Optical microcavity as one of the building blocks of photonic integrated circuits has enabled a variety of applications including nonlinear optics, low-threshold laser and single molecule detection, its small mode volume and high-quality factor (Q factor). Especially in unlabeled sensing or environmental monitoring as a great supplement for medical and environmental research. However, the most adapted sensing scheme of mode shifting and splitting relies critically on the cavity’s Q factor. Overcoming such limitations by introducing new principles to microcavity systems thus becomes urgent.

In this work, we propose a design method to improve the resolution of microcavity sensors through multi-mode coupling with a compact, on-chip integrated micro cavity system. Based on a waveguide to micro-racetrack structure, our design allows efficient and distinct inter-mode coupling at the 1520 to 1555 nm band for both racetrack quasi-TE and TM modes leading to frequency shifts and sharp lineshape, which helps to distinguish two modes during self-referenced sensing and breaks the sensitivity’s dependence on the Q factor that microcavity sensors always suffer from.

Conventionally, when two modes are weakly coupled, for instance one discrete mode and one continuous mode, the discrete mode would experience a frequency shift and linewidth sharpening determined by their detuned wavelength and coupling strength. While the coupling includes two discrete modes simultaneously,
they will likely experience different shifts for that they possess distinct coupling strength and eigenfrequencies. Thus by controlling the composition of three modes, we could manipulate the relative frequency difference of them after coupling happened, that in certain scenarios would help us to distinguish two discrete modes with higher resolution.

Figure 1 Schematic of the on-chip micro-racetrack multi-mode coupled system.

In experiments, the optimization of the pulley waveguide’s geometrical structure allows the resonance mode to be over-coupled. The coupling strength of TE modes also excels TM modes by nearly 10 times, which allows distinct mode coupling condition for different polarization. In the frequency domain, the coupling effect creates a sharp, asymmetric lineshape in the transmission spectrum. This leads to a 3 times higher mode resolution. While experiencing minor refractive index change tuned by temperature. The multi-mode coupled system offered a 7.2 times higher sensitivity at 44pm/℃. Meanwhile, the coupled system possesses a lineshape sensitive to the background phase, its lineshape contrast ratio shifted 6.46 × 10^-3/pm which is 24 times larger than the non-mode-coupled system manufactured in the same chip. Our work allows for the comprehensive enhancement on every dimension of the self-referenced thermal sensor with a highly integrated system.

Figure 2 (a) Transmission spectrum of the multi-mode coupled and (b) uncoupled system under temperature control. (c)Frequency shifts. (d)Changes in lineshape contrast ratio. (e) Transmission spectrum of the multi-mode coupled system. (f)Mode separation in the frequency domain.

This work is published at “Xueyi Wang, Tingge Yuan, Jiangwei Wu, Yuping Chen, and Xianfeng Chen,  Enhanced temperature sensing by multi-mode coupling in an on-chip microcavity system, Laser & Photonics Reviews, 2300760 (2024)”。

Link: https://doi.org/10.1002/lpor.202300760