Abstract:
Optical fiber ball resonators based on single-mode fibers in the infrared range are an
emerging technology for refractive index sensing and biosensing. These devices are easy and
rapid to fabricate using a CO2 laser splicer and yield a very low finesse reflection spectrum with
a quasi-random pattern. In addition, they can be functionalized for biosensing by using a thinfilm
sputtering method. A common problem of this type of device is that the spectral response is
substantially unknown, and poorly correlated with the size and shape of the spherical device. In
this work, we propose a detection method based on KarhunenLoeve transform (KLT), applied to
the undersampled spectrum measured by an optical backscatter reflectometer. We show that this
method correctly detects the response of the ball resonator in any working condition, without prior
knowledge of the sensor under interrogation. First, this method for refractive index sensing of a
gold-coated resonator is applied, showing 1594 RIU1 sensitivity; then, this concept is extended to a
biofunctionalized ball resonator, detecting CD44 cancer biomarker concentration with a picomolarlevel
limit of detection (19.7 pM) and high specificity (30–41%).