Professor Guo Tuan's research group at Jinan University has proposed a compact fiber optic sensor for in-situ and continuous turbidity monitoring, based on surface optical scattering of polarization vanishing waves from target particles. The sensor consists of a tilted fiber Bragg grating (TFBG) encapsulated within a microfluidic capillary. The transmission spectrum of TFBG provides a set of fine narrow clad resonant combs that are highly sensitive to turbidity, as they are local light scattering caused by polarized vanishing waves near the fiber surface (in contrast to traditional bulk/volume turbidity measurements). In addition, a transmission spectral region interrogation method was proposed, and the reproducible correlation between surface turbidity and optical spectral region response was quantified. We demonstrate that maximum sensitivity in turbidity response can be achieved when the wavelength of the sensing envelope resonance matches the size of the surrounding solid particles.

Fiber optic sensors, including micro nano fibers, TFBG, and Fabry Perot interferometers (FPI), have shown great potential in field detection in biomedical, environmental protection, and energy storage fields due to their low invasiveness, resistance to electromagnetic interference, and chemical corrosion. Traditional turbidity meters have an emitting fiber end and a receiving fiber end, used to measure the scattered light intensity at a certain angle to the incident beam. Turbidity is inferred from the degree to which a beam of light is scattered by particles when passing through a sample. In addition, diffuse reflectance ultraviolet visible spectroscopy has been used to monitor the turbidity of water samples. The light is irradiated into the test sample through the fused silica collimating lens, and the reflected light is collected to estimate the mass and concentration of the sample in a non-contact manner. Recently, a method has been proposed to simultaneously distinguish the temperature and turbidity of liquid samples. In summary, all of the above methods focus on measuring overall/volumetric turbidity by evaluating the transparency of impurities in liquid samples. However, there is still a great need for quantitative measurement of local turbidity, that is, the turbidity at the surface of the target sample. For example, a recent paper reported tracking the chemical kinetics/state and capacity loss of batteries by monitoring the turbidity of electrolytes, which is induced by particle induced light scattering and absorption at the electrolyte electrode interface.

Professor Guo Tuan's research group from Jida University proposed a new method for in-situ surface turbidity measurement based on TFBG. The transmission spectrum of TFBG provides a set of narrowband cladding resonance fine comb patterns that are highly sensitive to surface turbidity, due to the possible multiple scattering effects between cladding modes and particles attached to the TFBG surface. This scattering manifests as high total insertion loss. When the diameter of the particles is much smaller than the wavelength of the incident light, Rayleigh scattering becomes the dominant scattering mechanism. However, when the size of the particles is comparable to the wavelength of the incident light, Mie scattering is more likely to dominate. A new spectral region interrogation method has also been proposed, in which the total sum of spectral changes in sensor cladding patterns is used to accurately measure turbidity changes. We have successfully established the correspondence between the spectral characteristics, turbidity, and particle size of TFBG. An additional advantage of our proposed TFBG sensor is that it utilizes the core mode's insensitivity to scattering and absorption in the surrounding medium, while only being temperature sensitive, providing a promising temperature independent surface turbidity measurement method. The directional tilt of the grating plane can effectively couple the forward propagating core mode into hundreds of backward propagating cladding modes, resulting in a dense comb like transmission amplitude spectrum as shown in Figure 1. Among them, the high-order cladding modes on the short wavelength side have strong vanishing fields. When the refractive index of the surrounding medium changes within the vanishing field sampling region of TFBG, the resonance position and amplitude of the corresponding cladding mode will also change accordingly.

Source: Sensor Expert Network