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Three ultracompact integrated photonic devices are proposed, assisted by etched structures. A mode-order converter (MOC) is achieved with a footprint of 0.85 × 1.4 μm2, which exhibits high performances with insertion loss (IL) below 0.34 dB and mode purity (MP) above 90% within 100 nm waveband. Moreover, a mode blocking filter (MBF) is proposed based on a subwavelength grating with a footprint of 0.8 × 4.12 μm2, which allows the propagation of TM0 mode but blocks the TM1 mode with a remarkably high extinction ratio of 31.6 dB at the wavelength of 1550 nm. Finally, a compact reciprocal optical diode (ROD) is presented by cascading the abovementioned MOC and MBF with a footprint of 0.85 × 5.9 μm2. This ROD enables one-way transmissions for both TM0 mode and TM1 mode with contrast ratios of 19.74 dB and 32.04 dB, respectively. The design methodology paves a new way for mode manipulation in integrated multimode photonic circuits.