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Dipartimento di Fisica - Politecnico di Milano

Characterization of photonic and optofluidic devices

In this laboratory photonic and optofluidic devices are characterized. Several functional areas can be outlined:
 

Optical Microscope inspection

Visual characterization of the device is performed with an optical microscope that permits to clearly image micro features of the object. This analysis is important to view size, entity and topology of the permanent modifications induced by femtosecond laser pulses on transparent substrates, i.e. optical waveguides and microfluidic channels. To better emphasize the refractive index change induced or the surface structure, a differential interference contrast (DIC) microscopy is available.
 

Refractive index profile measurement

The refractive index change induced by femtosecond laser exposure can be directly measured using a refractive index profilometer (Rinck Electronik 2D). By the refractive near field method it gives refractive index changes with an accuracy of 10-4 and a spatial resolution of 0.5 µm.

 

Waveguide coupling, characterization and pigtailing
The characterization of an optical waveguide includes the measurement of the propagation losses and the analysis of the guided mode profile to estimate the coupling losses. Lasers at different wavelengths are available in the lab: He-Ne lasers (633 and 543 nm), semiconductor lasers (800 and 980nm, tunable in the 1.46–1.60 μm range), solid state lasers (532 nm and 473 nm). Either fiber-butt-coupling or end-fire coupling (with a microscope objective) can be implemented. Insertion losses and near-field of the guided modes are measured.

Waveguide birefringence is characterized by a specific set-up involving waveplates and polarizers. Transmission spectra can be acquired by an optical spectrum analyzer to characterize directional couplers or Mach-Zehnder interferometers.

To increase the robustness of the devices the optical fibers can be permanetly glued to the optical waveguides. This is done in a professional pigtailing workstation that can produce fiber-to-waveguide permanent coupling with excess losses as low as 0.5dB/facet.

 

Control of liquid flow in microfluidic networks

In microfluidic circuits we verify that a suitable flow of liquid can be generated and controlled. To this aim we exploit either hydrostatic pressure, by inducing a different height of the liquid at the two sides of the microchannel, or electroosmotic flow, by applying suitable electric fields along the channel. Another essential characterization tool of the optofluidic devices is the fluorescence microscope that allows us to image fluorescently labelled samples inside the microfluidic chips.