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A monolithic lab-on-a-chip fabricated by femtosecond laser micromachining capable of label-free biosensing is reported. The device is entirely made of fused silica, and consists of a microdisk resonator integrated inside a microfluidic channel. Whispering gallery modes are excited by the evanescent field of a circular suspended waveguide, also incorporated within the channel. Thermal annealing is performed to decrease the surface roughness of the microstructures to a nanometric scale, thereby reducing intrinsic losses and maximizing the Q-factor. Further, thermally-induced morphing is used to position, with submicrometric precision, the suspended waveguide tangent to the microresonator to enhance the spatial overlap between the evanescent field of both optical modes. With this fabrication method and geometry, the alignment between the waveguide and the resonator is robust and guaranteed at all instances. A maximum sensitivity of 121.5 nm/RIU was obtained at a refractive index of 1.363, whereas near the refractive index range of water-based solutions the sensitivity is 40 nm/RIU. A high Q-factor of 105 is kept throughout the entire measurement range.

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The fabrication of optical add-drop multiplexers in fused silica is demonstrated, for the first time to our knowledge, using the femtosecond laser direct writing technique. To achieve this, a Mach-Zehnder interferometer configuration was used for the signal routing by the implementation of 3-dB directional couplers, along with Bragg grating waveguides for wavelength selectivity. The fabrication of all individual devices required was optimized. The behavior of the fabricated add-drop multiplexer was characterized at around 1550 nm, where a 3-dB bandwidth of 0.19 ± 0.01 nm was obtained along with an intrachannel and adjacent interchannel crosstalk of -30 and -20 dB at Δλ = ± 0.75 nm, respectively. This study shows that such complex devices can be manufactured by femtosecond laser direct writing, with future improvements being discussed.Index Terms-Add-drop multiplexer, femtosecond laser, integrated optics, laser materials-processing applications, optical device fabrication, wavelength division multiplexing.