Study on Lithium Niobate Photonic Materials: Breakthrough in Thin Film Lithium Niobate Technology

1. Lithium niobate crystal: superior performance, mainly used in optoelectronic fields

Lithium niobate crystal has the characteristics of high photoelectric effect and strong performance adjustability. Lithium niobate (LiNbO3) is a compound of niobium, lithium, and oxygen, and is a negative crystal with a large spontaneous polarization (0.70 C/m2 at room temperature). According to Sun Jun et al.'s "Overview of Lithium Niobate Crystal and Its Applications," lithium niobate is the ferroelectric with the highest Curie temperature (1210°C) currently discovered. Lithium niobate crystal has the characteristics of high photoelectric effect, strong performance adjustability, stable physical and chemical properties, and wide light transmission range.

(1) Lithium niobate crystal has a variety of photoelectric properties, including piezoelectric effect, electro-optic effect, nonlinear optical effect, photoelastic effect, photovoltaic effect, and acousto-optic effect;

(2) The performance of lithium niobate crystal can be strongly adjustable, which is caused by the crystal lattice structure and rich defect structure of lithium niobate crystal. The (4) The light transmission range is wide, with a large double refraction, and it is easy to prepare high-quality optical waveguides. Therefore, acoustic surface wave filters, optical modulators, phase modulators, optical isolators, and electro-optic Q-switches based on lithium niobate crystals have been widely studied and put into practical use in fields such as electronic technology, optical communication technology, and laser technology.

The Development History: Nearly 100 Years of Research on Lithium Niobate Materials

The research on lithium niobate materials has been nearly 100 years, and can be divided into three stages:

The first stage (1928-1965): Foreign research on the growth process and crystal structure of lithium niobate. In 1928, mineralogist Zachariasian first conducted preliminary research on the structural properties of lithium niobate; in 1937, Sue et al. experimentally synthesized lithium niobate, but it did not receive widespread attention; until 1949, when the high-temperature ferroelectric properties of lithium niobate were discovered by Matthias and Remeika of the Bell Laboratory in the United States, it officially entered people's In 1980, Nankai University and the Southwest Institute of Technical Physics cooperated to discover the high anti-light damage performance of high-magnesium niobate lithium, which crystal was called "the star of China"; the same year, Nanjing University broke through the growth process of periodically polarized lithium niobate, realizing quasi-phase matching experimentally.

Classification application: mainly includes two types of acoustic and optical grade

Lithium niobate crystals can be divided into two types according to the application of acoustic grade and optical grade. Optical grade lithium niobate crystal: It has the advantages of large electro-optical and nonlinear optical coefficient, wide light transmission window, large birefringence index, can grow Kg bulk crystals, good machining performance, no delixing, etc. It is used in integrated optical waveguide, waveguide laser, wedge Angle plate for optical isolator, phase modulator, intensity modulator, quasi-phase matching laser frequency multiplier, optical parametric oscillator, etc. Acoustic grade lithium niobate crystal: has excellent piezoelectric properties, thermal stability, chemical stability and mechanical stability, mainly used in the field of surface acoustic wave filter.

Technical direction: Thin film has become an important development direction of materials

Lithium niobate thin film technology and its potential integrated photonics system have gradually become the "transformative" technology at the forefront of current photonics research. In the optical devices based on the bulk material lithium niobate crystal, the light is confined to the planar waveguide formed by ion diffusion or proton exchange, the refractive index difference is usually relatively small (~0.02), and the device size is relatively large, which is difficult to meet the needs of miniaturization and integration of optical devices. The thin film lithium niobate is a thin film lithium niobate material formed by stripping the lithium niobate film from the massive lithium niobate crystal through the way of "ion slicing", and bonding to the Si wafer with SiO2 buffer layer. Compared with traditional bulk material structure, thin film lithium niobate platform can achieve higher integration and better performance.

Field of application: Optical applications

Lithium niobate crystals have excellent electro-optical effects and nonlinear optical effects, and are widely used in electro-optical modulators. The photoelectric effect of lithium niobate crystal is very rich, among which the electro-optical effect and nonlinear optical effect are outstanding, and are also the most widely used. In addition, lithium niobate crystals can be prepared by proton exchange or titanium diffusion of high-quality optical waveguides, and can be prepared by polarization inversion of periodic polarization crystals, so it is widely used in electro-optical modulators, phase modulators, integrated optical switches, electro-optical Q-switched switches, electro-optical deflection, high voltage sensors, wave front detection, optical parametric oscillators and ferroelectric superlattice devices. Among them, the lithium niobate electro-optic modulator is realized by using the linear electro-optic effect of lithium niobate crystal, with the advantages of high speed, high extinction ratio, low chirp, etc., suitable for high-speed external modulator for optical communication, including amplitude modulator, phase modulator and other products.

Field of application: piezoelectric applications

Lithium niobate crystal is an excellent piezoelectric material, which is widely used in filter and other fields. Lithium niobate crystal is an excellent piezoelectric crystal material with high Curie temperature, small temperature coefficient of piezoelectric effect, high electromechanical coupling coefficient, low dielectric loss, stable physical and chemical properties, good processing performance, and easy to prepare large size and high quality crystals. Compared with commonly used piezoelectric crystal quartz, lithium niobate crystal has high sound speed and can prepare high-frequency devices, so lithium niobate crystal can be used in harmonics, transducers, delay lines, filters, etc., used in mobile communications, satellite communications, digital signal processing, television, radio, radar, remote sensing and other civilian fields as well as electronic countermeasures, fuze, guidance and other military fields. Among them, the most widely used is the surface acoustic wave filter.

The surface acoustic wave filter (SAW) is a kind of special filtering device which is made of piezoelectric materials such as quartz crystal and piezoelectric ceramics by using the physical characteristics of piezoelectric effect and surface wave propagation. The basic principle is that the wireless signal is converted to the acoustic signal by the piezoelectric effect at the input end to propagate on the medium surface, and the acoustic signal is converted to the wireless signal by the inverse piezoelectric effect at the output end. Lithium niobate piezoelectric wafer has excellent piezoelectric properties, thermal stability, chemical stability and mechanical stability, and is an ideal substrate material for making RF surface acoustic wave filters.

Industry chain: Domestic lithium niobate industry chain tends to improve

The upstream materials of the lithium niobate industry chain mainly include lithium niobate crystals and films, and the upstream equipment mainly includes electron beam direct writing and DUV lithography machines. The middle reaches of the industry chain are mainly lithium niobate modulator chips and devices, including bulk material lithium niobate modulator and thin film lithium niobate modulator; The downstream industry chain is mainly used in many fields such as optical communication, fiber optic gyroscope, ultrafast laser, cable TV (CATV).

Market size: In 22 years, the global lithium niobate crystal market size reached 1.13 billion yuan

The global lithium niobate crystal market is growing steadily, and the market size will reach 1.13 billion yuan in 2022. With the rapid development of the downstream industry, the lithium niobate crystal market space continues to expand. According to the "2023-2028 China lithium niobate crystal industry market in-depth research and development prospects forecast Report", the global lithium niobate crystal market size in 2022 reached 1.13 billion yuan, an increase of 3.1%.

Optical grade is the main type of lithium niobate crystal, accounting for about 60% in 2016. According to QYReseach public number data, in 2016, the global lithium niobate crystal market revenue was 124 million US dollars (about 800 million yuan), is expected to reach 146 million US dollars (about 1 billion yuan) in 2022, CAGR of 2.26%. Among them, optical grade is the main type of lithium niobate crystal, 2016 global optical grade lithium niobate crystal sales revenue of about $0.75 million, accounting for about 60% of global sales revenue. In addition, in 2016, Japan had the world's largest export volume and manufacturer of lithium niobate crystals, while Europe was the second largest sales market for lithium niobate crystals.

2. Lithium niobate modulator: The growth potential of thin film modulator is outstanding, welcoming the opportunity of localization

Lithium niobate modulator is the mainstream product in the electro-optic modulator market

An electro-optical modulator (EOM) is a modulator that utilizes the electro-optical effect of certain electro-optical crystals. Electro-optic effect means that when the electro-optic crystal is subjected to an applied electric field, the refractive index of the electro-optic crystal will change, and the light wave characteristics of the crystal will also change accordingly, so as to achieve the modulation of the amplitude, phase and polarization state of the optical signal. Electro-optic modulator includes phase modulator, intensity modulator, polarization modulator and so on.

The lithium niobate modulator has excellent performance and is mainly used in 100G-1.2T long-distance backbone coherent communication and single-wave 100/200G ultra-high-speed data centers. At present, there are three main optical modulation technologies in the industry: electro-optical modulators based on silicon light, indium phosphide and lithium niobate material platforms. Among them, silicon optical modulator has the advantages of low energy consumption and low cost, which is mainly used in short-range data communication transceiver module. Indium phosphide modulator is mainly used in medium and long distance optical communication network transceiver module; Lithium niobate modulator has the advantages of high bandwidth, good stability, high signal-to-noise ratio, low transmission loss, mature process, etc. It is the mainstream product in the current electro-optic modulator market, mainly used in long-distance backbone network coherent communication above 100Gbps to 1.2Tbps and single-wave 100/200Gbps ultra-high speed data center.

The future of lithium niobate modulators-thin film lithium niobate modulators

The bulk material lithium niobate modulator has a bottleneck in the improvement of key performance parameters, and the volume is large, which is not conducive to integration. The traditional lithium niobate modulator uses lithium niobate as the material. Although the bulk material lithium niobate modulator plays a key role in the transmission modulation of high-speed backbone network, it also has some shortcomings: (1) Due to the free carrier effect in lithium niobate materials, the improvement of key performance parameters such as signal quality, bandwidth, half-wave voltage and insertion loss of traditional lithium niobate based electro-optical modulators has gradually encountered bottlenecks, and is incompatible with CMOS technology; (2) Due to the lithium niobate material and process reasons, the size of the lithium niobate modulator cannot be reduced, and it is difficult to meet the requirements of "under the increasing requirements of the optical module for the port density, the size requirements of the optical device are getting smaller and smaller". This does not limit the application of lithium niobate in smaller and more demanding next-generation networks.

Thin film lithium niobate modulator has the advantages of large bandwidth, low power consumption and small size, and is expected to become an important development direction of modulators in the future. Compared with other optoelectronic materials, such as indium phosphide (cost limited), silicon light (performance power limited), lithium niobate crystal (size limited), thin film lithium niobate can achieve ultra-fast electro-optical efficiency and highly integrated optical waveguide, with large bandwidth, low power consumption, low loss, small size and other excellent characteristics, and can achieve large size wafer scale manufacturing. It is an ideal material for electro-optic modulator.

Demand: Long-term thin film lithium niobate modulator market space or more than 10 billion

By 2024, the global market for modulator chips and devices will reach $22.6 billion. According to the Optical Kuco 2021 annual report, citing WinterGreen Research data, it is predicted that the global modulator chip and device market (including communication, sensing and other) will grow to $22.6 billion in 2024.

We believe that with the rapid penetration of coherent optical communication, superimposed fiber optic gyro, ultrafine laser and other non-communication market demand, the long-term global thin film lithium niobate tuner market is expected to exceed 10 billion. 1) In the field of optical communication, according to our calculation, the long-term global market size of thin film lithium niobate modulator is about 5.9-8.8 billion yuan; 2) In the field of fiber optic gyro, according to our estimates, the market size of China's lithium niobate modulator will be about 5-6.7 billion yuan in 2024; 3) In the field of ultrafine lasers, according to our estimates, the global lithium niobate modulator market size is about 321 million US dollars (about 2.2 billion yuan) in 2024.

Supply: lithium niobate modulator industry high barriers to entry

Lithium niobate modulator belongs to the industry with high technology, heavy capital and long cycle, and the industry has high barriers to entry. Different from the structured division of labor mode of semiconductors, the current optoelectronics industry, especially the lithium niobate modulator industry, the global mainstream companies all adopt the IDM mode. The IDM model means that companies need to have a full range of capabilities from design, manufacturing, packaging and testing to sales. Lithium niobate modulator product design is difficult, the process is extremely complex, in the design, manufacturing process, packaging and other aspects, there are high thresholds, belongs to high technology, heavy capital, long cycle industry, it is difficult to have "light asset" companies out.