The optical Fano resonance (Fano resonance) phenomenon based on coupled microcavity systems originates from the interferometric interaction between discrete and (quasi-)continuous modes. When the phase shift between modes is varied in the pi range, the resonance line patterns in the transmission spectrum can precisely show an evolution from classical Lorentzian line patterns, to antisymmetric peaks, and even electromagnetically induced transparency-like (EIT-like) spikes. Depending on the different resonance properties, the above lineshapes are also used in the fields of optical sensing, all-optical light opening, and band-pass filtering, etc., respectively. If the flexible tuning of different line shapes can be realized in a single Fano resonance system, the functional range of the device will be significantly extended. However, most of the current modulation means for Fano line shapes mainly focus on the transmission phase between modes, change the effective refractive index difference between modes by introducing additive modulation means such as thermo-optic effect, or directly change the geometrical parameters of the structure, which has low modulation efficiency and limited range of change of line shapes, and the latter is difficult to be applied to the scenario of on-chip integration.

In this paper, we propose a grating coupler-assisted tapered waveguide-coupled microcircular resonant cavity structure to control the coupling and dispersion between modes by designing the geometrical parameters of each component, and utilize the interference between the non-resonant TE1 modes in the waveguide and TE0 modes in the microcavity to realize on-chip Fano resonance spectra on thin-film lithium niobate platforms with concurrently frequency-domain-space twofold dimensional tunability, and The theoretical tunability of the intermode phase shift in either dimension is up to 2 pi, which means that we can obtain arbitrary Fano line shapes in any resonant mode in the C-band.

Figure1 (a) Grating-assisted fiber to chip coupling system. (b-e) Scanning electron microscopic images of the waveguide coupled micro-ring resonator structure on thin film lithium niobate. (f)Transmission spectrum of the grating-coupled system without and (g) with the micro-ring resonator. (h)Theoretically fitted q factors of resonance modes, inset: corresponding phase-shift. (i) Fano fitting of the selected modes. (j)Numerical simulated lineshapes with α = 0.9998, t = 0.9917 and f2 = 0.011.

Figure 1 (a-e) shows the grating coupling system used in the experiment and a physical view of the Fano resonance structure on thin-film lithium niobate. The TE0 and TE1 modes within the on-chip waveguide are excited using a single-mode fiber coupled grating. As the width of the tapered waveguide is reduced from 15 um to 0.8 um in the coupling region, the dispersion between the two modes is further increased. As shown in Fig. 1(f), when only the conical waveguide exists, the background intensity of the transmission spectrum shows a complete oscillation period in the range of 1520-1570 nm, representing that the intermode phase shift can be changed by 2\pi through C-band sweeping only; when the above structure is coupled with the micro-annular resonance cavity, a series of resonance line shapes with different morphologies appear in the transmission spectrum. By fitting the individual resonant modes, the Fano factor q characterizing the asymmetry of the line patterns can be obtained, from which the corresponding intermodal phase shifts can be further calculated, as shown in Fig. 1(h). In addition, based on the transmission spectra measured in the experiment, we also estimate the parameters of the system, based on which the numerical simulation results are in good agreement with the experiment (see Fig. 1(j)).

Figure2 (a) Schematic of the grating-coupler. Inset: field distribution of the fundamental and second-order TE modes at the end of the taper-waveguide with the largest width of 15 μm. (b-e) Selective Fano lineshapes with different grating-coupling position and wavelength, and (f) corresponding phase-shifts.

On the other hand, we consider the transverse field phase characteristics of the anti-symmetric distribution of the waveguide TE1 mode. As shown in Fig. 2(a), the cross-field phase of the two flaps in TE1 mode naturally differs by \pi. this phase difference in the transverse space also affects the phase shift between it and TE0 mode. When we move the single-mode fiber laterally at the receiving end, we can flexibly and quickly achieve \pi range of modulation of the intermode phase shift, and even the transmission line pattern. On the basis of this, by adding the adjustment of the coupling position of the fiber at the incident end, we can further change the initial phase relationship during the mode excitation, thus extending the modulation range up to 2\pi. Fig. 2(b-e) shows the continuous change of the resonant line shape in one cycle when changing the coupling position of the fiber at the input and output ends on the experimental side, which is in good agreement with the theoretical prediction. Meanwhile, the relative phase relations between the resonant modes at different wavelengths remain almost unchanged in the process, showing the great potential of our structure in joint multi-dimensional tuning of the Fano line shape, which is expected to play an important role in the fields of chip-scale optical sensing and signal processing.

The research was published in “Tingge Yuan†, Xueyi Wang†, Jiangwei Wu, Yuping Chen, and Xianfeng Chen. Frequency-space selective Fano resonance based on a micro-ring resonator on lithium niobate on insulator, Laser & Photonics Reviews，2400457 (1 of 9), (2024)”.

Link: https://doi.org/10.1002/lpor.202400457