Compact Fourier‐Transform Spectrometer on Thin‐Film Lithium Niobate via Mode‐Circulating Modulation

Fourier‐transform spectrometers (FTSs) based on electro‐optic modulation offer a promising route toward compact and low‐power on‐chip spectroscopy. However, their spectral resolution is typically constrained by the physical length of the modulation arm. In this work, we present a compact on‐chip FTS on thin‐film lithium niobate that exploits mode‐circulating electro‐optic modulation. In this scheme, guided light is sequentially converted among multiple spatial modes to accumulate multiple phase modulation events within a single physical modulation arm. Using a three‐mode circulation scheme, a spectrometer with a modulation arm length of only 1 cm achieves a Fourier‐limited spectral resolution of 19 nm, comparable to that of conventional single‐pass designs requiring substantially longer electrodes. Furthermore, by exploiting the full transmission response through a regularized regression reconstruction framework, an effective spectral resolution of 0.4 nm is demonstrated over a 30 nm bandwidth in the C‐band. These results establish mode‐circulating modulation as a scalable and versatile strategy for realizing compact, low‐power, and high‐performance integrated spectrometers for on‐chip sensing and photonic applications.