The demand for greater link capacity in datacenters has pushed silicon photonic (SiP) optical interconnects from linear edge coupler arrays to two-dimensional grids of vertical grating couplers. To meet the challenge for low-profile, fiber-to-chip coupling, 3D optical waveguide circuits were integrated with total internal reflection (TIR) mirrors, enabling efficient horizontal fiber coupling and vertical SiP coupling through a fused silica interposer. TIR air disks of 30 µm diameter and ~3 µm thickness were fabricated up to 320 µm circuit depth by femtosecond laser irradiation followed by chemical etching (FLICE). The micro-mirror offered 0.54 to 1.2 dB reflection loss for waveguide-to-waveguide coupling within the interposer, as measured across the 1460 to 1625 nm telecom bands. The efficient TIR mirror lays the groundwork for flexible design of 3D photonic interposers to meet high-density interconnection requirements of SiP circuits to multicore fiber arrays for the telecom industry.