Lithium Niobate Optical Waveguides Research Articles

Thin-film lithium niobate electro-optic terahertz wave detector

The design, fabrication, and validation of a thin-film lithium niobate on insulator (LNOI) electro-optic (EO) time-domain terahertz (THz) wave detector is reported. LNOI offers unprecedented properties for the EO detection of freely propagating THz wave radiation pulses and transient electric fields because of the large EO coefficient of the material, engineering of the velocity matching of the THz wave and optical wave, and much reduced detector size. The proof-of-concept device is realized using thin-film lithium niobate optical waveguides forming a Mach–Zehnder interferometer with interferometer arms electrically poled in opposite directions. THz waves are coupled effectively to the fully dielectric device from free space without using antennas or plasmonics. The detection of THz waves with frequencies up to 800 GHz is successfully demonstrated. The detector allows for the detection of THz frequency electric fields up to 4.6 MV/m. The observed frequency response of the device agrees well with theoretical predictions.

Low-noise co-arm differential sensor for an optical frequency comb sampling an E-field test system.

Optical frequency comb (OFC) technology can realize the rapid measurement of electric fields (E-fields) with large bandwidth. However, this technology suffers from the problem of high intensity noise, resulting in low sensitivity and a blind frequency region. In order to solve the above problems, a dual-path optical E-field sensor with a common reference arm based on a lithium niobate optical waveguide is proposed. The introduction of the reference arm improves the balance of optical paths and the degree of integration. A segmented electrode is also designed to ensure the generation of reverse electrical signals on two Mach-Zehnder interferometers (MZIs). After exiting from the differential photodetector (PD), the intensity noise can be removed, and the sensitivity of the sensor can be improved. After testing, the maximum intensity noise reduction is about 37 dB, the average noise reduction is about 22.3 dB, and the blind frequency region can be eliminated with the co-arm differential optical E-field (CDOE) sensor in the process of measuring the signal. This sensor can be used in the 1 MHz-12 GHz bandwidth with a sensitivity better than 10 mV/m·√Hz.

X-Cut Lithium Niobate Optical Waveguide with High-Index Contrast and Low Loss Fabricated by Vapor Proton Exchange

Highly integrated and stable devices are appealing in optical communication and sensing. This appeal arises from the presence of high refractive index contrast and high-quality waveguides. In this study, we improved the vapor proton exchange (VPE) process, enabling large-scale waveguide fabrication and addressing the issue of liquid exchange during cooling. Additionally, we have prepared and characterized planar waveguides on X-cut lithium niobate (LN) crystals. The exchanged samples exhibit α and k1 phases, refractive index contrasts as high as 0.082, and exceptional refractive index uniformity. Furthermore, we utilized the same process to fabricate channel waveguides and Y-branch waveguides. We achieved low propagation losses in channel waveguides, accompanied by small mode sizes, and low-loss Y-branch waveguides with a highly uniform beam splitting ratio. All waveguides exhibited consistent performance across multiple preparations and tests, remaining free from aging effects for three months. Our results underscore the promising potential of VPE for creating Y-branch splitters and modulators in LN crystals.

TE/TM mode electro-optic conversion based on a titanium diffusion lithium niobate waveguide with a polarization-maintained fiber structure.

For the development of photonic integrated circuits and lithium niobate (L i N b O 3, LN) optical waveguide technology, the implementation and application of polarization devices based on LN are also becoming more widespread, where titanium (Ti)-diffused LN waveguides form the basis of many important electro-optic (EO) integrated optical devices. Moreover, utilizing polarization conversion has the potential to enhance both the effectiveness and capacity of optical transmission. Thus, we have presented an EO polarization mode converter packaging with PANDA polarization-maintaining optical fibers (PMFs) in the broadband wavelength range (1440-1620nm) to obtain the multiwavelength modulation, featuring the wavelength tunability. Additionally, the fabricated device is able to achieve transverse electric (TE) to transverse magnetic (TM) mode conversion efficiently with the applied voltage of ±, which provides high conversion efficiency. Importantly, our device also features a high-frequency response of about 600MHz with overall insertion loss below 5dB. The rapid development of LN-based polarization devices holds great promise for chip-integrated systems in the field of polarization telecommunication.

Traveling-Wave Mach-Zehnder Modulator Integrated With Electro-Optic Frequency-Domain Equalizer for Broadband Modulation

Broadband modulator is a key component of future high-capacity and high-baud-rate optical fiber communication. An electro-optic (EO) frequency-domain equalizer is proposed and investigated to increase the 3-dB bandwidth of optical modulators. The equalizer can be integrated with conventional traveling-wave modulators using a crossing waveguide, U-turn waveguide, or ferroelectric-domain-inversion. The 3-dB bandwidth of the modulator can be doubled by integrating the equalizer. As a theoretical consideration, we modeled several types of EO equalizers and investigated the function and parameter dependency of the equalizers in detail. For the experimental demonstration, a titanium-diffused lithium niobate optical waveguide modulator integrated with the EO equalizer was fabricated. The optical loss including fiber-coupling was as low as 5.4 dB, and the half-wave voltage was 3.7 V. The measured electro-optic response at 110 GHz is –0.4 dB, and that of the 3-dB bandwidth is larger than 110 GHz, which is the upper frequency limit of our measurement equipment. The 3-dB bandwidth of the modulator estimated using the fitting curve obtained from the measured electrical propagation loss of the modulator electrode was approximately 200 GHz.

Design and performance analysis of optical sequence detector using lithium niobate-based Mach–Zehnder interferometers

Abstract Present article suggested and proposed a new design of 4-bit optical sequence detector using an electro-optic effect by utilizing Lithium niobate (LiNbO3) based Mach–Zehnder interferometer, because of its consistency about production and stability. Suggested sequential detector is designed by OptiBPM (optical beam propagation method) software. This structure uses EO effect to alter the light upon lithium niobate optical waveguide via cross-state to bar-state and replica. Sequence detector is a sequential state machine, that extract an input string of bits and, when the targeted bit has been detected, then it gives an output as 1. In this paper the authors have focused on all-optical 4-bit sequential detector to produce the outcome as 1 when the bit sequence finds as 1101 within the given data. This all-optical device can be utilized in several optical switching and networking operations. The suggested design is replicated with MATLAB along with mathematical descriptions too and has been verified.

Design and Analysis of Broadband LiNbO3 Optical Waveguide Electric Field Sensor with Tapered Antenna.

The three-dimensional (3D) simulation model of a lithium niobate (LiNbO3, LN) optical waveguide (OWG) electric field sensor has been established by using the full-wave electromagnetic simulation software. The influences of the LN substrate and the packaging material on the resonance frequency of the integrated OWG electric field sensor have been simulated and analyzed. The simulation results show that the thickness of the LN substrate has a great influence on the resonant frequency of the sensor (≈33.4%). A sensor with a substrate thickness of 1 mm has been designed, fabricated, and experimentally investigated. Experimental results indicate that the measured resonance frequency is 7.5 GHz, which nearly coincides with the simulation results. Moreover, the sensor can be used for the measurement of the nanosecond electromagnetic impulse (NEMP) in the time domain from 1.29 kV/m to 100.97 kV/m.

Design of Flat Optical Frequency Comb Based on Lithium Niobate Optical Waveguide

Design of Flat Optical Frequency Comb Based on Lithium Niobate Optical Waveguide

Rejuvenating a Versatile Photonic Material: Thin‐Film Lithium Niobate

AbstractThe excellent optical and unique material properties of lithium niobate have long established it as a prevailing photonic material, especially for the long‐haul telecom modulator and wavelength‐converter applications. However, conventional lithium niobate optical waveguides are bulky, hence large‐scale photonic circuit implementations are impeded and high power requirements are imposed. To address these shortcomings, thin‐film lithium niobate technology has been a topic of intense research in the last few years and a plethora of ultracompact devices with significantly superior performances than the conventional counterparts have been demonstrated. These efforts have rejuvenated lithium niobate for novel electro‐, nonlinear‐, and quantum‐optic applications. Herein, the most recent advancements of this booming field are summarized and a perspective for future directions is given.

Лазерная модификация титановой пленки на поверхности оптических волноводов в ниобате лития

The availability of thin titanium film laser modification for precise loss control in lithium niobate optical waveguides was demonstrated. A simple ray model of the interaction between a nanosized metal cover on the top surface of a Ti-indiffused channel waveguide and orthogonal optical waveguide modes with changes in optical losses up to 0.95 and 1.05 dB/mm for TE- and TM-polarized modes respectively in case of 5 nm titanium film covering was suggested. The consistency between obtained theoretical losses and results of optical film modification by a 976 nm laser beam with the threshold intensity up to 1 kW/mm2 was demonstrated. The suggested method was applied for precise balance of optical field intensities in arms of Mach-Zehnder optical modulator; an associated increase of the extinction ratio from 30 to 48 dB was achieved.

An Integrated Photonic Electric-Field Sensor Utilizing a 1 × 2 YBB Mach-Zehnder Interferometric Modulator with a Titanium-Diffused Lithium Niobate Waveguide and a Dipole Patch Antenna

We studied photonic electric-field sensors using a 1 × 2 YBB-MZI modulator composed of two complementary outputs and a 3 dB directional coupler based on the electro-optic effect and titanium diffused lithium–niobate optical waveguides. The measured DC switching voltage and extinction ratio at the wavelength 1.3 μm were ~16.6 V and ~14.7 dB, respectively. The minimum detectable fields were ~1.12 V/m and ~3.3 V/m, corresponding to the ~22 dB and ~18 dB dynamic ranges of ~10 MHz and 50 MHz, respectively, for an rf power of 20 dBm. The sensor shows an almost linear response to the applied electric-field strength within the range of 0.29 V/m to 29.8 V/m.

Improved second harmonic performance in periodically poled LNOI waveguides through engineering of lateral leakage.

In this contribution, we investigate the impact of lateral leakage for linear and nonlinear optical waveguides in lithium niobate on an insulator (LNOI). Silicon nitride (SiN) loaded and direct patterned lithium niobate cross-sections are investigated. We show that lateral leakage can take place for the TE mode in LNOI ridge waveguides (X-cut lithium niobate), due to the birefringence of the material. This work gives guidelines for designing waveguides in LNOI that do not suffer from the lateral leakage effect. By applying these design considerations, we avoided the lateral leakage effect at the second harmonic wavelength of a nonlinear optical waveguide in LNOI and demonstrate a peak second harmonic generation conversion efficiency of ~1160% W-1cm-2.

Numerical simulation on measurement for optical arbitrary waveforms using blind frequency resolved optical gating based on PPLN

A novel scheme of blind frequency resolved optical gating (Blind-FROG) measurement for optical arbitrary waveforms, based on sum frequency effect in periodically poled lithium niobate (PPLN) optical waveguide, is proposed. The proposed scheme is very simple, low cost and good noise performance. Furthermore, it can simultaneously measure two arbitrary, potentially very complex pulses. Through theoretical analysis and numerical simulation, the performance of the scheme is studied. The results show that, the accuracy of measurement degrades with the increase of the two optical arbitrary waveforms complexity; Nevertheless, the smaller the polarization periodic mismatching is, the higher the measurement accuracy becomes; And the accuracy of the complete measurement system also can be improved by increasing the number of iterations.

Optical waveguides in lithium niobate: Recent developments and applications

The state of the art of optical waveguide fabrication in lithium niobate is reviewed, with particular emphasis on new technologies and recent applications. The attention is mainly devoted to recently developed fabrication methods, such as femtosecond laser writing, ion implantation, and smart cut waveguides as well as to the realization of waveguides with tailored functionalities, such as photorefractive or domain engineered structures. More exotic systems, such as reconfigurable and photorefractive soliton waveguides, are also considered. Classical techniques, such as Ti in-diffusion and proton exchange, are cited and briefly reviewed as a reference standpoint to highlight the recent developments. In all cases, the application-oriented point of view is preferred, in order to provide the reader with an up-to date panorama of the vast possibilities offered by lithium niobate to integrated photonics.

A superconducting nanowire single photon detector on lithium niobate

Superconducting nanowire single photon detectors (SNSPDs) are a key enabling technology for optical quantum information science. In this paper we demonstrate a SNSPD fabricated on lithium niobate, an important material for high speed integrated photonic circuits. We report a system detection efficiency of 0.15% at a 1 kHz dark count rate with a maximum of ∼1% close to the critical current at 1550 nm wavelength for a parallel wire SNSPD with front side illumination. There is clear scope for improving on this performance with further materials optimization. Detector integration with a lithium niobate optical waveguide is simulated, demonstrating the potential for high single photon detection efficiency in an integrated quantum optic circuit.

Analysis and optimization of propagation losses in LiNbO3 optical waveguides produced by swift heavy-ion irradiation

The propagation losses (PL) of lithium niobate optical planar waveguides fabricated by swift heavy-ion irradiation (SHI), an alternative to conventional ion implantation, have been investigated and optimized. For waveguide fabrication, congruently melting LiNbO3 substrates were irradiated with F ions at 20 MeV or 30 MeV and fluences in the range 1013–1014 cm−2. The influence of the temperature and time of post-irradiation annealing treatments has been systematically studied. Optimum propagation losses lower than 0.5 dB/cm have been obtained for both TE and TM modes, after a two-stage annealing treatment at 350 and 375∘C. Possible loss mechanisms are discussed.

Optical damage of Zr:LiNbO3 waveguides produced by proton implantation

We reported on the optical damage properties of the proton-implanted Zr doped lithium niobate (Zr:LiNbO3) optical waveguides. The waveguides in both planar and channel configuration were fabricated in z-cut wafers by the double-energy proton beam implantation at 470keV (at the fluence of 1×1016ions/cm2) and 500keV (at the fluence of 2×1016ions/cm2). The experimental results showed that the proton implanted waveguides in Zr:LiNbO3 crystals were with relatively high optical damage threshold at the wavelength of 532nm, which was one order of magnitude higher than that of the previously reported ion implanted waveguides in undoped LiNbO3. It was also found that the optical damage threshold (∼9.0×104W/cm2) of Zr:LiNbO3 channel waveguides was much higher than that of a planar waveguide (∼2.0×103W/cm2).

Fabrication of a Y-splitter Modulator Embedded in LiNbO3 with a Femtosecond Laser

We describe a technique to integrate embedded microelectrodes and optical waveguides in Lithium niobate (LiNbO3) using a femtosecond laser, and a Y-splitter modulator was demonstrated. The Y-splitter was fabricated by femtosecond laser direct writing and the embedded microelectrodes were fabricated by combining femtosecond laser ablation with selective electroless plating. The embedded microelectrodes give rise to homogeneous electric field across the optical waveguides, which could result in effective electro-optic overlap. The simple and flexible technique could open new opportunities for fabricating integrated 3D electro-optic devices.

Thick optical waveguides in lithium niobate induced by swift heavy ions (~10 MeV/amu) at ultralow fluences

Heavy mass ions, Kr and Xe, having energies in the approximately 10 MeV/amu range have been used to produce thick planar optical waveguides at the surface of lithium niobate (LiNbO3). The waveguides have a thickness of 40-50 micrometers, depending on ion energy and fluence, smooth profiles and refractive index jumps up to 0.04 (lambda = 633 nm). They propagate ordinary and extraordinary modes with low losses keeping a high nonlinear optical response (SHG) that makes them useful for many applications. Complementary RBS/C data provide consistent values for the partial amorphization and refractive index change at the surface. The proposed method is based on ion-induced damage caused by electronic excitation and essentially differs from the usual implantation technique using light ions (H and He) of MeV energies. It implies the generation of a buried low-index layer (acting as optical barrier), made up of amorphous nanotracks embedded into the crystalline lithium niobate crystal. An effective dielectric medium approach is developed to describe the index profiles of the waveguides. This first test demonstration could be extended to other crystalline materials and could be of great usefulness for mid-infrared applications.

Confocal Raman imaging of optical waveguides in LiNbO3 fabricated by ultrafast high-repetition rate laser-writing

We report on the confocal Raman characterization of the micro-structural lattice changes induced during the high-repetition rate ultrafast laser writing of buried optical waveguides in lithium niobate (LiNbO(3)) crystals. While the laser beam focal volume is characterized by a significant lattice expansion together with a high defect concentration, the adjacent waveguide zone is largely free of defects, undergoing only slight rearrangement of the oxygen octahedron in the LiNbO(3) lattice. The close proximity of these two zones has been found responsible for the propagation losses of the guided light. Subjacent laser-induced periodic micro-structures have been also observed inside the laser focal volume, and identified with a strong periodic distribution of lattice defects.