Lithium Niobate Waveguide Research Articles

Pulse distortion caused by waveguide inhomogeneity in nonlinear optical wavelength converters

Low-noise integrated all-optical wavelength converters that can be operated in short pulse regime are essential tools to overcome contention resolution in a modern communication network, based on wavelength division multiplexing. Any imperfect functionality in such devices causes non-ideal optical power transfer to the converted data pulses. All imperfections during the preparation and operation of the wavelength converters can be addressed to the waveguide inhomogeneity which distorts data pulses to be converted. This paper reports different waveguide inhomogeneity effects on the pulse distortion while using periodically poled lithium niobate waveguide as wavelength converters. Three types of chi ^{(2)}-based nonlinear optical processes, including second harmonic generation, difference frequency generation, and cascaded second harmonic generation/difference frequency generation are numerically studied to show that any constant, linear, and quadratic waveguide inhomogeneity causes short pulse (down to 1 ns) distortion in such wavelength converters. In addition, it is shown that the reconstruction of textrm{sech}^2-shaped generated pulses is possible, when suitable upside-down quadratic variations of obtained inhomogeneity are deliberately induced in the waveguide. Notably, for pulsed second harmonic generation, the generated pulse can be compressed using an upside-down quadratic phase mismatch.

Redeposition-free inductively-coupled plasma etching of lithium niobate for integrated photonics

Abstract Lithium niobate on insulator is being established as a versatile platform for a new generation of photonic integrated devices. Extensive progress has been made in recent years to improve the fabrication of integrated optical circuits from a research platform towards wafer-scale fabrication in commercial foundries, and optical losses have reached remarkably low values approaching material limits. In this context, argon etching of lithium niobate waveguides has been shown to provide the best optical quality, yet the process is still challenging to optimise due to its physical nature. Namely, the micro-masking effects introduced by the material redeposition and a close to one etch mask selectivity for deep etches. We present a workflow to identify the parameter set offering the best etching results independent of the plasma system being used. We show how to reach the redeposition-free regime and propose three methods to achieve redeposition-free lithium niobate etching with good quality sidewalls without need of wet chemistry for cleaning.

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.

Photon pair generation in lithium niobate waveguide periodically poled by femtosecond laser

Reliable generation of single photons is of key importance for fundamental physical experiments and quantum protocols. The periodically poled lithium niobate (LN) waveguide has shown promise for an integrated quantum source due to its large spectral tunability and high efficiency, benefiting from the quasi-phase-matching. Here we demonstrate photon-pair sources based on an LN waveguide periodically poled by a tightly focused femtosecond laser beam. The pair coincidence rate reaches ∼8000 counts per second for average pump power of 3.2 mW (peak power is 2.9 kW). Our results prove the possibility of application of the nonlinear photonics structure fabricated by femtosecond laser to the integrated quantum source. This method can be extended to three-dimensional domain structures, which provide a potential platform for steering the spatial degree of freedom of the entangled two-photon states.

Method for the simultaneous measurement of waveguide propagation loss and bending loss

Abstract: The propagation loss of waveguide is a key indicator to evaluate the performance of an integrated optical platform. The commonly used cut-back method to measure the propagation requires the introduction of the Spiral test structure. In order to remove the effect of bending loss, the bending radius is usually designed to be large enough, taking up a lot of foot print. In this paper, we suggested a method to measure the propagation loss and bending loss of waveguide simultaneously with cut-back structure. The bending loss can be exponentially fitted with the bending radius according to simulation, which can be further simplified as linear fitting between the natural logarithm of the bending loss and the bending radius. A genetic algorithm was used to fit the insertion loss curve of the cutback structure and calculate the propagation loss and bending loss. With this method, we measured a cut-back structure of lithium niobate waveguide and got the propagation loss of 0.558dB/cm and bending

Broadband Light Generation in Nonlinear Silicon Nitride Strip-Loaded Lithium Niobate Waveguides

This work demonstrates SiN strip-loaded lithium niobate waveguides with nonlinear optical properties, focusing on their performance when pumped at telecom wavelength. Experimental results show second and third harmonic generation in periodically poled SiN/LiNbO3. Furthermore, simulations reveal that these waveguides can be dispersion engineered to generate supercontinuum. The findings highlight the potential of SiN strip-loaded lithium niobate platform in sustaining broadband nonlinear light sources.

Polarization beam splitter based on the asymmetric directional coupler of lithium niobate film.

A polarization beam splitter (PBS) based on a lithium niobate film asymmetric directional coupler is proposed. The PBS is located on a lithium niobate platform on an insulator consisting of a silicon nitride-lithium niobate waveguide (SLW) and a lithium niobate waveguide (LNW). By rationally designing the SLW and LNW sizes, TE polarization satisfies the phase matching condition and TM polarization phase mismatch. The numerical simulation results show that the extinction ratio (ER) and insertion loss (IL) of PBS for TE mode are 30.57 and 0.66dB, respectively, and the ER and IL of PBS for TM mode are 28.15 and 0.11dB, respectively, at an operating wavelength of 1.55µm.

Improving the Signal-to-Noise Ratio of Photonic Frequency Conversion from 852 nm to 1560 nm Based on a Long-Wavelength Laser-Pumped PPLN Waveguide Module

The storage wavelength of quantum nodes based on atomic systems does not match the wavelength of optical fiber communication, which requires the establishment of an efficient conversion system between flying bits and storage bits. In this paper, based on the nonlinear wavelength conversion technology of the periodically poled lithium niobate waveguide, a low-noise conversion of 852-nm photons to 1560-nm photons was achieved by a 1878-nm pump laser. The generation mechanism and transmission mechanism of noise due to nonlinear process are analyzed theoretically. The noise photons introduced by the spontaneous parameter downconversion and spontaneous Raman scattering process of a strong pump laser are experimentally studied. The noise suppression near 1560 nm is realized by the fiber Bragg grating (FBG) filter. In the experiment, when the FBG bandwidth is reduced from 0.257 nm to 0.130 nm, the signal-to-noise ratio (SNR) increases from 52 to 90. Our results show that the SNR can be greatly improved by using a narrower band filter. Therefore, the quantum node is connected to the fiber channel, and the signal can be transmitted over long distances with low loss and high fidelity.

A photonic integrated continuous-travelling-wave parametric amplifier.

The ability to amplify optical signals is of pivotal importance across science and technology typically using rare-earth-doped fibres or gain media based on III-V semiconductors. A different physical process to amplify optical signals is to use the Kerr nonlinearity of optical fibres through parametric interactions1,2. Pioneering work demonstrated continuous-wave net-gain travelling-wave parametric amplification in fibres3, enabling, for example, phase-sensitive (that is, noiseless) amplification4, link span increase5, signal regeneration and nonlinear phase noise mitigation6. Despite great progress7-15, allphotonic integrated circuit-based demonstrations of net parametric gain have necessitated pulsed lasers, limiting their practical use. Until now, only bulk micromachined periodically poled lithium niobate (PPLN) waveguide chips have achieved continuous-wave gain16,17, yet their integration with silicon-wafer-based photonic circuits has not been shown. Here we demonstrate a photonic-integrated-circuit-based travelling-wave optical parametric amplifier with net signal gain in the continuous-wave regime. Using ultralow-loss, dispersion-engineered, metre-long, Si3N4 photonic integrated circuits18 on a silicon chip of dimensions 5 × 5 mm2, we achieve a continuous parametric gain of 12 dB that exceeds both the on-chip optical propagation loss and fibre-chip-fibre coupling losses in the telecommunication C band. Our work demonstrates the potential of photonic-integrated-circuit-based parametric amplifiers that have lithographically controlled gain spectrum, compact footprint, resilience to optical feedback and quantum-limited performance, and can operate in the wavelength ranges from visible to mid-infrared and outside conventional rare-earth amplification bands.

Dataset and Simulation Files for article "Stimulated Brillouin scattering by surface acoustic waves in lithium niobate waveguides"

Dataset and Simulation Files for article "Stimulated Brillouin scattering by surface acoustic waves in lithium niobate waveguides"

Short-period domain patterning by ion beam irradiation in lithium niobate waveguides produced by soft proton exchange

The lithium-niobate-based waveguides have emerged as one of the key platforms for enabling integrated optical technologies. The most common method for creating waveguides in lithium niobate is soft proton exchange (SPE) method. The range of the waveguides applications can be expanded further by creation of near-surface periodical structure of stripe ferroelectric domains within a waveguide (periodical poling) to realize various types of nonlinear optical interactions for creation of frequency conversion devices. However, the joint use of these two procedures (SPE and periodical poling) remains a challenging task. In this paper, we propose a focused ion beam (i-beam) periodical poling technique realized after SPE process for creation of an integrated optical frequency conversion device. The choice of i-beam poling parameters is based on the study of the features of domain structure evolution in lithium niobate after SPE process. We have found that SPE process leads to formation of shallow structure of isolated nanodomains. The revealed isotropic domain growth was caused by the sideways domain wall motion by step generation as a result of merging with these nanodomains. This type of domain kinetics is preferable for periodical poling as it decreases the probability of fast merging of stripe domains due to formation of fast domain walls. Eventually, we have created the 1-mm-long 2-μm-period domain structure in a channel waveguide. The obtained second harmonic generation efficiency for pulse pumping at 756 nm was up to 6·107% W−1 cm−2 for average power and 300 % W−1 cm−2 for peak power indicating the high quality of periodical poling.

Lithium niobate optical beam deflectors operating at both electro-optic and thermo-optic modes

A novel optical beam deflector based on a Lithium niobate (LN) waveguide is demonstrated, which can operate at both electro-optic and thermo-optic modes. At the electro-optic mode, the microstructured serrated electrodes induce inhomogeneous change in the refraction index of LN waveguide via the Pockels effect when a voltage is applied. The incident beam is thus deflected with the deflected distance changing linearly with the applied voltage. A high deflection speed up to 2.5 GHz is observed experimentally. The optical beam deflector can operate at the thermo-optic mode, where the deflected distance depends on the applied electric power; thus it varies quadratically with the applied voltage. The deflection capacity at the thermo-optic mode is much higher than electro-optic mode, since the deflection distances are respectively 27.19 and 2.53 μm for the thermo-optic and electro-optic modes at a 4 V applied voltage. Therefore, the distinct electro-optic and thermo-optic operating modes enable the LN optical beam deflector to be qualified for various tasks, such as optical scanning, optical modulator.

Efficient second harmonic generation by harnessing bound states in the continuum in semi-nonlinear etchless lithium niobate waveguides

Integrated photonics provides unprecedented opportunities to pursue advanced nonlinear light sources with low-power consumptions and small footprints in a scalable manner, such as microcombs, chip-scale optical parametric oscillators and integrated quantum light sources. Among a variety of nonlinear optical processes, high-efficiency second harmonic generation (SHG) on-chip is particularly appealing and yet challenging. In this work, we present efficient SHG in highly engineerable semi-nonlinear waveguides consisting of electron-beam resist waveguides and thin-film silicon nitride (SiN)/lithium niobate (LN). By carefully designing octave-separating bound states in the continuum (BICs) for the nonlinear interacting waves in such a hybrid structure, we have simultaneously optimized the losses for both fundamental frequency (FF) and second harmonic (SH) waves and achieved modal phasing matching and maximized the nonlinear modal overlap between the FF and SH waves, which results in an experimental conversion efficiency up to 4.05% W−1cm−2. Our work provides a versatile and fabrication-friendly platform to explore on-chip nonlinear optical processes with high efficiency in the context of nanophotonics and quantum optics.

Wafer-Scale Fabrication of Silicon Film on Lithium Niobate on Insulator (LNOI)

Hybrid integration of silicon photonics with lithium niobate (LN) devices provides a promising route to enable an excellent modulation performance in silicon photonic integrated circuits. To realize this purpose, a substrate containing a Si film on an LNOI substrate, called Si on the LNOI structure, was analyzed and fabricated. The mode propagation properties in the Si-on-LNOI structure were simulated in detail and a vertical adiabatic coupler (VAC) between the Si waveguide and LN waveguide was simulated to help in the determination of the dimension of this structure. A 4-inch wafer-scale Si on an LNOI hybrid structure was fabricated through the ion-cut process. This structure has a single-crystalline quality, high thickness uniformity, smooth surface, and sharp bonding interface, which are practical for realizing low loss and high coupling efficiency.

Red and green light generation in TPPLNOI ridge optical waveguide with 1550 nm picosecond laser

Periodically poled lithium niobate (PPLN) waveguides are a powerful platform for efficient frequency conversion. Conventional frequency doubling PPLN converters only generate second-harmonic light. However, we propose a trapezoidal periodically poled lithium niobate on insulator (TPPLNOI) ridge optical waveguide, simultaneously generating red and green light with 1550 nm picosecond laser by cascading second-harmonic generation and sum-frequency generation (SHG-SFG). According to the experimental data, we find that the second-harmonic (red light at 775 nm) and third-harmonic (green light at 517 nm) power increase with the pump light (1550 nm) power. We also verify experimentally that temperature can affect nonlinear frequency conversion of cascading SHG-SFG, and when the temperature increases from 20 °C to 70 °C, the normalized conversion efficiency of red light and green light rises from 154.79% W−1 cm −2, 129.24% W−1 cm −2 to 204.29% W−1 cm −2, 152.08% W−1 cm −2, respectively.

Thermally tunable and efficient second-harmonic generation on thin-film lithium niobate with integrated micro-heater.

In this Letter, we report thermo-optic tunable and efficient second-harmonic generation (SHG) based on an X-cut periodically poled lithium niobate (PPLN) waveguide. By applying an on-chip heater with thermo-isolation trenches and combining a type-0 quasi-phase matching mechanism, we experimentally achieve a high on-chip SHG conversion efficiency of 2500-3000% W-1 cm-2 and a large tuning power efficiency of 94 pm/mW inside a single 5-mm-long straight PPLN waveguide. Our design is for energy-efficient, high-performance nonlinear applications, such as wavelength conversion, highly tunable coherent light sources, and photon-pair generation.

Ultra-broadband mid-infrared generation in dispersion-engineered thin-film lithium niobate.

Thin-film lithium niobate (TFLN) is an emerging platform for compact, low-power nonlinear-optical devices, and has been used extensively for near-infrared frequency conversion. Recent work has extended these devices to mid-infrared wavelengths, where broadly tunable sources may be used for chemical sensing. To this end, we demonstrate efficient and broadband difference frequency generation between a fixed 1-µm pump and a tunable telecom source in uniformly-poled TFLN-on-sapphire by harnessing the dispersion-engineering available in tightly-confining waveguides. We show a simultaneous 1-2 order-of-magnitude improvement in conversion efficiency and ∼5-fold enhancement of operating bandwidth for mid-infrared generation when compared to equal-length conventional lithium niobate waveguides. We also examine the effects of mid-infrared loss from surface-adsorbed water on the performance of these devices.

All-optical wavelength conversion of a 92-Gb/s 16-QAM signal within the C-band in a single thin-film PPLN waveguide.

Tunable all-optical wavelength conversion (AOWC) within 151 nm bandwidth is demonstrated in a thin-film periodically poled lithium niobate (PPLN) waveguide, which utilizes the cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) process. Also, in the same waveguide, AOWC of a 92-Gb/s 16-ary quadrature amplitude modulated (16-QAM) signal within the C-band is successfully achieved. For Bit-error ratio (BER) measurements, we obtain a negligible optical signal-to-noise ratio (OSNR) penalty (<0.2 dB) for the converted idler wave at a BER of 1e-3.

Long-period gratings on lithium-niobate waveguides: phase-matching turning points for ultra-broadband mode conversion

We analyze a polymer long-period grating (LPG) formed on the surface of a z-cut lithium-niobate (LN) few-mode waveguide for TM00–TM10 mode conversion. We identify and explore the turning points along the phase-matching curves of the LPGs for the achievement of ultra-broadband mode conversion. We fabricate a number of gratings with annealed proton-exchanged LN waveguides and our experimental results agree well with the theoretical results. A typical fabricated grating operating near the turning point provides a mode extinction ratio larger than 20 dB from 1550 nm to a wavelength longer than 1700 nm, and another one provides a mode extinction ratio larger than 10 dB from 1464 nm to a wavelength longer than 1700 nm. Our gratings show good thermal stability. The structure of the LPG on LN waveguide could be applied to the realization of ultra-broadband high-speed mode switches for mode-multiplexing applications.

Reduced material loss in thin-film lithium niobate waveguides

Thin-film lithium niobate has shown promise for scalable applications ranging from single-photon sources to high-bandwidth data communication systems. Realization of the next generation high-performance classical and quantum devices, however, requires much lower optical losses than the current state of the art resonator (Q-factor of ∼10 million). Yet the material limitations of ion-sliced thin film lithium niobate have not been explored; therefore, it is unclear how high the quality factor can be achieved in this platform. Here, using our newly developed characterization method, we find out that the material limited quality factor of thin film lithium niobate photonic platform can be improved using post-fabrication annealing and can be as high as Q ≈ 1.6 × 108 at telecommunication wavelengths, corresponding to a propagation loss of 0.2 dB/m.