An Overview of Lithium Niobate Crystals and Their Applications

The introduction of "new infrastructure" has gradually brought 5G into our lives, and with it, the development of cloud computing, virtual reality, data communication, and high-definition video has been continuously advancing, driving the core optical network to upgrade towards ultra-high speed and ultra-long distance transmission. In this process, there is an indispensable core component - that is, the lithium niobate modulator (LiNbO3).

Figure 1: Lithium Niobate Modulator

It is reported that the lithium niobate modulator utilizes the electro-optic effect of lithium niobate crystals combined with optoelectronic integration technology to convert electronic data into photonicinformation, and is the core component for achieving electro-optic conversion. To understand its outstanding features, we must first start with the electro-optic effect and application of lithium niobate crystls.

About Lithium Niobate Crystals

Lithium niobate is a compound of niobium, lithium, and oxygen, a negative crystal with a high spontaneous polarization (0.70 C/m² at room temperature), and is the ferroelectric material with the highest Curie temperature (1210 ℃) found to date.

Figure 2: (a) 3-inch optical-grade nominal pure congruent lithium niobate crystal; (b) Iron-doped lithium niobate crystal

Lithium niobate crystals have two particularly noteworthy characteristics. First, they have a rich variety of electro-optic effects, including piezoelectric effect, electro-optic effect, nonlinear optical effect, photorefractive effect, photovoltaic effect, photoelastic effect, acousto-optic effect, and more. Second, the performance of lithium niobate crystals is highly tunable, which is due to the crystal lattice structure and rich defect structure of lithium niobate crystals. Many properties of lithium niobate crystals can be significantly adjusted through crystal composition, element doping, valence control, etc.

Additionally, lithium niobate crystals have stable physical and chemical properties, are easy to process, have a wide range of light transmission, have a large birefringence, and are easy to prepare high-quality optical waveguides. Therefore, optical modulators based on lithium niobate crystals have incomparable advantages in long-distance communication - not only have a very small chirp effect, high modulation bandwidth, and good extinction ratio, but also have excellent stability, making them the best in high-speed devices. Therefore, they are widely used in high-speed, high-bandwidth, long-distance communication.

Harvard once made such an evaluation of lithium niobate: If the center of the electronic revolution is named after silicon materials, then the birthplace of the photonics revolution is likely to be named after lithium niobate.

Preparation of Lithium Niobate Crystals

(1) Congruent lithium niobate crystals

For congruent lithium niobate crystals, the main preparation method is the Czochralski pulling method. Although methods such as the Bridgman method, the crucibleless method, and the temperature gradient method have also been used to prepare lithium niobate crystals, they do not have obvious advantages or clear application requirements compared with the Czochralski method, so they have not received widespread attention.

(2) Near-stoichiometric lithium niobate crystals

Although near-stoichiometric lithium niobate crystals have many excellent electro-optic properties, their ratio deviates from the congruent melting point of the liquid-solid, and high-quality crystals cannot be grown using the conventional Czochralski method. The main preparation methods currently used are the lithium-rich melt method, flux method, and diffusion method.

(3) Lithium niobate single crystal thin films

Lithium niobate single crystal thin films can be used in micro-nano structures such as optical waveguides and acoustic devices, as well as in the preparation of hybrid integrated devices such as silicon-based devices. People have been exploring the preparation of lithium niobate single crystal thin films for a long time, but the only method that has been truly applied is the "IonSlicing" technology, which has now been commercialized and can provide single crystal thin film products with a thickness of 300 to 900 nm.

Figure 3: Lithium Niobate Single Crystal Thin Film

Main Applications of Lithium Niobate Crystals

(1) Piezoelectric Applications

Lithium niobate crystals have a high Curie temperature, small temperature coefficient of the piezoelectric effect, high electromechanical coupling coefficient, low dielectric loss, stable physical and chemical properties, good processing performance, and are easy to prepare large-size high-quality crystals. They are an excellent piezoelectric crystal material.

Compared with the commonly used piezoelectric crystal quartz, lithium niobate crystals have a high sound speed and can prepare high-frequency devices. Therefore, lithium niobate crystals can be used for resonators, transducers, delay lines, filters, etc., and are applied in civil fields such as mobile communication, satellite communication, digital signal processing, television, broadcasting, radar, remote sensing and telemetry, as well as in military fields such as electronic countermeasures, fuses, guidance, etc., among which the most widely used are surface acoustic wave filter devices (SAWF).

Figure 4: (a) 2.4 GHz Surface Acoustic Wave Filter (SAW); (b) Miniature SAW Duplexers

(2) Optical Applications

In addition to the piezoelectric effect, lithium niobate crystals have a rich variety of electro-optic effects, among which the electro-optic effect and nonlinear optical effect are particularly prominent and widely used. Moreover, lithium niobate crystals can be prepared into high-quality optical waveguides through proton exchange or titanium diffusion, and periodic polarization crystals can be prepared through polarization reversal, so they are widely used in electro-optic modulators, phase modulators, integrated optical switches, electro-optic Q-switches, electro-optic deflection, high-voltage sensors, wavefront detection, optical parametric oscillators, and ferroelectric superlattices.

In addition, applications based on lithium niobate crystals such as birefringence wedge plates, holographic optical devices, infrared pyroelectric detectors, and erbium-doped waveguide lasers have also been reported.

Figure 5: Lithium Niobate Electro-Optic Modulator

(3) Dielectric Superlattices

In 1962, Armstrong et al. first proposed the concept of quasi-phase-matching (QPM), using the reciprocal 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 polarization χ2, and the periodic polarization domain structure with opposite polarization directions can be prepared in the ferroelectric to achieve quasi-phase matching technology. Crystals such as lithium niobate, lithium tantalate, and potassium titanyl phosphate can be prepared into periodic polarization crystals, among which lithium niobate crystals are the earliest and most widely used materials for the preparation and application of this technology.

The initial application of periodically poled lithium niobate crystals mainly considered laser frequency conversion. In 2014, Jin et al. designed an optical superlattice integrated photonic chip based on a reconfigurable lithium niobate waveguide optical path, and for the first time, high-efficiency generation of entangled photons and high-speed electro-optic modulation were achieved on the chip.

It can be said that the proposal and development of the dielectric superlattice theory have pushed the application of lithium niobate crystals and other ferroelectric crystals to a new height, with important application prospects in all-solid-state lasers, optical frequency combs, laser pulse compression, beam shaping, and entangled light sources in quantum communication.

Outlook for Lithium Niobate Crystals

(1) Acoustic Applications

The current fifth-generation mobile communication network (5G) deployment includes sub-6G frequency bands of 3-5 GHz and millimeter wave bands above 24 GHz. The increase in communication frequency not only requires that the piezoelectric performance of the crystal material can meet the requirements but also requires thinner crystal slices and smaller interdigital electrode spacing, posing a great challenge to the device manufacturing process.

Therefore, the surface acoustic wave filters widely used in the lithium niobate and lithium tantalate crystals before the 4G era are facing competition from bulk acoustic wave devices (BAW) and film bulk acoustic resonator devices (FBAR) in the 5G era.

Research on lithium niobate crystals for higher frequency filters is progressing rapidly, and the material and device manufacturing technology still shows great potential. With the development of lithium niobate single crystal thin film materials and new acoustic device technology, as one of the core devices for future 5G communication, front-end RF filters based on lithium niobate crystals have important application prospects.

(2) Optical Communication Applications

The optical modulator is a key device in high-speed optical communication networks. The future requirements for lithium niobate electro-optic modulators include higher modulation rates as well as miniaturization and integration.

At present, the commercial application of lithium niobate electro-optic modulators is mainly 40/100 Gbps,