0 Introduction

With the advantages of large bandwidth, strong parallel processing ability and low transmission loss, microwave photonic technology has the potential to break the technical bottleneck of traditional microwave systems and improve the performance of military electronic information equipment such as radar, electronic warfare, communications, measurement and control. And built with discrete devices.microwave photonlarge volume, heavy weight, poor stability and other problems, these problems seriously restrict the application of microwave photonic technology in spaceborne and airborne platforms. Therefore, the integrated microwave photonic technology is becoming an important support to break the application of microwave photons in military electronic information systems and give full play to the advantages of microwave photonic technology.

At present, Si-based photonic integration technology, InP-based photonic integration technology in the field of optical communication after years of development, has matured, and has been a batch of products on the market; but for microwave photonic applications, these two types of photonic integration technology have some problems: such as Si modulator, InP modulator in the non-linear electro-optic coefficient and microwave photonic technology in the pursuit of high linearity, large dynamic characteristics; for example, the silicon optical switch that realizes optical path switching, whether it is based on the thermo-optic effect, the piezoelectric effect, or the dispersion effect based on carrier injection, has the problems of slow switching speed, power consumption and excessive heat consumption, and cannot meet the fast beam scanning, large array-scale microwave photonic applications.

Lithium niobate material has been the choice of high-speed electro-optic modulation material for its excellent linear electro-optic effect. However, the traditional lithium niobate electro-optic modulator uses block-shaped lithium niobate crystal material, and the device size is very large, which cannot meet the needs of integrated microwave photonic technology. How to incorporate the lithium niobate material with linear electro-optic coefficient into the integrated microwave photonic technology system has become the pursuit goal of relevant researchers. In 2018, the research team of Harvard University in the United States first reported the photonic integration technology based on thin-film lithium niobate in Nature. Because this technology has the advantages of high integration, large electro-optical modulation bandwidth, and high linearity of electro-optical effect, it immediately attracted the attention of the academic and industrial circles in the field of photonic integration and microwave photonics. In this paper, from the point of view of the application of microwave photonics, the influence and significance of the photonic integration technology based on thin film lithium niobate may be brought to the development of microwave photonics technology.

1 thin film lithium niobate material and thin film lithium niobate modulator

In the past two years, a new type of lithium niobate material has appeared, that is, by means of "ion slicing", the lithium niobate thin film is stripped from the bulk lithium niobate crystal and bonded to the Si wafer with a silicon dioxide buffer layer to form LNOI (LiNbO3-On-Insulator) material [5], which is called thin film lithium niobate material in this paper; A ridge waveguide with a height of more than one hundred nanometers can be etched on a thin film lithium niobate material by using an optimized dry etching process, and the effective refractive index difference of the formed waveguide can reach more than 0.8 (far exceeding the refractive index difference of a conventional lithium niobate waveguide 0.02), as shown in FIG. 1. This strong confinement waveguide makes it easier to match the optical field with the microwave field when the modulator is designed, thereby facilitating the realization of a lower half-wave voltage and a larger modulation bandwidth in a shorter length.

The emergence of low-loss lithium niobate sub-micron waveguides breaks the bottleneck of the high driving voltage of traditional lithium niobate electro-optic modulators. The electrode spacing can be reduced to ~ 5 μm, the overlap between the electric field and the optical mode field is greatly improved, and the Vπ · L decreases from more than 20 V · cm to less than 2.8V · cm. Therefore, under the same half-wave voltage, the device length can be greatly reduced compared with the traditional modulator. At the same time, after optimizing the width, thickness, spacing and other parameters of the traveling wave electrode, as shown in Figure 2, the modulator can have the ability of ultra-high modulation bandwidth greater than 100 GHz.

Fig.1 (a) mode field and (B) cross section morphology of typical thin film lithium niobate waveguide

Fig.1 （a） calculated mode distribution and （b） image of the cross-section of LN waveguide

Figure 2 (a) Waveguide and electrode structure of thin-film lithium niobate modulator, (B) Thin-film lithium niobate modulator chip (with various structures)

Fig.2 （a） Waveguide and electrode structure and （b） coreplate of LN modulator

The pairs of thin-film lithium niobate modulators with existing high-speed electro-optical modulators such as traditional lithium niobate commercial modulators, silicon-based modulators and phosphide (InP) modulators are shown in Table 1:

(1) Half-wave voltage-length product (V-L,V-cm), which measures the modulation efficiency of the modulator, the smaller the value, the higher the modulation efficiency;

(2)3 dB modulation bandwidth (GHz), a measure of the modulator's response to high-frequency modulation;

(3) Optical insertion loss (dB) of the modulation region. It can be seen from the table that the thin film lithium niobate modulator has obvious advantages in modulation bandwidth, half-wave voltage, optical insertion loss and so on.

Table 1 Comparison of traditional lithium niobate, thin film lithium niobate, silicon and InP electro-optic modulators

Tab.1 comparison of modulator fabricated by various materials

As the cornerstone of integrated optoelectronics, silicon has been developed so far, the process is mature, its miniaturization is conducive to large-scale integration of active/passive devices, and its modulator has been widely and deeply studied in the field of optical communication. The electro-optic modulation mechanism of silicon is mainly carrier depletion (carrier deple-tion), carrier injection (carrier injection), carrier accumulation (carrier accumulation). Among them, the bandwidth of the modulator and the linear degree carrier depletion mechanism are optimal, but due to the non-uniform overlap of the optical field distribution and the depletion region, the effect will introduce nonlinear second-order distortion and third-order intermodulation distortion, plus the absorption of carriers to light, which will lead to the reduction of optical modulation amplitude and signal distortion.

The InP modulator has outstanding electro-optic effect, and the multi-layer quantum well structure can realize ultra-high speed and low driving voltage modulator, and Vπ · L can reach 0.156V · mm. However, the refractive index changes with the electric field contains linear and nonlinear terms, and the increase of electric field intensity will make the second-order effect prominent.

Therefore, silicon and InP electro-optic modulators need to apply a bias voltage to form a pn junction when working, and the pn junction will bring absorption loss to light. However, the modulator size of the two is small, and the size of the commercial InP modulator is 1/4 of that of the LN modulator. High modulation efficiency, suitable for high-density short-distance digital optical transmission networks such as data centers. The electro-optic effect of lithium niobate has no optical absorption mechanism and low loss, and is suitable for large-capacity, high-speed long-distance coherent optical communication. In the application of microwave photons, the electro-optic coefficients of Si and InP are nonlinear, which is not suitable for the microwave photonic system that pursues high linearity and large dynamics, while lithium niobate material is very suitable for the application of microwave photons because of its completely linear electro-optic modulation coefficient;

2 High linearity electro-optic modulator and microwave photonic applications.

With the increasing demand for communication systems, in order to further improve the efficiency of signal transmission, people fuse photons and electrons to achieve complementary advantages, and microwave photonics is born. The conversion of electricity to light in microwave photonic systems requires the useelectro-optic modulatorThis critical step usually determines the performance of the entire system. Since the conversion of RF signals to the optical domain is an analog signal process, and ordinary electro-optic modulators have inherent nonlinearity, there is a serious signal distortion in the conversion process. In order to achieve approximately linear modulation, the operating point of the modulator is usually fixed at the orthogonal bias point, but it still cannot meet the linearity requirements of the microwave photonic link on the modulator, and people urgently need electro-optic modulators with high linearity.

The high-speed refractive index modulation of silicon materials is usually achieved through the free carrier plasma dispersion (FCD) effect. Both the FCD effect and the PN junction modulation are non-linear, making the silicon modulator less linear than the lithium niobate modulator. The lithium niobate material has the Pockels effect, and thus can exhibit excellent electro-optical modulation characteristics. At the same time, lithium niobate material has the advantages of large bandwidth, good modulation characteristics, low loss, easy integration and compatibility with semiconductor technology, etc., the use of thin film lithium niobate to produce high-performance electro-optic modulators, compared to silicon-based almost no "short board", but also to achieve high linearity, thin film lithium niobate on insulator (LNOI) electro-optic modulator has become a very potential development direction. With the development of thin film lithium niobate material preparation technology and waveguide etching technology, thin film lithium niobate electro-optic modulator with high conversion efficiency and higher integration has become a field of research and development in the international academic and industrial circles.

3 thin film lithium niobate based photonic integration

In the planning of DAP AR in the United States, the following evaluation of lithium niobate materials has been made: if the center of the electronic revolution is named after the silicon material that makes it possible, then the birthplace of the photonics revolution is likely to be named after lithium niobate. This is because lithium niobate has the characteristics of light collecting effect, acousto-optic effect, piezoelectric effect, thermoelectric effect and photorefractive effect, as if it were a silicon material in the optical field.

For the application of microwave photons, the advantages and disadvantages of thin-film lithium niobate materials and InP, SiO2, silicon, silicon nitride and other materials commonly used in the current photonic integration process are compared in terms of microwave photonic integration characteristics, as shown in Table 2:

Table 2 Comparison of the characteristics of thin film lithium niobate and InP, SiO2, silicon, silicon nitride and other materials in microwave photonic integration

Tab.2 comparison of microwave photonics chip based on various materials

In terms of light transmission characteristics, InP material has the largest on-chip transmission loss due to its absorption of commonly used 1550nm band light. SiO2 and silicon nitride have the best transmission characteristics, and the loss can reach the level of ~ 0.01dB/cm. At present, the processing technology level of thin film lithium niobate waveguide can reach the level of 0.03dB/cm. With the continuous improvement of the technology level in the future, the loss of thin film lithium niobate waveguide has the potential to be further reduced; therefore, the thin film lithium niobate material will show good performance for passive optical structures such as photosynthetic circuits, shunts, and microring.

In terms of light generation, only InP has the ability to directly emit light; therefore, for microwave photonic applications, it is necessary to introduce an InP-based light source on an LNOI-based photonic integrated chip by flip-chip bonding or epitaxial growth.

In terms of optical modulation, the foregoing has focused on the fact that thin-film lithium niobate materials are easier to achieve larger modulation bandwidth, lower half-wave voltage and lower transmission loss than InP and Si; and the high-linearity electro-optical modulation of thin-film lithium niobate materials is essential for all microwave photonic applications.

In terms of optical routing, the high-speed electro-optical response characteristics of thin-film lithium niobate materials enable LNOI-based optical switches to have the ability of high-speed optical routing switching, and the power consumption of this high-speed switching is also very low. For the typical application of the integrated microwave photonic technology, which is an optically controlled beam forming chip, the ability of high-speed switching can well meet the requirements of fast beam scanning, the ultra-low power consumption is well adapted to the stringent power consumption requirements of large-scale phased array systems. Although InP-based optical switch can also realize high-speed optical path switching, it will introduce large noise, especially when multi-stage optical switches are cascaded, the noise coefficient will be seriously deteriorated. Silicon, SiO2 and silicon nitride materials can only realize optical path switching through thermo-optic effect or carrier dispersion effect, which has the disadvantages of large power consumption and slow switching speed. When the array scale of phased array is large, the requirements for power consumption cannot be met.

In terms of light amplification, InP-basedSemiconductor Optical Amplifier (SOA)It has been commercially available, but it has the disadvantages of large noise figure and low saturation output power, which is not conducive to the application of microwave photons. The parametric amplification process in the thin film lithium niobate waveguide based on periodic excitation flip can achieve low noise and high power on-chip optical amplification, which can well meet the needs of integrated microwave photonic technology for on-chip optical amplification.

In terms of light detection, thin film lithium niobate has good transmission characteristics for light in the 1550 nm band; it cannot achieve the function of photoelectric conversion, so for microwave photonic applications, in order to meet the needs of photoelectric conversion on the chip. It is necessary to introduce the detection unit of InGaAs or Ge-Si on the LNOI-based photonic integrated chip by means of flip-chip bonding or epitaxial growth.

In terms of coupling with optical fiber, because the optical fiber itself is SiO2 material, the mode field of SiO2 waveguide has the highest matching degree with the mode field of optical fiber, and the coupling is the most convenient. However, the mode field diameter of the strong confinement waveguide of thin film lithium niobate is about 1μm, which is quite different from the mode field of optical fiber. Therefore, appropriate spot transformation must be carried out to match the mode field of optical fiber.

In terms of integration, whether various materials have a high degree of integration potential depends mainly on the bending radius of the waveguide (affected by the degree of limitation of the waveguide mode field); strong confinement waveguides allow a smaller bending radius, which is more conducive to achieving a high degree of integration; therefore, thin film lithium niobate waveguides have the potential to achieve high integration.

Therefore, the emergence of thin-film lithium niobate makes it possible for lithium niobate materials to truly take on the important task of optical "silicon"; and for the application of microwave photons, the advantages of thin-film lithium niobate are even more prominent.

4 Conclusions

This paper summarizes the development trend and prospect of thin film lithium niobate crystal materials and high linearity electro-optic modulators for microwave photonic technology to achieve high linear modulation; by comparing the advantages of thin film lithium niobate materials with InP, SiO2, silicon, silicon nitride and other materials commonly used in the existing photonic integration process platform in terms of photonic integration characteristics, it is concluded that thin film lithium niobate will become an important cornerstone of integrated microwave photonic technology and is of great significance to the development of photonic integrated technology in the future.

Source: China Aerospace Journal Platform

Author: Sun Lijun; Jiang Cheng; Cai Xinlun; Li Hao; Peng Song; Zhang Yu