Microwave photonics (MWP) is an emerging interdisciplinary subject which study and utilizes the interplay between light waves and microwaves. It takes advantage of optical devices and systems to perform microwave signal generation, manipulation and transmission tasks. In recent years, the rise of integrated photonics has further promised to build integrated MWP systems that are more compact, integrated and scalable.

At the heart of almost all MWP systems sits the electro-optic (EO) modulators, which are responsible for faithfully encoding signals from the microwave to the optical domain. During this process, modulation linearity, often quantified as the spurious-free dynamic range (SFDR), is the most critical metric that ultimately determines the MWP link performance. To improve the linearity performance of modulators, a number of linearization strategies have been reported in popular integrated optical platforms, including silicon (Si) and indium phosphide (InP) platforms, which however are often limited by the intrinsically nonlinear modulation mechanisms in these materials.

Recently, with the burgeoning development of the lithium niobate on insulator (LNOI) platform, EO modulators with low drive voltages and wide bandwidths pave the way for next-generation optical communication systems. However, high linearity modulators for microwave photonic systems have not been reported in LNOI. The linearity of state-of-the-art LNOI modulators is limited by the inherent nonlinear transfer function of a Mach-Zehnder interferometer (MZI), with reported SFDR values ~ 100 dB·Hz^2/3, restricting the performance of MWP links.

To address the linearity issue of LNOI modulators, a group led by Prof. Cheng Wang at the City University of Hong Kong has experimentally demonstrated an ultra-high-linearity integrated lithium niobate modulator.. Relevant research results were published in Photonics Research, Volume 10, No. 10, 2022 (Hanke Feng, Ke Zhang, Wenzhao Sun, Yangming Ren, Yiwen Zhang, Wenfu Zhang, Cheng Wang. Ultra-high-linearity integrated lithium niobate electro-optic modulators[J]. Photonics Research, 2022, 10(10): 2366).

The linear modulator reported in this paper adopts a micoring-assisted MZI (RA-MZI) configuration, as shown in Figure 1. By carefully designing the coupling coefficient of the micoring, the phase response of the micoring is utilized to compensate for the nonlinear characteristics of the MZI, so as to improve the linearity of the transfer function.

Fig. 1 Schematic of the high-linearity RAMZI modulator.

The experimental results show that the transfer function of the RA-MZI modulator clearly exhibits a broadened linear regime compared with that of the reference MZI modulator, as shown in Fig. 2 (a). In the linearity performance test, the RA-MZI modulator effectively suppressed the cubic term of the third-order intermodulation distortion (IMD3), leading to SFDRs as high as 120 dB· HZ^4/5, which is ~ 20dB higher than the previously record in the LNOI platform [Fig. 2 (b)].

Fig. 2. (a) Measured optical transmissions of the RAMZI modulator (red) and a reference MZI modulator (blue). (b) Measured SFDR results for the RAMZI (red) and the reference MZI (blue).

Further integration of these linearized modulators with functional components on the LNOI platform, e.g. filters, phase shifts, true delay lines, could lead to chip-scale and high-performance MWP systems with more advanced functionalities.