Transillumination with non-ionizing radiation followed by the observation of transmitted and diffused light is the simplest, and probably the oldest method to obtain qualitative information on the internal structure of tissues or body sections. Although scattering precludes formation of high-definition image (unless complex techniques are employed), low resolution pictures complemented by information on the functional condition of the living sample can be extracted. In this context, we have investigated a portable optoelectronic instrumental configuration for efficient transillumination and image detection, even in ambient day-light, of in vivo samples with thickness up to 5 cm, sufficient for visualizing macroscopic structures. Tissue illumination is obtained with an extended source consisting in a matrix of 36 near infrared Vertical Cavity Surface Emitting Lasers (VCSELs) that is powered by a custom designed low-voltage current driver. In addition to the successful acquisition of morphological images of the hand dorsal vein pattern, functional detection of physiological parameters (breath and hearth rate) is achieved non-invasively by means of a monochrome camera, with a Complementary Metal Oxide Semiconductor (CMOS) sensor, turned into a wavelength selective image detector using narrow-band optical filtering.

Analysis and fine distinction of turbid fluids is challenging since standard techniques, usually applied to transparent liquids, fail to work properly due to scattering losses. The phenomenon of speckle pattern, which arises when such liquids are illuminated by coherent light, can be successfully applied to investigate the properties of these media. In this work, we exploit a simple instrumental configuration, combining a laser diode and a PC-interfaced, digital CMOS camera, for acquiring images and video of speckle pattern generated by illuminating turbid fluid samples that were obtained by dilution of opaque suspensions, such as commercial vegetable milk. By processing the acquired image sequences, we have been able to extract several statistical parameters that can be correlated with the concentration of the scattering elements of each sample, allowing an easy differentiation of the various fluids.

We present a flow-through refractive index sensor for measuring the concentration of glucose solutions based on the application of rectangular glass micro-capillaries, with channel depth of 50 µm and 30 µm. A custom designed and 3D printed polymeric shell protects the tiny capillaries, ensuring an easier handling and interconnection with the macro-fluidic path. By illuminating the capillary with broadband radiation centered at λ~1.55 µm, both the transmitted (T) and reflected (R) optical spectrum from the capillary are detected with an optical spectrum analyzer, exploiting an all-fiber setup. Monitoring the spectral shift of the ratio T/R in response to increasing concentration of glucose solutions in water we have obtained sensitivities up to 530.9 nm/RIU and limit of detection in the range of 10⁻⁵-10⁻⁴ RIU. Experimental results are in agreement with the theoretically predicted principle of operation. After the demonstration of amplitude detection at a single wavelength, we finally discuss the impact of the capillary parameters on the sensitivity.

In this work, we demonstrate spectral phase-shift interferometry operating in the near-infrared wavelength range for refractive index (RI) monitoring of fluidic samples in micro-capillaries. A detailed theoretical model was developed to calculate the phase-sensitive spectral reflectivity when low-cost rectangular glass micro-capillaries, filled with samples with different refractive indices, are placed at the end of the measurment arm of a Michelson interferometer. From the phase-sensitive spectral reflectivity, we recovered the cosine-shaped interferometric signal as a function of the wavelength, as well as its dependence on the sample RI. Using the readout radiation provided by a 40-nm wideband light source with a flat emission spectrum centered at 1.55 µm and a 2 × 1 fiberoptic coupler on the common input-output optical path, experimental results were found to be in good agreement with the expected theoretical behavior. The shift of the micro-capillary optical resonances, induced by RI variations in the filling fluids (comparing saline solution with respect to distilled water, and isopropanol with respect to ethanol) were clearly detected by monitoring the positions of steep phase jumps in the cosine-shaped interferometric signal recorded as a function of the wavelength. By adding a few optical components to the instrumental configuration previously demonstrated for the spectral amplitude detection of resonances, we achieved phase-sensitive detection of the wavelength positions of the resonances as a function of the filling fluid RI. The main advantage consists of recovering RI variations by detecting the wavelength shift of “sharp peaks”, with any amplitude above a threshold in the interferometric signal derivative, instead of “wide minima” in the reflected power spectra, which are more easily affected by uncertainties due to amplitude fluctuations.

In this work we present a smart micro-opto-fluidic platform based on rectangular-section glass micro-capillaries with integrated reflectors. The micro-opto-fluidic setup was exploited for absorption spectroscopy measurements in the near infrared range from 1.0 μm to 1.7 μm. The external flat sides of the micro-capillaries are coated with thin Aluminum layers to create multipath micro-devices in which light can bounce multiple times. Broadband light provided by a Tungsten lamp is coupled into a multimode optical fiber and shone onto the flat surface of the micro-capillary. After travelling inside the micro-channel filled with the sample, radiation is directed towards an optical spectrum analyzer. The line-shape of the output light spectrum depends on the absorption features of the sample and, in particular, on the concentration of water in alcoholic solutions. Experimental results are in good agreement with the prediction given by the developed theoretical model that describes light transmission through the fluidic micro-channel. Moreover, a responsivity parameter, defined as the output power ratio at two wavelengths, is retrieved: water sensitivity is greatly enhanced by the multipath effect since it is proved to be directly proportional to the light bounces in the micro-channel.