Integrated photonics on lithium niobate thin film (LNTF) has emerged as an outstanding approach for on-chip scalable optical signal manipulation and processing, which has attracted widespread attention and stimulated tremendous enthusiasm in recent years. In addition to the excellent optical properties of lithium niobate materials, LNTF provides the capability for dense integration of photonic integrated circuits (PICs) on the novel platform while compatible with conventional CMOS technology, resulting in various compact and superior devices that are inaccessible to their conventional counterparts. As one critical component among integration applications, the Mach–Zehnder interferometer (MZI), which can be balanced or unbalanced, has a wide range of applications. Amplitude modulators based on the balanced MZI structure are ubiquitous and play a significant role in optical switching, processing, and communication. Unbalanced MZIs (UMZIs), on the other hand, can be used to implement wavelength sensitive devices, such as comb filters, which have been demonstrated as the key component of sensors, filters, spectrometers in photonics, and neural and network applications. UMZIs on LNTF are particularly suitable for such tunable devices, as thermo-optic and electro-optic effects can be easily incorporated into the devices.

In this letter, we report on an optimized UMZI designed and fabricated in LNTF via electron beam lithography (EBL) and inductively coupled plasma (ICP) etching. A high extinction ratio (ER) of 32.4 dB and low extra loss (EL) of 1.14 dB is obtained on the basis of the total interferometer. Both the thermo-optic and electro-optic tunability of the fabricated UMZI is experimentally investigated. The demonstrated structure has applications for sensing and filtering in PICs on the novel platform. Figure 1(a) and 1(b) is the optical microscopy image of the fabricated UMZI and the SEM image of the cross-sectional view of the nanowaveguide. Figure 1(c) indicates the simulated electric field distribution of the TE0 mode at 1550 nm. To characterize the thermal and electro-optic tunability, we measured normalized transmission spectra of the UMZI at different temperatures and voltages, and the free spectral range of the spectra and the output variation of specific wavelengths were also described. The results of thermally and electrically tuned are displayed in Fig. 2 and Fig. 3, respectively. A linear tuning efficiency of 42.8 pm/°C and 55.2 pm/V is obtained. The demonstrated structure has applications for sensing and filtering in PICs on the novel platform.

Fig. 1. (a) Optical microscope image of the UMZI. Insets: SEM images of the grating coupler and MMI components, respectively. (b) False-color SEM image of the cross section of the ridge waveguide. (c) The simulation of the fundamental TE mode at 1550 nm.

Fig. 2 (a) Thermo-optic tuning performance of the UMZI. (b) The FSRs of the UMZI at temperatures from 20°C to 70°C. (c) Wavelength shift and FSR as a function of the temperature. (d) The measured output power varies with temperature and its fitting curve at 1534.34 nm.

Fig. 3. (a) Electro-optical tuning performance of the UMZI. (b) The FSRs of the UMZI at voltages from 20 V to 45 V. (c) Wavelength shift and FSR as a function of the voltage. (d) The measured output power varies with voltage and its fitting curve at 1549.3 nm.

This research is published by “Xuerui Sun, Yinan Wu, Chuanyi Lu, Yuting Zhang, Hao Li, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen, Experimental investigation on the unbalanced Mach–Zehnder interferometer on lithium niobate thin film, Chinese Optics Letters, 20(10), 101301 (2022)”.

Link: http://www.researching.cn/EN/Article/OJfb54df7d0e1f2678