Views: 9 Author: Site Editor Publish Time: 2022-05-05 Origin: Site
With the continuous development of services such as cloud computing, virtual reality, data communication and high-definition video, the core optical network has been upgraded to ultra-high-speed and ultra-long-distance transmission. In this process, there is a core device that is essential - that is the lithium niobate modulator (LiNbO3).
Figure 1 Lithium niobate modulator
Lithium niobate modulators are fabricated by utilizing the electro-optical effect of lithium niobate crystals combined with optoelectronic integration processes. They can convert electronic data into photonic information and are the core components for electro-optical conversion.
Lithium niobate is a compound of niobium, lithium and oxygen. It is a negative crystal with a large spontaneous polarization (0.70 C/m2 at room temperature), and is a ferroelectric with the highest Curie temperature (1210 °C) found so far.
Figure 2 (a) 3-inch optical grade nominally pure lithium niobate crystal; (b) iron-doped lithium niobate crystal
Lithium niobate crystals have two characteristics that are particularly attractive. First, lithium niobate crystals have many photoelectric effects, including piezoelectric effect, electro-optic effect, nonlinear optical effect, photorefractive effect, photovoltaic effect, and photoelastic effect. , acousto-optic effect and other optoelectronic properties; second, the performance of lithium niobate crystal is highly controllable, which is caused by the lattice structure and abundant defect structure of lithium niobate crystal, and many properties of lithium niobate crystal can be It can be greatly regulated by crystal composition, element doping, and valence control.
In addition, the physical and chemical properties of lithium niobate crystals are quite stable, easy to process, have a wide light transmission range, have large birefringence, and are easy to prepare high-quality optical waveguides. It has unparalleled advantages in communication - not only has a small chirp effect, high modulation bandwidth, good extinction ratio, but also has excellent stability, it is the leader in high-speed devices, so it is widely used in high-speed high-speed devices. bandwidth for long-distance communication.
Harvard University once commented on lithium niobate: if the center of the electronic revolution is named after silicon material, then the birthplace of the photonics revolution is likely to be named after lithium niobate.
(1) Lithium niobate crystal with the same composition
For the same composition of lithium niobate crystal, its preparation mainly adopts the pulling method. Although the bubble method, the guided mode method, and the temperature gradient method have also been used to prepare lithium niobate crystals, they have no obvious advantages or clear application requirements compared with the pulling method, so they have not received extensive attention. #FineWinwafers' lithium niobate crystals use the pulling method and have more than ten years of crystal growth experience, providing 2-6 inch diameter SAW and OPTICAL grade ingots, blocks, wafers.
(2) Near stoichiometric ratio of lithium niobate crystals
Lithium niobate crystals with near stoichiometric ratios have many excellent photoelectric properties, but their ratios deviate from the solid-liquid eutectic point of the same composition, so the conventional pulling method cannot be used to grow high-quality crystals. At present, the main preparation methods used are lithium-rich Melt method, flux method, diffusion method.
(3) Lithium niobate single crystal thin film
Lithium niobate single crystal thin films can be used in optical waveguides, acoustic devices and other micro-nano structures, as well as in the preparation of silicon-based and other hybrid integrated devices. People began to explore the preparation of lithium niobate single crystal thin films for a long time. Only "Ion Slicing" technology, which has been commercialized, can provide single crystal thin film products with a thickness of 300~900 nm.
At this stage, the production technology of lithium niobate crystal is mature, and the market share of leading enterprises is relatively large. In the global market, #EPCOS of Germany, #Sumitomo of Japan, and #KorthKristalle of Germany are the top three lithium niobate producers in terms of market share.
(1) Piezoelectric application
Lithium niobate crystal has high Curie temperature, small temperature coefficient of piezoelectric effect, high electromechanical coupling coefficient, low dielectric loss, stable physical and chemical properties of crystal, good processing performance, and easy to prepare large-sized and high-quality crystals. Piezoelectric crystal material.
Compared with the commonly used piezoelectric crystal quartz, lithium niobate crystal has high sound speed and can prepare high-frequency devices. Therefore, lithium niobate crystal can be used in resonators, transducers, delay lines, filters, etc., and is used in mobile communications, satellites, etc. Communication, digital signal processing, television, broadcasting, radar, remote sensing and telemetry and other civil fields as well as electronic countermeasures, fuze, guidance and other military fields, the most widely used is the surface acoustic wave filter (SAWF).
Figure 3 (a) 2.4 GHz surface acoustic filter (SAW); (b) small SAW duplexer
(2) Optical applications
In addition to the piezoelectric effect, the photoelectric effect of the lithium niobate crystal is very rich, among which the electro-optic effect and the nonlinear optical effect have outstanding performance and are the most widely used. Moreover, lithium niobate crystals can prepare high-quality optical waveguides by proton exchange or titanium diffusion, and can prepare periodic polarized crystals by polarization inversion. , electro-optical deflection, high-voltage sensors, wavefront detection, optical parametric oscillators and ferroelectric superlattices and other devices have been widely used.
In addition, applications based on lithium niobate crystals such as birefringent wedges, holographic optics, infrared pyroelectric detectors, and erbium-doped waveguide lasers have also been reported.
Figure 4 Lithium niobate electro-optic modulator
(3) Dielectric Superlattices
In 1962, Armstrong first proposed the concept of Quasi-Phase-Match (QPM, Quasi-Phase-Match), which uses the inverted lattice vector provided by the superlattice to compensate for the phase mismatch in the optical parametric process. The polarization direction of the ferroelectric determines the sign of the nonlinear polarizability χ2. The ferroelectric domain structure with the opposite periodic polarization direction can be prepared in the ferroelectric to realize the quasi-phase matching technology, including lithium niobate and lithium tantalate. Periodically polarized crystals can be prepared from crystals such as potassium titanyl phosphate and potassium titanyl phosphate. Among them, lithium niobate crystal is the earliest and most widely used material in the preparation and application of this technology.
The initial application of periodically polarized lithium niobate crystals is mainly considered to be applied to laser frequency conversion. In 2014, Kim designed an optical superlattice integrated photonic chip based on reconfigurable lithium niobate waveguide optical circuit, and realized the high efficiency of entangled photons on the chip for the first time. generation and high-speed electro-optic modulation.
It can be said that the proposal and development of dielectric superlattice theory has pushed the application of lithium niobate and other ferroelectric crystals to a new level, in all-solid-state lasers, optical frequency combs, laser pulse compression, beam shaping and quantum communication. The entangled light source has important application prospects.
(1) Acoustic application
The current fifth-generation mobile communication network (5G) deployment includes the sub-6G frequency band of 3 to 5 GHz and the millimeter wave frequency band above 24 GHz. The increase in communication frequency not only requires the piezoelectric properties of crystal materials to be satisfied, but also requires thinner wafers. , The interdigital electrode spacing is smaller, and the fabrication process of the device is greatly challenged.
Therefore, in the 4G era and before, the surface acoustic filters widely used in lithium niobate crystals and lithium tantalate crystals are facing the competition of bulk acoustic wave devices (BAW) and thin film cavity acoustic resonators (FBAR) in the 5G era. .
Lithium niobate crystals have made rapid progress in higher frequency filters, and materials and device fabrication techniques still show great potential. With the development of lithium niobate single crystal thin film materials and new acoustic device technologies, as one of the core devices of future 5G communications, front-end RF filters based on lithium niobate crystals have important application prospects.
(2) Optical communication applications
Optical modulators are the key components of high-speed optical communication networks. The future requirements for lithium niobate electro-optical modulators include higher modulation rates, miniaturization and integration.
At present, the lithium niobate electro-optical modulators used in commercial applications are mainly 40/100 Gbps, and higher-speed lithium niobate modulators have been developed. For example, in 2017, #Fujitsu released a 600 Gbps lithium niobate electro-optical modulator. Currently, 400 Gbps and 600 Gbps products are gradually entering the market.
Optical communication technology is an important part of the construction of the fifth-generation mobile communication network, and the lithium niobate electro-optical modulator, as the core device, will also usher in greater development.
(3) Photonic integrated chip
Photons have been widely used in high-capacity communication, optical storage, information transmission, information processing, detection and other fields. Like the development of electronics from discrete components to integrated circuits, the miniaturization, integration, and low power consumption of photonic devices As the requirements for power consumption, modularity, intelligence and high reliability are getting higher and higher, integrated photonics chips will inevitably replace discrete optical components. The development of integrated photonics chips in the early stage was mainly driven by the demand for optical communication, and researches were carried out around silicon-based photonics and indium phosphide-based integration.
Silicon-based photonics integrated chips have developed rapidly due to the huge mature semiconductor material and process technology system, but silicon-based laser preparation technology has always been one of the shortcomings, and currently relies on mixed integration with indium phosphide; some indium phosphide integrated photonic chips It has been commercialized and its performance is better than that of silicon-based photonics integrated chips, but it lacks a general process platform like silicon, and the process technology is complex and expensive.
Lithium niobate-based integrated photonics research, which is driven by the demand for optical communication, mainly focuses on Mach-Zehnder interference light modulators, phase modulators, and integrated optical switches.
In addition to the demand for integrated photonics in the field of optical communication, the future demand for photonics-based optical quantum information processing, optical computing, biosensing, imaging detection, signal processing, storage, 3D display, etc. The hybrid integration scheme is difficult to apply.
From the perspective of single technology development, almost all photonic components have been realized based on lithium niobate crystals, including:
Mode-locked lasers, Q-switched lasers and optical amplifiers realized by rare earth doping;
Optical waveguides realized by titanium diffusion and proton exchange, as well as integrated optical switches, optical crossovers, optical couplings, and single-photon detection;
Intensity, phase and polarization modulation, wavefront detection and optical pulse selection, etc., realized by electro-optic effect;
Optical frequency conversion and quantum entangled photon generation realized by nonlinear optical effects;
Gratings, holographic storage, phase conjugators, spatial light modulators, etc. realized by photorefractive effect;
New devices such as all-optical logic gates, half adders, and frequency combs developed from lithium niobate photonic crystals and lithium niobate optical microcavities;
The mutual conversion and sensing of force, heat, light and other signals are realized through piezoelectric effect, pyroelectric effect, and photoelastic effect.
In the currently developed optoelectronic material system, it is rare to develop so many basic optical components, photonics devices and optoelectronic devices based on the same host material, which also makes people have doubts about the future development of lithium niobate crystals in integrated photonics chips.
Fig.5 Metasurface micro-nano structure prepared by lithium niobate crystals (Source: Nankai University)
Lithium niobate crystals integrate a variety of optoelectronic properties and can meet practical performance requirements, which are very rare in optoelectronic materials. With the development and improvement of core technologies such as the theory, preparation and application of lithium niobate crystal integrated photonics chips, lithium niobate crystal has become an "optical silicon" material in the photonic era, providing a strategic foundation for the development of integrated photonics.
Source: Overview of Lithium Niobate Crystal and Its Applications, Sun Jun, Hao Yongxin, Zhang Ling, Xu Jingjun, Zhu Shining, Journal of Intraocular Crystals, 2020, 49(6)