Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels

Abstract

We report a simple optical sensing device capable of measuring the refractive index of liquids propagating in microfluidic channels. The sensor is based on a single-mode optical fiber that is tapered to submicrometer dimensions and immersed in a transparent curable soft polymer. A channel for liquid analyte is created in the immediate vicinity of the taper waist. Light propagating through the tapered section of the fiber extends into the channel, making the optical loss in the system sensitive to the refractive-index difference between the polymer and the liquid. The fabrication process and testing of the prototype sensing devices are described. The sensor can operate both as a highly responsive on-off device and in the continuous measurement mode, with an estimated accuracy of refractive-index measurement of approximately 5 x 10(-4).

Optical microfibers and nanofibers

“…Benefitting from the large fractional evanescent fields of a waveguiding MNF, the intensity-dependent MNF sensor usually offers high sensitivity in either air or liquid solutions. In 2005, Polynkin et al reported a MNF optical sensor for measuring the refractive index of liquids in microfluidic channels [23]. By measuring the refractive-index-dependent leakage loss of 1.5-μm-wavelength light guided in a 700-nm-diameter silica MNF, they realized an accuracy of refractive-index measurement of 5.3 × 10 -4 .…”

“…(2) Strong evanescent field Strong evanescent field offers strong near-field interaction between the MNF and its surroundings, making the MNF highly favorable for optical sensing [21][22][23][24][25][26][27][28][29][30][31][32] and evanescent coupling between the MNF and other waveguides (e.g., a semiconductor [33,34], metal [35,36] nanowire or planar waveguide [37]) or a substrate [30,35,38,39]. Based on the high-efficiency evanescent coupling, a variety of optical components or devices (e.g., loop and knots resonators [40][41][42][43][44][45][46][47][48][49][50][51], lasers [52][53][54][55][56][57][58], and sensors [21][22][23][24][25][26][27]…”

“…Based on the high-efficiency evanescent coupling, a variety of optical components or devices (e.g., loop and knots resonators [40][41][42][43][44][45][46][47][48][49][50][51], lasers [52][53][54][55][56][57][58], and sensors [21][22][23][24][25][26][27][28][29][30][31][32]) have been demonstrated. Meanwhile, since the high fractional evanescent filed is tightly confined around the MNF, it can produce steep field gradient which provides large optical gradient force for manipulating cold atoms [19,[59][60][61][62][63] or micro-/nano-particles [64,65], as well as large or manageable waveguide dispersion under single mode condition [11].…”

Microfluidic and bio-applications of optical microfibres

“…By embedding an OM in a transparent low refractive index polymer (Sylgard 184, also called PDMS) in proximity of a fluidic channel [9], the evanescent field of the mode propagating in the OM overlaps with the microfluidic channel, thus the refractive-index difference between the polymer and the fluid strongly affects the OM transmission. The accuracy in the loss measurement affects the minimum measurable refractive index change, which resulted to be of the order of ~5·10 -4 .…”

Optical microfiber passive components

“…Polymers have been used as sensing media because they can absorb gas and their optical properties change when impregnated by gases. Bare silica OMFs were used to detect sub-monolayers of chemicals absorbed on their surface [48]. Molecules and their agglomeration dynamics were recorded on a second to minute scale.…”