Microwave photonic (MWP) systems generate, manipulate, transmit, and measure high-speed radio-frequency (RF) signals in the optical domain. Converting RF signals in the optical domain improves the signal-processing bandwidth and speed and reduces power consumption by complex electronic systems. Optical modulation is the most important step in converting microwave signals into optical signals in all MWP systems. This usually determines the performance of the whole system, including its bandwidth, system loss, linearity, and dynamic range. Nonlinear distortion is introduced mainly by the nonlinearity of the Mach-Zehnder electro-optical modulator (MZM) used in MWP systems. The modulation curve of a typical MZM takes the form of a cosine function. To achieve the approximate linearity of the modulation, the working point of the modulator is fixed at an orthogonal offset point, where it cannot fulfill the requirements of a microwave photonic link. A stable and efficient MWP system requires the modulator to exhibit a low noise figure, i.e., less than 10 dB, and a high spurious free dynamic range (SFDR) exceeding 120 dB·Hz^2/3. The SFDRs of a typical silicon MZM and a microring modulator are approximately 97 dB·Hz^2/3 and 84 dB·Hz^2/3, respectively. Therefore, a high-linearity electro-optical modulator is urgently needed.

Two options exist for improving the linearity of an electro-optical modulator: the electrical and optical domains. In the electrical domain, one method is to use electronic predistortion. Introducing arcsine predistortion into the RF signal compensates for the cosine modulation. Another method is to employ electronic post compensation. This method uses digital sampling at the output to remove the distortion term produced by the modulator. These methods in the electrical domain do not fundamentally improve the linearity of the modulator, and they require an accurate control of the introduced distorted signal and additional high-speed electronic devices. With increasing working time, the MZM itself experiences unstable factors such as temperature drift. This requires electronic compensation to adapt dynamically to the change in the modulator, which leads to a complex system with limited performance.

In the optical domain, various methods can be used to improve the linearity of the modulator and achieve improved performance. These methods commonly include the dual-polarization control, MZM series/parallel, and microring-assisted MZM (RAMZM) methods. The basic idea of the dual-polarization method is to control the third-order distortion power of TE and TM light at the same strength but in opposite directions to cancel each other. This is usually achieved by adding a polarization controller at the input or output of the MZM modulator, and it produces limited enhancement. The dual-polarization control method can be combined with the MZM parallel method, which is called the polarization-multiplexing MZM parallel method. By combining three linearization methods—power-weighting control, polarization multiplexing, and bias control—it can enhance the third-order SFDR from 95.4 dB·Hz^2/3 to 112.3 dB·Hz^2/3 compared with a conventional MWP link, and the second-order SFDR is 94.6 dB·Hz^2/3(Fig. 9). The basic idea of the MZM series/parallel method is to use one MZM to compensate for the third-order distortion caused by another MZM. The two MZMs can be connected either in series or in parallel. The double-parallel MZM method utilizes two MZMs connected so that the third-order intermodulation distortions generated by the upper and lower MZMs cancel each other. This method has a wide optical bandwidth, high manufacturing tolerance, and temperature-variation tolerance. Specific control is provided by the driving RF and bias voltage. The electrical-power distribution ratio between the two modulators and DC bias angle of the RF signal in the two submodulators can be controlled appropriately to construct two nonlinear distortion signals with opposite phases, so as to cancel the IMD3 intermodulation distortion in the link. Using bias-voltage control, the nonlinear distortion term can be cancelled by controlling the phases of the RF electrical and input optical signals. The principle of the MZM series method is basically similar to that of the parallel method. The MZM series/parallel methods need to adjust the bias voltage and power or phase of the driving RF signals accurately. Because the modulator is sensitive to temperature, an additional control circuit is needed to stabilize the bias voltage and temperature. The microring-assisted MZM (RAMZM) method utilizes the superlinear phase modulation of the microring to compensate for the nonlinear cosine modulation function of the MZM. It has a simple structure and can achieve high linearity. The key point of RAMZM is to control the coupling coefficient between the microring and MZM arm. The fabrication tolerance of the directional coupler between the microring and modulation arm of the RAMZM is small, and the losses in the microring also affect the linearity. Lithium niobate exhibits the characteristics of high bandwidth, good modulation, and low loss. Further, it exhibits a high-linearity electro-optical effect that silicon does not. Recently, the thin films of lithium niobate on an insulator (LNOI) have become promising platforms for photonic integration. Using materials with high linearity, we can expect to improve the linearity of an electro-optical modulator further and make it practical.

Although linearization methods in the electrical domain are traditional and widely applicated, the required electronic control equipment is complex. These methods are used for linearization-compensation for traditional modulators with poor linearity. Conversely, optical-domain linearization is committed to eliminating nonlinear components through optical methods without introducing redundant electronics or compensation equipment. With the continuous development of integrated optics and on-chip integrated optoelectronic devices, the optical-domain linearization method has gradually become a research hot spot. Reported methods include the optical-polarization, MZM series/parallel, and microring-assisted MZM methods. These methods can be used in combination with each other. With the fast development of thin-film lithium niobate platforms, the prospect of fabricating high-performance electro-optical modulators with high linearity on LNOI appears promising.