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A physical micromachining technique is demonstrated in a low-loss flat-fiber substrate to fabricate a multimode interference (MMI) device. The flat-fiber substrate is a low-index-contrast material; however, by making use of two physically micromachined trenches, lateral confinement is achieved providing high index contrast for the MMI region. A 1 ?? 3 MMI device exhibiting 1.89 dB of excess loss has been demonstrated.