A new strain sensor is proposed which can measure both through-thickness and in-plane strains within a laminated fibre-reinforced composite material. The planar sensor uses novel embedded planar optic sensors and is demonstrated on a carbon fibre composite test coupon. The planar optical sensor was fabricated via flame hydrolysis deposition and was physically machined to reduce the substrate thickness to <50 µm. Strain components are decoupled through monitoring the transverse electric and transverse magnetic effective refractive indices of two orthogonal planar waveguides. The work investigates if the embedded planar sensors have any mechanical knockdown effects on the composite structure and demonstrates their effectiveness at decoupling three orthogonal strain components.