Optical filters are extensively employed in wavelength division multiplexing (WDM) systems, microwave photonics, external cavity lasers, and optical differentiators. The implementation of these filters relies on various photonic integrated circuits (PICs), such as a long-period grating (LPG), Bragg grating, grating-assisted directional coupler, Mach-Zehnder interference (MZI), micro-ring resonator (MRR), MRR-MZI, etc. To process flexibly the optical signals in the next-generation reconfigurable WDM net-work, optical filters with bandwidth and wavelength tunability simultaneously are highly desirable and have attracted ever-increasing attention. To enlarge the wavelength and the bandwidth tuning range, the optical filters based on MRR-cascaded, MRR-assisted-MZI, and MZI-assisted-MRR structures have been proposed. Most prior MZI-assisted-MRR filters are realized on mature material platforms such as silicon on insulator (SOI) and silicon nitride platforms.
As an emerging platform for PIC devices, lithium niobate (LN) on insulator (LNOI) is a low-loss material and capable of realizing high-index-contrast waveguides and hence compact PIC devices. Moreover, the LNOI platform offers the opportunity to implement simultaneously electro-optic (EO) and thermo-optic (TO) PIC devices with a moderate power consumption and a low driving voltage, as it features not only TO characteris-tics but also a strong electro-optic (EO) response (r33=30 pm/V). Up to now, various TO or EO devices or both on LNOI, including modulators, optical filters, interleavers, switches, and so on, have been demonstrated experimentally.
Prof. Kaixin Chen from the University of Electronic Science and Technology of China have designed and demonstrated experimentally an MZI-assisted MRR tunable filter based on the LNOI platform, verifying its performance advantages. The proposed filter combines the advantages of the MZI and the MRR structures, and the asymmetric MZI (AMZI) and MRR used in this work facilitate a periodically variable free spectral range (FSR) of the transmission spectrum at the through and drop ports, and hence have a tunable bandwidth. The research results are published in Chinese Optics Letters, Vol. 21, Issue 8, 2023: Wanzhen Wu, Yuzhe Sun, Hao Zhang, Zhefeng Hu, Jixin Chen, and Kaixin Chen. MZI-assisted-MRR filter with a tunable bandwidth and dip wavelength on the LNOI platform [J]. Chinese Optics Letters, 2025, 23(8): 081302.
The specific structure of the tunable filter is schematically shown in Figure 1. The filter consists of a straight waveguide and an MZI-assisted MRR formed by bending one arm (Arm1) of a 2 × 2 AMZI and connecting its two input and output waveguides. The MRR is coupled to the straight waveguide via a two-mode interferometer (TMI1). Here, the use of TMI couplers instead of directional couplers (DCs) eliminates the need for fabricating submicron-sized coupling gaps, thereby reducing the difficulty of the MRR fabrication, but the resulting MRR performance and dimensions also remain comparable to those achieved using DCs. The radius of the MRR is r. In addition, two TMIs (TMI2 and TMI3) are used to split and combine light signals in the aforementioned AMZI. These three TMIs have the same widths wT but different lengths li (i = 1, 2, 3). Except for the three TMIs, all other waveguides are single-mode waveguides (SMWs) with identical widths of wS. Considering that the splitting ratio of the three TMIs and the length difference Δl (=l5−l4) between Arm1 and Arm2 will affect the 3 dB bandwidth (W3-dB), extinction ratio (ER), and FSR of the transmission spectra, four electrode heaters are placed on three TMIs and Arm1 to provide TO tuning. In addition, a pair of tuning electrodes is placed on both sides of Arm2 to provide EO tuning.
Fig. 1. (a) Three-dimensional schematic diagram of the proposed filter incorporating waveguides and electrodes. (b) Top view of the waveguide layout of the proposed filter. (c) Cross-sectional view of the LNOI waveguides used in this work.
To characterize the fabricated filters, light from an amplified spontaneous emission (ASE) source (1530−1600 nm) was launched into the input port of the filter under test via a lensed single-mode fiber (SMF). The filter was placed on a metal platform with a thermoelectric cooler stuck underneath. The polarization state of the light was controlled with an inline fiber polarizer and a polarization controller (PC). The position of the fiber was adjusted carefully to launch the light exactly at the center of the LNOI core to excite only the fundamental mode. The output light from the through port or drop port was collected with another lensed SMF and monitored with an optical spectrum analyzer (OSA) (Anristu, MS97740A). With this method, the transmission spectra of all five samples are measured sequentially at a temperature of 22°C.
Experimental results demonstrate that the filter achieves a maximum load Q factor of 31942 and a maximum ER of 21.3 dB, together with the minimum and maximum 3 dB bandwidths of 27.86 and 31.74 GHz at the through port, respectively, and 14.68 and 30.69 GHz at the drop port. Meanwhile, the device exhibits excellent tuning performance, with TO tuning rates of the dip wavelength approximately −6.5 pm/mW and EO tuning rates approximately −65.09 pm/V.
The results indicate that the proposed filter can achieve EO and TO tuning for the bandwidth and the dip wavelength simultaneously. Moreover, the results also reveal significant variations in TO tuning efficiency across different regions, with higher efficiency when tuning the two arms. Similarly, EO tuning of the arm also achieves substantial tuning efficiency. This flexible and highly efficient tunable characteristic positions the proposed filter with significant potential for applications in optical communication and optical information processing systems, particularly in enabling multifunctional filtering operations.
