and the center wavelengths as desired. In addition, the temperature sensitivity of the channel wavelength is as low as 35 pm/°C and the measured eye diagrams are open with high quality for 50 Gbps data. The presented high-performance lithium-niobate photonic filter can work with higher bit rates and can also be integrated easily with other lithium-niobate photonic devices, such as optical modulators, showing great potential for high-capacity data transmissions. 

First Realization of a Three-Channel Lithium-Niobate Photonic Filter for 50G Passive Optical Networks

Abstract A twelve-channel local-area-network (LAN) wavelength-division multiplexing (LWDM) filter is proposed and demonstrated with a uniform channel spacing of 4.5 nm (800 GHz) in the O-band of 1270–1330 nm by using x-cut lithium-niobate-on-insulator (LNOI) photonic waveguides for the first time. The present LWDM filter consists of twelve single-channel bandpass filters based on multimode waveguide gratings (MWGs) assisted with a TE 0 /TE 1 mode (de)multiplexer. In particular, two stages of MWGs in cascade are introduced for each single-channel bandpass filter, in order to achieve high sidelobe suppression ratios, thus reducing interchannel crosstalk. For the fabricated twelve-channel LWDM filter, all the channels have very excellent box-like spectral responses with low excess losses of ∼0.6 dB, broad 1-dB bandwidths of ∼2.9–3.4 nm (which is close to 75 % of the channel spacing), and ultra-low interchannel crosstalk of &lt;−40 dB in experiments. In addition, the present device is highly tolerant to the random variations of the etching depth (±20 nm) and the grating waveguide width (±20 nm) of the LNOI photonic waveguides, showing great potential for high-capacity WDM systems. 

Twelve-channel LAN wavelength-division multiplexer on lithium niobate

Lithium niobate on insulator (LNOI) has emerged as a promising platform for photonic integrated circuits (PICs) due to the excellent electro‐optic (EO) properties of lithium niobate (LN). However, such a platform still lacks many critical passive components with superior performance for constructing PICs. The 3 dB power splitters are one of the important passive components in PICs for separating and combining optical energy to construct the other crucial photonic devices such as Mach–Zehnder modulators, optical switches, and optical phased arrays, et al. In this contribution, an ultra‐broadband and compact 3 dB power splitter is proposed and demonstrated using a tapered subwavelength grating (SWG)‐assisted Y‐branch on a silicon nitride (Si3N4) loaded LNOI platform. The SWG is used to alleviate fabrication challenges and reduce radiation loss while increasing the evanescent coupling. The simulation result shows an excellent operation bandwidth of 800 nm (from 1.2 to 2.0 µm) at the insertion loss (IL) of 0.12 dB. The proposed device with a compact footprint (9.6 × 3 µm2) is finally fabricated on a Si3N4 loaded LNOI platform, and the experimental result shows the IL of <0.2 dB in the wavelength ranges from 1.5 to 1.6 µm, which makes it suitable for broadband PICs required for future communication systems.

Demonstration of a High‐Performance 3 dB Power Splitter in Silicon Nitride Loaded Lithium Niobate on Insulator

We propose and demonstrate two types of 1 × 2 power splitters based on multimode interference (MMI), which are ultra-compact, fabrication friendly, and low loss. The contours of MMI and output tapers are optimized with Bezier curves, which can implement arbitrary ratio power splitters (ARPSs) and ultra-broadband dual-polarization power splitters (UDPSs). For ARPSs, the experimental results show that arbitrary power splitting ratios can be obtained with an average excess loss (EL) of 0.17 dB at 1550 nm for fundamental TE polarization. For UDPSs, the experimental results show that the ELs for fundamental TE and TM polarization are less than 0.63 dB and 0.44 dB over a large bandwidth of 415 nm (1260-1675 nm). The footprints of the proposed devices are less than 10 µm × 2.5 µm (without input straight waveguide) with large fabrication tolerance.

Ultra-broadband dual-polarization and arbitrary ratio power splitters based on Bezier curve optimized multimode interference.

A high-performance optical filter is proposed and realized with multimode waveguide grating (MWG) and two-mode multiplexers on the x-cut lithium-niobate-on-insulator (LNOI) platform for the first time, to the best of our knowledge. The present optical filter is designed appropriately to avoid material anisotropy as well as mode hybridness, and has a low excess loss of 0.05 dB and a high sidelobe suppression ratio (SLSR) of 32 dB in theory with Gaussian apodization. The fabricated filters show a box-like response with 1-dB bandwidth of 6-23 nm, excess loss of ∼0.15 dB, sidelobe suppression ratio of >26 dB. The device performance is further improved with a sidelobe suppression ratio as high as 48 dB and a low excess loss of ∼0.25 dB by cascading two identical MWGs.

High-performance lithium-niobate-on-insulator optical filter based on multimode waveguide gratings.

Heterodyne phase-sensitive dispersion spectrometer (HPSDS) retrieves the concentration of gas samples by measuring the refractive index fluctuations near the molecular resonance. Compared to previous HPSDS studies focusing on pure intensity modulation, it is attractive to investigate the performance of HPSDS sensor based on a distributed feedback (DFB) laser under conditions where frequency modulation is much higher than intensity modulation. In this work, we report the implementation of a near-infrared HPSDS for methane detection based on the direct modulation of a DFB laser. The performance of our HPSDS is assessed using the characteristic absorption peak of methane near 1653.7 nm. Long-time measurements show that our HPSDS has a detection limit (MDL) of 1.22 ppm at standard atmospheric pressure and room temperature. In the same experimental conditions, we have experimentally compared HPSDS to wavelength modulation spectroscopy (WMS) to evaluate the dynamical range, long-term stability, and precision limits of the two methods.

Methane detection with a near-infrared heterodyne phase-sensitive dispersion spectrometer at a stronger frequency modulation using direct injection-current dithering.

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Papers

Ultra-broadband dual-polarization and arbitrary ratio power splitters based on Bezier curve optimized multimode interference.

High-performance lithium-niobate-on-insulator optical filter based on multimode waveguide gratings.

Methane detection with a near-infrared heterodyne phase-sensitive dispersion spectrometer at a stronger frequency modulation using direct injection-current dithering.