Advantages and significance of thin film lithium niobate in integrated microwave photon technology

Microwave photon technology has the advantages of large working bandwidth, strong parallel processing ability and low transmission loss, which has the potential to break the technical bottleneck of traditional microwave system and improve the performance of military electronic information equipment such as radar, electronic warfare, communication and measurement and control. However, the microwave photon system based on discrete devices has some problems such as large volume, heavy weight and poor stability, which seriously restrict the application of microwave photon technology in spaceborne and airborne platforms. Therefore, integrated microwave photon technology is becoming an important support to break the application of microwave photon in military electronic information system and give full play to the advantages of microwave photon technology.

At present, SI-based photonic integration technology and INP-based photonic integration technology have become more and more mature after years of development in the field of optical communication, and a lot of products have been put into the market. However, for the application of microwave photon, there are some problems in these two kinds of photon integration technologies: for example, the nonlinear electro-optical coefficient of Si modulator and InP modulator is contrary to the high linearity and large dynamic characteristics pursued by microwave photon technology; For example, the silicon optical switch that realizes optical path switching, whether based on thermal-optical effect, piezoelectric effect, or carrier injection dispersion effect, has the problems of slow switching speed, power consumption and heat consumption, which can not meet the fast beam scanning and large array scale microwave photon applications.

Lithium niobate has always been the first choice for high speed electro-optic modulation materials because of its excellent linear electro-optic effect. However, the traditional lithium niobate electro-optical modulator is made of massive lithium niobate crystal material, and the device size is very large, which can not meet the needs of integrated microwave photon technology. How to integrate lithium niobate materials with linear electro-optical coefficient into the integrated microwave photon technology system has become the goal of relevant researchers. In 2018, a research team from Harvard University in the United States first reported the photonic integration technology based on thin film lithium niobate in Nature, because the technology has the advantages of high integration, large electro-optical modulation bandwidth, and high linearity of electro-optical effect, once launched, it immediately caused the academic and industrial attention in the field of photonic integration and microwave photonics. From the perspective of microwave photon application, this paper reviews the influence and significance of photon integration technology based on thin film lithium niobate on the development of microwave photon technology.

Thin film lithium niobate material and thin film lithium niobate modulator
In recent two years, a new type of lithium niobate material has emerged, that is, the lithium niobate film is exfoliated from the massive lithium niobate crystal by the method of "ion slicing" and bonded to the Si wafer with a silica buffer layer to form LNOI (LiNbO3-On-Insulator) material [5], which is called thin film lithium niobate material in this paper. Ridge waveguides with a height of more than 100 nanometers can be etched on thin film lithium niobate materials by optimized dry etching process, and the effective refractive index difference of the waveguides formed can reach more than 0.8 (far higher than the refractive index difference of traditional lithium niobate waveguides of 0.02), as shown in Figure 1. The strongly restricted waveguide makes it easier to match the light field with the microwave field when designing the modulator. Thus, it is beneficial to achieve lower half-wave voltage and larger modulation bandwidth in a shorter length.

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

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

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

The comparison of thin film lithium niobate modulators with traditional lithium niobate commercial modulators, silicon-based modulators and indium phosphide (InP) modulators and other existing high-speed electro-optical modulators, the main parameters of the comparison include:
(1) Half-wave volt-length product (vπ ·L, V·cm), measuring the modulation efficiency of the modulator, the smaller the value, the higher the modulation efficiency;
(2) 3 dB modulation bandwidth (GHz), which measures the response of the modulator to high-frequency modulation;
(3) Optical insertion loss (dB) in the modulation region. It can be seen from the table that thin film lithium niobate modulator has obvious advantages in modulation bandwidth, half-wave voltage, optical interpolation loss and so on.

Silicon, as the cornerstone of integrated optoelectronics, has been developed so far, the process is mature, its miniaturization is conducive to the large-scale integration of active/passive devices, and its modulator has been widely and deeply studied in the field of optical communication. The electro-optical modulation mechanism of silicon is mainly carrier depling-tion, carrier injection and carrier accumulation. Among them, the bandwidth of the modulator is optimal with the linear degree carrier depletion mechanism, but because the optical field distribution overlaps with the non-uniformity of the depletion region, this effect will introduce nonlinear second-order distortion and third-order intermodulation distortion terms, coupled with the absorption effect of the carrier on the light, which will lead to the reduction of the optical modulation amplitude and signal distortion.

The InP modulator has outstanding electro-optical effects, and the multi-layer quantum well structure can realize ultra-high rate and low driving voltage modulators with Vπ·L up to 0.156V · mm. However, the variation of refractive index with electric field includes 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 bias to form pn junction when they work, and pn junction will bring absorption loss to light. However, the modulator size of these two is small, the commercial InP modulator size is 1/4 of the LN modulator. High modulation efficiency, suitable for high density and short distance digital optical transmission networks such as data centers. The electro-optical effect of lithium niobate has no light absorption mechanism and low loss, which is suitable for long distance coherent optical communication with large capacity and high rate. In the microwave photon application, the electro-optical coefficients of Si and InP are nonlinear, which is not suitable for the microwave photon system which pursues high linearity and large dynamics. The lithium niobate material is very suitable for microwave photon application because of its completely linear electro-optic modulation coefficient.