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Utilizing the thermooptical effect of polymers, a compact variable optical attenuator (VOA) based on multimode interference (MMI) is demonstrated. By directly modulating in the MMI region according to overlapping imaging of MMI, variable attenuation is achieved. The device length is less than 6 mm. The experiment shows that the VOA has 25-dB attenuation at the driving power of only 7.8 mW. The response time is around 3-4 ms. The polarization-dependence loss is 0.3 and 0.8 dB when the attenuation is less than 15 dB and about 20 dB, respectively.