Polarimetric detection of cached objects and chiral solutes by light scattering in turbid media

Abstract

Photoelastic modulation (PEM) and synchronous detection of laser light scattered from an optically dense turbid medium can reveal the presence and topographical features of embedded targets invisible to naked-eye observation under ambient illumination, as well as provide a quantitative measure of the optical rotation, and therefore the concentration, of chiral constituents dissolved in a turbid fluid. A 544 nm helium-neon probe beam phase-modulated at f = 50 kHz was scanned across the front surface of a scattering cell containing an optically dense suspension of micron-sized polystyrene microspheres and different types of embedded targets. Backscattered light was analyzed for signals at the modulation frequency 1(f) and first harmonic I(2f), which gave nearly instantaneous measures (i.e. approximately over a modulation period T = 1 7 is) of the difference in intensities of orthogonal states of circular and linear polarizations, respectively. Examination of different targets showed sensitivity of polarimetnc imaging to edges, surface texture, and absorption. In another set of experiments the optical rotation and degree ofpolarization ofphase-modulated light was observed by forward, lateral, and back scattering from solutions of the enantiomer D-glucose containing a suspension of polystyrene microspheres. Optical rotations increased linearly with glucose concentration at a rate dependent on the microsphere concentration, and were large even at optical thicknesses sufficiently great to extinguish transmission of the incident beam. Applications of the techniques to remote viewing and biochemical analyses can be envisioned..

Real-time compensation method for robust polarimetric determination of glucose in turbid media

“…As an alternative to Faraday rotators, other modulators like Pockels cells [27] or photoelastic modulators (PEMs) [6,7,9,11,17] have been used in many other setups. A PEM is a piezoelectrically-driven crystal which modulates the phase of incident light along the modulation axis [17].…”

“…A PEM is a piezoelectrically-driven crystal which modulates the phase of incident light along the modulation axis [17]. The PEM is modulated sinusoidally, commonly with a frequency ω PEM = 50 kHz [6,7,9,11]. Similar to the Faraday-modulated polarimeter presented here, harmonics of ω PEM arise from the signal leading to frequency components of I(DC), I(ω PEM ) and I(2ω PEM ) [6,11,28].…”

“…The PEM is modulated sinusoidally, commonly with a frequency ω PEM = 50 kHz [6,7,9,11]. Similar to the Faraday-modulated polarimeter presented here, harmonics of ω PEM arise from the signal leading to frequency components of I(DC), I(ω PEM ) and I(2ω PEM ) [6,11,28]. Since the I(DC) part in the PEM approaches is a measure for absolute light intensity [18] while I(ω PEM ) and I(2ω PE M ) comprise only polarization information [11], the ratios I (ω PEM ) I(DC) [17], I (2ω PEM ) I(DC) [11] and I (ω PEM ) I (2ω PEM ) [17] have been successfully used to increase stability and to improve their signal-to-noise ratio (SNR).…”

“…Similar to the Faraday-modulated polarimeter presented here, harmonics of ω PEM arise from the signal leading to frequency components of I(DC), I(ω PEM ) and I(2ω PEM ) [6,11,28]. Since the I(DC) part in the PEM approaches is a measure for absolute light intensity [18] while I(ω PEM ) and I(2ω PE M ) comprise only polarization information [11], the ratios I (ω PEM ) I(DC) [17], I (2ω PEM ) I(DC) [11] and I (ω PEM ) I (2ω PEM ) [17] have been successfully used to increase stability and to improve their signal-to-noise ratio (SNR). We show that with I(ω) I(2ω) a similar method can be applied for Faraday-modulated polarimeters to increase the robustness in the presence of slightly turbid media and varying absolute light intensity.…”

“…Furthermore, we demonstrate that this method can be applied to reduce the disturbance impact of scattering by turbid media on glucose predictability. This is an alternative method to other approaches using closed loop systems [3,[13][14][15][16] and PEM-modulated polarimeters [7,9,11,17,18] as described in subsection 2.2.…”