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We propose a high-resolution compact spectrometer based on a multimode waveguide (MMW) integrated with a planar photonic lantern (PL). Four modes were launched into the MMW via asymmetric directional couplers (ADCs) and individually switched by four modulators. The mode field of the MMW was spatially sampled by four separate detectors. Measurement of the 64 possible states (16 switch states x 4 modes) allowed the spectrum of the optical signal to be recovered. Experimentally, we demonstrated spectral correlation width of 0.16-nm in the measurement at 1560 nm using a MMW with 0.5 mm length for both sparse and continuous spectra retrieval. To scale up, the spectrometer resolution can be estimated by δ λ = λ ² /Δ n _eff L while the operational bandwidth can be estimated to be 2 ^N × N × δ λ, with N denoting the number of modes supported in the whole system. This work may lead to ultra-compact …