Single Stage Ultrafast Laser Inscription of a Side-Polished Fiber-Like Waveguide Sensor

Ultrafast laser inscription (ULI) has been used to fabricate an 18 mm long sub-surface optical waveguide in fused silica. The waveguide started and terminated 75 below the substrate surface, approaching it to a minimum depth of for the central section. The device was fabricated in a single step, exhibiting a fiber-to-fiber insertion loss of 2.6 dB. When a high refractive index oil was placed on the substrate surface, the polarization-averaged waveguide insertion loss increased by 3.7 dB. This work demonstrates that a single ULI stage can be used to fabricate optical waveguides sufficiently close to a substrate surface for evanescent field sensing applications.